PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 292

for the Structural Lightweight Aggregate Concrete requires a 28-day com-
pressive strength of 17 MPa (2500 psi) and an air-dried unit weight of less
than for structural lightweight concrete. The use of
lightweight concrete in a structure is usually predicated on the overall cost
of the structure; the concrete may cost more, but the reduced dead weight
can reduce structural and foundation costs.
The mix proportions for lightweight concrete must compensate for the ab-
sorptive nature of the aggregates. Generally, lightweight aggregates are highly
absorptive and can continue to absorb water for an extended period of time
(Figures 7.42 and 7.43). This makes the determination of a water-cementitious
materials ratio problematical. In addition, the lightweight aggregates tend to
segregate by floating to the surface. A minimum slump mix, with air entrain-
ing, is used to mitigate this effect.
Nonstructural applications of very lightweight concrete have also been
developed. Concrete made with styrofoam “aggregates” has been used for in-
sulation in some building construction.

1850 kg/m 3 115 lb/ft 3

1 2

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7.6.5 Heavyweight Concrete

Biological shielding used for nuclear power plants, medical units, and atomic
research and test facilities requires massive walls to contain radiation. Con-
crete is an excellent shielding material. For biological shields, the mass of

FIGURE 7.42 Aggregate used for lightweight concrete.