Table 6 (cont'd).
C. Rigid Pavement Horizontal Stress Criteria
1) The Corps of Engineers (U.S. 1990):
Nh = 10 [(df -adon) / bdon]
where: Nh = allowable traffic based on horizontal stress
df = Rcon/σh
Rcon = flexural strength of the concrete, lb/in.2
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 ni = number of applications at strain level i
resultant strain calculations.
Ni = number of applications to cause fail-
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-
D = total cumulative damage.
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-
mine the states of points at different radii, it is
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 ni 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
CUMDAM
Ni the number of applications to failure at a par-
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),
i
ni
and Coetzee and Connor (1990). Three other dam-
=D
∑
(11)
Ni
age models for flexible pavements are based on
i =1
13
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