Foundation, Concrete and Earthquake Engineering

Requirements for Shrinkage and Temperature Reinforcement for Concrete Member

Development of Shrinkage:

Hardening of concrete always associate with change in volume resulting shrinkage stresses. It is advisable to minimize such shrinkage by using concrete with the smallest possible amounts of water and cement compatible with other requirements, like strength and workability, and by thorough moist-curing of sufficient during. But a certain degree of shrinkage is usually unavoidable irrespective of what precaution are taken.

In case of slab having moderate dimensions freely rested on its supports can contract to compensate the shortening of its length produced by shrinkage. But in normal case, slabs and other members are joined rigidly to other parts of the structure and restricted to contract, producing shrinkage stress. In outdoor structures like bridges, a decrease in temperature relative to that at which the slab was poured subjected to similar effect of shrinkage.

Requirement of Reinforcement:

As concrete is weak in tension, the resulting stress of shrinkage produce cracks. Cracks of this nature are not detrimental provided their size is limited which is known as hairline cracks. The evenly distributed cracks are achieved by placing reinforcement in the slab to resist contraction.

Function of Steel as Shrinkage Reinforcement:

When the concrete tends to shrink, such reinforcement resists the contraction, subjecting to compression. The total shrinkage in a slab so reinforced is less than that in one without reinforcement.

However, if crack is produced, will be smaller width and more evenly distributed by virtue of the reinforcement.

Placement of Reinforcement:In slabs, provided with reinforcement provided for bending moments has the desirable effect of reducing shrinkage and distribution cracks. however, as contraction take place equally in all directions, it is necessary to provide special reinforcement for shrinkage and temperature contraction in the direction perpendicular to the main reinforcement. This added steel is known as temperature or shrinkage reinforcement.

CODE Requirements:Minimum Shrinkage and temperature reinforcement normal to primary flexural reinforcement is required for structural floor and roof slabs (not slabs on ground) where the flexural reinforcement extends in one direction only.

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