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STRENGTH TESTS ON CONCRETE

(1) Compressive Strength Test (ASTM C 39)Basic procedure: Apply a compressive axial load to a cylindrical specimen at a prescribed rate until failure occurs. Calculate and report the compressive strength.

Compressive Strength = Maximum load /Cross-sectional area of specimen

Requirements for Cylindrical Test Specimens: (1) Length should be 2 times the diameter. Most commonly used: 12 inches in length & 6 inches in diameter. (2) Diameter should be at least 3 times the maximum aggregate size. (3) The ends should be ground or capped to provide smooth loading surfaces.

Making cylindrical test specimens in the field

Curing test specimens in a moist room

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Compressive strength test on a 6 x 12 inch concrete specimen

(1) Compressive Strength Test (Continued)Test specimens can be:(A) Molded from freshly mixed concrete and cured

in the field (ASTM C 31) or in the laboratory (ASTM C 192).

(B) cored from the hardened concrete in the field (ASTM C42).

(C) Made from cast-in-place cylinder molds (ASTM C873).

Effect of Moisture Condition:The standard procedure requires that the specimen be tested in a moist condition. Air-dried specimens can give 20 to 25% higher compressive strength than saturated specimens.

Effect of specimen diameter:

For the same length-to-diameter ratio, the compressive strength decreases as the diameter increases.

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Effect of Length-to-Diameter Ratio:

The compressive strength decreases as the L/D ratio increases.

(2) Flexural Strength Test(A) Using third-point loading (ASTM C 78)

Modulus of rupture, R = PL/bd2

where P = maximum applied load

(B) Using center-point loading (ASTM C 293)

Modulus of rupture, R = 3PL/2bd2

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Center-Point Loading Test on a Concrete Beam

(2) Flexural Strength Test (Continued)Test Specimen: The standard beam is 6 in. X 6 in. in cross section. The length of the beam should be at least 2 inches greater than 3 times the depth.Usage of test results:– The modulus of rupture by the third-point loading test is

usually used in design.– The modulus of rupture by center-point loading can be used

for quality control if relationship to third-point test results are known.

Relationship between modulus of rupture (R) and compressive strength (fc'):

ACI Equation:R= 7.5 √fc' (in psi)

(3) Splitting Tensile Strength Test (ASTM C 496)

Tensile Strength, T = 2P/πldP = maximum applied loadl = length of specimen; d = diameter of specimen

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(3) Splitting Tensile Strength Test (Continued)

Ratio of Tensile Strength to Compressive Strength= 10 to 11% for low strength concrete (<3000 psi)= 8 to 9% for medium strength concrete (3000-6000 psi)= 7% for high strength concrete (>6000 psi)

Ratio of Tensile Strength to Modulus of rupture = 48 to 53% for low strength concrete= 57 to 60% for medium strength concrete= 61 to 63% for high strength concrete

Factors Affecting Strength of Concrete(1) Effects of Porosity - Strength decreases as porosity increases.

:

Powers Equation:

fc = 34,000 x3

where:

x = solid/space ratio

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(2) Effects of Water/Cement Ratio

Abram’s w/c law:

Strength increases as w/c decreases.

(3) Effects of air entrainment - For a fixed w/c, strength decreases as air entrainment increases. The reduction in strength is more for higher strength concrete than for lower strength concrete.

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(4) Effects of aggregate size - For high-strength concrete, strength decreases as the maximum size of aggregate increases. The effect is less for lower-strength concrete.

(5) Effects of aggregate grading - When a change in aggregate grading causes a change in the consistency and the bleeding characteristics of the fresh concrete, the strength of the concrete can be adversely affected by the change in aggregate grading.

Mixes 1 & 2 had the same w/c

(6) Effects of Aggregate Type - Calcareous aggregate gives higher strength than siliceous aggregate. The difference is more substantial for high strength concrete. Rough-textured aggregate gives higher strength than smooth one.

Compressive Strength

X 103 psi

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(7) Effects of Curing Condition -

Strength development improves with moist curing

Effects of Curing Temperature - Strength development improves with higher curing temperature

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Effects of casting temperature - Lower casting temperature improves strength development

(8) Effects of using Seawater - Strength of concrete may be lower at later ages. - Increase the risk of corrosion of reinforcing steels.

Behavior of Concrete Under Various Stress States

(1) Behavior of Concrete Under Uniaxial Compression- The stress strain curve is linear up to 30% of the ultimate strength (fc’). The microcracks in the transition zone remain undisturbed up to this point.- For stresses from 30% to 50% of fc’, the microcracks in the transition zone show some extension, as seen from the increase in the curvature of the stress-strain plot. However, no cracking occurs in the mortar matrix.- For stresses from 50% to 75% of fc’, the cracks in the transition zone begin to grow. - The critical stress occurs around 75% of fc’, above which crack propagation becomes unstable, and concrete shows time-dependent fracture.

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Stress versus Axial Strain & Lateral Strain Plots for Concrete under Uniaxial Compression

Stress versus Volumetric Strain for Concrete under Uniaxial Compression

The volumetric strain reaches a maximum value at the critical stress, and reverses in direction beyond this point

Stress Strain plots for Concrete under Sustained Stress Conditions

Time-dependent fracture occurs at stresses above the critical stress level

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(1) Behavior of Concrete Under Uniaxial Compression (Continued)- The higher the rate of loading, the higher the observed strength value. However, within the range of customary testing, the effect of rate of loading on strength is not large.- Under repeated loading at stresses above 50% of fc’, the elastic modulus and the compressive strength decreases as the number of cycle increases.

(2) Behavior of Concrete Under Uniaxial Tension- The elastic modulus and Poisson’s ratio of concrete under tension are similar to those under compression.- The average load carrying area is reduced as new cracks develop in tension. Failure in tension is caused by a few bridging cracks rather than by numerous cracks.

Typical Mohr rupture diagram for concrete

(3) Shear strength of concrete = approximately 20% of the uniaxial compressive strength = the shear stress at the point the failure envelope intersects the vertical axis

(4) Behavior under Biaxial Stresses- When concrete is under compressive stresses in two directions, the compressive strength increases. The increase may be up to 27%.- When concrete is under compressive stress in one direction and tensile stress in another direction, the compressive strength decreases as the applied tensile stress increases.- When concrete is under tensile stresses in two directions, the tensile strength is approximately equal to the uniaxialtensile strength.

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Stress-Strain Plots of Concrete under Biaxial Compressive Stresses

Compressive strength increases when concrete is under biaxial compressive stresses.

Stress-Strain Plots of Concrete under Combined Tension-Compression Biaxial Stresses

Compressive strength decreases as applied tensile stress increases.

Stress-Strain Plots of Concrete under Biaxial Tensile Stresses

The tensile strength stays approximately the same.