By Danny Gierhart, P.E.
The ultimate goal for any asphalt pavement project is to provide smooth, quiet and safe service to the traveling public for many years. But, how many years is a reasonable expectation?
Thin lift asphalt overlays are equal to or less than 1.5 inches. A recent National Cooperative Highway Research Program (NCHRP) synthesis report showed information and agency responses indicating their service life ranging from five to over 12 years. That begs the question, “How can I make my thin lift overlay last closer to the 12 year side of that range?”
There are many things that go into providing a quality thin lift asphalt overlay, including:
• the type of asphalt mixture
• the quality of the component materials of the mix (aggregate, asphalt binder, additives, etc.)
• the robustness of the final mix design
• material handling practices at the plant
• laydown practices on the project
• the caliber of quality control and quality assurance.
However, the effectiveness of each of those important contributors to long service life can be derailed by something else before any plans are even drawn: poor project selection. Thin overlays placed on existing pavements with too much distress as a type of “band-aid” will not last long and will skew the reported average service life toward the low side. The overarching concept in project selection for thin asphalt overlays is that the surface to be overlaid must be structurally sound.
Before deciding upon whether or not a thin lift asphalt overlay is suitable for a particular project, some type of distress survey should be performed. “The Distress Identification Manual for the Long-Term Performance Program (LTPP)” is a useful tool for this endeavor and is available for free online. It includes a description of each distress type, illustrations to help the user classify the distress as “low,” “moderate,” or “high,” and guidelines regarding how to measure and record the distress. Some state agencies such as Oregon DOT have expanded on the LTPP document to produce their own manual on the subject. Alternatively, “ASTM D 6433, Standard Practice for Roads and Parking Lots Pavement Condition Index Surveys”, details a procedure for determining a Pavement Condition Index (PCI) based on distresses observed on the surface of the pavement.
The remainder of this article will discuss several common distress types and treatment options as they apply to thin asphalt overlays.
Longitudinal cracking in the wheelpath
This type of crack is normally associated with traffic loading. It’s a good idea to core cracks like this to determine whether the crack initiated at the surface or lower in the pavement structure. If the crack began at the surface, evaluate the severity of the crack. If the cracks are narrow and not very deep (low severity), a cleaning and tack coat may be all that are necessary before applying a thin overlay. If the cracks do not move under traffic but are a little wider and deeper (moderate severity), they may need to simply be sealed or routed out, cleaned and then sealed before overlaying. If the cracks are wide and actively move under traffic (high severity), thin overlays would not be a good option.
If the cracks are limited to the surface course, milling the entire lift often becomes a good option. Be aware that pavements are designed to a certain total thickness to resist the anticipated traffic loading. Therefore, the total pavement thickness should not be decreased by, for example, milling off 2 inches of asphalt and coming back with only 1 inch of overlay. In order to avoid leaving localized thin, weakly bonded layers in place after milling, it is best practice to mill a bit deeper than what you think you need. In this case, a thin asphalt overlay could be used to cap a more traditional overlay.
If a crack initiated in the intermediate layers or base of the pavement structure, some type of structural rehabilitation strategy will be needed.
Longitudinal cracking not in the wheelpath
This type of crack is often a result of poor longitudinal joint construction or one of several types of paver-induced segregation. These types of cracks tend not to be structural in nature, and good candidates for thin asphalt overlays. However, some longitudinal cracks near the edge of a pavement structure may be a result of subgrade slope degradation, which would need to be addressed before any type of overlay.
Alternatively, relatively straight cracks near the edge could be caused by shear failure from the original rolling operation during construction. This type of surface crack would typically appear more linear in nature than a ragged edge crack.
This type of crack is perpendicular to the roadway, and often at fairly uniform intervals. It typically occurs from the top down as a result of sudden contraction of the pavement surface due to cold weather events. If the crack is of low severity, it can often simply be cleaned and tacked before overlaying. Wider cracks would usually be cleaned and filled before tacking and overlay. If the edges of the crack are degrading, they may need to be routed and cleaned before filling.
However, if the existing pavement has binder that is too stiff to flex with the internal stresses caused by normal cold weather contraction, the resulting cracks will continue to open and close throughout the year as the temperature fluctuates between the seasons. In this case, the cracks will simply reflect through the overlay each time the pavement contracts. An overlay would not be a good option in this scenario.
This type of crack is propagated by the movement of PC concrete slabs underneath the asphalt. Slab movement must be halted by some method first, or the cracks will simply continue through the new overlay. Projects with this type of cracking are poor candidates for a thin asphalt overlay.
This type of interconnected crack is the result of excessive traffic loading and indicates a severe structural fatigue problem. If the alligator cracking is infrequent and in short runs, the pavement might be able to be salvaged by full-depth patching those areas. Any patching should be of sufficient quality to make the patch structurally sound. Best practices include extending the patch at least one foot beyond the distressed area, squaring off the sides, removing the entire depth of the distress (into the subgrade if necessary), cleaning, tacking the bottom and sides of the hole, and patching in lifts if the depth requires it. A roadway with structurally sound patches can be successfully overlaid with thin asphalt lifts.
Rutting is a surface depression in the wheelpaths. It is caused by some type of weakness either in the subgrade or in one or more of the asphalt layers. Rutting due to an unstable subgrade is not a good candidate for an overlay. This type of rutting is often distinguished by numerous cracks in the rutted section as the asphalt is forced to mold itself into the rutted subgrade. The cause of the unstable subgrade must be addressed and repaired, which is almost always a full-depth operation.
Rutting due to plastic movement in the top lift of asphalt is often distinguished by transverse displacement of asphalt along the sides of the rut. In this case, the entire plastic lift can be milled and the remaining pavement overlaid. Again, it is best not to decrease the total pavement thickness by overlaying with a thickness less than what was milled.
Projects with rutting due to plastic movement in a lower lift are not good candidates for an overlay, and require more extensive rehabilitation.
Utility cuts and/or patch failure
A patch, whether for a utility repair or permanent failure, is where is a portion of pavement has been removed and replaced. Patches can last a long time when constructed using best practices. If the patch has been constructed well enough to be structurally sound and perform as an integral part of the pavement structure, it can be overlaid after sweeping and tacking. With thin overlays, there is not enough new thickness to restore much smoothness. If there is a stable depression in the area of the patch, the area should be leveled before applying a thin overlay. Likewise, if there is a bump in the patched area, it would need to be milled down before a thin overlay.
Potholes are localized disintegration of the pavement which can be caused by several different problems. Potholes must be properly patched, with special attention given to first removing all of the unstable area well into the structurally sound pavement first. Thin asphalt overlays can readily be applied on surfaces with proper patches.
Raveling and weathering
As pavements ravel and weather, the pavement surface wears away because of separation of the aggregate particles from the asphalt binder. Raveling is usually found in the wheelpath, while weathering typically covers the entire pavement surface. These surface distresses are ideal candidates for thin asphalt overlays. The surface should be thoroughly swept to dislodge any loose particles before applying tack and overlaying.
Aggregates that have been worn to a smooth finish by traffic in the pavement surface are called polished aggregates. Once the aggregates polish, they become a skid hazard, especially when wet. A thin asphalt overlay can restore surface friction to a pavement, greatly enhancing traffic safety.
Bleeding or flushing
Bleeding is an uncommon distress that leaves a thin film of asphalt on the pavement surface resulting from excess bitumen and/or low air voids, giving the surface a slick, glossy appearance. Flushing is often regarded as a less extensive form of the distress with only a dark discoloration of the surface rather than an actual coating of binder on the surface. While a surface with actual bleeding is not a good candidate for a thin asphalt overlay, a surface with minor flushing could be readily overlaid.
Remember that roadway failures are rarely single catastrophic events. They typically occur over long periods of time. Sometimes an agency can get caught, and have to reactively spend a lot of budget money on a roadway which has fallen too far into disrepair before action was taken. By proactively monitoring the roadway and applying a thin asphalt overlay to the right pavement at the right time, it is possible to furnish a long-lasting, smooth, quiet and safe roadway to the traveling public.
Gierhart is an Asphalt Institute Regional Engineer based in Oklahoma.