Question

A rigid pavement is on a highway with two lanes
in one direction, and the pavement is conservatively
designed. The pavement has an 11-inch slab with a
modulus of elasticity of 5,000,000 lb/in2 and a concrete
modulus of rupture of 700 lb/in2, and it is on a soil with
a CBR of 25. The design drainage coefficient is 1.0, the
overall standard deviation is 0.3, and the load transfer
coefficient is 3.0. The pavement was designed to last 20
years (initial PSI of 4.7 and a final PSI of 2.5) with 95%
reliability carrying trucks with one 18-kip single axle
and one 28-kip tandem axle. However, after the
pavement was designed, one more lane was added in the
design direction (conservative design still used), and the
weight limits on the trucks were increased to a 20-kip
single and a 34-kip tandem axle (the slab thickness was
unchanged from the original two-lane design with
lighter trucks). If El Niño has caused the drainage
coefficient to drop to 0.8, how long will the pavement
last with the new loading and the additional lane (same
volume of truck traffic)?