The compressive strength of the concrete cylinder is one of the most common performance measures performed by the engineers in the structural design. Here, the compressive strength of concrete cylinders is determined by applying continuous load over the cylinder until failure occurs. The test is conducted on a compression-testing machine.
The cylinder specimens are cast in steel, cast iron or any mold made of non-absorbent material. Even under severe conditions, the moulds used must retain its original shape and dimensions. The mold must hold the concrete without any leakage. Before placing the concrete mix within the mold, the interior of the mold must be properly greased to facilitate easy removal of the hardened cylinder. The mixed concrete is placed into the molds in layers not less than 5cm deep. The strokes per layer during the compaction must not be less than 30 in number. Compaction must reach the underlying layers allowing the majority of the air voids to escape. The specimens are stored undisturbed in a place with at least 90% relative humidity at a temperature of 27° ± 2°C for 24 hours. After this period, the samples are taken and submerged in clean and fresh water until the testing age is reached.
Fig.1: Concrete Cylinder for Compression Test
Fig.2: Fractured Concrete Cylinder Specimen at Failure
The technician in charge of the test conducted must record the following observations before and after the compression test of the cylinder.
The strength of concrete increases with age. Table-1 shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.
Table.1: The strength of concrete at different ages in comparison with the strength at 28 days
Age | Strength per cent |
1 day | 16% |
3 days | 40% |
7 days | 65% |
14 days | 90% |
28 days | 99% |
Table.2: Compressive Strength of Different Grades of Concrete at 7 and 28 Days
Grade of Concrete | Minimum compressive strength N/mm 2 at 7 days | Specified characteristic compressive strength (N/mm 2 ) at 28 days |
M15 | 10 | 15 |
M20 | 13.5 | 20 |
M25 | 17 | 25 |
M30 | 20 | 30 |
M35 | 23.5 | 35 |
M40 | 27 | 40 |
M45 | 30 | 45 |