Pavement Failures and Maintenance Techniques
According to Mark Watson of MnDOT, the theory of pavement maintenance is best summed up with the adage, “a stitch in time saves nine.” “Essentially what we’re saying is that by keeping on top of repairs and starting them earlier on in the pavement’s life, there are more options for maintaining it,” he explained. “Regular maintenance starting when the pavement is still in good shape pushes out the life of the pavement just like regular oil changes can lengthen the life of a car.”
Watson next discussed some of the various types of pavement failures and likely causes of each. “When we know what is causing [the pavement] to fail, we can better identify the proper solutions,” Watson said. He referred to two manuals MnDOT crews typically use to correctly identify pavement distresses. One is called Pavement Rehabilitation: A Guide for Minnesota Cities and Counties from the Minnesota Local Road Research Board (LRRB) and the other is The Distress Identification Manual for Long-Term Pavement Performance from the Strategic Highway Research Program (SHRP). These manuals provide pictures of various pavement distresses along with a description of each and how to rate the severity of the problem.
Watson explained that there are two main groups of pavement distresses: cracking and surface deformation. Within these two groups are various subgroups, which he went on to briefly describe. One type of cracking, for example, is fatigue or alligator cracking because it looks like the back of an alligator in that the pavement is broken up into relatively small chunks. This type of cracking develops when the pavement has insufficient strength: perhaps it is too thin for the traffic loads or it does not have the proper base support.
Longitudinal joint cracking is another common pavement problem. This is characterized by cracks running predominantly parallel to the pavement centerline, lane division lines, or the lane shoulder division. These cracks present a particular safety hazard for motorcycles as the divots and ruts created are the right size to catch a motorcycle tire. This may cause the driver to lose control of the vehicle, possibly resulting in injury or death. These cracks also deteriorate the pavement markings, causing further safety problems. “We think [longitudinal cracking] is probably a result of construction problems relating to improper base compacting,” he explained. “Research testing suggests that applying sealants or other crack treatments can prevent this type of stressor from occurring. The main idea is to keep the water out and protect the surface.”
Block cracks are typically seen in lower volume roads and on shoulders, he continued, and are typically caused by oxidation. “The pavement material goes through chemical and physical change just by being outside…it gets more stiff and brittle as it is exposed to the sun and oxygen,” Watson said. “However, sometimes oxidation is a result of the material being mixed too long or at too high of a temperature at the plant.”
Several other distresses appear in pavement as various surface deformations. These include: rutting, shoving, bleeding, raveling, delaminating, patch deterioration, and of course, potholes. “All potholes start as a crack,” Watson explained. “As water gets in, it erodes away the support materials. As it loses support, goes through freeze-thaw cycles, and has to carry more and more traffic, the pavement gets fatigued. Eventually all of this forces material out, and we get a pothole.”
Preventive maintenance activities
While there are a variety of corrective and preventive maintenance activities DOTs and other agencies use to prolong pavement life, Al Brooker, technical services manager with Fahrner Asphalt Sealers, LLC, discussed a few of what he considers to be the “simpler fixes.” The first process he described is what is called blow or spay patching. This method provides a quick, mobile process of putting down patch material using a self-contained patch machine called a snorkel blow patcher. Everything needed to create the patch is contained in one unit, which allows a single operator to control the process from the driver’s seat of the machine. This involves first spraying the crack or pothole clean with compressed air, applying a tack coat of emulsion, and then mixing in rocks with the emulsion to build up to the thickness needed. Finally, the operator finishes the patch with a coating of rocks that are then raked level, making the surface traffic-ready immediately.
Brooker then described infrared patching, whereby crews heat up an area of distressed asphalt using a propane-fired heating unit. “It takes 10 to 20 minutes to heat the asphalt,” he explained. “Once it’s up to the proper temperature, crews scarify the hot asphalt, add in some fresh hot mix, level it off, and then roll it similarly to a hot patch.”
One of the most cost-effective methods of maintaining pavement longevity is crack sealing, he explained next. “Crack sealing helps keep water out of the pavement base, which is especially important during freeze-thaw cycles,” he said. However, to be effective, crack-sealing crews must first identify the right types of projects. “The pavement cannot be in too bad of shape,” Brooker said. “And not all types of cracks can be sealed. A transverse crack is a good fit for crack sealing, but if the pavement has too many cracks, maybe a chip seal would be better.”
When crack sealing is used, the crack itself must be properly prepared to accept the sealant, and the right sealant product and applicator also must be employed. In some cases, crews may prep the crack by using a router to clean out old material and create a reservoir that will accept the new sealant and allow the crack to expand and contract without debonding. “Crews can use a heat lance to blow away anything that’s loose in the crack, and the heat helps improve bond,” he added. “Finally, the crack should be filled as fast as possible, maybe using two people to speed up the process, to get the road reopened to traffic.”
Last, Brooker discussed using polymastic patch material for crack sealing more distressed surfaces with wider cracks and joints. This material also can be used on portland cement concrete for leveling cupped cracks, minor depressions, and spall repair. “It can be a good option for smaller potholes and utility cut repairs, but it’s probably too costly for bigger potholes,” he said. “This material is tough and durable…it’s really a hot-mix patch with a lot of asphalt in it.”
—Nancy Strege, LTAP freelancer