For a concrete highway pavement with temperature/steel reinforcement, what depth below the top surface is reinforcement generally placed to control temperature and shrinkage stresses?

Introduction / Context:
Reinforcement in cement concrete pavements is provided primarily to control temperature and shrinkage cracking. Its vertical position (cover from the top surface) significantly influences crack control and durability under traffic and environmental actions.

Given Data / Assumptions:

• Rigid pavement (concrete road slab) with temperature steel.
• Typical practice under Indian/standard highway guidelines.
• Focus on the usual nominal depth below the top surface.

Concept / Approach:
To effectively control surface-origin cracks due to thermal gradients and drying shrinkage, reinforcement is placed relatively near the surface. Too deep a placement reduces its crack-arresting efficiency; too shallow a placement risks corrosion and damage during construction and service.

Step-by-Step Solution:

Verification / Alternative check:
Design guides commonly specify cover values around 50 mm for pavements, balancing durability and crack control. Field experience corroborates satisfactory performance with ~5 cm placement.

Why Other Options Are Wrong:
6–9 cm lowers the steel, diminishing crack control for top-surface thermal stresses; 5 cm remains the accepted, effective compromise.

Common Pitfalls:
Confusing slab “mid-depth” reinforcement (used for other structural members) with pavement practice; ignoring the need for corrosion cover in deicing or coastal environments.

Final Answer:
5 cm