Thin-lift Sand Mix Stretches Maintenance Dollars

Seeking a way to properly maintain its streets while conserving cash, a small Illinois town adds specially designed polymer-modified asphalt mix to its maintenance program.

Article December 28, 2000
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Roads are deteriorating throughout the U.S. at the same time cities, counties and states are having difficulty finding the funds to keep their roads in good condition. Inherent in this situation is the need for a cost-effective method of maintaining aging pavements.

The costs of traditional hot-mix asphalt (HMA) overlays sometimes can be prohibitive for small cities and towns. Thicker overlays also can cause problems with curb heights and drainage. While emulsion solutions, such as chip seals and slurry seals, are cost-effective for preventive maintenance, they are not designed for pavement with more serious distresses.

A study performed by Fehr-Graham & Associates, Freeport, Ill., concluded that a high-performance thin lift might be the answer to the city of Freeport's pavement maintenance dilemma.

Innovative solution

Located in the northwestern region of the Land of Lincoln, the city of Freeport has a population of 26,000. A 1994 pavement management study completed by the city had touched on the possibility of a thin overlay. An analysis of eight-year-old pavement treatments showed little performance differences between thin and conventional thick overlays. As the pavement maintenance program was being developed for 1996, Craig LeBaron, the director of public works, favored the cost saving of thin lifts.

Fehr-Graham's Thomas Mathews proposed a polymer-modified binder coupled with a quality aggregate as a possible solution. Recent advances in modified asphalt, in aggregate quality standards, and in proof testing, have raised technology to a new level. This has resulted in the creation of alternatives for optimizing pavement designs and performance.

Mathews contacted Koch Materials Co., Wichita, Kan., who in conjunction with Chicago Testing Laboratory, Skokie, Ill., performed a mix design based on locally available aggregates and polymer-modified asphalt meeting Illinois SBS-10 specifications.

Continuing the feasibility study, Mathews contacted Bruce Helm of Civil Constructors Inc., Freeport, to determine the ability of local contractors to produce and lay the mix, and at what cost. The contractor said that problems had been experienced in the past with laying and rolling sand mixes. However, after further discussions that differentiated the high-performance mix from previous mixes, Helm said he thought it was possible to produce and construct the proposed 1/4-in. overlay.

Project specifications were finalized and the project was put out for bid. The contract included three maintenance operations: the polymer-modified sand mix thin overlay, a bituminous concrete surface course Class I mix with polymer-modified binder, and a seal coat using a high-float polymer-modified asphalt emulsion.

Because of the experimental nature of the high-performance sand mix, the city decided to use it on a variety of surface conditions ranging from minor cracking and segregation to heavy cracking, segregation and surface distortions.

Civil Constructors was low bidder for the project and performed the work in the summer of 1996. The city, which had feared the new materials would be more expensive, was pleased the winning bid was 5% below the original estimate. The polymer sand mix was 12% more expensive than the polymer bituminous concrete surface course, Class I mix, perhaps because of the unknowns of handling the experimental material. The cost of the sand mix is expected to decrease with experience.

The contractor decided to lay the conventional surface mix with polymer first, because he felt comfortable using the aggregate. A Cedarapids Grayhound CR 451 asphalt paver placed the material. When the time came to lay the polymerized sand mix, the contractor faced resistance from its crew. Comments were made, such as, "This stuff won't work."

The contractor expected some balling up of the materials and rolling problems. Representatives from the polymer supplier and the engineering firm were present to observe and help with any problems. The problems did not materialize, and the material was easily laid. The biggest problem was keeping traffic off of the newly paved streets until they were completely rolled.

A thin-lift Troxler Nuclear Density test gauge was used to determine the density and the number of roller passes required. The mix was 88% compacted behind the screed, and required three roller passes to bring it to 92% to 96% compaction.

Vibratory rolling was not used for the thin-lift polymer sand mix, but an Ingersoll-Rand DD90 vibratory steel drum roller was used on the polymer surface course. A Case Corp. Model 252 finish roller was used, but not really needed because the three roller passes of the breakdown/intermediate roller had already accomplished the compaction. However, the finishing roller was helpful at smoothing out marks at drive approaches when it was run perpendicular to the road.

At temperatures greater than 300 degrees F, the mat tended to stick to the roller, thus breakdown rolling was conducted at 270-275 degrees F. It also was necessary to do any hand work before the temperature decreased. If the mix dropped below 190 degrees F, the rollers were ineffective. If a ridge was left at this temperature, the roller could iron it out, but the ridge would spring back because of the elasticity of the polymer.

The sand mix was bid at $2.41/sq yd, compared to $3.28/sq yd for the 1/4-in. conventional surface course mix with polymer, and $0.90 average chip seal costs. The cost of the sand mix is expected to drop slightly as it moves from experimental status to a standard mix.

Innovation

The inclusion of an elastomeric polymer offers several advantages. It is the polymer that reduces and slows cracking, allowing a thinner and therefore more cost-effective overlay. The polymer-modified binder is less susceptible to oxidative aging and premature brittleness. It accommodates thicker films on the aggregate without danger of draindown.

The networked polymer is less temperature susceptible, resisting rutting at high temperatures and thermal cracking at low temperatures.

Koch tested the mix using a Hamburg wheel-tracking device. The test results confirmed that the proposed mix had the strength to resist rutting and stripping. The elastomeric binder also has greater durability in withstanding repeated loading in a flexible pavement. The mix was produced using a Barber Greene DM70 batch plant.

For the project, a chemically reacted styrene-butadiene block copolymer manufactured using Stylink technology and meeting Illinois' SBS-10 specifications was used. An elastomeric polymer, such as Stylink MAC-10, which meets elastic recovery and separation requirements gives the best results. The elastic recovery ensures the elasticity of the material. The homogeneity ensured by passing separation will prevent segregation of polymer and asphalt, and be easier to handle at the mix plant and in the

The aggregate is fine, and includes manufactured sands for strength. While this project used only crushed materials, subsequent testing has developed a mix using both crushed and natural sands. The crushed material gives skid resistance and the structural integrity to resist deformation. The natural sand increases the workability and decreases the cost. Windblown sands, baghouse fines, mineral fillers and dusty materials are to be avoided.

The mixture has relatively high binder content to enhance durability and interconnected voids to expel moisture and reduce the effects of hydroplaning and back spray. Skid numbers typically are in the low 60s if skid resistance materials are used, and the mix has AASHTO layer coefficients of approximately 0.40.

Use of a polymerized asphalt emulsion tack coat is strongly recommended on projects such as this to ensure bonding to the underlying surface.

Testing and photo documentation will continue on the sites in Freeport to track the long-term performance. To date, none of the sections show any significant signs of cracking.

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