In this article Learn : Workability of concrete and Compaction factor test.
WORKABILITY OF CONCRETE
It is the amount of work required to place concrete and to compact it thoroughly.
Workability of concrete increases with the addition of water but it reduces the strength and as such it is not a very desirable way of increasing the workability. Use of aggregates which are round and have smooth surfaces increases the workability.
Keeping the cement, aggregates ratio the same if the quantity of coarse aggregate is increased then the workability improves.
Workability could also be improved by adding Air-entraining agents such as Vinsol resin or Darex etc. Use of Lisapole liquid at the rat of 30 c.c. per bag of cement improves not only the workability but also the water tightness of concrete.
Slump test gives an idea of only the flowing property of wet concrete and not of the workability. Workability of concrete is better determined by compaction factor test.
Compaction factor test.
In this test the amount of compaction obtained by spending a known amount of energy is measured. This gives an idea of workability. The apparatus (Fig. 8.3.) used for the test consists of two hoppers A and B and a cylinder C. The hoppers are provided with hinged bottoms.
There is a certain clear distance between the hoppers A and B and the hopper B and cylinder C. Diameter of cylinder C is 15 cm and is of 30 cm height. Cement concrete to be tested is placed in the hopper A and its bottom released. The concrete falling in hopper B achieves some compaction. The bottom of hopper B is now released so that the concrete now falls in cylinder C. Surplus concrete is removed from the top of cylinder by simultaneously moving towards the centre of cylinder two trowels held horizontally in both the hands. Sharp blaes of the trowels rest on the top of cylinder. Concrete in the cylinder is now weighed. Let this weight be W1.
After cleaning the cylinder it is refilled with concrete in layers of 5 cm each. Every layer of concrete is thoroughly compacted with aniron rod. Any surplus concrete from the cylinder is removed in the manner explained before. The weight W2 of the concrete in the cylinder is found out.
The ratio of the two weights W1/W2 is known as the compaction factor.
A compaction factor of 0.85 represents a mix of poor workability, 0.92 represents medium and 0.95 good workability.