Finding Compressive Strength of Cement Concrete

Introduction:

It is very important that concrete mix under consideration must possess compressive strength equal to its minimum characteristic strength. To calculate this compression test is performed on cement concrete cubes.

Equipment:

1. Cube moulds of 150 mm size.
2. Compression testing machine of 300 ton capacity.
3. Vibrator
4. Electronic balance 

Procedure

1. Calculate out the weight of ingredients I.e. cement, sand, aggregates etc. as per nominal mix taken under consideration. In this case (1:1.5:3) M20 being used.
2. Mix all ingredients properly in dry state 
3. Add required amount of water as calculated.
4. Mix again it by using shovel in wet state properly.
5. Fill the mould properly with concrete mix by using a scoop.
6. Compact concrete properly by using vibrator needle
7. After about 1-2 hours cover the moulds by using wet gunny bags and keep them undisturbed for about 24 hours @ 27±2 oc and 90% relative humidity.
8. Take out the concrete cubes from moulds after 24 hours.
9. Submerge the cubes in fresh water maintained at  27±2 oc for 7, 14, 28 days. Water must be renewed after every 7 days
10. Take out the cubes from water half an hour prior to test. Dry them up to surface dry conditions.
11. Place cube under compression testing machine pressure plates centrally.
12. Apply load @ 140 kg/cm²/minute
13. Note down load at which specimen collapses.
14. There should not be more than ±15% variation between compressive strength of two individual samples.
15. Take at least three readings. And take average of their results

Compressive strength of concrete is given by;

Units: kg/cm²

Conclusion:

1. Compressive strength of concrete depends upon various factors like water cement ratio, proportion used, grading of aggregates etc.
2. To conclude that concrete mix is giving nominal strength or not; compression test on concrete is performed.
3. The test is good to ensure quality control in case of concrete construction.