CONCRETE FOR DAMS 173
achieve this is largely controlled by the physical characteristics of the fresh
concrete, notably its cohesiveness and workability. It is therefore again
related to the mix proportions, principally in terms of the water, cement
and fines contents.
Satisfactory mix proportions are dependent upon balancing the
several conflicting demands. The range within which each of the principal
parameters may be varied, is, in practice, subject to severe constraints.
Indicative mix proportions and properties are summarized in Table
3.10. It may be noted that it is the characteristics of the mature mass concrete
at ages in excess of 90–180 days which are significant in dam construction.
3.6.4 Concrete production and placingHigh production rates are necessary on large projects and require a care-
fully planned central materials handling facility and concrete batching
plant. Efficient utilization of the plant is made difficult by inevitable varia-
tions in demand caused by the need to programme pours in accordance
with the optimum monolith and lift construction schedules.
Transportation of concrete may be by travelling overhead cableway
and skips on a compact site. Trucks may be preferable where the site area
is extensive, and increasing interest is being shown in the use of conveyor
systems on large projects.
Concrete lifts are normally formed in at least two layers, and
compacted by poker vibrators. The cost efficiency and effectiveness of
the compaction process may be improved on larger dams by the use of
Table 3.10 Characteristics of mass concrete for dams
Characteristics Concrete mix
Hearting FacingCement (C)PFA (F) (kg m^3 ) 150–230 250–320
F/(CF) (%) 20–35 0–25
Water:(CF) ratio 0.50–0.70 0.45–0.65
90-day compressive strength, c(MN m^2 ) 18–30 25–40
(t/c) 0.10–0.15 0.07–0.10Unit weight, (^) c(kN m^3 ) 23–25
Modulus of elasticity, E(GN m^2 ) 30–45
Poisson ratio 0.15–0.22
Shrinkage (% at 1 year) 0.02–0.05
Coefficient of thermal expansion ( 10 6 °C^1 ) 9–12
Tensile strength
Compressive strength