TM 5-822-13/AFJMAN 32-1018
CHAPTER 5
DESIGN CRITERIA
5-1. Damage Factor.
The damage factor is defined as DF = n/N where n is the number of effective stress or strain repetitions and N
is the number of allowable stress or strain repetitions. The cumulative damage factor is the sum of the damage
factors for all vehicles. The value of n is determined from the number of vehicle operations, and the value of N is
determined from the computed stress or strain and the appropriate criteria. The pavement thickness is deter-
mined when the cumulative damage factor equals one.
5-2. Vehicle Operations.
When vehicle operations (passes) are given, an operation-per-coverage ratio is needed for the particular design
vehicle to convert operations to coverages. The operation-per-coverage ratios for the representative configura-
tions are shown in tables 5-1 and 5-2 for rigid and flexible pavements, respectively. In LEDROAD computer
program, the operations-per-coverage ratio is computed based on the standard deviations listed in the tables.
The computations are based on the assumption that the wheels wander in a normal distribution across the traffic
lane. The operations-per-coverage ratio computed in LEDROAD will be different from those listed in the tables
and consequently the values computed using LEDROAD may be slightly different from those presented in this
manual.
5-3. Concrete Pavement.
The limiting stress (fatigue) criteria are the basis for the design of concrete pavements. The thickness of the
portland cement concrete slab is selected so that the maximum tensile stress at the bottom of the slab does not
exceed the preselected value. The criteria are presented as a relationship between design factor and allowable
coverages by the equation:
N = 10x
(eq 5-1)
where
N = allowable coverages
X = (R/-A)
B
A = 0.2967 + 0.002267 (SCI)
B = 0.3881 + 0.000039 (SCI)
R = flexural strength, psi
1.33 times the maximum tensile stress at bottom of the slab computed with elastic layered method
SCI = Structural condition index. SCI = 80 for the first-crack condition, and SCI = 50 for the shattered
slab condition
5-4. Flexible Pavement.
Basically, there are two criteria for determining the allowable stress (or strain) repetitions N. The first is the
allowable number of operations as a function of the vertical strain at the top of the subgrade. The second is the
allowable number of operations as a function of the horizontal strain at the bottom of the asphalt layer. There is
no strain criteria for unbound base. In developing the procedure, it was assumed that unbound base and subbase
that meet CE guide specification for quality will perform satisfactorily.
a. Asphalt strain criteria.
(1) The primary means recommended for determining the limiting horizontal tensile strain for bituminous
concrete is the use of the repetitive load flexural beam test on laboratory-prepared specimens. Procedures for
the test are presented in detail in appendix C. Several tests are run at different stress levels and different
sample temperatures such that the number of load repetitions to fracture can be represented as a function of
5-1
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