By Kevin Carlson
The road network is one of the most valuable assets in the United States. It is vital to the movement of people and goods to support the economy.
According to the Federal Highway Administration (FHWA) statistics from 2011, there are approximately 4 million miles of roads in the United States. Asphalt accounts for approximately 93 percent of the paved road miles.
According to “An Economic Analysis of Transportation Infrastructure Investment” prepared by the National Economic Council and the President’s Council of Economic Advisers in July 2014, funding is not keeping pace with demands or the needs of our growing economy, for today or for future generations. The report also states that over the past 20 years, total federal, state and local investment in transportation has fallen as a share of GDP. Agencies are being faced with the challenge of trying to maintain a road network with less funding in many cases.
A concept that can help the road owners get the most for their money is pavement preservation. The premise of pavement preservation is to apply less costly maintenance treatments earlier in the pavement life to delay or postpone more expensive maintenance or reconstruction in the future.
A pavement preservation strategy can maintain the pavement condition with minimal cost preservation treatments versus doing nothing until the road needs major repairs or reconstruction. The FHWA estimates that every $1 spent on pavement preservation can delay or even eliminate the need to spend $6 – $10 on rehabilitation or reconstruction in the future. Fortunately for the road owners, asphalt provides a number of preservation treatments that can be applied quickly and at relatively low costs. The speed of construction and quick return to traffic make many of these asphalt pavement preservation treatments desirable.
Keys for a successful pavement preservation program
According to FP2, the success of a pavement preservation program is the application of the right treatment, to the right pavement at the right time to save or delay future expenditures. According to James Moulthrop, Executive Director of FP2, the goal of a pavement preservation program is to keep good roads good.
According to James Moulthrop, the key to setting up a successful pavement preservation program is you need to know the condition of your system. In particular, you need to know about the pavement structures you are dealing with, the age of the pavements, the condition the pavements are in, and what distresses you have so you can make good decisions on which treatments to use. A pavement management system is a good tool that can be used to store this information and compare the condition of your pavements.
A pavement management system can also be useful to implement a pavement preservation program. Pavement deterioration curves can be used to predict pavement conditions in the future and indicate when you may want to apply a preservation treatment. Trigger values can be set in the pavement management system based on age, pavement condition, type of distresses present along with amount and severity and traffic volume. This will aid you in scheduling your pavement preservation treatments as well as helping forecast the budget needed to apply those treatments.
Asphalt pavement preservation options
There are many options available for preservation treatments of your asphalt pavements. Most are easy to construct with minimal disruption to traffic. Here is a list of some of the most common asphalt pavement preservation treatments.
Crack sealing – Typically a polymer-modified asphalt material is applied to cracks on the pavement surface to keep water from entering the pavement structure and keeping debris out of the cracks. This is best done on pavements in good to average condition. Pavements in below average condition can be crack sealed but the effectiveness may be diminished if the pavement structure isn’t stable.
Fog seals – A light spray application of asphalt emulsion to protect the pavement surface from water infiltration as well as delaying wear and weathering of the pavement surface. Fog seals work on pavements that are in good condition with no structural problems. Fog seals are also becoming very popular to apply on a new chip seal to aid with chip retention and extend the life of the chip seal.
Sand seals – A spray application of emulsified asphalt that is covered with sand or a fine aggregate. In addition to protecting the pavement surface from water intrusion, it can also improve the friction resistance of the road. Sand seals work on pavements in good condition with no structural problems.
Chip seals – A spray application of emulsified asphalt or cutback asphalt that is covered with aggregate and rolled to seat the aggregate. A chip seal seals the surface from water intrusion and improves the friction resistance, as well as sealing small cracks and correcting minor surface distresses such as raveling. Chip seals work best on pavements in good structural condition with no more than minor surface distresses.
Scrub seals – A spray application of emulsified asphalt that is followed up closely by a series of brooms that help force the emulsion into cracks and other surface distresses. This is followed by an application of aggregate that depending on the size may also be broomed into the surface, followed by a rubber tire roller to seat the aggregate. This offers many of the same benefits as chip seals, but may be more effective at sealing cracks. Scrub seals work best on pavements in good structural condition with no more than medium severity surface distresses.
Sandwich seals – A series of two chip seals that are applied with larger aggregate seal being constructed first followed by a second chip seal with smaller aggregate. Sandwich seals work best on pavements in good structural condition with no more than minor surface distresses.
Slurry seals – A mixture of asphalt emulsion, aggregate, water, mineral filler and possibly other additives that is spread evenly over the pavement surface. Slurry seals are built with a thickness equal to the largest aggregate in the mixture. Slurry seals protect the surface from water intrusion and improve the friction resistance, as well as sealing small cracks and correcting minor surface distresses such as raveling. Slurry seals are smoother than chip seals, which may be more desirable in urban areas. Slurry seals work best on pavements in good structural condition with no more than minor surface distresses.
Micro surfacing – A mixture of polymer-modified asphalt emulsion, aggregate, mineral filler, water, and possibly other additives that is spread evenly over the pavement surface. Micro surfacing could be considered a high-performance version of slurry seal that can be placed up to 1” – 2” thick to fill ruts in the pavement as well as protecting the surface from water intrusion, improving the friction resistance, sealing small cracks, and correcting minor surface distresses such as raveling. Micro surfacing has a quicker return to traffic time than slurry seal. They have a smooth surface that may be desirable in urban areas. Micro surfacing works best on urban residential street pavements in good structural condition with no more than medium severity surface distresses, and pavement where the rut depths have stabilized.
Cape seals – A chip seal that is covered by slurry seal or micro surfacing. They offer the benefits of chip and slurry seals with a smooth pavement surface which is desirable in urban areas. Cape seals work best on pavements in good structural condition with no more than medium severity surface distresses.
Thin hot mix asphalt overlays – A mixture of aggregate and asphalt binder that is placed up to 1½” thick. Thin overlays can address low severity rutting and minor surface defects such as raveling. Cracks should be sealed prior to placing a thin overlay. They have a smooth surface similar to a new asphalt pavement. Thin hot mix asphalt overlays work best on pavements in good structural condition with no more than low severity surface distresses.
Pavement preservation treatments are a valuable tool for those in charge of maintaining a road network. Asphalt pavements have a number of preservation treatments available for use. They not only keep the roads in good condition, but also have potential to save much more money in the future by delaying or deferring major maintenance or reconstruction. With proper selection and timing of your preservation treatments, you should be able to achieve the ultimate goal of pavement preservation of keeping good roads in good condition at lower overall cost to the road owner.
Carlson is research and development general manager at Jebro Incorporated in Iowa.