Geosynthetics are touted by manufacturers in paving operations for their cost effectiveness and strength in paving operations.
By Carol Brzozowski
Now, the use of geosynthetics in paving operations may get a boost from the federal government. In late October 2011, US Reps. John Duncan (R-TN) and Frank LoBiondo (R-NJ)—both members of the House Transportation and Infrastructure Committee—sent a letter to the US Government Accountability Office, asking that office to conduct a study regarding geosynthetic materials in roadway systems.
The Geosynthetic Materials Association (GMA), a division of the Industrial Fabrics Association International, represents 80 member companies that manufacture, distribute, fabricate, and install geosynthetic materials.
The GMA has sought a federal study of the cost benefits of geosynthetic materials as separators and interlayers in pavement systems, with association managing director Andrew Aho pointing out that there have been a number of academic and industry case studies indicating the positive benefits of using the materials in road building.
One company, GSE, manufactures a civil nonwoven geotextile for the transportation industry for construction of paved and unpaved roadways, reinforcement and filtration functions, erosion control plans, or planning a subsurface drainage system.
Boyd Ramsey, chief engineer for GSE Lining Technology, points out there is a specification, AASHTO M-288 (The American Association of State Highway and Transportation Officials) that describes the efficacy of most geotextile materials.
One type that is most commonly used and has the longest history is the needle-punched nonwoven geotextile, Ramsey says.
“The other is a group of woven and geogrid products primarily designed to be stronger than the needle-punched nonwoven geotextiles,” he says. “They add more tensile strength and are more robust. These are used in two methods: one is reinforcement—if the road is required to be built with poor-grade soils, these products can reinforce and improve the strength of the road.
“The other one—and the greatest in volume—is called separation. As you’re constructing your road, you want to separate your base course from the upper layers of the road, be they concrete pavement or whatever material it is. The strength comes from the base course, but if the base course has fine materials from the upper layers of the road, then you lose the composite effect and the road becomes weaker, potholes more often, has more maintenance, and doesn’t last as long.”
Ramsey points out there are two cost benefits to the use of geosynthetics.
“In a lot of cases, geosynthetics will actually allow you to put down a 6-inch course of gravel layer with a geosynthetic layer on top of it to serve as separator reinforcement instead of a 6-inch course,” he says.
That provides a cost savings on the front end.
“There’s a maintenance longevity savings on the back end,” Ramsey says. “These roads tend to rut less. They tend to pothole less. On a per-mile maintenance cost basis, they’re much more effective.”
Another function for geosynthetics is drainage control.
“Geosynthetics are starting to be used more frequently not on the road itself, but in the side drains, particularly where there are bridges or heavy salting operations,” says Ramsey. “They’re using geosynthetics to line canals and divert that liquid that sometimes comes off the road with very high salt content.”
The geosynthetic material acts as a barrier in the channels to control and direct the liquid to where it should go to prevent roadside erosion and to protect roadside vegetation and any nearby water bodies that might be impacted as well, he adds.
Bryan Gee, roadway systems product manager for Tensar International, says Tensar’s geogrid products are applied in two types of situations: subgrade stabilization and base reinforcement.
“Subgrade stabilization gives you a firm foundation on which to build the road,” he says. “That could also apply to a parking lot or anywhere where you need a firm subgrade. That typically involves our geogrid product and the aggregate or sand or whatever material you’re placing on it that would create a stabilized subgrade.”
Base reinforcement—also referred to as paved application—is where geogrid is used to improve or optimize the pavement design itself, says Gee.
“You’re not trying to create a foundation on which to build, but you’re actually in the pavement design trying to improve it,” he says. “That might be adding a geogrid to the aggregate layer and leaving the layer at the same thickness, which would give you a longer life for the roadway.”
It may also mean adding a geogrid and decreasing the thickness of the aggregate layer to reduce the cost for the same performance, Gee adds.
“If you look at the roadway section as a whole by mechanically stabilizing the aggregate layer, you actually have the opportunity to look at the section as a whole and possibly reduce the asphalt thickness,” says Gee.
In using geogrid technology, “You can look at minimizing the upfront costs of the pavement section, optimizing the pavement section for a given cost or look at expanding the life of the pavement section by stabilizing the aggregate,” says Gee.
“The main key to understand is the aggregate layer and geogrid work together,” he adds. “The key to designing it so it doesn’t fail is knowing which geogrid to use and how it interacts with the aggregate.”
For pavement operations, Tensar International offers the Spectra Roadway Improvement System. It’s designed to increase cost efficiencies and performance of the subgrade, granular base, and asphalt, resulting in simplified and swift construction and a reduction in thickness of aggregate fill, asphalt base, and wearing course, and a reduction in undercut and overexcavation.
The Spectra Roadway Improvement System increases the strength of the pavement or unpaved working surface through the composite of a high-performance TriAx Geogrid and granular fill to form a mechanically stabilized layer.
“The other place where we work within the pavement section is with interlayers for asphalt paving,” says Gee. “We have a couple of products that are designed to be installed within or below an asphalt overlay. They’re rarely used in new construction, but they’re intended to manage fatigue and reflective cracking when you do an asphalt overlay on a roadway.”
|Photo: Propex US
Propex’s Geotex1341 unbonded concrete interlayer geotextile paving product over a cracked Minnesota pavement.
|Photo: Invisible Structures
Permeable pavers from Invisible Structures reduce runoff.
Tensar is the Americas distributor for the GlasGrid Pavement Reinforcement System from Saint-Gobain Technical Fabrics.
The GlasGrid System addresses rehabilitation of highways, airport aprons, runways, and parking lots experiencing reflective cracking distress due to concrete pavement longitudinal and transverse joints, thermal loads, lane widening, cement-treated or stabilized layer shrinkage cracks, block cracks, and asphalt construction joints.
The GlasGrid System is composed of fiberglass strands coated with an elastomeric polymer and a pressure-activated adhesive coating on the underside. It can be milled with traditional equipment and reused as a recycled asphalt product in road projects.
GlasGrid TF features a dry polymer tack film to eliminate the need for a traditional spray-applied tack coat and is designed to withstand overlay fatigue five times longer than an unreinforced overlay.
Tensar also is a distributor for Saint-Gobain Technical Fabrics’ GlasPave25 Waterproofing Paving Mat. Fiberglass mesh is embedded into the polyester mat to extend the life of pavement overlays through the delay of reflective cracking. It provides tensile strength at 2% strain.
The nonwoven polyester matrix structure of the GlasPave25 Mat enables asphalt binder to completely fill voids within the fabric, thereby limiting moisture infiltration and preventing water from entering into the pavement structure’s granular layers.
Tensar recently introduced a new drainage composite product, RoaDrain. It’s a synthetic geocomposite drainage layer for subsurface applications.
RoaDrain is designed to rapidly remove water permeating down through the pavement or up from the subgrade.
“It has a drainage net with nonwoven geotextile on either side,” says Gee. “It is engineered to remove water from the pavement structure, so if you have a high water table situation or a need for a capillary break in the design, then Road Drain is a composite that can be placed within the pavement section usually at the subgrade interface.”
RoaDrain daylights out the side of the section to get the water from the road out to the side, Gee says.
To address the challenges of haul roads, Tensar offers Tensar Biaxial (BX) Geogrids to internally reinforce structure and fill material.
Gee points out that the cost benefit of geosynthetics in the roadways is a function of aggregate and asphalt costs.
“If you have very low aggregate and asphalt costs, you may run into a situation where geosynthetics don’t add value because it’s cheaper to put more rock or more asphalt down,” he says. “As those costs rise—which they have been up and down for asphalt with petroleum costs—then the benefit of geogrid improves.
“While it’s different for subgrade stabilization, for the other pavement applications you pay for the geogrid now and the benefit is delivered over 20 years. It’s important to be sure the geogrid you’re paying for is going to deliver the expected benefit. The way to make that connection is full-scale research.”
Gee points out that AASHTO standards for geosynthetics rely on full-scale testing to determine the performance of a geosynthetic in a roadway section.
“One geosynthetic does not equal another geosynthetic,” he says. “You can’t pull a grid out and put a fabric in. You can’t pull one kind of grid out and put another kind of grid in and expect to get the same performance. That goes to how roadway design works.
“It’s largely empirically based, and it’s critical in selecting geosynthetics that you make sure you’re selecting something that was tested and that you understand the research and the testing is what supports the benefit of what you’re buying.”
NAUE America, whose parent company is NAUE GmbH & Co. KG of Germany, manufactures Secugrid and Combigrid for use in paved roads projects.
The products are engineered polypropylene biaxial geogrid products specifically designed to distribute loads in roads and other similar applications, says Tracy Tanner, sales and operations director for NAUE America.
Secugrid and Combigrid are manufactured via a patented vibratory-welded process and are offered in varying ultimate tensile strengths from 20 kN/m to 40 kN/m, with 20 kN/m and 30 kN/m being the most commonly used in North America, Tanner adds.
The products are also used in load platforms, airport runways/taxiways, parking lots, and building foundations, Tanner says.
“Secugrid and Combigrid benefit owners and developers by reducing overall construction costs in a number of ways,” says Tanner. “Typically, the geogrid layer replaces varying thicknesses of stone used in building the road foundation. Secugrid and Combigrid apertures allow for an ‘interlocking’ with the stone they are installed within. This locks the components together forming a firm build platform.”
Tanner points out that stone is often “very expensive,” with additional costs incurred when it has to be trucked in from far distances.
“By replacing several inches or more of gravel with a less-expensive engineered geogrid, and by shortening the time to install the alternative, significant cost savings are realized,” Tanner says. “The use of Secugrid and Combigrid yield less quarrying and trucking of stone, thus providing for a more ‘green’ construction process.”
Typical end users of the biaxial geogrids include the Federal Highway Administration, state Departments of Transportation, and local municipalities, as well as private owners and developers.
Challenges inherent in paving operations include poor soils that must be over-excavated—dug out and replaced with better material—and often attribute to a more costly construction, notes Tanner.
“Secugrid and Combigrid strengthen and reinforce these poor soils in lieu of having to dig them out, haul them away and replace them with more expensive suitable materials,” says Tanner. “Wet soils due to high water tables and/or poor drainage can often be dealt with by specifically using NAUE’s Combigrid geogrid.”
Combigrid is a composite material of a biaxial geogrid and a geotextile and provides a drainage layer via the geotextile and the reinforcement via the geogrid.
“Often, the stone layer road base is underlain by poorer, more fine-grain soils,” says Tanner. “If these materials are not separated during construction, over time the fine-grained soils migrate into the stone layer, causing it to weaken and ultimately cause a failure to occur. The geotextile component of Combigrid provides this imperative separation as well as the need reinforcement.”
L&M Supply Co.’s products are applicable to paving operations from start to finish. The company offers silt fencing to meet regulatory necessities for grading and earthmoving projects as well as erosion control blankets for the final step of wrapping up the project.
For the second phase of the project, L&M Supply Co. has available a 200- or 315-pound woven stabilization geotextile fabric used to provide a stabilization factor for roadway projects.
The company also provides needle-punched non-woven geotextiles for the paving phase of a roadway project.
“The grab tensile strength is paramount,” says Jim Taylor, national sales manager for L&M Supply Co. “It stabilizes and also separates.”
There are cost benefits to be derived through the use of geotextiles in paving projects, Taylor points out.
“Geotextile will save you on the amount of aggregate needed to be used, acting as a separation and a stabilization barrier between the site surface and the aggregate,” he says.
“Picture a subsurface that might be sandy or loose soil and you put aggregate on without a stabilization fabric or a separator. That aggregate can mix with the subsurface, and you end up losing some of your aggregate, so you have to put on more aggregate to give you the same depth that you are trying to achieve. With the geotextile fabric, you’re going to use less aggregate, therefore saving you money in the long run.”
Invisible Structures manufactures permeable pavers, including Grasspave2 and Gravelpave2. The product is used as an alternative to impermeable pavement such as concrete or asphalt.
“Our products essentially allow water to filter through the cross-section back into underground water supplies, mimicking the normal, natural water cycle,” notes Dustin Glist, media and information director for Invisible Structures.
The Grasspave2 porous pavement system is composed of a base course, Hydrogrow soil amendment and fertilizer, Grasspave2, sand, and thin-cut sod, washed sod, or hydroseeding.
It provides load-bearing strength while protecting vegetation root systems from compaction. High void spaces within the cross-section enable root development and storage capacity for rainfall from storm events.
Gravelpave2 porous pavement is a geotextile fabric injection molded to a ring and grid structure. It comes in four standard colors to match the aggregate fill and requires a base course.
The company has seen an increasing demand for its products in response to the influx of stormwater management regulations reducing runoff pollution.
“We’ve seen a great boost in both a need and an awareness for our products,” Glist says, adding that the growth is occurring not just with his company’s technology but with similar paving techniques with Invisible Structures products being used in combination with traditional paving applications.
Invisible Structure products are used in low-speed applications such as parking lots, fire lanes, and access roads. They are not applicable for highway or high-speed driving. They can be used in conjunction with traditional pavements such as asphalt to stabilize shoulders.
“When combined with traditional paving in a shoulder application, they take runoff water from the highways, capture it, and put it back into the water supply where it belongs instead of having it run into the creeks or the watersheds and needing industrial pollution removal,” Glist says.
Invisible Structures’s products have compressive strengths that when installed correctly are nearly twice that of two inches of concrete.
“They are 5,700 psi in the compressive strength area,” says Glist. “They also have been tested for pull-apart—the tensile strength—in their connectors, so they have a great horizontal strength as well. They can withstand any street legal vehicle traversing over the products and well above that.”
The products have been used in the shoulders of some LAX runways.
“While a jumbo jet airliner will drive on it is not a high probability, if it does happen to, it will definitely support the weight,” says Glist. “It’s strong both in the vertical and the horizontal.”
Pricing for the products is the same as asphalt paving and somewhat less than concrete, he says.
“If you take some of the other components that are sometimes needed in paving such as the shoulders, the drainage systems, and the stormwater management, you can have a good cost-saving benefit to the other components,” says Glist.
Invisible Structures’s products are based on recycled high-density polyethylene with UV inhibitors.
“If the products are well-maintained with vegetation coverage in the case of Grasspave2 or regularly kept filled with gravel in the Gravelpave2, they’ll outlast the traditional pavement,” says Glist.
He cites a University of South Alabama study showing that using the Grasspave2 and Gravelpave2 in lieu of traditional pavements can save $50,000 over a 20-year period due to the resurfacing that would be needed for asphalt and replacement costs for concrete.
“The Grasspave2 needs to have vegetation growing on it to maintain itself,” says Glist, adding that the vegetation secures the product to the base course below it and also provides cover from the sun.
“If it’s maintained regularly with an ample supply of water—depending on the climate, irrigation is sometimes needed—but if goes through the regular mowing and fertilization of any lawn or turf system that you’re used to, it can last 60 years and probably well beyond that,” says Glist.
Aggregate can move and get caught in tire treads or migrate throughout the surface, especially in high-traffic areas.
With Gravelpave2 being an aggregate containment system, if it is maintained with brooming and raking and kept covered, it should provide 25 years of service, Glist says.
Carol Brzozowski writes on technology, erosion, and construction.