ANS:B - 5 cm to 8 cm

The appropriate range for the thickness of the topping of a ribbed slab should allow for sufficient concrete cover over the ribs to provide adequate protection to the reinforcement, while also considering factors such as structural requirements, construction tolerances, and economy. 5 cm to 8 cm

1. Reinforcement Cover: The concrete topping needs to adequately cover the ribs and reinforcement to protect them from environmental factors such as corrosion. A minimum cover requirement is typically specified in design codes to ensure durability and structural integrity. A thickness of 5 cm to 8 cm provides sufficient cover for most reinforcement sizes commonly used in ribbed slabs.
2. Structural Requirements: The thickness of the topping should be adequate to provide the required strength and stiffness for the slab. A thicker topping contributes to the overall strength of the slab and helps distribute loads effectively. However, excessive thickness may lead to unnecessary weight and higher construction costs. Therefore, the range of 5 cm to 8 cm strikes a balance between structural requirements and economy.
3. Construction Tolerances: During construction, there may be variations in the levelness and evenness of the slab surface. A range of 5 cm to 8 cm allows for some tolerance in the concrete placement while still ensuring that the minimum thickness requirements are met uniformly across the slab.
4. Economy: Using a thinner topping within the specified range can help reduce the overall concrete volume required for the slab, leading to cost savings in materials and construction.