Cost Effective Pothole Repairs

By Stefanie R. Fishman, Project Assistant

    Potholes form when water becomes trapped beneath the pavement surface. Water can enter the road base through surface cracks or from road sides. The water freezes, often causing frost heaves. The ice melts from the top down, leaving a trapped pool of water. As vehicles run over it, the unsupported surface layer collapses. The pothole expands as traffic hits the hole.
    In the summer, highway departments can take preventive measures such as sealing cracks and improving drainage. In the winter and spring the only alternative is pothole patching. To ensure a longer-lasting pothole patch, crews must apply the right combination of materials and procedures.

Materials

    Although hot-mix asphalt patches last longer than cold-mix it, crews must keep the material hot during the patching operation therefore cold mix is usually used. Cold mix is less expensive, easier to use, and can be stockpiled.
    Three types of cold mix can be used: local cold mix, agency-specified cold mixes, and proprietary cold mixes. Local cold mix is a blend of local aggregates and liquid asphalts, either cutbacks or emulsions. When highway departments specify cold mixes, asphalt plants will produce the material according to these specifications using high-quality aggregates and modified liquid asphalts. Proprietary cold mixes are commercially produced blends of carefully matched aggregates, liquid asphalts, and additives. By applying cold-mix in the following techniques, highway departments can make repairs that will last for many months.

Repair Techniques

    Repairs can be performed during weather conditions, ranging from clear spring days to harsh winter storms, with temperatures from 0° to 100F. Repairs are generally performed as an emergency repair under harsh conditions or as a routine maintenance, scheduled for warmer and drier periods. Highway departments have successfully used three methods for pothole patching: throw-and-roll, semi-permanent, and spray injection. The method of choice depends on how much an agency can spend, the equipment available, and productivity needed.

The throw-and-roll method consists of the following steps:

• Place mixture into the pothole which may or may not be filled with water and debris. Use any type of hand tool such as a shovel or pitchfork to fill the hole. Fill the hole so that there is a crown in the center.
• Compact the material by rolling over it 6 to 8 times with truck tires. Some crews have found it useful to cover the patch with sand before rolling a truck over the patch to prevent material from sticking to tires.
• Check the level of the patch to make sure the center of the patch is ¼" to ½" above the pavement surface.
• If the patch is low add more cold mix and repeat the patching steps again.

    This method is similar to the standard "throw-and-go", "dump-and-run" or the "pitch-and-pat" methods except truck tires compact the patches. Compaction provides a tighter patch for traffic to drive over it without creating depressions and it provides better water runoff. The extra 1 to 2 minutes to compact the patches will produce a significantly better patch.

    Semi-permanent patching is the most widely recommended method of repair. It includes the following procedures:    

• Remove water and debris from the pothole, using a broom, shovel, compressed air or whatever is available.
• Straighten pothole edges making sides as vertical as possible. This can be done using a jackhammer, pavement saw, or milling machine, etc.
• Place the mix by hand using a shovel and rake. Placement should be made in no more than 3" lifts.
• Compact patch from the center towards the edges to provide better compaction at the edges and corners. Hand devices such as a vibratory plate compactor or single-drum vibratory rollers are recommended for this task.

    This repair requires more equipment and workers than the throw-and-roll or spray injection methods, but results in a very tightly compacted patch.

Spray Injection

    This method is quick, provides a long-lasting patch, and uses low cost materials. However, it requires a skilled operator to obtain a good patch and the equipment costs is higher than the other procedures. The spray-injection procedure consist of the following steps:

• Blow the hole clean and dry of water and debris.
• Spray a tack coat of binder on the sides and bottom of the pothole.
• Blow asphalt and aggregate into pothole. The compaction is provided by the velocity or the aggregate sprayed into the hole.
• Cover the patch with a layer of aggregate.

    The best results are obtained by scheduling repair work during dry, warm weather. However, potholes usually form of wet and cold weather. In such cases, careful selection of materials and procedures is important to obtain a long-lasting patch.
    Aggregates for winter patching should be high quality, crushed aggregate with few fines. The binder should be emulsified asphalts with some anti-strip additive to prevent stripping of the asphalt. The mixture should be workable at low temperatures to allow both easier handling and compaction. The most important aspect is that the binder-aggregate-additive mixture be compatible. Since winter patching seldom allows the time to use the semi-permanent procedure, use the throw-and-roll method with a high quality or highway department specified mix to provide a longer-lasting patch.

    Patches placed in the spring have a longer life than those in the winter because of the more favorable weather and the end of the freeze-thaw cycle. Spring patching can be done by any of the procedures discussed above: the throw-and-roll, semi-permanent, or spray injection procedures. Cost and the availability of equipment and workers should be the most important criteria.
    Managers should make sure that material stockpiled over the winter is workable in a range of temperatures. Materials workable at very low temperatures tend to be very sticky and hard to use at higher temperatures. High-quality crushed aggregate with few fines, and emulsified asphalt, should be used for spring patching. Antistripping additives are recommended to keep asphalt from stripping away from aggregates.

The SHRP Asphalt Pavement Repair Manual of Practice provides illustrations of the methods discussed above. Readers can obtain the book from the UNH T² Center.

"Fix It and Forget It: A Training Course in Pothole Patching" (L036) which is available from the Local Technical Assistance Program (American Public Works Association, 1301 Pennsylvania Avenue NW, Suite 501, Washington DC 20004; phone (202) 347-7267, fax (202) 737-9153; Email ltap@patriot.net)

"Materials and Procedures from the Repair of Potholes in Asphalt-Surfaced Pavement." Asphalt Pavement Repair Manual of Practice. Washington DC: Strategic Highway Research Program.

Return to UNH T2 Center, 33 College Road, Durham NH 03824 603-862-2826 Fax 603-862-2364