The Maximum Size of Reinforcement Bar for a 75mm Thick Concrete Slab

To determine the maximum size of reinforcement bar that can be used for a 75mm thick reinforced concrete slab, we need to consider the minimum concrete cover requirements, as well as the maximum aggregate size.

Minimum Concrete Cover Requirements
- Concrete cover is the distance between the outer surface of the reinforcement bar and the nearest surface of the concrete.
- It is important to provide sufficient concrete cover to protect the reinforcement from corrosion and provide fire resistance.
- The minimum concrete cover requirements are specified in design codes and standards.
- For slabs, the minimum concrete cover is typically around 20-25mm.

Maximum Aggregate Size
- The maximum aggregate size refers to the largest particle size of the aggregates used in the concrete mix.
- Larger aggregates are generally used in structural concrete to reduce the amount of cement paste required and improve workability.
- The maximum aggregate size is typically limited to one-fifth of the minimum thickness of the concrete member.
- In this case, the concrete slab is 75mm thick, so the maximum aggregate size should not exceed 75/5 = 15mm.

Determining the Maximum Reinforcement Bar Size
- To ensure adequate concrete cover and prevent aggregate interlock with the reinforcement, the maximum reinforcement bar size is limited by the minimum concrete cover and maximum aggregate size.
- In this case, the minimum concrete cover requirement is around 20-25mm.
- The maximum aggregate size is limited to 15mm.
- Therefore, the maximum size of reinforcement bar that can be used is limited by the smaller value between the minimum concrete cover and maximum aggregate size.
- Among the given options, the maximum size that satisfies both requirements is 8mm (option C).
- Option C (8mm diameter) is the correct answer.

Conclusion
The maximum size of reinforcement bar that can be used for a 75mm thick reinforced concrete slab is 8mm diameter. This is determined by considering the minimum concrete cover requirements and the maximum aggregate size. Providing adequate concrete cover and preventing aggregate interlock with the reinforcement are important for the long-term durability and performance of the concrete slab.