By John Davis
Pavement distresses accumulate as asphalt pavements age and traffic pounds them. If timely maintenance isn’t performed, distresses are compounded. Cracks become potholes and potholes become craters.
This article uses information from “MS-16 Asphalt in Pavement Preservation and Maintenance” to provide practical information about methods, procedures and terminology for properly sealing cracks and patching potholes. According to Larry Galehouse, director of the National Center for Pavement Preservation (NCPP), more and more private companies and local road agencies are conducting training sessions about methods and procedures to maintain and preserve asphalt pavements.
“The cost of addressing minor deficiencies is much less than addressing major deficiencies,” says Galehouse. “We have to do road maintenance when the roads are in fair-to-good condition, rather than waiting until they are in poor condition.”
Galehouse says it takes far fewer dollars to fix a good road in need of some maintenance rather than rehabilitating a bad road in need of a lot of maintenance. “Road agencies just don’t have the money to reconstruct bad roads anymore,” he adds.
A good time to do crack sealing is when an asphalt road or street is in fair to good condition. Along with proper drainage, crack sealing is probably the single most important maintenance activity. Most pavement distresses can be related to the intrusion of water into the pavement structure. If water is kept out of the pavement, the majority of distresses can be stopped or delayed.
Crack filling is done with liquid asphalt, cutbacks and asphalt emulsions and is considered temporary work. In this article, we will focus on crack sealing.
Crack sealing has two primary purposes:
- To prevent the intrusion of water through the crack into the underlying pavement structure.
- To prevent extraneous materials from entering the crack and causing further deterioration as the pavement expands and contracts with temperature changes.
Crack sealing where cracks are subject to expansion and contraction is done using a specially prepared hot-poured sealant. Depending on the climate, the materials used, the pavement conditions and the technique used, crack sealing will last three to eight years.
Cracks that are 1/8 inches (3 millimeters) or less in width are too small to seal effectively. If there are numerous hairline cracks over a large area, then a surface seal such as fog seal, chip seal, slurry seal or sand seal should be used. The particular surface seal must be fluid enough to flow into the all the hairline cracks.
Small, medium and large cracks
Cracks that are 1/8 inch or slightly larger are usually routed to a width of ¿ inch or greater to provide a reservoir for the sealant. The crack is then cleaned and sealed. If the cracks are more than 2 inches deep, a backer rod should be installed to conserve sealant.
Cracks that are ½ inch to ¾ inch wide usually need only cleaning and sealing. Install a backer rod if cracks are more than 2 inches deep. Cracks that are larger than 3/4-inches wide should be filled with an asphalt emulsion slurry seal, a hot mix asphalt sand mix, or a hot-poured sealant.
The time of year when the crack filling is done will affect the performance of the sealant. Most cracks will open and close, depending on the season of the year. Crack sealing should be carried out when the cracks are in the middle of their opening range, which usually equates to spring or fall. Cracks filled in summer, when they are at minimum width, will be under-filled in the winter. Cracks filled in the winter, when they are at maximum width, will be over-filled in the summer and traffic may pull the crack filling material out of the crack.
Crack sealing materials
Asphalt crack sealing materials must have good adhesion or bonding. They must be elastic yet resist softening. They must be easy to apply yet resist cracking, aging and weathering. Also, they must be compatible with asphalt pavement.
Asphalt emulsions, asphalt cements and fiberized asphalt are used for crack filling. Asphalt rubber, rubberized asphalt, low-modulus rubberized asphalt and self-leveling silicone are used for crack sealing.
Crack sealing procedures
For crack sealing, the most important aspect of the procedure is the preparation of the crack for treatment. Also, the season when the crack sealing is done will affect its performance.
If the cracks need to be routed or sawed to remove extraneous material, it should be done before cleaning the cracks. The routing or sawing is best accomplished using a vertical-spindle router, rotary-impact router, or a random-crack saw. After doing the routing or sawing, clean the cracks using high-pressure air, sandblasting, wire brushing, hot air blasting or high-pressure water.
Cleaning the cracks is an essential step to ensure that the sealant will adhere to the sides of the crack. After cleaning, check the cracks for depth. A backer rod should be placed in large deep cracks to conserve sealant. The backer rod should be a compressible, non-shrinking, non-absorbent material with a melting point higher than the temperature of the sealant. The backer rod should be about 25 percent wider than the crack, to prevent slipping or floating out after placing the sealant.
After the cracks are prepared, they are sealed with liquid asphalt. Equipment used for crack sealing or filling varies from truck-mounted pressure applicators with hand wands to pour pots. Each type of equipment can heat and maintain the temperature of the sealant in the 450¿F range.
No matter what type of equipment is used, the crack should be filled with sealant material from the bottom to the top of the crack to prevent air bubbles from forming. The air bubbles create weak spots in the sealant. Pour only the amount of material that will fill the crack. Don’t try to completely fill the crack because it is a waste of filler. Coat the vertical surfaces of the crack with a small excess of filler deposited in the bottom of the crack. To prevent tracking, the filler should be 1⁄8 to 1⁄4 inch below the top of the crack. If necessary, use a squeegee to remove excess sealant on the pavement surface, and then blot with sand or limestone dust.
Patching is the process of filling potholes or excavated areas in the asphalt pavement. Quick repair of potholes or other pavement disintegration helps control further deterioration and expensive repair of the pavement. Without timely patching, water can enter the subgrade and cause larger and more serious pavement failures.
A full-depth or deep patch is considered a permanent repair, while a thin surface patch or a “throw and go” pothole repair is usually temporary. Materials for patching include hot mix asphalt, asphalt emulsion mixes, stockpile patching mixes, and proprietary patching mixes with special blends of aggregate and modified binders.
Full-depth patching is the removal of the entire pavement surface layer, regardless of its thickness, over the patching area. Deep patching is the removal of four inches or more of the pavement surface course. Full-depth patching applies to either asphalt or concrete pavements, but deep patching applies only to asphalt pavements.
Provide firm support
In full-depth patching, the material in the repair area is removed to the depth necessary for reaching firm support. This means oftentimes removing some of the sub-grade. A full-depth patch may even require some additional drainage.
The excavation should extend at least one foot into the good pavement surrounding the patching area. Patches should be square-edged and the cuts rectangular in shape without having varying lengths or widths within the patch area. If the width of the patch is near the width of the lane, a full lane patch may be best because the contractor can use standard paving equipment rather than handwork and eliminate extraneous longitudinal joints. A pavement saw makes a fast and clean cut. When large and numerous patches are necessary, a medium-sized milling machine works well. When small and numerous patches are needed, use a small milling machine. After the material is removed and the patch area cleaned, apply an asphalt tack coat to the vertical faces of the patch.
A full-depth patch should be backfilled with a dense-graded hot mix asphalt. If hot mix asphalt is not available, an appropriate cold mix, specialty mix or proprietary mix can be used. If the patch is more than six inches deep, place the patching material in 4-inch layers, and compact each layer as it is placed.
Proper compaction is a critical factor in producing a permanent patch. A vibratory-plate compactor is excellent for small patches and mandatory for compacting corners. A medium-sized roller may be more practical for large patch areas. A properly compacted patch should be overfilled in anticipation of traffic compaction. A straightedge or string-line should be used to check the evenness of the surface. A patcher truck is useful if numerous patches are involved. The truck can contain a bin for hot mix asphalt or store liquid asphalt and aggregate to mix and dispense into the patch. Vibrating compactors can be part of or attached to the patching truck.
Surface patches are usually temporary patches. They are constructed by milling a portion of the pavement to a depth that removes all deteriorated material. The patch area should be milled to a minimum depth of at least three times the nominal maximum size of the aggregate used in the patch. Using a 3/8 inch size aggregate or ¼ inch size aggregate will minimize the necessary milling depth, help tie the patch to the existing pavement, and provide adequate hot mix thickness to reduce the possibility of raveling.
Spray-injection patching is a method of repairing small pavement defects with semi-permanent repairs, particularly during wet or cold weather. This method requires a truck or trailer-mounted unit that contains an emulsion tank, aggregate tank, heating components, high-volume blower, telescoping boom with injection head and the necessary controls. The operation consists of cleaning the patch area with compressed air to remove loose material and debris, applying a tack coat of hot asphalt emulsion, blowing the combined aggregate and hot emulsion into the patch with forced air, and then placing a dry coat of aggregate on top of the patch to prevent tracking.
The aggregate used in this method is usually a one-size stone similar to a chip-seal aggregate. Compaction is accomplished by the force of the air as the mix is sprayed into the patch in layers. The method is especially effective for pothole patching.
Infrared heater patching
Infrared heater patching requires fewer workers and is often faster and cheaper than full-depth patching. Infrared heaters are truck-mounted and heat the asphalt to a depth of 2 to 3 inches, which is comparable to a thin surface patch. The patch area is heated by the infrared heater and scarified. Rejuvenators can then be worked into the in-place asphalt or new asphalt mix can be worked into the existing material. After reworking the existing asphalt, it is compacted.
Cold weather emergency patching
Sometimes pothole repairs in an emergency situation or during cold or inclement weather are necessary. They are temporary in nature and are done quickly for the safety of motorists. There are four methods for this type of repair: throw-and-roll, throw-and-go, semi-permanent and spray injection.
The throw-and-roll method cleans the debris and water from the pothole with a stiff broom, fills the pothole with asphalt material and compacts it, leaving a 1/8 or ¼ inch crown. The material is compacted with a hand tamper or the truck tires.
Throw-and-go differs from throw-and-roll in that there is no compaction. The filled pothole is compacted by normal traffic.
The semi-permanent method requires that water and debris be removed from the pothole. The contractor must square up the sides of the patch and make sure the edge is cut back into good pavement. The asphalt mix is placed in the patch and compacted to produce a flush or nearly flush patch. More patch time is required but the patch will normally last longer. The spray injection method may also be used for emergency patching.
In both crack sealing and pothole patching, timing is important. Don’t wait until the road is in poor condition to schedule the work. Pavement distresses multiply if timely maintenance isn’t performed.