1.1 Various strength of concrete
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 (Fct) = 0.35 (Fck)1/2
Where,
Fck = 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 (Fst) = 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.
| Specified grade (1) | Mean of group of 4 non – overlapping consecutive test result in N/mm2 (2) | Individual test in results in N/mm2 (3) |
| M15 and above | Fck + 0.825 * (Established standard deviation) Or, Fck + 3 N/mm2 ; which ever is greater | Fck – 3 N/mm2 |
| M20 and above | Fck + 0.825 * (Established standard deviation) Or, Fck + 4 N/mm2 ; which ever is greater | Fck + 0.825 * (Established standard deviation) Or, Fck + 3 N/mm2 ; which ever is greater |
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.