Thin smoothie

March 17, 2009

Thin-lift asphalt mixes are making a resurgence in Ohio. A Federal Highway Administration (FHWA) emphasis on preventive maintenance, the need to use paving dollars judiciously and the drive to deliver better pavement conditions for the public have caused engineers to look to thin asphalt overlays as a rehabilitation strategy. And what they are seeing are advances in thin-lift mixes that lead to improved performance and sound economics.

Thin-lift asphalt mixes are making a resurgence in Ohio. A Federal Highway Administration (FHWA) emphasis on preventive maintenance, the need to use paving dollars judiciously and the drive to deliver better pavement conditions for the public have caused engineers to look to thin asphalt overlays as a rehabilitation strategy. And what they are seeing are advances in thin-lift mixes that lead to improved performance and sound economics.

Ever since the 1980s, Ohio engineers have given serious consideration to thin asphalt surfaces for major roadway projects. Back then the driving force for placing thin lifts was economics. Engineers were hard-pressed and looking for ways to stretch a buck. Having never lost that element of frugality, Ohio engineers have found that thin asphalt mixes are a useful tool to economically extend pavement life.

Ohio engineers have turned to Smoothseal for preventive maintenance. Making its debut in 1992, Smoothseal is officially listed in the Ohio Department of Transportation (ODOT) Construction & Materials Specification (C&MS) book as Item 424, Fine Graded Polymer Asphalt Concrete. The name Smoothseal is borrowed from an old Rockville, Md., preventive maintenance strategy. Smoothseal gained popularity beginning in 2002 when the FHWA placed an emphasis on preventive maintenance. From that point on, thin-lift asphalt overlays of Smoothseal have become a favorite of engineers looking for an economical preventive-maintenance treatment, one that yields excellent ride characteristics and adds some structure too.

Smoothseal was developed primarily by a designer dream team of three talented men: Gary Cobb, Bob Bailey and Pat Welsh. Their goal was to give Ohio’s asphalt paving industry a cost-competitive alternative to surface treatments. They knew the advantages of using polymers to “stretch” the performance of hot-mix asphalt (HMA) even further. And it worked better than anyone expected.

Designed for preventive maintenance

Asphalt overlays of various thicknesses are commonly used in Ohio. As a consequence, they were in many cases considered a minor rehabilitation strategy, and their usefulness as a preventive-maintenance tool was often overlooked. Smoothseal has changed that. This asphalt mix is specifically intended for preventive-maintenance applications.

There are two types of Smoothseal: Types A and B. The difference between the two is largely in particle size and binder content. Type A is a sand asphalt mix with 8.5% binder content. Type B is a blend of 0.5-in.-maximum-size coarse aggregate and sand-size particles with a minimum asphalt binder content of 6.4%. A silicon dioxide requirement for both mix types ensures good friction characteristics. Type B requires 100% two-faced crushed coarse aggregate for mixes used in heavy traffic conditions. The crushed aggregate provides internal mix friction, leading to greater stability. Complementing the mixture’s stability is its use of SBR latex-rubber polymer, or PG 76-22M—SBS polymer-modified asphalt binder. The synergy of using crushed aggregate and a polymer-modified binder results in durability superior to conventional fine-graded HMA.

“It’s a very attractive mix,” said Cobb of Shelly Materials, one of Smoothseal’s co-developers. “We relied upon the Marshall mix method back then to develop the product and that continues to this day. No. 8s, 9s, 50% manufactured sand and latex-modified binder comprises the mix—no big secret. Binder contents run in the high 6% range. With those materials we’re able to place the material at 3?4-in. thick with no problems and get very nice aesthetics.”

Dave Powers, ODOT asphalt materials engineer, remarked, “What’s attractive about Smoothseal is its smoothness and durability. We have a lot of experience in Ohio with using latex-modified asphalt. That experience goes all the way back to the mid-1960s when Firestone Tire and Rubber championed the use of rubber in asphalt mixes.”

Indeed, having Firestone’s home base in Akron, Ohio, made it opportune for experimentation on Ohio’s roadways. Harold Carlson, Firestone scientist and head of the company’s Synthetics Division, firmly believed that rubber in asphalt would pay for itself in greater pavement life. History has validated that belief, and the asphalt industry has wholly embraced polymer-modified asphalt as a means of extending pavement life and improving performance.

Cycle of life considered

Bailey, president of Kokosing Materials, another co-developer of Smoothseal, commented, “To be competitive in a market dominated by surface treatments, we needed a hot-mix product that was not proprietary.

“Agencies prefer to buy generic, locally available aggregates, especially the abundant natural sand found in many of Ohio’s gravel deposits. We also needed a mix that could be placed in thin lifts to reduce cost per square yard and that could outperform the competition.”

ODOT undertook a study to ascertain whether the benefits of applying preventive-maintenance treatment to a roadway justify the costs associated with the treatment and to identify the factors for which an individual preventive-maintenance treatment can be considered cost-effective. In its most current findings, 2007 data indicate that Smoothseal provided the best life-cycle cost of all preventive-maintenance treatments evaluated. The study, titled “Preventive Maintenance Process Analysis,” is yet to be finalized. The results received to date are preliminary and may change with the next round of updates. Thus far, HMA treatments have demonstrated an ability to be cost competitive both in life-cycle cost and cost-benefit analyses.

Table 1 provides a summary of cost-effectiveness as measured by life-cycle cost. Included is a “control” option, a nonpreventive-maintenance strategy. It serves as a benchmark by which comparisons can be made of the cost-effectiveness of the various preventive-maintenance treatments. The costs shown in the tables are in dollars per square yard. The lower the cost per square yard, the greater the savings in dollars and the more preferred the alternative. Highlighted cells in the table are the costs per square yard associated with treatments having the lowest life-cycle cost. Observations we can make from Table 1 are:

The Smoothseal treatment provided the lowest life-cycle cost for all pavement types, conditions and traffic levels except composite pavements in good condition having low traffic levels;

The control option (nonpreventive maintenance) provided the lowest life-cycle cost for the composite pavement in good condition having low traffic; and

The life-cycle costs of Smoothseal and chip seals placed on low-volume flexible pavements in both fair and good condition were nearly equivalent.

When to thin it

Not every distressed pavement is a good candidate for Smoothseal. Experience has taught us that thin-lift asphalt overlays work well on pavements that show the following characteristics:

Dry-looking, “bony” pavements that are porous or permeable;

Pavements that have begun to ravel;

Pavements with extensive cracking too fine for crack sealing; and

Pavements with cracking of the surface too extensive for crack sealing alone.

The pavement should have no fatigue damage. Also, it is important that the pavement should have sufficient remaining structural capacity to last the expected life of the preventive-maintenance treatment. Rapidly deteriorating projects are not good candidates for preventive maintenance, because the rapidly declining condition may indicate structural inadequacy. In summary, Smoothseal should be used wherever pavement preservation is the objective of a treatment. It should be placed on structurally sound pavements that are exhibiting only surface distress. Smoothseal is ideal when raveling and minor cracking caused by oxidation are the main distresses.

If significant rutting (greater than 1?4 in.) is present, the cause must be determined and corrected. Pavement layers exhibiting plastic deformation must be removed and replaced with materials having sufficient stability to resist the stress being applied. Structural or base deformation is an indicator of the need for a structural overlay (in other words, thick overlay) or pavement reconstruction.

A Smoothseal overlay will generally consist of a single-course overlay. Type A is placed 0.625 to 3?4-in. thick, and Type B is placed 3?4 to 1 in. thick. Sufficient thickness must be specified to permit placement and compaction of the overlay over the existing pavement irregularities without exceeding the material’s minimum or maximum layer thickness. Uniform courses are best for optimum compaction. The overlay should be at least 1.5 times the largest aggregate particle size over high spots, and not more than three times in the low spots. Pavement surfaces having greater variation will require planing or a leveling course prior to placement of Smoothseal.

The mix specified must be appropriate for the traffic conditions to which it will be subjected; that is, light, medium, heavy or high stress. Type A is suitable for medium traffic and urban applications. The high binder content and fine gradation make it exceptionally durable for applications where light traffic and tree canopy can oxidize pavement or accelerate deterioration. Type B mix may be specified for any and all applications. It has proven itself stout enough for even heavy-duty and high-speed applications.

The preventive-maintenance concept does not necessarily preclude the use of pavement planing or a leveling course, which can provide the advantages of a smoother ride, achieving greater density in a uniform thickness, or being able to maintain curb exposure, etc. If a leveling course is desired, a “scratch course” of conventional surface mix may be specified.

Modified mixing

The manufacturing process for Smoothseal is similar to any HMA mixture using polymer-modified binder. In general, polymer-modified mixes require greater heat during production, resulting in mixtures arriving at the project site with elevated temperatures when compared with conventional mixes. Temperature should not be so high as to cause the binder to drain off the aggregate. Warm-mix asphalt technologies can be used to reduce temperatures while still retaining the ability to compact the mix.

The paving process differs from conventional methods only in that using polymers requires increased attention to factors affecting pavement smoothness. In any asphalt paving job, obtaining a high-quality, smooth asphalt pavement requires the contractor to observe all matters affecting mix manufacturing, placement and compaction. With polymer-modified mixes, heightened sensitivity to these factors is necessary. Uniform mix production, uniform mix temperature, uniform delivery of material to the project, uniform head of material in front of the screed and uniform compaction all become critically important.

Handling and raking should be minimized when Smoothseal or any other polymer-modified mix is placed. The high binder and polymer content of Smoothseal causes it to be very, very sticky. Handwork is not easily accomplished and may harm the aesthetics of the mat. The same stickiness that causes difficulty in handling also may cause material to accumulate in delivery truck beds. However, this same material quality is what provides the extended pavement life desired from preventive-maintenance treatments.

For joint construction, butt joints are preferred. Keep handwork and feathering areas off public road surfaces, especially for polymer-modified materials that are more difficult to work by hand. Consider using conventional mixes for driveways and approaches where handwork or feathering is necessary.

What started out to be a mix used only by the Ohio DOT is now moving into local markets. Indeed, one municipality has switched from a slurry seal preventive-maintenance program to a Smoothseal program. Eric Smith, city manager for Englewood, Ohio, remarked that one of the few times he ever received a resident’s phone call complimenting him on his performance as city manager was when they called about their newly Smoothsealed street. Now Englewood can boast that with the completion of the 2008 construction season, all residential streets are paved with Smoothseal.

Thin asphalt overlays continue to grow in popularity. Their cost-effectiveness and ability to enhance pavement longevity and ride quality make them a very attractive strategy for highway engineers. Enhanced mixes like Smoothseal have long been an integral part of thin-lift mix success. Their use will continue to flourish as the industry advances new materials and new strategies.

About The Author: Ursich is president and executive director of Flexible Pavements of Ohio, Dublin, Ohio.

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