Question

Discuss why air-entrained concrete mixtures require lower water-to-cement ratio in comparison with non-air-entrained mixtures.

Discuss the importance of the slump test for fresh concrete.

Answer 1

Air entrained concrete is made either by using air entraining cement or by mixing a small quantity of
air entraining agent. These air entraining agent produce millions of micro air bubbles inside the concrete mix and these
act as flexible ball bearings. This result in modification of the properties of concrete like workability, segregation,
bleeding and finishing quality
Since the air bubbles form in water, this lower the w/c ratio of the paste and make it hard to entrain air in the paste.
On the other hand, if the concrete has higher w/c ratio , the small air voids can easily become large ones and then
escape from the paste. increasing w/c ratio leads to increases in the spacing in
hardened concrete and the air void system become worse

Slump test used for in situ determination of workability
It gives a measure of consistency
It is very useful in detecting variation in uniformity of mix of given nominal proportions
The consistency indicates how much water is present in the concrete mix
It is used for the measurement of property if fresh concrete
It is simple process and it measures behaviour of compacted inverted cone of concrete under gravity
It measures wetness of concrete
There are 3 types of concrete slump
1) collapse slump
2)shear slump
3)true slump
This test ensure uniformity for different batches of similar concrete under different field conditions
Ideal value of slump in dry sample is 1-2 inches
And 6-7 inches in wet condition