As pavement ages, it will inevitably begin to deteriorate due to weathering and traffic loading. The condition of the pavement may not be so severe as to warrant complete reconstruction. At this point in the pavement’s life is when rehabilitation techniques can be performed. Rehabilitation strategies are designed to help extend the life of an existing pavement.
Rehabilitation is carried out on pavements that exhibit distresses beyond the effectiveness of maintenance and preservation techniques, but not too severe to warrant the cost of complete reconstruction.
Hot Mix Asphalt Overlay
Hot mix asphalt (HMA) overlay is used for strengthening the existing pavement structure, correcting the surface defects, and improving pavement serviceability. It may be applied to the pavement structure more than once during the entire service life.
Hot mix asphalt the overlay is a paving method of applying a new layer of asphalt to a deteriorating surface. Rather than tearing up an old asphalt surface entirely, an asphalt overlay project will use the existing layers as a base for the new asphalt pavement. In this rehabilitation method, 1.5 to 2 inches of new asphalt pavement is placed over the existing pavement. If the project has curbs, the pavement needs to be edge-milled to allow for the new asphalt surface to match the existing curb. Overlays refresh the pavement’s surface and curb appeal but are only recommended for pavements that are still performing well.
Asphalt overlays have some distinct advantages compared to less costly surface maintenance treatments. In addition to sealing shallow fatigue cracks, overlays add strength to the existing structure and provide support for heavier loads with less chance of stress failures in the bottom layer of asphalt. Overlays can restore the proper transverse profile (slope) and improve longitudinal profile (smoothness). Overlays are an ideal tool to upgrade the existing pavement’s structure, as long as the increase in elevation doesn’t create a drainage or clearance issue.
Asphalt overlays are susceptible to reflection cracking, which are cracks that are caused by the existing cracks in the pavement (below the overlay). Areas with fatigue cracking or potholes should be corrected prior to performing an overlay. Overlays generally aren’t used if the pavement has full-depth cracks, block cracking, deep ruts in the wheel paths, or underlying base problems, such as moisture or deformation.
Overlays are a suitable solution when: a pavement surface is oxidized with minor surface cracking, there is a loss of skid resistance, wheel paths need to be leveled, or there has been minor water intrusion.
Mill and Overlay
Mill and Overlay (otherwise known as Mill and Fill) is the process of grinding off the top layer of existing asphalt pavement by means of a large milling machine and then the milled surface is overlaid with a new hot mix asphalt pavement. The typical depth of milling is between 1” and 2” or more, depending on the condition of the existing riding surface, depth of available curb reveal, and depth of existing asphalt pavement. A leveling course may be applied prior to the final riding surface to resolve rutting, depressions, or other roadway profile issues.
A mill and overlay refresh the pavement’s surface, much like a normal overlay, and also allows for the correction of drainage issues. A mill and overlay is susceptible to reflection cracking, and areas with fatigue cracking or potholes should be corrected prior to performing the mill and overlay. A mill and overlay do not address structural issues because it is only replacing the thickness of the pavement removed. It will not solve all road degradation issues, and if not done properly, the same damage can crop up in short order.
Mill and Overlay typically should be placed over the sound structural pavement with a good profile. The pavement may be exhibiting minor to moderate surface distress (cracking, bleeding, raveling, oxidation).
Cold-In-place recycling (CIR) is a rehabilitation technique where an existing HMA layer with distresses is milled, screened, processed, and mixed with an asphalt recycling agent and compacted with a paver.
The cold in-place recycling process involves milling 3"-6" depth of existing asphalt material and sizing of the existing aggregate, blending with a new binder and/or virgin aggregate, placement, and compaction of this "new" bituminous base course to the desired grade, slope, and profile. This is all accomplished with a single-pass equipment train. The benefits of cold in-place recycling include substantial cost savings, conservation and recycling of natural resources such as asphalt binder, energy, and aggregates, and reduced user delays. Applications include existing roads that may be rutted, fatigued, failing, potholed, trenched from utility work, or an undesirable existing grade or profile. Cold in Place Recycling is a 50% cost savings over reconstruction or mill and overlay practices. Cold in Place Recycling as a green process can generate 80% green savings over conventional practices.
Hot In-place Recycling
Hot In-Place Recycling (HIR) is an onsite rehabilitation method where the road surface layer is heated, rejuvenated, and repaved in-situ, therefore, eliminating the use of virgin materials. This method is practiced using a train system where there is a pre-heating unit followed by a heating unit that scarifies the hot mix asphalt, immediately followed by an auger that mixes the heated rejuvenated materials which are then leveled off with a screed. Finally, conventional compaction equipment is used to compact the newly recycled pavement. The advantage of this method is that the use of new materials is minimized since no new aggregate is added, only adding a rejuvenator (recycling agent).
Hot In-Place Recycling (HIR) is effective at correcting surface distresses that are limited to the top 1 in. to 2 in. (25 mm to 50 mm). Examples include rutting, corrugations, raveling, flushing/bleeding, loss of surface friction, minor thermal cracking, and minor load-associated cracking. The advantages of hot in-place recycling are that elevations and overhead clearances are preserved, it is comparatively economical, and needs less traffic control than the other rehabilitation techniques. This process can also be used to recoat stripped aggregates, re-establish crown and drainage, modify aggregate gradation and asphalt content, and improve surface frictional resistance.
Full Depth Reclamation
Full Depth Reclamation (FDR) is a rehabilitation method used for asphalt pavements with severe distresses. When total roadway reconstruction seems to be the only option, FDR should be considered since it removes all existing road surface distresses.
Full Depth Reclamation is a simple, quick, and economical method for constructing a new homogenous stabilized base. FDR 6"-12" depth is the mixing of the existing asphalt surfaces course as well as the underlying base or subbase materials with a stabilizing agent to produce a stabilized base course. This process is recommended for existing pavement sections that have insufficient base or subbase to support increasing traffic loads, or undesirable grade and cross slope. Depending on the existing materials, stabilizing agents could be bitumen emulsion, lime, or portland cement. Virgin aggregate or RAP can be added to meet gradation requirements. The steps normally involved in this process include: pulverization of the existing surface course including blending it into the base material, excavation & profiling to the desired grade and slope, stabilization with the recommended additives, fine grading, and profiling of a new surface course.
Full-depth reclamation is a high production, low-cost reconstruction alternative whose benefits include resistance to water and frost damage, improved ride quality, increased structural strength, elimination of disposal costs, conservation and recycling of raw materials and energy.?