Some tips why concrete fails:

1. Delay in transportation of concrete mixture to site.

2. Water to cement ratio was not maintained properly.

3. Mixing of aggregates and cement was not proper.

4. Improper ratio of aggregates and cement.

5. Lack of proper curing of concrete.

6. Usage of highly porous and very soft aggregates.

7. Using dirty aggregates. (aggregates  to be clean to avoid interference  with  cement hydration process)

8. Removal of form works before concrete got set and hardened.

9. Presence of voids between concretes.

10. Using dirty water to mix with cement. (impurities in water interferes in cement hydration process)

11. Concrete was not filled with fully compact level.

12. Using cement which was already got wet and dried. 

13. Usage of low quality aggregates and low quality RCC steel rods.

14. Using the cement mortar which has already started to set.