Question

An Existing 2-lane highway pavement serviceability index 4.2 comprising of 4 inch asphalt concrete surface (Mr=400000 psi) plus 11 inch granular base (CBR=55%) resting on a subgrade (CBR=6%), the current AADT is (2000 vpd) with aannual growth rate of 5%. Truck percent is 20% and truck factor is 0.8. Find the anticipated life time of the pavement to reach terminal serviceability index of 2.2. Take R= 80%, So = 0.4 and M2=1

Answer 1

Using the emprical equation of the 1993 AASHTO Flexible Pavement Structural Design,

W18	equals	predicted number of 80 kN (18,000 lb.) ESALs
	ZR	equals	standard normal deviate
	So	equals	combined standard error of the traffic prediction and performance prediction
	SN	equals	Structural Number (an index that is indicative of the total pavement thickness required)
		equals	a1D1 + a2D2m2 + a3D3m3+…ai = ithlayer coefficientDi = ithlayer thickness (inches)mi = ith layer drainage coefficient
	ΔPSI	equals	difference between the initial design serviceability index, po, and the design terminal serviceability index, pt
	MR	equals	subgrade resilient modulus (in psi)

using the equation we can get our desired value as 7.8 years.