Hardened Concrete Properties Testing of concrete

The basic method of verifying that concrete complies with the specifications is to test its strength using cubes or cylinders made from samples of fresh concrete.

concrete assumed as a brittle materialconcrete assumed as a brittle material

Compressive StrengthCompressive Strength

Cylinder : ASTM C470Cylinder : ASTM C470

Cubes : British standard 150x150x150 mmCubes : British standard 150x150x150 mm33

Other sizes:Other sizes: Cylinder: 100 Cylinder: 100 ×× 200 or 150 200 or 150× × 300 mm 300 mm

Cubes: Cubes: 100 100 ×× 100 100 ×× 100 mm 100 mm33 or or

A

PC

•For 150 mm cubes fill in 3 layers compact each layer 35 times.

•For 100 mm cubes fill in 3 layers compact each layer 25 times.

•No need for capping.

•For 150 x 300 mm cylinder, fill in 3 layers compact each layer 25 times.

•Capping to obtain a plane and smooth surface (thin layer ≈ 3mm), using:

Stiff Portland cement paste on freshly cast concrete, or mixture of sulphur and granular material, or high-strength gypsum plaster on hardened concrete.

Factors Affecting Measured Compressive StrengthFactors Affecting Measured Compressive Strength

1. Stress Distribution in Specimens.1. Stress Distribution in Specimens.

2. Effect of L/d ratio.2. Effect of L/d ratio.

3. Specimen Geometry.3. Specimen Geometry.

4. Rate of Loading.4. Rate of Loading.

5. Moisture Content.5. Moisture Content.

6. Temperature at Testing.6. Temperature at Testing.

Typical Failure Modes for Test Cubes: (a) Typical Failure Modes for Test Cubes: (a) Non-explosive; (b) explosiveNon-explosive; (b) explosive

Typical Failure Modes for Test Standard Cylinders: Typical Failure Modes for Test Standard Cylinders:

a) Splitting; a) Splitting; (b) Shear;(b) Shear; (c) Splitting and (c) Splitting and shear shear

(cone).(cone).

2. Effect of L/d ratio

The standard cylinder has a length to diameter ratio of 2.0 If L/D The standard cylinder has a length to diameter ratio of 2.0 If L/D ratio is other than 2.0 a correction factor must be applied to ratio is other than 2.0 a correction factor must be applied to count for the restrainment effect of the platens; discussed count for the restrainment effect of the platens; discussed earlier.earlier.

Reference Cylinder : L/D =2Reference Cylinder : L/D =2

Strength (L/D) = C.F x Strength (L/D=2)Strength (L/D) = C.F x Strength (L/D=2)

3. Specimen Geometry

Different geometries for a concrete specimen can be Different geometries for a concrete specimen can be used: Prisms, Cubes, and cylinders.used: Prisms, Cubes, and cylinders.

As stated before, cube are more confined by the As stated before, cube are more confined by the platens thus have higher strength than cylinder platens thus have higher strength than cylinder made of the same concrete. It has been found that made of the same concrete. It has been found that cc=1.25 =1.25 cyl .cyl .

As specimen size increases, strength decreases.As specimen size increases, strength decreases.

4. Rate of Loading Higher rate of lading Higher rate of lading higher strength. higher strength.

5. Moisture Content

Standards require testing of concrete in SSD conditions (ASTM C39).Standards require testing of concrete in SSD conditions (ASTM C39). 6. Temperature at Testing

Higher Temperature Higher Temperature lower strength lower strength

1. Direct Tensile: No standard Test 1. Direct Tensile: No standard Test

2. Indirect Tensile:2. Indirect Tensile: A. Splitting Tension Test.A. Splitting Tension Test.

The tensile strength of concrete is approximately equal to The tensile strength of concrete is approximately equal to 10% of its compressive strength.10% of its compressive strength.

Tensile strength:Tensile strength:

LD

P2sp

ff : : The test is useful since most concrete members is loaded in The test is useful since most concrete members is loaded in

bending rather than in axial tension. Thus, it represents the bending rather than in axial tension. Thus, it represents the concrete property of interest. concrete property of interest. ff is calculated as: is calculated as:

I

MC

I

MC

B.Flexural strengthB.Flexural strength

This test is mostly used for quality control of highways and airport This test is mostly used for quality control of highways and airport runways. It gives more useful information than do compression runways. It gives more useful information than do compression tests.tests.

Flexural strength:Flexural strength:Affected by:Affected by: - Specimen Size - Specimen Size strength strength - Temperature: Same as in compression.- Temperature: Same as in compression.

The tensile strength of concrete is approximately equal to 10% of The tensile strength of concrete is approximately equal to 10% of its compressive strength.its compressive strength.

Strength of concrete

Strength = ability to resist stress without failure.

Concrete strength made of:

1. Strength of paste or mortar.

2. Strength of CA-paste (mortar) interface.

3. Strength of CA.

Cracks at the bond between the aggregate, rebar, and paste (see arrows).

Factors Affecting Strength of ConcreteFactors Affecting Strength of Concrete

1. Water/Cement Ratio1. Water/Cement Ratio

Since the W/C ratio controls the porosity of concrete, it controls Since the W/C ratio controls the porosity of concrete, it controls the strength as well.the strength as well.

W/C W/C strength strength

2. Degree of Compaction2. Degree of Compaction

Strength = f (full compaction)Strength = f (full compaction)

Relation between strength and W/C ratioRelation between strength and W/C ratio

3. Curing Time:3. Curing Time: In practice, it is common to obtain 7-day as well as 28-day In practice, it is common to obtain 7-day as well as 28-day

compressive strength. compressive strength.

4. Cement:4. Cement:

The effect of Portland cement on concrete strength depends The effect of Portland cement on concrete strength depends on the chemical composition and fineness of the cement.on the chemical composition and fineness of the cement.

5. Aggregates:5. Aggregates:aggregates Shape and Textureaggregates Shape and Texture

Texture depends on whether aggregate is natural (gravel )or Texture depends on whether aggregate is natural (gravel )or crushed.crushed.

Dmax Dmax Reducing the specific surface area Reducing the specific surface area Less Bond Less Bond Strength Strength

II. Dmax II. Dmax More restraint on volume changes in the More restraint on volume changes in the pastepaste

Inducing additional stresses in paste Inducing additional stresses in paste Strength Strength

III. Dmax III. Dmax Water content Water content Strength Strength

C. Aggregate StrengthC. Aggregate Strength

B. maximum Aggregate Size (Dmax)

Stress-Strain Diagram of Concrete: