Most encountered problems in fresh concrete


Some of the major problems seen while preparation of fresh concrete is as follows;

1) Harshness:

It is the rough finishing property of concrete and causes improper proportioning, less compaction, larger size and shape of coarse aggregates, low water content, excess of medium sized aggregates, deficiency of sand etc.

Prevention:

Proper proportioning, good compaction, proper water cement ratio (w/c), proper amount of sand use of well-graded sand and coarse aggregates.

2) Segregation:

Separation of constituent materials of heterogeneous mixture of concrete so that their distribution is no longer uniform. The steps of segregation are as follows;

  • Coarse aggregates settles down
  • The paste separates from coarse aggregates
  • Separates out water from paste. This is due to the different value of specific gravity of materials.

Causes:

Improper grading/mix, excess w/c ratio, improper placing such as from height, cause of badly designed mixture, excessive vibration, etc.

Prevention:

Correct proportioning of the mix until it gets uniform, complex mix, proper handling, transporting, placing, compacting, finishing, remixing, use of workability agent to control w/c ratio etc, use of pozzolanic materials (makes concrete cohesive), use of air entrain agents.

Effects:

Concrete is not only going to be weak, but also due to lack of homogeneity will induce all undesirable properties in the hardened concrete.

3) Bleeding:

It is the phenomenon to come out of water from the concrete to the surface of concrete. This is because water ahs low specific gravity among all. While coming, water takes out cement particles from concrete the surface forming cement paste called “Laitance” which causes dust in summer and mud in raining and the weathering increases, shrinkage and cracks are liable to be seen/formed. It also forms very weak bond for another layer concreting. Water makes pores in concrete so it reduces bond among aggregate paste and reinforcement.

Prevention:

Use of proper proportioned, uniform and complex mix, use of fine cement enables low bleeding, use of pozzolanic materials (makes concrete cohesive), use of air entrain agents. Fine cement has more specific area which causes quick reaction making bleeding low. Also by adding C3A causes low bleeding.


Leave a Reply