Easy come, easy go: Smooth road repair
Arlie Long, a recently retired street superintendent for Sioux Falls, S.D., has seen virtually every pavement repair situation during his 45-year career. Like many engineers, he considers potholes in asphalt pavement to be the most annoying repair problem. “Eighty to ninety percent of the pothole patches were gone after one winter,” he says.
Long determined that the spray injection method of patching would be more effective than conventional hand patching because the technique requires less labor and has a longer patch life. In 1995, he convinced the mayor and city commission to purchase a truck-mounted spray injection machine.
Spray injection was almost immediately successful; local residents noticed that the patches were neater and longer-lasting than the previous cold mix patches. The mayor and commission were so pleased that, within two years, the city purchased a second machine.
Potholes are just one of the basic pavement repair problems city and county governments face. Rutting, roughness, pavement breakups and other forms of pavement distress also cause citizen dismay. By determining which repair techniques are best for local problems, officials can maintain city and county streets safely and efficiently.
The Strategic Highway Research Program (SHRP), a federal research initiative, determined in its 1991 study of patching methods that spray injection is the most effective method of repairing potholes because of a longer patch life. Spray injection does not require intensive labor and support equipment such as a cutter, tamper or roller. It also is accomplished via a towed unit or truck-mounted machine.
In Sioux Falls, a truck-mounted spray injection machine was chosen over the less expensive towed unit for two reasons. First, the mounted unit typically requires only one person to perform the patching. Second, it does not require operators to stand on the roadway, thereby lowering the risk of work site accidents.
Repair materials are as important as the equipment chosen for repairs. The best available aggregate and liquid asphalt – particularly the aggregates – will vary across the country. Quarry materials that are 100 percent fractured are superior to natural stone. Rapid cure emulsified liquid asphalt needs to be selected as a binder that is compatible with the stone, according to the Asphalt Institute, Lexington, Ky., and an SHRP study.
The use of high-performance, proprietary mixes is becoming routine in many areas, especially in cold regions of the country, since the mixes tend to remain workable at low temperatures. One drawback: the mixes work well in small potholes with vertical sides that confine the mix, but, in larger holes or utility cuts, they sometimes lack stability and can “shove” under heavy traffic.
Conventional cold-mixed asphalt still is used in many places. However, it must be used quickly after being mixed, and its “shelf life” when stored in a small stockpile is brief, especially in hot weather. Patch life is shortened dramatically when the asphalt mix becomes dry and hard to handle.
The use of a “hot trailer” can improve the performance of cold mix. Ron Olson, street and sanitation superintendent for Mitchell, S.D., has worked with hot trailers, first as a county highway superintendent and now in street maintenance. He recommends that patch crews heat the trailer to warm the mix, making it easier to handle and compact. Vapors from the mix can be used as a guide for mix temperature. Steam rising from the mix is simply vaporized moisture that got into the pile and should be removed. But, if blue smoke begins to rise from the mix, it is being overheated, and the heat must be reduced quickly. (Overheated mix hardens the asphalt, causing the asphalt binder in the patch mix to lose its elasticity. When that happens, the mix will not stay in a pothole.)
Pothole repair procedure
Application of the asphalt makes a difference in the quality of the repair. Olson emphasizes that 50 percent of patch performance is tied to good workmanship. Proper repair procedures include: * cleaning the hole and making sure it is as dry as possible; * adding light tack oil, which usually helps common hot or cold mixes perform better; * placing the mix in compacted lifts if the hole’s depth is more than 3 inches; and * shaping the surface with a lute or special rake for leveling asphalt. (Common garden rakes tend to segregate the stone and fines in the mix.)
Compacting the patch while the mix is warm and fresh will yield good density. A roller is not necessary for the task; if nothing else is available, the wheels of the trailer or truck will do.
Repair crews should not expect the traffic to pack the asphalt. A light passenger vehicle traveling at only 30 miles per hour can exert as little as 5 pounds per square inch (psi) of pressure, while slowly backing a lightly loaded pickup truck over the patch can exert an effective pressure of 20 psi on the asphalt.
After potholes, rutting and roughness are the bane of the street repair worker. Rutting causes problems with surface drainage and accelerates pavement deterioration, while roughness often prompts citizen complaints.
Rutting and roughness can be hard to repair. Since it is critical to determine the cause of the pavement distress, a good analysis by an experienced engineer is often needed. For an aging pavement that has slowly developed rutting over a period of 15 years or more, a simple thin overlay can restore the profile and extend pavement life. In Roberts County, S.D., Roger George, county highway superintendent, selected a 3-mile section of rural highway for thin overlay in 1989. Rutting of approximately 11/2 to 2 inches deep and some roughness had occurred. Constructed in the early 1960s, the road had never been rehabilitated. (It has a low traffic volume but carries heavy agricultural equipment and truck traffic during harvest.)
To repair the road, crews placed a thin overlay with a paver using conventional hot mix – sometimes referred to as a “scratch coat” – to fill ruts and depressions. Less than 700 tons per mile was used. A seal coat was applied later that season, and a full overlay was placed on the section during late summer 1999.
In other regions of the country, micro-surfacing has been used effectively to eliminate rutting of 11/2 inches or less. That process requires close control of aggregate and emulsified asphalt along with other additives. The substance is mixed on the road and placed by the same machine. The equipment is highly specialized, but the method can be effective in high-traffic areas that need to be reopened quickly. The material gains strength and stability quickly due to the rapid curing of emulsified liquid asphalt in the mix.
Milling off the existing asphalt surface to a depth just below the ruts and placing a new asphalt overlay on the entire surface is another alternative for rutting repair. That method often is used on urban sections so that pavement elevation does not get above the curb and gutters.
Premature rutting in pavements is a much more serious problem that is not so easily solved. Inadequate compaction at the time of construction, poor mix design or quality control may cause preliminary rutting, but, whatever the cause, expert advice is needed before proceeding with any type of repair. Simply filling the ruts will usually serve only as a short-term solution.
Major breakup areas
Extensive pavement damage will require far-reaching solutions. Simply removing the loose material and patching major pavement breakups is almost always a bad idea.
While the pavement may occasionally be at fault, poor support from the base and subgrade is more often the problem. The distress that appeared before actual breakup can be a good indicator. If moisture was present on the surface or severe alligator cracking appeared, particularly in the wheel tracks, usually the base and/or subgrade is the problem.
New techniques of soil stabilization and old techniques, such as incorporating lime into the subgrade or base, can help solve base problems. Additionally, installing underdrains in wet soils can be a good pre-repair option.
However, more managers and designers are turning to deep aggregate base repair, including geosynthetic stabilization. The deeper and stronger the aggregate base is, the longer the pavement will last. That option is much easier to justify in regions where aggregates are plentiful.
Toronto, S.D., a town of fewer than 250 people, has had long-term success with deep base repair. The city used the technique to repair one block of failed pavement after paying for contract surface repair three years in a row. In 1997, the town council, searching for a longer-lasting solution, obtained advice from the South Dakota Local Transportation Assistance Program office as well as from two experienced contractors.
A weak base was obviously the problem. Eighteen inches of the street was removed and hauled away. A needle-punched, non-woven geosynthetic or “fabric” was placed on the bottom of the undercut. Then, 1 foot of bank run coarse gravel was placed, topped with 6 inches of drainable, crushed gravel and compacted. The street was given an asphalt surface treatment and has not shown any distress since.
Street repair often is a balancing act, as city officials try to find the most inexpensive and longest-lasting solutions to pavement problems. While it pays to evaluate new products and methods as they become available, buying good materials and handling the job with skill and good workmanship will always be important for success.
Ken Skorseth is field operations manager for the South Dakota Local Transportation Assistance Program in Brookings, S.D.