SCOPE OF THE GUIDE

This Guide for the Design of Pavement Structures provides a comprehensive set of procedures which can be used for the design and rehabilitation of pavements; both rigid (portland cement concrete surface) and flexible (asphalt concrete surface) and aggregate surfaced for low-volume roads. The Guide has been developed to provide recommendations regarding the determination of the pavement structure as shown in Figure 1.1. These recommendations will include the determination of total thickness of the pavement structure as well as the thickness of the individual structural components. The procedures for design provide for the determination of alternate structures using a variety of materials and construction procedures.

A glossary of terms, as used in this Guide, is provided in Appendix A. It is recognized that some of the terms used herein may differ from those used in your local practice; however, it is necessary to establish standard terminology in order to facilitate preparation of the Guide for nationwide use. Insofar as is possible, AASHTO definitions have been used herein.

It should be remembered that the total set of considerations required to assure reliable performance of a pavement structure will include many factors other than the determination of layer thicknesses of the structural components. For example, material requirements, construction requirements, and quality control will significantly influence the ability of the pavement structure to perform according to design expectations. In other words, “pavement design” involves more than choosing thicknesses. Information concerning material and construction requirements will be briefly described in this Guide; however, a good pavement designer must be familiar with relevant publications of AASHTO and ASTM, as well as the local agencies, i.e., state agencies or counties, for whom the design is being prepared. It is extremely important that the designer prepare special provisions to the standard specifications when circumstances indicate that nonstandard conditions exist for a specific project. Examples of such a condition could involve a roadbed soil which is known to be expansive or nonstandard materials which are to be stabilized for use in the pavement structure or prepared roadbed.

Part I of this Guide has been prepared as general background material to assist the user in the proper interpretation of the design procedures and to provide an understanding of the concepts used in the development of the Guide. Detailed information related directly to a number of design considerations, e.g., reliability, drainage, life-cycle costs, traffic, and pavement type selection, will be found in the Appendices. References used in the preparation of the Guide can be found following each of the four major Parts.

Part I, Chapter 3 of the Guide provides information concerning economic evaluation of alternate pavement design strategies. It should not be concluded that the selection of a pavement design should be based on economics alone. There are a number of considerations involved in the final design selection. Appendix B of the Guide on pavement type selection provides an extensive list of guidelines which should be used in comparing alternate design strategies.

Part II of this Guide provides a detailed method for the design of new pavements or for reconstruction of existing pavements on the existing alignment with new or recycled materials.

Part III of this Guide provides alternative methods for pavement rehabilitation with or without the addition of an overlay. The methodology used in this part of the Guide represents the state of the knowledge regarding the deterioration of a pavement structure before and after an overlay has been applied. It is recognized that there are alternate methods for the determination of overlay requirements; a number of these methods are cited in Appendix C. The method included in Part III is somewhat more basic in concept than other existing methods and has the capability for broader application to different types of overlays, e.g., flexible on rigid, flexible on flexible, rigid on rigid, and rigid on flexible type pavements. The method is also compatible with the performance and design concepts used in Part II. In this way, consideration of such factors as drainage, reliability, and traffic is the same for both new and rehabilitated (overlayed) pavement structures.

State of the art procedures for rehabilitation of pavement structures without overlay, including drainage and the use of recycled material, are emphasized in Part III. These techniques represent an alternative to overlays which can reduce long-term costs and satisfy design constraints associated with specific design situations.

As an adjunct to pavement rehabilitation it is important to first determine what is wrong with the existing pavement structure. Details of the method for interpretation of the information are contained in Part III. A procedure for measuring or evaluating the condition of a pavement is given in Appendix K and Reference 1. It is beyond the scope of this Guide to discuss further the merits of different methods and equipment which can be used to evaluate the condition of a pavement. However, it is considered essential that a detailed condition survey be made before a set of plans and specifications are developed for a specific project. If at all possible, the designer should participate in the condition survey. In this way, it will be possible to determine if special treatments or methods may be appropriate for site conditions, specifically, if conditions warrant consideration of detailed investigations pertinent to the need for added drainage features.

Part IV of this Guide provides a framework for future developments for the design of pavement structures using mechanistic design procedures. The benefits associated with the development of these methods are discussed; a summary of existing procedures and a framework for development are the major concerns of that portion of the Guide.

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