Civil Engineering Reference
In-Depth Information
punchings. Batch sizes should be reduced to about 50% of
the rated mixer capacity. Because some heavy aggregates
are quite friable, excessive mixing should be avoided to
prevent aggregate breakup with resultant detrimental
effects on workability and bleeding.
Preplaced aggregate
methods can be used for placing
normal and high-density concrete in confined areas and
around embedded items; this will minimize segregation of
coarse aggregate, especially steel punchings or shot. The
method also reduces drying shrinkage and produces con-
crete of uniform density and composition. With this
method, the coarse aggregates are preplaced in the forms
and grout made of cement, sand, and water is then
pumped through pipes to fill the voids in the aggregate.
Pumping
of heavyweight concrete through pipelines
may be advantageous in locations where space is limited.
Heavyweight concretes cannot be pumped as far as
normal-weight concretes because of their higher densities.
Puddling
is a method whereby a 50-mm (2-in.) layer or
more of mortar is placed in the forms and then covered
with a layer of coarse aggregate that is rodded or internally
vibrated into the mortar. Care must be taken to ensure uni-
form distribution of aggregate throughout the concrete.
siderations to reduce heat of hydration and the resulting
temperature rise to avoid damaging the concrete through
excessive temperatures and temperature differences that
can result in thermal cracking (
Gajda and VanGeem, 2002
).
In mass concrete, temperature rise (Fig. 18-8) results
from the heat of hydration of cementitious materials. As
the interior concrete increases in temperature and
expands, the surface concrete may be cooling and con-
tracting. This causes tensile stresses that may result in
thermal cracks at the surface if the temperature differen-
MASS CONCRETE
Mass concrete is defined by
ACI Committee 116
as “Any
large volume of cast-in-place concrete with dimensions
large enough to require that measures be taken to cope
with the generation of heat and attendant volume change
to minimize cracking.” Mass concrete includes not only
low-cement-content concrete used in dams and other mas-
sive structures but also moderate- to high-cement-content
concrete in structural members of bridges and buildings
(Fig. 18-7). Mass concrete placements require special con-
Fig.18-7. Large foundation placements as shown require
mass-concrete precautions. (50918)
200
Level 1 - Bottom
Level 2 - Middle
Level 3 - To p
82
150
62
42
100
22
Cement, Type I
=
328 kg (553 lb)
50
Silica Fume
=
27 kg (45 lb)
Fly ash
=
87 kg (147 lb)
2
HRWR
=
7.86 L (266 fl oz)
w/cm
=
0.281
-18
0
0
100
200
300
400
500
600
Time from start of placement, hr
Fig. 18-8. (left) A drilled shaft (caisson), 3 m (10 ft) in diameter and 12.2 m (40 ft) in depth in which “low-heat” high-strength
concrete is placed and (right) temperatures of this concrete measured at the center and edge and at three different levels
in the caisson (
Burg and Fiorato 1999
). (57361)
