PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 275

materials ratio, high cement content, low coarse aggregate content, existence
of steel reinforcement, and aging. On the other hand, if concrete is cured
continuously in water after setting, concrete will swell very slightly due to
the absorption of water. Since swelling, if it happens, is very small, it does
not cause significant problems. Swelling is accompanied by a slight increase
in weight (Neville 1981).
How much drying shrinkage occurs depends on the size and shape of
the concrete structure. Also, nonuniform shrinkage could happen due to the
nonuniform loss of water. This may happen in mass concrete structures,
where more water is lost at the surface than at the interior. In cases such as
this, cracks may develop at the surface. In other cases, curling might devel-
op due to the nonuniform curing throughout the structure and, consequently,
nonuniform shrinkage.

■ 7.4.2 Creep Properties

Creep is defined as the gradual increase in strain, with time, under sustained
load. Creep of concrete is a long-term process, and it takes place over many
years. Although the amount of creep in concrete is relatively small, it could
affect the performance of structures. The effect of creep varies with the type
of structure. In simply supported reinforced concrete beams, creep increas-
es the deflection and, therefore, increases the stress in the steel. In rein-
forced concrete columns, creep results in a gradual transfer of load from the
concrete to the steel. Creep also could result in losing some of the prestress
in prestressed concrete structures, although the use of high-tensile stress
steel reduces this effect. Rheological models, discussed in Chapter 1, have
been used to analyze the creep response of concrete (Neville 1981).

■ Permeability

Permeability is an important factor that largely affects the durability of
hardened concrete. Permeable concrete allows water and chemicals to pen-
etrate, which, in turn, reduces the resistance of the concrete structure to frost,
alkali-aggregate reactivity, and other chemical attacks. Water that permeates
into reinforced concrete causes corrosion of steel rebars. Furthermore, im-
pervious concrete is a prerequisite in watertight structures, such as tanks
and dams.
Typically, the air voids in the cement paste and aggregates are small and
do not affect permeability. However, the air voids that do affect permeabili-
ty of hardened concrete are obtained from two main sources: incomplete
consolidation of fresh concrete and voids resulting from evaporation of mix-
ing water that is not used for hydration of cement.
Therefore, increasing the water-cementitious materials ratio in fresh
concrete has a severe effect on permeability. Figure 7.28 shows the typical
relationship between the water-cementitious materials ratio and the coeffi-
cient of permeability of mature cement paste (Powers 1954). It can be seen

7.4.3