Materials for engineering, 3rd Edition - (Malestrom)

152
Materials for engineering
larger, they will cause some reduction in the strength of the cement. The
major problem with porosity, however, arises from air bubbles resulting
from poor processing or pockets of water. These can be up to 1 mm in size
and can propagate under stress leading to premature failure of the component,
in accordance with the Griffith equation (equation [2.12]).
The porosity is essentially dependent upon the water/cement ratio, an
increase in which causes the strength of the cement to decrease. As the
amount of water increases above that necessary for the complete hydration
of the cement mixture, it produces a more porous structure, resulting in a
decrease in strength (Fig. 4.13). The amount of air in the concrete also
affects strength, but to a lesser extent.
In compression, a single large flaw is not fatal (as it is in tension). In a
compression test, cracks inclined to the compression axis experience a shear
stress which causes them to propagate stably. They twist out of their original
orientation to propagate parallel to the compression axis. They eventually
interlink to form a crush zone which develops at an angle of 30° to the
compression axis. The form of the resultant stress–strain curve is as shown
in Fig. 4.14.
Effect of the aggregate
The size and shape of the aggregate and the aggregate/cement ratio all affect
60
50
Compressive strength (MN m –2 )
40
30
20
10
0
0.3 0.6 0.9 1.2
Water/cement ratio by weight
4.13 Compressive strength vs. water/cement ratio.