Asphalt is the smooth, quiet, durable and sustainable surfacing material that has been around for hundreds of years.Today's asphalt is a scientifically designed product. It is rut, shoving and crack resistant. It is the most recycled material in the world.
Asphalt concrete structural overlay design can be broadly categorized into the following (modified after Monismith and Finn, 1984): 

Engineering judgment. This approach to overlay design selects an overlay thickness and the associated materials 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. This method is highly subjective and can be heavily influenced by political and budget constraints. 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 This approach to overlay design uses pavement surface deflection measurements 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. This approach to overlay design uses the same mechanistic-empirical methods that were discussed in the Structural Design section. These methods are quite versatile because they can evaluate different materials under various environments and pavement conditions. In many places these procedures have replaced limiting deflection overlay methods, since the latter do not account for subsurface material properties. 

Pavement maintenance describes all the methods and techniques used to prolong pavement life by slowing its deterioration rate. The performance of a pavement is directly tied to the timing, type and quality of the maintenance it receives. 

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