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Don’t tear out that concrete yet. Justbecause a cylinder breaks low

doesn’t mean that you have bad con-crete. Occasionally, cylinders aren’t

properly made or tested. Cores re-moved from the structure also can beused for acceptance. Contractors mustknow about coring to protect their in-place concrete investment. Cores canget you out of trouble.

Cores for acceptance

ACI 318 “Building Code Require-ments for Reinforced Concrete” (Ref.1) requires three cores removed fromthe concrete area in question. If theaverage of the three cores is equal to

at least 85% of the 28-day compres-sive strength and no core is less than75% of the 28-day compressivestrength, then the concrete is consid-ered to be structurally adequate.

These concrete acceptance criteriabased on core testing are quite com-mon. However, be careful; some own-ers require the average core strengthresults to be equal to the required 28-day compressive strength.

ASTM requirements

Never let a testing laboratory obtainand test a core that isn’t in confor-mance with ASTM C 42, StandardTest Method for Obtaining and TestingDrilled Cores and Sawed Beams of Concrete (Ref. 2). If the core doesn’tmeet this ASTM standard, the engi-neer or owner could question the va-lidity of the test and make the c o n t r a c-tor pay for another test. If for somereason a core is taken that violates

the ASTM standard, check with theengineer and the owner to make surethey will consider the core a satisfac-tory test.

C o re diameter. The core diametermust be at least twice the maximumnominal size of the coarse aggregate.H o w e v e r, it is preferable for the corediameter to be three times the maxi-mum nominal size of the coarse aggre-gate. For 11⁄ 2-inch coarse aggregate the

minimum core diameter would be 3inches and preferably 4 1⁄ 2 inches.

Be careful a 3-inch-diametercore barrel will provide a core with asmaller diameter. Core barrels aresized by outside diameter, not insidediameter. Select a core barrel greaterthan 3 inches to obtain a minimum 3-inch-diameter core.

C o re length. The length of thecore, when capped, should be abouttwice the diameter. If after capping

Questionable concrete?

Let cores prove your point!

By Bruce A. Suprenant

The compre s s i v estrength results ofthree cores can beused for acceptingc o n c rete. Don’t takecores from the top

layers of columns,walls, or footings.These top layers willbe 10% to 20%weaker than layersfrom the middle orlower portion.

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the core height is less than thecore diameter, then the cores h o u l d n ’t be tested. ASTMpermits a core length fromabout equal size to a maxi-mum of twice the size of thecore diameter.

Moisture conditioning.

The core should be submergedin lime-saturated water at73.4° F ± 3.0° F for at least 40

hours. Test the core promptlyafter removing it from thelime-water. If necessary, pro-tect the core with a wet burlapcover until the test.

If directed by the engineeror owner of the project, a corecan be preconditioned in an-other manner (dry), underASTM C 42. ACI 301 (Ref. 3)and ACI 318 (Ref. 1) both pro-vide guidance for conditioningcores.

Length-to-diameter ratio.The length of the capped coreand the core diameter must bemeasured to the nearest 0.01inch. Use the average of twocore diameter measurements atmid-height. These measure-ments are used to compute thelength-to-diameter ratio.ASTM provides length-to-di-ameter correction factors that are usedto reduce the compression strengthmeasure by the testing machine.

These correction factors are appli-

cable for concrete strengths rangingfrom 2000 to 6000 psi, lightweight(100 to 120 pounds per cubic foot[pcf]) and normal-weight concrete,and cores tested wet or dry. A corewith a length-to-diameter ratio from1.94 to 2.10 requires no correctionfactor. Correction factors not providedby Table 1 can be determined by inter-polation.

End preparation and capping.

Don’t forget end preparation and cap-ping. Sloppy capping and unplaned ormisaligned ends can affect corestrength. Cores, like cylinders, mustbe properly prepared and capped. Theprovisions for capping a core are the

same as for a cylinder.

Practical considerations

When to core ? For newconstruction, a low 7-day or28-day compressive strengthbreak usually initiates thecoring process. Don’t coretoo early; bond strength de-velops slower than compres-sive strength. ASTM C 42recommends waiting 14 daysuntil coring to avoid disturb-

ing the bond between thecoarse aggregate and mortar.For new concrete cured incold weather the time periodbefore coring may be longer.

W h e re to core ? A lowcylinder break usually leadsto drilling a core, butwhere? The National ReadyMixed Concrete Association(Ref. 4) says to avoid coringthe top layers of columns,walls, or footings. These toplayers will be 10% to 20%weaker than cores from themiddle or lower portion (seeFigure 1). For vertical struc-tures, ASTM C 42 recom-mends taking the core fromthe middle of the d e p o s i t

Table 1. Length-to -diameter Strength Correct ion Factor (Ref. 2)

Length/Diameter Ratio Strength Correction Factor

1.75 0.981.50 0.961.25 0.93

1.00 0.87

Table 2. Dry Versus Wet Compressive Strength

Tested Wet Tested Dry

Compressive Unit Weight Compressive Unit WeightStrength (psi) (pcf) Strength (psi) (pcf)

3380 142.4 4400 141.42820 138.8 4240 143.02970 143.8 3660 142.73280 141.9 2530 140.8

Avg. 3000 142.0 4100 142.0

F i g u re 1. C o res from the top of vertical pours can bemuch weaker than those from the middle or bottom ofthe concrete deposit. Also, cores obtained by drilling inthe direction of t he casting test about 8% higher thanc o res obtained by dri lling perpendicular to the dire c t i o nof concrete placement.

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and away from formed joints or edges of the concreteplacement.

Cores should be removed from the location in the con-crete structure that is considered to have low strength. Mosttesting laboratories use a rebound hammer for a relativecomparison of concrete strength to locate the potentiallyweak concrete. ACI 301 (Ref. 3) requires the engineer/ar-

chitect to determine the location of the cores to minimizedamage to the structure. If possible, avoid coring throughreinforcement. It decreases the life of the core barrel,makes core interpretation difficult, and may decrease thestrength of the member.

Test wet or dry ? ASTM C 42 requires the core to bes u b m e rged in lime-saturated water for a minimum of 40hours prior to testing, and only the project engineer or own-er can alter the conditioning requirements of the core. BothACI 301 and ACI 318 allow the core to be air-dried beforetesting if the concrete in the structure will be dry under ser-vice conditions.

Most engineers consider the difference between dry ver-

sus wet compressive strength to be around 10%. However,this may not always be true. Reference 5 reports that air-dried specimens can show a 20% to 25% higher strengththan corresponding specimens tested in a saturated condi-tion. On one project in Colorado, the compressive strengthof air-dried cores was 1100 psi higher than the compressivestrength of saturated cores (Table 2). The average unitweight of the cores for both test conditions was the same,142.0 pcf.

If the concrete will be dry in service, contractors mustmake sure that their cores will be tested dry in accordancewith ACI 301 and ACI 318. Testing wet, when not appropri-ate, could provide strength results leading to the rejection of 

good quality concrete.

Low-strength cores

Cores drilled through cracks or other defects shouldn’t betested for compressive strength and neither should a corethat breaks during drilling. Also, ACI 318 (Ref. 1) permitsthe testing of additional cores extracted from a location rep-resented by an erratic core strength result.

Occasionally, be prepared for low-strength cores. Howev-er, this doesn’t automatically require the concrete to be re-moved and replaced. Ask the engineer to review his calcula-tions to consider the effect of low-strength concrete. Theload capacity of many reinforced concrete members isn’tcontrolled by concrete but rather by the reinforcement.Quite often, the concrete can stay in place.

The cost of coring

Cores aren’t free. The cost of a two-man drilling crewruns about $65 an hour. Don’t forget travel time, core barrelwear, and finally testing. Besides compressive strength,most test laboratories also check the density and absorptionof the core. Don’t be surprised if the cost of obtaining andtesting three cores hits $500.

Who pays for the cores? This varies from region to re-gion. In some areas the contractor always pays for thecores. In other areas, who pays depends upon the strengthresults. If the strength results indicate good concrete, thenthe owner pays. If the strength results indicate bad orm a rginal concrete, then the contractor pays.

Coring questionable concrete usually is a good businesspractice. Most specifications automatically require cores if the cylinders break low. Following ASTM C 42 and know-ing when, where, and what to test, usually provides strengthresults that make contractors happy. Don’t have good quali-ty concrete rejectedtake cores to prove your point.

References

1. ACI 318, “Building Code Requirements for ReinforcedConcrete and Commentary,” American Concrete Institute(ACI), P.O. Box 19150, Detroit, MI 48219.

2. ASTM C 42, Standard Test Method for Obtaining andTesting Drilled Cores and Sawed Beams of Concrete, ASTM,1916 Race St., Philadelphia, PA 19103.

3. ACI 301, “Specifications for Structural Concrete for Build-ings.”

4. “What, Why & How? Strength of In-place Concrete,” Con-crete in Practice, CIP 10, National Ready Mixed ConcreteAssociation, 900 Spring St., Silver Spring, MD 20910.

5. P. K. Mehta, Concrete: Structure, Properties, and Materi-als, Prentice-Hall, New York, 1986.

6. D. L. Bloem, “Concrete Strength Measure m e n t C o re sVersus Cylinders,” P roceedings, ASTM, 1965, Vol. 65.

Bruce A. Suprenant is a consulting engineer, an associate

professor at the University of Colorado in Boulder and a

contributing editor for Concrete Construction.

PUBLICATION #C920437

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