PAVEMENT EVALUATION AND OVERLAY DESIGN: SUMMARY OF METHODS

A general framework for pavement maintenance and rehabilitation is shown in Figure 1, and Figure 2 lists a number of available alternatives. Maintenance refers to those processes-both preventive and correctivethat do not involve major alterations in the pavement structure. Rehabilitation encompasses the areas of reconstruction, overlays, and recycling and their combinations and can be used to restore or to improve the serviceability of the pavement structure. To assist the engineer in deciding what pavement maintenance or rehabilitation to do and when to perform it, pavement performance must be measured on a systematic and continuing basis. In this performance evaluation, both the functional and the structural performance of the pavement system should be considered. Functional performance describes how well the pavement serves the user, and structural performance is related to its ability to sustain load (which in turn affects its ability to serve the user). If the pavement becomes too rough, for example, it will be difficult for the user to operate the vehicle and functional performance will be unsatisfactory. However, although these characteristics are related, there is currently no welldefined relationship between structural distress and functional performance. Thus, at present, judgment must be used in deciding when structural deterioration will lead to a level of functional performance below that considered reasonable by the user of the facility (which obviously will vary among vehicles, users, and types of facilities). For street and highway pavements, particularly, it is appropriate to separate the performance-evaluation process into two phases (Figure 1). In the first, termed the network-monitoring phase, condition surveys are used to provide a basis for segregating those pavements that clearly do not require maintenance or rehabilitation from those that may and for which further information is required. The second phase is composed of more detailed or diagnostic investigations that can provide the data required to determine an appropriate strategy. These include measurements of physical condition (surface characteristics and structural response) and assessment of special problems such as drainage difficulties or thermal cracking. Vis ual condition-evaluation procedures described in Ur.is Record in the papers by Shahin, Darter, and Kohn for airfield pavements and by Phang for highway pavements are a necessary part of the network-monitoring process and can serve as a guide to the type of maintenance or rehabilitation to be accomplished. Also, as noted by Phang, these surveys can assist in the overall investment-programming process for maintenance and rehabilitation. The use of visual condition surveys is well established and should be a part of the maintenance and rehabilitation methodology of every organization that has responsibility for pavements. Structural performance can be measured by a number of nondestructive testing devices that have been developed in recent years and are being used as a part of the overlay-design methodologies of many organizations. The majority of these devices provide some measure of surface deflection. The various devices currently in use are summarized below.