UFC 3-260-03
15 Apr 01
b. Limiting Stresses and Strains. WESPAVE determines the limiting values of stress/strain for a
particular pavement type using the following:
(1) Flexible Pavements. Horizontal tensile strains at the bottom of the AC layer and vertical
subgrade strains are considered in the evaluation of flexible pavements. The limiting AC strain criterion
(shown graphically in figure 4-7) is as follows:
ALLOWABLE STRAINAC ' 10&A
(eq 4-4)
where
ALLOWABLE STRAINAC = allowable tensile strain at the bottom of the asphalt layer,
inches/inches
EAC
% 0.392
N % 2.665 LOG10
14.22
A=
5
N = LOG10 (aircraft coverages)
EAC = AC modulus, pounds per square inch
The allowable subgrade strain criterion (shown graphically in figure 4-8) is as follows:
1/B
10,000
ALLOWABLE STRAINSG '
A
(eq 4-5)
N
where
ALLOWABLE STRAINSG = allowable vertical strain at the top of the subgrade, inches/inches
N = aircraft repetitions (passes)
A = 0.000247 + 0.000245 LOG(ESG)
B = 0.0658 (ESG)0.559
ESG = subgrade modulus, pounds per square inch
(2) Rigid and Nonrigid Overlay on Rigid Pavements. WESPAVE assumes that an AC over
PCC structure to be evaluated as a rigid pavement. If the modulus of the PCC layer determined using
WESDEF is less than 1,000,000 psi, the pavement should be evaluated as a flexible pavement. The
evaluation of rigid and nonrigid overlay of rigid pavements is based on the tensile stress at the bottom of
the slab. The criteria provide for prediction of pavement deterioration in terms of a structural condition
index (SCI). The SCI is defined as follows:
SCI ' 100 & A( ( sum of structural deducts)
(eq 4-6)
where A is an adjustment factor based on the number of distress types with deduct values in excess of
five points determined from the condition survey, and the structural deducts are a function of distress
4-12
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