What is the critical thickness of a prestressed concrete pavement (using Westergaard’s Corner Load Formula) to support a maximum wheel load of 4200 kg? Allow 10% for impact. Tyre pressure may be taken as 7 kg/cm². Assume flexural strength of concrete as 50 kg/cm², factor of safety as 2, subgrade reaction for plastic mix road as 6 kg/cm³, and modulus of elasticity as 3 × 10⁵ kg/cm².