When it comes to designing pavements, its generally considered a good idea to keep water away. One method is to drain free water out through a permeable base. For some years now, states such as California and Iowa have used drainable bases and edge drains to remove water from beneath their pavements (see Highway Drainage Systems, Feb. 1996, p 34).
"Now in North America we realize weve got to get drainage under our pavements," said Dr. John Emery, president of John Emery Geotechnical Engineering Ltd., Etobicoke, Ont. He noted that many agencies are using open-graded drainage layers under all new concrete pavements, adding, "We as pavement engineers are saying, Lets get the water out of all pavements as quickly as possible."
In recent years, however, a related concept, one centering on the goal of waterproofing, has begun to cause considerable debate in pavement design circles. Since the late 60s, paving fabrics have been used in overlay systems of hot-mix asphaltprimarily to reduce or delay the rate of reflective cracking through a new overlay.
In total, the use of paving fabrics for several years now has surpassed 100 million sq meters per year, according to the Industrial Fabrics Association International, Roseville, Minn.
Waterproofing is now being viewed as a potential benefit of paving fabrics. In their 1999 Transportation Research Board circular entitled "Paving Fabric Interlayer as a Pavement Moisture Barrier," authors Mark L. Marienfeld and Thomas L. Baker noted that, "Although many engineers think the paving fabric system is mainly used as a stress-relieving interlayer to retard reflective and fatigue cracking, a principal function of the system is waterproofing."
To apply a fabric system, an existing asphalt pavement typically is first milled to remove cracks. A leveling course of hot-mix asphalt (HMA) is applied. Next a contractor sprays about 0.25 gal/sq yd (1.1 liter/sq meter) of hot asphalt cement tack coat onto the surface. A non-woven fabric is laid into the tack coat and an HMA overlay is placed on top of the fabric. In the words of Marienfeld and Baker, "The heat and pressure of the overlay reactivate the asphalt tack coat, drawing it up into the fabric and bonding it to the overlay. The resultant interlayer is a fairly thick asphalt-saturated fabric-reinforced layer. This layer forms a waterproofing membrane and a stress absorption layer."
These days, however, the very use of paving fabrics for waterproofing is being called into question by many pavement designers, contractors and authorities. A number of engineers have said that waterproofing membranes used in a pavement will not only trap water moving from the top down, but also will trap water moving from the subgrade upward.
"Even if I have an asphalt pavement with no cracks, which is resting on a clay or silty-clay subgrade soil," said consulting engineer Jim Scherocman, "the aggregate base underneath that pavement layer may still become saturated even though no water comes down through any cracks. That is because moisture will still come up into the aggregate base from the underlying soil due to capillary action and pore water pressure.
"Placing a fabric on top of the existing pavement before an overlay is constructed, to provide waterproofing, may keep water from coming through the cracks, but will not necessarily solve the problem of a wet pavement structure. Since a large portion of the country consists of subgrade soils that are clays or silty clays, the primary problem with the strength of the pavement structure is not water coming from the top downbut instead, water coming from the bottom up.
"Putting a waterproofing fabric on that type of pavement is like trying to fix a leaky roof on a house when the foundation of the house is crumbling," said Scherocman.
The NAVFAC experience
Water and the use of a paving fabric contributed to a runway pavement problem at one military airfield in the mid-90s, said Darrell Bryan, a civil engineer with Naval Facilities Engineering Command (NAVFAC), Norfolk, Va.
The project had its origin in the late 80s, when a fabric interlayer was installed and the runway was overlaid with asphalt. After several years, the runway had deteriorated and NAVFAC went back to resurface the pavement.
Milling was the first step. The existing asphalt was removed to within about 1/2 in. of the fabric, said Bryan. Next came the HMA overlay, and then the problem surfaced.
"The fabric had absorbed water," said Bryan. "Wherever we applied hot asphaltthe new surface coursethe water steamed and boiled and created blisters in the fabric. So we had to go in and mill off all the fabric. The fabric was the culprit. It held the water and you couldnt pave over the top of it without creating an unsatisfactory surface, so we had to get rid of the fabric.
"If you have a mechanism for the water to come to the fabric, then it will absorb water, as our fabric did," he added. "It could have been water from the top, or some migration from the base course; more likely than not it was from the top.
"Weve had the whole range of experiences with fabrics," explained Bryan of his experience at NAVFAC, which covers military airfields and roads over a large geographic area. "I dont look at waterproofing as a geotextiles primary function."
Most of NAVFACs use of paving fabrics is restricted to southern climates. There, the agency either uses fabrics with asphalt overlays over asphalt pavement or with an asphalt overlay placed over a "cracked-and-seated" concrete pavement.
With the crack-and-seat process, the concrete pavement is broken to produce shattered slabs with cracks on a 2- to 3-ft spacing. The cracked slab is rolled to "seat" the concrete chunks, and a 1 1/2-in. layer of HMA is placed as an intermediate stress relief layer. "Then we put the fabric down on top of that to help reduce cracks that might get through the asphalt," said Bryan. "Lastly, we put 21/2 in. of asphalt on top of that."
Such projects are just about the extent of NAVFACs paving fabric usage. "We generally dont use fabric in pavements from the mid-Atlantic northward," said Bryan. "I would shy away from using them in pavements that go through harsh winters. You get limited, mixed results from about New Jersey on northward."
The Caltrans experience
Despite some problems, the California Department of Transportation (Caltrans) uses paving fabrics when conditions warrant. Paving fabric provides "an excellent impermeable barrier for us when we do not want water to penetrate into the subgrade," said Stephan Wiley, an engineering associate in Caltrans Materials and Research Engineering Department.
"We have had success with (paving fabrics)," said Wiley. The tack-coat-and-fabric method "has been a routine procedure for us for about 10 years."
Is there evidence that waterproofing is working? "Absolutely, thats why weve been doing it for 10 years," said Wiley.
As evidence, California pavements with fabric interlayers have performed well. "Our paving fabric use is generally based on asphalt cement roads where reflection cracking control is the dominant problem," said Wiley. "And when we crack-and-seat a concrete pavement we use paving fabric in conjunction with that procedure."
According to Wiley, some stripping problems have developed on some District 2 highways in northern California. On the projects, fabric interlayers were installed under asphalt overlays. The fabric trapped water and the asphalt became saturated. Wiley said that the aggregates from certain quarries were found to be incompatible with the asphalt cement and thats why the two did not bond properly.
"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," said Wiley. As a result, he said Caltrans now does not use fabric in locations where the problems occurred.
Further explanation is found in the report of the Caltrans investigation. The principal investigator was Jack Van Kirk with co-principal investigator Shakir R. Shatnawi; the supervisor was Robert N. Doty. Following are some excerpts from their general discussion.
"Locations that were identified as having severe stripping were those with either/both PRFs (pavement reinforcing fabrics) and/or chip seals. In almost all cases, in the areas where PRFs were used, the bond between the PRF and the layer above and below it was non-existent. Cracks, in some cases, were observed to extend from the PRF to the surface. Most of these cracks did not appear to be reflecting through the PRF from the underlying layers."
An associated problem with the stripping, according to Caltrans, is that high air voids in the asphalt mixture allowed water to permeate the mix. From the report: "High air voids reduce fatigue life and cause the pavement to retain high water levels. This can result in stripping and premature deterioration. Other researchers have indicated that air voids in excess of 7% would allow the water to readily penetrate the mixture. Adequate compaction should produce air voids of less than 7% to reduce the continuity of the air void system, which will reduce the potential for stripping."
Among the reports conclusions and recommendations:
- Stripping was related to the use of moisture-susceptible aggregates in mixes that were placed with high air voids. High air voids reduce the strength, trap water and accelerate the rate of deterioration;
- The use of chip seals, stress-absorbing membrane interlayers (SAMIs) and pavement-reinforcing fabrics (PRFs) can increase moisture damage because they trap water in the pavement;
- Caltrans should investigate the condition of existing AC pavements regarding moisture levels and the susceptibility to moisture damage before placing PRFs, SAMIs or applying chip seal treatments;
- Do not use PRFs or SAMIs in places where free water problems exist. Use alternatives such as thicker AC overlays to reduce reflective cracking; and
- Change specifications for the compaction of AC mixes to ensure that at least 95% relative compaction is achieved in areas identified as having moisture damage potential.
Schools of thought
Schools of thought
As quality assurance manager for Teichert Aggregates, Ron Stickel works closely with the pavement construction process. Teichert Aggregates is the aggregates and asphalt division of A. Teichert & Son Inc., a Sacramento, Calif., construction company. Stickel breaks the fabric interlayer debate into two schools of thought.
One school says yes, its worthwhile to place a fabric interlayer and stop water from going below the fabric. The other school believes in free draining where no PRFs are used to hold the water. "If you trap water on top, you prevent it from going down. But you may also cause problems with the new asphalt overlayyou could waterlog it and cause it to strip," said Stickel. "I dont think people have looked at it enough."
In summary, certainly there are risks associated with the use of paving fabrics for waterproofing. Water can be trapped and asphalt can strip. And most experts agree that water can migrate up into the subgrade and base.
"At the same time you can trap water thats moving up, and do the same thing to the old asphalt (strip it) underneath the membrane," said Stickel. "Ive seen it cause stripping below the membrane in roads where you have the opportunity for water to come up through a number of layers."
The debate here is not over the need to reduce a pavements exposure to water. Everyone agrees that prolonged exposure to water is harmful to pavements. The debate concerns which is the best method to limit exposure to waterand which method entails the fewest risks. The specifying engineer needs to understand his pavements and evaluate the risks and benefits involved before deciding upon his design of choice.