Concrete TestingConcrete TestingMaterials of ConstructionMaterials of Construction

Dr. TALEB M. AL-ROUSANDr. TALEB M. AL-ROUSAN

IntroductionIntroduction Testing is the basic method to verify that concrete Testing is the basic method to verify that concrete

complies with the specifications.complies with the specifications. Strength of concrete is verified by testing samples Strength 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 been Disadvantage: 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 to Accelerated strength tests are some times used to

offset this disadvantage.offset this disadvantage. When samples fail in testing further investigation of When samples fail in testing further investigation of

concrete may be performed using non-destructive concrete may be performed using non-destructive testing.testing.

Precision of TestingPrecision of Testing

Concrete properties vary.Concrete properties vary. Precision: general term used for the closeness of Precision: 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 method between 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 be operator, 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 same between two single tests results, obtained with the same method on identical test material under different conditions method 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%).

Precision Cont.Precision Cont.

Values of repeatability & reproducibility are Values 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 up To verify the experimental technique of a lab is up

to requirements.to requirements. To compare results of tests performed on a sample To 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 and To compare test results obtained by a supplier and

by consumer on the same batch of material.by consumer on the same batch of material.

Analysis of Fresh ConcreteAnalysis of Fresh Concrete Determination of composition of concrete at an early age can Determination of composition of concrete at an early age can

be of benefit since knowing that actual proportioning be of benefit since knowing that actual proportioning correspond to those specified will conclude that there is little correspond 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 (mainly Properties 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 chemical Water content of fresh concrete can be found as in chemical method, or by rapid drying method (different in mass before method, or by rapid drying method (different in mass before and after heating).and after heating).

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 than Indirect methods yields higher strength values than

the true tensile strength under uniaxial loading for the true tensile strength under uniaxial loading for reason already stated. reason already stated.

Method of Testing Concrete In Method 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 concrete With 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 therefore and 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 distribution Both splitting and flexural test involve non-uniform stress distribution which impede the propagation of cracks, and therefore delay the which impede the propagation of cracks, and therefore delay the ultimate failure. Whereas, in the direct test, the stress distribution is ultimate failure. Whereas, in the direct test, the stress distribution is uniform, so that once a crack has formed, it can propagate quickly uniform, so that once a crack has formed, it can propagate quickly through the section of the specimen.through the section of the specimen.

Compressive StrengthCompressive Strength Determined using 150 x 300 mm (6 x 12 in) cylinders in US and Determined 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 to Molds (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 in For high-slump concrete: Concrete placed in cylindrical molds in

three layers, and each layer is compacted 25 times with a rod (3/8 three 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 and For 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 its Top 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)

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 fire Mixture 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 strength Cap 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 capping Caution: Toxic fumes are produced when capping with Sulphur mixtures.with Sulphur mixtures.

Compressive Strength/ CuringCompressive Strength/ Curing ASTM C 192-90a curing conditions for standard test ASTM C 192-90a curing conditions for standard test

cylinders.cylinders. Molded specimens are stored for not less than 20 and not more Molded specimens are stored for not less than 20 and not more

than 48 hrs at temp. of 23 +- 1.7 than 48 hrs at temp. of 23 +- 1.7 ooC so that moisture loss is C so that moisture loss is prevented.prevented.

After removing from molds, specimens are stored at the same After removing from molds, specimens are stored at the same temp. and under moist conditions or in a saturated lime water temp. 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 quality Service cylinders may be used to determine the actual quality

of concrete in the structure by being subjected to the same of concrete in the structure by being subjected to the same conditions as the structure.conditions as the structure.

Compressive Strength TestCompressive Strength Test

ASTM C39-86 compressive strength of a ASTM C39-86 compressive strength of a cylinder.cylinder.

Loading rate for hydraulically operated Loading 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 operated Deformation 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 / cross Compressive strength = (max. load / cross section area of cylinder) reported to the nearest section area of cylinder) reported to the nearest 0.05 MPa (10 psi).0.05 MPa (10 psi).

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 mm 35 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 +- 5 Cubes stored at temp. of 20 +- 5 ooC and with 90% moist C and with 90% moist

condition.condition. De-molding after 16 – 28 hrs and specimens stored in a curing De-molding after 16 – 28 hrs and specimens stored in a curing

tank at 20 +-2 tank at 20 +-2 ooC 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.

Platen RestraintPlaten Restraint Failure of concrete under pure uniaxial compression is the Failure of concrete under pure uniaxial compression is the

ideal mode of testing.ideal mode of testing. But compression test imposes more complex stress system But compression test imposes more complex stress system

because of lateral forces developed between the end surface of because of lateral forces developed between the end surface of the concrete specimen and the adjacent steel platen of the the 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 stiffer which 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 depends The degree pf platen restraint on the concrete section depends on the friction developed at the concrete platen interface, and on 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 to there 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.

Typical Failure Modes/ CubesTypical Failure Modes/ Cubes Influence of platen restraint can be seen from typical failure Influence 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 the Effect of shear is always present and decrease towards the

center of the cube, or disintegrate so as to leave undamaged center 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 explosive Less 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 to Explosive: 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 and Other modes failure are regarded as unsatisfactory and indicate a probable fault in the testing machine.indicate a probable fault in the testing machine.

Fig. 16.1 Typical Failure Modes of test CubesFig. 16.1 Typical Failure Modes of test Cubes

a) Non-Explosive b) Explosive

Typical Failure Modes Cont.Typical Failure Modes Cont.

When the ratio of height to width of the specimen When 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 lateral The 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 the Fig. 16.3 influence of height/ diameter ratio on the

apparent strength.apparent strength.

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

Influence of Height/ Diameter Ratio Influence of Height/ Diameter Ratio on the Apparent Strengthon the Apparent Strength

As (H/D) ratio increase the apparent strength will As (H/D) ratio increase the apparent strength will decrease.decrease.

Due to larger effect of platen restraint on cubes mode Due to larger effect of platen restraint on cubes mode of failure of failure Cube strength = 1.25 Cylinder strengthCube strength = 1.25 Cylinder strength

This relation depends also on strength level and This 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 true Cylinder strength is probably closer to the true uniaxial compressive strength of concrete than the uniaxial 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.

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

Tensile StrengthTensile Strength

Tensile strength:Tensile strength: Uniaxial tension (direct tension: very difficult Uniaxial tension (direct tension: very difficult

because the ends have to be gripped and bending because 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 values Indirect methods yields higher strength values than the true tensile strength under uniaxial than the true tensile strength under uniaxial loading for reason already stated.loading for reason already stated.

Flexural TestFlexural Test The theoritical max. tensile stress reached in the bottom The theoritical max. tensile stress reached in the bottom

fiber of a test beam is known as the “Modulus of fiber 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 the The value of the modulus of rupture depends on the

dimensions of the beam and on the arrangement of dimensions of the beam and on the arrangement of loading.loading.

Symmetrical two-point loading (at third point of the Symmetrical 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 the This produces a constant bending moment between the load points so that one third of the span is subjected to load 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 take Therefore it is there where cracking is likely to take place.place.

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 a Beams tested on their side (as-cast position), in a

moist condition.moist condition. Rate of increase stress in the bottom fiber (0.02 – 0.1 Rate of increase stress in the bottom fiber (0.02 – 0.1

MPa/s (2.9 – 14.5 psi/s), lower rate for low strength MPa/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.

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

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 the If 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 loadL: Span length between supportsL: Span length between supportsd: depth of the beamd: depth of the beamb: width of the beamb: width of the beam

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 an ASTM 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 loadL: Span length between supportsL: Span length between supportsd: depth of the beamd: depth of the beamb: width of the beamb: width of the beam

If failure occurs at a section such that ((L/3) – a) > 0.05 L, then If failure occurs at a section such that ((L/3) – a) > 0.05 L, then the results should be discarded.the results should be discarded.

Flexural Strength Using One-Point Flexural 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

Splitting TestSplitting Test Concrete cylinder (or less commonly cube) of the type used in Concrete 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 testing Placed, with its axis horizontal, between platens of a testing

machine.machine. Load is increased until failure takes place by splitting in the Load 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 preventd Plywood are placed between specimen and platen tp preventd

local compressive stresses at the load line.local compressive stresses at the load line.

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 tensile According 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. loadL: length of specimenL: length of specimend: diameter or width of specimend: diameter or width of specimen

Fig. 16.5 Jigs for Supporting Test Fig. 16.5 Jigs for Supporting Test Specimen in Splitting TestSpecimen in Splitting Test

a) Cube or Prism

a) Cylinder

Test CoresTest Cores

If strength of the standard compression test If strength of the standard compression test specimen is below the specified value then specimen 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 the Or specimens are not truly representative of the

concrete in the structure (test specimen not correctly concrete in the structure (test specimen not correctly prepared, handled or cured, or testing machine could prepared, handled or cured, or testing machine could be at fault).be at fault).

Argument is often resolved by testing cores of Argument is often resolved by testing cores of hardened concrete taken from the suspect part of the hardened concrete taken from the suspect part of the structure in order to its potential strength.structure in order to its potential strength.

Test Cores Cont.Test Cores Cont.

Potential Strength: the strength equivalent to the 28 Potential 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 test Note 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.

Test Cores Cont.Test Cores Cont. ASTM C42-90 prescribe method of determining the ASTM 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 the Normal cores refer to the cores being representative of the

concrete.concrete. ACI 318-89 considers the concrete in the structure is adequate ACI 318-89 considers the concrete in the structure is adequate

if the average strength of three cores is equal to at least 85% of if the average strength of three cores is equal to at least 85% of the specified strength, and if no single core has a strength the 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 higher ACI require testing in a dry state which leads to a higher strength than when tested in a moist condition as (ASTM &BS strength than when tested in a moist condition as (ASTM &BS specify).specify).

Table 16.1 lists tests other than compressive strength, which Table 16.1 lists tests other than compressive strength, which may be made on coresmay be made on cores