A 12-in concrete pavement slab (f'c=4000 psi) on a subgrade with k=150 pci is subjected to an 18-kip single axle load with 12-in dual tire spacing. Using AASHTO 1993, what is the allowable load repetitions before fatigue cracking exceeds 20%?

Why is this the answer?

The stress ratio (SR) is calculated by dividing the flexural stress (determined via Westergaard or PCA charts for the given slab thickness, k-value, and load) by the modulus of rupture (MR). For f'c=4000 psi, MR is approximately 550-600 psi. The stress ratio for an 18-kip axle on a 12-in slab is typically low, resulting in high allowable load repetitions (N) exceeding 4 million cycles according to AASHTO fatigue models.

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