For many old pavements, restoration, not preservation, provides the long term solution to cost, performance, and sustainability.
Public Work Agencies and other pavement stakeholders face a growing inventory of roadway pavements that have reached their useful design life. These roads have uneven ride, consume maintenance dollars and resemble the pictures below. At this stage of a road’s life, when distress emanates within the base structure, pavement preservation strategies become incrementally less effective. Pavements are unstable systems that are designed to fail without proper maintenance; in California that means only 20 to 30 years. From this standpoint, roads built before 1980 are living on borrowed time. The goal of successful pavement design should anticipate all roadway demands.
Conventional Pavements are constructed using standard empirical design methods. A Traffic Index (TI) is assigned for each pavement based on an anticipated loading and the structural response of the various pavement layers for a determined period of time. Structural values were given to each pavement layer based on an equivalent layer of gravel or gravel factor (Gf 1.0)
Once constructed, the aging behavior of pavements will be dictated by the various pavement layers’ stability. The traffic loading value becomes the only constant design value within the pavement structure matrix. However, with many communities, traffic values increase when rapid development occurs and traffic loads exceed the available capacity.
The pavement material layers become unstable due to environmental conditions. The aging of the pavement layers consist of oxidation of the asphalt binder which leads to hardness and brittleness of the surface. In saturated subgrade areas, the underlying material’s moisture sensitivity begins to reduce the structural integrity of the unbound aggregate and soil support structure.
Perpetual pavements are defined as requiring only periodic surface treatments dependent on the degree of oxidation and surface distress associated with asphalt hardening-- they are roadways that exhibit the effects of oxidation but maintain stability within the support. Consistent examples of perpetual pavements can be found on desert roads. These roads have two distinct advantages: they are built on inherently better structural soils including low expansion potential and high resistance values and they have less exposure to excessive moisture influences or fluctuations. The desert environment provides a stable system within the subbase support structure. However, these desert roadways have a higher surface oxidation rate.
A Stable Support System or foundation is essential for extended pavement life. When considering stability of buildings, a concrete base is required to be situated below the seasonal moisture fluctuation elevation. With traditional pavement design, initial stability is achieved by compacting unbound aggregate and subgrade soil. When water is introduced to this foundation system the unbound aggregate and soils become highly reactive to loading, which weakens the entire foundation structure. This explains why unstable pavements typically correlate with bad soils and excess water in the foundation areas.
When considering long-term restoration of these deteriorated pavements, the best solution is to rebuild the foundation. However, many agencies still believe that overlays will solve the problem. Smart stakeholders realize that removing the existing pavement materials and replacing with essentially the same material is not economical or environmentally sustainable. A proven method to rebuilding the road is in-place material recycling and chemical stabilization. This method is known as Full-Depth Rehabilitation (FDR). It consists of pulverizing the existing pavement materials, applying a stabilizing reagent (Portland cement) that is mixed/hydrated into the recycled materials and compacted. The new FDR section provides a solid foundation support, that is unaffected by excess water. A new wearing coarse of asphalt is placed over the FDR section.
The FDR Design is supported by proven engineering protocols and construction verification. The reconstructed pavement is assigned a Traffic Index based on conventional design methods. The FDR mix design verifies the unconfined compressive strength (UCS) that converts to an equivalent gravel factor. Moisture sensitivity is minimized by designing strength within the bound structural section which provides a reduction in the expansion potential of the stabilized material.
FDR provides a perpetual foundation support. Design optimization and field verification are required for developing and constructing FDR sections. Design it in the lab and recreate it in the field. Design goals should address concerns of over-rigidity, future excavate-ability and reduction of material shrinkage, which eliminates potential for shrinkage cracking, and incorporates constructability issues that may result in change orders during construction. Other than remove and replace, FDR is the only pavement rehabilitation process that follows established design methods utilizing Gravel Factors (Gf) to assign long-term performance. Properly designed and installed FDR sections will become perpetual pavements, maintaining structural integrity well beyond their intended design life.
The evidence is clear that FDR has a proven record of success on both light and heavy-duty pavements. From an engineering perspective, FDR provides measurable performance as determined by ASTM certified testing of strength and durability.
The individual driving a car equates successful pavement design and performance to a smooth ride. As vehicles become lighter to meet environmental efficiency goals, the performance of the road becomes equally important as the vehicle itself. The FDR process provides the lasting stable support to meet these performance goals in an economic and environmentally sustainable way.