Pavement Preservation with Thin Overlays

How thin and ultra-thin overlays fit in a pavement preservation program.
 A systematic pavement preservation program delivers more road-quality bang for the taxpayers’ buck.

by Kent Hansen

Of the many pavement preservation techniques available, hot-mix asphalt overlays are probably the most versatile. They add structural capacity, seal cracks, improve ride, enhance skid resistance, reduce noise, and improve drainage. No other treatment does all this.

Overlays can be placed in varying thicknesses. Thick overlays add substantial strength to a pavement when needed. Thin overlays (1.5 inches or less) also add structural capacity to pavement. Overlays can also be placed very thin, down to about 0.5-inch thick. The increase in structural value will vary depending on the thickness and condition of the existing pavement.

Using thin overlays

In general, thin overlays should only be placed on structurally sound pavements that only exhibit surface distresses such as low-severity transverse and longitudinal cracking. Isolated structural distresses such as alligator cracking and potholes must be patched prior to overlay.

If the existing pavement surface has significant rutting (over 0.25 inch), the cause of the rutting must be determined using cores or pavement trenches. Pavement layers responsible for the rutting must be removed and replaced with materials capable of withstanding the traffic loads. Pavements that are deteriorating rapidly probably have significant structural deficiencies or material problems and therefore are not good candidates for thin overlays.

Milling or profiling of the roadway prior to overlay should be considered prior to overlay to improve ride quality and drainage, and to remove surface distress such as rutting, and transverse and longitudinal cracking that may reflect through the thin surfacing.

There are many HMA mixtures that have proven successful in thin overlays. The general categories include dense-graded, open-graded friction courses, sand asphalt, and stone matrix asphalt. These mixes and their uses are discussed in more detail in Thin Hot-Mix Asphalt Surfacings from the National Asphalt Pavement Association (www.hotmix.org).

There are also non-typical HMA overlays that have been developed and used in some regions with great success. These include Smoothseal in Ohio, asphalt rubber in Arizona, and ultra-thin overlays in Michigan.

Ohio’s Smoothseal

A good example of a thin dense-graded overlay is the Smoothseal used in Ohio. There are two types of Smoothseal – Type A and Type B. The Type-A mixture is a fine sand-asphalt that uses a recipe specification where the contractor selects the aggregate proportions to meet the gradation tolerances. The Type-B mixture is a fine, dense-graded asphalt using the Marshall procedure for light (35 blows), medium (50 blows), or heavy (75 blows) traffic. The specifications for these mixtures are contained in the Ohio Department of Transportation Supplemental Specification 854.

Both Smoothseal mixtures require the use of a polymer-modified asphalt binder. The binder content for the Type-A mix is set at 8.5% by weight of total mix. The binder content for the Type-B mix is determined by the mix design, with a minimum asphalt binder content of 6.4%. Flexible Pavements of Ohio recommends minimum and maximum thickness of 0.625 to 1.125 inches for Type-A mixtures and 0.75 to 1.50 inches for Type-B mixtures. The Type-A mixture is recommended for medium traffic and urban applications. The Type-B mixture may be specified for all applications including heavy-duty and high-speed applications.

The Type A Smoothseal has been used in Ohio for many years. One of the early Smoothseal projects was on Shelburne Road in Shaker Heights. The tree-lined residential street was paved with Smoothseal in 1973 and in 2001 received a Master Craftsman Award from Flexible Pavements of Ohio.

The Type B Smoothseal was first introduced in 1993. Since that time, Type B has been used on ODOT maintenance projects throughout the state.

Arizona’s asphalt rubber

Another example of thin overlays using modified binders is the use of asphalt rubber in Arizona and elsewhere. Asphalt rubber is a blend of liquid asphalt and ground tire rubber. In Arizona, the asphalt rubber generally consists of 18 to 20% ground tire rubber blended with paving-grade asphalt. Asphalt rubber is used in gap-graded mixtures and open-graded friction courses.

The asphalt rubber binders used in Arizona are very viscous and therefore draindown of the binder is seldom a problem with these mixtures. For this reason, asphalt rubber mixtures have higher binder contents than many conventional mixtures. The binder contents for gap-graded mixtures generally are between 7.5 and 8.5%. Binder contents for open-graded mixtures are generally between 9 and 10%. The increased binder content increases the flexibility of both mixtures which helps Arizona accomplish three goals in its pavement program: reduce reflection cracking, improve the durability of surface courses, and reduce noise in urban areas.

One example of ADOT’s use of asphalt rubber is a section of I-19 south of Tucson where a 0.75-inch-thick open-graded asphalt-rubber mix was placed on a plain jointed concrete pavement in 1988. Not until 1996, eight years later, were a few reflection cracks noted.

Michigan’s ultra-thin overlay

Michigan has also recently developed an ultra-thin overlay for pavement preservation. This mixture was developed as an alternative to micro-surfacing. It is similar to a sand asphalt mix with 100% passing the 0.5-inch sieve and 99 to 100% passing the 0.375-inch sieve. Mixes are designed using the Marshall procedure. Mixes can be designed for low-, medium-, and high-volume traffic. For medium-and high-volume traffic, polymer-modified asphalt is required.

These are only a few examples of thin HMA surfacings that can be used for pavement preservation. Stone matrix asphalt and standard mixtures such as the 9.5-mm and soon-to-be-released 4.75-mm Superpave-designed mixes, or their pre-Superpave design counterpart, remain a preferred pavement preservation strategy for years.

An AASHTO survey of pavement preservation strategies used by states showed that “overlay techniques are the most frequently cited treatments.” The versatility of HMA is probably one of the major reasons for this choice. Mixtures can be selected to meet every need, including fine mixtures that can be placed very thin to minimize curb loss, and rich mixtures using modified binders to reduce reflection cracking and improve durability.

No matter which HMA mixture is selected, properly designed and constructed mixtures will enhance skid resistance, improve ride quality, increase structural strength, and improve pavement drainage. No other pavement preservation treatment can meet all these requirements.

Reprinted from Better Roads Magazine
June 2003

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