PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 276

140

120

100

80

60

40

20

0
0.2

FIGURE 7.28 Relation between water-cemen-
titious materials ratio and permeability of mature
cement paste.

0.3 0.4 0.5 0.6 0.7 0.8

Water-cementitions materials ratio

from the figure that increasing the water-cementitious materials ratio from
0.3 to 0.7 increases the coefficient of permeability by a factor of 1000. For a
concrete to be watertight, the water-cementitious materials ratio should not
exceed 0.48 for exposure to fresh water and should not be more than 0.44 for
exposure to seawater (American Concrete Institute 1975).
Other factors that affect the permeability include age of concrete, fine-
ness of cement particles, and air-entraining agents. Age reduces the perme-
ability, since hydration products fill the spaces between cement grains. The
finer the cement particles, the faster is the rate of hydration and the faster is
the development of impermeable concrete. Air-entraining agents indirectly
reduce the permeability, since they allow the use of a lower water-cementitious
materials ratio.

■ Stress-Strain Relationship

Typical stress-strain behavior of 28-day-old concrete with different water-
cementitious materials ratios are shown in Figure 7.29 (Hognestad et al. 1955).
It can be seen that increasing the water-cementitious materials ratio de-
creases both strength and stiffness of the concrete. The figure also shows that
the stress-strain behavior is close to linear at low stress levels, then becomes
nonlinear as stress increases. With a water-cementitious materials ratio of
0.50 or less and a strain of up to 0.0015, the stress-strain behavior is almost
linear. With higher water-cementitious materials ratios, the stress-strain behavior
becomes nonlinear at smaller strains. The curves also show that high-strength
concrete has sharp peaks and sudden failure characteristics when compared
to low-strength concrete.
As discussed in Chapter 1, the elastic limit can be defined as the largest
stress that does not cause a measurable permanent strain. When the concrete
is loaded slightly beyond the elastic range and then unloaded, a small
amount of strain might remain initially, but it may recover eventually due to
creep. Also, since concrete is not perfectly elastic, the rate of loading affects

7.4.4