destructive tests

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.Construction and Building Materials 15 2001 8192

Reliability of partially-destructive tests to assess the strengthof concrete on site

J.H. BungeyU, M.N. Soutsos

Uni ersity of Li erpool, Department of Ci il Engineeering, Brownlow Street, Li erpool L69 3GQ, UK

Abstract

The range of available tests for assessing the strength of insitu concrete based on measurements of surface zone properties isexamined, together with developments in supporting documentation. Attention is concentrated upon a number of recent researchprogrammes, including work undertaken as part of the European Concrete Building Project. These focus primarily upon pull-outand pull-off techniques, and encompass applications to early age strength assessment, lightweight and high strength concretes,and testing of repairs. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Concrete; Early-age; In-place; Near-surface; Partially-destructive, Strength; Testing; Variability

1. Introduction

There has been recognition of the need to testin-place concrete to estimate strength for more than 70years, but it is in the last 30 years that the mostsignificant developments of commercially available sys-tems have occurred. Dozens of techniques have beenproposed in an attempt to overcome the problems ofdamage and time associated with cutting, preparationand testing of cores.

Whilst acceptance testing of concrete has traditio-nally involved compression testing of cylinders or cubes,

these may not adequately reflect the compaction orcuring received by the insitu concrete and, of necessity,relate only to a small proportion of the concrete placed.In-place testing may be used to overcome some ofthese difficulties. In the majority of current applica-tions, however, testing is for one or two reasons:

. Evaluation of an existing structure new or old .

UCorresponding author. Tel.: q44-0151-794-5238; fax: q44-0151-

794-5218. .E-mail address: [email protected] J.H. Bungey .

Monitoring strength development during new con-struction.

Use of in-place tests for acceptance is, at present,limited but increased awareness of the importance ofthe surface zone in terms of durability has stimulatedinterest.

A number of test methods have become established,and standardised either in Europe or North America,which involve the measurement of a surface zoneproperty which can then be related to strength bymeans of an appropriate correlation. They typically

cause localised surface damage which, although lessthan that associated with cores, must be considered atthe planning stage. Consequently the term partially-destructive is sometimes used to describe these meth-ods.

Research at the University of Liverpool has focusedover recent years upon the interpretation of resultsprovided by a number of these methods, includingcorrelation procedures, and taking account of within-member strength variations. This work has encom-passed both laboratory and field studies and has con-sidered the suitability of techniques for:

0950-0618r01r$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. .PII: S 0 9 5 0 - 0 6 1 8 0 0 0 0 0 5 7 - X


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( )J.H. Bungey, M.N. Soutsos r Construction and Building Materials 15 2001 819282

Early age testing. Use with normal concretes with a range of aggre-

gate types. Use with Lightweight and High Strength concretes. Testing repairs.

The scope of these investigations and key findingsare considered in this paper, together with futureprospects for this group of techniques.

2. Available tests and documentation

Principal test methods are described and discussed inw xdetail by the principal author 1 and the American

w xConcrete Institute 2 together with other related tech-niques. Surface zone insitu strength tests which arestandardised in the USA and UK, are listed in Table 1

together with an indication of key features, whilst thoseforming the basis of this investigation are briefly de-tailed below. Standards for many of these techniquesalso exist in other countries. It should be noted thatbreak-off methods have received little attention in theUK and are not considered in this paper.

2.1. Rebound hammer

This simple test is probably the most widely knownand most commonly used of all the test methods. Itinvolves an assessment of localised surface hardness by

means of a hand-held mechanically operated device,although electronic digital-recording versions are avail-

w xable 3 . Energy levels are relatively low and this methodis very sensitive to a large number of factors whicheffectively limits its use to comparative applicationswhere it is a valuable preliminary to other forms of

testing.

2.2. Probe penetration resistance

Measurements are made of the depth of penetrationof a metal rod or pin that is forced into the surface ofthe hardened concrete by a driver unit. This is availablein two basic forms:

Pin-penetration test in which a 3.56-mm diameterhardened steel pin is driven by a spring-loadeddevice to create a hole up to 7.6-mm deep in the

w xmortar. This is not recommended 2 for compres-sive strengths above 28 MPa. Probe-penetration test, commonly known as the

Windsor Probe test in which a larger rod 6.35-mm.diameter for normal weight concrete is fired from a

specially designed gun using a smokeless powdercharge producing a constant energy level. Penetra-tion may be up to approximately 40 mm dependingupon the strength of the concrete, and correlationsto compressive strength are known to be influencedby aggregate hardness. Measurements are usuallybased on the exposed probe length.

Table 1Standardised test methods

Method Standards Principal features

ASTM BS 1881

Rebound hammer C805 202 Existing concrete, best usedcomparatively.

Probe penetration C803 207 Existing concrete, pin or probe.Strength correlation specific toaggregate.

Pull-out

Internal Fracture 207 Existing concrete, high variability. .Cast-in C900 207 New construction Lok-test -general

strength correlation possible. .Drilled C900 207 Existing concrete Capo-test care

with drilling and underreaming.

Pull-off 207 Existing concrete, surface orpartially cored, care with bonding,strength correlation specific to mix.

.Break-off C1150 207 New construction formed or .existing concrete drilled , strength

correlation specific to mix and testtype.


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( )J.H. Bungey, M.N. Soutsos r Construction and Building Materials 15 2001 8192 83

Fig. 1. Probe penetration test.

Theoretical analysis is complex and relates to theabsorption of the initial kinetic energy by the concrete.No rigorous studies have been reported, but it is gener-ally accepted that most of the energy is absorbed by acone-shaped fracture zone as illustrated in Fig. 1. Theremay be a risk of cracking slender members and safetyregulations may apply.

2.3. Pull-out

Measurements are made of the force required to pullan embedded metal insert with an enlarged head froma concrete surface. The pulling load is applied by atension jack bearing against the concrete surfacethrough a reaction ring concentric with the insert.Failure involves the fracture, and often removal, of anapproximately cone-shaped portion of concrete. Acommon feature of pull-out tests is that correlationwith compressive strength is relatively insensitive tomixture characteristics such as cement and aggregate

.type except lightweight , size and proportions. Theyare thus, particularly useful when assessing a concreteof unknown composition.

Several forms of this type of test have been devel-oped:

Internal fracture test developed in the UK in whicha 6-mm diameter expanding wedge anchor bolt isinserted to a depth of 20 mm in a predrilled hole asshown in Fig. 2. A torquemeter device may in thiscase provide the pulling force. The method wasdeveloped specifically for testing existing slender,

prestressed concrete elements and although theequipment is inexpensive and relatively quick touse, variability is high.

Lok-test, which has emerged as the most commonlyused technique involving an insert placed in theconcrete surface during construction. This may befixed to formwork or a flotation cup and the headwill typically be located 25 mm below the surface.The basic configuration is shown in Fig. 3. Preplan-ning is obviously required, thus, restricting themethod to new construction. Many studies havebeen undertaken to analyse theoretically the mech-anisms associated with this test, but there is no firm

Fig. 2. Internal fracture test.

agreement concerning that governing the ultimatefailure load. It is generally recognised however, thatwell-defined experimental correlations can be es-tablished with compressive strength. This method isparticularly suitable for strength development mon-itoring.

Capo-test has been developed as a version of theLok-Test that can be applied to existing concrete.Drilling and under-reaming operations are requiredto provide a groove into which a compressed steelring can be expanded to provide a similar testconfiguration, see Fig. 4. The load is applied bymeans of a similar jack, but the test is obviously

w xslower to perform. It is claimed however, 4 , thatidentical correlations with compressive strength maybe utilised.

2.4. Pull-off

This measures the direct tensile force required topull a metal disk, together with a layer of concrete,from the surface to which it has been bonded. Where itis desired to avoid surface effects such as carbonation,the failure may be caused at some depth below thesurface by partial coring, see Fig. 5. This method isparticularly suitable where a tensile strength value isrequired, such as testing bonding of repairs, but may be

.Fig. 3. Pull-out Lok test.


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.Fig. 4. Pull-out Capo test.

correlated empirically to compressive strength for aparticular mix. Many forms of this test are available

w xcommercially 3 but that most widely used in the UK is

the Limpet system developed at Queens University,Belfast.

2.5. Temperature measurement methods

Although outside of the scope of this paper, atten-tion must be drawn to the maturity method ASTM C

.1074 in which insitu strength may be estimated on thebasis of a time and temperature maturity function for aspecific concrete mixture. This is particularly suitablefor strength estimation at early ages and most com-monly utilises a temperature sensor located at an ap-

propriate position within the pour, although chemically .based inserts COMA-meter are also available. In theUK, attention has also been paid to temperature

.matched curing BS 1881 pt 130 , in which cubes arestored in a water-tank whose temperature is controlledby a sensor in the pour to ensure an identical tempera-ture history to the insitu concrete. Both of these meth-ods suffer potential practical disadvantages including

w xvulnerability to power supply, failure or vandalism 5 ,but may be particularly useful when used in combina-tion with other tests, the pull-out method in particular.

Fig. 5. Pull-off test.

3. Strength development monitoring

Two major programmes involving field testing havebeen undertaken to assess the suitability of differenttest methods for early age testing.

3.1. Cooling tower study

This was aimed primarily at cooling tower construc-tion with site work at Drax in North Yorkshire, sup-ported by extensive laboratory work. The objective wasto enable formwork to be removed with confidence,even in cold weather, to enable construction schedulesto be maintained. These studies demonstrated thatsurface hardness tests are unreliable at early ages,whilst ultrasonic pulse velocity measurements can yieldgood strength estimates, but usage is usually limited bythe need for access to two opposite faces for reliable

.measurements. Penetration resistance Windsor Probeis quick and suitable for large members such as slabs,but was shown to be unreliable at low strength values.Internal fracture tests are similarly unsuitable at earlyages because of their high variability. It was concludedw x5 that pull-out testing, maturity testing and tempera-ture-matched curing are the most reliable and practica-ble techniques for use at low strength levels, althoughit should be noted that pull-off tests were not con-sidered in this programme.

.Pull-out Lok tests were conducted through cut-outpanels in the timber shutters on 175-mm thick wallpanels at ages commencing at 15 h . On site, tests were

performed in-groups of three within 400=250-mm re-movable formwork panels. This dimension was con-strained by formwork design considerations and limitedthe clear spacing between inserts to 165 mm. This isless than the minimum recommended by standards, buttrials indicated that at the low strength levels associ-ated with such tests no significant influence was de-tectable. In the laboratory, wall panels 1.5 m high and2.0 m long with the same thickness, and identicalreinforcement were fabricated. In this case pull-outtests were made through individual circular cut-outpanels at each test point.

Correlation tests were made on 225-mm cubes togive adequate edge distances, in conjunction with 150-mm cubes for crushing at strengths as low as 1 MPa.Tests were performed on all six faces of the 225-mmcubes, and analysis showed that mean values werevirtually identical to those obtained if the top andbottom surface tests were discarded. Variability withinthe group of results was however, greater than whenjust the four side faces were considered. Correlationswere achieved by testing the gravel aggregate mixturesat varying ages, and by varying the mixture designs.Results of these tests are shown in Fig. 6a in whicheach correlation point represents the average of six


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. .Fig. 6. a Laboratory pull-out strength correlation. b Site andlaboratory wall pour pull-out results.

Lok-tests and three cube compressive strengths. Forstrengths up to 10 MPa, all points lie within 1 MPa ofthe mean line, but at higher strengths the sensitivity ofthe pull-out force to changes in cube strength is re-duced and scatter increases. A further feature ofconcern was that the time taken to carry out a test atvery low strengths might be significantly less than thatspecified by standards. This was addressed by compara-tive tests on 225-mm cubes which indicated that neither

magnitude of force nor variability will be affected pro-vided the load rate is such that the test exceeds 20 s.

Wall-pour results shown in Fig. 6b represent theaverage of six Lok-tests together with cube strengthvalues estimated from maturities computed from ap-propriately-located temperature sensors using a tem-

w xperature-time factor with y11C datum 2 . The gener-ally close agreement between both laboratory and sitewall pour results and correlation tests, especially atvery low strength levels, can be seen and is mostencouraging given the uncertainties introduced by theuse of maturities in determining insitu strengths. It is

surprising to note that the greatest discrepancies, whichcannot be easily explained, occur with the laboratorywall panels, whilst on site no strengths of less than 10MPa were encountered despite the cold weather. A keyfeature emerging from the insitu temperature measure-ments was the extent of the in-place differentials across

the height of the wall. The peak temperature was foundto occur at approximately mid-height of the 1.5-m highlift at approximately 1215 h after casting. This wastypically 20C higher than that experienced 100 mmbelow the top surface of the pour, and will lead tosignificant early-age strength differences.

3.2. Cardington study

This recent study has been based on the Europeanconcrete building project at the Cardington laborato-

ries of the building research establishment in the UK.This project involved the construction of a seven-storey,insitu reinforced flat-slab building frame, Fig. 7,utilising a range of construction methods with theaim of improving speed, reducing costs and im-proving the quality of such construction. Seehttp:rrwww.bre.co.ukrbrercardingtonrcardlab1.htmlfor further details on this project. The structure pro-vided the basis for a number of construction-phaseresearch projects including that undertaken jointly bythe University of Liverpool and Queens University,

w xBelfast 6 . In this project, pull-out testing was ex-tended to include the capo-test and the pull-off testwas also considered, supported by maturity measure-ments and temperature-matched curing to assist insitustrength estimates. Air-cured and water-cured cubeswere also available, with results for early ages in addi-tion to 28 days to support the aim of seeking toestimate 28-day strengths from early age insitu tests, aswell as enabling comparisons of insitu strengths withcubes experiencing different curing regimes.

A range of concrete mixtures incorporating different .aggregate types gravel and limestone and admixtures

.plasticiser and superplasticiser with nominal cubecompressive strengths of 37 and 85 MPa were used.The six different mixtures covered in this study aredetailed in Table 2. Some adjustments were madeduring construction by the concrete supplier to main-

.tain target mean strengths C37N-10 and C37N-11 , asis common practice during construction of a largeproject.

Tests were performed on columns at different heights .top, middle and bottom , and on slabs, both adjacent

.to columns and in mid-bay top and soffit . The se-lected test methods were each used at similar locationsto permit comparison of their practicality on site; forexample speed, relative cost, disruption and accuracy,


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Fig. 7. Cardington insitu concrete building.

and to assist determination of an optimum balancebetween insitu and cube testing.

The insitu tests using the pull-off, lok and capo

methods, were undertaken at 1 day or as soon as.practicable , 3 days, 7 days and 28 days after casting.

Temperature measurements were available from the

Table 2Concrete details

3 .Concrete types and dry batch weights per m

C85MS C85MK C37N-10 C37N-11 C37P C37FMicrosilica Metakaolin Normal Normal Plasticised Flowing

Location Columns Columns Columns Columns & Slab Slab Slab

.Portland cement kg 400 400 380 355 330 335

.Sand kg 732 717 755 785 815 810

520 mm coarse .aggregate kg .

Limestone 1170 1146Gravel 1025 1025 1010 1010

.Admixtures mlPlasticiser 1232 1232 990Superplasticiser 8800 8800 5000

.Cement replacements kgMicrosilica 40Metakaolin 40

Free WrC 0.25 0.32 0.50 0.53 0.52 0.52

Target .Slump mm 100 100 100

.Flow mm 550 600 550


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. .Fig. 8. Lok-test inserts. a High strength Lok-test insert. b Flotation cup.

time of casting at several locations on columns at adepth of 25 mm below the surface, and on slabs at 25mm above soffits and 50 mm below top surfaces. Re-sults for corresponding temperature-matched curedcubes were based on sensors at a depth of 50 mmbelow the tops of slabs midway between columns.High-strength Lok-test inserts were used for the high-strength mixtures, whilst standard flotation cups were

used to locate inserts on the top surfaces of slabs Fig..8 .

3.3. Lok-test strength correlations

Historically, most strength relationships have beenassumed to be straight lines, and ordinary least-squaresanalysis has been used to estimate the correspondingslopes and intercepts, as shown in Fig. 9. Each pointrepresents the average of four no. pull-out tests on theside faces of 200-mm cubes and three No. 100-mmcube compressive strength values. Results for individ-ual groups of similar mixtures show small scatter abouttheir respective correlation lines but the intercepts and

slopes vary considerably for each individual concretemixture. The combined correlation for all mixtures ishowever, surprisingly very close to the manufacturers

w xcorrelation 4 . This is consistent with previous asser-tions that a generalised correlation can be used, butthat confidence limits can be improved by specific cor-

w xrelation for the mixture concerned 7 . The manufac-turer does recommend that, in order to improve theaccuracy of the strength correlation, the range of con-crete strengths considered should be as wide as possi-ble. This, for commercial buildings, which are unlikelyto use so many different concretes, can be achieved bytesting at various ages.

The combined strength correlation crosses the man-ufacturers recommended correlation at a force of 10.5kN but because of its different slope, results in anestimated in-situ strength which is greater by 4.8 MPaat a force of 37 kN. Beyond this force the manufac-turers recommended bi-linear correlation has the sameslope as the Cardington correlation but the compres-sive strength intercept is less by 4.8 MPa. It was foundthat grouping all the results together to obtain onestrength correlation for all the mixtures that were

tested improved the confidence in the strength predic-tion; the 95% confidence interval was "4 MPa for aconcrete strength of 43 MPa. This single strength cor-relation, based on all the results, was therefore, usedfor estimating the in-situ strength from measured loktest values.

Fig. 9. Lok-test strength correlations.


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Fig. 10. Lok and capo-test strength correlations.

3.4. Capo-test strength correlations

The possibility of using the lok-test correlation tointerpret the capo-test results has been investigated.Correlations were developed in the same manner as forthe lok-tests and results have confirmed that onestrength correlation can be used for both the lok and

.capo-tests Fig. 10 , even for ages as low as 1 day. Theslight deviations of the two individual correlations ap-pear to be due to the inclusion of high-strength con-crete mixes for determining the strength correlation forthe lok-test. Only normal-strength concretes were tested

with the capo-test because of the limitations of theequipment mentioned below.

3.5. Pull-off strength correlations

The ordinary least-squares method has again beenused to determine the strength correlations which wereobtained from six no. side face measurements on 150-mm cubes. These are shown graphically in Fig. 11. It isapparent that the accuracy of these strength correla-tions has been affected by the limited number of sets ofdata, three for each mix, which was due to the dif-

ficulties encountered when testing concrete at one day.It was found that the epoxy-bonding compound did notharden quickly enough to allow one-day testing.

It is clear that a specific correlation is needed foreach mix but if this is available, then a reasonable levelof strength prediction can be achieved.

3.6. Insitu measurements

Insitu lok-test strength predictions for slabs are com-pared with temperature-matched cube results in Fig. 12based on the average of four no. lok-tests at a particu-

Fig. 11. Pull-off test strength correlations.

lar location together with between two and four no.compressive-strength cubes. It should be noted thatlok-test results are limited to those available from alocation adjacent to the controlling sensor but includeresults for both top and soffit of the slab, thus, signifi-cantly increasing variability. It was noted that insitustrengths, especially at the soffit, were generally foundto be greater in the slab region adjacent to columnsthan at mid-bay.

A much closer relationship is seen in Fig. 13 betweeninsitu predictions from lok-test and maturity resultsusing a similar function to that used in the cooling

.tower study which serves to confirm the reliability ofthe lok-test values. Comparisons of insitu and air-curedcube strength results in Fig. 14 however, clearly indi-cate the inadequacies of reliance on air-cured cubes atearly ages, at both low- and high-strength values.

.Fig. 12. Insitu strength Lok-test vs. temperature-matched cuberesults.


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. .Fig. 13. Insitu strength Lok-test vs. insitu strength maturity .

4. Testing mature concrete

4.1. Normal concretes

The applications of the range of test methods cov-ered by this paper to normal strength concretes withnatural aggregates have been well documented, and

w xreliabilities and limitations established 1,7 , based onextensive work at Liverpool and elsewhere.

4.2. High strength concrete

Published results for partially-destructive tests ap-

plied to high-strength concrete are very limited, al-though it has been suggested that probe penetration,internal fracture and break-off methods are unsuitable.Thus, pull-out and pull-off are the only realistic ap-proaches, and both have been assessed, to a limitedextent, in the Cardington project described above. Thereliability of the lok-test has been confirmed up to 110MPa but the capo-test is limited to an upper value of

.Fig. 14. Insitu strength Lok-test vs. air-cured cube strength.

Fig. 15. Lok-test strength correlations for high-strength concretes.

approximately 70 MPa due to the characteristics of thepull-bolt system. Results suggest that the differences inmixture components and composition have no signifi-cant effect upon pull-out forcercompressive-strengthcorrelations. Further studies at Liverpool have con-firmed the absence of influence due to aggregate type .graniterlimestone over the whole of the strength

w xrange, and this is confirmed by Price and Hynes 8 toalso include gravel aggregates. This is illustrated in Fig.15. Price and Hynes suggest however, that the pull-outtest is less sensitive at high-strength levels and attributethis to possible changes of failure mechanism linked tohigh paste content, good paste-aggregate bond, andaggregate fracture effects. Pull-off testing requires cor-

relations for the specific mixture, with the maximumstrength again limited by equipment capacity to approx-imately 100 MPa.

4.3. Lightweight concretes

An extensive laboratory study utilising large scale .2.2 m=0.5 m=0.3 m beam specimens together with

Fig. 16. Pull-out test strength correlations for lightweight concretesw x9 .


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Fig. 17. Pull-off test strength correlations for lightweight concretesw x9 .

cores and cubes has demonstrated that all methodsconsidered can be successfully applied to lightweightw xconcretes 9 . Correlations with strength differ from

those for dense concretes in all cases and for some,tests are further affected by the nature of the aggregate

.and the type of fine material sand or lightweight used.Windsor probe results using a special larger diameter

.probe show increasing scatter with strength level, whilstinternal fracture tests are heavily-dependent on loading

.method torquemeter or direct-pull and are unsuitable .for very soft aggregates. Lok-test results Fig. 16 , show

the closest agreement for different types of lightweightaggregate. Surface pull-off correlations are shown in

Fig. 17, with clear differences according to aggregatetype and it was found that correlations for partially-cored tests were significantly different in all cases.

Although a specific correlation is needed for anygiven situation, it was found that variability of resultswas in many cases less than to be expected with con-crete made from natural dense aggregates, and the

w xFig. 18. Effects of pull-off disk characteristics 13 .

accuracy of strength estimation was generally compara-w xble or improved. Insitu variability 10 was however,

found in some aspects to be higher than expected withnormal concretes, with surface zone strengths typically20% less than interior strengths, compared with 510%expected for gravel concrete. Different relationships

between insitu strengths and standard 28-day cubestrength were also noted. It is thus, particularly impor-tant that these issues are considered during planningand interpretation of testing on lightweight concrete.

4.4. Testing repairs

This is emerging as a key application of Pull-offtesting, utilising partial-coring into the substrate topermit an assessment of the tensile bond-strengthbetween substrate and repair. This is dealt with by a

w xCIRIA Report 11 and a minimum bond-strength mayw xbe specified as part of acceptance criteria 12 .

A key issue that has emerged from studies at Liver-w xpool University 13 is that of disk stiffness, as repre-

sented by proportions and material, relative to that ofthe material to which the disk is attached. This isillustrated by Fig. 18 in which it can be seen that agreater ratio of disk thickness to diameter is neededwhen the disk is made of aluminium rather than steel.To achieve comparable measured pull-off strengths, aminimum depth of partial-coring of 20 mm has alsobeen recommended for commonly used 50 mm diame-ter aluminium disks to ensure uniformity of stressdistributions at the base of the cored portion, although

this may be reduced for steel disks.These results were obtained from extensive labora-

tory experimental studies, supported by finite-elementmodelling. It is thus, particularly important that onlystandardised equipment is used or alternatively, thatthe configuration is specified together with the mini-mum acceptable test value. A laboratory-based study in

w xthe USA 14 has also confirmed the suitability of themethod for assessing the bonding of highway overlays,and the effects of freezing, thawing cycling and de-icingsalts.

5. Discussion

The results presented have confirmed that pull-outand pull-off tests can be successfully applied to con-crete in structures to yield estimates of insitu strength.The pull-out method in particular offers advantages ofbeing able to utilise a general strength correlation asprovided by the manufacturer. Concrete mixtures usedon site may not be finalised until just before the start ofconstruction, thus, hampering the development of spe-cific correlations. Similarly, with existing concrete, pre-cise details of the composition may be unknown and


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expensive, disruptive coring may be the only effectivemeans of producing a specific correlation.

For new construction, the lok-test has been shown tobe easy-to-use and reliable over a very wide range ofstrengths. If it is possible to develop a specific strengthcorrelation, possibly during construction, this may be

used to improve the accuracy with which insitu strengthestimates may be made. For lightweight concretes ageneral correlation based on Fig. 16 can be used,noting that this is different to the manufacturers corre-lation which relates only to concretes made with densenatural aggregates.

When testing is required on existing structures wind-sor probe, pull-off and capo tests may be considered.Although simple to use, the windsor probe requiresspecific correlation for the aggregate and doubts havebeen cast upon reliability both at low strength levelsand above 50 MPa. Pull-off tests require surface prepa-ration and careful bonding, but are otherwise quickand straightforward to use. Specific correlations mustbe developed for the concrete under investigation, butthe work at Cardington has confirmed that concretewith strengths up to 100 MPa can be successfully testedwith strength estimation accuracies generally of theorder of"10% based on the average of six no. tests.This value is comparable to that associated with pull-out

.tests four no. using specific correlations. Capo-testingis not simple and requires considerable skill to success-fully perform the coring and under-reaming operations.Nevertheless, it is most encouraging that work reportedhere confirms the ability to use the same general

correlation as the lok-test with similar likely accuracies ."20% based on four no. results . This method thus,offers the significant benefits noted above associatedwith the ability to avoid the development of a specificstrength correlation. It can also be used as a back up tothe lok-tests, if required, but the need to use a cov-ermeter to avoid reinforcement should be noted.

The application of statistics to assess the reliabilityof insitu strength estimates is fraught with difficulties.This issue has been considered at length by Carino andothers in the USA with findings summarised by ACI

w x228 2 . The subject has also been considered by RILEM

Committee TC 126-in place testing. Unfortunately,there are seldom enough in-place test results to es-tablish statistical confidence, whilst the number ofstrength levels available when preparing a strengthcorrelation will have a large effect upon the uncer-

w xtainty of estimated strength. ACI 228 2 recommendthat six to nine strength levels should be used inperforming correlations, providing a balance betweenprecision and cost. Work at Cardington has highlightedthe difficulties of achieving this in practice, even on acarefully-planned project. Where it is necessary to de-velop correlations from cores taken from an existingstructure, the difficulties escalate. Few reports exist of

reliable and systematic comparisons of in-place testsw xwith cores taken from structures 4 , thus, it has to be

recognised that commonly quoted accuracies are ef- .fectively based on confidence limits typically 95% of

laboratory correlations. It may well be that it would bemore realistic to quote lower-bound estimates of in-

situ strength.The work described above has further highlighted

the significance of insitu strength variations due tolocation within an element. This effect is especiallymarked at early ages due to temperature differentials,and must be considered when establishing test loca-tions. Care must be taken when using large correlation

.specimens eg. 200-mm cubes to ensure that tempera-ture histories match those of specimens used for crush-

.ing eg. 100-mm cubes .The use of early age tests to predict later age strength

is potentially a valuable application of testing. Analysisof results from the Cardington project continues, butpreliminary findings suggest that predictions based onlok-tests at 3 days may be sufficiently accurate toidentify situations requiring remedial action. If com-bined with maturity measurements, confidence in laterage predictions can be increased.

It is clear from the Authors experience that simplic-ity is the key to increased usage of in-place testing.Combining methods can be valuable, especially at early

.ages eg. maturity and pull-out but can seldom bew xjustified except on major projects. Bickley 15 has

demonstrated the economic benefits of in-place testingduring new construction, and experience both at Drax

and Cardington has confirmed the value of testing inconjunction with formwork renewal and speed of con-struction. It is in this area that the greatest scope forincreased usage lies at present, since in most parts ofthe world there is little will to move away from cube orcylinder testing for acceptance purposes.

6. Conclusions

It has been demonstrated that partially-destructivetests provide a viable approach to estimation of insitu

concrete strength for a range of concrete types andcircumstances. Cast-in pull-out tests have been shownto be particularly suitable, both in terms of ease-of-testand reliability, and for early-age testing. Pull-out andpull-off tests may also be reliably applied both tolightweight and high-strength concretes in structures.The pull-off test, with partial coring, is valuable inassessing the bonding of repairs but results may beinfluenced by the test configuration.

Development of a reliable correlation with compres-sive strength is a critical aspect influencing reliability ofinsitu strength predictions, which are difficult to definestatistically. Use of results of inplace testing must fur-


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thermore, take account of insitu strength variationsincluding surfacerinterior effects.

Results confirm that the drilled version of the pull-out .method capo-test produces results which may be re-

.garded as comparable to the cast-in version lok-testand this may be a worthwhile method to use when

assessing unknown existing concretes.There is considerable scope for future development

and usage related to new construction, where test sim-plicity and reliability are the key issues. Long-termstrength predictions from early age partially destructivetests alone are not yet, however, regarded as suffi-ciently accurate for acceptance purposes.

Acknowledgements

Thanks are due to Professor A.E. Long and Dr. G.D.Henderson of the Queens University of Belfast, fortheir collaboration relating to the Cardington project,and to Dr. R. Madandoust of Gilan University, Iran,for his contributions to several aspects of the workreported in this paper.

References

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J.H. Bungey holds a MSc degree in concrete structures fromImperial College, London and a PhD from Liverpool Univer-sity where he is currently Professor and Head of the CivilEngineering Department. His research interests encompassconcrete materials, design and performance including non-destructive testing. He has authored numerous papers andbooks on these topics.

ACI Member M.N. Soutsos received his BEng and PhD de-grees from the University College London. He is currently alecturer in the Department of Civil Engineering, The Univer-sity of Liverpool. His research interests include: cement re-placement materials, thermal stresses in concrete, nondestruc-tive testing, as well as mix design and rheological properties ofhigh strength concrete.

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