There are few documented composite pavement design procedures available to
determine the proper thickness ratios between the rigid base thickness and the flexible
surface thickness. One way to determine an equivalent composite thickness buildup
can be done using elastic layer theory. A convenient computer program called
ELSYM5 (public domain) can be used for analysis of different layer combinations,
provided the designer is willing to make some assumptions. By accepting the assumptions,
the designer is getting results that are only approximate but relative. ELSYM5 is
based on elastic layer theory, which is not entirely appropriate for rigid pavement,
since rigid pavement is not continuous, isotropic, and homogeneous in all directions.
The procedure involves calculating the required rigid slab thickness for the conditions
present where the composite pavement will be constructed. This is done using the
AASHTO Pavement Design Guide or another method. The second step is to analyze
the required rigid slab using ELSYM5 under the conditions designed for by calculating
the deflections, strains, or stresses predicted under the maximum legal loading configuration.
Finally, using a trial-and-error procedure, replace up to 3 in (75 mm) of the
rigid slab with enough thickness of asphalt to achieve the same deflections under the
same loading scheme. A rule of thumb is to replace the first 1 in (25 mm) of slab thickness
with 3 in (75 mm) of asphalt concrete. However, there is not a linear relationship of 1 in
(25 mm) of PCC to 3 in (75 mm) asphalt concrete; for additional reductions in the rigid
slab thickness, the elastic layer theory is relied upon to calculate equivalent deflections.