By Robert Rasmussen and Sabrina Garber
Recent research published by the Federal Highway Administration (FHWA), “Nonwoven Geotextile Interlayers for Separating Cementitious Pavement Layers: German Practice and U.S. Field Trials”1 focuses on innovative applications for nonwoven geotextiles in concrete pavements.
In May 2006, representatives from FHWA, the American Association of State Highway and Transportation Officials (AASHTO), and the National Cooperative Highway Research Program (NCHRP) embarked on a tour to study European design and construction methods for long-life pavements. During the stop in Germany, the representatives were introduced to a method for using nonwoven geotextiles as an interlayer between cementitious layers. This practice was identified as a having potential for application in the United States. As a result, FHWA sponsored a research project to further investigate the German methodology and to develop initial U.S. materials and construction standards for similar applications in concrete pavements.
The use of geotextiles in the construction of concrete pavements in the U.S. is not a new concept, but use of a nonwoven geotextile between cementitious layers is. Geotextiles are commonly incorporated into pavement systems as a filtering layer between the base (or subbase if there is one) and the subgrade. German engineering, however, has demonstrated that nonwoven geotextiles can be placed between a concrete pavement surface and cement stabilized base. There, it provides a layer of separation, reduces water ingress, and minimizes bearing stresses. Typical U.S. practice for providing a separating layer requires a layer of thin hot-mix asphalt (HMA) be constructed between the concrete surface and a cementitious stabilized base layer. With more than 25 years of experience using nonwoven geotextiles as an interlayer, the German highway community is convinced that proper selection and placement of nonwoven geotextile interlayers will result in improved pavement performance.
Based on published German standards and specifications, discussions with leading experts, and consideration for U.S. testing methods, initial recommendations for material specifications and better construction practices have been developed. This information, along with a summary of recent field trials in Missouri and Oklahoma, is summarized below.
Recommendations for material specifications
Proper specifications for a nonwoven geotextile used as an interlayer between cementitious layers are based on optimizing pavement performance.
The material properties of interest, their function, and how they affect pavement performance include:
- Separation, which prevents the surface and base layers from bonding to one another and further helps to minimize discontinuities (cracks or joints) in the underlying cementitious layer from reflecting to the surface layer. To function as an effective separation layer, the material must possess a degree of resilience to accommodate the anticipated movements in the base layer. Effective separation between cementitious layers can promote durability and longevity.
- Drainage, which allows water to migrate away from the base and subgrade layers. Water that penetrates the surface and filters down to the interlayer should drain into the interlayer, and then proceed along the cross-sloped surface to the pavement edge. German practice requires that the geotextile either terminate next to a drainage layer or be daylighted (allowing the egress of water). The drainage function requires that the geotextile have enough permeability (and transmissivity) to allow a minimum flow rate in three dimensions. The benefit to proper drainage of infiltrating water minimizes loss of base and subgrade support reducing the potential for premature failures.
- Cushioning layerwhich reduces bearing stresses and the effects of dynamic traffic loads. This function also requires a geotextile material to meet specific specifications. The benefit of cushioning is increased longevity as a result of a reduced potential for premature failures due to heavy traffic loads.
Recommended material specifications for nonwoven geotextile interlayers used in concrete pavement are listed in Table 1. Figure 1 illustrates a typical example of a nonwoven geotextile interlayer material that meets these specifications.
Recommendations for better construction practices
Pavement construction using the nonwoven geotextile interlayer has been reported successful in U.S. field trials so far. This complements German reports of ease in construction. The following is an overview of some of the better construction practices, largely drawn from those reported in various German guidelines and publications.
To begin, before placing the nonwoven geotextile, the surface on which it will be placed should be clear of any loose debris. If the geotextile is used in an unbonded concrete overlay application, proper repair procedures must be performed to the existing pavement first.
The geotextile should be placed by rolling it out onto the underlying surface and pulling the material tight enough so that no wrinkles or folds occur, but not so tight that the fibers pull apart. The geotextile should be placed just in advance of paving, and construction and any other traffic that should traverse over it should be minimized and controlled. For example, tight turns and excessive accelerations and braking should be avoided.
To secure the geotextile, pins or nails should be punched through 50- to 70-mm (2- to 2.75-in.) galvanized washers or discs every 2m (6ft) or less. Overlaps in adjacent rolls should measure 20 ± 5cm (8 ± 2in.), and no more than three layers of material should overlap at any location.
The free edge of the geotextile should extend beyond the edge of the new concrete and into a location that facilitates drainage. The extent of this should be at least 10cm (4in.), which has been found to be enough to enable drainage away from the pavement system. While some German reports suggest the material should be moist before concrete is appled, this practice may not be necessary and may be a function of the anticipated potential for moisture loss from the mixture.
Periodic sampling and testing of the material should be performed to ensure quality of the product. For basic guidance on quality assurance testing, refer to “Nonwoven Geotextile Interlayers for Separating Cementitious Pavement Layers: German Practice and U.S. Field Trials.”1
Field trials in Missouri and Oklahomaincorporated a nonwoven geotextile interlayer. This section summarizes these trials.
Missouri, Route D, Cass and Jackson counties
During summer 2008, Clarkson Construction constructed a 5-in. unbonded overlay over an existing 22-year-old concrete pavement along Route D (Holmes Road) south of Kansas City, Mo. A nonwoven geotextile interlayer was specified as an alternative to the conventional 1-in. HMA interlayer to separate the existing concrete pavement surface from the proposed new concrete overlay.
Prior to placing the nonwoven geotextile, initial repairs were made to the existing pavement including patching, joint repairs, and removal of loose debris. The use of a flowable fill made repairs quick and easy. Figures 2 and 3 show the before and after conditions of the pavement prior to placement of the nonwoven geotextile.
The nonwoven geotextile materials were delivered to the project on 300-ft (91-m) rolls, 15ft (4.5m) wide, and placed using a telescopic forklift as seen in Figures 4 and 5. Product availability and the desire for trial sections led to the decision to use of two different nonwoven geotextiles for this project. A gas-powered system was used to drive the fasteners (collated pins) into 2.25-in. (57-mm) metal discs, through the geotextile, and into the underlying concrete pavement. This is illustrated in Figures 6 and 7.
Immediately before construction, the geotextile material was wetted using a tanker truck. The surface was left saturated, and it was later concluded that less water would have been better.
In construction of the overlay, no problems were reported with the geotextile during the paving process. In general, the project proceeded as it would have using a conventional interlayer.
One of the cores taken after construction is shown in Figure 8. The geotextile bonded with the new concrete, but not with the underlying concrete pavement. The lack of bond to the existing pavement indicated that the nonwoven geotextile was useful in separating the cementitious layers.
Upon initial inspections, the Route D project appears to be successful. The long-term performance of this section will be monitored.
Oklahoma, Interstate 40
In another case, a nonwoven geotextile was used as an interlayer to separate a new cement-treated base (CTB) from a new concrete pavement. The project was constructed in Oklahoma along Interstate 40 during the fall of 2008.
The nonwoven geotextile in this case was 5m (5.4yd) wide. The geotextile was fastened to the underlying CTB using a powder-actuated system with fasteners (consisting of integrated pins and washers), as shown in Figures 9 and 10. The washers used on this project are smaller than those recommended in German practice and also smaller than those used at the Missouri site. Therefore, additional care was taken to ensure that the geotextile did not tear free of these fasteners during subsequent concrete placement.
Overall, paving went smoothly with no changes to normal paving procedures. Figure 11 shows haul trucks maneuvering carefully on the geotextile, backing up to the paver, and placing fresh concrete onto the geotextile in advance of the spreader.
Figure 12 shows a core sample removed from the I-40 project after its completion. It shows the separation of the two cementitious layers, along with a slight bonding of the geotextile to the new concrete pavement.
Upon initial inspections, the project components in Oklahoma appear to be working well. The contractor said the process seemed to save time and money without compromising performance.
Conclusions and recommendations
Detailed investigations into the German practice of using nonwoven geotextiles as an interlayer between cementitious layers in concrete pavements resulted in the development of U.S. material specifications and construction standards.
Optimized fundamental properties of the nonwoven geotextile make it a likely candidate as a viable alternative to an HMA interlayer. These properties include separation, drainage, and reduced bearing stresses and their functions are realized best when measures are taken to properly prepare an existing cementitious layer before the placement of a nonwoven geotextile.
U.S. field trials in Missouri and Oklahoma proved a nonwoven geotextile interlayer is a viable alternative to more conventional interlayer materials such as HMA. Benefits identified because of the field trials include the following:
- Lower costs for material and installation when compared to conventional HMA interlayers.
- Reportedly equivalent performance, at least for separation of cementitious bases and new concrete pavements.
- Ease of installation, requiring a minimum of training and equipment.
- Rapid installation, with firsthand observations revealing installation rates exceeding that of paving.
Currently in the U.S., nonwoven geotextiles are not commonly used in concrete pavements as an interlayer between cementitious layers. The German experience, however, offers more than 25 years of working with this process and has demonstrated that it offers improvements if done properly. Drawing on German expertise, and incorporating U.S. innovation, the FHWA hopes to prove the processes right by promoting this innovative practice and encouraging contractors to try it for themselves.