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

1.

The air intentionally introduced in the cement during its manufacture
or during making concrete is known as entrained air. It is different from entrapped air where
the continuous channels are formed, thus increasing the permeability. In the case of entrained
air, the voids formed are discontinuous and are less than 0.05 mm in diameter. Air entrainment
increases workability, thus water can be reduced so water cement ratio will be decrease and resistance of concrete to weathering. The possibility of bleeding,
segregation and laitance is also reduced. However, there is some loss in the strength of concrete.
The air content should be from 4 to 7%, by volume, according to the maximum size of aggregate
used. The air entrainment may be done by surface active agents, chemicals, or by cement
dispersing agents.

2 . Importance of slump test

fresh concrete—concrete in the plastic state, which can be moulded into desired shape—the
theoretical w/c ratio to meet the requirements of water for chemical combination with cement,
and to occupy the gel-space is about 0.4 for maximum strength. The w/c ratio used at site may
vary because of:
(a) the presence of free surface moisture in the aggregates, and (b) the absorption of moisture
by the dry or porous aggregates.
Because of the above limitations another characteristic workability, which is again a reflection
of w/c ratio, becomes important.

For determining the type of fresh concrete we have to do slump test.

It is important because we have to achieve max compaction in the concrete .

Defferent type of work required concrete with deferent slump value.