The question addresses the topic of reinforced concrete structures, specifically the spacing, or pitch, of the main reinforcement in a slab. Understanding the rules governing reinforcement spacing is pivotal for ensuring the structural integrity and performance of slabs.

In reinforced concrete design, the spacing of main reinforcement bars is crucial for maintaining the structural stability and controlling crack widths in concrete structures.

Key Concept: According to the guidelines provided in the Indian Standard (IS) code for concrete structures, IS 456:2000, the maximum spacing of the main bars in slabs should not exceed three times the effective depth of the slab or 300 mm, whichever is smaller. This limitation helps in controlling cracks and ensuring uniform stress distribution across the slab.

Explanation:

• The effective depth of a slab is the distance from the top surface of the slab to the centroid of the main reinforcement bars.
• IS 456:2000 stipulates that to avoid large crack widths and ensure proper load distribution, the spacing of main longitudinal reinforcement should be controlled.
• Following this code, the maximum permissible spacing for main reinforcement (pitch) is limited to three times the effective depth of the slab.

Conclusion:

• Given the options, "three times" the effective depth is the correct answer, aligning with the IS code requirements.
• Other options, such as "four times", "five times", or "two times", do not adhere to these guidelines as per IS 456:2000, specifically for slabs.

Therefore, the pitch of the main reinforcement in a slab should not exceed three times its effective depth.