This test is considered as the most popular test performed on concrete in construction as it gives a general idea on the all the characteristics of concrete. Based on this test, one can either accept or reject a concrete work.
Compressive strength as a concrete property depends on several factors related to quality of used materials, mix design and quality control during concrete production.
Depending on the applied code, the test sample may be cylinder [15 cm x 30 cm is common] or cube [15 cm x 15cm x 15 cm is the most common]. For example, as per ASTM C39, a standard test method is given to get the compressive strength of concrete cylinders.
Compressive Strength Test Idea:
1- Concrete is poured in the mould and compacted properly to reduce the amount of voids.
2- After 24 hours, moulds are removed and test specimens are then placed in water for curing.
3- After the specified curing period [3, 7, 28, 56, or 91 days], specimens are tested by the compression testing machine.
4- Load is applied gradually until specimen failure.
5- Divide failure load by cross-sectional area of specimen to get the compressive strength of concrete.
Compressive Strength Test Procedure:
TOOLS/APPARATUS
Compression test machine, moulds, mixer, trowel, tamping rod (steel bar 16 mm diameter and 60 cm long)
SAMPLE PREPARATION
Samples are taken from the concrete patches used in site or prepared with the same mixture as used in the field.
Specimen size: 3 cylinders [15 cm x 30 cm] or 3 cubes [15 cm x 15cm x 15 cm]
Minimum three specimens should be tested at each selected age. The Average of there specimens gives the strength of concrete.
Concrete Mixing:
- by laboratory batch mixer, or
- by hand: First, mix the cement and fine aggregate until the mixture is thoroughly blended. Then, add the coarse aggregate and mix until the coarse aggregate is uniformly distributed. Finally, add water and mix until the concrete appears to be homogeneous and of the required consistency.
Pouring Concrete into moulds:
1- Clean the moulds properly and finish it with oil so that concrete will not stick into the mould and make latter cleaning difficult.
2- Put concrete in the moulds layer by by layer. Each layer is around 5 cm thick. Each layer is compacted with a tamping rod [35 stroke]
3- Level the top surface with a trowel
Curing:
Test specimens are kept in the moulds and stored in moist air for 24 hours and then, the specimens are removed from the moulds and submerged in fresh water for the specified curing period.
TEST PROCEDURE
1- Get the specimen out of water after curing time and wipe out excessive water from the surface.
2- Measure the dimensions of the specimen are that will be subjected to load.
3- Place the specimen in the appropriate location in the test machine. Note the load will be applied to the face perpendicular to the direction of casting.
4- Align the specimen centrally on the base plate of the machine.
5- Make the movable portion of machine touches the specimen top surface.
6- Apply the load gradually at the rate of 140 kg/cm2/minute until the specimen fails
7- Record the maximum load.
CALCULATION OF COMPRESSIVE STRENGTH
Cube size =15 cm x 15 cm x 15 cm
Area of the specimen subjected to load=150 x 150 = 2250 mm2
Maximum load at failure at 7 or 28 days = ………. N
Characteristic compressive strength(f ck) at 7 or 28 days days = Failure Load / Area = ………….. N/mm2
Average compressive strength of the 3 concrete cubes = [strength of cube 1 + strength of cube 2 + strength of cube 3] / 3 = ………….N/ mm2 (at 7 or 28 days)
GUIDANCE
The following table gives the strength of concrete at different ages as a percentage of the strength at 28 days in order to accept the test result.
| Age | % of 28 day Strength |
| 1 day | 16% |
| 3 days | 40% |
| 7 days | 65% |
| 14 days | 90% |
| 28 days | 100% |