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What alterna-tive does thecontractor havewhen concrete testcylinders showstrengths arebelow specifica-tion? Is core test-ing required when-ever this happensor are nondestruc-

tive tests like the rebound hammersufficient? Who is responsible whenthe specified concrete strength is notmet? CONCRETE CONSTRUCTION askedNicholas J. Carino, a research engi-neer at the National Institute ofStandards and Technology inGaithersburg, Md., and Ronald G.Burg, vice president at ConstructionTechnology Laboratories in Skokie,Ill., to comment.

CC: Are concrete cores always

required if I have low cylinder strength?Carino: No. The usual trigger tothe taking and testing of cores is fail-ure of standard-cured molded cylin-ders to meet the acceptance criteria inACI 318. Before making the decisionto drill cores, the low strength resultsshould be scrutinized to ensure thatthere were no problems in the curingand testing of the cylinders. Specialattention should be paid to the tem-perature of the concrete cylinders dur-ing the initial curing period. As early-

age concrete temperature increases,the 28-day strength decreases. Thedegree of consolidation of the con-crete in the cylinders should bechecked also. This is easily done byrequiring that cylinders be weighedbefore testing. Finally, the end condi-tions of the tested cylinders have toconform to the requirements ofASTM Test Method C 39. The detri-mental effects of improper end con-ditions are magnified as concrete

strength increases.If the review of testing reveals no

obvious deficien-cies, there is agood chance thatthe in-place con-crete representedby the cylindersmay be understrength. Beforedeciding to drill

cores, the design professional shouldevaluate the consequences of lowstrength concrete on structural ade-

quacy. Depending on the type ofstructure and the controlling failuremechanism, strength less than thatspecified may be acceptable for struc-tural safety. There may be cases, how-ever, where durability is more impor-tant than strength, in which caseunder-strength concrete may reduceservice life.

Burg: Not necessarily. The deci-sion as to whether or not cores arerequired when measured cylinderstrength is less than specified concrete

strength is in the hands of the engi-neer of record for the project. ACI318 permits using cores to evaluateconcrete when the likelihood of lowstrength concrete is confirmed and theload-carrying capacity of the structureis significantly reduced. If it can be

shown that the measured cylinderstrength was in error, or that the load-carrying capacity of the structure hasnot been reduced to a degree impact-ing the safety of the structure, it maynot be necessary to test cores in allcases. However, it is ultimately thecontractors responsibility to demon-strate, in a manner accepted by thegoverning codes, statutes, or engineerof record, that the concrete in the

structures meets the specificationrequirements or will not compromisethe structure in any way.

CC: Can the rebound hammer beused to directly measure in-placestrength?

Burg: There are cases where in-place strength can be demonstratedby means other than testing drilledcores, and nondestructive tests aresometimes sufficient. The reboundhammer, however, cannot provide a

definitive value for in-place strengthof concrete.Carino: No. Often contractors

will suggest using nondestructive teststo establish that the in-place concretemeets strength requirements. Unfortu-nately this is not acceptable. Nonde-structive tests, such as the reboundhammer, Windsor probe, or pulse ve-locity, provide indirect measures ofconcrete strength. They can be usedto reliably esti-mate concrete

strength only if aproper correlationhas been estab-lished for the con-crete in the struc-ture. Correlationsprovided by the

Concrete Perspect ives

What is the biggest problem with concrete core testing?It is ultimately the contractors

responsibility to demonstrate, in a

manner accepted by the governing

codes, statutes, or engineer of

record, that the concrete in the

structures meets the specification.

Ronald Burg

Often contractors will suggest using nondestruc-

tive tests to establish that the in-place concrete

meets strength requirements. Unfortunately this

is not acceptable. Nicholas Carino

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vendors of test equipment are not reli-able. Nondestructive tests, however,are useful for comparing the relativequality of concrete in different por-tions of the structure. The results ofnondestructive tests are useful in iden-tifying suspect regions in the structurewhere cores should be drilled.

CC: What do you see as thebiggest problem with concrete coretesting in the field today?

Burg: I see two major problems:testing lab qualifications and lack ofconsensus on testing methods. Thefirst is not being careful when select-ing the firm that will be taking andtesting the cores. Given the significantcost that the result of core testing mayhave, it is always surprising to methat those responsible for selecting afirm spend so little time evaluating

their qualifications. This companywill be taking and testing the coresand will often be responsible forinterpreting the test data.

The second problem I see is afailure to have all parties involved(contractor, engineer/architect, owner,concrete supplier) agree on thespecifics of how to conduct tests andinterpret results. Issues such as thenumber, size, and location of cores,repair of core holes, transport of

cores to the laboratory, storage priorto testing, conditioning method, testwitnessing, acceptance criteria, andretention of tested and untested coresshould all be agreed upon before thefirst core is taken.

Carino: Core conditioning andsmall diameter cores are of concern.In the past, cores were tested aftersoaking for a least 40 hours or afterair drying for 7 days, depending onthe expected service conditions.Research in the mid-1990s showed

that such treatments produce mois-ture gradients in the cores, and thesegradients have adverse effects onmeasured strengths. Both ASTMTest Method C 42 and ACI 318 arebeing revised to reduce moisture gra-dients in tested cores. New proce-dures will require that the cores besurface-dried after drilling andstored in sealed bags until tested.The objective is to test the cores in acondition that is close to their in-

place moisture condition and to min-imize testing moisture gradients.

Another problem is the use ofsmall diameter cores. ASTM TestMethod C 42 requires core diametersto be at least 3.75 inches (95 mm) formaximum aggregate sizes less than1.5 inches (37.5 mm). Cores withsmaller diameters should only be usedwhen it is impossible to obtain cores

with a length-to-diameter ratiogreater than 1. Research has shownthat cores with diameters less than 2inches (50 mm) result in lower andmore variable strengths.

CC: Are ACI and ASTM stan-dards for obtaining and testing coresadequate?

Carino: Standards are limited byresearch. As is the case for many con-crete testing standards, core-testingrequirements have evolved in responseto research and experience. The use of

high-strength concrete has raisedquestions about the adequacy of stan-dards based on lower concretestrengths. In response, ASTMSubcommittee C09.61 on strengthtesting established a task group ontesting high-strength concrete.

Unfortunately, due to the lack ofknowledge, some standard test meth-ods may not deal adequately withissues related to testing high-strengthconcrete. An example is the length-to-

diameter (L/D) strength correctionfactors for cores with L/D less than1.75. Limited research indicates thatthe correction factors may need to beincreased for high-strength concrete,but there are no good data.

A major change in the concretetesting business is the use of unbond-ed, restrained, rubber pads on cylin-der ends instead of a capping com-pound. Currently, unbonded caps arenot permitted for testing cores. Thereare concerns about the effects of

unbonded caps on the measuredstrengths of cores with low L/D val-ues. In addition, there are questionsabout the effect of the increased gapbetween core and the retaining ring if3.75-inch-diameter (95-mm) cores aretested with the same hardware usedfor 4-inch (102 mm) cores.

In a previous question, I dis-cussed the issue of moisture gradientsand their effect on measured corestrengths. Again, data are lacking on

the effects of different moisture condi-

tioning procedures on the moisturegradients in high-strength concrete.Because high-strength concrete ismuch less permeable than ordinarystrength concrete, the effects of differ-ent moisture conditioning proceduresmay not be the same as with normal-strength concrete.

A standard procedure for statisti-cal analysis of core strength results isalso lacking. The ACI 318 acceptancecriteria are based on research done inthe 1960s and they may not be appli-cable to modern concretes.

Burg: Standards are tools. Ibelieve the ACI and ASTM standardsadequately set a minimum basis fortesting cores and evaluating testresults, but everyone should recognizethat within the standards there arevarious alternatives that must be

selected to establish a specific testprotocol. For example, ACI andASTM standards present severalmethods for moisture conditioningbefore testing, ranging from soakingunder water to drying in laboratoryair. The different conditioning meth-ods will result in significantly differ-ent measured core strengths.

While the standards can, and do,prescribe a systematic means toobtain and test cores, they should not

be viewed as taking the place ofsound engineering judgment, theyshould be viewed as one importanttool that should be used when it isnecessary to evaluate the strength ofin-place concrete.

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