Rigid pavement constructed with an asphalt overlay is referred to as composite pavement. The advantage of constructing an asphalt overlay on a rigid pavement is solely in the areas of ridability and noise. Rigid pavements are considered by most to create more road noise inside a vehicle than flexible pavements. This phenomenon is largely due to the surface texture specified for rigid pavements to ensure proper skid resistance. By specifying an asphalt overlay with the rigid base, surface texture requirements can be relaxed and noise can be reduced.
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.
Because a composite pavement behaves more like a rigid pavement, special treatment is required for the transverse joints. Reflective cracking, cracks that propagate from the rigid pavement joint through the asphalt overlay, can be an intolerable distress which induces a rough riding pavement. The reflective crack allows water to enter, which induces stripping in the asphalt and slow deterioration into a spalled pothole. The suggested treatment to counter the reflective cracking is to saw and seal the asphalt concrete overlay directly above the concrete joint. The joints should be sawed as soon as the asphalt overlay is placed, and the joint sealant reservoir should be constructed the same way as discussed previously. Figure 3.8 shows a joint in a composite pavement that has been properly sawed and sealed.
