concrete materials and properties- problems and solutions

Concrete Materials

Copyright 2003 by Taylor & Francis Group. All rights Reserved.

To Evelyn


Concrete Materials

Problems and solutions

M.LevittPhD, FIQA, MICT

E & FN SPONAn Imprint of Thomson Professional

London • Weinheim • New York • Tokyo • Melbourne • Madras


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Contents

Preface

1 Concrete materials1.1 OPC, strength gain and sulfate and frost resistance1.2 OPC and curing1.3 Aggregates and frost damage1.4 Air-entraining agents and frost damage1.5 Alkali-silica reaction1.6 Calcium chloride1.7 Aluminous cement1.8 Steel reinforcement: additional requirements1.9 Excess steel reinforcement1.10 GRC and alkali-glass reaction1.11 Fibre-reinforced sandwich panels1.12 Delayed ettringite formation

2 Health and safety2.1 Cement eczema2.2 Cement burns2.3 Pumping grout

3 Concrete on site3.1 Covercrete d or k3.2 Spacers for rebars3.3 Tiling and moisture in floors3.4 Flatness of floors3.5 Flatness of formwork3.6 Joints between precast paving and between kerbs3.7 Tolerances3.8 Rising damp and a chemical barrier3.9 Cracking in the non-visual zone


3.10 Large-area scaling of floors3.11 Silanes

4 Specification problems4.1 The CE mark4.2 Durability4.3 Concrete quality4.4 Specifying strength

5 Precast concrete5.1 Hydration staining5.2 Lime bloom5.3 Colour variations5.4 Cracking and slenderness ratio5.5 Thermal cracking in pipes5.6 Tunnel segment impact damage5.7 Tesserae detachment

6 Testing6.1 Labcrete or realcrete6.2 Design or performance6.3 Camouflage testing6.4 Repeatability and reproducibility6.5 Changes in testing6.6 Testing fixation6.7 Testing accuracy

Glossary


Preface

In more than 40 years’ experience in the construction industry, I have spenta lot of time in what is commonly known as ‘troubleshooting’: on site withworks in progress, on completed projects, and sometimes even on anoccupied building or civil engineering project.

The advice I gave to the party raising the problem was usually based onmaterials science. My main aims were to explain the mechanisms that Ithought had caused the effect, and to suggest possible remedial measures.With the wide spectrum of problems that I have encountered, I felt that Icould make a useful contribution to the concrete industry by setting theseexperiences down on paper.

This book is the result. Each chapter deals with a particular problemarea, organised into sections that are generally relevant to that chapter.While I was deciding how best to organise the material, my attention wasdrawn to the Building Research Establishment’s Defect Action Sheets.These DASs ceased issue in March 1990, but their format—illustrating aproblem, dealing with its cause(s), and offering remedial proposals—wasvery useful. Following this vein, I have described each problem on thebasis of personal experience, with sections discussing identification,remedial measures, and avoidance. In most sections these three items formthe final paragraphs. However, in problem areas that generated secondaryproblems, the three items have sometimes been discussed with each of thesubproblems.

The book reflects personal experience, with a description of the bestspecific solutions that were found. The hardest part of problem solving isidentification, and this is often aggravated by there being more than onedamaging mechanism present. Therefore I make no guarantee that therecommendations in this book will always work.

A significant degree of overlap between chapters was unavoidable. If aproblem is described in a particular chapter, it does not mean that theproblem could not or does not occur elsewhere. For example, lime bloomis discussed in Chapter 5 (Precast concrete), but it is also known to occur


with in-situ concrete. However, it is not as much of a problem as it is forprecast concrete products.

A basic knowledge and appreciation of aspects of materials science isessential in dealing with these problems. Architectural preferences andengineering matters are not my discipline, and have been largely avoided.Even so, most problems that arose were probably due to a lack of mutualunderstanding between the various professions involved.

The causes of most problems are to be found in design, workmanship ormaterials. The division between design and workmanship is not verydistinct, but together these two account for over 80% of problems. It isunfortunate that materials have received, and continue to receive, too muchattention. When I analysed troubleshooting problems recently, I found thatmaterials accounted for only 18% of the total.

More often than not, the construction industry has problems withconcrete for the apparently simple reason that many procedures areundertaken in ignorance of the requirements of the other professions. Ingeneral, the solution is to identify the problem area and put the rightquestions to the right person(s). Although I have spent 20 years in theprecast concrete industry, and a similar time in general construction, theviews, recommendations and comments made in this book are mine alone.They do not necessarily reflect the views or policy of the precast concretetrade federations, nor of any companies within the Laing Group.

I have used the minimum number of references necessary to supportthe text. The advice given should stand alone, as being based upon goodfaith and experience.


Concrete materials 1

INTRODUCTION

Although this chapter and Chapters 3 and 5 are devoted to materials, innearly every case it was the deployment of these materials that gave rise toproblems, and not the materials themselves.

The introductions to the remaining chapters are much longer than thisone. This chapter covers a large range of materials problems, and eachsection has its own specific introduction. The introductions to theremaining chapters have a somewhat different function, in that each hasthe role of colouring in the background to what follows in the individualsections.

1.1 OPC, STRENGTH GAIN AND SULFATE AND FROSTRESISTANCE

A Concrete Society technical report (Concrete Society, 1987a) comparedcement property changes in the years 1960, 1974 and 1983. More recentdata has not been published, as far as I know, and I have made somecomments based upon individual cement test certificates that have comeinto my possession.

The principal changes have been in the percentages of the two calciumsilicates that are the main contributors to strength through their formationof calcium silicate hydrate (CSH) in the cement’s chemical reaction withwater. These are tri-calcium silicate (C3S) and di-calcium silicate (C2S).Typical contents of these in the late 1940s were 40% and 35% respectively.Representative figures for 1990s production could be 60% and 15%. Thesepercentages are intended to illustrate medians covering a range of cementplants and in-plant variations; I have a not-so typical cement test certificatein which the C3S and C2S contents are reported as 70% and 7% respectively.

Because C3S hydrates much more rapidly than C2S, strength is builtup much more quickly, but the 28-day strength is not significantly


affected. In effect, the shape of the strength-time curve has changed.This more rapid strength gain has had most effect in the in-situ concreteindustry, in that formwork can be stripped earlier, but the attractionhas been that the usual stripping times have been maintained whileusing less cement than was the case before cements were so high inC3S.

This is the main cause of the problems discussed in this section. Thedecision to decrease the cement content for an equivalent early strengthbehaviour ignores the detrimental effect that it can have on resistance todurability risks such as sulfate and frost attack. Decreased cement contentpromotes more voids in the concrete matrix that would otherwise havebeen filled up with cement paste. These voids result in increased spacewithin the concrete or mortar for the ingress of sulfates, frost-prone waterand/or de-icing salts/chemicals.

Changes over the years in the equivalent Na2O and alkali-silica reaction

are discussed more fully in section 1.5.Cement fineness also increased in the two decades reported from 1960

to 1983 (Concrete Society, 1987a), and this change also added to a morerapid hydration rate but with no significant effect upon the 28-day strength.The effect that this, coupled with the C3S and C2S changes, has had onthermal and moisture curing is outlined in section 1.2.

A problem that disappeared with the cessation of precast concrete pipemanufacture by the spinning process was that too fine a cement resulted intoo much cement separating at the inside of the bore. Although cement hasa higher density than conventional aggregate as well as water for the veryfine powders that are spun, segregation is more a function of particle sizethan of density. Lighting columns are commonly made by the spinningprocess, but the problem of segregation of cement has not been brought tomy attention. The original problem with spinning pipes was solved by themanufacturer by purchasing a cement with a specific surface of about260m2/kg and sold as ‘coarse-ground Portland cement’. A typical currentOPC specific surface is about 330m2/kg.

In the following sections, sulfate attack has been discussed singularly,but frost attack has been divided into two groups: direct frost attack withno other considerations, and attack accompanied by de-icing or anti-frostchemicals. Weathering due to the effects of wind and rain alone has alsobeen included.

1.1.1 IDENTIFICATION

(a) Sulfate attack

A softening and loss of the surface layers of the concrete is due to theformation of calcium alumino-sulfate (ettringite) from the reaction of


ground or air/rain-borne sulfate with the tri-calcium aluminate (C3A)component of Portland cement. The reaction is expansive, because theettringite takes up more space than the C3A hydrate with which the sulfatereacted. Delayed ettringite formation is discussed in section 1.12.

(b) Frost attack without de-icing salts or anti-frost chemicals

The original international research (RILEM, 1977) explained the meaning ofcritical saturation and specified the relevant tests. In simple terms, when thewater content in the water-accessible void/capillary structure reaches acritical level, if frost occurs damage can take place because of ice formation.For example, if the critical saturation of a specific concrete is 80% and itstotal water-accessible space is 15% by volume (approximately equal to 6%by mass), then frost damage is possible when the water saturation levelreaches 0.8×15=12% by volume (about 5% by mass) or more.

Frost damage usually manifests itself as surface spalling and/orsoftening, resulting in exposure of the aggregate. Less commonly, damageoccurs not by any apparent surface loss but by a decrease in the elasticmodulus, which for concrete used in structural applications could result indeflection beyond that designed.

Attempts to look for frost damage in open-textured concrete, such astypical concrete blocks and no-fines concrete, are generally fruitless. Thisis probably because such concretes have plenty of time to drain out to belowcritical saturation level before frost occurs.

In addition, in very severe winters problem areas of frost damage needbe identified only when salts or chemicals have been used. This is because,for ordinary frost damage without salts or chemicals being involved, mildwinters tend to cause more problems as more freeze-thaw cycles areinvolved than during very cold weather.

The other area of frost damage identification that can safely be put torest is that of water in blowholes being subject to freezing. No case of frostdamage due to such action has ever been reported; ice in a blowhole has afree face out of which it can expand as water freezes.

(c) Frost attack with de-icing salts or anti-frost chemicals

Identification is generally in the form of severe surface spalling and acommon ‘wet’ appearance. The use of salts or chemicals aggravates andaccelerates frost damage for two main reasons and one subsidiary one.

First, the use of salts (commonly salt as NaCl) and chemicals such asethylene glycol or urea depresses the freezing point to below 0°C. Themethod of broadcasting these materials onto concrete results inconcentration variations, which passing traffic can exacerbate. Additionalfreeze-thaw cycles can take place with no change in external temperature.


Second, when common salt dissolves into solution the reaction isendothermic, which can cause concrete just above freezing point to freezefor a while as it cools the surface.

Third, although ethylene glycol is not deliberately used to de-iceconcrete (it is used for spraying onto aircraft) it is an additional risk. Notonly can any chemical that drops onto the concrete cause a similar freezingpoint depression, but the glycol itself has a slow dissolution effect on thecalcium salts in the hydrated cement.

(d) Weathering

This is a dusting and softening of a 0–2mm depth of the surface, whichtakes place over many years, and is noticeable on unsheltered surfaces.

1.1.2 REMEDIAL

Remove all suspect and degraded concrete, and patch or full repair using(preferably) a polymer mortar (Concrete Society, 1984; Perkins, 1986).

1.1.3 AVOIDANCE

The cementitious content should be kept in the range 375–450kg/m3.

1.2 OPC AND CURING

Hydration of cement is an exothermic reaction: heat is produced, and themore rapid the reaction process the more rapid the heat production. Thehigher contents of C3S now common in OPC (as well as the form of themore finely ground rapid-hardening Portland cement), together with thehigher fineness levels of modern cements, have caused concrete to emitheat more quickly than in previous years. These factors have resulted intwo potential problem areas that have given increased cause for concern.

The first of these is high thermal gradients, which can give rise to thermalcracking, particularly in thick sections of OPC concrete. This cracking isthought to be caused because the strain set up by the thermal gradientexceeds the strain capacity of the concrete in its hardening state (Harrison,1992). Thermal cracking can also arise when concrete is cast against a coldsubstrate such as older concrete.

The second risk area is too rapid a loss of moisture at the surface (Birt,1985). This can give rise to drying shrinkage cracks, and can also result insurface softening, caused respectively by shrinkage from too high a rate ofmoisture loss and by there being insufficient water in the surface area tohydrate the cement effectively.


It may be seen, therefore, that the subject of curing problems alwaysneeds to be qualified by reference to whether the curing is for thermal ormoisture reasons or both.


(a) Thermal cracking

This is most noticeable when there is a maximum temperature gradientthrough the section, which for thick (over 0.5m) plain OPC concrete occurstypically at 40–50 hours. Crack apertures up to 2mm wide are notuncommon. However, as the temperature gradient becomes less steep,these cracks largely close down to apertures of about 0.5mm. These crackpositions often coincide with the main rebars, and the cracks penetrate asfar as the steel. However, in an in-depth examination of marine-exposedconcrete I observed that the crack width tapered rapidly from the surfacedown to about 10mm deep, where it kept to a consistent value of about0.1mm. Chloride penetration from exposure to sea water did not occurbelow this 10mm depth in the crack, and was marginal in all other areas.

Thermal cracks occurring at the junction of new and old concreteinterfaces (the old concrete being known as a ‘heat sink’) may generally beseen to occur at right angles to the interface and horizontal and parallelfurther up the ‘wall’.

(b) Drying shrinkage cracking

This has typical crack apertures in the range 0.1-0.5mm, tapering down tozero at depths of about 10mm. The cracking commonly manifests itself as aseries of parallel lines about 100–200mm apart and at right angles to the longestaxis of the section. For example, on a precast concrete column, the dryingshrinkage cracks would be seen as a number of ‘bracelets’ along the unit.

As with crazing in cast stone and other concretes, drying shrinkagecracks can seal themselves by autogenous healing, and this effect isillustrated in Figure 1.1 for the crazing on a cast stone pedestal.

1.2.2 REMEDIAL


A low-viscosity resin can be used (Concrete Society, 1984) if the cracksare considered to be a risk. This can be assessed by estimating the crackdepth by means of the ultrasonic pulse velocity (UPV) method (BS 1881Part 203:1986). This method is practical only if the cracks are nominallyfree of water and the average crack width over all positions is at least


0.2mm. The efficacy of the repair system used can be examined using theinitial surface absorption test (BS 1881 Part 208:1996). Repaired andunrepaired areas can be compared to eliminate the effect of the absorptioncharacteristics of uncracked concrete.


These cracks are generally aesthetic defects rather than a corrosion hazard,so as a rule no repair is required. If the effect needs to be masked, thensurface grinding followed by a flood application with a silicone (BS6477:1992) is recommended. As far as the lifetime of this form of remedialmeasure is concerned, the longest in my experience was shown by caststone lintels on a school, which were treated in 1958. The lintels wereexamined in 1996 and found still to be in good condition.

1.2.3 AVOIDANCE


Use formwork with built-in thermal insulation and/or leave thermallyinsulating covers in place until both the concrete surface temperature andthe temperature gradient have reached acceptable levels. As far as isknown, no quantitative guidance is available on concrete surfacetemperature, because much depends on air temperature, wind speed,

Fig. 1.1 Autogenous healing of crazing on cast stone plinth.


relative humidity and the possible risk of rain. Taking wind speed andwind chill into account, a difference of 10 deg C is probably the maximumacceptable.

As far as temperature gradient acceptability in the concrete section isconcerned, the recommendations suggested for precast concrete productsshould be similar to those for in-situ concrete (Levitt, 1982). Recommendedtargets are a maximum temperature gradient of 0.1 deg C/mm and amaximum cooling rate of 15 deg C/hr. These maxima could probably berelaxed (that is, increased) for lightweight aggregate or limestone aggregateconcrete, but might need to be more severe for flint gravel or igneous rockaggregate concretes.

As far as is known there is no published method for measuring thesurface temperature of concrete. My experience has shown that an ordinarycopper-constantan thermocouple spirally wound for its last 30–50mm andstapled to the face of a hand-held cork is suitable.

In addition to or in place of using thermally insulating formwork and/or covering the surface, it is worth considering procedures such as theinclusion in the concrete mix of additives such as PFA (BS 3892 Part 1:1993)or GGBS (BS 6699:1992), or the use of a retarding admixture, which willboth modify the exotherm-time curve with less heat being produced in thecritical period.


Good practice in mix design and procedures to inhibit moisture loss (Birt,1985; Neville, 1995) inhibit or prevent this form of cracking. Typicalmethods of moisture curing are by covering with soundly fixed polythene,or with hessian kept wet for at least the first 48 hours. These methods ofcuring should not be used for visual concrete such as cast stone andexposed-aggregate concrete, because they can promote hydration staining,lime bloom and colour variations (see sections 5.1, 5.2 and 5.3 respectively).

Figure 1.2 illustrates typical locations in construction of both thermaland drying shrinkage cracking.

The necessity for directing attention to curing being required for thermalor moisture reasons or both is repeated.

1.3 AGGREGATES AND FROST DAMAGE

Aggregates such as flint gravels, and igneous rocks such as granite andbasalt, are generally resistant to frost, but they have been observed to causepop-outs when de-icing salt has been used (Fig. 1.3).

In my experience the occurrence of these pop-outs is a functionneither of concrete quality nor of the method of manufacture. It has been


observed on in-situ concrete (both air-entrained and non-entrained) as wellas on wet-cast vibrated and hydraulically pressed precast concrete units,and for both flint gravel and granite aggregates.

The mechanism of frost damage and critical saturation has already beendiscussed in section 1.1, and it is possible that there are two processes thatsingly or jointly could exacerbate this mechanism.

First, concrete that has been broadcast with de-icing salt will havebecome more hydrophilic (attracting water) because of the presence of saltin its voids and capillaries.

Second, particles of aggregate of flint or granite and the like will have ahigher specific heat than the surrounding mortar, and will subsequentlyrespond more quickly to changes in temperature. This will result in theseparticles being colder nearer the surface of the concrete, and becomingwetter around their peripheries. These higher saturation levels would bedue to the moisture in the surrounding area moving towards the particles,

Fig. 1.2 Typical locations for thermal and drying shrinkage cracks (not to scale).(After C.J.Turton)

Fig. 1.3 Example of frost-induced pop-out at piece of aggregate.


which because of their lower temperature have a lower vapour pressure.(This is why, in a refrigerator, food will dry out unless it is encased, becausemoisture will migrate towards the colder condenser area.)


These are aesthetic defects in the form of pop-outs typically 10–20mm indiameter and 1–3mm deep, with the aggregate particle visible at the bottomof the pop-out. In general the pop-outs occur only at the largest pieces ofaggregate. For example, if the aggregate maximum size is 20mm, pop-outswould as a rule be observed at the near-surface 20mm particles.

1.3.2 REMEDIAL

This is a difficult matter to address because, provided the presence of pop-outs can be accepted, no remedial action is necessary. Any attempt atremedial work in the form of a paint or mortar coating could exacerbatethe occurrence of pop-outs, because moisture could become trapped orinhibited from evaporation. A possible way of dealing with the aestheticproblem of these pop-outs would be to feature the aggregate exposurerather than try and hide it. This could be achieved by exposing theaggregate over the whole of the visual face, possibly using a flametreatment or another suitable method.

The use of de-icing salt can be minimised by ensuring well-maintaineddrainage.

1.3.3 AVOIDANCE

Assuming that de-icing salts are likely to be used, and that efficientdrainage (including its maintenance) is unlikely to be installed, it can beadvantageous to select a coarse aggregate with a low specific heat, such aslimestone or sandstone. An alternative or addition to this precaution wouldbe to silicone (BS 6477:1992) the concrete on site, or to incorporate a water-repellent admixture such as 1–2% m/m cement of stearic acid powder. If awater-repellent admixture is being considered and another admixture isalso planned to be used, their compatibility needs to be assessed beforeconcrete production (see sections 5.2 and 5.3).

Some so-called limestones and sandstones are prone to frost attackbecause they have a significantly high water retention capacity of theirown, and a study of their track history is recommended. Flint gravels witha thick cortex surface coating (white, 1–3mm thick typical layer) can bealso prone to frost damage. Any such risk with these aggregates can beindicated by a simple test. Take exactly 100 pieces of the largest aggregate


size to be used, soak them in tap water for about 24 hours, then drain offand place in a polythene bag. Seal the bag and place it in a deep freezeovernight; take the bag out the following morning, thaw, and recount. Ifthere are more than 100 particles then pop-outs can be predicted, and thenumber above 100 gives an indication of the intensity of the pop-out frostdamage.

1.4 AIR-ENTRAINING AGENTS AND FROST DAMAGE

This section is solely concerned with the reliance placed upon the rathermisconceived idea that having the specified amount of air in the freshconcrete always means that the concrete, in its hardened state, has good orimproved frost resistance. I have been involved in several cases oftroubleshooting where frost damage, in the form of severe surface spalling,has occurred for concretes with the correctly specified fresh concrete aircontent.

It has been known for over 40 years (Powers and Helmuth, 1953) thatthe critical characteristics are optimum bubble sizes and spacing factors,and not the total amount of air in the compacted concrete. The freshconcrete air content test (BS 1881/106:1983) does not give any informationon bubble size, nor on the bubble-spacing factor. An added disadvantageis that the BS test does not differentiate between entrapped air andentrained air; entrapped air contributes insignificantly to frost protection.

These bubble geometries are usually verified by a microscopic method(ASTM, 1990) on slices taken from samples (usually drilled cores) of thehardened concrete. In all the troubleshooting cases that used this USstandard to examine the air void structure, the total amount of air wasfound to comply with the specification, but the recommended bubble sizesand spacing factors did not obtain.

Reference can be made to the British Standard (BS 5075 Part 2:1982),both for the recommended bubble geometries and for a freeze-thaw teston samples of ‘labcrete’. Typical diameters of the spherical air-entrainedbubbles would lie in the range 0.5–0.05mm and the spacing factor in therange 0.2–0.3mm. The freeze-thaw test in this standard does reflect thebubble geometries, but the test is on a specified laboratory concrete—‘labcrete’—and not on ‘realcrete’.

A European standard (BS prEN 480–11:1996) is in course of preparationthat specifies a microscopic test based upon the ASTM standard.

This section is solely concerned with the form (geometries) of theentrained air, and is not concerned with workmanship factors.

An example of misapplication is described in section 3.10, to whichreference may be made when large-area scaling occurs without theoccurrence of frost.



This defect shows itself as surface spalling, typically to a depth of 10–20mmwith exposure of the larger particles of aggregate. Sometimes the spallingis in the form of isolated areas, but at other times larger areas can spall.Confusion with the pop-outs discussed in the previous section is unlikelybecause frost spalling does not, as a rule, result in conical craters asillustrated in Fig. 1.3. Also, the large-area scaling discussed in section 3.10is unlikely to be blamed, because that scaling is typically in the form ofsheets 1–3mm thick and covering areas from 0.1m2 up to complete baysize.

1.4.2 REMEDIAL

Remove all degraded material; wash the surface thoroughly, and refacewith a polymer mortar or a frost-resistant concrete overlay. Considercomplete concrete replacement only if there are doubts about the propertyof the apparently unaffected material.

1.4.3 AVOIDANCE

If an air-entraining admixture is to be used it should comply with BS 5075Part 2, and should be assessed on the actual concrete to be used in theworks, taking into account all the variables that are likely to occur on site.The recommended method of assessment is by either microscopic or freeze-thaw tests on representative hardened concrete samples.

The alternative to using an air-entraining agent is to use a well-mixedand compacted and cured (thermal and/or moisture) concrete with acementitious content in the range 375–450kg/m3.

The cost considerations of the selected path would need to be discussed.

1.5 ALKALI-SILICA REACTION

Although the few recorded cases of alkali-silica reaction have generallyresulted in severe cracking, leading to some loss of structural integrity, theproblem discussed in this section is not this (chemical) reaction. It is, rather,the human reaction of a large cross-section of those involved in constructionto the possibility of this chemically expansive reaction causing problemswith their concrete.

Alkali-silica reaction, or ASR as it is commonly known, is generally along-term chemical reaction that takes place between the alkali in thecement and other ingredients in the aggregate (Concrete Society, 1987b;Swamy, 1991). Although virtually all siliceous aggregates contain some ofthis alkali-prone material, it is generally those containing strained quartz


such as opaline that cause most problems. The reaction that takes place isthe formation of sodium and potassium silicates by a slow and expansivereaction of the sodium and potassium hydroxides with the available silica.

As there will always be some available silica, whichever aggregate isused, the thing that needs to concern those involved is the question: Is theASR expansion that will occur going to be a problem? The longevity of somany constructions in the UK without any manifestation of such problemsindicates that the general answer to this question is ‘No’.

There is a requirement for updating on concrete performance in respectof ASR, because CSTR29 (Concrete Society, 1987a) referred to OPC propertychanges only by comparing data from three years: 1960, 1974 and 1983.The alkali equivalent of cement from those data, given as Na

2Oequiv.,

remained constant at about 0.6%m/m. Not only did these data exclude theperiod since 1983, but also they did not take into account the properties ofthe (then and now) cements imported from Greece and Spain. A notuntypical analysis of one of these imported cements gave a batch-to-batchNa

2Oequiv. of 1.2%.

Notwithstanding comparisons between UK-manufactured andimported cements in their Na

2Oequiv. contents, the few cases of damage

reported (Concrete Society, 1987a) indicate that ASR is a problem whenspecific aggregate location sources are likely to be used. Suppression ofthis reaction is relatively simple, and recommendations are made in section1.5.3. The few cases reported seem to indicate that cements are not themain problem.


On the few occasions that ASR damage has been observed it takes the formof generally random cracking, with crack apertures up to 10mm. However,the crack widths taper rapidly with depth. Secondary cracking often occursin the form of map cracking with crack apertures typically up to 1mm. Agel-like exudation similar to ‘egg-glass’ (waterglass) may be seen at theapertures.

1.5.2 REMEDIAL

If the problem is with the specification wording, which could be somethinglike ‘Not more than 3kg Na

2Oequiv. per m3 concrete’, draw the attention of

the parties concerned to the low—or zero-risk situation if that concrete hasalready shown a good track record.

If the problem is an actual risk, and damage has occurred, then eitherremoval and replacement of the damaged concrete and/or a protectivewaterproofing render will generally inhibit further damaging reaction.


1.5.3 AVOIDANCE

If it is definitely known that there is a risk of damaging ASR then this canbe inhibited by specifying an additive such as PFA, GGBS or MS in theconcrete mix design.

It is worth observing that, many decades ago, ships delivering cargo toLondon from the USA went back with Thames Valley aggregates andcements as ballast, from which much of the currently standing New YorkHarbour was built. These concretes are still performing well, even thoughthey are subject to wetting and drying conditions and sea water effects.

1.6 CALCIUM CHLORIDE

The use of calcium chloride is now generally banned in specifications. Themain problem that used to be encountered was that corrosion of thereinforcement was accelerated if water and air penetrated to the rebars.Calcium-chloride-induced corrosion rarely occurs nowadays, but instancesstill come to light. Calcium chloride admixture accelerates both the settingand the hardening rates of Portland cement concrete. The main attractionof this was to the precast concrete industry, and the use of calcium chloridewas concentrated in the 1950s and 1960s. It enabled manufacturers toachieve adequate demoulding strengths at earlier ages and/or a decreasein the cement content. This latter route was often chosen, and led toconcrete that contained more voids than would otherwise have been thecase, thus giving less protection to the reinforcement.

Steel reinforcement has a passive oxide layer on its surface when it issurrounded by a highly alkaline environment such as uncarbonatedcement. This high alkalinity can be reduced by a carbonation front reachingthe steel or, much more rapidly, by chloride reaching the steel. Once thispassive oxide layer breaks down, owing to either or both forms of ingress,the steel becomes subject to corrosion.

CP 110, the old (and now superseded) code of practice for structuralconcrete, used to recommend a maximum of 1.5% anhydrous calciumchloride m/m cement. There also used to be a commercially available OPCinto which this level of calcium chloride had been added. The materialwas sold as ‘417 Cement’. It is possible that if this admixture had beenaccompanied by a recommendation on the minimum cement content, andthis had been adhered to, the status quo might be different. The structuralconcrete division of one large precast manufacturer used to use calciumchloride regularly in structural concrete units. As far is known, no case ofcorrosion of reinforcement in any of the company’s units has ever beenbrought to light. The reason for this probably lay in that company’s strictattention to quality control, and in its use of a high minimum cementcontent.


Although it is a dubious thought, because the addition of calciumchloride would accelerate the corrosion risk with mediocre or poor-quality concrete, it could be a useful means of accelerating trouble thatwould otherwise have occurred rather late in the day. The facade shownin Fig. 1.4 was of white cement precast concrete architectural units, whichbecame corrosion damaged at 2–3 years old. The word ‘quality’ glimpsedin the lower right corner of this photograph obviously applied to thecars, but it has been misused as an adjective (see section 4.3) instead of anoun.


This defect is seen as spalling and/or cracking, with cracks commonlymirroring the rebar positions, together with rust staining marks at the crackapertures. When pieces of concrete are broken off to expose rebars, a slimy

Fig. 1.4 Chloride-accelerated corrosion in precast concrete cladding.


green coating may be seen on the steel surface. This is ferrous chloride,which, within some seconds’ exposure to air, turns into the brown ferricchloride. An additional test for chloride is to rub a finger over the steel andtaste it, or even lick a piece of spalled concrete. This will generally give asalty taste.

1.6.2 REMEDIAL

Following the Concrete Society’s recommendations (Concrete Society,1984), effect a repair after removing all degraded and suspect concrete andapplying a protective coating to the rebars. Even concrete where there isno visible damage needs to be assessed, as those areas may have corrosionoccurring at the steel, and/or the quality of the covercrete might not beacceptable.

1.6.3 AVOIDANCE

The answer is simple: do not use calcium chloride admixture. If excesschloride is likely to find its way into the mix, either by deliberate additionor by the use of contaminated materials, then the use of additives isrecommended. Where this is to be applied as a remedial action then thecementitious content should lie in the range 375–450kg/m3 and have 30%PFA, 70% GGBS or about 7% MS cement replacement.

1.7 ALUMINOUS CEMENT

This is the updated description for what used to be known as ‘high-aluminacement’ or HAC. The problem with this cement is that it is subject to whatis known as ‘conversion’. This is an additional chemical change (followingthe initial hydrates formation) in the calcium alumino-hydrates, promotedunder conditions of high humidity and temperature. These later changesgenerally cause a significant loss in strength (20–50%), and—possibly moreimportantly—this is coupled with an increase in the permeability.

Virtually all aluminous cement concretes were produced as precast unitsby three main manufacturers specialising in the production of prestressed,pretensioned concrete products. This production was undertaken mainlyin the years 1946–1975 but generally ceased at the time of the bans and partbans produced by the Government in the mid-1970s, following the partialcollapse of two constructions. The popularity of this cement was probablydue to its rapid build-up of strength: for example, 28-day equivalent OPCconcrete strengths were obtained after 2–3 days. The setting times ofaluminous cement concrete were (and are) not significantly different fromthose of OPC concretes; only the hardening times differed.


Many organisations involved in consultancy or testing have carriedout examinations and tests on buildings where aluminous cementconcrete had been used. The prime purpose of these exercises was toassess the concrete from the structural viewpoint—that is, to assess theeffect of any conversion on the strength of the concrete—rather than tostudy the permeability changes or any other design implications.Troubleshooting work has been undertaken in both the conversion andthe permeability areas.

In all, about 1000 constructions were examined (for this purpose I havecategorised a number of dwellings on a housing estate as a singleconstruction). Over the three decades referred to above about 15 000 000precast units were manufactured, and these were built into 60000constructions. The number of constructions would probably lie in theseveral hundreds of thousands if each dwelling on a housing estate hadbeen separately counted. So a very small proportion of the total number ofconstructions was examined: about 1.5%.

About four constructions out of these estimated 1000 were foundwanting because of either loss of structural integrity or degradation of theconcrete by chemical action. Four out of 60 000 is a very small proportionof the total number of units manufactured.

Personal experience in site examination was directed towards examiningsamples of concrete in the laboratory, using differential thermal analysis toestimate the degrees of conversion. Most analyses gave a range ofconversion levels of 70–90% with no noticeable distress due to structuralcauses. Webs of T-section precast concrete beams were examined usingUPV but, again, problems were seldom found, and, when present, werenot at significant levels. The problems found were in two areas: detailingat supports and the previously mentioned chemical attack. This risk hasnever previously been emphasised in any warnings as far as is known.

As an illustration of the first problem area, visits to two schools with theprecast units open to view (not hidden by a suspended ceiling or similar)showed that the beam ends sometimes rested on column haunch bedlengths of 1–2mm. This was observed with children and teachers in theclass. The classrooms so concerned were immediately evacuated, andremedial work was put in hand. No other undue distress or deflexions ofthese precast elements were observed.

The second problem area was highlighted in concentrating surveys inareas of construction where there were high humidities and warmconditions, such as kitchens, bathrooms and roof areas. It was in roof areasthat several cases of chemical degradation were observed. In each case,woodwool slabs were used above the units, and condensation and/orleakage was taking place. It was ascertained that the wood fibres inwoodwool slabs were commonly stabilised (to prevent organic growth) bytreatment with calcium chloride. Effluent from condensate or leakage


resulted in chloride attack on concrete made more prone to attack byconversion, causing increased permeability. The attack was mainly due tothe formation of calcium alumino-chloride, and the surface had becomesoftened to a depth of about 10mm. Although in some cases chloride wasfound to have reached the prestressing wires, no problems had occurred.

Although, like OPC concrete, aluminous cement concrete is subject tocarbonation (BRE, 1981a) this mechanism was not found to have caused aproblem.


Problems with aluminous cement can be identified by: (a) insufficient support bed lengths;(b) chloride chemical attack, resulting in surface softening.

1.7.2 REMEDIAL

Where (a) applies, stainless steel or equivalent brackets should be fixed tothe columns to extend the bedding lengths to an acceptable figure. Shimsof stainless steel or similar would probably need to be placed in the gapsand grouted up to give continuity. Any shims used should not give rise tobimetallic corrosion reaction with the brackets.

Where (b) applies, carry out remedial work to inhibit or preferably stopcondensation and/or leakage. Units may have their degraded surfacematerial removed if required and refaced with a polymer mortar.

1.7.3 AVOIDANCE

The best way of avoiding problem (a) is probably to design adequatetolerances and to ensure in both precast and in-situ concrete work thatthese tolerances are achieved.

Concerning (b), it is best to ensure that areas such as roof spaces,kitchens, bathrooms and similar risk zones have their precast units in anadequately ventilated and leak-and condensation-free environment.

1.8 STEEL REINFORCEMENT: ADDITIONAL REQUIREMENTS

In addition to the normal structural need for reinforcing steel in concrete,two other problem areas were often met where steel was found to benecessary for other reasons. Although the two instances applied to precastconcrete, there could well be cases when a similar need could arise forinsitu concrete.


The first additional requirement was for steel in the compression zonefor handling purposes. The problem concerned precast concrete step units,as illustrated in Fig. 1.5. Structural reinforcement had been placed in thetop of each step to counteract cantilever forces. What had not beenconsidered was that, although the units were delivered to site in goodcondition, each unit was lifted manually from the truck and taken to thestorage area. The two operatives involved in this lifting exercise causedthe units to behave as end-supported beams, and each unit cracked underthe bottom shoulder adjoining the boss.

Although this particular saga continued in a manner unrelated to thissection heading it shows how important it is to pay attention to handling.There were 14 of these units made for a spiral staircase to be prestressed toa ground anchor after installation. After making, scrapping and remaking(with additional bottom of step steel), the units were put in place andprestressed without any packing mortar between the moulded andtrowelled faces. The spalling that occurred was the outcome. At theinstallation of the third remake units with 10mm thick mortar packingbetween the joints, only 13 could be installed up to the landing level,

Fig. 1.5 Compression-induced spalling in step units.


leaving a gap in the spiral. As far as is known these units were dismantledand a wooden staircase was installed.

The second problem was for a steel requirement in what was thought tobe the compression zone. This need was probably because of differentialmoisture and thermal movement between the top and bottom of thesection. The units in question were architecturally faced duct covers, eachabout 1m square in plan by 80mm thick. The top of each unit was facedwith a cast stone finish of about 30mm nominal thickness, and theremainder was made up of an OPC concrete with 10mm maximum sizeaggregate.

Figure 1.6 illustrates the placement of each duct cover over anenvironment of high humidity and a reasonably constant temperature,with the visual face exposed to the weather. Shrinkage cracking occurredon the top face of virtually all the units, and although these cracks did notapproach anywhere near the interface between the two mixes, let alonethe steel in the backing concrete, the units were rejected. They wereremanufactured with a light stainless steel mesh at about mid-depth in thefacing, and no further cracking was reported, as far as is known.


The symptom is cracking in the design compression zone. This crackingtends to exhibit a rather random pattern, generally unrelated to the dryingshrinkage and thermal cracking discussed in sections 1.1 and 1.2.

1.8.2 REMEDIAL

If the crack apertures remain nominally static, and crack depths are notconsidered to be a corrosion risk, then consider a grinding to clean offdetritus at the crack apertures, followed by a silicone treatment. If there arestructural implications because of the cracking, seek the advice of achartered engineer concerning such actions as replacement, stitching inextra steel, or other solutions.

Fig. 1.6 Visual-faced concrete duct cover unit (not to scale).


1.8.3 AVOIDANCE

When detailing reinforcement requirements, consider the need foradditional steel to withstand either handling needs and/or differentialthermal/moisture movements.

1.9 EXCESS STEEL REINFORCEMENT

Reinforcement is generally put into concrete to cater for its relativeweakness in tension compared with compression. The term ‘cross-sectionalarea’ (CSA) is used to refer to the area of the section under consideration,both for the concrete and for the steel. The ratio of the area of the steel tothat of the concrete is the percentage of reinforcement, which for concretesections such as slabs, beams or columns could typically be 3–5%.

Many cases have been encountered where percentages of reinforcementof up to 25% have been used. These have led to problems on site, in theprecast concrete factory and in mix design at the preliminary stage.

One problem that was examined concerned precast columns in abuilding (Fig. 1.7). The columns were about 3.5m tall and 0.3m squaresection in plan. Four 40mm diameter bars, one at each corner with 40mmnominal cover, made up the main reinforcement. The main rebars were lappedby 10mm diameter stirrups at approximately 0.3m centres. The mix was a20mm/10mm/5mm to dust limestone aggregate with a 450kg/m3 whitePortland cement content and a total water/cement ratio of about 0.5 with a

Fig.1.7 Uncorroded rebar-induced spalling in new precast concrete column (notscale).


slump of 100mm. About three months after installation on site, severevertical cracking with no steel corrosion occurred in lines withaccompanying spalling in lengths up to 0.5m long. This was diagnosed asprobably being due to there being too much rebar restraining influence ona concrete with a high initial hydration shrinkage potential.

Another example where excess reinforcement affected mix designconcerned bifurcated in-situ white concrete columns, where congestion ofreinforcing bars at the crossover resulted in there being about 25% steel ofthe CSA in plan at the throat. The original mix design, using a 20mmaggregate with a 75mm slump, had to be changed to a 10mm aggregatewith collapse slump. Fortunately, a Portland limestone aggregate was used,and the 30–60 minute aggregate suction effect on the excess water contentgave cube results that built up quite significantly after four days. Thespecified cube strength was obtained about a week later.


The problems that occur from using excess reinforcement are numerous.The list below highlights those that have been commonly experienced:

(a) pieces of tie wire and detritus on the soffit;(b) cracking mirroring the main rebars, without steel corrosion;(c) shrinkage cracking due mainly to the use of too much water and/or

too dusty an aggregate in the mix;(d) honeycombing above the steel due to the close-packing of the rebars,

allowing fine material sole passage.

1.9.2 REMEDIAL

(a) Remove tie wires, detritus etc. from the face of the concrete as soon aspossible, and reface cut-out areas with mortar of the same mix as thefine material in the substrate concrete.

(b) On the assumption that by the time the problem is observed most ofthe hydration shrinkage considered responsible for the cracking willhave taken place, remove all unsound concrete and repair (ConcreteSociety, 1984).

(c) Repair shrinkage cracking, if considered necessary, as described insection 1.2.2(b).

(d) Cut out and replace honeycombed zones with concrete or mortar togive a weathering match.

1.9.3 AVOIDANCE

The general way to avoid most of the problems listed is either to ensurethat no more reinforcement is put in than is needed and/or to distribute


the placement of the main bars so as to spread stress shrinkage effects ontothe steel more uniformly. In addition, allow as much room as possible forconcrete to flow through the reinforcement, and use workability-promotingadmixtures and/or additives where possible.

Where high-workability mixes are necessary, consider the use ofaggregates with a suction ‘Vacuum concrete’ effect. Not only can this assistin extracting some of the excess water, but the increased wetness of theaggregate can also assist in the moisture curing of the concrete.

1.10 GRC AND ALKALI-GLASS REACTION

Publications on this subject (Swamy and Barr, 1989; Majumdar and Laws,1991) have tended to concentrate on the design and use of GRC andcomposites rather than on materials science considerations. The problemencountered on several sites has been slight surface softening to a depth ofless than a millimetre but with exposure of fibre ends, which exhibited abrittle nature. The detriment was solely aesthetic in nature.

Concerning the chemistry of cement and glass, this form of degradationwas predictable, but a little explanation at a basic level may be useful. Whencement, mortar or concrete is splashed or otherwise brought into contactwith window glass, etching occurs. This is because the alkali in cementattacks some of the silicates that are used in glass manufacture. The stockused in making glass fibres has better alkali resistance than window glassbecause zirconia is used as one of the constituents.

In order to improve the alkali resistance further, a polymer coating isapplied to the fibre as it is drawn from the melt and before it is choppedinto strands or rolled into spools. The chopping process can take place atthe manufacturing stage, or in the works, or on site during sprayapplication from the spool of glass fibre. Whichever method of cutting isused, the cut ends as well as polymer-bruised fibre sides (bruised duringthe mixing and/or compaction stages) lose some of the coatingprotection.

There is also a thermodynamic process concerning heats of solutionof cement and glass. It means that more heat is required per unit weightto make cement than to make glass. The typical clinkering temperatureof cement is about 1200°C, and the melt temperature for glass is about800°C. When the two materials are put side by side, especially in anenvironment where chemical reaction can take place, there will be atransfer of heat energy from the cement to the glass in the form of achemical reaction.

It can therefore be predicted that unprotected GRC exposed to theweather, or to other conditions where water or other solutes can contactthe surface, will have the fibre ends and ‘bruised’ areas attacked.



The symptom is exposure of fibre ends on the surface, with accompanyingslight dusting to a fraction of a millimetre deep. With the assistance of amagnifying glass it might be also possible to observe some hollowing outof the fibre ends. This would be the residual thin skin of the polymerappearing as a ‘pipe’.

1.10.2 REMEDIAL

Rub down with a fine-grade disc, carborundum stone or similar toremove all softened material. Apply a silicate-based or polymer-basedpaint system to give an approved finish. The silicate systems are preferredas they have a track record of good performance exceeding 100 years,and this thus avoids continuous maintenance apart, possibly, fromcleaning.

1.10.3 AVOIDANCE

If GRC is likely to be subject to the exposure risk mentioned above, the useof a protective silicate-based or polymer-based paint would greatlyimprove the performance. The alternative would be to apply a rich (e.g. 1/1) fine mortar 1–2mm thick to the risk face before laying up the GRCsystem. A protective coating, as described above, could also be consideredas an additional precaution.

1.11 FIBRE-REINFORCED SANDWICH PANELS

Although the problem met on several sites was with GRC sandwich panelsthe same situation would almost certainly have arisen if the fibres hadbeen of fibrillated polypropylene, steel or carbon or other fibre. Thesandwich panels in question were of half to full-storey height, and hadinner and outer fibre-reinforced sheets about 8–12mm thick as well as sidewalls of the same material. The sandwich consisted of an expanded plasticsor mineral wool with a thickness of about 100mm.

The problem that occurred was of cracks of up to 2mm aperture at thevisual face edges. Where moisture had gained ingress into the sandwichthis cracking was sometimes accompanied by a lime leachate exudation atthe crack apertures.

As this cracking had not been reported from the returns or corners forcases of single-skin cladding, it seemed that temperature variations couldwell have been responsible. Not only was the outer skin exposed to theexternal temperature and general weathering conditions, but its responsetime was rapid compared with a monolithic solid typical concrete system.


The webs and rear skins of the panel would have remained comparativelytemperature static at most times, and there would therefore have beenconsiderable thermal differential strain at the face-web junctions.


The symptom is cracking at the face-web return junctions, sometimesaccompanied by a lime or carbonated lime leachate from the cracks.Differential movement may also cause the panels to bow in a convexmanner. This could be assessed with a straightedge, but should not beconfused with any convexity that may have been there when the unitswere made.

1.11.2 REMEDIAL

Although no remedial work was undertaken as far as is known on the sitesvisited, it is difficult to recommend anything that can cater for suchcracking. If, as is likely, the cause is a thermal one, then the cracking islikely to be ‘live’. Therefore, if a crack-filling material is to be used, thisshould be a low-modulus material capable of moving with the changingcrack apertures. The repair should be effected when the cracking is at themaximum opening so that the sealant used will generally be undercompression.

1.11.3 AVOIDANCE

Wherever possible, avoid the use of sandwich panels and consider usingsingle-skin cladding. If sandwich panels have to be used they could bedesigned so that the web-face skin junction is hinged rather than rigid.

1.12 DELAYED ETTRINGITE FORMATION

The problem encountered was more one of trying to find instances of DEFthan of DEF causing distress. Time will tell whether DEF is a real problem;at the time of writing this book there is little or no indication of this.Ettringite is calcium alumino-sulfate, and in cement it is formed by thereaction of sulfate with the calcium aluminate in the cement. Ettringite isdeliberately present as a reacted product in Portland cement concrete andmortar because gypsum (calcium sulfate) is added during the clinker-grinding process to control the setting and hardening properties. Withoutthis addition, the speed of hydration would be too great for practical use.This ettringite is harmless, because it is probably in the form of well-dispersed microcrystalline particles. For ettringite to cause distress it would


have to be larger than microcrystalline in size, and would need to form inthe hardened concrete. If such a reaction occurred solely at the surface ofthe concrete the effect would probably be that of surface softening, asdiscussed in section 1.1.1 (a).

The only observed case that could possibly have been attributed to DBFwas in the exposure of sections of hydraulically pressed kerbs containinghigh proportions of high-sulfate PFAs. This observation referred to a long-term laboratory study of units made in precast concrete works. The PFAloadings that caused trouble were 1.5–2.0 times the cement contents, andwith sulfate/PFA levels up to 1.8% (Levitt, 1982, section 4.2). After about10 years’ weathering, with no vehicular access, two of these kerb sectionswere found to have split cleanly through their mid-sections with no sign ofimpact. However, white crystalline deposits of up to 10mm in diameterwere observed at the broken faces. These deposits were found to be rich insulfate.

The kerbs manufactured by hydraulic pressing were partnered by kerbsmade by Kango hammer tamping. None of these hammer-compacted kerbsections exhibited distress on weathering. This could have been due to theirweaker and more elastic properties, coupled with the relatively largernumber of voids that could have accommodated expansion products.

Manufacturers of hydraulically pressed products would be unlikely touse such high PFA loadings, because this would necessitate an unacceptableincrease in the pressing time.


If this problem is ever found it is likely to be in the form of severe crackingand/or spalling, with visible ettringite-rich white crystalline deposits atthe broken face. This is a prediction based upon a single observation inlaboratory samples and not from a site.

1.12.2 REMEDIAL

Not enough experience is available to address this point. If damage wereto occur on site in similar vein to that observed on the hydraulically pressedkerb samples (assuming that it was due to DBF) then it is likely that majorremedial work or replacement would be necessary.

1.12.3 AVOIDANCE

Until actual site problems become documented and the mechanism isunderstood and accepted it is not possible to take avoiding action. As faras is known at present, there does not appear to be anything to avoid.


Health and safety 2

INTRODUCTION

The three examples in this short chapter reflect personal experience in theindustry coupled with involvement in litigation work. Health and safetymatters such as protection from falling objects, safety clothing, and the useof mechanical and electrical machines are well documented in companyprocedures, and are not covered here. The three risk areas discussed heredo not seem to have attracted the attention that they should. They shouldinterest not only safety officers but also site managers, operatives andpersonnel officers.

Because the first two sections refer to cement chemistry, it may be helpfulto put cement into perspective by comparing it with another chemical thatis commonly found on site and in the precast works: concentratedhydrochloric acid.

Not only is hydrochloric acid a simple two-element chemical, but it isone of the few acids that is a reducing agent and not an oxidising agent likenitric or sulfuric acid. Hydrochloric acid’s fuming property, its pungentsmell and (usually) delivery in glass carboys, coupled with its ability toetch concrete with accompanying bubbling, cause it to be respected.However, because human skin is generally acidic (except for the eyes), andbecause that acid is dilute hydrochloric, then provided the skin has nolesions, spillage of fuming hydrochloric acid onto the skin causes littleharm. It is not a caustic oxidising acid: it does not attack flesh.

Compare cement, which arrives as a dry powder, and commands verylittle respect. However, it should be treated with a level of safetyconsciousness that makes hydrochloric acid pale into insignificance. Thereare two reasons for this, and they are both based upon the fact that water isto be added to cement, which is a multicomponent chemical. First, one ofthese components is water-soluble hexavalent chromium. Second, causticalkali is released. These two risk areas are discussed in the first two sections.


2.1 CEMENT ECZEMA

Chromium (chemical symbol Cr) is a minor ingredient in OPC. It is presentat a typical concentration of about 6mg/kg or 6ppm (parts per million). Asfar as is known, this figure represents the total Cr and not the water-solublepart. I have encountered only a few cases of skin problems, and so it isreasonable to assume that UK cements have low contents of solublechromate. Chromium does not occur in the form of a heavy metal but as achromate salt. When this salt is present in its hexavalent Cr form it is water-soluble, and it is in this form that—for some personnel-eczema, dermatitisor, more rarely, skin cancer can occur.

There is nothing new in the knowledge of this risk factor; it was firstreported over 30 years ago (Burrows and Calnan, 1965). In Denmark,research has been undertaken more recently into ways of inhibiting this risk(Aunstorp, 1989a, b, c). Ferrous sulfate was added to Danish cement, andthe effects on operatives before and after this addition were reported. Danishlegislation then followed, with a limit on the maximum amount of solublechromate salt permitted in Portland cements, expressed as ppm of Cr.

Aunstorp found that the allergic reaction to the chromate in the cementwas a much more significant factor than attrition brought about, forinstance, by fresh concrete or mortar being rubbed onto the skin. TheDanish research was based upon a real situation, and was not a laboratoryassessment. Interestingly, the study found inter alia that the use of protectivecreams before starting work with concrete did not ease the irritant effect ofcontact with wet cement. The wearing of protective gloves had only amarginal effect in counteracting any allergy or in inhibiting irritation.Irritation, in all cases, would be the first sign of a skin reaction. The lastexamination of the Danish cement specification revealed that themaximum limit for the soluble chromate salt, as Cr, was 2ppm.

Publications by the Health and Safety Commission/Executive (HSC,1988a, 1988b; HSE, 1988) drew, seemingly, rather marginal attention tothese risks. However, at the time of writing it seems that an updateddocument is due for publication.


The symptoms of cement eczema are irritation of the contact area, probablyaccompanied by discoloration and a blotchy appearance. If the effect ofcement burns is occurring at the same time (see section 2.2), skin irritationis unlikely to be felt because of the damage to nerve ends.

2.1.2 REMEDIAL

Wash affected areas immediately with copious quantities of water, andremove cement-stained clothing. Seek medical advice as soon as possible


after this, and ensure that there will be no future work likely to result incontact with fresh concrete, mortar or grout.

2.1.3 AVOIDANCE

Most people are relatively insensitive to the risk of cement eczema, and soit is advisable to question all personnel at the pre-employment ordeployment stages. Has the person and/or any member of that person’sfamily ever been prone to allergic skin reactions or complaints? The 1965study reported earlier indicated that hereditary factors can play a role in aperson’s proneness to a reaction. For adequate documentation, the posingof these questions and the answers received need to be accurately recorded.

As far as protective handwear is concerned, only waterproof glovesknown to be unreactive to chromates should be used. Handcreams onlyprevent the skin from drying out; they do not offer resistance to thechromate salt.

2.2 CEMENT BURNS

Throughout this section the term ‘cement burns’ has been used. In USAlitigation reports and published articles the term ‘concrete burns’ isgenerally used. It is the chemistry of the wet cement that causes burns, andso ‘cement burns’ is probably the more explicit term. Abrasion by theaggregate and/or cement on the skin exacerbates the caustic chemicalmechanism involving necrosis that is considered to be responsible.

The UK problems I have encountered have generally been whereoperatives have been kneeling and carrying out floor-topping work. Othercases, such as concrete getting inside a Wellington boot, have beenillustrated in the UK press, and these are identified later. Severe injury wassuffered in all cases, resulting in the inability of the persons concerned tocarry out further manual work. Continuous, permanent pain and unsightlyskin grafts were also common.

The incidence of cement burns was recorded nearly six decades ago inan American medical journal (Meherin and Schomaker, 1939), but thereappears to have been a dearth of reports in the literature for more than twodecades thereafter. Rowe (1962) described cement burns as ‘unusual’. Manycases of litigation took place in the USA, with Erlin, Hime Associates, aconsultancy practice in Illinois, not only acting for claimants but alsobuilding up a useful dossier of case histories and references to relevant USlegislation. At the beginning of my involvement as an expert witness actingfor claimants in a county court case, a close liaison was set up with Erlin,Hime Associates. Their earlier 1980s publication (Hime and Erlin, 1982)had a significant impact upon both US and UK litigation. This US


publication did not pursue a detailed mechanism of the form that isproposed later. It will be argued that there is a simple theory capable ofexplaining the cement burn mechanism and of showing why the risk is farmore serious than previously considered.

Although section 2.1 refers to the possible issue of an updated Healthand Safety Executive publication, the two information sheets publishednearly a decade ago appear at present to be the sole UK guidance (HSC,1988a, 1988b). The references therein, together with warnings in documentssuch as delivery notes and safety instructions, will be highlighted later inthe proposed theory as not being detailed enough.

One of these publications contains a photograph of a cement burn to theknee of an operative. Three further publications appeared a few years later(Anon, 1993a, b, c). No reference can be made to personal case historyexperience here as one of the cases was settled out of court and anotherone is current as at early 1997.

It is not known how many cases of cement burns have occurred in theUK without being reported in the press. The pressures being placed uponcontractors and subcontractors could have resulted in precautions beingsacrificed for the sake of speed and/or economy. However, the apparentlylow incidence of cement burns in the UK implies that a high quality of careis being exercised on site, in the works and plants.

In 1988 Hime and Erlin reported a study where research by others wasreferenced, without much detail. This stated that trouser material acts as achemical buffer and increases the alkalinity at the skin, with an increase inpH from about 12.8 to nearly 14. It was not understood how material fabriccould cause this, and I have conducted laboratory tests using fresh cementmortar on one side of various fabrics such as worsted, linen, denim andnylon. In all cases the pH was found to be about 12.5 on both sides of allmaterials tested, with no significant gradients.

It is therefore logical to turn to what is known about personnel sufferingcement burns on site. Two factors seem to be necessary. First, the spillageneeds to be static and in contact with the skin for at least half an hour.Second, the skin where the spillage has taken place needs to be on arelatively warm part of the body. Cement burns do not appear to occurwhen the operative’s hands are in and out of concrete.

These two conditions need to be correlated with the known cause of cementburns, which is necrosis, or in simple terms a caustic burning effect on skin,nerves and muscle. The most critical of these three effects is the destruction ofnerve ends, because this dulls any feeling of burning or irritation. This isprobably why operatives carry on working after spillage, not knowing thatflesh burning is continuing. The 1993 Construction News article (Anon, 1993c)describing spillage into a Wellington boot is an example of this.

The presence of alkali in the forms of sodium and potassium hydroxideswas mentioned in section 1.5 and expressed as Na

2Oequiv. A typical level


for OPC would be about 0.6%. This is equivalent to approximately 0.4% assodium hydroxide. This is known as caustic (i.e. it burns flesh) soda. Boththe alkaline lime and the caustic soda contribute to the alkalinity of cementpaste. What is important is that lime is alkaline without being an alkali,but caustic soda is both. Furthermore, lime is only slightly soluble in water,whereas the alkalis are very soluble.

Consider now a realistic scenario: an operative kneeling on freshconcrete so that the chemicals in 1kg of concrete can access the skin. If thecement content is say 350kg/m3, then that 1kg of concrete will containabout 150g of cement, assuming a fresh wet concrete density of 2350kg/m3. At a 0.4% equivalent caustic soda level the operative is in contact with0.6g of one of the most caustic chemicals known.

Its uniform distribution throughout the contact area explains why bodyheat and time are jointly required. As body heat causes the water in theconcrete to evaporate, the effective concentrations of both lime and thealkalis increase. Lime, being only fractionally soluble in the moisture onthe skin, will not cause much distress. The approximate half gram of causticalkali would be responsible for the necrosis that occurs.

This explains why the combined conditions of both warmth and timeare necessary, and why a continuous replenishment and/or renewal offresh concrete with the skin does not apparently cause the same distress.The alkalis do not have the chance to become concentrated by evaporationof the water. In addition, it may be predicted that the abrasion effect ofcontact with aggregate will have an exacerbating effect on flesh alreadyundergoing necrosis.


The symptoms are soreness experienced after some hours, followed,usually the following day, by severe pain and ulceration, with a flesh colourvarying from green to purple and, generally, an accompanying discharge.

2.2.2 REMEDIAL

It is possible to ameliorate the situation slightly by immediate washingof the affected area with copious quantities of water, coupled by theremoval of all affected clothing. This clothing should be washed ordiscarded. The US articles referred to earlier advise not to cover the areanor to apply any form of dressing. Immediate hospitalisation is advisedwith, preferably, a department qualified to deal with cement burns. Thepatient’s contact with cement and caustic alkalis needs to be mentionedto the hospital staff.


2.2.3 AVOIDANCE

Protective waterproof (the latter word being commonly omitted in safetyguidance) clothing should always be worn. If operatives have to kneel onfresh cement then special knee covers or string-held cut-outs of car tyresshould be used. Any cement ingress behind these should be treated as insection 2.2.2.

2.3 PUMPING GROUT

Typical uses of grout pumps are for operations such as filling the annulibetween prestressing post-tensioning strands/wires and the duct tubes,and for filling gaps under large machine baseplates. It was in the formerapplication that an operative suffered injury to an eye that nearly resultedin blindness of that eye. A blockage occurred in the tube line to theprestressed unit, and a coupling was loosened to open up the tube.Admittedly the person concerned should have been wearing eyeprotection, but he assumed that because the machine had been turned intothe recirculation mode, pressure in the feed line had been relieved. He didnot appreciate that, with the particular machine in use, the pressure hadnot been taken off the feed line.

The pump supplier, when advised of this mishap, informed the projectmanager that there was a third position for the control tap. In addition tothe ‘12 o’clock’ recirculation and the ‘3 o’clock’ line pressure positions, therewas another one between 1 and 2 o’clock that took the pressure off the feedline. It was admitted by the pump supplier that this omission in theinstructions for use was an error, and that it would be rectified.

Figure 2.1 shows a simple schematic outline of the system.

Fig. 2.1 Grout pump system (not to scale).



This potential problem can be identified as non-existent or inadequatesupplier’s guidance on how to deal with blockages in the system.

2.3.2 REMEDIAL

Ask the supplier for written instructions on how to deal with pressure lineblockages. If a coupling in the pressure line has to be undone because of ablockage, and no instructions are available, run the pump with the tap inapproximately the midway position between the recirculation andpressurising positions. Ensure that the operatives wear full eye and handprotection at all times, and that there is a convenient optical douche nearby.

2.3.3 AVOIDANCE

Use grout pumps only where there are distinct instructions on how torelieve pressure in the feed line should a blockage occur. Wear onlyapproved eye and hand protection, and ensure that there is a convenientoptical douche station.


Concrete on site 3

INTRODUCTION

Most of the main problems encountered with in-situ concrete are groupedin this chapter, but inevitably there is some overlap with Chapters 1 and 5.It will be seen in Chapter 5 that the many instances of troubleshootinginvolving precast concrete products could have also applied to in-situconcrete. However, materials faults were seldom involved; the problemsgenerally related to design and/or workmanship.

It is easy to blame the material, because it is usually covered by astringent specification. Design and workmanship are dealt with by codesof practice, which are state-of-the-art documents, and can be interpretedin different ways. They also relate to a service rather than a product ormaterial, and are therefore difficult to define. When using materials on sitethere is a resulting tendency to concentrate unfairly on the standardisedmaterial ‘shall’ wording, and to use or sometimes misuse the guidanceCodes ‘should’ clauses.

As referred to in the Preface, less than 18% of all troubleshooting casesencountered were attributable solely to the material. This is not only myexperience, but also applies to many decades of Laing library problemanalyses. It leads inevitably to the conclusion that there is a misdirection ofeffort in construction quality control. The main focus of testing should beon the construction rather than on the material or product. Why is there aconcentration on specifications to resolve less than 18% of the problems?Presumably because it is fairly easy to specify materials, but difficult tospecify or control design and workmanship. Whether this situation willchange in the future remains to be seen.

3.1 COVERCRETE d OR k

Although covercrete d and k are defined in the Glossary, some qualificationmay be useful. Concentration on the depth of cover, d, is universal; there is


little or no interest in the quality, k, of the concrete in that cover. Thesevariables are generally recorded as d, the depth of cover in mm, and thelesser-known k, the permeability, in cm/s.

The problem encountered on site was due to non-compliance with a dspecification. This had secondary repercussions. First, it was not clearwhether the specification requirements were for ‘minimum’, ‘nominal’ or‘actual’ cover. Second, where cover to the main steel was of concern, littleaccount seemed to have been taken of cover to the stirrups. As may be seenin Fig. 3.1, this cover is reduced by at least that of the stirrup steel diameter.

In general, no consideration was given to the quality of the allegedlyreduced cover area(s). Where this quality was considered to be good,attempts were made (sometimes with success) to achieve a deemed-to-satisfy situation with no other work being carried out.

Let us consider each of these variables, d and k, separately.

3.1.1 COVERCRETE d

Why have a d in the specification? The simplest answer might be ‘to protectthe steel from corrosion’. Unfortunately, this reasoning seems tacitly toadmit an inability to qualify the answer. What is not apparently designedis whether d maintains its protective property over the planned lifetime, orwhether it gradually loses it.

From site experience, it seems that the underlying philosophy of a dspecification is ‘defer the fatal date’. In effect, we could well assume that dshould be maximised, because the cover will lose its protectiveness withtime. However, there are drawbacks to having too much cover: • fewer cracks, but each with an enlarged crack aperture;• less restraint against shrinkage cracks (see section 1.8).

Fig. 3.1 Difference between strirpup and main rebar cover (all dimesions in mm).


The loss of protectiveness of d is generally thought to be due to theadvance of a carbonation front. If chlorides and/or aggressive de-icing oranti-frost chemicals are present, this loss of protection may be far morerapid.

Carbonation is a reaction between carbon dioxide in the atmosphereand the alkaline hydroxides of hydrated cement. The pH value of the matrixin concrete unaffected by external elements is of the order of 12.5, and atany pH value above approximately 9 the rebars will retain a protectivepassive iron oxide film on their surfaces. This protection is lost at slightlyalkaline pH values of 7–9, and much more rapidly under acidic conditionsand/or in the presence of chlorides; corrosion can ensue. Note the word‘can’: corrosion of steel in or at the carbonated zone front needs bothmoisture and air to be present.

The modern design philosophy is possibly to specify d as thougheffectiveness would be lost according to a rule of thumb such as 1mm gives1 year. However, there does appear to be an underlying assumption thatthe cementitious part of the specification will ensure that adequate effectived is left at the end of the planned lifetime. This implies that considerablereliance is actually being placed on k. However, k is still generally placed ina secondary role to d, and it could be argued that, for corrosion durabilityof reinforced and prestressed concrete, these roles should be reversed.

It can be misleading to predict a linear relationship between the speedof advance of carbonation and time. For instance, if 40mm becomescarbonated after 40 years it would be useful to say that 50mm would farelikewise at 50 years. However, in general, the rate of advance of thecarbonation front follows the square root of time (BRE, 1981b). If this isapplied to the above example then 50mm would become carbonated within60 and not 50 years.

Another problem encountered on site was a one-off, but it is a risk areathat is easy to overlook. For carbonation of cement to take place, moistureand air need to be present. If the void/pore structure of the concrete is toodry then the reaction cannot proceed. At the other extreme, if the voidstructure contains enough water to inhibit or stop the passage of the carbon-dioxide-containing air then, similarly, carbonation is inhibited or cannotoccur. Carbonation tends to proceed most rapidly when the relativehumidity of the void structure is about 75% (BRE, 1981b). This could wellmean that the reinforcement at the back of concrete exposed to a cavitysituation could be more at risk than the rebars nearer the visual face. Thiswas found to be the case when the rear faces of precast panels were exposedfor examination by removal of the insulation and partition boards.

It is not known what guidance is given in codes concerning this, but therisk factor has been taken into account in the latest revision of the caststone standard (BS 1217:1997), which gives mandatory requirements for


cover to all exposed faces. The term ‘exposed’ is defined as unprotected byany mortar or similar material, and so applies to visual and cavity faces.

It is probable that the only instance when d is of importance is in fireresistance, where the purpose of having cover is to keep the steel sufficiently‘cool’ to avoid loss of strength caused by a phase change in the metal.

3.1.2 COVERCRETE k

It is commonplace to specify minimum cement or cementitious contents;there seems to be a growing tendency to control k with little knowledge ofwhat it actually is. At the same time, it could well be an admission thatthere will be variations in the actual steel covers being achieved, but thateven when these are at their minima the concrete left as covercrete will stillgive good performance over the planned lifetime.

The main problem with k is that knowledge already exists on how tomeasure permeability, but it takes too long (at least 3 months) before usefultest data emerge. It can be argued that d can be easily confirmed bycovermeter tests or similar, but there are errors involved in such testing (aswith any testing). More importantly, if the concrete has hardened, whataction can be taken to deal with non-complying areas?

A strict mix design specification, which can be easily verified bysupervision, can generally give compliance with a k requirement. Thisapplies whether the k design is for general weathering durabilityrequirements or for protection when chlorides are present.

Long-term research can establish how the permeability characteristicsof concrete can be controlled under a variety of durability hazards. It thusfollows that a contractually viable solution to producing the optimum kfor any risk situation lies in the area of mix design.


The problem is manifested as non-compliance with minimum coverrequirements (assuming that the parties concerned are agreed on what ismeant by ‘minimum cover’) coupled with the covercrete being potentiallygood enough to give adequate performance: in effect, a low d butaccompanied by a low k.

3.1.4 REMEDIAL

Illustrate by such means as the history performance of similar concreteand/or by agreed testing of the covercrete that a deemed-to-satisfysituation exists. Proposed action, such as protective coatings or othersurface treatments, may have an apparently remedial effect, but it is almostcertain to be a waste of resources if the k value is too low.


If the quality of the covercrete is mediocre or poor, but that concrete hasto be retained, remedial measures such as polymer-mortar-based renderingor siliconing might need to be considered.

3.1.5 AVOIDANCE

For carbonation, chloride attack or fire resistance a logical appreciation ofboth d and k would seem to be essential.

The carbonation advance with the square root of time, mentioned above,is a general rule, which appears to apply to concretes whose cementitiouscontent is below about 375kg/m3. Above this content, carbonation isgenerally a few millimetres deep after many years. Therefore, if carbonationis the sole risk, the solution seems to be a minimum cementitious content ofthe order of 375kg/m3.

When chlorides and other corrosion-promoting chemicals are additional(to carbonation) risks, then the mix to be used needs strict specificationand control. For example, in marine or de-icing salt exposure, the suggested375kg/m3 minimum cementitious content needs to be supplemented byspecifying that the cementitious content have 30% PFA, 70% GGBS or 7%MS cement replacement.

If fire resistance is the only requirement then d is almost certainly the onlyvariable that needs to be specified. Design factors such as secondary mesh-type ‘holding’ reinforcement in the covercrete zone may also need to beconsidered.

The most difficult recommendation to make for carbonation andcorrosion-promoting chemical hazards is what the minimum value of d andthe maximum for k should be. For the present, assuming that the minimumcementitious content has been achieved and that it has the right ingredients,then a few millimetres of cover seem to be all that is required. However, inorder to allow for workmanship and the risk of ingress of aggressive materialat the interfaces between mortar and aggregate facets, the minimum covershould logically be twice the maximum aggregate size. This would cater forthere being a rebar-aggregate-aggregate path to the exposed face.

The definition of ‘minimum cover’ still remains to be established. Thispossibly needs to be resolved at Eurocode level, but for a single meaningthe wording ‘actual minimum cover to any steel’ might suffice. This formof wording does not have any obvious interpretative modes.

3.2 SPACERS FOR REBARS

Six problems have been experienced with both in-situ and precast concrete;these have been identified separately below as (a)–(f). None of these hasbeen found to predominate over the other five. Only example (d) referredto spacers made from mortar; all the others were experienced with thecommonly used plastics spacers.


(a) The use of trestle-type spacers on vertical and other non-horizontalrebars instead of wheel-type spacers. Figure 3.2 illustrates thedifference between these two main types of spacer. It can be easilyseen that, under the action of filling with concrete and/or compaction,a trestle-type spacer placed onto a vertical bar can rotate and lose some(or, when dislodged, all) of the intended cover design.

(b) Because of overloading, spacer feet either punched into relatively softformwork/mould material or, when the concrete was formed againststeel or similar surfaces, distorted spacer ‘feet’ sprung on stripping. Inboth cases, spacer feet protruded from the surface of the concrete, butin the latter case this was sometimes accompanied by minute spalling.In the former case (relatively soft formwork), the holes left in this readas ‘pimples’ in subsequent form uses unless the holes were made goodbefore each reuse.

(c) Following aggregate exposure processes, such as grit-blasting, spacerfeet were observed to protrude; the blasting had little or no effect onthe plastics material. This resistance was predictable, because plastics,and polythene in particular (the common base material for spacers),have good energy-absorption characteristics. Thus concrete will wearaway more quickly than polythene, because it does not possess goodenergy absorption.

(d) Mortar spacers either too impermeable or too permeable. In the firstcase (too impermeable), poor bonding occurred between the freshconcrete and the spacer mortar. This resulted in a gap at the interface,allowing moisture and air to enter and corrosion to take place. In thesecond case (too permeable) the bond was generally found to be good,but the spacer itself allowed moisture and air penetration, with ensuingrebar corrosion.

(e) Because of an inadequately pierced cross-section, and under the actionof fire, the plastics material was easily degraded, and the rebar at theposition where the spacer had been became too hot.

Fig. 3.2 Trestle-type and wheel-type plastics rebar spacers.


(f) As (e) but, under the action of normal weathering, spalling occurred atspacer positions with no steel corrosion but with exposure of the rebarto the elements. This problem was commonly observed during the firstfew months of site exposure in hot weather. The mechanism consideredto be responsible was the differential in thermal expansions andresponses between the polythene in the spacers and the surroundingconcrete. Polythene has about 16 times the thermal expansioncoefficient of a typical concrete. A well-pierced spacer section wouldhave had interwoven concrete, causing it to act compositely instead ofindividually.

All these examples have been described at length (Levitt and Herbert, 1970),and Concrete Society (1989) touches on (b) and matters concerned more withdesign than with troubleshooting. Further references together withphotographs are given in the author’s book on precast concrete (Levitt, 1982).


(a) Reinforcement cage dislodged with the spacer in the wrongconfiguration or lying loose on the soffit. Detection of this problemcould be by a simple visual inspection or, more commonly, by meansof a covermeter survey.

(b) Spacer feet protruding from the concrete’s surface, with holes inrelatively soft formwork and minute spalling possible for concreteformed against steel or similar hard, unyielding material.

(c) Unsightly protrusion of spacer feet or bases in abrasion-blastedfinishes.

(d) Corrosion rusting, either in lines at the interface between spacer andconcrete boundaries or fairly uniformly over the spacer base.

(e) A blackened hole of easily removed carbon, giving direct access to thesteel. If the steel has suffered a phase change because of the heat,additional deflection may also have occurred.

(f) An approximately conical-shaped spall, exposing the spacer base and,generally, the rebar; the rebar has no corrosion on it at the time thatspalling took place.

3.2.2 REMEDIAL

(a) Treat as in section 3.1.4 in attempting to achieve a reduced-coverdeemed-to-satisfy agreement, or undertake remedial work, such asapplying extra cover (Concrete Society, 1984).

(b) Knife-cut protruding spacer feet and make good holes in the formwork.The use of heat to remove these protrusions is not recommendedbecause carbon staining could occur, and it is also possible that


aggregate being calcined could spit out of the surface, constituting asafety risk.

(c) Remedy as (b).(d) Cut out mortar spacer down to the rebar and repair with suitable

polymer mortar (Concrete Society, 1984).(e) Suspected structural damage due to fire having gained direct access to

the steel through the passage where the spacer used to be should bereferred to a chartered civil engineer for advice before any decision ismade concerning remedial work.

(f) Remove as much of the plastics spacer as possible, as well as spalledand unsound concrete, and repair (Concrete Society, 1984).

3.2.3 AVOIDANCE

(a) Use only trestle-type spacers on horizontal bars in contact with the baseof the form or mould. Wheel-type spacers can be used in all applications,bearing in mind that some types are not strong enough for use on thebottom steel. Wheel-type spacers can be prevented from slipping downvertical and other non-horizontal rebars by holding each spacer in placewith an elastic band, which is then left in the concrete.

(b) Ensure that the load distribution on spacers is such that the tendencyto punch into formwork is minimised. At the same time ensure thatnot too many spacers are used, because they can encourage planes ofpossible weakness in the concrete.

(c) Avoid using plastics spacers for exposed-aggregate finishes. Mortarspacers are preferred with spacers having aggregate exposure by thesame process planned for the concrete.

(d) Mortar spacers should be made with a ‘sand’/cement ratio between1.5 and 2.0, preferably of the same fine aggregate (‘sand’) planned forthe concrete. Curing of the spacers under damp conditions for at leastthe first 48 hours is recommended.

(e), (f) Ensure that the seating area of both trestle-type and wheel-typespacers is pierced to at least 25% of its section. This permits concrete,under the action of compaction, to interweave with the plastics andcause each spacer to act compositely rather than as an individual pieceof material. This caters both for the thermal expansion differential andfor degradation under the action of fire.

3.3 TILING AND MOISTURE IN FLOORS

Concrete construction processes involve the use of water, and as a rulemore water is used than is necessary to hydrate the cement. The result ofconcrete’s losing or trying to lose this excess water has been problems in


the application of floor coverings such as PVC tiles and sheeting, rubbertiles and synthetic-backed carpet tiles.

The problem has been generated from an advisory clause in a particularcode of practice (BS 8203:1996). This recommends that, when the surfacerelative humidity (RH) is measured by a specified means, tiling should notbe laid until the RH drops below 75%. Putting aside the validity orotherwise of this recommendation, this 75% is commonly invoked as acontractual mandatory clause.

The problem arises because the waiting time necessary to reach thismaximum is generally impracticable on site. On the rare occasions whensuch delay can be countenanced, adhesion and/or floor covering failuresare rare. However, it is not definitive that 75% is the right number, nor thatit is being measured the correct way, nor that the property of RH is theright one to address, nor that efforts should be directed towards the suitableadhesive for different conditions of ‘dampness’ of the floor.

Another area of difficulty was that of the actual failures, and the five ormore causes that could have been responsible, either singly or jointly: (a) moisture in the concrete softening the adhesive;(b) an alkaline concrete-adhesive reaction;(c) moisture pressure gradient pushing the coverings upwards;(d) inability of the adhesive to stick to the concrete;(e) temperature gradient across the thickness of the slab, causing mois

ture to be driven to the colder lower vapour pressure face, and pushingthe covering off of the surface.

The mechanism of (e) is shown in Fig. 3.3.The main problem with the 75% RH recommendation is contractual,

and lies within the remits of the main and specialist flooring subcontractors.Because they have to work to stringent time requirements, usually derived

Fig. 3.3 Tiling on ground and supended floors.


from the site schedule, they tend to withhold any form of guarantee becauseit is virtually impossible to achieve that maximum. The rule of thumbcommonly applied to predict how long a typical concrete slab will take todry to a moisture condition complying with the 75% maximum is to waitone week for every millimetre thickness of slab. This implies that a wait oftwo years would be required for a 100mm thick slab. Contractually, this isunacceptable.

Clearly, a host of problems arise from this code and the way it is applied,not least the five listed above. It can also be argued that, irrespective of thevalue of any specification number, there is a tendency to cling to thatnumber just because there is a number there. This is discussed in depth insection 6.6.

As far as the causes listed under (a)–(e) above are concerned, a projectscheduled to last about 30 months began at the end of 1996. It is jointlysponsored by the Department of the Environment under its Partners inTechnology (PIT) Scheme and by the Concrete Society, which acts as themain contractor. It is the aim that a technical report (a CSTR) will bepublished by the Society in 1999, and that the findings will be aimed atmaking clauses in BS 8203 and other relevant codes more realistic. Theauthor is the convenor of the working party that is controlling the work.

Feedback is essential, and the Society will possibly have one or morelaunches of the CSTR in a preliminary draft form for discussion at seminarsand workshops. The project working party reports to a more widelyconstituted steering group; no matter how wide these representations are,there will possibly be points to report that have not been covered. Some ofthese points will now be discussed in terms of experience by others; thismay well trigger additional feedback.

The role of chemical reactions as the cause of adhesion failures has beenmentioned earlier, but some reactions also lead to the emission of ratherunpleasant gases. Reports on this subject would be useful to the ConcreteSociety, together with any related research work by organic chemists.

Another matter of interest is the growing use of plasticisers andsuperplasticisers, both in the concrete and in the screeding and levellingmaterials laid on top. The main advantage of these admixtures is that, fora given workability, they permit large reductions in the total water/cementratio. This in turn reduces the excess water waiting to dry out, so that the75% should be reached more quickly. In general, the use of these admixturesin concrete does not seem to have caused any problems. However, theadmixtures for some screeds and underlayment levelling compounds arepolymer based. Although these will have very low water contents, andhence a lower initial RH, they can also have low permeabilities to thepassage of moisture, with a consequently reduced loss rate.

Other than by using a vacuum concrete process, methods of trying toeliminate excess water have not proven fruitful. The use of hot air blowers


has been found to have no beneficial effect on drying rates. The processdrives the water from the surface into the body of the slab but, within asshort a period as one day after turning the heating off, the original RH isreinstated as the temperature becomes reasonably uniform in the slab.

Moisture moves towards the colder face because of the vapour pressuregradient. Knowledge of this mechanism could be useful both in laying floorcoverings and in heating buildings. The aim is to keep moisture as far awayas possible from the face to be covered, so consideration could be given totiling and heating the top floor of a building first and working down to theground floor as the last to be covered.


This is the discovery that a 75% RH limit has been specified for laying floorcoverings, but that this cannot be met within the site limits as laid down ina bar chart.

3.3.2 REMEDIAL

The only possible avenues that may be worth pursuing are either to get thespecifier to accept the risk situation, or possibly to allow a variationresulting in the use of a more expensive adhesive that can tolerate highlevels of water in the floor.

3.3.3 AVOIDANCE

The use of plasticising or superplasticising admixtures in the concrete isworth examining as an interim measure and, possibly, as a long-term onepending the findings of the CSTR in 1999. Specifiers should have theirattention drawn to the problems generated by the 75% specification, andshould be told inter alia that there is no cheap way of getting over theproblem.

3.4 FLATNESS OF FLOORS

How do we define and specify ‘flatness’? This question has created a hostof site problems, not least in measuring whether compliance has beenachieved. Well-known publications (Concrete Society, 1988; Perkins, 1988)have discussed many of these facets at length, but a simpler approach hasbeen adopted in this section, based on dividing the problem into three maingroups identified by increasing dimensions, with a fourth group containingramps and relatively large and severely exposed floor areas such as multi-storey car parks:


(a) short-distance flatness, 0–100mm;(b) middle-distance flatness, 0.1–5.0m;(c) large-distance flatness, >5.0m;(d) multi-storey car parks.

‘Flatness’ here refers to achieving the target of having any three points onthe surface lying on a single straight line. In (d) the overall floor design is aslope towards drainage, but the term ‘flatness’ still applies.

3.4.1 SHORT-DISTANCE FLATNESS, 0–100MM

This problem shows up as sinkings commonly known as ‘elephantfootprints’, caused by poorly compacted and/or mixed areas of screedunderneath PVC tiles or sheeting. PVC is light reflective, which makesthese fairly easy to observe, and the edges of the sinkings are liable toattrition damage. The ‘footprints’ are generally 1–3mm deep and of randompattern. The problem has been commonly encountered in hospitals, wherethis form of construction has been used for corridors, operating theatresand preparation rooms. The general quality of husbandry in thesebuildings does not seem to have helped the situation, with the movementof vehicles, concentrated loads from operating trolleys, and anaesthetistssitting on three-legged stools not necessarily with all three legs in contactwith the ground.

Figure 3.4 illustrates the effect of an ‘elephant footprint’.Remedial work can be carried out, preferably before applying the

smoothing compound and/or the tiling, by cutting out defective patchesof screed material and (avoiding any feather-edging) making good withnew screed material and, most importantly, ensuring full compaction. Itis common to have this defect brought to one’s attention after the tiling.In such cases, when the tiling/sheeting have been removed, defectivecompaction in the screed has been found, and the screed has to be

Fig. 3.4 ‘Elephant footprint’ on PVC tiles’.


repaired and the surface retiled (see section 3.3) when ready to receivecovering. The building’s maintenance department should keep a supplyof new floor coverings to cater for the likelihood of repair. These coveringsshould be identical to the original, to avoid replacements being made withPVC tiling that does not match in colour or shade.

The short-distance flatness problem can be avoided by strict supervisionof screed laying and compaction. The screeds generally used where thisproblem has occurred are known as ‘semi-dry’, and good workmanship inlaying them is essential. Self-levelling screeds are becoming increasinglypopular, as they are not so sensitive to the standard of workmanship.Screeds used to be commonly laid ‘wet’, and the use of superplasticisersseems to have been instrumental in bringing about a return to this practice.

3.4.2 MIDDLE-DISTANCE FLATNESS, 0.1–5.0M

This problem has been identified on site with in-bay and bay-to-bayflatnesses, where a flatness-measuring device is used. Typical wording in aspecification might be ‘a deviation of not more than 3mm under a 3mstraightedge’. Straightedges are commonly made from a hardened steel,and their straightness can be certified by companies registered with theUnited Kingdom Accreditation Service. On site, a reasonably accuratemethod of checking the straightness of a straightedge is simply to lookalong the edge.

It has been found quite easy to record a failure with the 3mmrequirement when adjoining bays have been cast to very good individualflatnesses, but they are not horizontal. Figure 3.5 illustrates this, wherethere is an inverted V at the joint.

Failure can also be indicated where the adjoining bays have the samesloping directions, because there is a step at the joint. Problems at steppedjoints due to wear and tear are common, and therefore it is sensible tocheck flatness tolerances both within bays and from bay to bay.

Remedial work has been found to be best directed at removal ofexcesses rather than addition to the deficient zones. This can be achievedby grinding or other suitable means, accepting the aesthetic defect of aterrazzo-like appearance at the ground-off areas. Thin mortar applicationsto cater for small negative tolerances have generally been found tohave mediocre to poor performance. A longer-lasting mortar repair can

Fig. 3.5 Planeness at joint.


be achieved when at least 10mm of the surface is removed and a repair ofat least that thickness can be effected without feather-edging.

Many of these middle-distance flatness problems can be avoided byaccurate setting-up of stop-ends, rails etc. to which bays have beendesigned to be cast. Optical setting-out both from site datum levels andfrom bay-to-bay levellings is recommended. The use of laser alignment onone site to align the floor of a tunnel led to problems because temperaturevariations along the tunnel caused the laser beam to refract. Forced aircirculation had to be used to create a more uniform temperaturedistribution.

3.4.3 LARGE-DISTANCE FLATNESS, >5M

This problem has been identified in buildings such as warehouses and coldstores, where a lack of (mainly) bay-to-bay flatness leads to stepping atjoints and interference in the mechanics of mobile racking systems. Storageareas such as these are subject to relatively heavy wear and tear. Even a3mm step, which could well be permitted under the example specificationin section 3.4.2 (not more than 3mm under a 3m straightedge), would be atrisk of spalling. Any out-of-flatnesses, especially at joints, will be at riskunder the action of moving vehicles, sliding racks and similar.

Remedial work would probably be best undertaken following therecommendations listed in section 3.4.2. Severer structural requirementsmay be under consideration than for smaller floor areas. If so, then full-depth replacement of all or part of a bay may be necessary.

3.4.4 MULTI-STOREY CAR PARKS

Identification of the problem has commonly taken the form of puddling,frost damage and/or reinforcement corrosion where the flatness datumhas not been to a slope that is effective for drainage.

Although any roof with a slope of less than 10° is defined as ‘flat’, roofsof car parks as well as other buildings are commonly designed to a ‘flatslope’: 1:70 is typical in a specification. Roofs examined in troubleshootingexercises with a slope of less than 1:60 have commonly been observed tosuffer degradation due to inadequate drainage. Clearly, the target for suchroofs must be a combination of ‘slope’ and flatness. Carriage in of waterand de-icing salts by vehicles is common, and if drainage is poor, collectionof water can promote rebar corrosion.

Another observation made on site concerned control joints that shouldhave been placed at high or intermediate parts of a slope or slopes. Sealant-filled joints, not all of which were performing well, had been placed in thebottom of drainage gullies. Ingress into the construction had occurred.


Remedial work is difficult to recommend, bearing in mind that thecommon aim of such work would be to improve drainage by, for instance,cutting drainage grooves in the slope. Care would needed in such cuttingwork, in view of the likely effect of this on the covercrete d.

The problem can be avoided by paying special attention to drainagedetails: not only the slope and flatness of bays, but bay-to-bay slopeuniformity, drainage and positioning of control joints. Manual car-washingfacilities have been introduced in some car parks. Special drainage isimportant in these areas to ensure that the effluent from washingoperations is removed. Collection of water, especially when cars are beingwashed in cold weather, can lead to personnel risk if the surfaces becomeslippery because of water containing detergent or because of icing.

3.5 FLATNESS OF FORMWORK

The problem encountered on site concerned the use of untreated birch-facedplywood formwork for in-situ concrete wall construction in concretingsections about 3.5m high by 4.0m wide. A concreting subcontractor wasassembling the panels of formwork, then cleaning and reassembling themfor four or five reuses in an unprotected enclosure on the site. Each panel offormwork consisted of about eight separate pieces of plywood.

Significant movement of the birch was observed in the form of bowingand dishing of individual sheets and lack of alignment at the butt joints inthe formwork panels. Figures 3.6 and 3.7 illustrate respectively theproblems with the concrete and the formwork that caused them.

A Class B finish to BS 8110 Structural use of concrete had been specified, andalthough the walls were for internal parts of a building and were to be painted,the stepping was unacceptable to the architect. A limit of 2mm maximumstepping was eventually agreed; the wall areas that did not comply with thislimit were ground down at these joints to remove positive tolerance.

Observations on site indicated that there was minimal supervision ofthe subcontractor by the main contractor. It is debatable whether any ofthe problems would have arisen if the main contractor had realised thatresponsibility for the subcontract work was in the contractor’s remit, asthe expertise should have been there for this sort of work. Perhaps if solarreflectors, escalators or other specialist subcontract work had beenundertaken the involvement of the main contractor might have beenexpected to have been more restricted.


The problem is identified by stepping or lipping at formwork joints, aswell as convexity or concavity, discernible by the naked eye, by a


Fig. 3.7 Steps mirrored onot concrete.

Fig. 3.6 Steps in fromwork.


straightedge or by a vertical spirit level. Both types of defect have beenfound easier to observe in reflected light.

3.5.2 REMEDIAL

Grind down lips and other unacceptable high areas. If the appearance ofthe ground areas is a problem, then grinding the whole area could beconsidered, because a patchy terrazzo-like effect would result fromgrinding solely at the joints.

3.5.3 AVOIDANCE

Use good-quality timber formwork with known relatively stable moistureand temperature behaviour. Protect formwork from the effects of the weatheras much as possible, as well as from direct timber contact with concrete andmould-release agents. For the latter in-use protection, seek the formworksupplier’s views on the suitability of painting. If no information can beobtained on a suitable paint, a pigmented paint (Levitt, 1982) could beapplied to both the inside and the outside of the forms. The type of paintselected would depend upon how many reuses are planned. Whicheveractive ingredient base is used in the paint, performance has been found to bemore a function of the presence of a pigment than of the base.

Consideration could also well be given to improved setting-out of panelsof formwork. It is reasonable to assume that with each use there will besome movement. Therefore jig-setting before each use, with the necessaryadjustment of soldiers, whalings and shims, could well promote animproved planeness of finish.

3.6 JOINTS BETWEEN PRECAST PAVING AND BETWEENKERBS

Three problems have been encountered on site with the use of both thesetypes of precast concrete product:

• uneven bedding of paving flags;• joints between vertical faces of flags or kerbs;• jointing kerbs on a hill.

As in section 3.4, the identification, remedial and avoidance commentshave been discussed separately with each of the problems. Some of therecommendations repeat and qualify those published in the mid-1970s(Concrete Society, 1974). For some reason, no reference was made to thisuseful technical report in the concrete industry’s well-known annualreference book, the Concrete Year Book (1997).


The three problems listed above are often invoked to cast aspersions onthe properties of the precast concrete units. Any troubleshooting problemneeds to be examined carefully, and these cases have been found to causeconsiderable aggravation in this respect. It is not common for the singlecauses of the three problems in this section to be identified individually.

3.6.1 UNEVEN BEDDING OF PAVING FLAGS

Identification of this problem is simple, in general unevenness and steppingat joints (Fig. 3.8).

Cost rather than cost-effectiveness considerations have generally beenfound to be the reason for this problem, in that the base and sub-base werelow-cost materials, and a minimum of labour was used in compaction. Inaddition, the practice has been observed of placing a dab of mortar at eachcorner of a prepared seating area, placing a paving flag on top, andapplying a bolster at the centre to settle the unit in place. In effect, an impactflexural test is being conducted: breakage of the paving flag is the commonresult.

The danger to pedestrians from tripping cannot be overemphasised.Although it is not known how many cases are brought against localauthorities for personal injury, my experience with some authoritiesindicates that the departments dealing with installation, maintenance andlegal matters seem to operate in isolation from each other. Informationbrought to my attention in the mid-1990s indicated that one local authoritytook maintenance action only when the stepping at paving flag jointsexceeded 25mm.

The recommended remedial work is to remove all affected slabs, baseand sub-base material and reinstate with well-compacted Type 2 sub-basematerial followed by a minimum of 50mm of well-compacted dry leanconcrete. When this has hardened sufficiently, apply a full bed of a sand/cement 4/1–5/1 mortar and re-bed flags, leaving vertical joints unfilledand the thickness of a trowel’s blade apart.

Fig.3.8 Paving slabs on unsuitable base.


The problem is probably best avoided by having a sub-base, base andbedding specification, as suggested in the previous paragraph. Althoughthis is probably the most expensive method of laying paving flags, thereduced cost of maintenance work and the potential costs for personalinjury should also be taken into account.

3.6.2 JOINTS BETWEEN VERTICAL FACES OF FLAGS OR KERBS

This problem is commonly found in the form of wedge-shaped spalls orcompression cracking at joints. If the products butt, or a stone becomestrapped in the nose, or a line of mortar has been forced into the nose, thenpoint stresses are set up (Fig. 3.9).

Paving flags or kerbs laid during cold weather tend to exhibit thisspalling more than products laid in warm weather. Even though kerbs arecommonly bedded on a backing haunch of in-situ concrete there will stillbe a weather-exposed face responsive to solar-induced thermal movement.

Remedial work is fairly straightforward but, as with most work, thecause has to be removed or inhibited from acting further. All butting jointsshould be sawn down as far as possible, targeting full product depth, withas thin a blade as possible (2mm, for example).

Remove spalled areas and feather-edging, and repair with an SBR orsimilarly approved mortar. Apart from the possible use of an antifoliant,leave the joint untreated. Spall-repair joints with an aperture wider than2mm as above, and fill the joint (cut wider if necessary to 3mm or more)completely with a 3/1 to 4/1 sand/cement mortar having an aggregatemaximum size of 2mm.

The object of leaving a joint of 2mm aperture or narrower unfilled is topromote air-borne dust forming a natural control joint. The filling of widerjoints with mortar is to prevent joint-butting or stone-trapping fromcausing stress-raisers.

Fig. 3.9 Spalling at kerb noses.


The problem is probably best avoided by specifying that all products belaid to an unfilled trowel’s-thickness joint, or that joints be deliberatelydesigned to a nominal 5–10mm width and completely filled with mortarduring or after the laying sequence.

Although not directly related to this problem, control joints placedthrough the haunch concrete that backs a kerb should be continued througha kerb-kerb joint and be visible on the face. Control joints in roadconstruction work where precast concrete units are being used are probablybest designed to cater for separate and individual movements in bespokesections of the road.

3.6.3 JOINTING KERBS ON A HILL

This problem manifested itself as kerbs moving slowly down the hillbecause there was little or no restraint. Kerb joints opened up along theincline and dislodged at radius kerbs or similar at the bottom of the hilland/or at corner junctions. The problem appeared to be confined torelatively steep hills: on the few occasions that it has been observed thehills had inclines of 1 in 20 or steeper.

Remedial work was not undertaken in any of these cases as far as isknown. Common sense suggests that one answer could be to replace orrealign dislodged kerbs, coupled with some form of restraint. The restraintcould be effected by concreting in a vertical butting unit. This could be asmall column, or even a kerb on its end, so that runs of kerb could buttagainst these restraints. The problem could be avoided by taking thisprinciple of ‘column’ restraints into the design. An engineering appraisalmight be needed to calculate at which kerb centres these restraints wouldneed to be placed.

I have had only hearsay evidence of problems with precast interlockingpaving blocks, and am therefore unable to include any personal experiencehere. Most reports have been of these blocks sinking below datum level.Therefore much of what is recommended in section 3.6.1 would appear to bethe way in which the main problem of units sinking could have been avoided.

3.7 TOLERANCES

The main problems encountered on site with tolerances seem to have beenassociated with a general lack of appreciation that, although building lociare never at exact and predictable points in space, their variations andcatering for these variations are both within the realms of the designer,builder and/or manufacturer. In general, the problems investigated fellinto two broad categories:

• positive and negative tolerances;• matching tolerances.


When a dimension is put into a drawing, tolerances need to be putalongside, because what has been asked for has to be both achievable andbuildable. As far as tolerances of achievability are concerned, if what hasbeen dimensioned is something like a window, for example, then it is notlogical either to omit tolerances or to specify tolerances that cannot beachieved by the manufacturer. Concerning buildability, and using the samewindow as the example, if there is too much positive tolerance for thewindow and/or too much negative tolerance for the opening designed toreceive it then it is very likely not to fit.

If the case under consideration was a precast concrete cladding unit, thenin addition to the geometry of the opening and the dimensions of thecladding unit, there would be the tolerances (for both support and restraint)of the fixings.

There is a useful general rule: tolerance is an easy thing to find on aconstruction site but a difficult thing to lose. More difficulty has been foundin assembling parts of a building because they were too large than becausethey were too small. There are a variety of gap-filling materials available tocater for undersizing, but mechanical or similar removal is all that isgenerally available to deal with too much positive tolerance. There arenotable but rather singular exceptions to this guidance: the aluminouscement beams resting on column haunches described in section 1.7 are anexample where significant positive tolerance was critical.

I have been involved in many cases where troubleshooting revolvedaround the subject of tolerances. Of the three examples that follow, the firsttwo relate to positive and negative tolerances and the third to matchingtolerances.

Example 1

A prestige building had a canopy of single-skin GRC cladding panels fixed toa stainless steel subframe. It had apparently been assumed that the specialist-manufactured subframe would be made to strict tolerances because, possibly,of the use of an expensive alloy. Unfortunately, this did not turn out to be thecase; and, probably worse than that, the whole frame was under-dimensioned,and great difficulty was experienced in fixing the GRC panels in theirdesignated positions. Much of the excess negative tolerance was countered bythe use of large numbers of stainless steel shims at the fixings. In some cases asmany as 12 shims, 3mm thick, were used. The only thing found ‘right’ on thiscontract was that, in the other two space dimensions, the fixing points werefound to be within an acceptable tolerance.

Example 2

The problem concerned rain penetration around the window details of anoffice block gable end constructed of cast stone ashlar cladding, sill and


lintel units. On leaning out of one of the top-storey windows where leakagehad been occurring, it was found that both the sill and ashlar unit belowcould be easily rocked by hand. Arrangements were made to have thesetaken off the building before a second visit.

It was found during the second visit that there were no fixings to theinsitu wall relative to the removed stones, nor, with the aid of a torch, couldany be seen to the adjacent stones. The rain ingress points at the sill wereeasily discernible; a direct mortar path allowed passage under the sill.

When all 10 storeys of gable end stonework were removed, it was foundthat less than 10% of the total number of units had been secured to the in-situ concrete wall. The main reason for this lack of fixings appeared to bethat the in-situ wall was so far out of vertical that the Abbey Dovetail tailswere unable to reach the slots cast into the wall. Units were reinstated withcountersunk stainless steel expansion bolts both securing the cast stoneunits and fixing to the in-situ concrete.

Example 3

This was an instance of troubleshooting at the design stage, and concernedmatching tolerances. Precast concrete units were to be fixed usingproprietary cast-in stainless steel slots in the units to locate with floor boltscast into the in-situ concrete. The problem was that the site, being arefurbishment in a busy metropolis, had no storage room and could nottolerate any delay. Fixing had to be direct from the delivery vehicle ontothe facade, and had to be timed for when the tower crane was available.

It was agreed that, in order to correlate the precast unit dimensions andthe tolerances for the positions of both the site and unit fixing points, theopening (the gap to receive the unit) centre lines rather than column centreswould be used for location. Precast units were dimensioned from the centrevertical and horizontal lines, both for their overall sizes and for the cast-infixing slots. Few problems were encountered on site.

Figure 3.10 illustrates how this system was used.


The problem shows itself as difficulty or inability in putting parts of aconstruction together.

3.7.2 REMEDIAL

This is difficult to address, because each case probably needs to beconsidered individually. Fortuitously, the 1980s and 1990s have seen theadvent of many new types of proprietary fixing, which can be used, forinstance, as single or double locking or expansion devices. In addition,


polymer adhesive systems have some applications where injection orgravity feed are possible. Endoprobes can be used to inspect the efficacy ofmany of these remedial actions.

Sometimes difficulties are known to be likely because of poor alignment,and on-site welding of stainless steel fixings or similar might be a viableremedial measure.

3.7.3 AVOIDANCE

Most of the problem areas mentioned above can be avoided by applyingrealistic detailing and dimensioning, always bearing in mind thattolerances are important all the way from material to product to completedconstruction. Buildability could possibly be improved by aiming to keeppositive tolerances as small as possible and allowing variations to becatered for in the negative tolerance range.

Fig. 3.10 Tolerances for a slot fixing in a precast concret cladding unit; x and y each±2mm (not to scale).


3.8 RISING DAMP AND A CHEMICAL BARRIER

The problem experienced on site related to concrete block masonry wallsexhibiting apparent rising damp and how to rectify the situation. Therewas little point in addressing a rising damp problem unless it could beproved beyond reasonable doubt that the dampness was due to water beingdrawn up from the ground by capillary attraction. Dampness can be causedby one or more of the following:

(a) rising damp from the ground;(b) water penetration through the wall from outside;(c) water penetration through the roof;(d) condensation.

Attention was drawn to a British Standards publication (DD 205:1991) thatsuggests specific tests for identifying (a) with respect to the other threecauses. However, rising damp tends to exhibit a common behaviour,whether it is on brickwork, blockwork or rendered faces: the dampnessrises to only 1.0–1.5m from ground level, and the damp area is generallycovered with black mildew.

On the site visited it was found that the damp-proof membrane hadbeen omitted. The dampness was diagnosed as rising damp, and it wasrecommended that a chemical damp-proof course be installed followingthe guide lines in the code of practice (BS 6576:1985). Apparently, aninexperienced company was chosen to undertake the injection work, anda site re-visit was requested to comment on the possible reasons for theapparent lack of success of the remedial work.

Most of the advice given in BS 6576 had apparently been ignored, andthe errors that were observed on site included the following:

• The silicone used (an aluminium stearate might also have beensuitable) had a low solids content (about 5% is a typical content).

• The internal rendering had been reinstated too soon after the injection.The chemical should have been left to become fully effective. Thisnormally takes a year or a little longer.

• The rendering reinstatement should have been undertaken using awater-repellent mortar, stopping this at the injection line.

• Where party wall work was being undertaken the neighbour had notbeen advised of the smell of solvent permeating the wall, nor of theneed for ventilation.


The problem is characterised by dampness and mildew growth to a heightof 1.0–1.5m above ground, and chemical injection remedial work is of littleapparent benefit.


3.8.2 REMEDIAL

Using a contractor proficient in the application of the British Standardrecommendations, step drill and silicone inject the masonry wall as shownin Fig. 3.11, and reinstate mortar and decorations after a period of at least ayear.

3.8.3 AVOIDANCE

Take special care when installing mechanical damp-proof courses andtrays. Many problems found on site have been associated with damagedmembranes, as well as with mortar bridging dpm lines. The masonry codeof practice (BS 5628 Part 3:1985) recommends that something like 5mm ofmembrane should be left protruding from the face of the masonry. This isnot as common as it probably ought to be, and membranes tend to getbridged by mortar, allowing a path for rising damp.

A non-concrete case of dealing with rising damp was studied at theinstallation date and again just over a year later. It concerned a buildingthat was 150 years old, with a single skin of clay brickwork 320mm thick.

Fig. 3.11 Chemical injection steps for a damp-proof membrance.


The system studied used an electrical method by placing a direct currenfield across the base of the wall just above ground level. The systemallegedly relied on the electrical properties of the rising damp, and put anopposing field into the wall. Little or no improvement was observed a thesecond visit.

A possible lesson to be learnt from this is that processes should beexamined and understood as far as possible. The best test assessment isprobably track history of performance if knowledge of the mechanisms islimited.

3.9 CRACKING IN THE NON-VISUAL ZONE

The Glossary defines both ‘crack width’ and ‘crack aperture’; the problemsfound on site all related to crack aperture, as it was this that was visible.The crack aperture was generally assumed to reflect both the crack-widthand the number of cracks beneath the surface in the concrete. For example,it was generally thought that if a crack of 0.5mm aperture was present thenthe crack width was constant at 0.5mm right down to the steel, and thatthere was just that one crack. However, the various types of crack geometryshown in Fig. 3.12 demonstrate that this assumption can be misleading.

Once a crack has been seen, one or more of the following possibilitiesarise: (a) Water and air can access the rebar and initiate corrosion.(b) Dirt can collect at the apertures, giving the surface an unsightly

appearance.(c) Structural distress may be occurring.(d) The crack may be static or dynamic.

Fig. 3.12 Various crack geometries surface crack widths in mm (not to scale).


In general, the problem manifests itself on site as (a). Once a crack isseen it is commonly assumed that there is a corrosion risk, and thatsomething needs to be done about it. Guidance in the British Standard (BS8110 Part 1:1985) refers to a limiting crack aperture of 0.2mm as acceptablefrom the point of view of corrosion. The code of practice for water-retainingstructures (BS 8007:1987) suggests a limiting crack width of 0.1mm.However, little or no attention appears to have been paid in these guidancedocuments to the crack geometry. The reference to crack apertures alone,with no reference to what the crack does as it goes into the concrete, mightapply to one or two crack-forming mechanisms but cannot cover all causes.Section 1.2 referred to the observed geometry of thermal cracks that,although traversing down to the steel at a constant width of about 0.1mm,had not promoted carbonation, nor chloride-induced corrosion (it was acase of marine exposure), nor additional chloride ingress.

There are many publications that refer to crack width geometries as wellas to crack aperture, but it probably serves little purpose on thetroubleshooting trail to list these. Cracking in concrete is common, and thereare very few cases where either macro or micro-cracking is not present; yetmost concrete carries on doing the task for which it was intended withoutexhibiting distress. Reinforced concrete is designed to transfer its weaknessin tension to the rebars placed in the tension zone. The steel controls theextent of the concrete yield; it does not stop cracks. As pointed out in a paperpublished in the 1980s (Richardson, 1986): ‘If it isn’t cracked, it isn’t working.’The aim must be to identify whether or not a crack aperture is a risk.

The indirect method of the application of UPV at the surface (BS 1881Part 203:1986) can indicate both depth and direction fairly accurately if thecrack is not bridged by sound-conducting material such as water ordetritus. However, one needs to know the risk of water getting to the rebarsvia the crack only if there is a reasonable indication that the crack is singularand might be as deep as the rebars. The initial surface absorption of concrete(BS 1881 Part 208:1996) has been used on site quite successfully to assessboth cracked and uncracked areas. Because of the point-to-point variabilityin surface permeability it has not yet been possible to suggest a comparisonlimit for acceptance or rejection. The small amount of site data collectedindicates that, if the crack could allow water ingress, significantly higherresults would be obtained from cracked zones than would be expected forgeneral surface variabilities.

Crack aperture has generally been found to be irrelevant to the dirtretention risk referred to in (b). The mechanism observed was that capillaryattraction of rain at the crack aperture resulted in water being drawn in,and the dirt from the water was left on the surface (Fig. 3.13).

What this amounts to is that all crack apertures appear larger than theyreally are. Rubbing one’s finger across the crack to remove the dirt revealsa much narrower defect.


The structural and crack mobility factors referred to in (c) and (d) fallwithin the province of an engineering assessment rather than being amaterial problem. There is little point in dealing with cracks from any ofthe points of view of identification, remedy or avoidance without areasonably clear picture of the mechanism(s) that caused the cracking, andof the crack geometry.

The two common types of crack-measuring device used on site aretransparent plastics cards with different thicknesses of lines for layingalongside cracks, and the crack microscope. Both of these measure crackaperture and not crack width. The crack microscope is possibly too precisea tool, because the need to measure crack aperture beyond a 0.05mmaccuracy (which can fairly easily be judged with the human eye against areference line on the card) is debatable. Furthermore, site conditions andergonomics need to be considered. There are attractions to using a cheapcard that is pocket sized (100mm×40mm, say), sufficiently accurate, andvirtually unbreakable. However, whichever measuring device is used, theinformation it provides has been shown to have little value.


The problem is identifiable in one of two forms: either there is somewording in the specification limiting crack apertures, or visual cracks areobserved whose position, crack apertures or times of occurrence are not asexpected.

3.9.2 REMEDIAL

For static cracks that are not considered to be a corrosion risk, rub down toremove all dirt, and apply a flood coat of silicone water-repellent to therecommendations of BS 6477:1992. Static cracks that are considered to be acorrosion risk may be amenable to repair by injecting an epoxide resin. The

Fig. 3.13 Dirt retention at crack aperture (not to scale).


injection equipment should be connected to a pressure gauge, because it isquite easy, using ordinary hand-pump actuation, to make the damage worse.

Cracks exhibiting dynamic behaviour (that is, ‘live’ cracks) needindividual consideration depending upon the site conditions. If the crackaperture is in a narrow range (0.05–0.25mm), a copolymer capillaryintroduction may be successful. Wider crack apertures may be amenableto repair using a silicone or similar sealant.

3.9.3 AVOIDANCE

Although good design and workmanship minimise the occurrence ofcracks, it is essential—both at specification stage and during the work-notto raise problems when the risks are insignificant or unfounded. The bestapproach is probably for the parties to agree at a preliminary stage whichcrack apertures necessitate further investigation and possible action, as wellas which cracks do not need attention.

3.10 LARGE-AREA SCALING OF FLOORS

The problem occurred with an industrial in-situ concrete floor where asuperplasticiser complying with BS 5075 Part 3:1985 had been specified.The admixture was based on a chemical that was added into the truckmixer in the specification range 0.7–1.2% m/m cement at the delivery pointon site. It was added at a concentration of about 1.0% m/m cement for thisparticular site. As is common with truck-mixed concrete, the admixturewas stored in a tank above the mixer drum, and was added and truck-mixed on site just before the concrete was discharged.

Because of an error at the ready-mix plant, the admixture tank wasinadvertently charged with the same volume (about 20L) of an air-entraining agent based upon a neutralised vinsol resin. This type ofadmixture is normally added at one tenth of the concentration of asuperplasticiser: 0.1% m/m cement is typical. More importantly, it isrecommended to be added into the mixing water when the truck mixer ischarged at the plant.

The day after casting the bays with the 6m3 of concrete from this truckload, large-area scaling of sheets of surface material occurred down to athickness of 1–3mm. The scaled material was darker than the remainingconcrete, and at the interface there was a concentration of coagulatedbubbles with accompanying delamination parallel to the surface. Theremaining concrete was uniformly air-entrained, but the total air contentin the hardened concrete was about 12%. Of this percentage, about 1.0%consisted of irregularly shaped entrapped air voids; the remainderconsisted of the common spherically shaped air-entrained voids in the


diameter range 0.05–0.50mm. The coagulated bubbles at the interface weremostly in the diameter range 0.05–0.20mm. The bays affected were replacedwith concrete complying with the specification.

Although the causes of this scaling were not discussed at the time of thediscovery, a comparison of the mechanisms of the action ofsuperplasticisers and air-entraining agents could yield a relevant clue. Theplasticising mechanism of a superplasticiser is generally accepted aselectrostatic action in setting up repulsive forces between aggregate andcement particles. Thus underdosing or overdosing such admixtures wouldtend to cause less or more film-over-particle formation respectively. Air-entraining agents generally rely on the mixing action’s trapping bubblesthat are electrostatically bonded between cement particles and thesurrounding aggregate-rich matrix. So an overdosing of air-entrainingagent could result in a concentration of bubbles, which might join togetherunder the same forces and form a pseudo-entrained/entrapped system ofplates of bubbles. These plates might have only enough buoyancy underthe action of site compaction processes to rise to just below the surface.

If this mechanism is relevant, then if there were cement-rich areas in theready-mix truck due, perhaps, to insufficient mixing, similar large-areascaling would be expected. However, the scaled areas would be in patches,because if cement-rich areas were responsible they would be unlikely to beuniform in the truck mixer. Furthermore, the amount of air-entrainingagent overdosing would not need to be particularly high, because a bubble-trapping mechanism would tend to operate at the cement-rich areas.


The problem would cause large areas of scaling up to 3mm deep of surfacelayer, exhibiting little air entrainment but with a weak interface of coagulatedlaminar bubble plates also showing laminar cracking. The remainingsubstrate concrete from the same batch of concrete would tend to exhibitfairly uniform air bubble distribution. The surface planarity would notchange significantly over the scaled areas. The scaling observed, andillustrated in Fig. 3.14, could not have been misdiagnosed as the familiarblistering or small-area scaling, about which much has been published.

3.10.2 REMEDIAL

Depending upon the degree of unsound material, a decision may be takento cut out the top layer to a depth of, say, 10–20mm and resurface with newconcrete if the underlying substrate is acceptable. If there are more seriousdoubts (as there were with the case mentioned) then complete replacementmay have to be considered.


3.10.3 AVOIDANCE

All admixtures should be used in accordance with known and acceptablegood practice. Those added at the plant or works should be fed in with themixing water. Superplasticising admixtures added on site should be fed inonly after the concrete in the drum has been uniformly mixed and is asclose as possible to the required discharge position.

3.11 SILANES

The problem encountered on site was with large concrete units precast onsite for placement in and above tidal water. The specification required thatvirtually all concrete be treated with silane according to governmentrecommendations (DoT, 1990). The nature of the problem was twofold.First, the concrete was too impermeable to absorb more than a marginalamount of the two ‘coat’ specified silane applications. Second, what littlemanaged to get into the concrete penetrated only about 1mm deep.

As far as was known at that time (and probably at the time of preparationof this book), silanes had little or no track history in the UK. In contrast, thelong-established silicones have had a proven performance over at least 40years. It is of interest to note that the superseded 1984 edition of the currentstandard on masonry water repellents (BS 6477:1992) replaced a standard(BS 3826:1969) that referred to silicone only in its title. The aim of thesetypes of treatment is to impregnate (not coat) concrete with a chemical thathas or endows the concrete with water-repellent properties. With silanes,one has to wait for a significant amount of time to obtain this property.Therefore the concrete or other material being treated must be capable ofbeing impregnated to a depth that will give an acceptable performance:that is, not so shallow as to be at risk of losing its efficacy too quicklybecause of wear or weathering. In addition, what impregnates the concretemust be designed to act as a water repellent and not be subject to nordesigned for water under more than a nominal pressure.

Only silicones, usually based in a volatile solvent such as white spirit,act immediately as water repellents by lining the sides of voids andcapillaries with a hydrophobic layer. Silanes have much lower viscosities

Fig. 3.14 Large-area scaling on a slab.


than silicones in solvent, and should penetrate further if viscosity is thesole criterion. However, silanes are not water repellents, and have to behydrolysed (chemical transformation by reaction with moisture in theconcrete or air) to form silicone.

It would be expected that water-repellent properties would only becomeapparent with time. This was observed on site, where the thin layer ofpenetration was observed to be water repellent only after several months’weathering.

An additional factor to bear in mind is based on the physics of fluidpenetration. Viscosity controls the speed at which fluids pass through acapillary system, but there has to be pressure driving that fluid. The‘pressure’ for fluids in contact with permeable materials such as concretearises from the capillary attraction force, which is mainly a function of thesurface tension of the fluid and of the capillary radii of the permeablematerial. There is considerable knowledge of capillary sizes in concreteand of the viscosity of silane and silicones, but nothing on the surfacetension of either silanes or silicones.

Another potential disadvantage of silane is that because hydrolysis isthe mechanism that converts the chemical to the water-repellent siliconeform, moisture cannot get past the hydrolysed material to hydrolyse theunderlying silane.

Silanes are virtually solvent-free systems, and have very low boilingpoints, which can, at 35°C, sometimes be lower than the temperature ofthe concrete. Not only does this make application troublesome in warmweather, but it may also be necessary to spray the concrete with water topromote hydrolysis if its own free moisture availability is low.

Another consideration is what to do with the unabsorbable run-offmaterial. A typical cost of silane is about £3 per litre; from the two specifiedapplications of 300mL/m2 each, it would not be unusual to have 500mL/m2 or more being lost. In the case in question, gutters, filters and collectiontanks were installed, and the silane saved was reused.


The problem is flagged by a silane application requirement in thespecification.

3.11.2 REMEDIAL

Advise the specifier of the following: • The concrete will have to be of mediocre or poor quality for the silane

to be able to penetrate to a significant depth.


• A wait of some months will be required for the silane to hydrolyse intothe form of a silicone water-repellent.

• It may not be possible to achieve the specified coverage rates with littleor no rejection of the silane.

• Problems are likely to be encountered in hot weather.• Silane is expensive.• Silicone in a solvent may be a better proposition.

3.11.3 AVOIDANCE

Pre-tender discussion is advised. The contractual aims may well beachievable by better-known and tried means.


Specification problems 4

INTRODUCTION

Before going into the details of the four problem areas discussed in thischapter, it may be helpful to describe some of the technical and materialsaspects that have coloured the current activity. Every effort has been madeto keep to materials problems, although it is difficult to avoid thecontractual aspects of construction. Hopefully, what will emerge is the needfor an interdisciplinary appreciation of all the interlocking features of theconstruction programme. It is much more effective to get involved inproblem solving and troubleshooting at the preliminary stages rather thanduring the work on site or after it has been completed.

Some of the general problems encountered tend to have commonfeatures related to the technical requirements of the contract:

• a tendency to use wording that is either vague or subject to debatableinterpretation;

• general reference to standards without detailing the relevant parts orclauses;

• a misguided concentration on initial cost, often at the expense ofscientific or technical needs;

• contradictions between the specification and other submitteddocuments in matters such as references and drawings;

• reluctance by the party receiving tender documents to comment ontechnical and materials preferences or buildability factors;

• reluctance by the party preparing tender documents to consult therelevant disciplines on these matters before submission for pricing;

• reticence in the application of scientific and technical state-of-the-artknowledge to construction.

There are possibly a few more factors that could be invoked. However, it ishoped that optimism for the future of the industry will result in a sensible


appreciation of the roles that the technical and scientific parties shouldplay in construction.

4.1 THE CE MARK

The problem related to what the reader understood when the CE markwas seen written on goods, literature, invoices or delivery notes. All toooften the interpretation was that what was being offered or supplied washigh-quality, reliable articles. This may well have been so, but it is essentialto understand what the claim means. I gained considerable experience inthis by representing the Chartered Institute of Building on the Departmentof the Environment’s Joint Advisory Committee.

The basis of the CE mark is the safety requirements contained in aHarmonized Standard, a Standard or an Agremént Certificate (the lattertwo documents do not have a European equivalent). The Joint AdvisoryCommittee, with members constituting trade associations, professionalinstitutions and the like, advised the UK government’s representatives onthe European Standing Committee concerning the policy of Directives.

For the construction industry the main Directive of relevance is theConstruction Products Directive, which was issued as a StatutoryInstrument (HMSO, 1990). There are other Statutory Instruments basedupon other Directives, such as Low Voltage Equipment or Gas Appliances,which also apply—sometimes in contradiction to the CPDbut the CPDpredominates for construction.

The CE mark for construction products relates to six EssentialRequirements: mechanical, fire, health, noise, energy and safety. These referto the behaviour of the product in the works, and not to the product itself.How these six features apply to the European Standards TechnicalCommittees who receive their instructions from the Standing Committeehas been subject to a great deal of interpretation. Eight InterpretativeDocuments have been published (fire has three: structural, noxiousemission and extinguishers), which are an order of magnitude larger thanthe original CPD.

The Harmonized Standard on which a CE claim is based is a normativeAnnex derived from the CPD’s Essential Requirements. Therefore goodsclaimed to be CE marked need to be studied for qualification as to the basisof that claim. The following points merit investigation:

• Against what Harmonized or equivalent Standard is the claim made?• Do the clauses in that Standard meet both the technical requirements

and the health and safety needs of the product when installed in theworks?

• Is it appreciated that the CE mark applies only to the EssentialRequirements, and that if there is another contract specification, suchas an architectural one, this is unlikely to be covered?


• Is it appreciated that the CE mark refers to the suitability of the productin the works, but contains no reference to the construction processesdirectly?

• Is it appreciated that the main basis of a CE mark claim is that theproduct complies when assessed under the clauses relating to healthand safety in the Harmonized Standard (or equivalent document)? Thisis irrespective of whether or not those clauses meet the end-user’s‘essential requirements’.

In effect, the CE mark is a ‘safe to use’ claim based upon safety defined byothers; it is not a ‘fit for purpose’ claim. Although a product manufacturerplacing the CE mark on goods generally has to have external assessment,the goods do not, in my opinion, generate the same degree of confidencethat the British Standard Kitemark gives. The Kitemark specifies thestandard to which the product is being assessed, and how regularly it isbeing examined.


The problem arises when the CE mark appears in the contract documentationwithout the parties concerned knowing to which Harmonized Standard orequivalent specification the product is specified, or how all this relates to therequired performance of the product in the works.

4.1.2 REMEDIAL

Advise the party or parties concerned of the limitations of having onlypart of the knowledge necessary for the construction.

4.1.3 AVOIDANCE

CE marked products should be specified only when it is clear which arethe supporting documents, and what their relevance is to the jobrequirements. The better-tried route of invoking relevant codes of practicecoupled with supporting standards and adequate supervision on site mightbe more cost-effective.

4.2 DURABILITY

The problem is to define the word ‘durability’. It has been used for manyyears in relation to concrete and concrete structures, but all too often it canmean any of the following:

• strength, usually with respect to the cube results;• sulfate resistance, usually for concrete placed in the ground;


• freeze/thaw resistance, for pavement-quality concrete;• a generally vague reference to how long it ‘lasts’.

The best way of tackling the problem could well be to pose the questionsthat seem relevant, and propose how these questions may be answered.This has manifested itself in four main ways on site:

• What is meant by ‘durable concrete’?• What is/are the durability risk(s) and effect(s)?• How is durability to be assessed?• For how long is the concrete required to be durable?

4.2.1 DEFINITION OF DURABILITY

The simple dictionary definition of ‘durable’—‘strong and long-lasting’—is possibly what has resulted in the use of concrete strength to defineconcrete durability. However, the relationship between concrete strengthand any particular durability risk is, at best, tenuous. As far as is knownthe word ‘durability’ has not been defined in the Standard (BS 6100 Part6.2:1986), and the following might be suitable and relevant:

The ability of concrete to perform in the manner expected underthe defined conditions of use for the time expected.

This definition is subject to interpretation with respect to such phrases as‘manner expected’ and ‘time expected’. However, if it is accepted that theseaspects of durability are relevant then they need to be described asaccurately as possible. The phrase ‘defined conditions’ is probably the partof the definition that is simplest to understand. For example, if concrete isdamaged by a durability hazard arising from an unexpected change ofuse, then the concrete was and still is durable by its basic definition. Thefollowing sections should clarify the meaning of ‘manner expected’ and‘time expected’.

4.2.2 RISKS AND EFFECTS OF DURABILITY HAZARDS

There are at least a dozen durability risk factors that, either singly or incombination, could be relevant to any particular assessment:

• Sulfate attack: weakens surface or can cause splitting.• Chloride ingress: promotes rapid unprotected rebar corrosion.• Carbonation ingress: permits rebar corrosion.• Freeze-thaw (F/T) without de-icers: degrades surface slightly.• F/T with de-icers: severe surface degradation.• F/T with anti-frost chemicals: surface softening and dissolution.• Inorganic chemicals: dissolution in depth.• Organic chemicals: generally a more rapid dissolution than inorganic

chemicals. (Note: The calcium salts resulting from some organic acid


reactions can form an acid-insoluble protective layer during thereaction. Citric acid tends to behave in this manner; the initial rapidreaction tends to stop within a minute or two.)

• Attrition: loss of surface and dusting.• Impact: spalling, cracking or shattering.• Fire: integrity loss, spalling or cracking.• Weathering: change in appearance. (Note: Architectural durability

should be considered alongside the other durability factors. Anappendix in the cast stone standard (BS 1217:1997) tabulates theseconsiderations: this is possibly the first time that this matter has beenaddressed in a BS.)

• ASR: severe cracking and loss of integrity when the reaction isunacceptably expansive.

Note: Refer to section 1.12 for the reason why DEF is omitted from this list.

4.2.3 DURABILITY ASSESSMENT

Although it seems desirable to assess the concrete’s degree of durability bya test, the apparent logic of this does not always prevail. An example thatarose from a recent site experience related to concrete in an estuarineenvironment, with chloride ingress being the main risk. Section 3.1discussed the diffusion characteristics and the symbol k, where test datatook at least 6 months to yield indicative data. Contractually, it is muchmore acceptable to use a strictly supervised mix design regime with itsknown track record of good site performance. The same considerationslargely apply to freeze-thaw testing and ASR assessments.

It is not the purpose of this section to discuss how to deal with each ofthe 13 risks listed above, but simply to suggest that although testing has itsattractions it is not always the most effective or acceptable. There are well-established track records of specific recipes of concrete, and with the needto ensure good mixing, compaction and curing procedures, it is attractiveto specify and control mix design. If the parties concerned with theassessment of durability agree that these control aspects are those thatprevail, then the incorporation of supervisory items in the contractprobably merits further discussion.

4.2.4 DURABLE LIFETIME

This difficult subject is possibly best tackled from the point of view of trackrecord, coupled with on-site monitoring to check predictions. This may beachieved by concrete mix/concreting specification, or testing, or both. Areasonable prediction can be made of the onset of something unacceptable,or of the need for maintenance or remedial work; then a reasonable


assurance of a 10-, 25-, 50- or 100-year life will result. Concrete has been inuse for well over 100 years, and most of it is documented as performingsatisfactorily. There is enough information available to explain why thingsgo wrong when they do.

The main problem with concrete is that it cannot be designed like anelectric lamp, for example, to give 1000, 5000 or 10 000 hours’ burning time.Most of my troubleshooting experiences have related to concrete under 20years old, with most of the problems becoming manifest within the firstfive years. The inference from all this is that concrete should be designedto last as long as possible. It seems too risky to design concrete deliberatelyto show durability distress within a limited time, such as 10–20 years.

When it comes to proving the point that the planned durability hasprevailed over the years, there are, in addition to visual examination, alarge number of monitoring methods. For example, carbonation depth,chloride gradient, half-cell potential and the initial surface absorption test(ISAT) have all been found to be useful on-site tools. Of particular notewas an article referring to the use of the ISAT in limit-state design for thetime of the onset of rebar corrosion (Levitt, 1985).


Look out for the words ‘durable’ or ‘durability’ appearing in documentswith no qualification, risk description or assessment/control.

4.2.6 REMEDIAL

Probably the best way to deal with this is to ask for the qualification. Whenit comes to dealing with durability as defined in 4.2.1, there appear to betwo camps. The first is the ‘cater’ camp, which accepts that something isgoing to go wrong and caters for it either by repair or by replacement. Thesecond is the apparently rarer ‘cure’ camp, which ensures that the concreteneeds no maintenance apart, possibly, from occasional cleaning.

4.2.7 AVOIDANCE

Pre-contract liaison between the members of the construction team seemsto be the best way to promote an understanding of the relevant durabilityconsiderations, and of the best way to deal with them in each specific case.

4.3 CONCRETE QUALITY

The problem is concerned specifically with the word ‘quality’, and withwhat much of the specifying and purchasing communities think the wordmeans. Quality, and all that the word engenders, has resulted in a growth


industry in both the manufacturing and servicing sectors. The subject iscomplex, and a simplified step-by-step approach may help to throw somelight into a grey area.

Many users of the identical EN 29000 or ISO 9000 series of standards areas likely to be misled by the inference of the word ‘quality’ as by thedeliberately badly worded title of this section.

Quality is defined (BS 4778 Part 3:1991) as

The totality of features and characteristics of a product or servicethat bear on its ability to satisfy stated or implied needs.

In my opinion this definition could be substantially improved, but firstnote that the word ‘quality’ is a noun. It is generally misinterpreted as anadjective, to mean that the thing it refers to is ‘good’, ‘better’ or ‘excellent’in some way. This initiates a discussion on the interpretations of thedefinition of the word, which will lead naturally into a description of thepit-falls awaiting the unwary. Most people seem to accept that the word isalways a guarantee of something good.

4.3.1 QUALITY: THE NOUN

As a noun ‘quality’ needs an adjective to describe the quality on offer, suchas ‘good’, ‘mediocre’ or ‘poor’. It does not matter whether what is beingconsidered is a product or a service. The current common use of the word‘quality’ as an adjective before either of these words—or any other word—is either meaningless or misleading. It gives no information as to the sortof quality on offer, and it implies, without supporting information, thatwhat it refers to is superior or good.

4.3.2 PRODUCT AND/OR SERVICE

The ‘and/’ part of the title of this section does not occur in the definition.Usually this will have no effect on a product manufacturer, nor on thedesigner or specifier. However, when it comes to a contractor or asubcontractor the definition fails, because what is under consideration is amixture of both product and service.

The simple solution would be to change the definition to include ‘and/or’. A more complicated approach might be to restrict the contractor orsubcontractor’s work to quality assurance of the work undertaken as aservice. This would leave any product involved subject to separate assurance.

4.3.3 IMPLIED NEED

The words ‘or implied’ in the definition introduce a grey area ofinterpretation. It would be far less complicated if these two words hadbeen omitted from the definition.


4.3.4 QUALITY PROBLEM MANIFESTATIONS

Great store is currently being set by the optimism that seems to beengendered by specifying quality in some form or the other in products orservices. However, when a product or service is ‘quality assured’, all that isknown is that there is some independently assessed paperwork confirmingthat a claimed level of quality is being achieved. Thus, if a quality-assuredproduct or service is specified or offered, the parties concerned have areasonable assurance of consistency, and an implied expectation ofsomething that is good or very good.

The impression created by a framed ISO 9000 certificate in a foyer or aclaim in trade literature seems to be out of proportion to what thatcertificate or claim actually means. The information has to be accompaniedby one or more appendix pages stating what quality is on offer, and howthe person making the offer substantiates the claim. It is only for a serviceorganisation such as an architectural practice that the lead certificate couldcarry words such as ‘Complete range of architectural services’ and providesufficient information on a single piece of paper.

As an example of the complications manifested by the subject of quality,consider a specifier asking for a product from an organisation with ISO9000 registration. The interest is solely in the product, not in the way it is tobe used. If the logical assessment of the suitability of that product is in itsproperties, then surely a BSI Kitemark is sufficient?

A second example that might be more relevant to the constructionindustry would be a ‘manufacture and install’ requirement. The first ‘head’of this two-headed specification could probably be dealt with by aKitemark. The second ‘head’ needs to be a service-oriented control by theISO 9000 route, or by a known track record, or by adequate site control andsupervision.

Hence there needs to be a clear definition in a series of specificationclauses that define, preferably with little or no ambiguity, what is requiredto be purchased. Where a product and/or service is on offer, in the samelight, that product and/or service requires a fully qualified andunambiguous description. There are also grounds for discussion outsidethe subject matter of this section for the inclusion of the control of qualityas specific cost items in the bill of quantities. Although it can be expensiveto install a quality system, a well-organised and well-run system can soonbe shown to be cost-effective, and can pay for itself in the long run.

The most dangerous trap awaiting the unwary is probably tied up withthe word ‘complacency’. In-place quality systems generate a considerableamount of paperwork, the need for some of which is questionable. An ISO9000 claim or statement of compliance with an established qualityprocedure conveys an aura of well-being to the uninitiated recipient. In


spite of all this, it is still incumbent on all associated with the quality sceneto understand what is being asked for and what is supplied.

Products and services are fit for purpose only if the parties concernedare fully aware of what it is they are buying or selling, and realise thatfitness remains within the constancy of that claimed and specifiedthroughout the contract.

The author, with many years’ experience as a test house quality manager(how one ‘manages’ quality could well be subject to another and longerdiscussion) and a Fellow of the Institute of Quality Assurance, condonesthe application of quality principles in any industry or service.Nevertheless, the 1980s and 1990s have seen a tendency to over-react tothe assumed benefits of registration.

A recent publication in the Institute’s journal (Seddon, 1994) discussedISO 9000 in general terms. Seddon stated that registration was related tocompetitiveness in a tendering position, and that the results of a survey byVanguard Consulting painted a picture of a standard that was not makinga strong positive contribution to quality in UK Ltd.

Before ending with the usual three sections, there is another popularphrase that deserves criticism: ‘quality improvement’. Great care isnecessary before embarking on an all-round application of this principle,because dangers can lurk. It may be relevant in the training of operativesto carry out specific processes, but for a product there could well behazards.

Consider the case of a paint that has a proven track record of at least 20years’ satisfactory use on site. Should the manufacturer change to a‘quality-improved’ product? There could be two arguments against this:

• Twenty years was good enough in any case.• Do we wait for more than 20 years to see if it is improved?There is nothing against improving something when an improvement isnecessary, but nothing for it simply for the sole aim of improvement. Manycases of the unfortunate loss of track records have probably been causedby unnecessary improvements.


Look out for the use of the word ‘quality’ in any context without the partiesconcerned knowing in unambiguous terms what the word implies in termsof product and/or service.

4.3.6 REMEDIAL

Ensure that what is under consideration is qualified by a full description.If this is not forthcoming, explain why a problem area still exists, and howthis may affect the work.


4.3.7 AVOIDANCE

Four points are listed below as suggested constructive criticisms in tackling the problem at the pre-tender or tender stages. However, overridingall this discussion is the possibility that quality assurance is the wrong pegon which to hang one’s hat.

Complaints are the indicators that there is something wrong withquality. The way to avoid complaints or keep them minimal is to ensurethat quality is controlled (not assured) at all stages: that is, total qualitycontrol. The installation of such a system, coupled with its supervisionwould seem to have a lot in its favour without many of the disadvantagesassociated with quality assurance.

It is difficult to summarise the avoidance of the problems describedearlier. The following points could form the basis for future discussion asavoidance targets: • The specifier or purchaser needs to state what is wanted as exactly as

possible, by test data, drawings, specification wording, description anddimensions.

• The specifier or purchaser should neither specify nor describe how toachieve what is wanted.

• The supplier’s suitability needs to be confirmed by a strict form ofcontrol.

• The way this control is achieved needs to be documented and availablefor audit by the other contract parties.

4.4 SPECIFYING STRENGTH

Concrete strength in a contractual supply situation is commonly specifiedby the term ‘characteristic strength’. The term applies only to cubes orcylinders made from that concrete, and not to the concrete in the structure.The term is statistical (BS 8110 Part 1:1985); it means that not more than10% of all cube results would be expected to fail the specification level. Forexample, if 40 cubes of a C40-specified concrete are tested at 28 days, andfour of these cubes give results below 40MPa, that concrete can still comply.The relative ease of using this form of specification, coupled with theapparent relative simplicity of the manufacturing and testing regimes, hasgenerated serious problems on site, mainly with in-situ concrete.

Problems of this nature do not usually arise with precast concreteproducts because, apart from beams, columns, cladding panels, cast stoneand similar, most precast products are specified by tests on the product.This is the fundamental difference between type testing (a test on theconcrete supplied) and proof testing (a test on the actual finished concrete).The terms are not to be confused with words ‘realcrete’ and ‘labcrete’


(defined in the Glossary). Proof testing of precast concrete products suchas masonry units, kerbs and pipes is on realcrete. As cubes or cylinders forstrength testing are usually made on site and then tested in a laboratory,they do not fall into either the realcrete or the labcrete category.

4.4.1 VALUE OF CUBE/CYLINDER STRENGTH

From the foregoing discussion, it may be concluded that the value of a testresult is solely in the potential compressive strength. The test cannot caterfor realcrete factors such as: (a) how well the concrete has been compacted;(b) how well the concrete has been cured (thermal and/or moisture);(c) the effect of segregation, honeycombing and similar;(d) the potential difficulty in applying the term ‘characteristic’ to high-

strength specifications;(e) the potential problem of the ‘10% below’ all being in a single and critical

place on site;(f) the likelihood of specifying strength when strength is an unnecessary

specification requirement.

An in-depth analysis of the pros and cons of the subject matter could wellextend this list, but the discussion here is restricted to my own experience.For convenience (a), (b) and (c) have been integrated into this section, (d)and (e) into 4.4.2, and (f), by itself, in 4.4.3.

My experience in the real world of site, plant and works concrete hasbeen coloured by the observation that people rarely follow specifiedprocedures to the letter of the specification. Happily, where deviations haveoccurred, these have usually had a marginal or insignificant effect on theresult.

Identification of the problem area is in two parts: the laboratory data,and the strength that the concrete has in the construction. The first partmanifests itself in questions about the validity of the data. For example,confidence can be undermined when a cube test certificate gives twodecimal places of strength or, for that matter, comments on a pass-or-failsituation.

The second part of the problem is much more common, and isencountered when a site dispute arises over aspects such as compaction,curing or materials in general. This form of dispute can be triggered fromthe cube or cylinder strength data, from a site inspection of the concrete, orperhaps from some form of behaviour at a later date giving grounds forsuspicion.

It is remarkable how frequently data from cube or cylinder testing areemphasised with little concentration on any part of the manufacture, cure


or testing of these items. Remedial measures seem to be tied up withsensible and total quality control all the way down the line. If as muchattention was paid to the manufacture and testing of test items as to theresults that the tests produce, then many of the problems would disappear.

Remedying queries relating to the realcrete on site is a different matter,and one that should probably be included at the tender stage, to avoid orinhibit the likelihood of disputes. There are a number of non-destructiveand semi-non-destructive tests that could be agreed as criteria. Forexample, cores could be tested (BS 1881 Part 120:1983), or the concrete couldbe examined for surface hardness (BS 1881 Part 202:1986). The latter testwould need to be supported by correlation data, and the former test wouldneed to be subject to a pre-agreed interpretation, both with respect to thespecified strength.

The way to avoid the problem is to treat all aspects of testing, fromsampling of the concrete right through to reporting, as part of a strictlycontrolled regime. The primary documentation needs to spell out themethod for dealing with disputes on site relating to the quality of theconcrete in the construction. If all parties agree a detailed procedure fortackling subsequent problems then much late and unnecessary argumentcan be avoided. The parties probably also need to accept that amulticomponent material emanating from a number of different disciplinesis always likely to create problems.

4.4.2 THE DISADVANTAGES OF SPECIFYING ‘CHARACTERISTIC’

Two problems have been met in connection with this statistically basedspecification. They are both easily identified.

The first problem concerns high-strength specifications such as C50 andhigher when the coarse aggregate used or specified has an ultimate yieldcompressive strength not much higher than 50MPa. In order to meet thespecification, a manufacturer or supplier of concrete would, for example,strive to achieve an average strength of at least SOMPa plus two standarddeviations. It is assumed that all cube results would sit within a bell-shapedGaussian (normal) distribution, and that not more than 10% of the total datawould fall below SOMPa. Thus with a standard deviation of, say, 3MPa anaverage strength of 56MPa would be targeted. Most importantly, thestatistical concept would assume a mirror reflection of data on either side ofthe average. Because 80% of the data would be expected to lie within therange 50–62MPa (there would be 10% ‘failing’ the upper limit), any aggregatestrength below 62MPa would annul the characteristic concept. The newdistribution of data would be skew and possibly of a Poisson type.

The second problem concerns the potential risk that all of a 10% failurebatch might find its way into a single and possibly critical part of the


structure. As an example, a part-load of 2m3 of ready-mixed concrete or aneven smaller load of site-mixed concrete could be used in concrete piles. Ifthe total amount of concrete used in the piles on the contract was 20m3 andthe cubes from the 2m3 were the only failures then the concrete could besaid to have complied. A chartered engineer would need to advise on howcritical the 2m3 pile positions were and on their relation to the cube strengthdata.

A remedy for the first problem—the inapplicability of the Gaussiandistribution to high-strength concrete—is suggested by a current case,where the solution was to vary the requirement of BS 8100 from ‘not morethan 10%’ to ‘not more than 5%’.

As far as the second problem is concerned, if the cube data relative tosome pile positions are suspect although the specification is met, then anengineer’s appreciation of that concrete is required. The non-destructiveor semi-non-destructive tests referred to in section 4.4.1 might be of help.

The avoidance of both types of problem is easier to address. If the gradeof strength required means that the Gaussian distribution has a significantoverlap with the ultimate aggregate strength, then either a Poissonstatistical basis of specification needs to be introduced, or the specificationcould be based on minimum strength as the criterion or on some otheracceptable scheme. For the second form of the problem, it is possible that aminimum rather than a characteristic strength specification would addressthe risk. Remember that the ‘10% below’ refers to the number of resultsand not to the level of each result. Because it is a habit to specify concretecube or cylinder strength on the characteristic basis it does not have to bemandatory.

4.4.3 THE NEED FOR A STRENGTH SPECIFICATION

This problem has generally concerned contractual situations where theconcrete is expected to perform as a function of cube or cylinder test data.The performance requirement has been either unrelated or only tenuouslyrelated to strength. The problem arising from the reliance on test samplestrength data was aggravated by having to wait for several days for thenumbers to arrive on site, by which time the construction would haveprobably advanced by several stages. The following are examples of thepart or full inapplicability of strength tests:

(a) situations such as chloride resistance where the selection of mixingredients is the most important aspect;

(b) situations where concrete is likely to be subject to deformation, and itselasticity is more important;

(c) concrete subject to the hazards of fire, impact, explosion and impactsound insulation;


(d) situations where the concrete is subject to an aesthetic weatheringdurability risk, and its water and dirt resistance are more importantthan strength.

The only possible remedy would be to introduce an assessment related tothe performance requirement, or to have the strength specificationmodified. As far as waiting for strength data to arrive on site is concerned,the use of a non-destructive test such as the rebound hammer (BS 1881Part 202:1983) could be considered. However, at the remedial stage, it isunlikely that better than a 15% accuracy can be placed upon strengthprediction, because a calibration relationship for the specific concrete isunlikely to be available.

As in section 4.4.2, the problem is fairly easy to avoid, because actioncan usually be taken at tendering or pre-contract stage. The partiesconcerned with the technical requirements need to agree what performancecharacteristics are relevant, how these are to be assessed, and with whatlimits. The specification of mix design as a priority over the properties ofthe hardened concrete could also yield fruitful results, provided that thiswas coupled with a realcrete assessment to cater for such things as curingand compaction.


Precast concrete 5

INTRODUCTION

In this chapter I have taken the opportunity to update several items frommy first book, Levitt (1982), as well as to add knowledge accumulated bothin the time spent with the Laing organisation and since then.

There are no clear dividing lines between many of the problemsencountered with precast concrete products and those encountered within-situ concrete. This chapter describes problems that have been observedwith precast products, but where the same problems are met with in-situconcrete there is no reason to think that the same approaches toidentification, remedy and avoidance cannot be taken, as with aggregatesand frost damage discussed in section 1.3, for example.

As in Chapters 1 and 3, the actual material being used is rarely the reasonfor the problem. Matters of design or workmanship nearly always lie atthe root of the problem.

5.1 HYDRATION STAINING

Levitt (1982) described this problem at length, but this virtually irremovableand most unsightly of aesthetic defects continues to occur. It has beenobserved in both in-situ and precast concrete finishes, although thecomplaints recorded have referred mainly to precast concrete products.This is probably because the surface finish requirements and expectationsfor precast products are more stringent than those for in-situ concrete. Also,precast products are commonly stacked at an early age. Mould smoothnessor polish and the effect of stacker packs have both been found to besignificant factors in causing this problem, and so it was considered best toplace this section in this chapter.

Before listing the factors associated with hydration staining, it is worthrestating the underlying mechanism suggested in Levitt (1982) to be the


cause of this problem. The words ‘macro’ and ‘micro’ have been used atvarious places in this book, and will be found in many treatises dealingwith concrete and other materials. About four orders of magnitude belowthe micrometre (µm), atomic forces are encountered at the ångstrom levelof length measurement (1Å=10-10m). Strong levels of atomic attractiondue to van der Waals’ forces are known to operate at atomic distances. Areasonably smooth surface of a mould or a stacker piece is likely to haveareas of atomically smooth surface. Therefore fresh or fairly youngconcrete in contact with such a surface will tend to adopt the same macro,micro and atomic smoothness as that of the surface with which it is incontact.

A relevant manifestation of the strength of these forces may have beenexperienced when mounting 35mm photographic slides. In addition to thetwo plastics locking frames for each slide there is a stack of tissue-wrappedsmooth glass plates, of which two are used to encase each transparency.Each piece of glass can generally be removed only by a sideways slidingaction. It is almost impossible to pull a piece of glass off vertically alongthe same axis as the stack of glass plates.

Another less well-known example of these forces occurs when twopieces of pure copper have their ends polished to a mirror smoothnessin a nitrogen box (to avoid oxidation), and these polished faces arebrought together under light hand pressure. A bond as strong as abrazed or soldered joint is obtained, and the two pieces of copper cannotbe parted.

The main factors involved in hydration staining are as follows: • Concrete shrinks during the hydration process of the cement.• The shrinkage away from the mould or formwork leaves a small gap,

which is nevertheless large enough for air with its small carbon dioxidecontent to enter.

• For typical OPC concrete this results in a light grey colour because ofslight surface carbonation.

• Such concrete cured under air-free conditions exhibits a colour betweendark grey and black.

• White Portland cement concrete and cast stone exhibit a light blue-grey colour.

• Smooth polish-finish, gloss-painted moulds or formwork will promotehydration staining, because atomic smoothness can be obtained overquite large areas.

• Smooth-faced packing pieces or stacking blocks placed against visualfaces before the concrete is about 2 days old will promote the sameproblem.

• Hydration staining has been found to bear no relation to the type oramount of mould release agent or oil used.


• The glossier the finish of the mould or formwork the worse theproblem.

• The strong attraction forces between the concrete and the mould orformwork make demoulding or stripping difficult.

• Weathering does not ameliorate the effect; if anything, the concretesurrounding the stained area tends to lighten in colour while the stainremains virtually unaffected. This makes the stain stand out more incontrast.

Figure 5.1 illustrates an example of hydration staining on Portland-finishcast stone units, caused by stacking planks. This twin-line staining wasfirst observed in 1991, and when last inspected in 1996 showed no sign ofimprovement.


The symptom is dark glossy area(s) on the surface, usually accompaniedby difficulty in demoulding or stripping. This staining typically penetrates10–30mm into the concrete. Weathering, if anything, tends to emphasisethe contrast between stained and unstained areas. Hydration staining hasbeen found to have no similarity to the staining caused by leaves—planksof timber used for stacking after the concrete is a few days old—wheresaponification by the lime in cement generally causes such stains to fade ordisappear.

5.1.2 REMEDIAL

Once hydration staining has occurred there is no direct remedial actionthat can be taken. The areas affected can be cut out and ‘made good’, but

Fig. 5.1 Hydration staining on a case stone unit.


the aesthetics of the made-good concrete would need to be discussed. Thealternative indirect treatment would be to paint the concrete with a low-maintenance paint such as a silicate or acrylic-based system, ensuring that thesurface can still breathe: that is, can allow the passage of water in vapour form.

5.1.3 AVOIDANCE

Avoid the use of gloss-finish moulds or formwork. Where paints are usedthey should be matt finish and pigmented, with the pigment colourchanged if more than one coat is to be applied. The pigment in paint ismore significant in promoting wear resistance than the type of paint used(Levitt, 1982), and changing the colours of subsequent coats facilitates theassessment of paint wear rates.

Where the mould or formwork has a high gloss finish, two or threeconsecutive daily mortar applications and removals help to ‘weather’ thesurface into a consistent use behaviour. The alternative is to sandblast thesurface lightly so as to produce a matt finish.

Where stacking pieces are to be used they should not be deployed untilthe concrete is 48 hours old. Even after this time, for visual faces, thestacking items used should preferably allow part-air ingress. For example,expanded polystyrene blocks would be preferable to polythene-wrappedpieces of timber.

The associated experience of the concrete’s sticking to the mould orformwork can often be alleviated by incorporating a valve into whichcompressed air is blown during demoulding or stripping. The sticking wouldbe due to the van der Waals’ attraction forces, which at a simulated totalvacuum suction level could reach 0.1MPa. Over an area of concrete of say,0.1m2, the force required to release this area of adhesion would be 1 tonne.

5.2 LIME BLOOM

The most abundant alkaline material in hardened concrete is calciumhydroxide, often referred to as lime. It is slightly soluble in water, and canmove through the void structure. When it reaches the surface it quicklyreacts with the carbon dioxide in the air to form calcium carbonate. This iswhite in colour, and is known as ‘lime bloom’. It should not be confusedwith efflorescence, which is normally in the form of a white crystallinedeposit of calcium sulfate, and is a problem (outside the scope of this book)that is associated with some clay bricks.

In general, the simple difference between calcium carbonate as limebloom and calcium sulfate as efflorescence is that when each is placed inturn in dilute hydrochloric acid, lime bloom bubbles whereas efflorescencedoes not.


However, the occurrence of lime bloom on the face of concrete is aserious problem, which all too often encourages the use of an apparentlycheap cleaning agent such as hydrochloric acid. The drawbacks of this arediscussed later. The lime bloom forms a patchy appearance, which for dark-coloured backgrounds looks very unsightly if aesthetics is a consideration.Calcium carbonate is almost insoluble in water, although it is very slightlysoluble in carbonic acid (water containing carbon dioxide). In practice, therate at which lime reaches the surface tends to decrease with time, becausehydration products and carbonated lime tend to obstruct the voids.Therefore a long wait, of the order of several months or even a year ormore depending upon the site exposure conditions, is necessary before thebloom lessens or disappears.

Lime bloom is most likely to occur in mid-spring, commonly at the end ofApril and in early May. At that time of the year the UK experiences its lowestrelative humidities; I have measured levels as low as 30% with a whirlinghygrometer. Associated with these low humidities are low temperatures,which tend to be below the temperature of the body of the concrete. The dry,low-temperature surface zone will have a low saturated vapour pressurecompared with the concrete or cast stone below, and so the calciumhydroxide in solution will be drawn towards the surface in an attempt toreach a pressure equilibrium. This mechanism, which is virtually the sameas that described in section 3.3, exacerbates the formation of lime bloom.

In summer the average relative humidity is high, and it would only beat night time—when the surface is colder than the body of the concretethatlime bloom would be promoted. Acting against this process could well bethe nightly comparatively high relative humidity, promoting surfacedampness, and a slow diffusion process would have to take place betweenthe calcium hydroxide in solution in the body of the matrix and therelatively fresh and new water in the surface layers.


Lime bloom consists of a white powdery deposit on the surface, which isinsoluble in water but soluble in dilute hydrochloric acid with theformation of bubbles. (Note: The person carrying out this test would notbe expected to identify the bubbles as being carbon dioxide.)

5.2.2 REMEDIAL

Lime bloom can either be removed by dry brushing, or left to the slowdissolution process described previously. Although brushing will removemost of the bloom there is still an abundant source of lime from the cementhydrates waiting to go into a very dilute lime solution and replace the


bloom on the surface. Once the lime bloom has been brushed off, a siliconesolution complying with BS 6477 is very effective as a more permanentremedy.

The use of hydrochloric acid to remove lime bloom is not advised, eventhough it appears to be very effective. Even if the concrete or cast stone isthoroughly wetted both before and after the acid application (in order toinhibit acid penetration) the treatment will leave the reaction productcalcium chloride in the void structure. Calcium chloride is very soluble inwater, but it is also very hydrophilic, and promotes surface dampness. Thisin turn will allow calcium hydroxide to get to the surface and, if anything,will exacerbate lime bloom formation.

5.2.3 AVOIDANCE

Use a water-repellent admixture such as stearic acid in wet-cast concrete,or calcium or aluminium stearate in earth-moist products such as mostcast stone production. Do not use this type of admixture with any otheradmixture unless representative tests have shown that the two admixturesare compatible in all properties. The alternative would be to use a siliconeas in section 5.2.2 as soon as possible after manufacture. In this case carewould be needed to avoid spillage onto bedding faces. Provided other partsof the construction are adequately protected, site application can have itsattractions, most obviously the fact that mortar joints would be siliconedat the same time.

5.3 COLOUR VARIATIONS

The word ‘colour’ is used here to embrace the many terms invokedarchitecturally to describe the visual effect that the surface of concrete hason the human eye. There is considerable subjectivity in this assessment,because factors associated with the surface, such as texture, tone andreflectance, all play parts. Superimposed on all this are the conditions whenviewing takes place, such as:

• direct sunlight on the surface;• sunlight on a surface adjoining the one being assessed;• wetness, dampness or dryness of the surface;• the appearance under cloudy, misty or foggy conditions;• the effect of adjoining coloured material, such as a window surround.

It may be seen that the view likely to be reached is a function of two groupsof variables: those that seem to lie with the producer, and those that aresubject to the viewing conditions. The problem of colour variations ismainly concerned with the first of these, because the viewing conditionsshould be controllable on site or during storage.


The problem has arisen because concrete, including cast stone, is a multi-ingredient material subject to a wide variety of circumstances before it isused on site. No matter how competent the producer is, colour variationswill occur from product to product as well as within a product. This alsoapplies within small areas of the face of a section of visual insitu concrete.Consider a few of the relevant variables that affect colour, tone and texture,and which the most realistic of control systems cannot control to give auniform ‘sameness’ to the surface appearance:

• slight changes in the fine aggregate grading;• slight changes in the colour of the fine aggregate;• small variations in the cement content;• small changes across the concrete mix in water/cement ratios;• slight changes in the surface curing conditions;• small variations in cement colour.

The words ‘small’ and ‘slight’ apply solely to the region of ultra-fine control,which is not within the practical control that the strictest of manufacturerscan exercise.

As far as the relationship between manufacturer/producer andpurchaser/user is concerned, what this amounts to is an acceptance thatthere will be control but not at a level that would be impossible to obtain.Therefore any agreement on what colour variations are acceptable needsto be based upon samples that represent both the limits within which themanufacturer can operate and any effects that the geometry of the units inquestion may have.

To cover the first aspect, samples ideally need to be unit-sized andreplicated for both works and site comparison. Each sample, as well as acomparison of one sample with another, should demonstrate thevariabilities within which the full production can work.

In dealing with the second aspect, two of the many factors that have tobe considered are the inclusion of insulation, causing differential curingrates, and the differing aggregate facets shown by vertically cast andhorizontally cast faces of exposed-aggregate concrete.

The subject of colour leads on to concrete deliberately coloured by theinclusion of pigments in the mix. These materials are generally finepowders of iron oxide for the blacks, reds, yellows and browns, but alsoinclude green chromic oxide and blue cobalt oxide. All these have the sameorder of fineness as Portland cement, and are nominally spherical particles.The exception is yellow pigment, which has dendrite-shaped particles(shaped, microscopically, like the branch of a fir tree).

Yellow iron oxide pigment (actually an iron oxide/iron hydroxidecomplex) needs extra care in dispensing and mixing. Carbon black isavailable as a pigment but has two general disadvantages. First, it iscommonly an order of magnitude or more finer than the metallic oxide


pigments, and the low dosage needed is difficult to control, which oftenresults in large colour variations. Second, and more important, carbon blackpigment is not only irregular in shape but has a sponge-like porous matrix,which occludes any lime bloom that forms much more than the imperviousand generally spherical metallic oxide pigments. This has resulted incarbon-pigmented concrete being wrongly labelled with a colour fadedescription. Although carbon can slowly oxidise at weatheringtemperatures it does not fade in the short term; it is masked by lime bloom.Figure 5.2 shows a dwelling with tiles pigmented with both carbon andiron oxide. The carbon-pigmented tiles are mainly those further from thecamera.

Recommendations for avoiding colour variation are listed at the end ofthis section. However, it has always seemed incongruous to me that amaterial costing 5–500 times the price of cement should be put into aconcrete mix with little or no plan in mind to promote colour retentionduring the planned aesthetic durability period. The most expensivepigment is blue cobalt oxide, for which a daily price would probably haveto be obtained as it is based on a semi-precious metal. A blue organic coppercomplex pigment is available, but its colour tends to change to green as theconcrete carbonates. The natural blue mineral ultramarine is unstable whenused as a pigment, and fades completely after only a few weeks’ exposure.

Concerning applied colours in the form of paints, no problems havebeen found with the inorganic silicate-based systems. They not only requirenominally zero maintenance (apart from washdown needs), but have agood performance track record of more than a century. Maintenance hasbeen found necessary with organic chemical-based paints, mainly becauseof ultra-violet effects.

Fig. 5.2 Lime bloom on tiles pigmented with iron oxide and with carbon.


Concerning the second group of variables relating to site conditions, anoptimum method of recording colour and its change with time is providedby standardised night-time flash photography during a dry moonlessnight, using the same film, camera and position each time.


There is an apparent lightening in colour with age, and a variation in colour,texture, shade and reflectance, both within distances as small as a fewcentimetres within a unit and from location to location and unit to unit.The lightening with age tends to promote an eventual uniformity, whichcan occur after only a few months on a south or west unprotected face, andafter several months or a year or two on other faces.

5.3.2 REMEDIAL

Either apply a silicone to BS 6477 after any remedial work, or do nothing.Any other form of surface treatment, including acid etching, might seemto be beneficial in the short term, but nearly always makes matters worse.

5.3.3 AVOIDANCE

In addition to exercising the best manufacturing control, any concrete(pigmented or otherwise) will generally benefit from the use of stearic acid inwet-cast mixes, or aluminium or calcium stearate in earth-moist mixes. Boththese admixtures endow the concrete with water-repellent properties, andtherefore, if control is mediocre or poor, there will be minimal lightening dueto weathering, and defects in colour variation will be visible for a long time.

Consideration could also be given to using pigments in pre-blendedform, where the pigment and the water-repellent admixture are mixed withthe cement in a powder disperser, as illustrated in Fig. 5.3. The advantagesof this quickly offset the cost of the blender, because not only can thepigment content be reduced by up to 50% for the same staining power as ifit had been added to the mixer, but the blend containing the water repellentbecomes a hydrophobic cement. During storage it would not be subject tothe typical caking or sack-hardening associated with untreated cement.

Mixes containing water-repellent admixtures need to be mixed by direct-action as in pan-type mixers. Drum mixers generally do not have enoughenergy to mix effectively when stearine-type admixtures are used.

5.4 CRACKING AND SLENDERNESS RATIO

The problem of cracking has been encountered mainly with cast stone sillsand lintels, either during handling and storage or when built into the


structure. The cause was almost certainly the specification of units thathad a large z length dimension compared with the x and y sectiondimensions. This is also related to the false expectation that a producthaving a cube strength in the range 30–40MPa is as ‘structural’ as a 40–60MPa wet-cast concrete unit.

As far as the product is concerned, cast stone, especially the commonmoist-mix design products, is generally a mortar mix and not a typicalconcrete mix. The bending strength of concrete is about 10–14% of thecompressive strength. For the mortar-mix cast stone this ratio has beenfound to concentrate at the 10% end of the range. Therefore, if a cast stonewith 40MPa compressive cube strength is being made, the highest bendingstrength will be about 4MPa. However, this assumes that the compactionand strength of the unit are the same as the cube, but this is rarely likely tobe the case. The safety factor that can be ostensibly applied has not beenspecified, and it is only in the standard for cast stone (BS 1217:1997) thatmaximum limits for z have been specified compared with the x and ydimensions. This part of the cast stone specification is based upon x and ybeing represented by an inscribed or superscribed circle in the xy section,depending both on shape and on whether x or y is vertical when theproduct is handled.

As far as handling, transport and storage are concerned, it is obviousthat if a long unit is being lifted at its ends and x is larger than y, then it is

Fig. 5.3 Elutriation blending of powders inot cement.


best moved with the x axis vertical. However, when lifted by twooperatives, such a unit is more comfortable to hold with the shorter y axisvertical. This will result in a larger bending moment, and therefore therecommendations in BS 1217 need to be accompanied by strict supervision.

Specifications for cast stone sills and lintels have perhaps beeninfluenced by the use of prestressed units, which not only can bemanufactured and installed virtually crack-free with high slendernessratios, but also can support large loads of superimposed masonry. Theproblem at the manufacturing stage for cast stone or similar products (otherthan prestressed) is that producers have been loath to tell the specifier thatthere is a cracking risk due to the unit’s being too slender. This problemwill remain as both a specifier and manufacturer remit unless the specifierreceives a documented risk warning. Figure 5.4 shows a lintel of cast stonethat has a high slenderness ratio.

On site, in addition to picking up a unit with the shorter of the x and ydimensions in the vertical direction, there is the extra bending momentbrought about by the manual or cranage acceleration in changing from therest position to a moving one. As far as I know no data have been publishedon the effect of this, but consider the effect of this acceleration on the lintelin Fig. 5.4.

Take the following reasonable assumptions:

• a cube compressive strength of 40MPa;• an idealised bending strength (equivalent prism) of 4MPa;• a realcrete bending strength of 2MPa;• a cast stone density of 2100kg/m3;

• acceleration at the first lift that is double the static bending moment;• unit weight=95kg.

Fig. 5.4 Cast stone lintel with slenderness ratio (not to scale).


The calculations give the following approximate data:

• Dynamic (during lifting) bending moment=0.50×106Nmm;• Moment of inertia divided by distance of neutral axis=0.3×106mm3

• Flexural stress applied at lifting stage=2MPa approx.

This is the same as the assumed realcrete bending strength, and so the unitis likely to crack.

An additional problem with cast stone on many sites is that suchproducts are treated with little respect. Not only have storage and handlingconditions been commonly found to be poor, but the installation isgenerally carried out by bricklayers. If natural stone was being installed itwould be handled by masons. Although cast stone is generally cheaperthan natural stone it is made (by its BS 1217 definition) to be used in placeof natural stone. It therefore seems illogical to use price when comparinginstallation processes.


Look for one or more cracks near the centre of the unit, tending to narrowor disappear towards the top of the section. End-lifting a sill or lintel in itsupside position rather than its design installation position would result inseverer cracking, as there would probably not be any reinforcement in thetensile zone created during the lifting.

5.4.2 REMEDIAL

A polymer-based system is often suitable, provided that an engineer hasadvised on the suitability for repair rather than rejection. The selectiondepends upon whether or not the cracks are dead or live. Whichever typeof repair is selected, it is an aesthetic advantage to cut a trench along thecrack about 10mm wide and deep, and to make this good after attendingto the crack. The epoxide-based resins are suitable for dead cracks in manycases, and the copolymer emulsions for live cracks. The trench repair canbe undertaken for the two types of repair material with a polymer matching(after weathering) mortar or a sealant respectively. These are broadrecommendations, but each case has to be treated individually. There aremany geometrical variations with concrete and cast stone sills and lintels,which make it difficult to generalise.

5.4.3 AVOIDANCE

Units should be designed and manufactured with limiting slendernessratios. Cast stone units need to be designed to the specified clause of BS1217, and the conditions laid down therein could well be good starting


points for other types of precast concrete unit. On site, contractors need tobe aware that the product being handled, especially if it is cast stone, needscareful consideration at all stages. When the products are cast stone,masons should be employed for the installation.

5.5 THERMAL CRACKING IN PIPES

The problem related to concrete pipes that exhibited cracks at their inverts,with crack apertures up to 0.5mm. As far as was known, in all cases thepipes were of well-designed, manufactured and cured concretes, and theincidence was not found to be a function of the method of manufacture.However, the cracking took place only during the hot summer months,which indicated that a thermal effect was responsible. Of more and, as itturned out, relevant interest was the opinion of one or two manufacturersthat storing pipes with their axes in the north-south direction seemed tocause more cracking.

To investigate this possible link, the author tested works-manufacturedpipe rings of about 1m diameter by 0.5m long in the laboratory. Heat wasapplied from an arc battery of infrared lamps, which could be rotated tosimulated solar radiation with both north-south and east-west axisorientations. The movement caused by this was measured by mechanicalstrain gauges round the periphery of each pipe. Surface temperatures werealso measured, at locations referring to the centre of each strain gaugelength.

In the north-south simulated orientation there was a significanttendency for the heated side of the pipe ring to become ellipsoidal in shape;in some cases cracking was induced at the inverts similar to that observedin the works. For pipes tested in a simulated east-west storage directionthere was a slight tendency towards an ellipsoidal shape, but themovements observed were not significant.

Figure 5.5 illustrates the pipe ring positions during the test.It was also of interest to observe in the measurements of surface

temperature that, in a laboratory running at about 20°C, concrete surfacetemperatures up to 60°C were recorded. The response of concrete to solar(or other thermal) radiation is fairly well known. On a hot summer’s day,dark-coloured or black concrete will feel warmer than grey or lighter-coloured concrete. At night-time, the converse is observed.

A body that is 100% efficient in absorbing (and consequently in emitting)radiation is known as a ‘black body radiator’. The author calculated theradiation coefficient for the grey-coloured concrete rings and found it to beabout 0.8. This meant that ordinary grey concrete was almost as efficient inabsorbing or emitting heat as a perfect black body.

From this small apparent difference between grey and a theoreticalblack-coloured concrete it can be predicted that white concrete would not


be all that much cooler on a summer’s day than grey concrete. It wouldtherefore follow, if research supported this prediction, that thermalprotection covers or enclosures/containers should be silver-coloured ratherthan white or light in colour.


The problem shows up as a single crack at one or both of the inverts, withcrack apertures in the range 0.2–0.5mm. The cracks are common in the toprow of pipes and in those stored with their main axes in a north-southdirection.

5.5.2 REMEDIAL

Subject to an engineer’s acceptance of remedial action, repair the crackswith an epoxide or polyester resin system, and re-store pipes with theiraxes in an east-west orientation. If the pipes cannot be stored in this way,protect them from direct sunlight with a suitable covering.

5.5.3 AVOIDANCE

Store pipes with their main axes in an east-west orientation. If this is notpossible, protect them from direct sunlight with a suitable covering.

Fig. 5.5 Thermal cracking in concrete pipe.


5.6 TUNNEL SEGMENT IMPACT DAMAGE

The problem was observed on a site where contra-shapedtrapeziumsectioned concrete pipe ring units were being hydraulicallyrammed into the forming of a tunnel lining. The precision required in thetrapezium shape made it necessary for the manufacturer to choose concretemoulds because of their stability. The concrete in the precast units wasspecified to have an average cube strength of not less than 45MPa. In allother respects the concrete was observed to have been well designed,compacted and cured.

The problem manifested itself during the ramming as edge-spalling andcracking, with most of the damage taking place with the final installedunit, namely the top dead centre locking piece. To counter the problem,which was apparently considered to be due to lack of adequate strength,the minimum average cube strength specification was increased from 45to 50MPa. This resulted in a significant increase in the incidence of spallingand cracking.

A subsequent discussion on the energy-absorption characteristics gavesome credence to the attraction of using a more elastically behaving weakerconcrete. It was thought that this property would apply to both slow andfast (impact) strain induction. The latter applied on this particular site.Figure 5.6 illustrates how weak concrete compares with strong concrete;the area under the curves is a function of the energy absorption.

Fig. 5.6 Stress versus strain and energy absorption.


As a site trial, a batch of segments was manufactured with the specifiedminimum average strength decreased from 50 to 35MPa. It was found thatthe spalling and cracking problems virtually disappeared. From then on,all concrete segments were made to this weaker specification.

A similar mechanism was discussed in section 3.2, where, under theimpact of sandblasting, the face of the concrete receded but the elasticenergy-absorptive plastics spacers were left proud of the surface. In section4.2 impact was listed as one of the durability hazards. These observationsall seem to imply that impact durability is inversely related to resistance.


The problem shows up as spalling and cracking occurring during handlingor installation, especially when high-strength concrete is being used.

5.6.2 REMEDIAL

Subject to an engineer’s consideration, repairs can be undertaken followingthe general principles of CSTR26 (Concrete Society, 1984).

5.6.3 AVOIDANCE

Where concrete, precast or in situ, is at impact risk and has not been fibre-’reinforced’ for that risk, characteristic strengths are probably best keptwithin the range 30–40MPa.

5.7 TESSERAE DETACHMENT

A tessera (plural tesserae) is generally a square or rectangular-shaped pieceof ceramic or glass that is used to form a multi-tessera mosaic on the face ofconcrete or other materials. The problem met has been the detachment ofthese and/or hollowness, and has been commonly observed in precastconcrete units, but has also occurred with in-situ concrete.

It is common for mosaic to be used in sheets (e.g. 300mm square), withpaper stuck to the visual face. For precast products, these sheets arenormally laid paper-face down at the bottom of the mould, which then hasa mortar applied, followed by concrete. The paper is washed or scrubbedoff after demoulding, as the glue on the paper is water soluble. For in-situconcrete facades, the sheets are generally pressed onto an applied polymermortar; when the mortar has hardened sufficiently, the paper is removedsimilarly.

Both processes are very sensitive to the quality of workmanship, andthis, together with specific design considerations, has been found to affectthe bond detrimentally in the following ways:


Fig. 5.7 Mosaic subject to edge weathering.

Fig. 5.8 Mosaic subject to compression at a joint.


(a) inadequate surface preparation;(b) adhesive mortar allowed to stiffen or harden before applying the

individual tesserae or sheets of mosaic;(c) the use of an adhesive mortar with inadequate adhesion;(d) weathering at an exposed mosaic edge return, as shown in Fig. 5.7;(e) tessera compression failure across a joint, as shown in Fig. 5.8. Note: A failure due to (c) alone would be a design failure, not a mater ialfailure.


The symptom is detached tesserae found on the pavement, ground orsubroofs. Hollowness may also be present, and can be detected by tapping.This form of testing is best concentrated at the edges or joints of precastunits and at random areas on in-situ concrete with, possibly, concentrationat discontinuities such as windows, doors and corners.

5.7.2 REMEDIAL

With reference to the above five reasons for probable bond failure, thefollowing remedies are generally effective:

(a)–(e) Remove tesserae from suspect or hollow areas. The use of heatin the form of a propane gas torch is very effective over large areas.The heat debonds the mosaic much more quickly than mechanicalmethods do. Take precautions to protect personnel and the buildingfrom falling hot tesserae.

(a), (b) and (c) For (b) and (c) remove all mortar, and for all three treatthe substrate mechanically, by grit-blasting, mechanical tooling orby heat calcination of the aggregate, to produce an exposedaggregate finish. At this stage or before aggregate exposure thesubstrate concrete could usefully be surveyed for the depth of coverof the steel, depth of carbonation and other properties. This wouldconfirm that the concrete was suitable to receive remedial mosaicwork without the possible later risk of failure due to rebar corrosionor other mechanisms. Once the surface has been prepared, applyan SBR mortar in areas no bigger than can be mosaic-appliedwithout the mortar becoming dry or stiffening. All these activitiesshould be undertaken by specialists.

(d) and (e) Remove affected tesserae and mortar and prepare thesubstrate surface, preferably by mechanical means, to produce anexposed-aggregate finish. For (e) cut the joint to give an openingof 5–10mm and point up with a suitable sealant. For both (d) and


(e) replace the tesserae to within about 50mm of the edge or joint,and finish to the edge with the same mortar as used for beddingthe tesserae to form a margin.

Note: For buildings constructed before about 1970, the common squaretessera size was 0.75 inches. Replacement square tesserae are now 20mmin size, and filling in patches can be a problem, because a metric tessera isabout 0.5mm larger than the imperial one it replaces.

5.7.3 AVOIDANCE

This can be usefully set out as a number of do’s and don’ts in a formatsimilar to a code.

Do not:

(a) design or apply mosaic right up to a joint or edge;(b) allow tesserae to butt across a joint.

Do:

(c) have a mortar margin about 50mm wide beside a joint or edge;(d) for mosaic to be applied to a hardened concrete face, expose the

aggregate either by the use of a suitable retarder or by mechanical orthermal methods;

(e) use polymer mortars in preference to plain mortars for sticking orcasting against mosaics (the SBR-based systems have a good trackrecord);

(f) for glass mosaic apply an epoxide resin followed by a sand blindingto the adhesion face, and allow to harden before sticking onto theconcrete as in (e);

(g) in the rare case of using net-backed (as distinct from paper-faced)mosaic, follow the same procedure as in (f), ensuring compatibilitybetween the epoxide and the glue used in the net.

Note: Mosaics have a good track record, going back over two millennia.These ancient structures (the mosaic floor in the Roman villa at Bignornear Bognor Regis is a good example) indicate that good workmanship isthe significant factor.


Testing 6

INTRODUCTION

Materials form the sole subject matter of this book, and so it is notsurprising that testing so often comes into the picture. So often, problemshave arisen in testing when the focus has been on the numbers or resultsthat a test produces, with little or no attention paid to such matters as:

• the relevance of the test to the performance required;• the value of the test result as affected by the tester;• misleading interpretation of test data based on selective sampling of

results;• sanguine acceptance of complying results when there may be hazards

unaccounted for;• omission of relevant test requirements.

The reader has probably experienced other problems, but the followingsections refer to my own ‘hands-on’ problem encounters. My hope is thatthese discussions will encourage the construction team members toquestion, discuss and offer suggestions before or at the tender stage. Inaddition, rather than look upon testing as a built-in item overhead, itsimportance and relevance to performance in practice might be better servedby the incorporation of specific bill items.

6.1 LABCRETE OR REALCRETE

The term ‘labcrete’ is often used to define concrete or mortar that is madeand tested under strict laboratory conditions, whereas ‘realcrete’ appliesto concrete made on site or in the works and used on site. The grey areahere is that of concrete samples (such as cubes) made on site and thentested in a laboratory. The problems described here, with both laboratory-


made and tested concrete and with site-produced samples, were in thevalidity and applicability of the results obtained.

Consider, first, laboratory-made and tested concrete, taking admixturesas an example. The standard for plasticising admixtures (BS 5075 Part 1:1982)specifies inter alia that nominated concrete ingredients shall be used in acertain way to produce test data that confirm or deny that the admixturecomplies with the standard. The appendix of that standard (like those of theother admixtures standards) warns the reader of the need for site trials toassess the suitability of that admixture for the conditions on site or in theworks. However, site or works conditions will not emulate the BSspecification, and so compliance of any admixture with the standard doesnot necessarily mean that it will produce the target performance in realcrete.

If samples are made on site and tested in the laboratory—a mixture oflabcrete and realcrete—the results obtained are likely to be moremeaningful than those from labcrete alone. However, the attention paid tothe manufacture of the simple and small size of a cube or prism comparedwith, say, that of a column, is likely to give rise to doubts.

Probably the best way to describe these two cases is to say that thelabcrete admixtures standards give assurance of classification, coupledwith potential for use, whereas the realcrete-labcrete hybrid indicates themaximum potential compressive strength of the realcrete.

The specifier has a choice:

• Use data from labcrete or labcrete-realcrete (site-made cubes, forexample) as a be-all and end-all, with or without the application ofsafety factors.

• Use realcrete data alone.

The second choice applies to a minority of concrete made: dimensionallycoordinated precast concrete products, where the product itself is tested. (Iprefer the phrase ‘dimensionally coordinated’ to ‘standardised’ becausethere could be 100 diagrams in a standard deemed to comply, but possiblyonly a few could be used with each other.) In-situ concrete and bespokeprecast units such as cladding and cast stone are generally assessed forstrength in a specification by a cube test. The latest standard for cast stone(BS 1217:1997) accepts this, and has a division between type tests (labcrete-realcrete) and proof tests (realcrete).

The main points to be addressed are the validity and applicability ofrealcrete and labcrete information, and this has to include the commonrealcrete-labcrete hybrid, generally known as a cube. To discuss thematerials science and technical requirements of this problem, the test needscan usefully be listed under three headings:

• The test must be meaningful.• The test must be accepted by all parties.


• The test data must be accepted as final, with minimum or nointerpretation.

6.1.1 MEANINGFUL

It would be logical to assume that the purpose of carrying out a test is toproduce data that relate either directly or, in a constant manner, indirectlyto a necessary or desirable performance characteristic. If strength(compressive, tensile or shear) is in question, then it would be simple toassume that a cube or prism result is sufficient. This is difficult to accept,because a labcrete-realcrete test usually gives maximum potential strengthand little else. The use of the cube or prism density figures (specified to becalculated and reported) generally gives misleading information. This isbecause nominal cubes are tested, and these are not necessarilygeometrically true cubes: up to 1% deviations are permitted on alldimensions. Thus nominal cubes, all from the same concrete and virtuallyequally compacted, with a true density of 2350kg/m3, can have nominaldensities in the range 2280–2420kg/m3.

Therefore, apart from an indication of the maximum potential strength,there is a risk of sacrificing the target of ‘meaningful’ on the altar oftraditionalism and the attractive cheapness of the cube test. The codes ofpractice, such as BS 8110 Part 1:1985, generally apply safety factors to thecube data to cater for structural design purposes. It could be argued, withhindsight, that if the rebound hammer had been invented before thecrushing machine this problem would not exist.

This leads to the interim conclusion that, wherever possible, preferenceshould be given to realcrete testing, if there is any way in which it can beshown to be of use.

If realcrete testing is the preference for producing meaningful data, thenthe next question is: which of the durability hazards listed in section 4.2are relevant to the concrete being tested? It follows that the partiesconcerned with the test regime as well as the testers need to set up a matrixof properties versus tests so that a sensible application of the available teststo the concrete can be made.

6.1.2 ACCEPTABILITY

Scientific and technical development of labcrete and realcrete in theconstruction industry will proceed only when three factors are addressed: (a) The test methods (included as costed bill items) are agreed in the

specification.(b) Test limits or ranges are agreed. If interpretation is likely, this wording

should also be agreed at a preliminary stage.


(c) Preserving anonymity of the information source, data are fed back tothe BSI Committee Secretary so that the revision of standards maymake full use of the state of the art.

The current example of the tardiness of acceptability is the BS 1881/200series (non-destructive testing), all of which are currently‘recommendations’, and not mandatory.

An example of where (b) above has caused arguments is in theinterpretation of site-drilled, laboratory-tested cores. In an attempt to dealwith the interpretation of cores tested to BS 1881 Part 120, the ConcreteSociety produced guidance in the form of a report (Concrete Society, 1976;Addendum, 1987). The Society has accepted that this report (CSTR11)needs updating, and is currently carrying out research with this aim inmind.

CSTR11 suggests various interpretative approaches in trying, amongstother exercises, to relate core strengths to the strength of cubes that wouldhave been made from that concrete. However, the suggested operatingfactors are based on a small quantity of data, and definitive dogmatismshould be avoided. The other factor relating to acceptability is that nomatter how relevant any test procedure is to the property in question, thereis the contractual matter of timing to consider. For example, if it takes sixmonths to produce data relating to resistance to chloride ingress, and thetrack record shows that this resistance can be achieved by the use of PFA,GGBS or MS additives, then it makes sense for a testing specification toconcede to a mix design specification.

6.1.3 FINALITY OF DATA

Arguments often arise over the finality of data; many of these are based onlack of knowledge of the test criteria, including the status of the testingfacility. If the interpretation aspect discussed in section 6.1.2 is broughtinto the picture, then it is possible that the wording was somewhat loose.Therefore it is probably best to aim at a test regime that has a minimum ofor, preferably, no interpretative clauses.

This reflects the discussion in section 4.3, and implies that total qualitycontrol at all stages—from specification to handover—would present thefewest obstacles to agreement on the finality of the data. The finality wouldnaturally be based upon the three steps of the test regime’s beingmeaningful, acceptable and final.


The problem reveals itself in the form of contract data invoking labcrete,labcrete-realcrete and/or realcrete tests that have little or no relation to the


properties required and/or are capable of misinterpretation and/or do notcarry bill items to cater for testing.

6.1.5 REMEDIAL

No remedy appears to be possible; the situation would be acontemporaneous one, and not one that occurs at the tender or pre-tenderstage. A contract review could perhaps be undertaken, in order to dealwith possible problems to come, but this is in the contractual field andhence outside the remit of this book.

6.1.6 AVOIDANCE

The main thing to avoid is a contractual dispute over any of the items insections 6.1.1–6.1.3. One way to achieve this might be for the tenderedparties to adopt a more proactive role, coupled with strong liaison betweenall members of the construction team. The setting up of a properties-versus-tests matrix, mentioned earlier, could well have much to commend it.

6.2 DESIGN OR PERFORMANCE

I have commented in several sections on the testing specification beingdesign based or performance based. Where the problem in this subject hasreared its head is in a tendency to ignore the factors relating to this choiceand to concentrate—wrongly—on performance testing.

It is only partly logical to conclude that if concrete is required to performin a specific manner then a performance test should apply. This conclusionignores the many scientific, technical, architectural, engineering andcontractual requirements that also apply. Compounding all this is theslackness that is sometimes met in the format of those parts of thecontractual documents relating to the materials: slackness in

• the words used;• the intended meaning of those words;• the interpretation of the words by the receiving party;• whether or not the words addressed the property requirement.

It is highly unlikely that concrete would be needed in the construction withonly one property requirement. Thus each property needs to be discussedin the light of the boundary conditions that pertain. There is growingpressure from the European standards organisations to concentrate onperformance testing, with an apparent disregard of other considerations.This could create future problems. This pressure should be resisted;performance-based specifications should be supported only when theyhave minimum interference with buildability, and they relate to sensible


property targets. This may mean that a specification has to have a mixtureof design-based and performance-based clauses, but if the fiverequirements listed below are considered, this will form the basis for alogical materials approach:

(a) funder—price, speed and financial return;(b) specifier—unambiguous, relevant and sensible clauses;(c) specialists—appreciation of services involved;(d) contractor and subcontractor—buildability;(e) tester—timing of data returns and meaningful tests.

Other factors may also be relevant, but it is these five that, singly or incombination, have led to problems and discussions. None of theserequirements is an independent variable; altering one of them will almostcertainly affect one or more of the others. Chloride diffusion control bydesign rather than as a performance-based specification is an example:input at (b) involves (d) and (e).

Another example commonly met in troubleshooting is the use of airentrainment to produce frost-resistant concrete (see also section 1.4). Atypical specification for a concrete with 20mm maximum size aggregatewould be 3.5–7.5% total air in the fresh concrete (BS 1881 Part 106:1983).This specification is design based, but has the aura of a performance test. Itpossibly comes into the category of the next section. Consider how thisspecification relates to (a)–(e) on the reasonable assumption that theconstruction team members wish to have a frost-resistant concrete (puttingaside other property targets such as strength, flatness and appearance):

(a) The funder is unlikely to be affected by the price or speed of puttingthe admixture into the concrete. Where the funder may be concernedis with costs arising after handover or completion due to (b)-orientedproblems.

(b) The specifier will not know whether compliance with the air contentrequirement means that the air bubbles are present in the optimumsizes and geometrical distribution.

(c) Data from the specialist would probably emanate from the admixturemanufacturer and are likely to be misapplied because we are dealingwith labcrete, not realcrete or a hybrid.

(d) The contractor’s buildability is unlikely to be affected unless (b) applies,in which case remedial or replacement work might be required.

(e) On the basis of (b) and (c) the testing is not likely to be meaningful, buta delayed timing of data—to wait for petrographic results—willprobably need to be considered if pre-works data have not beenobtained.

There are dangers of ‘tunnel vision’, in concentrating on design at theexpense of performance testing (or vice versa), as well as in considering


only one of the five requirements listed and discussed above and over-looking the others. The choice of design testing, performance testing, orboth, depends upon what the concrete has to achieve in cost-effectiveperformance terms.


Look for any documentation in which design testing should have beenspecified instead of performance testing, or vice versa, as well as a lack ofconsideration of any one or more of (a)–(e) listed above.

6.2.2 REMEDIAL

The only possible remedy for a current situation is to try and obtain acontractual variation or instruction to cater for the obstructing matters.

6.2.3 AVOIDANCE

Pre-contract discussions or comments at tender stage seem to be the wayto address specific cases. In general, the properties versus materials matrixproposed in section 6.1 could be used and qualified by method statementsand test data limits. The benefits of having standard contract clausesaddressing each of the construction targets could also be discussed.

6.3 CAMOUFLAGE TESTING

This was one of the types of testing listed in Levitt (1985), which dealt withthe philosophy of testing. Camouflage testing may be defined as any testrequirements or procedures that are completely irrelevant to reasonableand sensible materials property targets. The problem with camouflagetesting is that it is largely irrelevant, misleading, dishonest, and defies logic.There are a number of bases for camouflage:

(a) trying to impress others by having a test clause;(b) copying something that has been done before without checking its

relevance;(c) catering for a problem by invoking a test that has little or no relevance

to that problem;(d) promotion of a test facility;(e) promotion of a proprietary product.

An example of (a) was an instance where one of the construction party’saims was to set out before another member of the building team aconsiderable amount of test data in order to impress by the amount ofpaperwork.


Probably (b) is the most insidious form of camouflage testing, because itreflects strongly on the traditionalism that pervades the constructionindustry. The reader may be able to pick out examples in the previous text,but the author has had to be careful not to mention specifics.

In (c) the case was where a client experiencing a problem with theconcrete was ‘satisfied’ with additional but irrelevant testing. Personsbecoming unwillingly involved in such a situation should record anddocument their views to the relevant party.

Both (d) and (e) need no qualification, and examples can be found in theearlier text.


The problem reveals itself in the inclusion of a test requirement (methodand/or limits) that is completely irrelevant to a property that should beunder consideration.

6.3.2 REMEDIAL

Unless the requirement is deleted or altered, no remedy is possible.

6.3.3 AVOIDANCE

As with so many of the other problems described earlier, a sensiblediscussion between the construction team members at pre-tender or tenderstage is suggested.

6.4 REPEATABILITY AND REPRODUCIBILITY

There is a growing trend to include data on these two properties in bothBritish and American standards. Briefly, the meanings of these two wordsare as follows:

Repeatability refers to the production of data by a specific centre, eitherby the repetition of testing on the same sample (non-destructive tests,for example) or by replicate tests on subsamples from the one mastersample. These data can be produced by more than one operativeworking in that centre. Repeatability is commonly described instatistical terms such as variance, standard deviation or range.

Reproducibility refers to the production of data on nominallyidentical subsamples or samples tested at more than one centre, andcompares the results within the group of centres. Again, as withrepeatability, a statistical method is generally used to comparenumbers.


The term often used in standards and other documents to describerepeatable and reproducible data is ‘precision data’.

Concrete is a multi-component (sand, coarse aggregate, water, cement,admixtures, additives), multi-variable (mixing, compaction, curing)material. The problems that this causes are twofold. First, whatever test isbeing considered there will be variations, and the demand for stringencyin repeatability limits has to be realistic. Second, although statisticiansprefer relatively large numbers of centres to be involved for reproducibilitystudies, there have been instances when inferences or ‘conclusions’ havebeen drawn from as few as six cooperating laboratories. In my view, thenumber should be at least 12.

Therefore, as far as repeatability is concerned, it is possible for a singlecentre to produce enough data for a statistical analysis to be meaningful(assuming that the test being examined is one for which that centre canproduce the required quantity of data with acceptable interference on itsother commitments). However, the test has to be of a common genre andcommon to a large number of centres.

So, for concrete testing laboratories, it follows that a study ofrepeatability and reproducibility would be feasible for data such as cubestrengths, and aggregate specific gravities, but restrictions could well beencountered for petrographic tests, oxygen diffusion tests and the like.

Caution is necessary when using statistics, because it is an applied andnot a pure form of mathematics. Because assumptions are made in themathematical treatment of data, any results produced are not definitive;statistics does not ‘prove’ or ‘show’ anything. The results can only indicatelikelihood, comparison, relationship or trend. Section 4.4 discussed theinadequacy of the normal or Gaussian distribution in catering for cube orcylinder strength when the target strength is close to the aggregate crushingstrength. For UK aggregates, ultimate strengths in the range 60–100MPacould be assumed as typical, and so C50 and higher specifications forconcrete strength might well require a different approach for bothspecifying and drawing inferences. This application of data would applyto repeatability tests inter alia.

Example 1

This example concerned the use of the Brinel hardness pistol to assess thestrength of prestressed concrete units in a factory. The pistol used to be incommon use as a hand-held test tool for hardness testing of metals andalloys. Its principle was to impact a hardened steel ball against the surfaceunder test; the hardness of the metal was assessed by the diameter of thespherical impression. (The same principle is now used for metal testing,but a diamond with strict geometry to its facets is used. All modern testequipment is in the form of a composite machine.)


In the precast concrete works, the manufacturer’s target strength was45MPa at 28 days. Strict total quality control was exercised, and the cubestrengths obtained lay in the range 40–50MPa. Each time the units weretested with the pistol an average impression diameter of 3mm wasrecorded, with a range from about 2.7 to 3.3mm.

As these data referred to a specific strength-repetitive concrete, a rangeof concrete cubes were made in another laboratory with cube strengthtargets ranging from 15 to 60MPa at 28 days old. Just before crushing, eachcube was tested with 10 pistol impressions, and the average of these wascompared with each cube result. It was found that, irrespective of thestrength, the impression diameter was always about 3mm.

Two points arise out of this. First, consistency of data can be misleading.Second, as discussed in section 5.6, the weaker concrete could have beenpredicted to have improved resistance, because its energy absorptioncharacteristic would have been better than that of the stronger concrete.

As an aside, this leads to an apparent anomaly, in that the reboundhammer generally gives a positive relationship between rebound andstrength; the rebound numbers increase with increasing strength. Thereason for this may be the difference between the relatively large area ofimpact of the hammer and the 6mm diameter steel ball in the old Brinelhardness pistol.

Example 2

The problem relates to the recently issued recommendation for non-destructive testing of concrete using initial surface absorption (BS 1881 Part208:1986). The standard refers to the omission of precision data, as therewas not enough information to hand when the standard was prepared.The ISAT, by its nature, generally measures only the surface voidageproperty, and at a relatively short interval from the start of the test.

Observation of a typical concrete surface drying out after rain wouldreveal a patchy appearance over distances as small as a few millimetres,caused by variations in the absorption properties. The sensitivity of theISAT would be expected to reflect this variation, and experience has shownthis to be so.

ISAT units are specified to be recorded in units of mL/m2.s, and theapparatus has minimum and maximum range limits of 0.01 and 3.0 of theseunits respectively. At the lower end, the result can be read to an accuracy of0.01, and at the higher end to 0.2.

In practice it has been found that, taking readings at 10 minutes asexamples, concrete averaging 0.01 will vary from zero to 0.03. The morepermeable example would vary from 2.6 to ‘too fast to measure’. This, inmy opinion, indicates that precision data will be difficult to obtain for theISAT, and that it is unrealistic to expect ‘ideal’ repeatability and


reproducibility. An additional problem found on site with the low (say0.01) results is that if the reading is taken as the sun starts to shine on theequipment, a liquid expansion occurs in the cap and a ‘negative’ absorptioncan be recorded.


There may be pressure to ask for precision data when they are either notjustified or irrelevant, as well as the use or tabulation of data based upon asmall number of results.

6.4.2 REMEDIAL

In a current situation there would appear to be no remedy apart, possibly,from a review of or amendment to the conditions of application.

6.4.3 AVOIDANCE

The recipients of precision documentation in standards, specifications andregulations preparation should take a proactive role. A defensive, reactiveresponse to the receipt of such data is not constructive.

6.5 CHANGES IN TESTING

The problem is, quite simply, tradition. This takes the general form of strongresistance to anything new. I neither condone nor condemn this attitude,but I cannot agree with a generalisation either way. If a test has beenestablished for a long time this neither means that it is the right test (see6.1–6.4) nor that there is necessarily a better test that could take its place.The problem is probably exacerbated by the lack of use of the currentlyavailable mechanisms to correct the problem. It is logical for members ofthe construction team to accept that testing needs to have a nominatedposition in the control of material properties. If testing is a weak link in thechain joining performance to materials, design and workmanship thenscience and technology will have little or nothing to contribute toconstruction. The best way to tackle this problem is to study each testrequirement on the basis of the matrix suggested earlier in this chapter,and then do one of the following:

(a) Confirm and/or reinforce that test.(b) Replace it with a different test.(c) Run a new test alongside the existing test: that is, (a)+(b).(d) Remove the test requirement completely.(e) Introduce a test where there was no test before.


It is in respect of (c) that the future appears to be the most attractive. BothBritish and European standards have a tendency for a specific test to be the‘reference’, with other tests being subsidiary. For several British Standards,where an alternative test to the reference test is included, users arerequested to submit data (but obviously not contract details) to the BSI.This is a good idea, but lacks the power to change things. It might be betterto make both the reference and the alternative tests mandatory, with allresults to be sent to the BSI. (The BSI would be the secretariat for nationalas well as European and international standards).

The complete removal of a test, as listed under (d), can form a largediscussion platform. Over the years many revised standards have omittedearlier test specifications. Reasons for the omission of a test would be givenin the revised standard. There is no reason to conclude that this process iscomplete; there are still some tests that have no reason for their presenceother than tradition.

By the same token, under (e), there is no reason to conclude that everytest necessary to define a property or performance need is present in everyStandard. If the matrix approach suggested in sections 6.1–6.3 is acceptable,a method of dealing with the problem and its spin-offs could result.


The problem reveals itself as a reliance on inappropriate or misplaced tests,often coupled with a resistance to consider or accept anything new ordifferent.

6.5.2 REMEDIAL

Apart from discussing the possibility of variations to the test requirementsthere seems to be little that can be done to remedy a current problem.

6.5.3 AVOIDANCE

Refer to section 6.4.3 for a nominally identical approach. BSI publicationssuch as BSI News provide a monthly update on the progress of British,European and international standards. In addition, any person canpurchase a draft at the public comment stage and submit their opinion tothe relevant secretariat.

6.6 TESTING FIXATION

Although this title implies that the problem is a mixture of the earlierdiscussion in sections 6.3 and 6.5, there is in fact a completely different


facet that warrants exposure. The problem encountered was a dogmaticinsistence that wherever or whenever a property or requirement was underconsideration there had to be a test accompanying that part of thespecification. This insistence was often found to generate a spin-offproblem in the form of a reversal, which commonly manifested itself as aninsistence on some form of property requirement so that a test could beproposed to accompany it.

An example of test insistence that in my opinion was (and still is) largelyunjustified was described in reference to aluminous cement in section 1.7.The test was differential thermal analysis on drilled powder samples takenfrom precast pretensioned concrete beams, made of high alumina cement(as it was then called and is still known), in order to ascertain the degree ofconversion. Virtually every construction examined in my experienceshowed 70–90% conversion, with stable performance of the precast unitsand the construction.

An example of testing that was not really sensible was described insection 3.1 in reference to chloride diffusion, where track records haveshown that good performance has been achieved by the mix design route.Testing would have not furnished data of significance for about 6 months,and such a potential contract delay to await test results would have beenunacceptable to most parties in the construction team.

It is debatable whether either the alkali-silica reaction (section 1.5) ordelayed ettringite formation (section 1.12) comes into the spin-off categoryreferred to above. In my experience damage has been almost certainly dueto ASR on only three constructions. As far as DEF is concerned, apart fromthe possibility of its having been the cause of the splitting observed inexperimental kerbs described in section 1.12, no case on site has beenexperienced.


Someone will insist on the presence of a test and/or call up a property,whether relevant or not, so as to have a test to address that property.

6.6.2 REMEDIAL

If discussion is possible, and logic can be applied, a change in the wordingto the testing or property documentation should be attempted.

6.6.3 AVOIDANCE

The most fruitful approach would seem to be full discussion at committee,institution or authority levels before requirements are put into formaldocuments.


6.7 TESTING ACCURACY

The problem refers to the data produced from a test initiated at thespecifying stage by the demand for an impossible or unreasonable accuracy,and at the reporting stage by a form of impressionism. Examples are givenbelow.

6.7.1 PROBLEMS AT SPECIFYING

A common example of this family of problems is in typical specificationwording such as ‘The concrete cube strength shall be SOMPa at 28 daysold’. There are two virtual impossibilities here. First, even under the strictestform of production, it is impossible to get each cube to reach the exactstrength of 30MPa at that age. Second, cube strengths are specified to bereported to the nearest 0.5MPa, so the 30MPa (if it had been possible toachieve) should be 30.0MPa. The omission of that SOMPa being specifiedto be a minimum, maximum or average could also be called into question.

In other instances, dimensions have been specified to an accuracy of1mm, and tolerances have been completely omitted from the drawings. Inthe former case, the contractor or producer was being asked to work to theunachievable; in the latter case, no tolerance would seem to be permissiblein the work.

6.7.2 PROBLEMS AT REPORTING

An example of this is with a typical cube-testing machine that ‘locks in’ thefailing load reading to the nearest 1kN. Thus it would appear that a 100mm(nominal) cube failing at 424kN load could be reported to have had a42.4MPa strength. Putting aside the specification requirement of reportingto a 0.5MPa accuracy, this report ignores the machine accuracy. At the bestthis would be no better than 1% under a Class 1 machine certification. Italso ignores the cube’s being only nominal in size, with a 1% allowance onall dimensions. Therefore the 42.4 could be anywhere between 42.0 and42.8 on the machine accuracy. The crushing area of the cube could be up to2% larger or smaller than the specified nominal size calculation. Takingthe largest negative and positive cube area sizes on the final load range,the actual cube strength (ignoring other testing variables) could lieanywhere in the range 41.2–43.6MPa. So although the specified reportingaccuracy for this cube gives a strength of 42.5MPa, it is still subject to anerror of about 1MPa.

Another example of a reporting problem is with a water absorption teston, say, an approximately ‘cubic’ sample of 100mm ‘side’ cut from concrete.It, and its weight changes from oven drying to wetting, can usually bemeasured to an accuracy of 1g. For a sample weight of about 2kg, this


represents an accuracy of 0.05%. If an operative carried out 30 minuteabsorption tests on three subsamples and obtained readings of 3.50%, 3.50%and 3.55%, an average of 3.52% could be reported. The lesson here is thatreporting accuracy should not be based upon unnecessary mathematics,which can give answers indicating a form of superiority.


Look for an unreasonable or impossible accuracy specified or an unjustifiedaccuracy in the data reported.

6.7.4 REMEDIAL

In the first example, the specifier should be advised of the impossibility orinapplicability of the requirement; in the second example, the report shouldbe returned to the issuing activity. The corrected replacement report shouldhave the same report reference number as the superseded one but bemarked ‘Rev’ or ‘Superseding Report No......’ or similar, and the supersededreport should be marked as such.

6.7.5 AVOIDANCE

Both specifiers and testers should be aware of the problems that can begenerated, and should take appropriate steps to avoid them. Othermembers of the construction team should also draw the attention of thespecifier or the testing authority to any cases that come into their remits.


Glossary

This glossary describes most of the acronyms and lesser-known words,omitting the commonly used terms. There are no contradictions with theterms used in the British Standard (BS 6100:1984–1992), and the reader isreferred to the standards sections for detailed descriptions.

AEA Air-entraining agent(s).alkali Soluble form of sodium or potassium.alkaline, alkalinity Refers to a pH from above 7 to 14.ASR Alkali-silica reaction.ASTM American Society for Testing and Materials.autogenous healing Sealing of cracks and/or crazing with calcium

carbonate and/or lime.BS British Standard (specification or code of practice).BRE Building Research Establishment (formerly Building Research Station,

BRS).caustic Destructive or corrosive to flesh.CE mark Mark denoting compliance with a Harmonized European

Standard.CIRIA Construction Industry Research and Information Association.covercreteConcrete that lies between the rebars and the exposed face.covercrete d Covercrete distance from the rebars to the exposed face.covercrete k Permeability of the covercrete.crack aperture Crack width at a surface.crack width Crack width at a stated position.CSA Cross-sectional area.CSTR Concrete Society Technical Report.DAS Defect Action Sheet from BRE.DEF Delayed ettringite formation.efflorescence Crystalline sulfate deposit associated with clay bricks.


endoprobe Optical device for viewing inside cavities and similar places.ettringite Calcium alumino-sulfate salt formed from the reaction of sulfates

and the calcium alumino-hydrate from cement.GGBS Granulated ground blastfurnace slag.GRC Glass-‘reinforced’ cement or mortar.ISAT Initial surface absorption test as specified in BS 1881/208:1996.labcrete Concrete made and tested under laboratory conditions.lime bloom White calcium carbonate deposit formed by the reaction of

atmospheric carbon dioxide with lime from cement.MS Microsilica, a by-product of the ferrosilicon industry.necrosis Death or decay of living flesh, nerves or muscle.OPC Ordinary Portland cement.PFA Pulverised fuel ash, a by-product from the burning of pulverised coal

in the production of electricity.pH A measure of the acidity or alkalinity of a system.realcrete Concrete made and used in the works or on site.rebar Reinforcing bar.RILEM International Union of Testing and Research Laboratories for

Materials and Structures.SBR Styrene butadiene rubber; used normally as a white emulsion.troubleshooting Problem investigation and advice.UKAS United Kingdom Accreditation Service (formerly NAMAS).UPV Ultrasonic pulse velocity.
