Civil Engineering Reference
In-Depth Information
Table 1-1. Observed Average Density of Fresh Concrete (SI Units)*
Density, kg/m
3
**
Maximum
size of
Air
Relative density of aggregate
†
aggregate,
content,
Water,
Cement,
mm
percent
kg/m
3
kg/m
3
2.55
2.60
2.65
2.70
2.75
19
6.0
168
336
2194
2227
2259
2291
2323
37.5
4.5
145
291
2259
2291
2339
2371
2403
75
3.5
121
242
2307
2355
2387
2435
2467
* Source:
Bureau of Reclamation 1981
, Table 4.
** Air-entrained concrete with indicated air content.
† On saturated surface-dry basis. Multiply relative density by 1000 to obtain density of aggregate particles in kg/m
3
.
Table 1-1. Observed Average Density of Fresh Concrete (Inch-Pound Units)*
Density, lb/ft
3
**
Maximum
size of
Air
Specific gravity of aggregate
†
aggregate,
content,
Water,
Cement,
in.
percent
lb/yd
3
lb/yd
3
2.55
2.60
2.65
2.70
2.75
3
⁄
4
6.0
283
566
137
139
141
143
145
1
1
⁄
2
4.5
245
490
141
143
146
148
150
3
3.5
204
408
144
147
149
152
154
* Source:
Bureau of Reclamation 1981
, Table 4.
** Air-entrained concrete with indicated air content.
† On saturated surface-dry basis. Multiply specific gravity by 62.4 to obtain density of aggregate particles in lb/ft
3
.
The weight of dry concrete equals the weight of the
freshly mixed concrete ingredients less the weight of mix
water that evaporates into the air. Some of the mix water
combines chemically with the cement during the hydration
process, converting the cement into cement gel. Also, some
of the water remains tightly held in pores and capillaries
and does not evaporate under normal conditions. The
amount of mix water that will evaporate from concrete
exposed to ambient air at 50% relative humidity is about
1
⁄
2
% to 3% of the concrete weight; the actual amount
depends on initial water content of the concrete, absorption
characteristics of the aggregates, and size and shape of the
concrete element.
Aside from conventional concrete, there is a wide spec-
trum of special concretes to meet various needs; they range
from lightweight insulating concretes with a density of as
little as 240 kg/m
3
(15 lb/ft
3
) to heavyweight concrete with
a density of up to 6000 kg/m
3
(375 lb/ft
3
) used for coun-
terweights or radiation shielding.
penetration by water or other substances (liquid, gas, or
ions). Generally, the same properties of concrete that make
it less permeable also make it more watertight.
The overall permeability of concrete to water is a func-
tion of: (1) the permeability of the paste; (2) the permeabil-
ity and gradation of the aggregate; (3) the quality of the
paste and aggregate transition zone; and (4) the relative
proportion of paste to aggregate. Decreased permeability
improves concrete's resistance to freezing and thawing,
resaturation, sulfate, and chloride-ion penetration, and
other chemical attack.
The permeability of the paste is particularly important
because the paste envelops all constituents in the concrete.
Paste permeability is related to water-cement ratio, degree
of cement hydration, and length of moist curing. A low-
permeability concrete requires a low water-cement ratio
and an adequate moist-curing period. Air entrainment aids
watertightness but has little effect on permeability.
Permeability increases with drying.
The permeability of mature hardened cement paste
kept continuously moist ranges from 0.1 x 10
-12
to 120 x 10
-12
cm per sec. for water-cement ratios ranging from 0.3 to 0.7
(
Powers and others 1954
).The permeability of rock
commonly used as concrete aggregate varies from approx-
imately 1.7 x 10
-9
to 3.5 x 10
-13
cm per sec. The permeability
of mature, good-quality concrete is approximately 1 x 10
-10
cm per sec.
The relationship between permeability, water-cement
ratio, and initial curing for 100 x 200-mm (4 x 8-in.) cylin-
drical concrete specimens tested after 90 days of air drying
Permeability and Watertightness
Concrete used in water-retaining structures or exposed to
weather or other severe exposure conditions must be virtu-
ally impermeable or watertight. Watertightness is often
referred to as the ability of concrete to hold back or retain
water without visible leakage. Permeability refers to the
amount of water migration through concrete when the
water is under pressure or to the ability of concrete to resist
