# WHAT ARE THE FACTORS INFLUENCING COMPRESSIVE STRENGTH TEST RESULTS OF CONCRETE?

## Factors affecting strength test results of concrete

Compressive strength is the most important property of concrete. The compressive strength of concrete is determined in the laboratory under controlled conditions. On the basis of this test result we judge the quality of concrete. But sometimes strength test results vary so widely that it becomes difficult to reach a conclusion. This change in the test results can be avoided by taking necessary steps.

There are 5 factors that affect the strength of concrete when it is tested for compressive strength. These factors are mentioned below.

1. Size and shape of test specimens
2. height to diameter ratio
4. Moisture content in the test sample
5. material used for capping

### 1. Size and shape of test specimens

The size and shape of the specimens largely affect the strength test result of the concrete. If two cubes of different size but prepared from the same concrete are tested, they will show different test results. For example, the strength of a cubic sample of size 10 cm is 10% less than the strength of a cube of size 15 cm.

If two cubes of different sizes (such as a cube and a cylinder) are tested, they will show different test results. It has been found from the experiment that the strength of a cylinder 15 cm in diameter and 30 cm long is equal to 0.8 times the strength of a 15 cm cube.

### 2. Height to Diameter Ratio

Normally for testing cylindrical concrete specimens, the height to diameter ratio is kept as 2. But sometimes it is not possible to keep the height/diameter ratio equal to 2 when the core is cut from any part of the road pavement or airspace or structure.

If the core length is too long, it can be trimmed to an H/D ratio of 2:1 before testing. But if the length of the original sample is short, it is necessary to apply a correction factor to the test result.

After capping a correction factor according to the height/diameter ratio of the sample is obtained from the curve as shown below.

The product of this correction factor and the measured compressive strength will give the true compressive strength. This modified compressive strength is equal to the strength of a cylinder with a height/diameter ratio of 2.

### 3. Rate of Application of the Load

The rate of application of the load has a significant effect on the strength test results. If the rate of application of the load is slow, or there is some time lag. Then this will result in lower values ​​of power. The reason behind this is creep. Due to the slow application of load, the sample will undergo some amount of creep which in turn increases the strain. And this increased stress is responsible for the failure of the test specimen, resulting in low power value.

### 4. Moisture content in the test sample

The presence of moisture content in the test sample also affects the test result to a great extent. If two cubes prepared from the same concrete (one is wet and the other is dry) are tested at the same age, the dry cube will give greater strength than the wet cube. This may be due to a decrease in the cohesion of the solid material due to the presence of water.

To obtain reproducible results, it is advised that concrete cubes or cylinders be tested immediately upon removal from the curing tank. Because if you test the concrete in dry condition the test results will vary substantially.

### 5. Material Used for Capping

Various methods are available for capping concrete cylinders such as, Sulfur capping, Gypsum capping and Cement mortar capping etc. (Click here to know more about capping). The type of capping material used also has some effect on the value obtained from the strength test. So it is suggested that for a particular construction project, the method of capping concrete cylinders is a must. And that’s one way it should never change. By employing this method we can avoid wide variation in test results and judge the quality of concrete.

Er. Mukesh Kumar

Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.