30 June 2001
F = maximum computed tensile stress with elastic-layered model such as JULEA computer
program, MPa (psi)
Co = coverage level at which the SCI begins to decrease from 100
Cf = coverage level at which the SCI becomes 0
SCI = the structural condition index desired at the end of the pavement design life
c. When aircraft passes are given, then the pass-per-coverage ratio for the particular design aircraft
will be used to convert passes to coverages. The engineer is cautioned that Equations 19-1 and 19-2
were formulated based on accelerated traffic tests with volumes less than 10,000 coverages. The use of
the relationship to design for traffic volume greater than 10,000 coverages, which will frequently be the
case for current traffic volumes, will require extrapolation of the linear relationship. The pass-per-
coverage ratios for some aircraft are shown in Table 17-1.
6. FROST CONSIDERATION. Two methods have been developed for determining the thickness
design of a pavement in frost areas. One method is to limit subgrade frost penetration and the other is to
design the pavement for reduced subgrade strength. The first method is directed specifically to the
control of pavement distortion caused by frost heave. It requires a sufficient thickness of pavement,
base, and subbase to limit the penetration of frost into the frost-susceptible subgrade to an acceptable
amount. Complete frost penetration prevention is nearly always uneconomical and unnecessary except
in regions with a low design freezing index or where the pavement is designed for heavy-load aircraft.
When the rigid airfield pavement is designed by the reduced subgrade strength method, a minimum
thickness of 102 millimeters (4 inches) of granular unbound base will be used. A mechanistic procedure
for seasonal frost is being developed. Until it is available, the method in Chapter 20 should be used.
7. ALTERNATE OVERLAY DESIGN PROCEDURE. A methodology for the design of rigid overlays of
rigid pavements has been developed that predicts pavement structural deterioration from load induced
stresses. The performance of the pavement is expressed in terms of an SCI which relates the type,
degree, and severity of pavement cracking and spalling on a scale from 0 to 100. The design
methodology for rigid overlays uses the layered-elastic analytical model and the analysis of fatigue
methodology predicts performance, an accurate characterization of the materials, structural pavement
condition, and fatigue are required. The steps for designing rigid overlays of rigid pavements are
illustrated in Figure 19-1 and are implemented in the LEDRRO group of programs.
a. Material Properties. Each layer of the pavement must be described by a modulus of elasticity
and a Poisson's ratio.
(1) The modulus value for the concrete can be determined in the laboratory or conservatively
estimated as 27,576 MPa (4,000,000 psi).
(2) Modulus values for subgrade soils are often estimated from correlations with existing tests.
(3) Flexural strength of concrete overlays should be determined as part of the mixture
proportioning studies. The flexural strength of the base slab may be determined from historical data,
flexural beams cut from the base pavement, or approximate correlations between flexural strength and
tests run on cores taken from the base pavement.