Shrinkage Deflection in Rectangular Beams and Slabs

Shrinkage deflection is a common issue in concrete structures, particularly in rectangular beams and slabs. It occurs due to the drying and shrinkage of concrete, which leads to a reduction in volume and subsequent deflection of the structure. To eliminate or minimize shrinkage deflection, certain measures need to be taken, such as the use of compression steel.

Compression Steel and Shrinkage Deflection

Compression steel refers to the reinforcement provided in the compression zone of a concrete structure. In the case of rectangular beams and slabs, placing compression steel can help counteract the effects of shrinkage deflection. The correct approach is to provide compression steel in an amount greater than the tensile steel.

Explanation of the Correct Answer (Option B)

The correct answer to the question is option B, which states that compression steel should be more than tensile steel. This approach is adopted to prevent excessive deflection and cracking due to shrinkage.

Reasoning behind Option B

1. Neutral Axis and Tension Zone: In a rectangular beam or slab, the neutral axis is located closer to the tension zone. As such, the tensile steel is primarily responsible for resisting the bending moment and controlling deflection.
2. Shrinkage Effects: Shrinkage causes a reduction in the volume of concrete, leading to a decrease in the length of the member. This reduction in length induces tensile stresses in the concrete, resulting in deflection and cracking.
3. Compression Steel: By providing compression steel in an amount greater than the tensile steel, we increase the capacity of the compression zone to resist the shrinkage-induced tensile stresses. This helps balance the forces and minimize deflection.
4. Increased Stiffness: The presence of compression steel increases the overall stiffness of the structure, reducing the deflection caused by shrinkage. This is because the compression steel effectively resists the shrinkage-induced tensile forces, preventing excessive bending and deflection.
5. Control of Cracking: By ensuring that the compression steel is greater than the tensile steel, we can control the width and extent of cracking caused by shrinkage. The compression steel helps distribute and absorb the tensile stresses, reducing the likelihood of cracks forming.

Therefore, option B is the correct answer as providing compression steel in an amount greater than the tensile steel helps eliminate or minimize shrinkage deflection in rectangular beams and slabs.