An isolated concrete pavement slab of length $L$ is resting on a frictionless base. The temperature of the top and bottom fibre of the slab are $T_t$ and $T_b$, respectively. Given: the coefficient of thermal expansion $=\alpha$ and the elastic modulus $=E$. Assuming $T_t > T_b$ and the unit weight of concrete as zero, the maximum thermal stress is calculated as

1. $L \alpha (T_t-T_b)$
2. $E \alpha (T_t-T_b)$
3. $\dfrac{E \alpha (T_t-T_b)}{2}\\$
4. zero