In this article we will discuss about the placement of large concrete masses.
When large volumes of plain (un-reinforced) concrete are placed in position such as in gravity dams or spillways, the concrete is known as mass concrete. In mass concrete there is always a danger of thermal cracking due to restraint to contraction of concrete on cooling from a high temperature caused by the heat of hydration of cement. Such a cracking may take several weeks to develop. On the other hand there is a danger of early age thermal cracking in thinner sections, unless appropriately reinforced.
Thermal cracking takes place due to the variation in temperature. It should be clearly distinguished from plastic cracking. The plastic cracking occurs on or near the surface of concrete due to rapid evaporation of water from the concrete while it is still in a plastic state. Drying of concrete can also cause shrinkage cracking which normally takes place at a later stage than thermal cracking.
When a concrete mass is not insulated from the atmosphere, a temperature gradient exists within the concrete due to the fact that its interior becomes hot due to the rise in temperature by the evaluation of heat of hydration, whilst the surface loses heat to the surrounding atmosphere.
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If the difference in temperature between the interior and the exterior is large enough, cracking will develop. During the period of rise in temperature the cracks will develop in interior portion, as shown in Fig.18.4 while in the course of cooling cracking will develop on surface as shown in Fig.18.5.
The cracking will develop if the temperature difference is more than 20°C. Cracks may also develop if concrete is placed against a cold surface. The interior is thus restrained from full thermal expansion and compressive stress is developed in the interior, which is balanced by tensile stress in the exterior. Though both stresses are relieved to some extent by the creep, but the tensile stress still may be sufficient to cause surface cracking. As the concrete starts cooling it contracts, the tensile stress in the exterior is relieved and if any surface crack closes, it renders the concrete harmless.
Since the interior wants to contract more than the exterior, the strain in the interior is restrained and tensile stress is developed, with a balancing compressive stress in the exterior. During this cooling phase, there is less relief of stress by creep than in the heating phase as the concrete is more mature.
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Thus the induced tensile stress caused by internal restraint on cooling may be large enough to cause cracking in the interior of concrete mass. Thus if cracking in concrete is to be avoided, the temperature differential or temperature gradient should be limited. According to Gibbon, limit to gradient should be about 20°C when gravel aggregate is used.
On the other hand if the entire mass of concrete is insulated from the outside air or earth so that the temperature is uniform throughout. In this case cracking will take place only if the total mass is wholly or partly externally restrained from contracting during the cooling period. This form of restrained is known as external restraint. Thus to avoid cracking of concrete it is necessary to minimize the difference between the peak temperature of the concrete and the ambient temperature or to minimize the restraint.
The tolerable temperature deferential or temperature gradient between the peak temperature and the final ambient temperature depends upon the type of aggregate used in concrete, if gravel aggregate is used, then tolerable temperature gradient should not be more than 20°C (36°F). In case certain lime stone aggregates are used, then it may be taken upto 40°C (72°F). In case light weight aggregates are used then it can be as high as 130°C (234°F).