QIT f2w
coarse aggregate are used a finer combined grading should be preferred, while for richer mixes
a coarser combined grading is more desirable and economical.
It is also a fact that for any mix design there is no singly overall grading which is best for all
sets of conditions. A finer grading should be used for leaner mixes, more workable mixes and
for concretes made with poorly shaped aggregate particles, i.e., angular, flaky, or elongated
particles with rough texture. A larger proportion of fine particles should also be employed in
mixes where the maximum size of aggregate is small.
Likewise, no single grading is the best for any given type of mix. A finer grading should be
used for condition which tends to promote segregation, e.g., transporting in jolting or vibrating
containers over long distances by pneumatic placer or on a series of conveyor belts; discharging
down inclined chutes into a heap; dropping through a considerable height or over reinforcement;
and placing in form work, which is not mortar tight. When concrete is to be placed underwater
or pumped there should be enough fine aggregate to provide a more plastic and cohesive mix
than may otherwise be required. The higher the workability of concrete the greater is the
proportion of fine aggregate needed.
The overall grading may also be governed by site conditions for considerations other than
those of cohesiveness. Where concrete quality cannot be controlled the average proportion of
fine aggregate should be increased to provide a greater margin of protection against any mix
being under sanded. The proportion of fine aggregate may have to be increased or decreased
for concrete whose appearance is of prime importance. The selection of a suitable grading
requires sound judgement and experience. Nevertheless the final proportions of fine and
coarse aggregate should be based on the behavior of the concrete when trial mixes are made
under full scale conditions (i.e., one full bag of cement and a concrete mixer of adequate
capacity), and it should then be apparent whether the mix is appreciably over or undersanded.
In case of doubt about the most suitable proportion of fine aggregate it is usually safer to use
a too high, rather than a too low proportion ever though it may increase the richness of the mix
slightly so that there will be adequate protection against segregation, harshness, and therefore,
p 2II S q2 2 2IHE
2
&
21 2 2