1) The Corps of Engineers (U.S. 1990):

where: *N*h = allowable traffic based on horizontal stress

σh = horizontal stress at the base of the concrete, lb/in.2

adon = 0.2967 + 0.002267 SCI

bdon = 0.3881 + 0.000039 SCI

SCI = surface condition index of the pavement when failed

showed that this did not significantly affect the

where *n*i = number of applications at strain level *i*

resultant strain calculations.

The solution computed by NELAPAV is an

ure at strain level *i*, based on damage

approximation of the exact solution. In reality, the

model predictions

stress state changes from point to point. There-

fore, the modulus of a nonlinear material varies

both vertically and horizontally. While NELAPAV

In this relation, failure can occur when *D *equals

recomputes the set of compatible moduli to deter-

or exceeds 1.0. Thus, for a section to last its de-

sign life, the value of *D *should not accumulate to

bound by the assumption that the moduli are con-

1.0 until the design period expires.

stant everywhere in the layers. An exact theory

The value *n*i relates to the design traffic, or

for nonlinear materials would allow the modulus

applications, in 8165-kg (18,000-lb) equivalent

to vary horizontally within the layer in accordance

standard axle loadings (ESALs). For the first two

with the nonlinear model.

simulation series in this study, anticipated traffic

Output from NELAPAV is one file for each

was considered to be 2,815,000 ESALs during a

day. The files include: 1) a repeat of the input

5-year period. Since our incrementation was on a

information, 2) compatible moduli of the layers

daily basis, we applied this as a constant loading

resulting from calculations, and 3) stress condi-

of 1542 ESALs per day. In the final simulation

tions for all points specified. For this study, the

series the anticipated traffic was revised upwards

points specified were located 0.01 in. above the

to 3,300,00 ESALs over five years, applied at a

bottom of the asphalt or PCC and 0.01 in. below

rate of 1808 ESALs per day.

the top of the subgrade.

CUMDAM includes several damage models

previously developed by others for determining

The program CUMDAM calculates cumulative

ticular strain/stress condition (Table 6). Of the

damage to the pavement structure, and was devel-

damage models for flexible pavements, four are

oped at CRREL. No report has been prepared that

based on horizontal strain at the bottom of the

discusses its function and operation. In general

asphalt layer, and relate to damage effects that

form, the procedure used for CUMDAM's calcu-

result in pavement cracking. These models were

lations is the linear summation of cycle ratios,

developed by the Asphalt Institute (1982), Witczak

referred to as Miner's rule, which may be stated as:

(1972), the Corps of Engineers (U.S. Army 1988),

and Coetzee and Connor (1990). Three other dam-

=*D*

∑

(11)

age models for flexible pavements are based on

13