What is Slump Loss in Concrete?
Slump value is of great importance when concrete is in its fresh state. Slump value represents the workability property of concrete. Due to its importance, the value of slump is generally specified by the designer. And while working on site, the engineer’s responsibility is to make sure that the concrete should show the same slump value as specified. It is therefore required to check slump before placing concrete. But due to several reasons, the slump value gradually decreases as time elapsed. This reduction in value of slump with time is called slump loss in concrete.
Methods to Control Slump Loss in Concrete
The following methods are helpful to manage slump loss in fresh concrete.
- Initial high slump
- Using retarders
- Using plasticizers or super plasticizers
- By repetitive dose
- By dosing at final point
- By keeping temperature low
- By using compatible super plasticizers with cement
1. Initial High Slump
In this method a very high slump is managed at the mixing point, so that after slump loss takes place, the residual slump is still satisfactory for placing concrete. This method is not economical & hence not good. But sometime this method is adopted.
2. Using Retarders
In this method retarders are used at the time of mixing. This will help concrete to remain in a plastic condition over a long period of time.
3. Using Retarding Plasticizers or Super-plasticizers
Before placing the concrete, add an appropriate dose of plasticizers or super-plasticizers, which will give desirable value of slump. This method is adopted only when concrete is conveyed by transit mixers.
4. By Repetitive Dose
This is one of the common methods to manage slump loss in concrete. In this method repetitive doses is given in intervals and thereby maintaining the required slump for long time. The time interval should be chosen in such a way that the concrete will have such a residual slump value which can be boosted up.
5. By Dosing at Final Point
Sometimes a small dose of super-plasticizers is added at the beginning and the slump is increased up. After concrete arrives at the pouring point, it will still have some residual slump but not good enough for placing by specified methods (such as pumping or by tremie). At this point an appropriate dose of super-plasticizers is added to boost up the slump to required level.
6. By Keeping Temperature Low
Hydration process of cement generates heat, which is also a cause of slump loss. The process of hydration can be retarded by keeping the temperature of the concrete low. At low temperature the slump loss is also slow. Use of ice flakes by replacing some amount of water is resorted to reduce the slump loss.
7. By Using Compatible Super-plasticizers with Cement
Use of highly compatible admixture with the given cement will also reduce the problem of slump loss. Cement with low C3A content will be of use in this regard.