Structural HMA Overlays for HMA Pavement

Structural overlays are used to increase pavement structural capacity. Therefore, they are considered rehabilitation, although they typically have some maintenance-type benefits as well. Asphalt concrete structural overlay design can be broadly categorized into the following (modified after Monismith and Finn, 1984^[1]):

• Engineering judgment
• Component analysis
• Non-destructive testing with limiting deflection criteria
• Mechanistic-empirical analysis

Each of the above categories will be briefly described.

Engineering Judgment

This classification of overlay design is the most subjective of the four listed and can be heavily influenced by political and budget constraints. Selection of overlay thickness and the associated materials is often based on local knowledge of existing conditions, which can result in cost effective solutions; however, local expertise is fragile and subject to retirements, agency reorganizations, etc. Currently, more agencies appear to be relying on quantifiable overlay design approaches but tempered with local expertise.

Component Analysis

This approach to overlay design essentially requires that the total pavement structure be developed as a new design for the specified service conditions and then compared to the existing pavement structure (taking into account pavement condition, type, and thickness of the pavement layers). Current component design procedures require substantial judgment to effectively use them. This judgment is mainly associated with selection of “weighting factors” to use in evaluating the structural adequacy of the existing pavement layers (i.e., each layer of the pavement structure is assigned a layer coefficient often on the basis of experience).

Non-Destructive Testing with Limiting Deflection Criteria

Pavement surface deflection measurements can be used to determine pavement structural properties, which can then be used to determine the required amount of additional pavement structure. Basically, a pavement’s surface deflection in response to a known loading is used as a measure of effective strength. This “effective strength” is influenced by a variety of factors including material properties (including subgrade), thickness of pavement layers, and environmental effects. Most currently used deflection based overlay design procedures do not attempt to isolate material properties of individual pavement layers.

Mechanistic-Empirical Analysis

Mechanistic-empirical based design methods are useful in overlay design as well as new pavement design. Their greatest advantage is the versatility provided in evaluating different materials under various environments and pavement conditions. Mechanistic-empirical procedures provide a basis for rationally modeling pavement systems. As these models improve, better correlations can be expected between design and performance parameters. In many places these procedures have replaced limiting deflection overlay methods, since the latter do not account for subsurface material properties. Mechanistic-empirical overlay design is essentially the same as mechanistic-empirical structural design for new pavements but with the addition of more evaluation locations. Mechanistic-Empirical covers this design method.