by Jonathan Thomas
Broken pavement and potholes have plagued area drivers for many years. However, recent pavement overlays have provided smoother travel on some of these state and local roads.
But would earlier preventive sealing of pavement cracks have been a cost-effective method of keeping some roads from becoming so deteriorated?
The causes of potholes and broken pavement are not a mystery. Natural aging and stresses such as heavy traffic cause asphalt pavement to begin to crack after two or three years.
A major cause of further pavement failure is water passing through these early pavement cracks. “Every crack wants to be a pothole when it grows up,” says the Web site of a manufacturer of equipment for sealing pavement cracks to prevent water to getting through a crack.
Other studies, such as one done by the research lab of the U.S. Army Corps of Engineers, agree that open cracks put a pavement at risk. One study from the Corps says, “Cracks over 1/4” wide are open sufficiently to allow water to flow easily by gravity into the pavement surface and then into the base course. Once water enters, it quickly deteriorates the pavement where it causes extensive and expensive damage.”
The report continues, “Once a roadway starts to disintegrate, potholes quickly develop.”
Is sealing of cracks the best surface treatment to preserve pavement, and is it cheaper than a pavement overlay? The short answer is, “Maybe.” A more complete answer is, “It depends on a combination of factors.”
What is the condition of the road base? One factor is how well the road was built and how good the drainage is.
Maine Department of Transportation project manager Denis Lovely in Bangor, and paving manager Brian Luce in Augusta, told Penobscot Bay Press that the state has generally found it to be not cost-effective to seal state-maintained roads that have not yet been built up to current state standards.
Senator Dennis Damon, chairman of the legislature’s transportation committee, said that about 4,200 miles of state-maintained roads (about a third of the total), have not been built to state standards. He said the MDOT had a plan to annually budget $25 million to upgrade another 600 miles yearly of those roads each year until they are all at that level. However, because of declining revenues in the current budget, there is only $8 million available to do $25 million worth of work.
When there are insufficient funds to build deteriorating roads up to standards before the surface begins to break up, the state has found it necessary to apply a pavement overlay to keep these roads usable and safe.
What is the condition of the pavement’s surface layer? Another factor in deciding whether to crack seal pavement is how badly cracked the pavement is at the moment. When there are too many cracks in a pavement it becomes costly and inefficient to seal them individually. A fuller description of the crack sealing process would help explain why this is true.
What is crack sealing? It is a process that uses a specially formulated liquid rubberized asphalt sealant, heated to about 400 degrees Fahrenheit.
MDOT’s Luce said that the state is now recommending a new sealant that has improved adhesive qualities for sticking tightly to the sides of a crack.
An operator, holding a long nozzle attached to an insulated hose, applies the sealant to the top of a crack. Depending on conditions, another person may follow with a long handled tool to force more of the sealant into the crack, and smooth the overband. The still-warm sealant surface will then be given a coating to prevent it from sticking to vehicle tires as traffic on the road is resumed.
Just prior to the sealing process, a worker holding a hot compressed-air lance, blows out the dirt and occasional plant life that may be in the crack. Using propane as the fuel, the lance operates like a blowtorch on a long handle to remove any moisture and preheat the crack walls of the pavement.
Which cracks should be sealed? It is not efficient to use this sealing process on either very fine hairline cracks, or cracks wider than about an inch.
If there are wide cracks, they should be filled separately with an asphalt pavement patching mix and then compacted.
As indicated earlier, when there are many cracks in a small area that resemble an alligator skin or zebra stripes, that pavement is considered too deteriorated to be worth sealing the cracks individually.
The maximum benefit from crack sealing is achieved when crack sealing is done two or three years after a pavement has been laid. At that time in a pavement’s life, natural aging cracks usually running from shoulder to centerline or shoulder to shoulder at intervals of about 10 to 50 feet, depending on the condition of the base layer.
Some cracks parallel to the centerline, called longitudinal cracks, may begin to develop about this time. They result from stress on the pavement, such as traffic loads on a base that is not firm.
When 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 do crack sealing.
Who should do crack sealing? Larger municipalities purchase their own equipment and do the work themselves. Smaller communities are more likely to hire a contractor. Stonington Town Manager Kathleen Billings-Pezaris said that for an airport runway project, a vendor she used made the equipment available as part of the purchase price for the sealant she bought.
The primary piece of equipment is a large sealant melter or “kettle” to melt and heat the sealant, and a pump and insulated hose leading to a nozzle. These are permanently mounted onto a trailer that is towed just ahead of the crewmember applying the sealant.
The other major piece of equipment is the trailer carrying an air compressor and a tank of propane to power the hot-air lance that cleans and pre-heats the cracks. This trailer is towed just ahead of the trailer with the sealant melter.
(Billings-Pezaris said that the Stonington airport had purchased a small melter with donated funds. She said it worked well on smaller jobs, but was not really suitable for extensive town road work.)
Why is crack sealing not more commonly used? Penobscot Bay Press talked to some state and local officials asking why they were not making greater use of the crack seal as a preventive maintenance procedure. Some local road officials contacted cited concern over costs, but did not have specific information.
Billings-Pezaris had the town crew seal cracks in the airport runway last year and was pleased with the results. She said that her crew is checking the pavement conditions, and that she will be including a recommendation for crack sealing in next year’s budget. She said she that her goal is to extend the life of existing pavements where possible because of the high costs of overlay.
Castine Town Manager Dale Abernethy said in that his town has done crack sealing in the past, and more of it is recommended in the Castine’s new roadway master plan.
MDOT’s Lovely and Luce said that they recommend timely crack sealing on state roads that have been built to state standards. However, they acknowledge that it has been difficult to adhere to that policy because of both state budget constraints and also certain federal restrictions.
How does federal policy influence MDOT decisions on crack sealing? Lovely and Luce said that the MDOT gets 80 percent federal matching funds for some of its work. However, crack sealing is considered a maintenance procedure that is not eligible for federal highway funds intended for construction. In order to maximize federal revenue the MDOT has been putting what funds are available into work that is 80 percent reimbursable, instead of doing crack sealing that might otherwise be done. Senator Damon said the state is trying to get this federal regulation changed.
How does one determine whether pavement crack sealing is cost effective? Because of the number of variables and factors involved, a “one size fits all” answer is not possible.
Various state highway departments, sometimes in association with universities, have done many studies. Some have also been some federally funded, such as the one quoted above. Their general conclusion: crack sealing is cost effective when done on the right roads at the right time using a quality sealant that has been properly applied.
In order to give more specific answers that would be useful to local officials and other readers, PBP has prepared a worksheet showing that crack sealing costing $1,000 on a “typical” mile of road would add two years of life, adding about $8,400 of value to a pavement overlay.
A tool for estimating the costs and benefits of crack sealing is available. Because most roads are not “typical” and would differ in one or more respects from the prepared model, PBP is making its calculations available as a downloadable Excel worksheet at penobscotbaypress.com; click on Community News for the link.
A town official (or other interested person) can enter differing specifications such as frequency of cracks or bid prices from vendors and immediately see the change in the cost savings amount.
In the “typical” example given, cracks with an average spacing of 20’ produced the $8,400 savings shown. If the user estimates that there are twice as many cracks, the calculated savings drops slightly below $7,400 per mile while extending the pavement life by the same two years.
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