Importing pavement innovation

October 2009 » Features » PROGRESSIVE ENGINEERING
A visit to Germany prompts U.S. tests of nonwoven geotextiles for concrete pavement interlayers.
Sabrina I. Garber, MSE,
A Missouri project used a nonwoven geotextile as part of a concrete overlay application.

The use of geotextiles in pavements is hardly a new thing. They can be commonly found between and within subgrade and base layers to mitigate the effects of poor quality materials, as a filter around subdrains, and along embankments for stabilization. However, the use of geotextiles as an interlayer between cementitious layers in concrete pavements is not common in the United States — at least until now.

New concrete pavements commonly include at least three layers: subgrade, base, and a concrete surface (see Figure 1). The subgrade consists of existing soil that is sometimes improved to improve stability and uniformity and to help mitigate environmental effects. Since, in most cases, the existing subgrade material is not of a high enough quality, especially when heavy traffic loading is expected, a base layer is incorporated into the pavement design.

Figure 1: Typical concrete pavement cross section

When that base material is treated with cement, a thin layer of hot-mixed asphalt (HMA) is sometimes placed over it before the concrete surface is constructed. The purpose of the HMA is to provide a separation between the two cementitious layers, which can be important if the cement-treated base (CTB) cracks excessively. The HMA interlayer also functions as a water barrier that impedes water ingress from the surface to the underlying layers, and it provides a level of bedding that cushions the base from the effects of dynamic loads caused by heavy traffic.

Workers installed a nonwoven geotextile interlayer on route D south of kansas City, Mo., as recommended without wrinkles or folds.

Use of the HMA interlayer is common in the United States and has proven effective, provided the material is of good quality and the interlayer is designed and constructed correctly. Unfortunately, HMA has become increasingly expensive. Furthermore, the application of this layer often means a unique operation that can convolute construction schedules. The need for suitable alternatives has become apparent.

But, for concrete pavements to remain competitive on a cost basis without compromising long-term durability, alternative interlayers must be selected carefully. Ideally, they should be selected based on experience. A recent discovery made by U.S. highway practitioners while visiting with their German counterparts identified the use of nonwoven geotextiles as such an alternative. German engineers have more than a quarter of a century’s worth of experience using nonwoven geotextile interlayers, both in concrete overlays and new concrete pavement construction projects. During that time, they have made great strides in perfecting it.

Use of nonwoven geotextiles in Germany
The Germans first began experimenting with nonwoven geotextiles as interlayers between cementitious pavement layers more than 25 years ago. In 1981, German engineers were in the midst of general repairs to many of their primary roadways. The Autobahn 5 (A5) was one such motorway on the list for repairs. On this project, it was discovered that most of the damage to the pavement was the result of poor drainage and subsequent loss of support in the base and subgrade layers. The standard repair included removing the existing concrete surface, replacing the support layers as needed, and patching with a new concrete surface. However, this would not have corrected the root of the problem, and thus something else had to be done.

On an experimental whim, German engineers chose to use a nonwoven geotextile as part of their repair procedure. Their hope was that the transmissivity of the nonwoven geotextile would offer lateral drainage from beneath the surface of the concrete, and thus minimize the potential for saturation directly beneath the concrete surface.

Their hunch was correct. The A5 repair project was a success, and during the next few years, the use of nonwoven geotextiles became a common practice not only in repair, but also as part of new construction projects. During this time, German engineers further realized that the benefit of nonwoven geotextile interlayers was not limited to transmissivity. Instead, the geotextiles serve three critical functions: drainage, separation, and bedding. Furthermore, to work properly, structural design of the pavement requires special attention, considering that the compliance of the support is different than without the interlayer. A slight (1-cm) increase in the surface thickness was all that was necessary to accommodate this (Leykauf and Birmann).

During the next 25 years, German engineers learned how to adjust the manufacture of nonwoven geotextiles to create a material with a combination of specific characteristics that would perform the best under concrete pavement surfaces. Standards and specifications were eventually developed by the Bundesministeriums für Verkehr, Bau und Stadtentwicklung (BMVBS), the German equivalent of the Federal Highway Administration (FHWA). As the German experience continues to grow, BMVBS continues to update these publications accordingly.

Importing the technology
In 2006, representatives of FHWA, the American Association of State Highway and Transportation Officials (AASHTO), and the National Cooperative Highway Research Program (NCHRP) toured Europe (Hall et.al., 2007) and met with their counterparts there. During that tour, the team learned about the German technique of using nonwoven geotextiles and further recognized it for potential application in the United States. Subsequent to the tour, FHWA sponsored a small project to focus on documenting the accomplishments of the Germans and the results of initial U.S. implementation efforts of this practice (Rasmussen and Garber, 2009)

Steel nails and fasteners are required to secure the geotextile to the base. This photo shows the devices used on the Missouri project.
This (upside down) core from the Missouri project shows the nonwoven geotextile bonded properly to the concrete surface and not to the base.

During this project, it was revealed that the success in the German experience stemmed from proper guidance that was built on their experience. Using translations of published specifications and standards, along with the results of interviews with German practitioners, initial recommendations for the proper selection of material, design methods, and construction techniques were developed. These recommendations are documented in the FHWA report, “Nonwoven Geotextile Interlayers for Separating Cementitious Pavement Layers: German Practice and U.S. Field Trials.” Also documented in the report are estimated material costs and the results from recent field trials in Missouri and Oklahoma.

According to the FHWA report, the installed cost per square yard of nonwoven geotextile ranges from $1.25 to $2.00. Subsequent to the report, the interest and use of this technique has rapidly grown in the United States, which in turn has fueled competition between geotextile manufacturers. This has led to even lower prices and a larger number of sources. When compared with the traditional HMA alternative, nonwoven geotextiles are therefore appealing for use as a concrete pavement interlayer.

Field trials in Missouri and Oklahoma demonstrated successes in incorporating nonwoven geotextiles in typical construction practice. The Missouri project, located on Route D south of Kansas City, implemented a nonwoven geotextile as part of a concrete overlay application. The project in Oklahoma, along Interstate 40, implemented the nonwoven into a new construction application similar to the common German application. In both instances, minimal adjustments to typical construction methods were introduced, and the contractors were pleased with the ease with which the nonwoven geotextile could be placed.

Recommendations for construction practices identified in the FHWA report were largely adhered to during both field trials. Some of those recommendations include placing and installing the nonwoven geotextile on a reasonably clean surface free of loose debris, securing the material to the underlying base material without any excessive wrinkles or folds, and minimizing traffic over the material after installation.

Placement of the nonwoven geotextile for the Missouri and oklahoma demonstration projects involved simple equipment and a small number of laborers to minimize wrinkles and folds.

Contractors deemed both projects successful, reporting reduced overall costs and ease in construction. Long-term success will be measured by pavement performance in the years to come. While there is no reason to doubt it, time will tell if the applications in the United States share the same success as the German experience.

The future
The innovative German technique of using nonwoven geotextiles as an alternative to HMA interlayers in concrete pavements is gaining momentum in the United States. Currently, there are more implementation projects underway throughout the country. Expectations are hopeful that nonwoven geotextile interlayers will become widespread among all states, particularly in the application of unbonded concrete overlays as a pavement rehabilitation technique.

As the United States develops its own expertise through continued implementation projects, further research, and continued communication with German experts, recommendations for material specifications and testing, as well as better practices for construction, will continue to evolve.

Nonwoven geotextiles can be a successful alternative to HMA interlayers as long as proper materials are selected and best-practice construction methods are adopted. German experience has proven that it works; FHWA supports it, and the concrete paving industry needs it.

References

  • Forschungsgesellschaft für Straßen- und Verkehrswesen Arbeitsgruppe Betonstaßen, 2001, Richtlinie für die Standardisierung des Oberbaues von Verkehrsflächen (RStO).
  • Forschungsgesellschaft für Straßen- und Verkehrswesen Arbeitsgruppe Betonstaßen, 2001, Zusätzliche Technische Vertragsbedingungen und Richtlinien für den Bau von Fahrbahndecken aus Beton (ZTV Beton StB), updated 2007.
  • Leykauf, G., and D. Birmann, Concrete Pavements with Nonwoven Geotextile Interlayer in Germany – Measurements and Long Term Behavior, Munich University of Technology, Munich, Germany.
  • Hall, K., et. al., 2007, Long-Life Concrete Pavements in Europe and Canada, Report FHWA-PL-07-027, Federal Highway Administration, Washington.
  • Rasmussen, Robert Otto, and Sabrina I. Garber, 2009, Nonwoven Geotextile Interlayers for Separating Cementitious Pavement Layers: German Practice and U.S. Field Trials, Federal Highway Administration, U.S. Department of Transportation.

Sabrina I. Garber, MSE, is project manager, and Robert Otto Rasmussen, Ph.D., INCE, P.E., is vice president and chief engineer for The Transtec Group, Inc. They can be contacted at sgarber@thetranstecgroup.com and rrasmussen@thetranstecgroup.com, respectively.


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