1. Tensile strength:

• Due to brittle nature of concrete it is very weak in tension.
• Concrete develops cracks when subjected to tensile force. So it important to determine the tensile strength of concrete.

Tensile strength (F_ct) = 0.35 (F_ck)^1/2

Where,

F_ck = Characteristics compressive strength

2. Compressive strength:

• Concrete has high compressive strength.
• Most of the concrete structure are designed by adopting the value of compressive strength.

3. Shear strength:

• Occurs to the combination of tensile and compressive stress or due to the application of torsion.
• Shear strength of concrete is about 12% of compressive strength of concrete.

4. Bond strength:

• The resistance to slip of the steel reinforcement bars which are embedded in concrete.
• Higher the grade of concrete higher the bond strength.

1.2 Compressive, tensile and bond strength tests

Compressive strength tests:

1. Cube test:

• Specimen of 150 mm * 150 mm * 150 mm cube is used.
• The cube filled in three layers and well compacted.
• The top is made smooth and the finished surface is left undisturbed for 24 hours at room temperature.
• Then the mould is removed and the specimen is stored in water for curing usually up to 28 days.
• The testing of specimen is taken at 1,3,7,14,28,90 and 365 days.
• The test is performed by standard uniaxial compression test and at each time, minimum three sample are tested.

Compressive strength of concrete = Force at failure / Area

2. Cylinder test:

• The test is carried out using concrete cylinder of diameter 150 mm and 300mm height.
• The cylinder is filled in three layer and well compacted.
• Cube strength is higher than cylinder strength.

Tensile strength test:

• It is very complex to apply uniaxial tension to a concrete specimen so the tensile strength of concrete is determined by two test.

The test which are performed for tensile strength test are as follows:

1. Flexural test:

• Evaluates the tensile strength of concrete indirectly.
• Test the ability of unreinforced concrete beam or slab to withstand failure in bending.

2. Splitting test:

• Method of determining tensile strength of concrete using a cylinder which splits across vertical diameter.
• Concrete cylinder is placed horizontally.

Horizontal tensile strength (F_st) = 2P/ πLD

Where,

P = Applied load

D = Diameter of cylinder

L = Length of cylinder

Bond strength test:

• Bond test can be done by pullout test.

Pullout test:

• Destructive technique that involves drilling a hole into the hardened concrete and inserting oversized screw into that hole.

1.3 Non – destructive tests

• Test which are performed without destruction of the sample are known as non – destructive test.

Objective:

• To determine the strength of concrete.
• To determine cracks in concrete.
• To determine permeability of concrete.
• To determine the thickness of concrete layer.

Merits:

• Less wastage of material.
• Accident prevention.

Demerits:

• Expensive.
• Dangerous (Radiation hazards)

Method of non – destructive test are:

1. Rebound hammer test:

• Done to estimate the compressive strength of concrete.
• Also known as Schmidt hammer test.
• It is a device used to measure the elastic properties or strength of concrete.

Merits:

• Easy to carry out.
• Cheap.
• Skilled manpower not required.

Demerits:

• Interpretation of data is difficult.
• Only measure strength of concrete close to the surface.

2. Ultrasonic pulse velocity method:

• Used to determine the integrity and quality of concrete by measuring the speed and attenuation of ultrasonic wave passing through the element being tested.
• There are three ways of measuring pulse velocity through concrete. They are as follows:

a. Direct transmission

b. Indirect transmission

c. Surface transmission

1.4 Variability of concrete strength and acceptance criteria.

Variability of concrete strength:

• Strength concrete varies from batch to batch over a period of time.

Factors affecting variation of concrete strength:

• Quality of raw material.
• W/C ratio.
• Age of concrete.
• Compaction of concrete.
• Curing.

Acceptance criteria:

a. Compressive strength:

• The mean strength determined from any group of four non – overlapping consecutive test results, complied with the appropriate limits col 2 of table.
• Any individual test results complies with the appropriate limits in col 3 of table.

b. Flexural strength:

• Mean compressive strength of four consecutive test results must be more than equal to characteristic strength of concrete +3 N/mm².
• Any test results is not less than specified characteristic strength -0.3 N/mm².

1.5 Quality control and quality assurance

Quality control:

• Quality means degree of excellence.
• Good design is not sufficient but its actual implementation is required for fully quality work and good performance of structure.

Quality can be controlled by:

• Appropriate mixing.
• Proper compaction.
• Correct placing.
• Adequate curing.

Quality assurance:

• Systematic process of determining whether the concrete structure meets specified requirement is called quality assurance.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

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