“A stitch in time saves nine.” Our grandmothers would say this in an era when it was common to mend a garment when it first began to show signs of wear.
But what does this have to do with pavements? Traditional pavement maintenance has been to limited to patching and filling potholes when necessary. When a section pavement became too rutted, rough, and broken, it would be resurfaced. These practices are sometimes called corrective maintenance, and are done in reaction to a specific problem.
This article is about preventive maintenance, about making that “stitch in time.” For pavements, that may mean sealing those early cracks. Doing so will delay having to make those “nine” stitches of corrective maintenance such as patching and filling pot holes. Of course resurfacing will eventually be necessary, because no pavement lasts forever.
These short questions and answers about crack sealing will be followed by more detailed information:
What is crack sealing? Crack sealing is the insertion of a specialized material such as hot liquid rubberized asphalt into a pavement crack to create a flexible seal to prevent water from penetrating the pavement surface.
Why should crack sealing be done? Water passing below a pavement surface can destroy a pavement from beneath, by erosion of the pavement itself, by freeze-thaw actions, and by breaking down the base layers below the pavement.
Which cracks should be sealed? Sealing the shoulder-to-centerline or shoulder-to-shoulder cracks (known as transverse cracks) that normally occur on pavements that are two to three years old provides the greatest return for the cost, by extending the pavement life two years or more.
When and How should cracks be sealed? The spring and fall seasons, when pavements are midway between their maximum thermal expansion and contraction, are the best times to prepare cracks and apply the sealant.
Who should do crack sealing? Larger municipalities can purchase their own equipment, while smaller municipalities are more likely to hire private contractors.
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What is crack sealing?
Nothing is simple. There are different kinds of pavement cracks that are treated in different ways. New pavements usually begin to develop transverse cracks within two or three years. These are caused by thermal stresses and aging of the asphalt. Transverse cracks are described as “working cracks” because they widen and narrow in width as the seasonal temperature changes. This article will focus on the importance of treating transverse cracks soon after they appear.
Other types of cracks include: longitudinal cracks (running lengthwise on the pavement), which are often caused by heavy loads; block cracks, from pavement fatigue from heavy loads or high traffic volume, or failure of road base; reflective cracking, from subsurface imperfections. Alligator cracking is an indication of a “seriously deteriorated road … that cannot be saved by crack sealing.” (Eaton & Ashcraft)
As stated earlier, crack sealing is the insertion of a specialized material into a crack, sometimes with an overband on the top surface. When sealing a ‘working crack,’ one that widens and narrows, it is important to use a sealant that is both flexible and sticky enough to adhere to the sides of a crack to maintain a watertight seal.
On older pavements with wider more random cracks (that are often non-working cracks), a process called crack filling is used. This process requires less crack preparation, and uses cheaper less flexible materials with a shorter useful life.
Why should crack sealing be done?
On roads that do not have a well-drained base course and good sub- grade material, any water coming through breaks in the pavement surface will collect under the pavement. Eaton and Ashcraftwrite that “Hairline cracks, and cracks up to ¼ inch admit water through the pumping action of traffic. Water lying on the roadway is pushed down into the cracks when vehicles pass over the cracks.” When cracks are wider than ¼ inch, water will flow down onto the base course by gravity. Water under the pavement will quickly deteriorate it, causing “extensive and expensive damage.”
Eaton and Ashcraft refer to a 1987 study by researchers with the transportation ministry in Ontario, Canada, which describes this damage in detail. “The combination of water and heavy traffic accelerates the deterioration because the traffic forces more water into the cracks than would occur naturally. The continuous pumping action eats away the underside of the asphalt and support for the upper layer is eliminated.”
The salt and calcium chloride used on winter roads worsen the problem, according to their research. The brine reaching the base course through the cracks “accentuates the freeze-thaw problem by slightly melting the base course. This produces even more thermal movement within the crack, and the structure, weakened by a saturated base, will experience localized failures when subjected to traffic.”
High traffic volumes and heavy loads increase these problems. “Once a roadway starts to disintegrate, potholes quickly develop. During freezing, a frost-susceptible base course can be pushed upward by the expansion of the water in the base. When the base course thaws, it is saturated and has reduced load-carrying capacity. As traffic passes over this saturated material, the pavement repeatedly flexes and cracks and creates a pothole.” (Eaton and Ashcraft)
From the above discussion, it becomes obvious that keeping water from passing through pavement cracks into the base course is worth doing. The question becomes which treatment to use to accomplish this. Options include crack sealing, surface sealing, such as with a slurry or chip seal, or a full overlay. Because of the complex number of variables involved, there is some range in specific cost benefit numbers. But in general crack sealing, when done at the right time, seems to be most cost-effective.
The Eaton and Ashcraft report (published in 1992) contains a summary of cost data from 19 state departments of transportation (in a variety of climate zones) that responded to a nation-wide survey.
The conclusions were:
• “[A] surface seal will cost between 3 and 14 times as much as sealing the cracks.”
• “An overlay will cost 8 to 26 times as much as crack sealing.”
• “Although surface sealing or overlaying improves the road’s appearance, the problems have been solved only temporarily because the cracks will reappear within a 1 to 2-year span.”
• “If the cracks have not been sealed before the surface seal or overlay is applied, the water will enter the new surface and continue through the older surface to the base course.”
“On the other hand, if the cracks are properly sealed, the states indicated [in the survey] an average gain of three years of extra service. One state reported as much as eight years of extra life. By eliminating the entrance of water, one also eliminates the disintegration of the subsurface materials.” The writers say that reflective cracks, which are caused by problems within the base course cannot be eliminated “without addressing the initial cause of the problem. However, they still must be sealed to prevent further damage to the base course.”
In a 1996 article entitled “Crack Sealing and Flexible Pavements: A Life Cycle Cost Analysis”, authors Ponniah and Kennepohl describe a long term study in the cold Ontario climate. They write that “In general, the consequences of not sealing cracks in flexible[asphalt] pavements are increased rehabilitation cost and decreased service life.” The study also concludes that “Routing and sealing cracks can minimize secondary crack growth and increase service life by at least 2 years … [and] is a cost-effective pavement maintenance procedure.”
There may be readers of this article who have seen the new transverse cracks in roads that were resurfaced a few years ago and have dismissed them as inconsequential. Other roads in town may already be in bad shape which must be fixed with the limited available money.
Engineering consultant Ann Johnson, P.E. may have been thinking of these people when she writes: “Although the benefits of crack sealing may not be obvious immediately, they will be evident several years later when a sealed pavement shows fewer signs of deterioration than an unsealed pavement.” Johnson is the author of the “Best Practices Handbook on Asphalt Pavement Maintenance” published by the University of Minnesota and the Minnesota Dept. of Transportation. The next section will present more of her recommendations.
Which cracks should be sealed?
Some readers may now be thinking, “I am willing to consider that crack sealing might be a good thing, but how do I know which roads to do? Do I start with the roads in poor condition or the roads in better condition?”
Johnson devotes the first chapter of the “Best Practices Handbook …” to helping to road professionals make these decisions about doing “the right repair on the right road at the right time.” She says that historically the emphasis has been on building new roads. But because of increasing budget constraints, “the new focus is on maintaining and preserving existing pavement surfaces. This shift has resulted in three types of pavement maintenance operations.”
She describes the three types as:
Preventive Maintenance, which is “performed to improve or extend the functional life of a pavement” using “surface treatments and operations intended to retard progressive failures and reduce the need for routine maintenance and service activities.” (Although Johnson includes seal coats in this category, this article is limited to discussion of crack sealing.)
The second type is Corrective Maintenance, which is “performed after a deficiency occurs in the pavement, such as …moderate to severe rutting, or extensive cracking.” This type of maintenance is considered “reactive” maintenance.
Emergency Maintenance is the third type of maintenance, which would include repair of severe potholes or other work that needed to be done immediately, “until more permanent repairs can be performed.”
The practice of pavement crack sealing, the subject of this article, can be considered either as “preventive” or as “corrective” maintenance, depending on when it is performed. Johnson writes that “the main difference is the condition of the pavement when the treatment is applied,” but that there are “no clear boundaries” between when a treatment is preventive or corrective.
From casual observation, and talks with local officials, it appears that most crack sealing is done as a corrective measure on pavements with numerous cracks. The objective is getting a little longer life out of these aging pavements while putting together enough money to resurface them.
One public works director interviewed for this article who does use crack sealing as a preventive maintenance procedure is Jim Plummer of Freeport. Plummer said that he asks his contractor to seal the minor cracks in recently paved roads as soon as he can, and that it is “without a doubt a benefit.” Plummer said that although he has not performed any actual cost/benefit study, he believes that it has “saved me a lot” and “protected the roads.” Town Manager Dale Olmstead said that Freeport has been doing crack sealing for at least five years.
Naples Town Manager Derik Goodine, formerly in Levant, also uses crack sealing as preventive maintenance. “I like it!” he said with enthusiasm. During his tenure as Levant’s town manager, he crack sealed all his roads, except for those sections with alligator cracking that needed major repair. Once all the Levant roads were crack sealed, Goodine said that he made sure that they stayed sealed.
The sealant itself does age in a few years, and there are losses from winter plowing. Goodine said that his loss from plows was about 5% per year. (10% annual loss is common. See Masson et al). Goodine said that his annual program in Levant was to seal (or re-seal) every crack on the town’s roads. He noted that at first the budget committee was skeptical, perhaps because of prior experiences with earlier, less effective sealants, but later supported the program.
Goodine said that he has initiated a similar crack sealing program in Naples, although there are some sections of road that will need major repair work instead. He said his goal is to be able to extend the life of pavements to about “15 years,” about the time when the asphalt itself begins to break down from a normal aging process.
The “Best Practices Handbook”, referred to above, “focuses on preventive maintenance activities because these are the most cost-effective and offer the best means for prolonging pavement service life.”Johnson writes that “by extending the life of a pavement section until it can be rehabilitated, preventive maintenance allows an agency to even out its maintenance budget from year to year, which can otherwise vary greatly.” She cites the Michigan DOT as an agency that “has used preventive maintenance to balance out its construction program.”
Johnson observes that more preventive work is not being done in the past because “information was lacking about the performance and cost-effectiveness of preventive maintenance practices.” Also, highway agencies are reluctant to fund “treatments on pavement in good condition when there is a large backlog of pavement in poor condition within the system.”
MDOT officials interviewed during the preparation of this article said that they would like to be doing more crack sealing on state roads. However, because of funding constraints in recent years they have had only limited funds for crack sealing. As a result, they have had to be very selective in the roads they could crack seal, and have tried to “get the most bang for the buck” with the funds they had.
It is not just public officials and budget committees that need to be informed about preventive maintenance programs. Johnson writes that the general public also needs to understand the purpose, what is being done and why, so that passing motorists won’t “complain when they see work crews fixing a road that seems to be in fine shape.”
It is important to note that there are times when it is NOT economical to crack seal. A federally-funded research project concluded that “In general, a large number of cracks or severely deteriorated cracks indicate a pavement in an advanced state of decay. Crack sealing or filling in these circumstances is both uneconomical and technically unsound, as it does little to delay the need for more extensive corrective actions.” (Smith and Romine)
When and How should cracks be sealed?
As with many human activities, different people have differing opinions on how something should be done. Pavement crack sealing is no different. One 1999 report said that “crack sealing is still an evolving technique.” (Masson et al) . The recommendations that follow describe commonly accepted practices, but they are not intended to imply that they are the only way to do something.
Sealants. Hot pour rubberized asphalt sealants, applied at about 400 degrees F, are formulated for the climate in which they will be used. When in place, sealants are designed to have a specific flexibility, and to be sticky enough to adhere to the sidewalls of a crack within a certain temperature range. MDOT currently requires that sealants meet the ASTM D-3405 specification.
Sealants materials are brought to the site in blocks or chunks weighing about 20 pounds, and melted in a double-boiler type kettle mounted on a trailer. The melter or kettle has thermostatic controls, continuous mechanical stirrers, and a pump to deliver the melted sealant through an insulated/heated hose to an application wand with a nozzle for injecting the sealant into cracks.
Seasons. Most authorities say that the spring and fall seasons, when pavements are midway between their maximum thermal expansion and contraction, are the best times to seal cracks. Pavement contracts in winter, causing cracks to widen, which puts tension on the sealant. If enough sealant is within the crack, it can stretch and fill the expanding space without pulling away from the side walls of a crack.
In summer, cracks are narrowed because of pavement expansion. If they are filled during those times, there is increased risk of sealant failure because the narrowed cracks may not hold enough sealant to stretch across the crack when it widens in cold temperatures.
If cracks are filled when widened by cold temperatures, there is risk of the sealant being forced out of the cracks when the pavement expands in the summer heat. However, Dana Wardwell, Bangor’s public works director, has not found this to be a significant problem. Wardwell said that on winter days when the temperature is around 30 degrees and crews are less busy, he will assign them to do crack sealing with city-owned equipment. He said that although the failure rate may be a little higher, it is better to do some crack sealing then, because in warmer weather there is often not enough time.
Routing. After a decision has been made to seal cracks, the next decision is whether or not to “rout” the cracks before applying the sealant. Eaton and Ashcraft define routing as “mechanically cutting the pavement along the crack lines.” The purposes of routing are to widen the top of narrow cracks to create a “reservoir” to hold more sealant, and “to provide an intact side wall for sealant adhesion in badly deteriorated cracks.” A typical rout is ¾ inch wide and ¾ inch deep, but specifications vary. Routing is usually done with one of two types of machines. A spindle router has cutters mounted on a vertical shaft. A star wheel router has cutters arranged on a horizontal shaft.
Eaton and Ashcraft write that “There is considerable debate over whether the benefits of routing outweigh the disadvantages.” Routing before sealing cracks adds to the cost, but extends the life of a seal. Smith and Romine write that “Rubberized asphalt sealants placed flush or overbanded in routed working cracks can provide 5-9 years of service, whereas placed overbanded in non-routed working cracks, they can provide 2.5 to 5 years of service.” Because of the variability in both road conditions and relative costs, this article suggests that a decision on whether or not to rout before sealing be made on site in consultation with the contractor.
The MDOT put out contracts for crack sealing in its five regions in 2006. Only one of those regional contracts requires routing. None of the several municipalities contacted for this article reported that they do routing, but there may be some municipalities in Maine which do.
Finishing. Sealant may be applied flush to the pavement surface, but it is more commonly left with an “overband” on the pavement surface. Overbands that are too wide or thick are more likely to catch on a plow, cause traction problems for motorcycles, and waste material. The MDOT’s Guy Berthiaume recommends keeping overbands small to minimize bumps on higher speed roads. Eaton and Ashcraft write that “a thin 1- to 1½-in. overband on each side of the crack is plenty.”
During the application process, some crews will have someone follow the applicator wand with a ‘U’ or ‘V’ shaped squeegee on a stick that will confine the width of an overband. On roads that have a complex crack pattern, this is less practical, because the application process moves quickly.
The final step in the sealing process is spraying the fresh sealant with a special emulsion that prevents the material from tracking or being picked up by tires driving over it before it is completely cooled and hardened.
Preparation. Crack preparation is critical to the success of any crack sealing, and is being discussed last for emphasis. It is more important than the seasonal temperatures, whether or not there is routing, or the shape of the banding. Before sealing, the crack surfaces must be made clean, dust free, and dry.
A major cause of sealant failure is moisture left in an open crack, which prevents a tight bond of the sealant to the asphalt. The preferred way to remove this moisture is with the use of a hot compressed air lance, which applies high pressure compressed air, heated with a propane flame. The temperature will be at least 2000 degrees F. When properly used, a hot air lance will blow out embedded sand and dirt, burn away any weeds or grass growing in a crack, and evaporate moisture. Finally, a hot air lance will pre-heat the asphalt on both sides of the crack to provide better adhesion when the hot sealant is applied. To take advantage of this pre-heating of the pavement, the state bid specifications require that the lance be not more than 50 feet ahead of the sealing unit, as they progress down the pavement, so that the crack area does not have time to cool or take up new moisture from below.
Who should do crack sealing?
The City of Bangor, as previously mentioned, has its own sealing equipment. A few other larger Maine municipalities may also own their own equipment, but most smaller communities are likely to hire private contractors. There are both in-state and out of state contractors interested in work in Maine communities. The MDOT puts much, if not all, of its crack sealing out to bid.
The Greater Portland Council of Governments’ Carol Mackenzie, when soliciting bids for paving in its joint purchasing agreement process, also includes crack sealing. Other councils of governments or regional planning commissions may do the same.
Charges for crack sealing work are usually based on the volume (gallons) or weight (pounds) of the sealant applied. Of course neither the writer of this article, nor the Maine Municipal Association can recommend private vendors. As with professional or contracted services in general, checking of references is recommended.
Conclusion
• A large portion of the cost of road resurfacing is the cost of the asphalt, with smaller portions being the cost of the heavy equipment required, and the labor.
• A large portion of the cost of crack sealing is labor costs, followed by lesser equipment costs, and finally the asphalt based sealant itself.
• Labor costs are relatively stable, at least in comparison with the rapidly increasing cost of crude oil, the source of asphalt.
• Tighter municipal budgets will continue to require maximizing the life of existing pavements.
Given this framework, crack sealing will become over time an even more cost-effective preventive maintenance procedure than it is today.