1) Cube Test
This is the conventional method of testing the compressive strength of concrete. This can be described as a “destructive” method of testing as casted concrete cubes are crushed to determine the compressive strength.
In this testing procedure concrete cubes are casted in a standard way such that they represent the batches of concrete used for concreting. The cubes are of the size 150x150x150mm and each cube is casted in three layers with specified compaction to each layer. Cubes are properly cured and crushed at the ages of 7, 28 etc. Three cubes are crushed at a time and their mean crushing strength is taken as the compressive strength of concrete.
2) Impact Hammer Test
This is a test specially designed for non-destructive testing of concrete structures. Using this test large areas can be tested for compressive strength consuming less time and money. Possibility of determination of concrete quality between different sections is also an advantage of this test.
The testing is done by pressing a impact plunger against a solid surface of concrete. The maximum height of rebound is recorded and a rebound number is obtained according to the scale of Impact Hammer. This rebound number can then be converted in to compressive strength via conversion tables provided with the equipment. These conversion values vary with the angle of testing and therefore angular corrections should be applied when necessary.
3) Ultrasonic Test.
Ultrasonic test for concrete is a field test for quality control and inspection of concrete structures. It uses measurement of the speed of ultrasonic pulses through the concrete to correlate concrete strength to standard strength. Using this test we can identify non-homogeneous conditions in the structure such as honeycombs, voids, cracks and frozen concrete.
First the instrument is calibrated using a standard calibration bar and the receiving and the transmitting ends are coated with a thin uniform grease layer. The concrete surface is cleaned well to make it dust free and uniformly distributed grease layer is applied on the concrete surface as well. The transmitting and receiving ends of the apparatus is then placed on marked locations of the structure and the machine is turned on to get the reading. The test is repeated after switching the positions of the transmitting and receiving ends. Then obtained readings are compared with the standard values to arrive at a conclusion about the concrete quality.