Tests for checking quality of concrete should be done for the following possible purposes:
1. To detect the variation of quality of concrete being supplied for a given specification.
2. To establish whether the concrete has attained a sufficient strength or concrete has set sufficiently for stripping, stressing, de-propping, opening to traffic etc.
3. To establish whether the concrete has gained sufficient strength for the intended purpose.
There are so many tests available for testing different qualities of concrete. Different tests give results for their respective quality of concrete. Thus it is not possible to conduct all the tests as it involves cost and time. Thus, it is very important to be sure about purpose of quality tests for concrete. The most important test for quality check of concrete is to detect the variation of concrete quality with the given specification and mix design during concrete mixing and placement. It will ensure that right quality of concrete is being placed at site and with checks for concrete placement in place, the quality of constructed concrete members will be as desired.
Following are the lists of various tests conducted for Concrete Quality:
Tests on hardened concrete:
- Compressive strength (cylinder, cube, core)
- Tensile strength: Direct tension
- Modulus of rupture
- Indirect (splitting) Test
- Density
- Shrinkage
- Creep
- Modulus of elasticity
- Absorption
- Permeability Tests on Concrete
- Freeze/thaw resistance
- Resistance to aggressive chemicals
- Resistance to abrasion
- Bond to reinforcement
- Analysis for cement content and proportions
- In situ tests: Schmidt Hammer, Concrete pull-out, break-off, cones etc.
- Ultrasonic, nuclear.
Tests on fresh concrete:
- Workability Tests (slump test and others)
- Bleeding
- Air content
- Setting time
- Segregation resistance
- Unit weight
- Wet analysis
- Temperature
- Heat generation
Of these many tests for concrete quality, in practice well over 90% of all routine tests on concrete are concentrated on compression tests and slump tests. It is also desirable to conduct fresh concrete temperature and hardened concrete density determination tests.
The reasons for the selection of compressive strength test and slump test in practice for quality control testing of concrete are:
1. All or most other properties of concrete are related to its compressive strength.
2. Compressive strength test is the easiest, most economical or most accurately determinable test.
3. Compressive strength testing is the best means available to determine the variability of concrete.
4. Slump tests also checks for variation of construction materials in mix, mainly water-cement ratio.
5. Slump test is easy and fast to determine quality of concrete before placement based on recommended slump values for the type of construction.
6. Slump test is most economical because it is done at site and does not require any laboratory or expensive testing machine.
7. Slump tests is done before the placement of concrete, so the quality of control is high as rejected mix can be discarded before pouring into the structural member. So, dismantling or repair of defective concrete members can be avoided.