Rehabilitation can be defined as (ITS, 2000):
" Measures to improve, strengthen or salvage existing deficient pavements to continue service with only routine maintenance. Deficient pavements exhibit distress in excess of what can be handled through routine maintenance."
In other words, although maintenance can slow the rate of pavement deterioration, it cannot stop it. Therefore eventually the effects of deterioration need to be reversed by adding or replacing material in the existing pavement structure. This is called rehabilitation. Rehabilitation options depend upon local conditions and pavement distress types but typically include:
- HMA overlays. Overlays are used for two primary purposes:
- Structural overlays are designed to add structural support to the existing pavement. Because of this, they are structurally designed and are thicker than non-structural overlays.
- Non-structural overlays are designed to add or replace the existing pavement wearing course. Because of this they contribute very little to the pavement structure and are generally assumed to provide no additional structural support. Because most agencies consider non-structural overlays to be maintenance items, they are discussed on the Maintenance page.
- In-place recycling. There are two types of common in-place recycling:
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):
- 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.