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Concrete Testing Materials of Construction Dr. TALEB M. AL-ROUSAN
Introduction
Testing is the basic method to that concrete complies with the specifications. Strength of concrete is verified by testing samples (cubes, cylinders, or prisms) made of fresh concrete. Disadvantage: Suspected concrete may have been placed and hardened when testing take place. Accelerated strength tests are some times used to offset this disadvantage. When samples fail in testing further investigation of concrete may be performed using non-destructive testing.
Precision of Testing
Concrete properties vary. Precision: general term used for the closeness of agreement between replicate test results.
Repeatability: the value below which the absolute difference between two single test results obtained with the same method on identical test material under the same conditions (i.e. same: operator, apparatus, Lab, and short interval of time) may be expected to lie within a specified probability (usually 95%). Reproducibility: the value below which the absolute difference between two single tests results, obtained with the same method on identical test material under different conditions (i.e. different: operators, apparatus, Labs, time) may be expected to lie within a specified probability (usually 95%).
Precision Cont.
Values of repeatability & reproducibility are applied in a variety of ways:
To the experimental technique of a lab is up to requirements. To compare results of tests performed on a sample from a batch of material with specification. To compare test results obtained by a supplier and by consumer on the same batch of material.
Analysis of Fresh Concrete
Determination of composition of concrete at an early age can be of benefit since knowing that actual proportioning correspond to those specified will conclude that there is little need for testing the strength of hardened concrete. Properties of interest: (W/c) ratio & cement content (mainly responsible for ensuring that concrete is strong & durable). BS suggest five methods to assess cement content:
Buoyancy method Chemical method Constant volume method Physical separation method Pressure filter method
Water content of fresh concrete can be found as in chemical method, or by rapid drying method (different in mass before and after heating).
Strength Tests
Compressive strength Tensile strength:
Uniaxial tension (direct tension: very difficult) Flexure test (Indirect) Splitting test (Indirect) Indirect methods yields higher strength values than the true tensile strength under uniaxial loading for reason already stated.
Method of Testing Concrete In Tension
Different test methods yield numerically different results, ordered as follows: Direct tension < Splitting < flexural tension
Reasons for that: 1.
2.
With the usual size of Lab. Specimen, the volume of concrete subjected to tensile stress decrease in the order listed above. Statistically there is a greater chance of a weak element and therefore of failure for larger volume than in a small volume. Both splitting and flexural test involve non-uniform stress distribution which impede the propagation of cracks, and therefore delay the ultimate failure. Whereas, in the direct test, the stress distribution is uniform, so that once a crack has formed, it can propagate quickly through the section of the specimen.
Compressive Strength
Determined using 150 x 300 mm (6 x 12 in) cylinders in US and 150 mm (6 in) cubes in UK. Smaller specimen sizes can be used based on Agg. Max. size. Molds (re-usable or non re-usable) must be oiled from inside to prevent bond with concrete. ACCORDING TO ASTM (Cylindrical molds are used). For high-slump concrete: Concrete placed in cylindrical molds in three layers, and each layer is compacted 25 times with a rod (3/8 in D). For low-slump concrete: Concrete placed in two layers and compacted using internal or external vibration. Top surface of cylinders must be plane, smooth, and normal to its axis. Plane surface can be obtained using two methods”
Grinding Capping (expensive)
Compressive Strength/ Capping
Materials used:
Stiff Portland cement paste on freshly-cast concrete. Mixture of Sulphur and granular material (e.g. milled fire clay)…… Best capping material Mixture of sulphur and high strength Gypsum plaster.
Cap should be thin (1.5 to 3 mm) and has strength similar to that of concrete. Caution: Toxic fumes are produced when capping with Sulphur mixtures.
Compressive Strength/ Curing
ASTM C 192-90a curing conditions for standard test cylinders. Molded specimens are stored for not less than 20 and not more than 48 hrs at temp. of 23 +- 1.7 oC so that moisture loss is prevented. After removing from molds, specimens are stored at the same temp. and under moist conditions or in a saturated lime water until the prescribed age of testing. Cured cylinders give potential strength. Service cylinders may be used to determine the actual quality of concrete in the structure by being subjected to the same conditions as the structure.
Compressive Strength Test
ASTM C39-86 compressive strength of a cylinder. Loading rate for hydraulically operated machines is 0.15 – 0.34 MPa/ s (20 -50 psi/s) Deformation rate for mechanically operated machines is 1.3 mm/min (0.05 in/min). Compressive strength = (max. load / cross section area of cylinder) reported to the nearest 0.05 MPa (10 psi).
Compressive Strength Cont.
ACCORDING TO BS (Cube molds are used). Molds filled in 3 layers. 35 strokes/ layer for 150 mm cubes, or 25 strokes for 100 mm cubes, using a 25 mm (1 in) square steel punner. Alternatively vibration can be used. Top surface finished by a trowel. Cubes stored at temp. of 20 +- 5 oC and with 90% moist condition. De-molding after 16 – 28 hrs and specimens stored in a curing tank at 20 +-2 oC until test age. Testing ages: common 3, 7, 28 days, uncommon 1, 2, 14 days, 13, 26 weeks, and 1 year.
Platen Restraint
Failure of concrete under pure uniaxial compression is the ideal mode of testing. But compression test imposes more complex stress system because of lateral forces developed between the end surface of the concrete specimen and the adjacent steel platen of the testing machine. These forces are induced by the restraint of the concrete, which attempts to expand laterally, by the several time stiffer steel, which has a much smaller lateral expansion. The degree pf platen restraint on the concrete section depends on the friction developed at the concrete platen interface, and on the distance of the end surfaces of the concrete. Consequently, in addition to the imposed uniaxial compresion, there is a lateral shearing stress, the effect of which is to increase the apparent compressive strength of concrete.
Typical Failure Modes/ Cubes
Influence of platen restraint can be seen from typical failure modes. Fig. 16.1 Failure modes in test cubes. Effect of shear is always present and decrease towards the center of the cube, or disintegrate so as to leave undamaged central core (Non-explosive). Testing machine is rigid. Less rigid machine can store more energy so that the explosive failure is possible. Explosive: One face touching the platen disintegrate so as to leave a pyramid or a cone. Other modes failure are regarded as unsatisfactory and indicate a probable fault in the testing machine.
Fig. 16.1 Typical Failure Modes of test Cubes
a) Non-Explosive
b) Explosive
Typical Failure Modes Cont.
When the ratio of height to width of the specimen increase, the influence of shear becomes smaller. The central part of the specimen may fail by lateral splitting. Situation in cylinders (H/W = 2). Possible modes of failure in Cylinders: Splitting, Shear, and splitting & shear. Fig. 16.2 Failure modes in test cylinders. Fig. 16.3 influence of height/ diameter ratio on the apparent strength.
Fig. 16.2 Typical Failure Modes of Test Cylinders
a) Splitting
b) Shear (Cone)
c) Splitting & Shear
Influence of Height/ Diameter Ratio on the Apparent Strength
As (H/D) ratio increase the apparent strength will decrease. Due to larger effect of platen restraint on cubes mode of failure Cube strength = 1.25 Cylinder strength
This relation depends also on strength level and moisture condition of concrete when tested. Cylinder strength is probably closer to the true uniaxial compressive strength of concrete than the cube strength because:
Less end restraint. More uniform distribution of stress over the cross section.
Fig. 16.3 Influence of Height/ Diameter Ratio on The Apparent Strength of A cylinder
Tensile Strength
Tensile strength:
Uniaxial tension (direct tension: very difficult because the ends have to be gripped and bending must be avoided) Flexure test (Indirect) Splitting test (Indirect)
Indirect methods yields higher strength values than the true tensile strength under uniaxial loading for reason already stated.
Flexural Test
The theoritical max. tensile stress reached in the bottom fiber of a test beam is known as the “Modulus of Rupture”. Relevant to the design of highways and airfields. The value of the modulus of rupture depends on the dimensions of the beam and on the arrangement of loading. Symmetrical two-point loading (at third point of the span) is used in UK ans US. This produces a constant bending moment between the load points so that one third of the span is subjected to the max. stress. Therefore it is there where cracking is likely to take place.
Flexural Test Cont.
BS
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. Curing as specified in BS. Beams tested on their side (as-cast position), in a moist condition. 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 concrete and higher rate for high strength concrete.
Fig. 16.4 Arrangement for The Modulus of Rupture Test
Flexural Test Cont.
ASTM C78- 84
Similar flexural test as in BS except: 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).
If fracture occurs within the middle one-third of the beam, the modulus of rupture (fbt) is given by: Fbt = (P L) / (b d2) P: Max total load L: Span length between s d: depth of the beam b: width of the beam
Flexural Test Cont.
If fracture takes place outside the middle one-third of the beam, then:
According to BS the test result should be discarded. ASTM C78-84 allows for failure outside the load points, say, at at an average distance (a) from the nearest . modulus of rupture (fbt) is given by: Fbt = (3 P a) / (b d2) P: Max total load L: Span length between s d: depth of the beam b: width of the beam
If failure occurs at a section such that ((L/3) – a) > 0.05 L, then the results should be discarded.
Flexural Strength Using One-Point Loading
Modulus of rupture (fbt) is given by: Fbt = (3 P L) / (2 b d2) P: Max total load L: Span length between s d: depth of the beam b: width of the beam
Splitting Test
Concrete cylinder (or less commonly cube) of the type used in compressive strength testing. Placed, with its axis horizontal, between platens of a testing machine. Load is increased until failure takes place by splitting in the plane containing the vertical diameter of the specimen Fig. 16.5 Jigs for ing test specimen in splitting test ASTM C496 -90 Plywood are placed between specimen and platen tp preventd local compressive stresses at the load line.
Splitting Test Cont.
Rate of loading:
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).
According to ASTM C 496 -90 the tensile splitting strength (fst) is given by: fst = 2P/ πLd P: Max. load L: length of specimen d: diameter or width of specimen
Fig. 16.5 Jigs for ing Test Specimen in Splitting Test a) Cylinder
a) Cube or Prism
Test Cores
If strength of the standard compression test specimen is below the specified value then either:
The concrete in the actual structure is unsatisfactory. Or specimens are not truly representative of the concrete in the structure (test specimen not correctly prepared, handled or cured, or testing machine could be at fault). Argument is often resolved by testing cores of hardened concrete taken from the suspect part of the structure in order to its potential strength.
Test Cores Cont.
Potential Strength: the strength equivalent to the 28 days strength of the standard test specimen. When translating core strength into potential strength, take into differences in:
Type of specimen and curing conditions. Age Degree of compaction
Note that core taking damages the structure, so test cores should be taken only when other, nondestructive, methods are inadequate.
Test Cores Cont.
ASTM C42-90 prescribe method of determining the compressive strength of cores. It is desirable to obtain cores free from reinforcement. Normal cores refer to the cores being representative of the concrete. 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 the specified strength, and if no single core has a strength lower than 75% of the specified value. ACI require testing in a dry state which leads to a higher strength than when tested in a moist condition as (ASTM &BS specify). Table 16.1 lists tests other than compressive strength, which may be made on cores
Introduction
Testing is the basic method to that concrete complies with the specifications. Strength of concrete is verified by testing samples (cubes, cylinders, or prisms) made of fresh concrete. Disadvantage: Suspected concrete may have been placed and hardened when testing take place. Accelerated strength tests are some times used to offset this disadvantage. When samples fail in testing further investigation of concrete may be performed using non-destructive testing.
Precision of Testing
Concrete properties vary. Precision: general term used for the closeness of agreement between replicate test results.
Repeatability: the value below which the absolute difference between two single test results obtained with the same method on identical test material under the same conditions (i.e. same: operator, apparatus, Lab, and short interval of time) may be expected to lie within a specified probability (usually 95%). Reproducibility: the value below which the absolute difference between two single tests results, obtained with the same method on identical test material under different conditions (i.e. different: operators, apparatus, Labs, time) may be expected to lie within a specified probability (usually 95%).
Precision Cont.
Values of repeatability & reproducibility are applied in a variety of ways:
To the experimental technique of a lab is up to requirements. To compare results of tests performed on a sample from a batch of material with specification. To compare test results obtained by a supplier and by consumer on the same batch of material.
Analysis of Fresh Concrete
Determination of composition of concrete at an early age can be of benefit since knowing that actual proportioning correspond to those specified will conclude that there is little need for testing the strength of hardened concrete. Properties of interest: (W/c) ratio & cement content (mainly responsible for ensuring that concrete is strong & durable). BS suggest five methods to assess cement content:
Buoyancy method Chemical method Constant volume method Physical separation method Pressure filter method
Water content of fresh concrete can be found as in chemical method, or by rapid drying method (different in mass before and after heating).
Strength Tests
Compressive strength Tensile strength:
Uniaxial tension (direct tension: very difficult) Flexure test (Indirect) Splitting test (Indirect) Indirect methods yields higher strength values than the true tensile strength under uniaxial loading for reason already stated.
Method of Testing Concrete In Tension
Different test methods yield numerically different results, ordered as follows: Direct tension < Splitting < flexural tension
Reasons for that: 1.
2.
With the usual size of Lab. Specimen, the volume of concrete subjected to tensile stress decrease in the order listed above. Statistically there is a greater chance of a weak element and therefore of failure for larger volume than in a small volume. Both splitting and flexural test involve non-uniform stress distribution which impede the propagation of cracks, and therefore delay the ultimate failure. Whereas, in the direct test, the stress distribution is uniform, so that once a crack has formed, it can propagate quickly through the section of the specimen.
Compressive Strength
Determined using 150 x 300 mm (6 x 12 in) cylinders in US and 150 mm (6 in) cubes in UK. Smaller specimen sizes can be used based on Agg. Max. size. Molds (re-usable or non re-usable) must be oiled from inside to prevent bond with concrete. ACCORDING TO ASTM (Cylindrical molds are used). For high-slump concrete: Concrete placed in cylindrical molds in three layers, and each layer is compacted 25 times with a rod (3/8 in D). For low-slump concrete: Concrete placed in two layers and compacted using internal or external vibration. Top surface of cylinders must be plane, smooth, and normal to its axis. Plane surface can be obtained using two methods”
Grinding Capping (expensive)
Compressive Strength/ Capping
Materials used:
Stiff Portland cement paste on freshly-cast concrete. Mixture of Sulphur and granular material (e.g. milled fire clay)…… Best capping material Mixture of sulphur and high strength Gypsum plaster.
Cap should be thin (1.5 to 3 mm) and has strength similar to that of concrete. Caution: Toxic fumes are produced when capping with Sulphur mixtures.
Compressive Strength/ Curing
ASTM C 192-90a curing conditions for standard test cylinders. Molded specimens are stored for not less than 20 and not more than 48 hrs at temp. of 23 +- 1.7 oC so that moisture loss is prevented. After removing from molds, specimens are stored at the same temp. and under moist conditions or in a saturated lime water until the prescribed age of testing. Cured cylinders give potential strength. Service cylinders may be used to determine the actual quality of concrete in the structure by being subjected to the same conditions as the structure.
Compressive Strength Test
ASTM C39-86 compressive strength of a cylinder. Loading rate for hydraulically operated machines is 0.15 – 0.34 MPa/ s (20 -50 psi/s) Deformation rate for mechanically operated machines is 1.3 mm/min (0.05 in/min). Compressive strength = (max. load / cross section area of cylinder) reported to the nearest 0.05 MPa (10 psi).
Compressive Strength Cont.
ACCORDING TO BS (Cube molds are used). Molds filled in 3 layers. 35 strokes/ layer for 150 mm cubes, or 25 strokes for 100 mm cubes, using a 25 mm (1 in) square steel punner. Alternatively vibration can be used. Top surface finished by a trowel. Cubes stored at temp. of 20 +- 5 oC and with 90% moist condition. De-molding after 16 – 28 hrs and specimens stored in a curing tank at 20 +-2 oC until test age. Testing ages: common 3, 7, 28 days, uncommon 1, 2, 14 days, 13, 26 weeks, and 1 year.
Platen Restraint
Failure of concrete under pure uniaxial compression is the ideal mode of testing. But compression test imposes more complex stress system because of lateral forces developed between the end surface of the concrete specimen and the adjacent steel platen of the testing machine. These forces are induced by the restraint of the concrete, which attempts to expand laterally, by the several time stiffer steel, which has a much smaller lateral expansion. The degree pf platen restraint on the concrete section depends on the friction developed at the concrete platen interface, and on the distance of the end surfaces of the concrete. Consequently, in addition to the imposed uniaxial compresion, there is a lateral shearing stress, the effect of which is to increase the apparent compressive strength of concrete.
Typical Failure Modes/ Cubes
Influence of platen restraint can be seen from typical failure modes. Fig. 16.1 Failure modes in test cubes. Effect of shear is always present and decrease towards the center of the cube, or disintegrate so as to leave undamaged central core (Non-explosive). Testing machine is rigid. Less rigid machine can store more energy so that the explosive failure is possible. Explosive: One face touching the platen disintegrate so as to leave a pyramid or a cone. Other modes failure are regarded as unsatisfactory and indicate a probable fault in the testing machine.
Fig. 16.1 Typical Failure Modes of test Cubes
a) Non-Explosive
b) Explosive
Typical Failure Modes Cont.
When the ratio of height to width of the specimen increase, the influence of shear becomes smaller. The central part of the specimen may fail by lateral splitting. Situation in cylinders (H/W = 2). Possible modes of failure in Cylinders: Splitting, Shear, and splitting & shear. Fig. 16.2 Failure modes in test cylinders. Fig. 16.3 influence of height/ diameter ratio on the apparent strength.
Fig. 16.2 Typical Failure Modes of Test Cylinders
a) Splitting
b) Shear (Cone)
c) Splitting & Shear
Influence of Height/ Diameter Ratio on the Apparent Strength
As (H/D) ratio increase the apparent strength will decrease. Due to larger effect of platen restraint on cubes mode of failure Cube strength = 1.25 Cylinder strength
This relation depends also on strength level and moisture condition of concrete when tested. Cylinder strength is probably closer to the true uniaxial compressive strength of concrete than the cube strength because:
Less end restraint. More uniform distribution of stress over the cross section.
Fig. 16.3 Influence of Height/ Diameter Ratio on The Apparent Strength of A cylinder
Tensile Strength
Tensile strength:
Uniaxial tension (direct tension: very difficult because the ends have to be gripped and bending must be avoided) Flexure test (Indirect) Splitting test (Indirect)
Indirect methods yields higher strength values than the true tensile strength under uniaxial loading for reason already stated.
Flexural Test
The theoritical max. tensile stress reached in the bottom fiber of a test beam is known as the “Modulus of Rupture”. Relevant to the design of highways and airfields. The value of the modulus of rupture depends on the dimensions of the beam and on the arrangement of loading. Symmetrical two-point loading (at third point of the span) is used in UK ans US. This produces a constant bending moment between the load points so that one third of the span is subjected to the max. stress. Therefore it is there where cracking is likely to take place.
Flexural Test Cont.
BS
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. Curing as specified in BS. Beams tested on their side (as-cast position), in a moist condition. 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 concrete and higher rate for high strength concrete.
Fig. 16.4 Arrangement for The Modulus of Rupture Test
Flexural Test Cont.
ASTM C78- 84
Similar flexural test as in BS except: 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).
If fracture occurs within the middle one-third of the beam, the modulus of rupture (fbt) is given by: Fbt = (P L) / (b d2) P: Max total load L: Span length between s d: depth of the beam b: width of the beam
Flexural Test Cont.
If fracture takes place outside the middle one-third of the beam, then:
According to BS the test result should be discarded. ASTM C78-84 allows for failure outside the load points, say, at at an average distance (a) from the nearest . modulus of rupture (fbt) is given by: Fbt = (3 P a) / (b d2) P: Max total load L: Span length between s d: depth of the beam b: width of the beam
If failure occurs at a section such that ((L/3) – a) > 0.05 L, then the results should be discarded.
Flexural Strength Using One-Point Loading
Modulus of rupture (fbt) is given by: Fbt = (3 P L) / (2 b d2) P: Max total load L: Span length between s d: depth of the beam b: width of the beam
Splitting Test
Concrete cylinder (or less commonly cube) of the type used in compressive strength testing. Placed, with its axis horizontal, between platens of a testing machine. Load is increased until failure takes place by splitting in the plane containing the vertical diameter of the specimen Fig. 16.5 Jigs for ing test specimen in splitting test ASTM C496 -90 Plywood are placed between specimen and platen tp preventd local compressive stresses at the load line.
Splitting Test Cont.
Rate of loading:
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).
According to ASTM C 496 -90 the tensile splitting strength (fst) is given by: fst = 2P/ πLd P: Max. load L: length of specimen d: diameter or width of specimen
Fig. 16.5 Jigs for ing Test Specimen in Splitting Test a) Cylinder
a) Cube or Prism
Test Cores
If strength of the standard compression test specimen is below the specified value then either:
The concrete in the actual structure is unsatisfactory. Or specimens are not truly representative of the concrete in the structure (test specimen not correctly prepared, handled or cured, or testing machine could be at fault). Argument is often resolved by testing cores of hardened concrete taken from the suspect part of the structure in order to its potential strength.
Test Cores Cont.
Potential Strength: the strength equivalent to the 28 days strength of the standard test specimen. When translating core strength into potential strength, take into differences in:
Type of specimen and curing conditions. Age Degree of compaction
Note that core taking damages the structure, so test cores should be taken only when other, nondestructive, methods are inadequate.
Test Cores Cont.
ASTM C42-90 prescribe method of determining the compressive strength of cores. It is desirable to obtain cores free from reinforcement. Normal cores refer to the cores being representative of the concrete. 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 the specified strength, and if no single core has a strength lower than 75% of the specified value. ACI require testing in a dry state which leads to a higher strength than when tested in a moist condition as (ASTM &BS specify). Table 16.1 lists tests other than compressive strength, which may be made on cores
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