Solving fabric problems in road repair
Over the past several years, a number of failures have cropped up with paving fabric placed as an interlayer between an older asphalt pavement and a new asphalt overlay. For example, Stephan Wiley, an engineering associate in Caltrans’ Materials and Research Engineering Department, cites stripping problems that developed on some District 2 highways in northern California. Fabric interlayers installed under asphalt overlays trapped water, and the asphalt became saturated.
“Sometimes the asphalt glue or binder is not compatible with the aggregates, and, in the presence of water, that lack of compatibility is enhanced, which results in stripping. The road will fall apart,” Wiley says. As a result, Caltrans now does not use fabric in locations where the problems occurred.
Problems with paving fabrics can occur when the fabric fails to make a proper bond between the new overlay and the old pavement. The overlay can delaminate; asphalt will loosen in patches; and the pavement will ravel and fail at that location.
“Based on the predominance of non-performing systems, we went back and looked at the causes of the failures,” says Ray Myers, executive director of the Asphalt Interlayer Association (AIA), Sacramento, Calif. “We think the predominant reason [for project failures] is that the systems are not being installed properly. We were not getting optimum performance because of poor construction techniques.”
A typical fabric installation involves milling the old asphalt pavement to remove cracks. Next, a leveling course of hot mix asphalt (HMA) is applied, followed by the spraying of 0.25 gallons per square yard (1.1 liters per square meter) of hot asphalt cement tack coat. A non-woven fabric is laid into the tack coat, and an HMA overlay is placed on top of the fabric.
The designed “sandwich” is formed when the heat and pressure of the HMA overlay reactivate the asphalt tack coat. The overlay draws up the tack coat of asphalt cement and bonds to it. The end result is a fairly thick, asphalt-saturated, fabric-reinforced interlayer.
That is how the installation process is designed to work, but a rash of problems has cropped up around the application of the fabric into the tack coat. The main problem is that, on a hot day or with excessively hot tack coating, the liquid asphalt cement will “bleed” through the paving fabric. Frequently, the same truck that is spraying the tack coat is laying down the fabric. That, in turn, means the fabric is being embedded in oil hot enough to promote or cause bleeding. Yet the oil from the truck has to stay hot – about 280 degrees Fahrenheit or more – so that it will not stick in the valves of the applicator truck. “Almost all of [the installation] problems occur as a result of elevated ambient temperatures and pavement temperatures,” Myers says. “When the contractor or fabric installer starts in the morning, and the temperature is 50 degrees Fahrenheit, everything goes well. But if the ambient temperature gets up to 95 or 110 degrees and the pavement temperature is 160 degrees, now all of a sudden you’ve got this bleeding problem. It’s not a result of excess binder quantity, it’s a function of temperature.”
AIA has now devised five recommendations to reduce what it calls premature fabric saturation, or the bleeding of the tack coat. According to the association, contractors should:
1. Use a high-viscosity binder as a tack coat. AIA recommends a 2,000-poise asphalt.
2. Specify the AASHTO minimum weight fabric – 4.1 ounces per square yard with 0.25 gallons per square yard of residual asphalt, with a tolerance of 0.03 gallons per square yard.
3. Lower the embedment temperature of the tack coat when the fabric is applied. According to the “Fabric Interlayer Guide,” updated by the AIA on June 6, 1999, “Polypropylene fabrics may damage or shrink at temperatures that are too high. Proper saturation will occur when a minimum 1.5-inch lift of compacted hot mix is placed over the binder and fabric when the binder is less than 180 degrees Fahrenheit. If fabric is embedded in binder that is hotter than 180 degrees Fahrenheit (especially in hot weather), the fabric may become prematurely saturated and cause construction problems.”
4. Lower the embedment pressure. Every fabric applicator has a broom that pushes the fabric into the tack coat as it comes off the roll. If the broom applies too much pressure, it promotes bleeding. The fabric does not need to be forced into the binder.
5. Spread hot mix over the in-place fabric and ahead of the paver manually or with a wheel loader. That should be done if bleeding still occurs after the first four recommendations have been followed.
Considerable controversy has centered on AIA’s third point – embedment temperature. While some fabric manufacturers contend that there are no shrinkage problems with paving fabrics, others have reported problems.
One of the earliest reports of heat shrinkage problems with polypropylene fabrics was in the July/August 1990 “Geotechnical Fabrics Report,” the publication of the Industrial Fabrics Association International, Sacramento, Calif. “The most serious shrinkage problems as observed in the field … have occurred when the fabric is placed into a hot oil that exceeds the fabric shrinkage temperature, rather than the melting point,” says author Mounque Barazone, president of Geotextile Apparatus, San Diego. “Texas (DOT), Los Angeles County and Caltrans also have documented shrinkage of polypropylene fabrics when placed in hot oils over 250 degrees Fahrenheit (121 degrees Celsius).”
According to Barazone, the key to embedding the fabric at the proper oil temperature is to allow enough time for the oil to cool, once it is sprayed from the tanker truck, before unrolling the fabric into the oil. “The oil comes out of the truck at 280 to 285 degrees, and cools to 250 degrees within a second or two when the weather is cool, early in the morning,” he says. “Later in the day, it could take a lot of seconds to cool to 250 degrees.”
To give the oil more time to cool, some agencies use a second machine to unroll the fabric after the tanker truck places the oil. The most common applicator is a tractor that carries the fabric roll on the front.
Certainly any pavement engineer who specifies a paving fabric must be aware of certain risks associated with the product’s use. As for waterproofing, fabric can trap water in the top course of asphalt and allow the asphalt to strip, causing pavement failure. Those risks and the need for proper installation should be acknowledged before the project begins.
Mike Phillips is president of MVP Marketing, Weston, Ontario.