Civil Engineering Reference
In-Depth Information
Figure 3.9
Diagrammatic representation of temperature rise and strength gain of setting
concrete
concrete as it cools will stress it in tension, and may cause it to crack. It is ironic that
this exothermic reaction is by far the most signifi cant cause of prejudicial cracking of
concrete, while the greatest design time, and code activity, is devoted to limiting the
less-damaging cracking in bending. It is most important that the designer understands
this characteristic of concrete. (It should be noted that this cooling-induced shrinkage
of concrete is quite distinct from long-term drying shrinkage.)
3.6.2 The cracking of thick sections of concrete due to temperature
gradients within the mass
When a concrete member is cast, the heat of the reaction is dissipated by surfaces in
contact with the air or the shuttering. In thick sections (anything over about 800 mm)
there will be a signifi cant temperature gradient through the thickness of the member,
with the skin being cooler than the core. Once the concrete has set, the hotter core
has to cool down from a higher temperature than the skin, and thus is put into tension
and tends to crack. It has been reported that the strength of concrete in the core of
thick sections is weaker than small test specimens of the same mix, and this is probably
due to this effect. The author does not believe that designers need to worry overmuch
about such a loss of strength; the material factors of the codes of practice may be
assumed to include for this effect. This internal cracking is likely, however, to create
leakage paths within the concrete that may cause problems for underground structures
subject to water pressure.
The temperature gradients that cause this cracking may be reduced by insulating the
free surfaces of the pour, although this can have the effect of increasing the maximum
temperature attained by the setting concrete, and may exacerbate some of the other
causes of cracking described hereafter.
3.6.3 The cracking of concrete caused by the presence of reinforcement
If the steel reinforcement in a concrete mass is entirely enclosed, there is no reason
why it should not follow closely the rise and fall of the concrete's temperature. If it
