Compressive strength of concrete refers to the strength of hardened concrete measured by the concrete compression testing machine. This strength is a measure of the ability of concrete to resist loads that tend to compress it. The measurement is conducted by crushing of specimens of cylindrical concrete in the machine.
Concrete’s compressive strength can be estimated by the division of failure load by area of the cross-section that resists the load, which is reported as Pounds per Square Inch (PSI) in the US and Mega Pascals (MPa) in the SI units measures. The concrete’s compressive strength can differ from 17 MPa (2500 psi) for residential application and 28 MPa (4000 psi) and more in commercial buildings. If there are higher strengths, exceeding 10,000 psi or 70 MPa, they are ideal for some other applications.
Results of compressive strength tests are mainly used to ascertain that the concrete mix delivered on-site meets the needs of specified strength fc’, as per the specification of the job. Cylinders tested for quality control and acceptance are made and cured as per procedures spelt out for standard-cured specimens in ASTM-C31 (global standards for making and curing concrete).
Test results are the average of at least 2 specimens with standard curing, derived from the identical batch of concrete which have been tested in the same period. In the majority of cases, the strength needs of concrete are 28 days.
Compressive strength test using concrete testing apparatus is the common test conducted on concrete material in the arena of construction because it offers a basic notion of concrete qualities. On the basis of such a test, the concrete work can be either rejected or accepted.
Compressive strength depends on many factors like quality of materials used, quality control all through production, and mix design. Based on the code of application, the sample for test may be Cylindrical (15 cm x 30 cm as standard) or Cube (which is 15 cm x 15cm x 15 cm).
Procedure for testing
- The concrete is tipped into the mould. It is properly compacted to reduce the quantum of voids in it.
- Following 24 hours, the moulds are taken away, and specimens for testing are kept inside water for the process of curing.
- Following the end of the curing period (91/56/28/7/3 days), the apparatus for testing concrete compression are used to test specimens.
- Gradually, the load is spread over till the failure of the specimen.
- One must do a division of failure load by area of cross-section of specimen for calculating compressive strength of the concrete.
These include a Compression testing machine, mixer, moulds, tamping rod (60 m long steel bar), and trowel.
Preparation Of Sample
From the patches of concrete, samples are taken or prepared using the same mixture in the area. Size of specimens is 3 cubes (15 cm x 15 cm x 15 cm) or 3 cylinders (15 cm x 30 cm).
A minimum of 3 specimens must be tested for each age chosen. The average value of the 3 specimens provides the measure of the strength of the concrete.
These are all some of the basic steps meant for testing of compressive strength of concrete. A point to note is that when concrete fails to meet specific needs, the failure can also be due to the testing procedure. This is true if testing, curing, handling and fabrication of cubes or cylinders have not been conducted as per standard procedures.