Testing concrete samples is the primary method to verify that concrete meets specifications. Strength is tested by crushing samples like cubes or cylinders. Precision of testing refers to the consistency of results between replicate tests. There are various failure modes in compression testing depending on the sample shape, with cubes more affected by lateral restraint at the platens which can increase the apparent strength. Cylinders have a ratio of height to width that reduces this effect.

1. Concrete TestingConcrete Testing
Materials of ConstructionMaterials of Construction
AVISH SINGHAVISH SINGH

2. IntroductionIntroduction
 Testing is the basic method to verify that concreteTesting is the basic method to verify that concrete
complies with the specifications.complies with the specifications.
 Strength of concrete is verified by testing samplesStrength of concrete is verified by testing samples
(cubes, cylinders, or prisms) made of fresh concrete.(cubes, cylinders, or prisms) made of fresh concrete.
 Disadvantage: Suspected concrete may have beenDisadvantage: Suspected concrete may have been
placed and hardened when testing take place.placed and hardened when testing take place.
 Accelerated strength tests are some times used toAccelerated strength tests are some times used to
offset this disadvantage.offset this disadvantage.
 When samples fail in testing further investigation ofWhen samples fail in testing further investigation of
concrete may be performed using non-destructiveconcrete may be performed using non-destructive
testing.testing.

3. Precision of TestingPrecision of Testing
 Concrete properties vary.Concrete properties vary.
 Precision: general term used for the closeness ofPrecision: general term used for the closeness of
agreement between replicate test results.agreement between replicate test results.
 RepeatabilityRepeatability: the value below which the absolute difference: the value below which the absolute difference
between two single test results obtained with the same methodbetween two single test results obtained with the same method
on identical test material under the same conditions (i.e. same:on identical test material under the same conditions (i.e. same:
operator, apparatus, Lab, and short interval of time) may beoperator, apparatus, Lab, and short interval of time) may be
expected to lie within a specified probability (usually 95%).expected to lie within a specified probability (usually 95%).
 ReproducibilityReproducibility: the value below which the absolute difference: the value below which the absolute difference
between two single tests results, obtained with the samebetween two single tests results, obtained with the same
method on identical test material under different conditionsmethod on identical test material under different conditions
(i.e. different: operators, apparatus, Labs, time) may be(i.e. different: operators, apparatus, Labs, time) may be
expected to lie within a specified probability (usually 95%).expected to lie within a specified probability (usually 95%).

4. Precision Cont.Precision Cont.
 Values of repeatability & reproducibility areValues of repeatability & reproducibility are
applied in a variety of ways:applied in a variety of ways:
 To verify the experimental technique of a lab is upTo verify the experimental technique of a lab is up
to requirements.to requirements.
 To compare results of tests performed on a sampleTo compare results of tests performed on a sample
from a batch of material with specification.from a batch of material with specification.
 To compare test results obtained by a supplier andTo compare test results obtained by a supplier and
by consumer on the same batch of material.by consumer on the same batch of material.

5. Analysis of Fresh ConcreteAnalysis of Fresh Concrete
 Determination of composition of concrete at an early age canDetermination of composition of concrete at an early age can
be of benefit since knowing that actual proportioningbe of benefit since knowing that actual proportioning
correspond to those specified will conclude that there is littlecorrespond to those specified will conclude that there is little
need for testing the strength of hardened concrete.need for testing the strength of hardened concrete.
 Properties of interest: (W/c) ratio & cement content (mainlyProperties of interest: (W/c) ratio & cement content (mainly
responsible for ensuring that concrete is strong & durable).responsible for ensuring that concrete is strong & durable).
 BS suggest five methods to assess cement content:BS suggest five methods to assess cement content:
 Buoyancy methodBuoyancy method
 Chemical methodChemical method
 Constant volume methodConstant volume method
 Physical separation methodPhysical separation method
 Pressure filter methodPressure filter method
 Water content of fresh concrete can be found as in chemicalWater content of fresh concrete can be found as in chemical
method, or by rapid drying method (different in mass beforemethod, or by rapid drying method (different in mass before
and after heating).and after heating).

6. Strength TestsStrength Tests
 Compressive strengthCompressive strength
 Tensile strength:Tensile strength:
 Uniaxial tension (direct tension: very difficult)Uniaxial tension (direct tension: very difficult)
 Flexure test (Indirect)Flexure test (Indirect)
 Splitting test (Indirect)Splitting test (Indirect)
 Indirect methods yields higher strength values thanIndirect methods yields higher strength values than
the true tensile strength under uniaxial loading forthe true tensile strength under uniaxial loading for
reason already stated.reason already stated.

7. Method of Testing Concrete InMethod of Testing Concrete In
TensionTension
 Different test methods yield numerically different results,Different test methods yield numerically different results,
ordered as follows:ordered as follows:
Direct tension < Splitting < flexural tensionDirect tension < Splitting < flexural tension
 Reasons for that:Reasons for that:
1.1. With the usual size of Lab. Specimen, the volume of concreteWith the usual size of Lab. Specimen, the volume of concrete
subjected to tensile stress decrease in the order listed above.subjected to tensile stress decrease in the order listed above.
Statistically there is a greater chance of a weak elementStatistically there is a greater chance of a weak element and thereforeand therefore
of failure for larger volume than in a small volume.of failure for larger volume than in a small volume.
2.2. Both splitting and flexural test involve non-uniform stress distributionBoth splitting and flexural test involve non-uniform stress distribution
which impede the propagation of cracks, and therefore delay thewhich impede the propagation of cracks, and therefore delay the
ultimate failure. Whereas, in the direct test, the stress distribution isultimate failure. Whereas, in the direct test, the stress distribution is
uniform, so that once a crack has formed, it can propagate quicklyuniform, so that once a crack has formed, it can propagate quickly
through the section of the specimen.through the section of the specimen.

8. Compressive StrengthCompressive Strength
 Determined using 150 x 300 mm (6 x 12 in) cylinders in US andDetermined using 150 x 300 mm (6 x 12 in) cylinders in US and
150 mm (6 in) cubes in UK.150 mm (6 in) cubes in UK.
 Smaller specimen sizes can be used based on Agg. Max. size.Smaller specimen sizes can be used based on Agg. Max. size.
 Molds (re-usable or non re-usable) must be oiled from inside toMolds (re-usable or non re-usable) must be oiled from inside to
prevent bond with concrete.prevent bond with concrete.
 ACCORDING TO ASTM (Cylindrical molds are used).ACCORDING TO ASTM (Cylindrical molds are used).
 For high-slump concrete: Concrete placed in cylindrical molds inFor high-slump concrete: Concrete placed in cylindrical molds in
three layers, and each layer is compacted 25 times with a rod (3/8three layers, and each layer is compacted 25 times with a rod (3/8
in D).in D).
 For low-slump concrete: Concrete placed in two layers andFor low-slump concrete: Concrete placed in two layers and
compacted using internal or external vibration.compacted using internal or external vibration.
 Top surface of cylinders must be plane, smooth, and normal to itsTop surface of cylinders must be plane, smooth, and normal to its
axis.axis.
 Plane surface can be obtained using two methods”Plane surface can be obtained using two methods”
 GrindingGrinding
 Capping (expensive)Capping (expensive)

9. Compressive Strength/ CappingCompressive Strength/ Capping
 Materials used:Materials used:
 Stiff Portland cement paste on freshly-cast concrete.Stiff Portland cement paste on freshly-cast concrete.
 Mixture of Sulphur and granular material (e.g. milled fireMixture of Sulphur and granular material (e.g. milled fire
clay)……clay)…… Best capping materialBest capping material
 Mixture of sulphur and high strength Gypsum plaster.Mixture of sulphur and high strength Gypsum plaster.
 Cap should be thin (1.5 to 3 mm) and has strengthCap should be thin (1.5 to 3 mm) and has strength
similar to that of concrete.similar to that of concrete.
 Caution: Toxic fumes are produced when cappingCaution: Toxic fumes are produced when capping
with Sulphur mixtures.with Sulphur mixtures.

10. Compressive Strength/ CuringCompressive Strength/ Curing
 ASTM C 192-90a curing conditions for standard testASTM C 192-90a curing conditions for standard test
cylinders.cylinders.
 Molded specimens are stored for not less than 20 and not moreMolded specimens are stored for not less than 20 and not more
than 48 hrs at temp. of 23 +- 1.7than 48 hrs at temp. of 23 +- 1.7 oo
C so that moisture loss isC so that moisture loss is
prevented.prevented.
 After removing from molds, specimens are stored at the sameAfter removing from molds, specimens are stored at the same
temp. and under moist conditions or in a saturated lime watertemp. and under moist conditions or in a saturated lime water
until the prescribed age of testing.until the prescribed age of testing.
 Cured cylinders give potential strength.Cured cylinders give potential strength.
 Service cylinders may be used to determine the actual qualityService cylinders may be used to determine the actual quality
of concrete in the structure by being subjected to the sameof concrete in the structure by being subjected to the same
conditions as the structure.conditions as the structure.

11. Compressive Strength TestCompressive Strength Test
 ASTM C39-86 compressive strength of aASTM C39-86 compressive strength of a
cylinder.cylinder.
 Loading rate for hydraulically operatedLoading rate for hydraulically operated
machines is 0.15 – 0.34 MPa/ s (20 -50 psi/s)machines is 0.15 – 0.34 MPa/ s (20 -50 psi/s)
 Deformation rate for mechanically operatedDeformation rate for mechanically operated
machines is 1.3 mm/min (0.05 in/min).machines is 1.3 mm/min (0.05 in/min).
 Compressive strength = (max. load / crossCompressive strength = (max. load / cross
section area of cylinder) reported to the nearestsection area of cylinder) reported to the nearest
0.05 MPa (10 psi).0.05 MPa (10 psi).

12. Compressive Strength Cont.Compressive Strength Cont.
 ACCORDING TO BS (Cube molds are used).ACCORDING TO BS (Cube molds are used).
 Molds filled in 3 layers.Molds filled in 3 layers.
 35 strokes/ layer for 150 mm cubes, or 25 strokes for 100 mm35 strokes/ layer for 150 mm cubes, or 25 strokes for 100 mm
cubes, using a 25 mm (1 in) square steel punner.cubes, using a 25 mm (1 in) square steel punner.
 Alternatively vibration can be used.Alternatively vibration can be used.
 Top surface finished by a trowel.Top surface finished by a trowel.
 Cubes stored at temp. of 20 +- 5Cubes stored at temp. of 20 +- 5 oo
C and with 90% moistC and with 90% moist
condition.condition.
 De-molding after 16 – 28 hrs and specimens stored in a curingDe-molding after 16 – 28 hrs and specimens stored in a curing
tank at 20 +-2tank at 20 +-2 oo
C until test age.C until test age.
 Testing ages: common 3, 7, 28 days, uncommon 1, 2, 14 days,Testing ages: common 3, 7, 28 days, uncommon 1, 2, 14 days,
13, 26 weeks, and 1 year.13, 26 weeks, and 1 year.

13. Platen RestraintPlaten Restraint
 Failure of concrete under pure uniaxial compression is theFailure of concrete under pure uniaxial compression is the
ideal mode of testing.ideal mode of testing.
 But compression test imposes more complex stress systemBut compression test imposes more complex stress system
because of lateral forces developed between the end surface ofbecause of lateral forces developed between the end surface of
the concrete specimen and the adjacent steel platen of thethe concrete specimen and the adjacent steel platen of the
testing machine.testing machine.
 These forces are induced by the restraint of the concrete,These forces are induced by the restraint of the concrete,
which attempts to expand laterally, by the several time stifferwhich attempts to expand laterally, by the several time stiffer
steel, which has a much smaller lateral expansion.steel, which has a much smaller lateral expansion.
 The degree pf platen restraint on the concrete section dependsThe degree pf platen restraint on the concrete section depends
on the friction developed at the concrete platen interface, andon the friction developed at the concrete platen interface, and
on the distance of the end surfaces of the concrete.on the distance of the end surfaces of the concrete.
 Consequently, in addition to the imposed uniaxial compresion,Consequently, in addition to the imposed uniaxial compresion,
there is a lateral shearing stress, the effect of which is tothere is a lateral shearing stress, the effect of which is to
increase the apparent compressive strength of concrete.increase the apparent compressive strength of concrete.

14. Typical Failure Modes/ CubesTypical Failure Modes/ Cubes
 Influence of platen restraint can be seen from typical failureInfluence of platen restraint can be seen from typical failure
modes.modes.
 Fig. 16.1 Failure modes in test cubesFig. 16.1 Failure modes in test cubes..
 Effect of shear is always present and decrease towards theEffect of shear is always present and decrease towards the
center of the cube, or disintegrate so as to leave undamagedcenter of the cube, or disintegrate so as to leave undamaged
central core (Non-explosive). Testing machine is rigid.central core (Non-explosive). Testing machine is rigid.
 Less rigid machine can store more energy so that the explosiveLess rigid machine can store more energy so that the explosive
failure is possible.failure is possible.
 Explosive: One face touching the platen disintegrate so as toExplosive: One face touching the platen disintegrate so as to
leave a pyramid or a cone.leave a pyramid or a cone.
 Other modes failure are regarded as unsatisfactory andOther modes failure are regarded as unsatisfactory and
indicate a probable fault in the testing machine.indicate a probable fault in the testing machine.

15. Fig. 16.1 Typical Failure Modes of test CubesFig. 16.1 Typical Failure Modes of test Cubes
a) Non-Explosive b) Explosive

16. Typical Failure Modes Cont.Typical Failure Modes Cont.
 When the ratio of height to width of the specimenWhen the ratio of height to width of the specimen
increase, the influence of shear becomes smaller.increase, the influence of shear becomes smaller.
 The central part of the specimen may fail by lateralThe central part of the specimen may fail by lateral
splitting.splitting.
 Situation in cylinders (H/W = 2).Situation in cylinders (H/W = 2).
 Possible modes of failure in Cylinders: Splitting,Possible modes of failure in Cylinders: Splitting,
Shear, and splitting & shear.Shear, and splitting & shear.
 Fig. 16.2 Failure modes in test cylinders.Fig. 16.2 Failure modes in test cylinders.
 Fig. 16.3 influence of height/ diameter ratio on theFig. 16.3 influence of height/ diameter ratio on the
apparent strength.apparent strength.

17. Fig. 16.2 Typical Failure Modes of Test CylindersFig. 16.2 Typical Failure Modes of Test Cylinders
a) Splitting b) Shear
(Cone)
c) Splitting & Shear

18. Influence of Height/ Diameter RatioInfluence of Height/ Diameter Ratio
on the Apparent Strengthon the Apparent Strength
 As (H/D) ratio increase the apparent strength willAs (H/D) ratio increase the apparent strength will
decrease.decrease.
 Due to larger effect of platen restraint on cubes modeDue to larger effect of platen restraint on cubes mode
of failureof failure
Cube strength = 1.25 Cylinder strengthCube strength = 1.25 Cylinder strength
 This relation depends also on strength level andThis relation depends also on strength level and
moisture condition of concrete when tested.moisture condition of concrete when tested.
 Cylinder strength is probably closer to the trueCylinder strength is probably closer to the true
uniaxial compressive strength of concrete than theuniaxial compressive strength of concrete than the
cube strength because:cube strength because:
 Less end restraint.Less end restraint.
 More uniform distribution of stress over the cross section.More uniform distribution of stress over the cross section.

19. Fig. 16.3 Influence of Height/ Diameter RatioFig. 16.3 Influence of Height/ Diameter Ratio
on The Apparent Strength of A cylinderon The Apparent Strength of A cylinder

20. Tensile StrengthTensile Strength
 Tensile strength:Tensile strength:
 Uniaxial tension (direct tension: very difficultUniaxial tension (direct tension: very difficult
because the ends have to be gripped and bendingbecause the ends have to be gripped and bending
must be avoided)must be avoided)
 Flexure test (Indirect)Flexure test (Indirect)
 Splitting test (Indirect)Splitting test (Indirect)
 Indirect methods yields higher strength valuesIndirect methods yields higher strength values
than the true tensile strength under uniaxialthan the true tensile strength under uniaxial
loading for reason already stated.loading for reason already stated.

21. Flexural TestFlexural Test
 The theoritical max. tensile stress reached in the bottomThe theoritical max. tensile stress reached in the bottom
fiber of a test beam is known as the “Modulus offiber of a test beam is known as the “Modulus of
Rupture”.Rupture”.
 Relevant to the design of highways and airfields.Relevant to the design of highways and airfields.
 The value of the modulus of rupture depends on theThe value of the modulus of rupture depends on the
dimensions of the beam and on the arrangement ofdimensions of the beam and on the arrangement of
loading.loading.
 Symmetrical two-point loading (at third point of theSymmetrical two-point loading (at third point of the
span) is used in UK ans US.span) is used in UK ans US.
 This produces a constant bending moment between theThis produces a constant bending moment between the
load points so that one third of the span is subjected toload points so that one third of the span is subjected to
the max. stress.the max. stress.
 Therefore it is there where cracking is likely to takeTherefore it is there where cracking is likely to take
place.place.

22. Flexural Test Cont.Flexural Test Cont.
 BSBS
 Beam 150 x 150 x 750 mm (6 x 6 x 30 in)Beam 150 x 150 x 750 mm (6 x 6 x 30 in)
 Or beam 100 x 100 x 500 mm for max. agg < 25 mm.Or beam 100 x 100 x 500 mm for max. agg < 25 mm.
 Curing as specified in BS.Curing as specified in BS.
 Beams tested on their side (as-cast position), in aBeams tested on their side (as-cast position), in a
moist condition.moist condition.
 Rate of increase stress in the bottom fiber (0.02 – 0.1Rate of increase stress in the bottom fiber (0.02 – 0.1
MPa/s (2.9 – 14.5 psi/s), lower rate for low strengthMPa/s (2.9 – 14.5 psi/s), lower rate for low strength
concrete and higher rate for high strength concrete.concrete and higher rate for high strength concrete.

23. Fig. 16.4 Arrangement for The ModulusFig. 16.4 Arrangement for The Modulus
of Rupture Testof Rupture Test

24. Flexural Test Cont.Flexural Test Cont.
 ASTM C78- 84ASTM C78- 84
 Similar flexural test as in BS except:Similar flexural test as in BS except:
 Beam 152 x 152 x 508 mm (6 x 6 x 20 in).Beam 152 x 152 x 508 mm (6 x 6 x 20 in).
 Rate of loading 0.0143 – 0.02 MPa/s (2.1 – 2.9 psi/s).Rate of loading 0.0143 – 0.02 MPa/s (2.1 – 2.9 psi/s).
 If fracture occurs within the middle one-third of theIf fracture occurs within the middle one-third of the
beam, the modulus of rupture (fbeam, the modulus of rupture (fbtbt) is given by:) is given by:
FFbtbt = (P L) / (b d= (P L) / (b d22
))
P: Max total loadP: Max total load
L: Span length between supportsL: Span length between supports
d: depth of the beamd: depth of the beam
b: width of the beamb: width of the beam

25. Flexural Test Cont.Flexural Test Cont.
 If fracture takes place outside the middle one-third of the beam,If fracture takes place outside the middle one-third of the beam,
then:then:
 According to BS the test result should be discarded.According to BS the test result should be discarded.
 ASTM C78-84 allows for failure outside the load points, say, at at anASTM C78-84 allows for failure outside the load points, say, at at an
average distance (a) from the nearest support.average distance (a) from the nearest support.
 modulus of rupture (fmodulus of rupture (fbtbt) is given by:) is given by:
FFbtbt = (3 P a) / (b d= (3 P a) / (b d22
))
P: Max total loadP: Max total load
L: Span length between supportsL: Span length between supports
d: depth of the beamd: depth of the beam
b: width of the beamb: width of the beam
 If failure occurs at a section such that ((L/3) – a) > 0.05 L, thenIf failure occurs at a section such that ((L/3) – a) > 0.05 L, then
the results should be discarded.the results should be discarded.

26. Flexural Strength Using One-PointFlexural Strength Using One-Point
LoadingLoading
 Modulus of rupture (fModulus of rupture (fbtbt) is given by:) is given by:
FFbtbt = (3 P L) / (2 b d= (3 P L) / (2 b d22
))
P: Max total loadP: Max total load
L: Span length between supportsL: Span length between supports
d: depth of the beamd: depth of the beam
b: width of the beamb: width of the beam

27. Splitting TestSplitting Test
 Concrete cylinder (or less commonly cube) of the type used inConcrete cylinder (or less commonly cube) of the type used in
compressive strength testing.compressive strength testing.
 Placed, with its axis horizontal, between platens of a testingPlaced, with its axis horizontal, between platens of a testing
machine.machine.
 Load is increased until failure takes place by splitting in theLoad is increased until failure takes place by splitting in the
plane containing the vertical diameter of the specimenplane containing the vertical diameter of the specimen
 Fig. 16.5 Jigs for supporting test specimen in splitting testFig. 16.5 Jigs for supporting test specimen in splitting test
 ASTM C496 -90ASTM C496 -90
 Plywood are placed between specimen and platen tp preventdPlywood are placed between specimen and platen tp preventd
local compressive stresses at the load line.local compressive stresses at the load line.

28. Splitting Test Cont.Splitting Test Cont.
 Rate of loading:Rate of loading:
 BS: 0.02 – 0.4 MPa/s (2.9 – 5.8 psi/s).BS: 0.02 – 0.4 MPa/s (2.9 – 5.8 psi/s).
 ASTM: 0.011 – 0.023 MPa/s (1.7 – 3.3 psi/s).ASTM: 0.011 – 0.023 MPa/s (1.7 – 3.3 psi/s).
 According to ASTM C 496 -90 the tensileAccording to ASTM C 496 -90 the tensile
splitting strength (fsplitting strength (fstst) is given by:) is given by:
ffstst = 2P/= 2P/ ππLdLd
P: Max. loadP: Max. load
L: length of specimenL: length of specimen
d: diameter or width of specimend: diameter or width of specimen

29. Fig. 16.5 Jigs for Supporting TestFig. 16.5 Jigs for Supporting Test
Specimen in Splitting TestSpecimen in Splitting Test
a) Cube or
Prism
a) Cylinder

30. Test CoresTest Cores
 If strength of the standard compression testIf strength of the standard compression test
specimen is below the specified value thenspecimen is below the specified value then
either:either:
 The concrete in the actual structure is unsatisfactory.The concrete in the actual structure is unsatisfactory.
 Or specimens are not truly representative of theOr specimens are not truly representative of the
concrete in the structure (test specimen not correctlyconcrete in the structure (test specimen not correctly
prepared, handled or cured, or testing machine couldprepared, handled or cured, or testing machine could
be at fault).be at fault).
 Argument is often resolved by testing cores ofArgument is often resolved by testing cores of
hardened concrete taken from the suspect part of thehardened concrete taken from the suspect part of the
structure in order to its potential strength.structure in order to its potential strength.

31. Test Cores Cont.Test Cores Cont.
 Potential Strength: the strength equivalent to the 28Potential Strength: the strength equivalent to the 28
days strength of the standard test specimen.days strength of the standard test specimen.
 When translating core strength into potential strength,When translating core strength into potential strength,
take into account differences in:take into account differences in:
 Type of specimen and curing conditions.Type of specimen and curing conditions.
 AgeAge
 Degree of compactionDegree of compaction
 Note that core taking damages the structure, so testNote that core taking damages the structure, so test
cores should be taken only when other, non-cores should be taken only when other, non-
destructive, methods are inadequate.destructive, methods are inadequate.

32. Test Cores Cont.Test Cores Cont.
 ASTM C42-90 prescribe method of determining theASTM C42-90 prescribe method of determining the
compressive strength of cores.compressive strength of cores.
 It is desirable to obtain cores free from reinforcement.It is desirable to obtain cores free from reinforcement.
 Normal cores refer to the cores being representative of theNormal cores refer to the cores being representative of the
concrete.concrete.
 ACI 318-89 considers the concrete in the structure is adequateACI 318-89 considers the concrete in the structure is adequate
if the average strength of three cores is equal to at least 85% ofif the average strength of three cores is equal to at least 85% of
the specified strength, and if no single core has a strengththe specified strength, and if no single core has a strength
lower than 75% of the specified value.lower than 75% of the specified value.
 ACI require testing in a dry state which leads to a higherACI require testing in a dry state which leads to a higher
strength than when tested in a moist condition as (ASTM &BSstrength than when tested in a moist condition as (ASTM &BS
specify).specify).
 Table 16.1 lists tests other than compressive strength, whichTable 16.1 lists tests other than compressive strength, which
may be made on coresmay be made on cores