Hugging the road

Chip seal application holds on tight to aggregate

Preventive Maintenance Article June 16, 2004
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Preventive maintenance, asset management and pavement preservation are all terms used to describe what everybody knows: Effective maintenance saves money and preserves your investment whether it’s changing oil in your car, painting your house or preventive maintenance projects on highways. I-90 is the longest interstate in the U.S., with 3,112 miles stretching from Boston to Seattle. The South Dakota Department of Transportation (SDDOT), which is responsible for 412 miles of I-90, understands its importance and is seeking new and better ways of maintaining the interstate while saving taxpayer dollars.

According to Gill L. Hedman, SDDOT pavement design engineer, “We were looking for a surface treatment that could be applied to high-speed, high-volume pavements. We currently have approximately 300 two-lane-miles of asphalt-surfaced interstate highways and see the need for a viable maintenance treatment to extend the lives of these pavements.”

On a 20-mile section of I-90 from Salem to west of Humboldt, the SDDOT did an experimental project sealing the surface with a new process called macro-surfacing.

Watch for flying . . .

In a Long Term Pavement Performance (LTPP) SPS-3 study by the Federal Highway Administration (FHWA), the Kansas Department of Transportation found that chip seals are one of the most cost-effective means of pavement preservation. South Dakota has a very aggressive seal coat program for their non-interstate system. New surfaces are seal coated within three years of placement, and usually a second application is needed prior to rehabilitation. Federal, state and local agencies apply over 800 million square meters of chip seals each year in the U.S.

However, the field performance does not always meet expectations. A 1994 report stated that slow curing times, unpredictable quality and vehicle damage from unbound chips have prevented them from being used by most agencies on high-volume roads. A 2001 FHWA-sponsored international scanning tour of pavement preservation techniques revealed that countries such as Australia and South Africa are using chip seals on high-volume roads much more readily than the U.S., and the report concluded their use should be increased, if quality and reliability are improved, specifically using polymer-modified binders and higher quality aggregates.

A new way to sweep test

There are two processes that must occur for an asphalt emulsion to build the strength necessary to hold aggregates in surface treatments. First, the emulsion must break, or release the water as the emulsion destabilizes and reverts to separate asphalt and water phases. Second, the asphalt particles must coalesce, forming cohesive strength within the asphalt matrix as well as adhesive strength to bond to the underlying surface and the aggregates. A further consideration is for the asphalt emulsion residue to hold the chips firmly in place throughout the life of the chip seal.

Traditional asphalt emulsion specifications are adept at characterizing both the emulsion properties and residual bitumen properties, but have not been very accurate at predicting field performance with the aggregate. Recognizing this, ASTM recently accepted a new method, D 7000, Standard Test Method for Sweep Test of Bituminous Emulsion Surface Treatment Samples. This performance-related test characterizes the breaking, curing and chip retention of chip-seal emulsions with the project aggregate.

ASTM D 7000 is similar to the wet track abrasion test. A measured application rate of the emulsion to be used on the project is applied to a circle of roofing felt, followed by application of the project aggregate. After standard curing conditions and time, the sample is weighed and then subjected to abrasion by a brush. Unlike the wet track abrasion test, the test is not run underwater. The testing equipment consists of an A-120T Hobart mixer fitted with an attachment to hold a nylon brush and an applied load of 1,550 g. At the end of one minute, the test is stopped, any loose aggregate is removed and the sample is reweighed. Duplicate samples are run for each test. The result is the average percent mass loss.

Holding the largest

The biggest difference between the traditional chip seals and the new process is the use of performance-related specifications, which include a mix design protocol based on performance tests, stricter aggregate specifications, a polymer-modified emulsion developed for the process and a construction process requiring synchronization of the application of asphalt emulsion and aggregate.

Bruce Batzer, president of Asphalt Surface Technologies Corp. (ASTech), said that the process used on I-90 is “unlike the chip seal of yesteryear.” According to Batzer, the design process brings surface treatments into the type of program used for hot-mix asphalt.

The theory is that the liquid application should be sufficiently thick to hold the largest-size aggregate in place. The performance requirements result in a “very different proportion of liquid asphalt to stone,” or an emulsion application roughly 1.35 times greater than traditional chip seals, and an aggregate application rate that is much lower. The chips are then firmly held by the emulsion, and there are not as many excess chips that can dislodge in traffic and do vehicle damage. The specifications also call for a higher quality and more uniformly sized chips.

Batzer said that these differences were highly apparent during and after construction of the I-90 project. The heavier asphalt application also should give more sealing protection to the existing pavement.

Hedman and Craig Smith, SDDOT Sioux Falls area engineer, added, “Many of the variables that lead to chip seal failures can be controlled because of the use of a single piece of equipment applying both aggregate and emulsion.” They also like the “cubical one-size chip and the faster breaking and setting emulsion allowing chips to be swept off and the surface released to traffic more quickly without the fear of loose chips flying around.”

Macro results

The existing pavement was a composite, with a continuously reinforced portland cement concrete structure with an approximately five-year-old hot-mix asphalt overlay. The SDDOT did an excellent job of pavement preparation. While in generally good condition, there were about 20 small areas where the concrete had blown out. These areas were treated to full-depth concrete repair followed by hot-mix asphalt repair. All the existing cracks were sealed. ASTech, the largest surface contractor in the region, applied the macro-surfacing in September 2003. A special machine manufactured by E. D. Etnyre & Co., Oregon, Ill., was used to both spray the emulsion and apply the aggregate. Koch Pavement Solutions’ Marshall, Minn., plant supplied the polymer-modified asphalt emulsion. The aggregate was a high-quality quartzite from Spencer Quarries, Spencer, S.D.

The performance-related specifications required synchronization of the emulsion and chip application. By using the special machine this requirement was met, and there was no construction traffic disturbing the uncovered emulsion or the finished product.

ASTech found that while they needed five passes of the roller on the chip seal sections, only one initial pass was needed on the macro-surfacing section. The macro-surfacing process was continuous, with live-bottom trailers equipped to supply both emulsion and aggregate to the laydown machine. These support trailers docked and undocked with the machine on-the-fly, resulting in very fast construction. The curing time varied from one to three hours depending upon the weather conditions. Once cured, any excess aggregate was swept off.

A fog seal was applied to both conventional chip seal and macro-surfacing sections the morning after placement of the surface treatments. According to Tom Wood, Koch Pavement Solutions field engineer, the fog seal is “like frosting on the cake. It looks like a new black surface, it gives extra embedment from the extra binder, it makes the pavement markings more visible and it helps prevent snowplow damage.”

Surviving the winter

The macro-surfacing project has met performance expectations on the 10,000 ADT highway, and South Dakota was scheduled to let another macro-surfacing project on another section of I-90 in May. The macro-surfacing is slightly more expensive than the conventional chip seal, but not as expensive as other options considered, such as thin hot-mix overlays. The SDDOT will continue to monitor the sections in this project.

After the first winter, the project can be “considered as a performance success,” according to Batzer. “We’ve done this project in South Dakota and three more projects, all as successful, in Minnesota. We have faith in the future of this tool and think it is the most cost-effective method for preventive maintenance on higher ADT highways.” SDDOT’s Smith said, “The performance to date is very good, and we are going to construct another 12-mile section of westbound I-90 in Jones County this summer.”

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