Labcrete and Realcrete_ch06

Testing 6

INTRODUCTION

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

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

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

unaccounted for;• omission of relevant test requirements.

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

6.1 LABCRETE OR REALCRETE

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

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

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

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

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

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

The specifier has a choice:

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

• Use realcrete data alone.

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

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

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


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

6.1.1 MEANINGFUL

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

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

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

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

6.1.2 ACCEPTABILITY

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

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

should also be agreed at a preliminary stage.


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

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

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

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

6.1.3 FINALITY OF DATA

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

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

6.1.4 IDENTIFICATION

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


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

6.1.5 REMEDIAL

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

6.1.6 AVOIDANCE

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

6.2 DESIGN OR PERFORMANCE

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

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

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

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


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

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

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

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

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

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

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

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

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

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


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


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

6.2.2 REMEDIAL

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

6.2.3 AVOIDANCE

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

6.3 CAMOUFLAGE TESTING

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

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

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

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

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


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

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

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


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

6.3.2 REMEDIAL

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

6.3.3 AVOIDANCE

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

6.4 REPEATABILITY AND REPRODUCIBILITY

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

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

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


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

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

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

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

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

Example 1

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


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

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

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

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

Example 2

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

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

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

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


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


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

6.4.2 REMEDIAL

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

6.4.3 AVOIDANCE

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

6.5 CHANGES IN TESTING

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

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


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

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

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


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

6.5.2 REMEDIAL

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

6.5.3 AVOIDANCE

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

6.6 TESTING FIXATION

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


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

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

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

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


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

6.6.2 REMEDIAL

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

6.6.3 AVOIDANCE

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


6.7 TESTING ACCURACY

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

6.7.1 PROBLEMS AT SPECIFYING

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

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

6.7.2 PROBLEMS AT REPORTING

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

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


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


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

6.7.4 REMEDIAL

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

6.7.5 AVOIDANCE

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


Labcrete and Realcrete_ch06

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