CONCRETE FOR DAMS 171
durability are intimately linked for a dense, well-proportioned concrete
mix. Less immediate relationships can similarly be established in other
instances, e.g. between volume stability and resistance to cracking.
Concrete technology is a wide-ranging discipline with an extensive
literature. For a general but comprehensive introduction to the subject,
reference should be made to Neville and Brooks (1987). The coverage
given in this section is necessarily brief, and is confined to outlining ele-
ments of concrete technology immediately relevant to dam construction. It
may be supplemented by reference to the specialist treatment of mass con-
crete provided in ACI (1970) and to the review given in Jansen (1988).
The development of roller-compacted concretes (RCCs) and the
prospects they offer for continuous and economic construction are intro-
duced separately in Section 3.7.
3.6.2 Constituent materialsThe primary constituents of concrete are cement, mineral aggregate and
water. Secondary constituents employed in mass concrete for dams include
pozzolans and other selected admixtures.
- Cements.The hydration of unmodified Portland cements (ASTM
type I equivalent) is strongly exothermic. The resulting temperature
rise and heat gain in large pours is unacceptable in relation to conse-
quent problems of shrinkage, heat dissipation and cracking. It is
therefore preferable to employ a low heat (ASTM type IV) or modi-
fied Portland cement (ASTM type II) if available. Thermal problems
can also be alleviated by the use of pozzolan-blended Portland
cements (ASTM type 1P). In the absence of special cements, partial
replacement with pulverized fuel ash (PFA) and/or cooling are
also effective in containing heat build-up. (PFA for concrete is a
quality-assured and processed waste product from coal-fired power
stations.) - Aggregates.The function of the coarse and fine aggregate is to act as
a cheap, inert bulk filler in the concrete mix. A maximum size
aggregate (MSA) of 75–100 mm is considered the optimum, with
rounded or irregular natural gravels generally preferable to crushed
rock aggregates. In the fine aggregate range, i.e. 4.67 mm size,
natural sands are similarly preferable to crushed fines. Aggregates
should be clean and free from surface weathering or impurities. The
petrographical, thermal and moisture characteristics should be com-
patible with hydrated cement paste. A smooth, well-graded particle
size distribution curve for the combined aggregates will ensure
maximum packing density for the compacted concrete mix.