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.

Asphalt concrete pavement, also referred to as flexible pavement, is a mixture of sand, aggregate, a filler material, and asphalt cement combined in a controlled process, placed, and compacted. The filler material can range from quarry crushing dust and asphalt-plant baghouse fines to wood fibers (cellulose). There are many additives that can be used in asphalt concrete mixes to encourage thicker cement coatings, more elastic mixes, stiffer mixes, and less temperature-sensitive mixes. Flexible pavements can be of a type constructed on a prepared subgrade, which is called full-depth asphalt concrete pavement (FDACP), or of a type built on an untreated granular base, which is not as carefully identified by the industry but is referred to herein as deep-strength asphalt concrete pavement (DSACP). (See Arts. 1.5.3 and 1.5.5.)

Flexible pavements are designed to bend and rebound with the subgrade. The design concept is to place sufficient layers of base and intermediate courses of pavement so as to control the strains in the subgrade so that no permanent deflections result. Loading of an asphalt pavement requires the stiffest layers to be placed at the surface with successively weaker layers down to the subgrade. The types and thicknesses of subbase materials placed above the subgrade should be selected with consideration of the strength of the subgrade. Very weak subgrades, after compaction, can lose compaction when very stiff aggregate bases are placed above. It is often advantageous to place a granular subbase, which is much weaker than an aggregate base, above weak subgrades to ensure that com­paction is sustained. Most, if not all, flexible pavement design procedures are based on a combination of elastic layer theory and experience. The elastic layer theory is used to cal­culate strains in each of the layers so as to ensure that excessive deflections will not occur. The experience is related to performance parameters that predict the number of times the pavement can bend (loadings) until cracking results.

As the name implies, continuously reinforced concrete (CRC) pavement is a rigid pave­ment constructed with continuous longitudinal reinforcement. No transverse joints are installed. Instead, the pavement is allowed to develop random transverse cracks, and the steel reinforcement holds the cracked sections together. The size and spacing of the cracks are influenced by the percentage of reinforcing steel used. Current practice calls for 0.6 to 0.7 percent of the slab cross-section area. The design of the reinforce­ment is covered in the AASHTO Pavement Design Guide. The thickness of the slab is determined the same way as for other concrete pavements.

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FIGURE 3.7 Typical preformed joint seal in rigid pavement. Conversion: 1 in = 25.4 mm.