Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 60

Another hotly debated question is the relative merit of gap and
continuous gradings. A basic difference is in segregation resistance and
pumpability. High slump and pump mixes require continuous gradings
but low slump, nonpump mixes compact faster with gap gradings. Two
further points worth noting are that single-sized aggregates do not seg-
regate in stockpiles and that it is more critical that the exact optimum
sand percentage be used in the case of a gap grading than in the case of a
continuous grading.
3.3 LIGHTWEIGHT AGGREGATES
Many types of lightweight aggregates are in use and full coverage is beyond
the scope of this topic. However some indication of the possibilities may
be of assistance. Nonstructural lightweight concrete is not only outside the
scope of the topic, but also outside the scope of the mix design and quality
control (QC) systems with which the topic is mainly concerned. Such con-
cretes are produced either by the use of foaming agents or the introduction
of extremely lightweight aggregates such as polystyrene foam or expanded
vermiculite or perlite. The range of lightweight concretes is a continuous
one. It is difficult to say where nonstructural stops and structural starts.
There may indeed be some overlap, with some concretes strong enough
to be regarded as structural being lighter than others not having enough
strength for structural purposes.
Structural lightweight concrete may be regarded as concrete having a
strength at least 10 MPa and, perhaps more important, having a good
degree of durability. It should also be capable of bonding to and protect-
ing reinforcement. Such concrete is likely to have a density in the range
of 1200 to 2000 kg/m 3 . Coarse aggregates used include naturally occur-
ring pumice and scoria (of volcanic origin), cinders from coal burning,
and manufactured aggregates produced by expanding clay or shale in
rotary kilns similar to (and often formerly used as) cement kilns or air-
cooled slag.
The main difficulty with lightweight aggregates is usually that they
have a very high water absorption. Some aggregates, especially those
manufactured in kilns, may have a relatively low permeability, sealed
surface. Those that are supplied as crushed material, especially the natu-
ral materials, may absorb 20% or more of their own weight. Such mate-
rials must be used in a fully saturated state if handling difficulties are
to be avoided. If this is not done, water will be absorbed during mixing,
transporting, and placing, with consequent rapid loss of workability. A
particular difficulty is that of pumping such concrete. Once under pres-
sure in the pipeline, water will be forced into any unsaturated aggre-
gate particles. This tends to cause pump blockages through severe loss