Civil Engineering Reference
In-Depth Information
Figure 12.14
Effect of heat of hydration tensile stresses
The principal reason for this cracking was the effect of heat of hydration (
3.6
).
Whereas the thin top slab had a heating and cooling cycle that lasted approximately
36 hours, the massive ribs had a cycle of some three days. By the time the rib cooled,
the slab was already stable, and restrained the shortening of the rib causing it to be
stressed in tension. The tensile bending stresses acted as the trigger that exhausted
the tensile strength of the rib, and defi ned the location of the cracks. The length of
the cracks corresponded to the stress diagram caused by the differential temperature
of rib and slab. The cracks were so wide because the reinforcement on the bottom
fi bre, although respecting the code of practice, was below the threshold required to
give ductile behaviour, consisting only of the stirrup corner bars, and small-diameter
distribution steel. The cracking was made worse by the fact that in the absence of
signifi cant bottom fi bre reinforcement, the cracked inertia was locally reduced, raising
the neutral axis, and increasing the positive prestress parasitic moment.
