Under contract with the Geotextile Division of the Industrial Fabrics Association International, Maxim Technologies, Inc. investigated the use, performance, and design considerations for asphalt membrane interlayer systems utilizing a nonwoven paving fabric. This is a widely used technology with more than 15,000 lane miles installed each year in North America. Although performance of the system has obviously sustained this amount of annual usage, very little controlled testing and scientific verification has been performed on the system since its inception 30 years ago. Maxim chose to investigate the paving fabric interlayer system by reviewing all available literature on the subject as well as by interviewing many experts who have worked with the system for years. Enough data was available on which to form a strongly backed technical opinion on the effectiveness of nonwoven paving fabric systems. The results of this “expert system” analysis of nonwoven paving fabrics are presented in this report and summarized in this executive summary of the report.
The literature review examined more than 200 reports on the use and performance of nonwoven paving fabric interlayer systems. This effort was followed by Maxim’s personal interviewing of over 50 expert users of the paving fabric system. This data collection also revealed a data base of over 100 pavement sections on which performance of the system was monitored. The data search found 4 principal end use applications of the nonwoven paving fabric system: 1) Paving fabric with a chip seal over unpaved roads and subgrades, 2) Paving fabric and chip seal over existing AC (Asphalt Cement Concrete) pavements, 3) Paving fabric and AC overlay over existing PCC (Portland Cement Concrete) pavements, and 4) Paving fabric and AC overlay over existing AC pavements. Most applications included an asphalt cement concrete overlay (Conditions 3 and 4) and the comments of this summary are based on those conditions.
Performance of the nonwoven paving fabric system was generally measured by evaluating the condition of overlays placed over the system compared to overlays where the system was not used. This performance was reported in two ways. It was reported by comparing the performance of the paving fabric/overlay sections to control sections with no paving fabric or control sections with thicker AC overlays, for a given period of time. The second way the performance was compared was by looking at the difference in service life of the different overlay treatments.
As a result of our investigation we found that the inclusion of a nonwoven paving fabric interlayer system significantly improves the performance of AC overlays. As reported by the experts, this performance improvement is a result of both the waterproofing capabilities and the stress absorption capabilities of the paving fabric system. When comparing thinner AC overlays with the nonwoven paving fabric interlayer system to thicker overlays without the system, there was a good consensus among the literature and the interviewed experts. Their conclusion was that the paving fabric system gives additional overlay performance equivalent to increased overlay thickness of 1.0 to 1.8 inches with an average performance equivalency of approximately 1.3 inches. The paving fabric interlayer is not meant to be a structural layer to make up pavement structural thickness deficiencies. Also, this amount of AC overlay equivalency does assume that the existing pavement is stable, the paving fabric system is properly installed, and a minimum overlay thickness of 1.5 inches is placed. However, even at this thickness the inclusion of fabric has been shown to increase service life. In almost all reports where the paving fabric system was shown to be of limited or no benefit, either the covered pavement was too deteriorated or too thin of an overlay was used.
In northern climates, the recurrence of thermal cracking often occurred even over a paving fabric system. This is because the thermal expansion and contraction is occurring within the overlay itself causing cracking. Thermal cracking is not generally a reflective cracking. It was found, however, that although some thermal cracking returned, the pavement was still waterproofed to minimize freeze/thaw damage and overall pavement service life was improved.
The second phase of the project was to develop a pavement design model which is consistent with the actual monitored performance of the nonwoven paving fabric interlayer system. The design parameters are based on the field verified benefits of the system. These benefits are provided in two general ways, structural and environmental. The structural design parameters are based on how the use of a paving fabric system effectively improves the structural performance of a pavement system. This structural improvement is due to the waterproofing function of the paving fabric system and due to the stress absorbing interlayer function.
The effective strength of the road base components is improved by controlling the precipitation infiltration with the pavement moisture barrier paving fabric system. Both AC and PCC pavements are quite permeable and will normally allow up to 50% of all precipitation to infiltrate to damage the base and subgrade. Stopping the moisture infiltration achieves the same result as effective base drainage–to keep moisture from holding in the base. By maintaining a lower moisture content in the roadbase materials the effective strength or support provided by that roadbase is improved. This improvement is quantified in AASHTO’s Design for Flexible Pavements, 1993. Well drained (dry) bases provide up to 2.5 times the pavement support of poorly drained (wet) pavement bases. The waterproofing benefit of the paving fabric system was conservatively assigned design parameters comparable to a one level increase in drainage coefficients by the AASHTO design method. Increasing the effective strength of the road base materials can lower the strength/thickness requirements for the overlay.
The second structural benefit afforded by the paving fabric system is the stress absorption function it provides. An installed asphalt saturated nonwoven paving fabric layer allows for slight differential movement between the top of the old pavement and the bottom of the AC overlay. This retards the development of reflective cracking since some movement associated with old cracking will be absorbed by the paving fabric layer and not transferred up into the overlay. The creation of a layered pavement system with a paving fabric interlayer also minimizes harmful tensile stresses. Overlays directly placed on old pavement surfaces simply create a thicker monolithic pavement while overlays over a nonwoven paving fabric system allow a bonded but layered system with less tensile stress per layer. The paving fabric interlayer allows many times the amount of traffic loading flexures before pavement cracking occurs. This is similar to the flexibility of laminated timbers versus solid timbers.
The effect of this low modulus paving fabric interlayer was input into the structural analysis algorithm along with the increased moduli of the pavement base layers with lower moisture contents. The combined effect varied somewhat based on pavement design and thickness of pavement and base layers. However, the average structural effect on the pavement by the nonwoven paving fabric system is equivalent to placing an additional overlay thickness of about 0.8 inches. This effect ranged from about 0.5 to 1.75 inches in equivalent overlay thickness benefits.
Changes in environmental factors due to the presence of a nonwoven paving fabric interlayer system, supported the second area of design parameters verifying the performance of the system. There are numerous environmental factors which influence pavement design and pavement performance. The moisture control function of the paving fabric system affects the environmental parameters of; general subgrade moisture conditions, swelling soils and freeze/thaw susceptible soils. The control of these factors was input into algorithms which calculated the equivalent overlay thickness attributable to the paving fabric system. The results varied slightly due to the soil types, geographic environmental zone and depth to rigid layer, but the environmental equivalent thickness benefit is generally about 0.5 inches.
The effect of the non-woven fabric is therefore on average 1.3″ with 0.5″ environmental equivalent thickness benefit and 0.8″ structural equivalent thickness benefit. This compares well with literature review results. This is significant since the cost of non-woven paving fabric in place is approximately equivalent to 0.5″ of hot mix asphalt concrete resulting to a savings equivalent to the cost of at least 0.8″ of hot mix asphalt concrete.
The algorithms representing both the structural benefit and the environmental benefit of a nonwoven paving fabric system are additive and make up the basis for the design model. The design model agrees with the field performance data reviewed. Once this design model was developed and verified, a regression equation was created that could be easily adopted into a typical pavement management system. This equation is presented in the report. It is the professional opinion of the authors that nonwoven paving fabric systems provide a technically sound, economical option which should be considered when evaluating AC overlays. Therefore, it is an important input factor for pavement management and should be included in pavement management systems. The use of nonwoven paving fabrics beneath chip seals was reported by the literature and experts to give excellent results, but the benefit in this application was not quantified by a design model in this report.
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