Asphalt Maintenance Best Practices
By Mark Weber
Crack sealing serves several purposes. Its primary purpose is to keep moisture from entering the pavement structure. This helps to maintain the integrity of the pavement and can prevent base failures, pothole development, crack deterioration, and increased roughness. When performed in a timely and effective manner, crack sealing is expected to extend the life of a hot mix asphalt (HMA) pavement. In an established maintenance program, crack sealing will be performed on a regular and periodic basis over the life of the HMA pavement.
Several factors can cause cracks in asphalt and diagnosing your pavement will allow a paving contractor to properly construct a paving solution. Paving contractors use the terms below to identify types of cracking and suggest appropriate asphalt repairs:
Fatigue cracking is sometimes called alligator cracking due to the interconnected cracks which resemble an alligator skin. Fatigue cracking is caused by load-related deterioration resulting from a weakened base course or subgrade, too little pavement thickness, overloading, or a combination of these factors.
Block cracking is a series of large (typically one foot or more), rectangular cracks on an asphalt pavement surface. This type of cracking typically covers large areas and may occur in areas where there is no traffic. Block cracking is typically caused by shrinkage of the asphalt pavement due to temperature cycles.
Edge cracks are longitudinal cracks which develop within one or two feet of the outer edge of a pavement. These cracks form because of a lack of support at the pavement edge.
Longitudinal cracks occur parallel to the centerline of the pavement. They can be caused by: a poorly constructed joint. shrinkage of the asphalt layer, cracks reflecting up from an underlying layer, and longitudinal segregation due to improper paver operation. These cracks are not load related.
Transverse cracks occur roughly perpendicular to the centerline of the pavement. They can be caused by shrinkage of the asphalt layer or reflection from an existing crack. They are not load related.
Reflection cracks are cracks that form in asphalt over joints or cracks in a concrete pavement below or in an asphalt overlay of a deteriorated asphalt pavement. The cracks form because of movement of the old underlying pavement.
Paving contractors often use the terms “crack sealing” and “crack filling” interchangeably, however there are some unique differences. Crack filling is defined as the placement of sealing materials into non-working cracks (cracks that undergo <1/8 inch of annual horizontal movement) to reduce the infiltration of water into the pavement structure. Crack filling also reinforces the adjacent pavement and helps to keep debris out of the pavement structure. Crack sealing is the placement of higher quality materials into working cracks to reduce the infiltration of water into the pavement structure and reinforce the adjacent pavement.
Crack sealing is different from crack filling in that the sealant material used must adhere to the crack walls while the crack is opening and closing. Crack sealing often includes more extensive crack preparation methods than crack filling and using higher quality sealant materials. While both crack sealing and crack filling are essential in asphalt maintenance this article will focus on crack sealing predominantly. Understanding the type of crack, size of crack, and geographical location will all factor together when discussing best installation practices.
Asphalt paving companies across the globe agree that before material installation begins, a preparation process must occur which will result in the area being clean and dry. This provides the greatest probability that a strong, permanent bond will be formed between the pavement and the sealant material. The presence of dirt, debris, and moisture will not allow sealing material to create a strong bond. There are several methods used to prepare a crack for material installation. All vegetation that may exist must be removed from cracks as this will prevent sealing material from bonding to the walls of the cracks the use of herbicide two weeks prior to a project start date can prevent issues with weeds down the road. Although not used by Colorado Department of Transportation (CDOT), many highway agencies use a router on cracks less than 3/4 inch in width creating a reservoir for the sealant material. When used, rout dimensions are commonly 3/4 inch wide by 3/4 inch deep, and the routing equipment must be designed so that it can easily follow cracks without excessively damaging the surrounding asphalt. Routing cracks brings much debate among paving companies as many feel the technique is a waste of labor with little benefit to the life of the seal.
Compressed air can prepare cracks and eliminate the adverse impacts of routing, and is more commonly used. Crack cleaning with compressed air is used to remove as much loose material, dust, dirt, and other debris as possible from the crack. The airflow should have a minimum pressure of 100 psi and a maximum of 150 psi. The compressor should use a filter so that oil is not sprayed into the crack. A cloth can be held over the compressed air flow to check the cleanliness of the discharge. To verify that the crack is clean (if it is wide enough), rub a finger or clean black cloth along the crack wall; if it does not collect dust it is ready for sealing. A hot-air lance is another tool used to both clean out and dry cracks. This tool is especially useful where there is some moisture in the crack, but the pavement is not saturated. A hot-air lance warms and dries the surface of the pavement, which helps contribute to a stronger bond of the material. This technique is seen most often in wet climates or when there are high levels of humidity or rain fall. Once a crack is clean, paving contractors will decide on material placement configuration. Flush fill, over band, and recessed are the three types of placement configurations-the way the sealant will be positioned in the crack. Configurations may change from crack to crack or job to job.
In the flush fill approach, the sealant is placed in the crack such that it level with the pavement surface. The flush fill can be used in conjunction with routing to produce a reservoir. It is not recommended that the flush fill configuration be used for pavements that will be overlaid.
In the overband configuration, the sealant material is placed into and over an un-routed crack. The material over the crack can be left unshaped, or may be squeegeed to produce a thin band of material that extends several inches on either side of the crack. The overband configuration is not recommended for pavements that will be overlaid or where carbide-tipped plow blades are used for snow removal. In addition, pavements with an excessively high overband or with a high number of over-banded cracks may contribute to increased ride roughness levels.
The recessed configuration leaves the top of the sealant beneath the pavement surface. This configuration should be used if the pavement is being overlaid shortly after sealant placement or if cracks are sealed when ambient temperatures are very low. The recessed configuration should be considered if excess sealant on the pavement surface is contributing to a rough ride.
Crack sealant should only be applied once the material has reached the recommended application temperature and several initial cracks have been cleaned and prepared. During the application process, it’s important to maintain your material at the temperature recommended by the manufacturer- usually between 380- and 410-degrees Fahrenheit. Under-heated sealant will affect adhesion, and over-heated sealant will degrade the properties of the sealant. If sealant is under-heated, sealing operations should stop until the sealant is able to reach proper application temperature as recommended by the manufacturer. Some over-heated material will be thick and stringy, or thin, depending on the sealant formula. Over-heated sealant must be disposed of. You should check the temperature periodically throughout the job to ensure proper temperatures and keep the melter partially full. Keeping the material melter ¾ full will help maintain temperature and aid in the effort to achieve a consistent crack seal material. Adding sealant blocks one at a time, rather than all at once will not only assist in temperature regulation but also increase productivity of project. Crack sealing operations will not need to stop and wait for proper temperature as you add blocks, allowing crews to continue working on non-sealed cracks. This technique is often overlooked as projects progress. You should keep traffic off the crack sealed surface until the sealant has cooled to avoid tracking. If you need to open the pavement to traffic quickly, blot the crack using either a detaching agent or black sand. Once blotted, the area can be opened to traffic immediately.
The best season for crack sealing is late summer to middle autumn. Application of the sealant material shall take place only when the pavement temperature is 45 degrees F and rising and the pavement is free of moisture. If sealing is scheduled during or near the end of winter months, paving contractors should clean all cracks of magnesium chloride or other related ice melting chemicals
When an asphalt pavement is exposed to sun, wind, and water, the asphalt hardens, or oxidizes. This causes the pavement to become brittle, cracking the pavement. A seal coat provides a waterproof membrane that not only slows down the oxidation process but also helps the pavement shed water, preventing it from entering the base material. Seal coating is a cost-effective asphalt maintenance service which prolongs the life of your pavement.
For proper bonding of the sealcoating, it is essential that the surface be cleaned as thoroughly as possible to remove dirt and debris. If the surface is not clean, the coating will not bond, and with time and usage it will flake or peel off. The extent of cleaning of the pavement will depend upon its condition. Simple air blowing, more involved scraping or wire brushing, and pressure washing are all possibilities. Seal coatings are not repair products; they are coatings to protect and preserve the surface. Oil spots in pavement will cause bonding issues for the seal coat material. Seal coating material will not bond to oil spots if they have not been properly cleaned and primed. Oil spot primer can be applied using a brush or a garden type sprayer, and once primer is dry, seal coat can be applied. Primer provides barrier so oil spots will not bleed up through freshly applied seal coat.
Once a surface is cleaned and appropriate areas are primed, material is ready to be applied. Ambient and surface temperatures should be at least 50 degrees F and rising. An application rate of 40-50 square feet per gallon is recommended. The exact coverage may vary depending on the application method and the roughness of the surface. The first coat will dry to the touch in a few hours. The second coat may be applied when the first coat is dry to the touch. The final coat will cure within 24 hours under ideal conditions (70 F and 50% humidity). Silica sand is often added to the seal coat process to improve skid resistance and durability. Five pounds of clean dry silica sand with each gallon of material is a standard ratio for application. Sand is not always used and is determined by paving crew members. New asphalt surfaces or patches need to cure 60 days before sealcoating. If the sealed lot is to be striped, a latex traffic paint is recommended as oil-based paints will bleed and discolor.
Asphalt maintenance contractors have several application methods to offer their customers. A spray system, ride-on equipment with squeegee and/or spray application option, or a squeegee/broom to apply material by hand are all possibilities. Paving contractors will agree both squeegee and spray methods have their own set of advantages. The pressure from the squeegee application method allows the sealant to fill any cracks creating a high-quality bond with the pavement system. The spray method allows crew members to control how much material is being used. Paving contractors have found using both methods together may achieve the best results. Applying the squeegee method for the first coat can create a proper bond to the asphalt but can leave behind pin holes and other imperfections. Spraying the second coat can mask these imperfections and allow the surface to have a clean appearance by eliminating squeegee marks and blotches.
Potholes are generally caused by moisture, freeze-thaw action, traffic, poor underlying support, or some combination of these factors. Pothole repair is necessary in those situations where potholes compromise safety and pavement rideability. When repairing potholes paving companies can use a few different methods. Depending upon project budget, project length, and condition of pavement system, contractors will pick one or more of the methods below. Many paving contractors use the potholing method for repairing potholes.
Although not considered the best way to patch potholes, it is the most commonly used method because of its high rate of production. Consisting of the following steps, this method can be quick and efficient for failing pavement systems or projects with limited budgets.
Clean the hole with air and tac the vertical edges.
Place asphalt into the pothole
Compact the patch using truck tires or vibrating plate.
Verify that the compacted patch has some crown between 3 and 6 mm.
Move on to the next pothole.
To achieve a longer lasting fix, the use of a pavement saw, along with full depth removal is required. This procedure includes the following steps.
Full Depth Repair
1. Remove water and debris from the pothole.
2.Saw cut around the patch area and square-up the sides so that vertical sides exist in reasonably sound pavement, typically 1’from the pothole. Remove debris. Apply tac material to vertical sides.
3. Place HMA in hole.
4. Compact with a device smaller than the patch area. (Single-drum vibratory rollers and vibratory plate compactors work best.)
After the cut is made and the defective pavement and/or base material is removed, level and compact the base material. Apply a tack coat of asphalt to the vertical sides of the hole to assure a good bond and seal between old and new pavements. Making sure dust and debris have been removed from vertical edge is imperative. The tac on these vertical edges is vital to the quality of the overall patch. A 10 ton or larger steel-wheeled roller is valuable to an asphalt patching operation. A lighter vibratory roller is a poor substitute, as this can waste valuable time and the asphalt will cool too rapidly. On larger paving projects, one roller may be used to do the breakdown rolling or initial compaction, with another used for the finish rolling. Compaction is among the most important phases of the operation. All asphalt patches must be compacted to achieve optimal pavement strength and consolidation of material. A properly compacted asphalt patch will be tough and dense and will stand up to the wear of traffic and weather much better than if compaction is neglected. Compaction should begin as soon as the paving material is laid.
Technological sides of crack sealing in asphalt pavements – Article
Smith, K. L. and A. R. Romine. 1999. LTPP Pavement Maintenance Materials: SHRP Crack Treatment Experiment. FHWA-RD-99-143. Federal Highway Administration, Washington, DC.