Cube test of concrete is performed to calculate compressive strength of concrete.
In this article, we will study the compressive strength of concrete by using the cube test. The cube test is the most popular method to determine the compressive strength of concrete. Cube test determines whether poured concrete has desirable strength or not.
Table of Contents
Introduction Of Concrete Cube Test:
We can find characteristics of concrete with the single compressive strength of the concrete cube test. With this single test, one can find whether concreting is done properly or not.
For general construction work, concrete compressive strength varies from 15 Mpa (2200Psi) to 30 Mpa (4400psi) and even higher in commercial and industrial structures. The production of concrete compressive strength of concrete depends on different factors such as cement strength, quality of concrete material, water-cement ratio, etc.
A compressive strength test is carried out either on a cylinder or cube. Different standard codes recommend a concrete cylinder or concrete hexahedrons as the standard specimen for the test.
Definition Of Concrete Compressive Strength:
The ability of material or structure to carry the loads on its surface without any crack and deflection is called Concrete compressive strength. Under compression, the material tends to reduce the size while the size elongates in tension.
The formula for Compressive strength:–
Compressive Strength = Load/Cross Sectional Area
It is the Load applied at the point of failure to the cross-sectional area of the face on which load is applied.
Test Procedure Of Compressive Strength Of Concrete Cubes:
There are two types of specimens either cubes of 10 cm*10 cm*10 cm or 15 cm*15 cm*15 cm both depend on the sizes of aggregates used. Commonly sizes of 15 cm*15cm*15 cm cubical molds are used for most of the works.
Concrete is poured and suitably forced so that there will be no voids. After 24 hours specimen is put into water for curing after removing molds. Specimens’ top surface should be smooth and made even. To get the smooth texture of the specimen cement paste is spread smoothly on the whole area.
After 7 or 28 days of curing, the specimen is tested by the compression testing machine. A load of 140 kg/cm2 per minute is gradually applied until the specimen fails. So we get the compressive strength of concrete, by the load at the failure divided by the area of the specimen.
Steps For Testing The Compressive Strength Of Concrete Cubes:
1. Apparatus for Cube Test
Compression testing machine.
2. Concrete Cube Preparation specimen
Proportion and material for making the test specimen should be the same concrete used in the field.
3. Specimen
15 cm size of 6 cubes size Mix. M15 or above to be taken. See the below image.
4. Mixing of Concrete for Cube test
Concrete can be mixed either by hand or by laboratory batch mixer
5. Procedure for concrete mixing
- In the non-absorbent platform and in the waterproof platform a cement and fine aggregates are mixed until the mixture is thoroughly blended and is a uniform color.
- Add the cement and fine aggregates and mix with coarse aggregate till the coarse aggregate is uniformly distributed through the batch.
For appropriate and desired consistency add water and mix it until the concrete appears to be homogeneous.
6. Procedure to make Cubes for test
- Clean the mould and apply oil.
- Approximately 5 cm thick layers of concrete are to be filled in the mould.
- Each layer is compact with not less than 35 strokes per layer with a tamping rod.
- The top surface to be leveled and smoothen by a trowel.
7. Curing of Cubes
The specimen to be tested is to be stored in moist air for 24hours and after specimen should be marked and removed from the molds and in clean fresh water, it is submerged.
The temperature of the water must be at 27 plus minus 2 degrees Celsius.
8. Concrete cube test Procedure
- After the Specified curing time remove the specimen from water and excess water is to be wiped out from the surface.
- Take the dimension to be nearest 0.2 m.
- The bearing surface of the testing machine is to be cleaned.
- The specimen in the machine is to be put in such a manner that the load should be applied to the opposite sides of the cube cast.
- The specimen should be aligned centrally on the base plate of the machine.
- The movable portion should be rotated gently with hands so that it touches the top surface of the specimen.
- Load should be applied gradually without the shock at the rate of 140kg per cm2 per minute continuously till the specimen gets fail.
- Maximum load is to be recorded and unusual features in the type of failure are noted.
Note:
- Minimum 3 specimens should be tested at each selected age.
- Rejection of those specimens is taken into account whose strength varies by more than 15% of average strength.
- An average of 3 specimens gives us the crushing strength of concrete.
Cube Tests Reports:
- Identification Marks
- Date of test
- Age of Specimen
- Date of Completion of specimen and curing conditions
- Showing of Fractured faces of concrete and the type of fracture if they are not usual
Concrete Compressive Strength At Various Interval:
With age Concrete compressive strength increases. Here is the table to show the concrete compressive strength at different stages after casting.
| Day | Strength |
|---|---|
| 1 DAY | 16% |
| 3 DAYS | 40% |
| 7 DAYS | 65% |
| 14 DAYS | 90 % |
| 28 DAYS | 99% |
Different Grades Of Concrete Compressive Strength At 7 Days And 28 Days:
| Concrete Grade | Minimum Compressive strength N/mm2 at 7 days | Specified Characteristics compressive strength at 28 days in N/mm2 |
|---|---|---|
| M15 | 10 | 15 |
| M20 | 13.5 | 20 |
| M25 | 17 | 25 |
| M30 | 20 | 30 |
| M35 | 23.5 | 35 |
| M40 | 27 | 40 |
Conclusion:
The concrete compressive strength test or cube test represents the quality of poured concrete on the construction site. If concrete cubes do not acquire the required strength then poured concrete is repaired by different methods such as grouting or injection of chemicals to increase the strength of the concrete.
FAQS
The purpose of the cube test is to determine the compressive strength of concrete.
A cube mold, concrete mix, curing conditions, and a compression testing machine are required.
Usually, three concrete cubes are tested at a time to get a more accurate average compressive strength value.
The concrete cubes should be cured for a minimum of 28 days before testing.
The standard size of concrete cubes used for the test is 150 mm x 150 mm x 150 mm.
The load applied on the concrete cube during testing is gradually increased until the cube fails.
The compressive strength of concrete is calculated by dividing the maximum load applied on the cube by its cross-sectional area.
The acceptable compressive strength of concrete varies based on the type of structure and its location. It is usually between 20 to 40 MPa.
Factors that can affect the compressive strength of concrete include the water-cement ratio, curing conditions, and the type of aggregate used.
It is important to perform the cube test for compressive strength of concrete to ensure the quality and strength of the concrete used in construction.
Approx 99 percent.