efflorescence and the discoloration of concrete (1983)

EFFLORESCENCE ANDTHE DISCOLORATION

OF CONCRETEby

Peter Russell BSc FICE FIStructE FIHE FIOBformerly Chief Engineer in Scotland of the

Advisory Divisionof the Cement and Concrete Association

A VIEWPOINT PUBLICATION

VIEWPOINT PUBLICATIONS

Books published in the Viewpoint Publications series deal with allpractical aspects of concrete, concrete technology and allied subjects inrelation to civil and structural engineering, building and architecture.

13.026 First published 1983

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Any recommendations made and opinions expressed in this book are theauthor’s, based on his own personal experience. No liability or

responsibility of any kind (including liability for negligence) is accepted bythe Publisher, its servants or agents.

THE AUTHOR

Peter Russell was Chief Engineer in Scotland for the Cement & ConcreteAssociation from 1948 to 1978. He is now engaged in the preparation of advisorydocuments for the building industry and is writing a book covering those thirtyproductive years from hydro-electric schemes to oil platforms, for which hereceived the Silver Jubilee Medal. He has contributed to professional journals andis currently talking at some universities on the theme of construction and theenvironment.

Peter Russell has been Chairman of the Institutions of Civil, Structural andHighway Engineers, and of the Concrete Society, serving also on the Board ofthe Edinburgh College of Art. He played a part in the attainment of aesthetic aswell as structural quality over the past generation in a northern climate which lentitself to a special study of the subject now under review.

His thanks are due to the Cement & Concrete Association of whose excellentwork he has intimate knowledge, and valuable assistance was further given by thestaff of the Building Research Establishment.

PREFACE

Disfigurement of any concrete surface is at least a source of annoyance, and thispublication attempts to mitigate the problem in its chemical and physical aspects.It deals with the broad scope of elemental attack by the vagaries of weather andby salts as they move to the face of a building to result in discoloration and, at times,disruption. Much research has in particular been devoted to the cause and natureof efflorescence, an expressive yet ambiguous word which suggests flowering andfruition but describes an effect that almost vies with vandalism. While elusive,however, it is not an intractable phenomenon and can be cured in more sensesthan one. Its stalactites or bloom can be alleviated by common sense and scientificforethought, given all the rules of quality control. This largely aesthetic problemis conditioned by factors such as temperature, absorption and lack of imagination,with cement and aggregates of lesser importance. The text covers staining ofmany kinds, superficial but unsightly, and due to faulty workmanship or detailing.Concrete is too readily assumed to have strength and durability only, with anacceptable countenance underestimated or ignored. This work should helptowards an appreciation of its potential in visual as well as structural terms.

Peter RussellSeptember 1982

Plate 1: A retaining wall of engineering brickwork with disfigurement at the mortar jointsas the result of salts finding their way from the soil behind. These should have been isolatedby a waterproofing barrier over the full height of the wall, and it is possible that the mortarwas too rich, with consequent shrinkage and fine cracking.

v

CONTENTS

THE AUTHOR iii

PREFACE iv

CONTENTS vi

1 INTRODUCTION 1

2 CAUSES 3

2.1 Basic factors 3

2.1.1 Mechanism 5

2.1.2 Hydration 6

2.1.3 Formwork 6

2.1.4 Rainwater 7

2.1.5 Weathering 7

2.2 Blockwork 8

2.3 Brickwork 9

2.4 Mortar 9

3 SOLUBLE SALTS 13

3.1 Carbonation 14

3.2 Contamination 15

4 PREVENTION 17

4.1 Concrete quality 18

4.2 Admixtures 18

4.3 Blockwork 19

4.4 Cladding 19

4.5 Mortar 20

4.6 Joints 21

4.7 Pigments 22

4.8 Detailing 22

4.9 Damp-proofing 23

4.10 Formwork 23

4.11 Rendering 24

4.12 Urban environments 24

4.13 Surface treatment 25

5 REMOVAL 29

5.1 Washing and brushing 29

5.2 Acids 30

6 CONCLUSION 33

7 BIBLIOGRAPHY 35

8 APPENDIX: CHECKLIST TABLE 41

vii

1INTRODUCTION

Efflorescence is a white coating appearing on the external face of a wall as a resultof the migration of salts in solution to the surface where they crystallize unevenly.These blemishes may be derived either from the constituents of building materialsbased on Portland cement or from alkalis in the ground, and are salt depositscaused by the evaporation of their saturated solution. In normal atmosphericconditions they leave a harmless, superficial and often temporary discolorationwhich is nevertheless disturbing, and whose irregular configuration may persist fora number of years. Disruptive spalling of the surface can result if the salts are deepseated. Discoloration of concrete has many manifestations and the common causeis excessive ingress of rainwater due to faulty detailing. An effect similar toefflorescence may be caused by rain cleaning part of the wall, but this pattern isgenerally more extensive and is dependent on the extent of glazed areas andprojections on the face.

Chemistry plays an important part in that calcium hydroxide, liberated by thehydration of cement, is carried in solution to the surface of concrete or mortarwhere carbonation takes place. The location and extent of this relatively insolublesubstance is largely dependent on the porosity of the external layers. Materialswith an open texture allow air to penetrate some distance below the surface, anda saturated solution moving from the interior can be carbonated before it reachesthe outer skin. Concrete of intermediate porosity having a close texture,permeable to salt solutions but not permitting air to enter, may suffer a visibledeposit on the outer surface.

Efflorescence is the result of a combination of events occurring inside andoutside a wall. The presence of even small amounts of salt, with attendantmoisture, initiates evaporation or hydrostatic pressure causing movement of thesolution to the exterior. The resulting quantity and effect of leaching of theconcentration depends on its composition and solubility. The extent ofefflorescence is also determined by temperature and humidity, and it is morelikely to occur in seasons with a slower rate of drying.

Bloom is another phenomenon, similar in appearance to efflorescence, which iscaused by carbon dioxide in the atmosphere reacting with free lime producedduring the setting of cement and forming insoluble calcium carbonatedeposits. Sometimes this is of appreciable thickness, so that encrustation may be a

better description. It is naturally more noticeable on a dark surface than light, andexemplified by streaking down the face of engineering bricks from the mortarjoint or a paint-like deposit on concrete.

While the causes of efflorescence are relatively easy to explain without thecomplexity of chemical equations, some knowledge is required of the nature ofsoluble salts and their aesthetic and physical results. Almost any salt can appear,but each may react differently with atmospheric gases which are themselvesvariable. Another source is from the earth behind retaining walls or under solidfloors, so that this should be carefully isolated. Above all, imaginative detailing ofa structure is essential to shed water from the face or to dissipate its flow. At best,efflorescence is an irritation and at worst an agent of disruption, but while thelayer is annoying and generally unacceptable, it is barely noticeable on concretemade with white cement and in some cases can be advantageous in reducingporosity, and sealing fine cracks or crazing on the surface. Efflorescence is moreapparent in the spring, although remedial conditions are less severe during thisseason and showery weather is likely to aid the washing process.

Because concrete as a structural medium is relatively new, it suffers by man’sinability to forestall elemental attack; he does not make the same allowance fordiscoloration as he would with other materials long subject to the most blatant ofblemishes. Sporadic cracking and faulty jointing are of importance, and inanticipating the effects of weather every opening must be considered suspect,whether glazed or not.

All building materials have to meet a number of requirements when exposed toa variety of polluting and destructive elements which stem from the atmosphere,the soil or the materials themselves. There must, therefore, be a clearunderstanding not only of the properties of the constituent parts of the structurebut also of the factors which influence their performance both integrally andsuperficially. Uninterrupted areas are especially vulnerable.

Concrete has indeed prospered in an infinite range of function and form, but inits wake has come the problem of lack of attention to the facade and ignorance ofthe effects of salts and their deposits. It is an indispensable medium, but suffersvariability at the hands of planner and builder, and by climatic conditions in thelong term. More attention must be given to arresting or redirecting moisture byeffective barriers, and to forestalling cracking by due provision for expansion andcontraction.

2 INTRODUCTION

2CAUSES

2.1BASIC FACTORS

Portland cement combines with water in concrete mixes to produce highlyalkaline products and this chemical reaction is known as hydration. The make andcontent of cement has little effect on efflorescence, except that a very rich mixlends itself to crazing and, if too weak, is less than durable. Larger quantities oflime than usual need not cause a serious problem, but it is always desirable to useminimum free water consistent with full compaction. Dry air can increase the riskof blemish whereas humid air reduces it. A low temperature retards the build-upof a protective carbonate layer. Organic impurities in loam and clay may lowerthe strength, and estuarine aggregates should be well washed. The essentialrequirements of good concrete are impermeability, continuity of placing anduniformity of quality, with any protective coatings firmly bonded. All facesexposed to sea-spray are vulnerable and many industrial processes disgorge saltswhich are transferred to porous materials by the polluted atmosphere. The natureof stored materials such as fertilizers must be taken into account, as should thelikelihood of leakage from sinks and services in general.

Although concrete is versatile with a wide range of functions, the action ofweather can be complimentary or otherwise, depending on presentation to theenvironment and the experience of the designer. Documentation abounds inprinted and photographic form on the general theme of discoloration, butefflorescence as the end-product of climate and chemistry is rarely discussed. Thelocation of a building relative to its neighbours is a crucial factor in the planninganalysis and in predicting the intensity of wind and rain. Today’s pollution, albeitlinked with technical progress, enhances discoloration by its contribution toamorphous staining, made more prominent by its contrast with the normal colourof concrete.

When all the accepted rules of mixing and placing are strictly obeyed,discoloration is still possible, but it is axiomatic that the denser the concrete theless chance there will be for extraneous water to be absorbed into it and, onevaporation, to bring out the defiling chemicals. Uniformity of concrete quality is

crucial and the constituents should if possible be taken from constant sources.Admix tures do not adversely affect staining, and the use of integral pigments maygive better colour control, minimize surface variations and aid workability.Consideration of appearance and of overall durability impose even stricter limitson workmanship, whilst curing, as later amplified, is of paramount importance.

Apart from isolated areas of formwork leakage or irregular absorption by thelining, other causes of discoloration include concreting in cold weather, casting inexcessively thick layers and, above all, release of water to the surface duringplacing and compaction. Another factor is mixing time, and longer periods mayreduce the extent to which water and cement separate, as may extended vibration.The atmosphere may also contribute industrial effluents and traffic fumes toproduce chaotic patterns around corbels and ledges, which in any caseconcentrate rainwater. The outer skin of concrete, however skilfully moulded,takes the full force of attack and must of necessity be durable, whether air-entrained or otherwise. Appearance is largely determined by the properties of thecement paste, which in practice is regularly wetted so that dirt lodges in its pores;the rich laitance is etched to change its colour towards that of the fine aggregate.As the grime builds up it will give a measure of protection against further etching,but may show more contrasting efflorescence if the salt source is still operative.

A more encrusted growth is produced by water percolating through cracks in awall bringing lime to the surface, but this to some extent protects reinforcement.Efflorescence can be confused with streaking caused by rainwater which cleansalong irregular lines, and here again experience will indicate profiling or at leastdiffusion.

Concrete, in the same way as any other material, becomes dirty, and with tallbuildings care must be taken to counteract the effects of wind turbulence.Painting may have to be considered but this should always be seen as a secondline of defence. What is unforgiveable is the recurrent blemishing of an otherwisewelldesigned structure simply because of a careless attitude to long termperformance. Efflorescence is liable to occur on hardened concrete if, during thepouring of successive lifts, the joint is not perfectly sealed, as free water can carrya concentration of salts along myriad paths. Weather patterns are so oftendetermined by the configuration and efficiency of movement joints, so that theirlocation must be carefully established. Daywork joints must also be positionedeconomically and intelligently.

Cracking of in situ walls or across a building unit allows moisture toconcentrate along channels which feed the fissures and highlight those alreadyevident at the end of sills and at badly formed joints between precast units.Structural elements, such as the abutments of bridges, are prone to the migrationof groundwater into them, and retaining walls are targets for outpourings frommortar.

4 EFFLORESCENCE AND THE DISCOLORATION OF CONCRETE

2.1.1Mechanism

The formation of efflorescence depends upon a number of chemical and physicalprocesses involving the nature and solubility of salts and their transfer throughporous concrete. Local conditions dictate how these evolve, so that the depositscan be unpredictable in extent and location. Once a solution has becomesaturated, cooling will cause most salts to separate out in solid form, but loss ofwater by evaporation will also result in deposition, even without change intemperature. There may be a time lapse between the completion of a buildingand the appearance of the effects of salts, and their immediate location is nocertain guide to provenance, but simply a convenient drying surface.

The structure of concrete will dictate its drying rate and the place ofdisfiguration. With a fine-pored surface, capillary forces will draw free waterthrough the wall, crystals will appear on the exposed face and variable porositywill result in sporadic staining. With an open-textured surface, water willevaporate short of the face and the salts remain just within the wall, possiblycausing disruption. Salt-laden moisture may leave precipitations taking the formof localized striation corresponding with joints or architectural features. It isessential to keep track of such movement and its degree of replenishment, tostudy the origin and nature of the salts, and to have prior knowledge of spillage ordefects in drainage. The unpredictable festoon can be seen in buildings soon aftercompletion but will gradually fade away with the aid of wind and rain if its sourcehas been stemmed.

As concrete is a mixture of natural constituents, attention must be paid toselection of raw materials and their proportioning, compaction and curing. The lastof these does not imply a remedial process but ensures adequate hydration of theimmediate surface for consistency in colour and durability. The containing shuttermust itself be treated with due deference, as should rigidity, striking time andrelease agent. It has been said that there are nearly a hundred kinds of concreteblemish, many of which have a common cause, such as variable shrinkage andabsorption, with the catalyst so often the entry of soluble salts. Other forms aresand discoloration, blowholes, aggregate transparency, mottling, scouring andscaling.

Water in the concrete mix is normally distributed across the wall and can beadversely augmented if protective measures are not taken against absorption fromthe ground or rainfall. Efflorescence is more likely to occur if the building iserected during winter and if drying through the external wall is rapid in the earlypart of the year. When salts disappear from a surface, they have not necessarilybeen washed away, but have sometimes been absorbed back into the wall and willreappear at progressively lower concentrations. If, after precautions such ascovering the works have been taken, they recur on drying, then excessive wateris entering the wall through faulty detailing or waterproofing.

CAUSES 5

2.1.2Hydration

The whole subject of salts is discussed later but it is worth noting that in additionto the alkalis of potash and soda, cements contain sulphate as gypsum added atgrinding to control the set and on mixing with water these go into solution. Aftera few hours the sulphate is fixed insolubly, leaving the alkalis as hydroxides whichare gradually converted into carbonates by the atmosphere to leave a surfacedeposit. If the sulphate is transferred with the mixing water by capillary action toan adjoining building unit, the quantity of salt in that unit will also be increased.In other cases staining is caused by the movement of soluble calcium sulphate tothe concrete surface, this generally emanating from soil in contact with it.

The reaction of hydrating cement with carbon dioxide in the airreduces alkalinity. Such carbonation also increases shrinkage of the concrete andcan lead to cracking, although not penetrating too deeply. Aggregates near thesurface which are not well graded may also cause cracking, distinguishable fromthat caused by drying shrinkage in that it takes longer to develop. Only in veryporous concrete is there a problem, and here an assurance of adequate cover toreinforcement is essential. Cracking through which salts find ingress can also arisefrom differential settlement.

2.1.3Formwork

The time of stripping formwork is not necessarily significant, but blemishes maybe caused by warping or deflection of the form which leave an air gap allowingrapid localized drying and a finish inconsistent with the rest of the surface.Careful selection of form face can inhibit most types of staining although somelinings are unsuitable and easily damaged. A dark coloration is common if castingtemperatures are low but this can be offset by insulation. Variable surfaceappearance is a difficult problem, but in broad terms quality depends on goodplacing and an evenly applied release agent. Efflorescence is not so likely to occuron surfaces cast against absorptive forms, which encourage the water in the mix tocarry cement particles to the surface where they are deposited as dense skin of lowpermeability, so restricting the migration of lime and reducing carbonation.Impervious forms, on the other hand, trap excess water at the face. In some caseslight coloured areas tend to occur near the top of a lift and not lower downwhere the pressure of the plastic concrete brings a relatively greater flow of waterinto the receptive lining.

Exposure of concrete to the air leads to evaporation of the pore water, and if thisis rapid with only vapour reaching the surface, the carbonate will be depositedbelow the surface with possible disruption. There is also an intermediate statecausing localized effect by gross variations in porosity due to poor compaction and


segregation. In this case water reaches the surface through relatively large channelsand leaves an erratic deposit, although much less common on precast elements.

2.1.4Rainwater

Rainwater absorbed into any porous material is released by natural drying, thesignificant factors being solar radiation, wind speed, atmospheric humidity and theaspect of a building. The nature of concrete itself plays a large part in determiningloss of water from a surface and it is difficult to measure the overall affect withsuch a variety of influences. Penetration is obviously greater in regions where therain index is high and the walls have less chance of drying out betweendownpours. The criterion by which the degree of exposure can be judged isdescribed in BRE Digest 127. Sites may be graded between sheltered and severebut, in areas of moderate exposure, high buildings which stand above theirsurroundings or on hilltops should be regarded as being one grade more exposedthan indicated. Although incidence of rain conforms to a rough pattern for anygiven locality, short intense periods may be experienced from any direction andmuch of the penetration of walls occurs during a few prolonged stormsaccompanied by strong wind. The annual mean index gives a fair indication ofthe total amount to be expected, but while helpful in gauging the likely frequencyand intensity, is less useful in assessing run-off from surfaces or entry throughjoints. Experience of the behaviour of concrete in any situation is of critical value,as is guidance from the Meteorological Office.

The resistance of a solid wall is appreciably increased by the application ofrendering in accordance with BS 5262, while blockwork may also benefit in areasof severe exposure by the prevention of ingress through joints, cracks or the unititself. Walls with a cavity afford a more effective barrier than a single leaf,provided the quality of materials and workmanship is equally high.

2.1.5Weathering

Concrete does not normally weather well over long periods in terms ofdiscoloration; a comparison should, therefore, be made of the design of similarprojects with a vew to controlled variation and intentional highlighting ofdetailing to distract the eye. It exhibits its age by symptoms such as dirtaccumulation, growth of algae and efflorescence, some being superficial butothers deep seated. The surface obviously changes from its initial freshness, withproperties that alter in porosity and frost susceptibility. This outside layer varies,absorption determining the ability to hold the grime or resist aggression from theair, so that exposure may be uniform but the result patchy.

Weathering should ideally enhance the appearance of a building if only bytraditional mellowing, but the very lightness in tone of concrete in plain walling

CAUSES 7

is a disadvantage in combatting darker stains, while the modular patterns ofprecast units should render these less prominent. Mortar is another matter in thatclay brickwork, and even blockwork, can be disfigured to almost paint-pot degreeif a balance is not reached as to its constituents. Curing is the final act of surfaceattention, by water spray or otherwise, to ensure that the susceptible externallayer is not dried too quickly, by hydrating the cement particles at the surface andminimizing vulnerable laitance.

2.2BLOCKWORK

Two forms of efflorescence may be found on concrete blocks—sodium/potassium carbonates and calcium carbonate. Each is derived from correspondingfree hydroxides brought to the surface during hardening. Sodium and potassiumcarbonates appear as a soft deposit which is easily washed or brushed off,particularly if the unit has been exposed to soaking and slow drying in a stockyard.Calcium carbonate is not so common and its harder crust is more difficult toremove. Masonry can bring together a wet mortar and a block containing variablemoisture such that the water content of the wall is relatively high and depositmore likely to form on completion of the work.

Concrete blocks, lightweight or otherwise, should never be set in unnecessarilystrong mortar as this can lead to cracking in or near the joints, whereas a weakermix yields enough to accept small movements in a wall, and hairline cracking willbe distributed or less permeable. Strong cement/sand mixes are appropriate forheavy engineering works in units of comparable strength or in foundations andbelow damp-proof courses, but may display more efflorescence than a mixcontaining lime. While the latter contributes more salt, the denser joints offermore resistance to the passage of moisture and the precipitate sometimes spreadsover the blockwork on evaporation from the wall as a whole. In the same way awhite deposit round the edges of blocks suggests that the units have absorbedwater from a mortar containing alkaline sulphates. Even if there was noabsorption from the joints during laying, these sulphates may still be transferred tothe units if an unfinished wall is not covered in heavy rain. The answer lies inmatching the qualities of blocks and mortar by using cement/lime/sand ormasonry mixes which are not stronger than necessary. It is worthy of note that atechnical survey was undertaken by the International Concrete BlockCommission to collect data on the improvement of long term colour integrity ofmasonry.

Efflorescence on precast products in general may appear as white patches or asan overall lightening of colour. Where the units are exposed, acidic rainwater willslowly dissolve the film, and in the case of paving slabs, abrasion by foot trafficwill remove it more rapidly.


2.3BRICKWORK

Efflorescence on brickwork can appear as a loose white powder or a hard glossysubstance, partly covering and penetrating the face. It can be a seasonaloccurrence and become less marked in ensuing years, but often affects retainingand freestanding walls and is removed only slowly by rain. Magnesium sulphate isparticularly disruptive and its deposition not readily washed from either the wholeface or under copings from which lime has leached. The only satisfactorytreatment may be to render the wall after removing all loose material and rakingout the mortar which may itself be impregnated.

Certain brickwork has very low porosity and little sulphate content, whereas ahighly porous mortar will allow even a minute quantity of salt diffused along itscourses to be shown clearly against the background. Flettons may be highlyabsorptive with appreciable sulphates, and within themselves may result in saltsappearing on the surface over a wide area of the wall. A white deposit is broughtto the surface by the dry winds of April, sucking salts into its external face eitherthrough the joints or the units depending on relative percolation. The greatestrisk is from wind-blown rain finding its way through cracks in joints which are asstrong as the bricks but on shrinking permit water movement. Many tarnishedmulti-storey buildings are victims of this lack of knowledge as to the interaction ofvarious parts of a wall at different shrinkage and drying rates.

Clay products are prone to a legacy of lime, with deposits more prominent ontheir relatively dark surface and exhibiting the same crystallization of salts whichhave percolated in solution to the face. Again the extent depends on the attendantamounts of both salt and water, and on their chemical nature. Salts of calcium,sodium and potassium are, in the main, superficial but spoiling, some of thesealready being present in the clay or formed at firing, with calcium sulphate of lowsolubility constituting the major source. Salts in brickwork can have an influenceon adjacent concrete in transferring from one material to another.

2.4MORTAR

Cement-based mortars may be attacked by sulphates derived from clay bricks aswell as from external sources such as flue gases. The onset is gradual and onlyoccurs when the brickwork is consistently damp. The most common effect ofsoluble salts is to produce a deposit which, although unsightly, is usuallytemporary and harmless. A high percentage of calcium sulphate may cause littleefflorescence, while a lower porportion of sodium or magnesium sulphate cangive rise to heavy impregnation at the end of a long wet spell. Crystallization ofthe salts within the brickwork may, however, cause spalling of the face similar tofrost attack. While keeping winter working in mind, it must never be assumed

CAUSES 9

that the higher the strength of the mortar the better, because when it is too strongit simply concentrates movement into fewer but wider cracks.

The use of low alkali masonry cement will reduce the staining capacity ofmortar but the overall design of a building must ensure that the walls are kept asdry as possible by, say, an overhanging roof and the shielding of surfaces fromrainwater generally. Drains should be of ample capacity and maintained inserviceable condition. Present day mortars are fortunately much more durable andlikely to resist erosion in highly contaminated air. Guidance is given in BS 3921as to discoloration of brickwork.

Although cement contains less salts than hydraulic lime, care should be taken toprevent migration of mixing water from mortar to bricks or blocks before thecement has hardened, as salts in solution switch easily from one part to another bycapillary action. It is quite possible for a wall which is initially free of salts to becontaminated by their movement from the mortar, as indicated by thepredominance of sodium or potassium compounds at the surface. There is littleevidence to suggest that mortar plasticizers contribute significantly to efflorescencebecause they are added in such small amounts.


Plate 2: A marked contrast between the upper vertically diverting fluting and the plainrendered base which should have been similarly treated. Alternatively, the lower wall mighthave been covered with exposed aggregate panels.

CAUSES 11

3SOLUBLE SALTS

Salt is defined as a compound of molecules which in the solid state aregeometrically packed as crystals, but which lead an independent existence inaqueous solution. The action of salts is in part associated with cement-basedmaterials as the source of alkalis according to the location and porosity of themedium. The resulting deposit is the product of their migration through concreteof poor quality, and of subsequent evaporation which can build up an appreciablecrust on or within the surface. The readiness with which this forms depends onthe nature of the salts, and if the first fine crystals in the pores are such that stillsmaller capillary passages are left, further crystals may surmount these. Saltsderived from external sources, or moving from one part of a wall to another, areof fundamental importance, and their composition is to a great extent determinedby their origin. Portland cement and hydraulic lime in mortars provide thesulphates and carbonates of sodium and potassium, whereas the presence ofnitrates or chlorides points to groundwater as the source. Chlorides may alsocome from rubble fill and polluted atmospheres. Sulphates are found in buildingmaterials, industrial waste and some clay soils although the top few feet are generallyfree. All these salts, together with calcium oxide, are very soluble in water, andeven calcium sulphate dissolves to some extent.

The two main constituents of cement are dicalcium and tricalcium silicates,which react with water in concrete to give products which are highly alkaline.Carbon dioxide in the air reacts with the surface layer of the concrete to reducealkalinity in the process known as carbonation. This often intractable film is theresult of the crystallization or precipitation of salts in solution, replenished frominside the wall and found also within the pores in company with excess water.The chemical action takes place more rapidly when the concrete is at anintermediate stage between saturated and completely dry, but less so if it is wellcured.

In the initial stage, calcium hydroxide or free lime is released from the cementduring the setting process and migrates to the surface where it is converted intocalcium carbonate or chalk which is not readily removed because of its lowsolubility. In the early stages of hydration, lime remains in solution until theconcrete hardens, and efflorescence can be caused by very small concentrations. Ascarbonation proceeds in the normal course of weathering, the remaining

hydration products, including calcium silicates and aluminates, are slowlydecomposed by sulphur dioxide from industrial airborne sources, leading tocalcium sulphate. In good quality concrete this produces little change inappearance, as the sulphate acts as a binder and, while retaining dirt in shelteredplaces, helps to keep washed areas clean because of its slight solubility.

Magnesium sulphate is responsible for many of the failures resulting fromsoluble salts, although it is not present in great quantities and is readily washedaway except in sheltered parts. Potassium sulphate forms a hard film, whereassodium sulphate produces a fluffy deposit when it crystallizes. Surface failure isoften most severe when the salts are in more than one state of hydration, andsolar heat may convert these to a form occupying several times the earliervolume, with excessive pore pressure and powdering.

Depending on their origin, nature, distribution and quantity, salts may result inthe following effects:

(a) temporary but unsightly fluffy encrustation;(b) a glossy skin that causes blistering of the surface;(c) a reaction with certain compounds in the cement which gives rise to

softening and possible sulphate attack;(d) crystallization within the surface pores resulting in pitting and disruption and

known as crypto-efflorescence.

The source and solubility of salts vary, their path is devious and their movementdictated by moisture and humidity. Ability to dissolve increases with temperature,and calcium hydroxide is among the few salts which act more freely with coldwater than hot.

3.1CARBONATION

Air in permeating concrete decomposes the accessible hydrated compounds toreduce alkalinity in the process known as carbonation. This is normally limited tothe exposed surface and, if the concrete is well compacted and of lowpermeability, the resulting white deposit is built up very slowly in the presence ofmoisture. Its occurrence also depends on the aggregates, cement and admixturecontent, water/cement ratio, curing and exposure. The proportion of carbondioxide in the air is not usually significant, but may be influenced by pollution. Onthe other hand a degree of carbonation and corresponding reduction in alkalinityhas a bearing on the corrosion of reinforcement. Normal mixing water has only asmall percentage of carbonic acid dissolved in it and has little aggressive effect.

Since partly hydrated cement paste is more permeable, factors which delayhydration such as cold working conditions may increase the surface deposit.Although there is minimal transfer of calcium hydroxide from the interior of a well-built concrete wall, there are always localized effects caused by variations in


porosity, in which case water is channelled into the surface, bringing with itdissolved hydroxide and leading to heavy isolated excrescence. Moreover,rain can dissolve calcium carbonate or calcium sulphate from concrete copings toleave salts lower down if the wall is not designed to eject rainwater clear of thesurface or at least dissipate it.

3.2CONTAMINATION

Concrete can be contaminated by colliery shale used as hardcore in foundationsand under floor slabs, while the soil itself provides a possible reservoir which iscontinually renewed. Some aggregates may contain sulphur compounds andsoluble alkalis, but crushed stone, gravel and sand from well-established quarriesare normally free of deleterious chemicals. Unwashed sea-sand should not be used,for apart from its salt content, it will absorb moisture from the air to aggravate thesituation. Precast products are only prone to attack from outside sources. Troubleis not usually experienced with cladding unless it is located against salt-bearingbrickwork and it should in any case be fixed when both surfaces are dry.

Plate 3: A good example of striated walling bush-hammered to expose aggregate anddiverting the flow of rainwater within its channels. The facade is eyecatching, withefflorescence submerged with the fluting and masked by the bold exposure of theaggregate between the windows.

SOLUBLE SALTS 15

Plate 4: This fault is common in structures such as car parks where the joint detail is badlydesigned and the workmanship poor. The edge beam could have been effectively coveredby exposed aggregate panels or fluted to mask the junction between road slab andsupporting cantilevered beam. An uncontrolled flow of water is finding a ready exit forsalts in solution within the concrete or from outside.


4PREVENTION

Designer, operative and occupier all have a part to play in avoiding disfigurementwhich may become evident sooner or later. Control of migrating salts is essentialand water must be denied entry to a wall. These are the enemies of visualacceptance and conservation, but can be subdued by imaginative direction of rain,even in temperamental weather. Precautions should be taken at all times againstthe transfer of soluble chemicals into the fabric of a building, and it must beremembered that atmospheric gases adversely affect calcerous materials. Whendeciding on mix proportions it may be better to have a slightly lower fines contentand a richer mix, but too little sand will allow segregation. Placing should be asrapid as possible consistent with full compaction, and the head of concrete shouldnormally be less than 300 mm above the level of vibration.

The designer must attempt to predict weather conditions over many years, andhis aim should be to let each section of the building be cleaned by rain along lineswhich may actually improve its appearance. Vertical ribbing and fluting willcontrol the flow and discourage concentration, and a finish can profitably bespecified by reference to an existing structure in similar location and time. Thewater course must be predicted to its ultimate destination, and strategies shouldinclude intense modelling to overwhelm staining. This channelling willaccumulate dirt in sheltered unwashed positions, but more evenly.

The effect of weather on concrete may take the form of crazing or frostscaling, and the most vulnerable parts are those with laitance on the surface. Longterm appearance can be improved by exposing new rough-textured surfacescontaining a high proportion of inert particles, and the alternative approach is toconstrain the flow along pre-determined paths which will be visually rewarding.

CP 121 for walling provides a driving rain index for each of eight winddirections at 20 stations, while diagrams include details of copings to cavity wallsand recommendations for bonding, jointing and pointing. Tables offer suggestionsfor mortar mixes, methods of reducing cracking, exclusion of rain and damp-proofcourses. Rainwater must be evenly organized with attention to string course andcope; the minimum of run-off from flat roofs should be ensured by upstands atleast 200 mm high. Below windows it may be expedient to inisist on hiddengutters behind panels so that these weather evenly, while a system of concealedflow may be successfully conceived by the use of neoprene tubing.

Concrete is unpredictable in surface coloration even when every care is takenin its casting. An understanding of the value of curing is essential in preventingtoo rapid drying which can lead to dusting and a friable finish. One approach iscertainly the provision of vertical fluting at constant or variable spacings, withboldly expressed panels separated by distinct joints; the accent must be on patternand profile to control rain dispersal and hide its hallmarks. It is sometimesbeneficial to provide a separate, narrow feature at returns or corners near the topof a building, one side of which will be washed relative to the other dependingon the prevailing wind, but eye-catching by its contrast with the adjoining mainwalls.

Key words are detailing and diffusion, bearing in mind the consequential lossof face in more senses than one if salts are allowed to have their way. Weather insuch cases is not the discourse of fools; its graceless outcome must be sidetracked,while rain and the natural ingredients of the ground as the arch enemies must besensibly directed. Efflorescence is the end product of design and execution whichare less than adequate, but can be controlled if not altogether prevented. Ourgraffitic generation should at least try to eliminate it, for concrete so afflicted isnatures own vandalism; fortunately it can yield to scientific if not to psychologicaltreatment.

4.1CONCRETE QUALITY

It is crucial that the concrete should be as dense and impenetrable as possible asthe rate of salt attack is dependent upon the ease with which water can enter itssurface or move within its mass. Particular attention should be given to optimumcement content, proportioning of aggregates and workability consistent with fullcompaction. It must be remembered that concrete is quantifiable on paper butnot on site; the age at which it is put into use is another factor, as the material isvulnerable when still green, and external protective measures may be required inexceptional cases. To ensure minimum porosity and a hard, durable surface,particular care is required from the very beginning. A new understanding ofconcrete rheology is needed for a reduction in segregation and bleeding duringplacing and in its defacement from whatever cause. Quality control begins withthe selection of its constituents and ends with thorough external treatment.

4.2ADMIXTURES

Well-dispensed admixtures can reduce permeability with increased durability sothat rain penetration is reduced and colour retention achieved; good mix designand constant care are the basic guarantees against such contingencies.Airentraining or water-reducing admixtures and plasticizers will improveresistance to frost and to sulphates; workability aids assist in compaction and allow


lower water/cement ratios. Repellents reduce the passage of moisture bycountering capillary action, butyl stearate permits it to be better distributedthrough the matrix as an emulsion, while integral waterproofers can inhibit themigration of salts and lessen the need for cleaning. The subject is further coveredin the section on mortar.

4.3BLOCKWORK

A building should be dimensioned to suit the precast module aesthetically andpractically. Blocks should be carefully unloaded before laying, preferably above 4°C, and have joints of appropriate strength. Even units designed for facing workcan be penetrated by driving rain, but effective membranes and cavities dealsatisfactorily with excessive surface flow. Movement joints should be provided atchanges of height and wall thickness to anticipate expansion of floor slabs, and atall openings. Common defects include unfilled perpends, bonding patterns notmaintained, faulty lintel bearings and blocks built into a wall when excessivelywet. Flexible damp-proof materials should be stored away from heat, andprotected from damage by squashing.

Blockwork should be divided into rectangular panels, bed-joint reinforcementinserted in courses above and below windows, and slip-planes inserted betweenthe wall and a concrete roof. The units must be kept dry to minimize shrinkagequite apart from discoloration, and covers should not restrict the circulation ofair, as otherwise condensation may form. The blocks should be used in order ofdelivery, permit inspection and sampling, and must be protected fromsulphatebearing ground to avoid chemicals being drawn into them.

Precast elements may be contaminated on site or in the makers yard, anddifferent kinds should be kept apart. It is wise to stack them on a platform abovethe ground and to guard against corrosion of metals, frost action and change inmoisture content. Lintels should have matured and dried before being built intothe wall to prevent cracking at the ends due to shrinkage, while those cast in situshould be propped and allowed time to develop strength before carrying a load.Long units should be supported on damp-proof courses to permit movement.Care should be taken in the accurate setting out of the first course to avoidsubsequent inaccuracy of the superstructure, and all units laid on a full bed ofmortar with joints adequately finished. A careful study should be made of overalldesign concepts and the effective damp-proofing of walls subject to excessivesurface flow and rain ingress.

4.4CLADDING

The choice of cladding will be dictated by a combination of practical, economicand aesthetic factors. Concrete can vary considerably in its weathering

PREVENTION 19

characteristics and the designer should be aware of the likely changes in colour ofboth the units and their jointing. Conspicuous staining can be caused by unevenrain washing, particularly in association with glazed areas, and it should beanticipated that there will be some rain penetration and condensation within thecavity. Adequate provision for drainage and damp-proofing over openingsshould, therefore, be ensured, as any trapped water will find a way out, bringingsalts with it.

Cladding should normally be left to weather naturally, as cleaningsystems sometimes adversely affect appearance. The migration of salts may beprevented by coating adjoining brickwork with a waterproofing agent before thecladding is fixed, providing a cavity of width recommended in CP 297 and CP298. In certain cases efflorescence can be removed by a dilute acid followed bycopious washing, and there is evidence that it may initially be reduced oreliminated by the application of a silicone-based waterproofer. BS 3826recommends that waterrepellents should be applied to completed walls rather thanto individual building units to avoid inadvertent treatment of beds which willaffect mortar bonding, and to ensure that the faces of joints are also treated. Thesurface of precast units covered with silicone will give protection for some years.

4.5MORTAR

The main functions of mortar are to provide even load distribution, resist externalforces, release stresses that might otherwise crack the wall and control alignment.Without admixtures, the best compromise is 1/1/6 of cement/lime/sand,although a 1/2/9 mix may be used for lower strength lightweight blocks whenthere is no risk of freezing during construction. A mortar should be no strongerthan necessary for structural and durability purposes. Other properties such asworkability and rate of hardening are also important, although mortar need onlysupport the given load and have adequate weather resistance. Movements duringconstruction can be taken up by a slow-setting mortar with the minimum ofcracking and drying shrinkage, further reduced by the use of air-entrainingplasticizers. A mix which is too rich may result in porosity of the jointing byhairline cracks which encourage penetration, whilst one which is too wet maysoak the building units, unnecessarily promoting efflorescence by increasingmoisture movement and dangerous if frost is expected.

Most plasticizers contain an additive which improves the bond to high suctionblocks. Masonry cements consist of Portland clinker and a small proportion of air-entraining agent, together with a mineral filler which increases cohesion withoutundue water demand. These cements are usually lighter in colour than normaland have lower shrinkage and a better resistance to freeze/thaw conditions.Movement of excess sulphate-laden water from the mortar into blocks or brickswill occur if the units are very dry at the time of laying, but on the other hand


prior soaking may lead to loss of adhesion, so that the right balance must besought.

Mortars containing admixtures can be more durable, assist in pigmented colourretention and have a water-retaining capacity which provides a tighter bond andprevents passage of moisture. By this means re-wetting of the mortar to keep itworkable is unnecessary, and greater plasticity allows a longer spread. Someadmixtures are supplied in liquid form ready for use, with less water needed forgauging, no difference in colour and the absence of chemical action. A mix of 1part cement to 6 parts sand, plus a plasticizer, will improve resistance to rainpenetration and frost attack, but in exposed conditions the cement content shouldbe increased by richening to 4½ parts sand.

Efflorescence forming round the edges of building units suggests that these haveabsorbed water from the mortar before it hardened and that its constituents havebeen converted into alkaline sulphates. There must be a happy balance betweenthe porosities, but always consistent with overall durability. Even when nomoisture has been absorbed from mortar during laying, these compounds may stillbe transferred to the blocks if the partly finished wall is left unprotected duringrain, resulting in the formation of white bands. A similar effect is seen if the unititself is impermeable but the mortar is less so.

The chief danger to brickwork from the presence of salts is that of sulphateattack, showing itself as expansion of the mortar or horizontal cracking of therendering which becomes hollow and falls away. If the wall has to be rebuilt andsalts are likely to persist, the mortar should be made with sulphate-resisting cement,but the basic aim is to prevent further attack by the provision of a damp-proofcourse at all levels. Unnecessarily strong mortars can lead to wide cracks, while aweaker but still durable mix yields sufficiently to accommodate small movementsin a wall, with distribution into cracking which is less permeable. Mortar made onsite should preferably be mixed by a machine which is regularly cleaned to avoidcontamination, or if by hand in small quantities on watertight platforms. Pointingis also significant as a drying surface, for when it is denser than the mortar,evaporation at the joints will be restricted. Calcium chloride is not effective inprotecting from frost by heat evolution, and may lead to dampness and corrosionof wall ties.

4.6JOINTS

The greater use of thin easily assembled concrete units, the more the need toconsider the dangers of water ingress. Wall thickness no longer overcomes thenatural absorbency of the material or its constituent parts, so that jointing needsvery special attention. Ideally a feature should be made of all joints and, althoughgrooves may give rise to streaking, the division of large areas into panels ofappropriate size will distract sufficiently by their composite appeal. When precastpanels are being fixed, accurate alignment to agreed tolerance is essential so that

PREVENTION 21

there is minimum interruption to the flow of rainwater. The performance ofsealants is important, as the failure of mastics in expansion joints by loss ofadhesion or by restraint must be avoided. If, in spite of precautions, a light depositdoes appear, there is obviously faulty damp-proofing or drainage. No remedialaction will hide a poorly made junction between lifts, and this is best attained bycasting against a slightly roughened face which is dry and not treated with apriming layer of mortar. Contraction joints may also present a visual problem inthat they will appear as strongly marked lines and must be regularly spaced andbear some relation to aspects of the elevation such as sills and lintels. Sometimes asmall lip is left which, if not buffed off, can divert the flow of water to result inundesirable staining.

4.7PIGMENTS

Pigments minimize surface differences, contribute visual interest and should beregarded as inert fines. Sufficient intensity of colour is obtainable by a smalladdition, although consistency can be affected by differing vibration, leadingto local concentration. Moreover, it will in time be masked by soot deposits andshould, therefore, be secondary to form, texture and three-dimensional featurescapable of dictating water paths. In coastal areas it may be advisable to specify ashade lighter than would otherwise be chosen, as sea salts play a role in thedisfiguration of darker surfaces.

4.8DETAILING

New concrete has a lightness of tone which emphasizes differential grime, towhich efflorescence will make its own contribution if the surface is ill-conceived.Detailing cannot be dismissed as irrelevant and there has justifiably been a long-standing practice of capping which, for good measure, gives a strong outline.Adequate throating will throw the water clear and although it will probably blowback at a lower level, it will at least be scattered and less pronounced.Consideration must be given to the relationship of all the materials and featuresmaking up the structure, with particular emphasis on openings and projections;although forming a pattern themselves, their individual treatment is crucial, aseven a small crack can cause discoloration if it is fed by rain on a large area ofglazing. Care must also be taken to anticipate the impact of contaminated salts, airpollution and the general susceptibility of wallings, rendering and backings, beforedeciding on suitable protective measures.

Traditional units were smaller than the present day concrete monoliths and,while each element varied slightly, the overall effect was often strong enough tosubdue weather marking. The modern preference for cement in mortar has alsoupset this balance. Lack of attention to the disciplines of practical construction


using sensitive detailing has led to some criticism of concrete, especially in thecurrent quest for new shapes and finishes which are not always proof against therigours of weathering. For example, a mastic joint would allow for slightmovement to prevent ingress and it may be advisable to specify sulphate-resistingcement where chemically vulnerable in wetter districts. There is often a finelybalanced combination of direction and duration of rain with variable evaporationand in all prior assessment of this movement, salt-laden or otherwise, previousexperience of any locality is invaluable and salutary. Such a study should beimplemented by a consideration of all surface projections and by the provision ofmembranes below ground floors and around foundations. BS 4315 lists methods ofmeasuring resistance to rain penetration by recording the increase in area ofdampness and amount of leakage. Transfer of dirt from a horizontal to a verticalsurface can be avoided by raking backwards, while reveals may be recessed andsills inclined to grooves discharging into a drainage pipe. Rain splashings frompavements can often contribute to the disruption of plinths, and there is a case forpolished granite in this situation.

4.9DAMP-PROOFING

Damp-proof courses should be inserted just above ground level and belowoverhangs, with flashings of impervious sheeting firmly bedded and provided withadequate lapping to cover any intersection or vulnerable joint. Copings shouldbe specified for walls to throw rainwater clear of the faces, well-dripped andprevented from displacement. The aim is to keep any form of moisture out of thestructure and to reduce concrete shrinkage; it is necessary to recognize the effectof leakage other than that due to cracking, and to foresee all points of ingress.

4.10FORMWORK

The performance of a concrete face is largely conditioned by formwork. Its effecton discoloration arises from variations such as lining absorption, displacement ofrelease agent and workmanship during and after compaction. The form should bewell supported to ensure rigidity in countering excessive vibration; lack of abilityis sometimes evident along bridge parapets where a regular pattern coincides withvertical supports, and this can be aggravated by carelessness at the erection andremoval stages.

After striking, a carbonation layer is established on exposure to air; the moreporous the concrete the thicker the layer will be. The surface of structures to bebackfilled should, therefore, be left for a few days to develop this protective skin.Free water on a newly cast, non-carbonated surface is undesirable and it is often adisadvantage to strip the shutter at an early age, quite apart from mechanicaldamage caused by premature removal. Recommendations as to winter concreting

PREVENTION 23

are given in CP 110 where a strength of 5 N/mm2 is suggested at a concretetemperature not less than 5°C, and striking times are tabulated in CIRIA Report36. For high quality fairfaced concrete the goal is a surface as struck without theneed to make good. The number of uses should be restricted, all fixings appliedfrom behind and the holes set out to an agreeable design. Joints in the shuttershould be well taped to avoid grout runs spoiling the lift below, while releaseagents should be carefully selected and consistently applied.

4.11RENDERING

Rendering mixes should be weaker than the surfaces to which they are appliedand each successive coat no stronger nor thicker than its predecessor. New workshould be kept damp for the first three days and the second coat delayed until thefirst has hardened. Although slight watering may be necessary to reduced suction,free moisture should never be left on the surface, and suitable admixtures may beused to reduce loss of water and improve workability. Dense coatings such as 1/3cement/sand should not be applied to walls with appreciable sulphate content andin danger of shrinkage cracking. If so damaged, the facing should be stripped offand the brickwork allowed to dry before re-application with a more porous mixof 1/1/6 cement/lime/sand which permits readier evaporation, and using asulphate-resisting cement for both rendering and jointing mortar.

4.12URBAN ENVIRONMENTS

The designer in an urban area should regard pollution as inevitable. The extent ofsulphur dioxide in the air can be regrettably high and to offset this, colouredconcrete or exposed aggregates should be specified. The cement content ofall facings must, of course, be consistent with durability. The other key factors inassessment of any kind are location, aspect, severity of climatic conditions and theeffects of aggressive salts. While one approach may lie in cleaning the buildingregularly, this will accentuate the dirt but mask a light deposit. A hopeful maximis that the rougher a wall the less the signs of staining, so that it may be better tospecify a coarse-textured dissipating surface.

Discoloration of concrete can be caused by chemical changes in adjacentmaterials or emerging deposits washed by rainwater from windows above,however much these openings divert from its overall pattern. Backward slopingsurfaces have a tendency to streak because of partial water flow, while forwardslopes may collect silt at their bases. Ideally the elevation should be modelled toattract this in areas of greater shadow, if by so doing the other parts are keptcleaner. The art of concealment and prevention should be practised, withattention given to extreme weather attack on exposed faces or relativelyunwashed alcoves and returns.


Concrete is generally less permeable than traditional materials, but itsabsorbency is not uniform and it colours unevenly. No building is self-cleansingand smooth surfaces tend to weather in the manner dictated by rainfall alongrandom paths. The remedy is to influence this movement by regulating its sequenceor to scatter by the use of say abrasive blasting. The striations of a ribbed profilehide imperfections, and when the edges are hammered off they assume theappearance of hewn rock. Streaking may only occasionally be acceptable as, forexample, on the soffit of bridges, but even this can be objectionable where it isobviously the result of poor design.

When considering surface permeability it should be remembered that calcinedflints are themselves minutely crazed in baking and absorb water in hairlinecracks, while cement paste between fine aggregate particles will also absorb unlessconscientiously cured. A good concrete can be marred by dark patches with alower water/cement ratio than the body of the wall, and it is difficult to ensurecomplete uniformity over large smooth areas. It may be advisable to emphasizeshadow effects and even the lifts themselves without excessive obtrusion. Theeffect of disfiguration can also be reduced by planning contrast between adjacentfaces using panels alternating in colour and profile. If the concrete is at risk fromchemical or corrosive attack at the rear face it would be sensible to coat with awaterproof backing, well-bonded and flexible enough to inhibit cracks or faultyjoints through which salts might filter.

4.13SURFACE TREATMENT

The specified requirements for concrete surfaces to suit extreme weathering arestringent, but must be offset against the cost of cleaning or repair. Efflorescence isonly one form of tarnish in a range from scaffolding rust to grout spillage but,unlike others, is often an in-built irritation. It may be necessary to paint thesurface after careful preparation, as has profitably been done on motorways, wherediscoloration is offensive even at speed, but concrete can be restored to somethingapproaching its original appearance. Painting may be considered for aestheticreasons or for easier run-off. The colour scheme should be wellbalanced using themany shades available, but their number kept to a minimum for any givenproject.

Another approach is to use transparent coatings such as silicones which fill thepores as water-repellents but can result in subdued evaporation and the possiblebuild-up of disruptive crystalline growth under the surface. Such coatings shouldnot be used to improve the durability of the concrete if already suspect by virtueof salt action. A soluble silicate in powder form may give reduced permeabilityand protection against the creation of further crystals in the pores, but thisassumes good mix design and quality control.

All walls containing cementitious products are vulnerable to alkali attack andeither non-saponifiable paints or alkali-resistant agents may be employed.

PREVENTION 25

Emulsions are also valuable in that they are formulated with polymers of similarproperties; when used for rendering they should be applied thinly and shouldpenetrate the surface rather than create a glossy film to which paint may notadhere. Efflorescence can be difficult to scrape off and may disrupt an imperviousdecorative layer, but can be covered over if a suitable primer is applied.

Plate 5: The result of not extending the exposed aggregate panels to cover the edge beam.Efflorescence in the lower part has been dissipated by the bold rounded aggregate. There hasbeen failure to ensure a satisfactory bond between slab and beam, and to prevent ingress ofwater. Another approach would have been vertical fluting instead of concrete off a plainshutter.


Plate 6: This illustrates the failure to prevent efflorescence at the junction of otherwiseattractive panels by a suitable mortar mix, with the resulting entry of water from the deck.

PREVENTION 27

5REMOVAL

5.1WASHING AND BRUSHING

Feathery or fluffy efflorescence should be allowed to dry out before removal witha bristle brush; even if glossy and encrusted it is often possible to eradicate byscraping when it is not strongly keyed, first softening the deposits and thendiscarding them before washing the surface with water. This should preferably becarried out by men skilled in the procedure and with a knowledge of localconditions. If some of the salts soak back into the wall and re-appear, the sametechniques should be adopted until their movement ceases or they at least becomeacceptable. Continued physical treatment, vigorous or otherwise, demandssensitive workmanship to avoid damage, and restoration of important buildingsshould only be undertaken by specialist firms. Before dealing with an extensiveoutbreak of efflorescence, it is advisable to consider the effect of cleaning a smallsection which may show up another. It is better to leave certain areas until theconcrete is, say, a month old to let nature take its course. In any case, little can bedone to remove hydration discoloration which is a more deep-seated contrast inshade.

While efflorescence can often be removed by water only, and possibly withjetting, any treatment aimed at suppressing evaporation may lead to crystallizationbelow the surface, entrapment in the pores and spalling. In sheltered spots,deposits not reached by rain can sometimes be removed by repeated washing. It isimportant to remember that dirt at any point not only affects appearance but actsas a reservoir for harmful chemicals and hides evidence of decay. All may bedissolved in due course by natural means, but if it is too tenacious and the timefactor important, other remedies must be applied.

Particular care should be taken with structures of architectural or historicalimportance when cleaning by mechanical means such as grit-blasting and grindingor buffing discs, always guarding against dust if the surface contains free silica. If indoubt it is better to use hand tools, non-ferrous brushes or abrasive blocks ratherthan rotary power unless the staining is deep-seated and widespread. Wirebrushing should only be applied in extreme cases and never to sand-faced surfaces

likely to be damaged by attention of this kind. If cleaning by water spray workingfrom the top of the building, it should be directed only in sufficient quantity tokeep the deposits moist until they soften, generally in a few minutes. They maybe amenable to pressure hosing, supplemented by careful and timely brushing, adisadvantage being possible leakage into the actual building, although moderntechniques have made this unlikely. Operations should be restricted to those areasgiving most offence, but at the risk of blotching, as attempts to fade out are rarelysuccessful.

5.2ACIDS

If brushing is unsuccessful, treatment with a 5–10% solution of hydrochloric acidis normally effective. The surface would first be wetted down and the acid appliedwith a soft brush or broom. Then, as soon as the deposit has dissolved andchemical action ceased, the area should be washed thoroughly, starting fromabove and ensuring that galvanized and aluminium metalwork is protected. Forintegrally coloured concrete a more dilute 2% solution will prevent unduepenetration which may otherwise expose the aggregate in places and radicallychange the texture. Again the face should be dampened beforehand to preventabsorption of the acid. In all cases a trial should be carried out on aninconspicuous part, applying the solution to areas about a 1 m2 or less, waiting afew minutes before removing the salt with a stiff brush and cleansing copiously. Inthe main operation, entire walls should, if necessary, be treated to avoidaggravating the situation.

Among other forms of agent are those containing hydrofluoric acid, leavingbehind no soluble chemicals but running the risk of unsightly etching ofwindows. Also recommended for cement-based surfaces is the application of a10% solution of phosphoric acid which converts lime to an insoluble compoundbefore suitable priming and painting. If the wall cannot be obtained in a drycondition it must be covered with a porous alkali-resistant coating, ensuring thatconditions of heat and ventilation are good. Although proprietary materials areavailable for the removal of mortar stains, there is no chemical which willcompletely inhibit the effects of a saturated salt solution, even if the wall is sealedby silicone-based repellents. A typical cleaner, stored in plastic or rubbercontainers, is applied by brush and immediately washed. Coverage of this mildacid is about 20 m2 per 5 litres depending on surface porosity and the thicknessand extent of deposits. It will also deal with algae growth and normal atmosphericstaining, and there is no residual chemical reaction when it is used either neat orin equal parts with water. However, in the first place the use of a softer brush anddilute solution over a restricted area is recommended and the surface should bequickly washed down, as indeed is good practice after every application of thiskind, whatever the concentration. At joints, while stains caused by leaching fromfresh mortar or soaked blocks can also be removed by dilute acid, it may be


advisable to paint the surface of rendering rather than attempt to clean it if thedeposit is difficult to eradicate or likely to re-appear.

Corrosive substances should be handled with rubber gloves, and goggles andother protective clothing worn. Care must also be taken to wipe splashes off theskin with bicarbonate of soda or at least soap and water. Above all, it is bestto seek expert advice and to engage specialist operatives, as acids can permanentlyscarify and must be treated with respect. The cleaning of a building should besensibly planned, including safety measures, and a study made of all availablemethods relative to the extent and origin of the stains.

REMOVAL 31

6CONCLUSION

Discoloration of concrete surfaces, however transient, can present a complexproblem to designer and contractor alike in its cause and prevention. The degreeof disfigurement is determined by vagaries in weather and workmanship butinitially results from lack of knowledge on how best to disperse rainwater over astructure. Many blemishes arise from defective detailing, damp-proofing andconstruction joints as the crucial factors, whilst localized staining can be caused byvariation in porosity.

It is often difficult to prevent the movement of salts which pass into solutionand on evaporation become more concentrated into solids which are superficialbut unsightly; while porous materials are more likely to bring these to the surface,there can be disruption if evaporation is inhibited. All elements of a building shouldbe selected as to minimum salt content, and saturation prevented by sensiblestacking and protection of work as it proceeds. When rendering a wall, too muchsand or excessive trowelling should be avoided. Unduly strong jointing mortarcan lead to fine cracks, through which the alkaline solution may filter and leaveits mark.

The answer also lies in breaking up the surface by attractive profiling or theimaginative use of panels and exposed aggregate. Above all, care should be takento ensure that the concrete is of high quality and reliable durability, with attentiongiven to grading of the mix, optimum cement content and effective curing.

Plate 7: A close-up view of a parapet where fluting has certainly helped to break up thesurface and direct the flow of rainwater down the grooves at varied centres, but the featurehas not been taken down to the foot of the wall and has failed to prevent correspondingstains on the lowest quarter of the face.


7BIBLIOGRAPHY

VIEWPOINT PUBLICATIONS

The following are available from Eyre & Spottiswoode Publications Limited,Distribution Centre, George Philip Services Limited, Arndale Road, Wick,Littlehampton, West Sussex BN17 7EN.

TATTERSHALL, G.H. The workability of concrete. 1976, 138 pages, A5. Order No: 11.008RICHARDSON, J.G. Concrete notebook. 1974, 92 pages, A4. Order No: 12.063RICHARDSON, J.G. Formwork construction and practice. 1977, 275 pages, A4. Order No:

13.019RICHARDSON, J.G. Formwork notebook. 1982, 2nd edition, 120 pages, A4. Order No:

12.082

CEMENT & CONCRETE ASSOCIATION

The following are available from the Publications Sales Unit, Cement & ConcreteAssociation, Wexham Springs, Slough SL3 6PL, Berkshire.

42.313 The effect of weather on the formation of efflorescence

42.380 Concrete surface blemishes

42.384 The influence of concrete mix proportions and type of form face on theappearance of concrete

42.414 Construction joints in concrete

42.467 A survey of literature relating to the properties and use of concrete blocks

42.480 The crazing of concrete

42.493 An investigation into the incidence of colour variation in formed concretesurfaces

42.505 Mechanical damage to concrete by early removal of formwork

45.007 Winter concreting

45.012 The determination of proportions of aggregates

45.013 Concreting in hot weather

45.015 Chemical methods of removing stains from concrete

45.016 Impurities in aggregates for concrete

45.020 Controlling algae and other growths on concrete

45.022 Air-entrained concrete

45.030 Superplasticizing admixtures in concrete

45.031 Concrete mixes for general purposes

45.034 Concrete in sulphate-bearing ground

Man on the job leaflets

45.101 Cements

45.102 Aggregates—delivery and storage

45.103 Testing aggregates

45.104 Concrete admixtures

45.105 Reinforcement

45.106 Batching and mixing concrete on site

45.107 Transporting and pumping concrete

45.108 Placing and compacting concrete

45.109 Curing concrete

45.110 Making good and finishing

45.111 Tooling concrete

45.112 Testing for workability

45.113 Concrete test cubes

45.114 Construction joints

45.115 Formwork

45.116 Ready-mixed concrete

45.117 Dry lean concrete

45.118 Concreting in cold weather

46.001 Concrete finishes for highway structures

46.019 The appearance of concrete highway structures

46.504 Watertight concrete construction

47.008 Abrasive blasting of concrete surfaces

47.010 Specification for high quality finishes

47.018 Striated finishes for in situ concrete

47.020 The curing of concrete

47.101 Visual concrete—design and production

47.102 External rendering

47.103 The control of blemishes in concrete

36 BIBLIOGRAPHY

47.104 Efflorescence on concrete

48.010 White concrete

48.037 Concrete practice

48.041 Precast concrete cladding

48.043 Model specification for concrete blockwork

94.017 The brighter face of concrete

REPRINT 1/80 Structural concrete finishes: A guide to selection and production

CATALOGUE 1982: Part 1—Publications, slide sets and films

Part 2—Research, development and technical reports

BUILDING RESEARCH ESTABLISHMENT

The following are produced by the Building Research Establishment of theDepartment of the Environment, and are published by H M Stationery Office.

BRE Digest No 45: Design and appearance—1

No 46: Design and appearance—2

No 54: Damp-proofing solid floors

No 77: Damp-proof courses

No 85: Joints between concrete wall panels: open-drained joints

No 113: Cleaning external surfaces of buildings

No 125: Colourless treatments for masonry

No 127: An index of exposure to driving rain

No 137: Principles of joint design

No 160: Mortars for bricklaying

No 176: Failure patterns and implications

No 177: Decay and conservation of stone masonry

No 196: External rendered finishes

No 237: Materials for concrete

No 244: Concrete mixes

No 245: Rising damp in walls

No 250: Concrete in sulphate-bearing soils

No 255: Index of digests

Current Paper 23/77: Chemical resistance of concreteInformation Paper 6/81: Carbonation of concrete made with natural aggregatesBRE Information Directory 1982HMSO Sectional list No 61: Construction 1982

BIBLIOGRAPHY 37

BRITISH STANDARDS INSTITUTION

The following British Standards and Codes of Practice are published by theBritish Standards Institution, 101 Pentonville Road, London N1 9ND.

BS 12: 1978 Ordinary and rapid-hardening Portland cement

BS 743: 1970 Materials for damp-proof courses

BS 882: 1975 Aggregates from natural sources for concrete

BS 890: 1972 Building limes

BS 1014: 1975 Pigments for Portland cement and Portland cement products

BS 1180: 1972 Concrete bricks and fixing bricks

BS 1198: 1976 Building sands from natural sources

BS 1199: 1976 Sands for external renderings

BS 1200: 1976 Sands for mortars

BS 2028, 1364: 1968 Precast concrete blocks

BS 3826: 1969 Silicone-based water repellents for masonry

BS 3921: 1974 Clay bricks and blocks

BS 4315: Part 2: 1970 Methods of test for resistance to air and water penetration

BS 4551: 1980 Methods of testing mortars and specification for mortar sand

BS 4887: 1973 Mortar plasticizers

BS 5075: 1982 Concrete admixtures

BS 5262: 1976 Code of practice for external rendered finishes

CP 110: 1972 The structural use of concrete

CP 111: 1970 Structural recommendations for loadbearing walls

CP 121: Part 1: 1973 Brick and block masonry

CP 297: 1972 Precast concrete cladding (non-loadbearing)

CP 298: 1972 Natural stone cladding (non-loadbearing)

YEARBOOK 1983 Summaries of British Standards

ADDITIONAL BIBLIOGRAPHY

PERKINS, P.H. Concrete structures: repair, waterproofing and protection. Applied SciencePublishers Ltd., London, 1977.

GAGE, M. & KIRKBRIDE, T. Design in blockwork. Architectural Press Ltd., London,1980

WILSON, J.G. Exposed concrete finishes. Vol 1—Finishes in in situ concrete and Vol 2—Finishes to precast concrete. G R Books Ltd., London, 1962.

GAGE, M. Guide to exposed concrete finishes. Architectural Press Ltd., London, 1970NEVILLE, A.M.and CHATTERTON, M. New concrete technologies and building design.

Pitman Publishing Ltd., London, 1979.

38 BIBLIOGRAPHY

ADDLESON, L. Materials for building. Vol 3—Water and effects. Iliffe, London, 1972.BUILDING RESEARCH ESTABLISHMENT. Principles of modern building. Vol 1—The

building and the wall, 1975 and Vol 2—Floors and roofs, 1977. HMSO, London.CONCRETE SOCIETY. The weathering of concrete. Symposium, London, 1977.CONCRETE SOCIETY. Guide to chemical admixtures. London, 1980.SPECIFICATION. Building materials and products. Architectural Press Ltd., London, 1982.SEALANT MANUFACTURERS CONFERENCE. Manual of good sealant practice.

London, 1976.MARSH, P. Air and rain penetration of buildings. Construction Press Ltd., Lancaster, 1977.

BIBLIOGRAPHY 39

Plate 8: Good example of fluting and tiling on a self-cleansing facade.

40 BIBLIOGRAPHY

8APPENDIX: CHECKLIST TABLE

42 APPENDIX: CHECKLIST TABLE

APPENDIX: CHECKLIST TABLE 43

efflorescence and the discoloration of concrete (1983)

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