Geosynthetics as Separators and Interlayers

Analyzing Life Cycle Cost-benefits of Pavements Incorporating Geosynthetics as Separators and Interlayers


The proposed report will address key elements utilizing geosynthetics in the roadway pavement systems that affect long-term performance, including separation/drainage and reflective crack reduction/moisture intrusion prevention. Pavement system design for long-term performance and longer life cycle pavement preservation techniques continues to be a high priority in Departments of Transportation across the country. 

The key element in separation/drainage utilizing a geotextile is to maintain the independent integrity of subgrades from the overlying base or sub-base while continuing to provide adequate internal pavement section drainage. The separation/drainage benefit will reduce long-term maintenance costs as a result of increased life and improved pavement performance.

The key elements in reflective crack reduction/moisture intrusion prevention utilizing a geosynthetic interlayer are to reinforce the asphalt overlay, retarding reflective cracking and to provide a barrier to further moisture intrusion from the surface. The reflective crack reduction/moisture intrusion prevention benefit, by lengthening the life cycle of the overlay and reducing the continued deterioration of the subgrade, will increase the lane miles of rehabilitation available for repair.

Industry background

Geosynthetics are available in a wide range of forms and materials and are used in many applications, often by highway agencies, in conjunction with unbound base layers (i.e., within the layer or as a subgrade/base interface layer) and as an interlayer reinforcement/moisture barrier with asphalt overlays, as a means for enhancing the performance of flexible and rigid pavements. Although a great deal of research has been performed on the properties of these materials and their use in pavement structures, limited reporting has summarized life-cycle cost benefit influence on pavement performance in a manner that would quantify the payoff obtained by the influence of geosynthetic.

Research problem statement

The benefits realized by the utilization of geotextiles as separators in roadway construction have been qualitatively proven through their performance both during and after construction. However, there is a need to quantitatively evaluate pavement designs utilizing geotextiles to ascertain their cost-benefit. Geotextile separators have been used empirically with great success over the past 30 years in the civil engineering industry, yet the design engineer has understandably been wary of using geotextiles in applications where very little analytical data is available to substantiate their use. The function of the separator in roadways is to prevent the ingress of fines into the sub-base, therefore maintaining the original aggregate thickness, and to allow the transverse drainage of excess water. A geotextile separation layer reduces maintenance costs, enables better compaction during the construction stage and the required base course strength can be achieved using less aggregate (because less aggregate is lost into the poor subgrade). This can be achieved because in the case of paved roads many of the empirical pavement design methods have included additional aggregate to compensate for the losses. The geotextile acts as a filtration layer and prevents the ingress of fines from the subgrade, which would otherwise reduce the quality and bearing capacity of the layer above. There is tremendous potential for technical improvement and improved long-term performance resulting in cost savings on a nationwide basis through the use of geotextiles, but only limited adoption of geotextiles in pavement design occurs because the cost-benefit has not been demonstrated. 

Premature cracking of asphalt concrete overlays is a serious national problem. State DOTs have employed a number of different techniques in an attempt to delay premature cracking of pavement overlays. Techniques that have been tried include the use of paving fabrics, geogrids, special additives in the asphalt concrete and the use of selected types of asphalt. There are no proven design models available to DOT designers to provide guidance for selection of the most appropriate crack mitigation technique. The objective of the NCHRP project 1‐41(FY3002), Selection, Calibration, and Validation of a Reflective Cracking Model for Asphalt Concrete Overlays, is to validate mechanistic‐based model for reflective cracking in AC overlays. However, the influence of techniques to mitigate premature cracking is not specifically addressed. Many state and local transportation agencies continue to use these crack mitigation techniques with little guidance on the cost benefit of preventing or delaying the development of reflective cracking. The engineer must use past experience and judgment often based on field experiments conducted locally. There are a wide variety of costs associated with using crack mitigation techniques. A research study is needed to quantify these costs related to the long long‐term effectiveness of these techniques.

Pavement problems related to the presence of too much water have traditionally been addressed through drainage of the pavement system. However, drainage is often ineffective when there are low permeability bases, lack of adequate water collection, and when water is frozen or trapped by frozen layers. Previous research indicates that in most roads, the primary source of water in upper pavement sections is from precipitation that has infiltrated through the pavement surface. A possible solution to the related damage would be to place a moisture barrier within the pavement to stop the water from entering the pavement section. Pavement interlayers have been investigated and shown to be effective pavement moisture barriers in TRB electronic circular EC006. Most moisture barrier research has been done inside agencies and has not been published. Research is needed to specifically investigate the aforementioned effectiveness of the moisture related pavement problems. If proven an effective deterrent against moisture related problems, the cost effectiveness of interlayer systems could be quantified for agency implementation.

Report objective

The objective of the report is to detail the long-term performance of pavement designs which incorporate geosynthetics in the pavement section as a separator between the subgrade and base or sub-base material and as an interlayer reinforcement/moisture barrier and reflective crack reduction in an asphalt overlay. This performance will be evaluated on the basis of determining the extended life of pavement sections incorporating geosynthetics, the reduction of maintenance required during the pavement design life, the speed of construction and the cost-benefits compared to alternatives such as increased thickness in pavement sections, sections without geosynthetics and various pavement rehabilitation techniques. This report will also identify the most promising existing techniques for delaying reflection cracking and detail cost benefits of interlayer reinforcement/moisture barrier techniques.

Elements of the report

  • Compile a list of existing sites and available data on the cost-benefits of sites using geosynthetics in the separation and interlayer applications.
  • Perform a thorough literature search to identify geosynthetic reflective crack mitigation techniques. Interaction with the FHWA Long-term Pavement Performance Program may assist in this and the other phases of the project.
  • Perform a survey of state, county, and municipal transportation agencies to determine types of separation and interlayer techniques used, site condition, maintenance history, performance and costs associated with them. 
  • Existing test sections that included performance monitoring should be identified. A database of sites should be developed for performance validation. 
  • Evaluation to the extent of the cost benefit/extended life cycle due to the use of geosynthetic separators and interlayers in existing pavement systems.
  • Evaluation to the extent of the cost benefit comparisons of geotextile separators verses alternative methods of maintaining base course structural strength, such as, but not limited to, additional aggregate thickness.
  • Evaluation to the extent of the cost benefit comparison between geosynthetic interlayers verses alternative methods of providing moisture barrier and crack mitigation/reflective crack reduction within the pavement layer.
  • Evaluation may include obtaining additional updated data from appropriate field sites. State pavement management systems may be able to provide updated data.
  • Analysis of the extent to which the uncertainty of the continued use of geosynthetic separators and interlayers in roadway construction may result in higher costs, delayed completion and lower quality of pavements.
  • Analysis of the extent to which any modifications to existing laws or regulations may be necessary to expedite the efficient use of geosynthetic separators and interlayer systems to insure the long term performance and increased life cycle of pavements.

Urgency, payoff potential and implementation

The benefits of this research can be measured by reduced long-term maintenance and pavement rehabilitation costs as a result of increased and improved pavement performance. Transportation agencies are constantly attempting to extend the life of existing pavements to stretch limited funds. Reflective cracking has been identified as one of the distresses that reduce pavement life. To combat this distress, agencies continue to spend taxpayer dollars on techniques that attempt to prevent premature cracking. The costs associated with using these methods have not been effectively quantified. As serviceability of the highways continues to decrease and construction and maintenance funds continue to shrink, the need for extending the life of existing pavements becomes extremely important. This project will help identify justifiable levels of additional expenditures for the additional pavement life gained from delaying reflective cracking and prevention of moisture penetration. It will also provide guidelines for the most appropriate techniques available depending on pavement types and conditions. It will also identify techniques that are destined for failure and thereby prevent wasting valuable funds.