Concrete Bleeding – Bleeding of Concrete
Concrete bleeding is the appearance of a watery scum (also called laitance) on the surface of a concrete after compaction. It is- an indication that there is too much water or deficiency of fine material in the mix, or that too much tamping, floating or troweling has been done. The result is a porous, dusty and weak surface. This scum should be removed. Bleeding makes weak joints between successive lifts in structural work. Concrete bleeding can be reduced by using less water, a finer sand, or by adding a finely ground inert material (stone dust).
The aggregate commonly used are seldom found in a perfectly dry state in the field. Moreover, aggregates have to be washed very often for removing impurities which further add to the moisture content. The moisture content varies considerably from time to time with the changing weather conditions, and this is especially so in the case of sand. The aggregate when dry will absorb water from the concrete and when wet at the surface the mixture will, have excess of water. Therefore, while computing the quantity of water due consideration must be given to the surface conditions of the aggregate that would exist at the time of preparing the mix.
Small size of aggregate need more water than big size and angular aggregate need more than rounded aggregate. In other words, a concrete containing a finely graded aggregate will require more water for a given workability than one containing an aggregate with a coarser grading. Consequently, the more finely graded aggregate, or that containing a larger proportion of fine aggregate and similarly a concrete with angular aggregate will produce a weaker concrete.