In my earlier article, we discussed “Tests for Fresh Concrete” in detail which will reduce the probability of failure that a building undergoes after the concrete hardens. In this article, we are going to discuss the “Tests for Hardened Concrete in Detail”.
Main objective of tests is to ensure properties of concrete vis-a-vis control quality and to adhere to the specifications. Properties of concrete are a function of time and ambient humidity.
Specimens used – Cube, cylinders and prisms.
- Tested normal (perpendicular) to the position of cart steel moulds of 150mm x 150mm x 150mm. Apply oil inside. Mould to be filled in 3 layers. Cone is filled to overflow and then it is compacted. Steel scale rule is used to chop off surplus (gives homogeneity)
- Each layer is compacted with 35 strokes of 25 diameter steel squares pinner.
- The specimen is stored for 24 hours at 18 to 22 degrees at relative humidity not less than 90%.
- Mould is the removed and cured (simulating field conditions)
- In compression test, cube is in contact with platens of the testing machine load is applied at constant rate of stern (15Mpa/minute)
- Crushing strength is reported nearest to 0.5Mpa.
Effect of end conditions of specimen on strength reported –
- Undulation in the cast surface (even 0.25mm) reduces the strength by one third.
- Convex surfaces (due to greater stress concentration) show greater strength reduction.
- Therefore proper surface shall be ensured. Maximum unevenness shall not be more than 0.05mm.
- End surfaces shall be free of sand grain and dust, which leads to splitting and premature failure.