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 in controlled conditions. On the basis of this test result we judge the quality of concrete. But sometimes the strength test results vary so widely that it becomes difficult to reach at any conclusion. This variation in test results can be avoided by taking necessary steps.
There are 5 factors which influence strength of concrete when tested for compressive strength. These factors are mentioned below.
Shape & Size of Test Specimens
Height to Diameter Ratio
Rate of Application of Load
Moisture Content in the Test Specimen
Material Used for Capping
1. Shape & Size of Test Specimens
Shape & size of specimens affect the strength test result of concrete to a large extent. If two cubes of different sizes but prepared from the same concrete, are tested then they will show different test results. For example, strength of a cube specimen having 10 cm in size is 10% less than strength of a cube specimen having 15 cm in size.
If two cubes of different shapes (such as cube & cylinder) are tested, then they will show different test results. From experiment it has been found that strength of the cylinder of size 15 cm diameter and 30 cm long is equal to 0.8 times the strength of 15 cm cube.
2. Height to Diameter Ratio
Generally for testing cylindrical concrete specimen, the height to diameter ratio is kept 2. But sometimes it is not possible to keep the height/diameter ratio equal to 2 when the core is cut from road pavement or airfield or any part of the structure.
If the length of the core is too long, it can be trimmed to h/d ratio of 2:1, before testing. But if the length of core specimen is short, it is necessary to apply a correction factor to the test result.
A correction factor according to the height / diameter ratio of the specimen after capping is obtained from the curve as shown below.
The product of this correction factor and the measured compressive strength will give the corrected compressive strength. This corrected compressive strength is equivalent to the strength of a cylinder having height / diameter ratio of 2.
3. Rate of Application of Load
Rate of application of load has a considerable influence on the strength test results. If the rate of application of load is slow, or there is some time lag,. Then it will result into lower values of strength. The reason behind this is creep. Due to slower application of load, the specimen will undergo some amount of creep which in turn increases the strain. And this increased strain is responsible for failure of test sample, resulting lower strength values.
4. Moisture Content in the Test Specimen
The presence of moisture content in the test specimen also affects the test result to a great extent. If two cubes (one is wet & another is dry) prepared from the same concrete, are tested at the same age, then the dry cube will give higher strength than the wet cube. This may be caused due to the reduction of cohesion of concrete ingredients due to presence of water.
To get reproducible results, it is advised that the concrete cubes or cylinders should be tested immediately on removal from the curing tank. Because if you test concrete in dry condition then the test results will vary largely.
5. Material Used for Capping
There are various methods available for capping concrete cylinders such as, sulphur capping, gypsum capping & cement mortar capping etc (click here to know more about capping). The type of capping material used has also some amount of effect on the value obtained from strength test. So it is suggested that for a particular construction project, the method of capping the concrete cylinders should be one. And this one method should not be changed at any times. By employing this method we can avoid wide variations in test results and can judge the quality of concrete.