Choosing the appropriate slump for the job
The information on this page is drawn from material produced by CPWR. The original pdf is available at the bottom of the page. 6/1/09
Background
In its simplest form, concrete is a mixture of paste and aggregate (sand and rock). The paste, composed of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete. This mixture and the ratios are critical to the workability, integrity and quality of the end product. The increased forces needed to screed/rod and rake can take their toll on the worker. The forces required to spread concrete with different "slump" values increase with a lower slump. In other words, the lower the slump, the higher the required force. For example: with a 1" slump, the mean force to pull material with a rake was: 46 pounds. In comparison, a 3" slump required an average of 27 pounds and a 6" slump required 20 pounds. The differences in these forces will take their toll on the worker over the course of a work shift.
Repetitive, awkward, high force motions may eventually lead to lower back, shoulder and/or wrist pain and/or fatigue.
- Low slump increases the forces needed for rodding/screeding and may encourage more back bending.
- Low slump increases the force needed to pull (rake) the material.
Examples of different slumps
A one inch slump
Working with a one inch slump
A three inch slump
Working with a three inch slump
Recommended practice
Reduce unnecessary work by using the appropriate slump for the job. If a mixture with a higher slump can be used, it will reduce the pulling forces required to rake the material.
Always use concrete with the highest slump that will ensure quality and strength while at the same time, reduce unnecessary work.
Result
When workers report feeling less tired at the end of the day they are more productive. Reducing the force required to spread concrete throughout the course of the work day will have a positive effect, is less fatiguing and is certainly safer.
