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Rehabilitation
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."
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| Figures 1 and 2: Pavement Rehabilitation |
 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.
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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.
- Hot in-place recycling
(HIPR). Covered in the Pavement Types section.
- Cold in-place recycling
(CIPR). Covered in the Pavement Types section. Full-depth CIPR, known as full-depth reclamation (FDR)
is considered reconstruction.
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.
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