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Petromat is a paving fabric that is used as a moisture barrier and stress absorbing interlayer beneath flexible pavements such as asphalt overlays or a chip seal. Petromat provides protection against moisture and cracking. It is a AASHTO/NTPEP compliant fabric that double the service life of a road.


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Consider this Propex mat For Your Moisture Barrier & Stress Absorbing Interlayer Underneath Flexible Pavements

This Propex product is a paving fabric used as moisture barriers and stress absorbing interlayers underneath flexible pavements such as asphalt overlays or a chip seal. It is unchallenged in the industry for asphalt pavement interlayer applications. It's AASHTO/NTPEP compliant and comes in a variety of styles to fit your specific project and specifications.

Here are the Benefits & Features

• Offers maximum protection against moisture and cracking.
• Creates a stress-absorbing interlayer to slow down the progress of reflective cracks.
• Will replace 1.5-inches of asphalt, saving around $50 per lane mile.
• Doubles the life of road service.
• Increases durability ad performance within rural chip seal pavement surface treatments.
• Reduces the requirements for additional maintenance.
• The width of one roll will cover one lane in a single pass.
• Easy to install, reduces time and labor costs.


• For asphalt pavement overlay
• For chip seal

About The Following Engineering Bulletin:

The Engineering Bulletin is a summary of a more detailed paper prepared for the Transportation Research Board or TRB, The original paper contains a compilation of research and references related to problems caused by pavement moisture and said paving fabric interlayer systems can be obtained from Propex.

The Purpose For Using Paving Fabric

This paving fabric system has been used for many years to boost the performance of asphalt concrete overlays. The system achieved this process through two mechanisms.

First, it acts as a stress absorbing interlayer to prevent the progression of cracks moving up through the overlay.

Secondly, the system forms a good moisture barrier to stop the infiltration of water passing through the pavement and into the base and subgrade materials. When there is less exposure to moisture, the base and subgrade will be stronger, increasing the support provided to the pavement and will improve the pavement's performance. The asphalt cement tack coat that is used in the installation of the paving fabric offers waterproofing and the fabric stabilizers. This holds the tack coat in place giving it the durability to provide waterproofing for many years to come.

Water Problems In Pavement

Research has shown over and over that two-thirds of water falling on pavement can seep through the pavement into the base and subgrade materials. In turn, excessive water will lead to poor support from the pavement. A lack of support can lead to excessive pore pressure inside the base materials, softening of cohesive subgrades, spurting of lines through cracks in the pavement, and erosion caused by the forming of ice, known as freeze-thaw.

To correct this, the most used pavement design approach, AASHTO, 1993 applies an increase of pavement thickness to make up for poor drainage conditions. AASHTO succeeds in doing this by using a drainage factor that measures a range from 1.2 to 1.4 for excellent drainage vs 0.4 to 0.95 for poor drainage. These coefficients are multiplied by the strength value of the base or subbase, improving or lowering the design structural strength of the layers. A free-draining aggregate base layer can be applied more than twice the structural design strength as the same thickness of an aggregate base that drains poorly. This explains the need for proportionately thick pavements where drainage is not properly applied.

Most base materials do not offer adequate drainage for pavement and the results in poor to only fair drainage conditions as defined by AASHTO. The limit in which a base will drain seems to be approximately 7% passing the #200 sieve. With content that is greater than this, the permeability of the base will drop off to levels that offer insignificant degrees of drainage. Because most bases and base courses that are being built have low permeability, the chances of improving support by improving drainage can be very difficult even if edge drains are used.

How To Improve The Drainage Of Pavement

When the existing pavement needs improvement, there are only a few approaches available. The overlay can be made thicker to offer a higher structural capacity or drainage can be improved to get more support from the existing structure. In some cases, retrofitting of edge drains can be used to improve the drainage of the pavement. This is achieved by essentially shortening the seepage path to get the water out of the base faster. To achieve this, is usually greater than what is available in the pavement structures. The Petromat paving fabric system is a reasonable alternative to increase the relative drainage of the base. Adequately reducing the amount of water getting through the pavement, will cut down on the length of time that the base will be saturated. Reducing the saturation time equates to improved support. Allowing less water to get into the base, the paving fabric moisture barrier accomplishes the same end results for improved drainage. It's keeping water out of the pavement layers. The use of a paving fabric membrane interlayer system offers the same AASHTO design benefits as improved drainage.

There are circumstances that would benefit from edge drains including where water is primarily entering the road base as groundwater from the roadsides or if the water table is extremely high and must be lowered. That said, most roads develop based on moisture issues are due to direct infiltration through the pavement.

Requirements for a hydraulic barrier within a pavement can be assessed based on standard infiltration rates.

FHWA Designing with Geosynthetics – Robert M. Koener 1994:

Koener observed the approximate time it would take to saturate a base. Even though any water in the base can weaken the subgrade soils, saturation of the base can cause greater structural failure. A pavement with a saturated base, even at 10% of the time, will only achieve 50% of the design life.

The rates for a typical pavement infiltration might be approximately 0.25” to 0.75” an hour. Taking into consideration the pavement's width, slope, base thickness, and the quality of the base porosity, saturation of the base material can take around 1 to 5 hours. Even at a low state or quality of the material allowing liquids to pass through the base could take anywhere from 60 days to over a year for the base to drain back down to 50% saturation. During that period of time, additional rain may re-saturate the base and many bases will never fully drain back to 50%.

A moisture barrier can reduce the infiltration rate by an order of magnitude, one-tenth the original would increase the length of time by 10 times as long. In turn, that would increase the length of time for the rain to saturate the pavement base by 10 to 50 hrs. It will become less likely that rainfall over that duration will recur frequently enough that the base cannot drain. In order to be effective, the moisture barrier should reduce the pavement permeability by at least one order of magnitude. Changes in the asphalt concrete mixes using polymers or rubber and increasing the densities can reduce the pavement quality of the material to allow liquids to pass through but only slightly. That said, it will fall short of effectively limiting infiltration.

There have been many studies performed in the lab as well as the field on the effectiveness of the Petromat paving fabric interlayer systems for lowering the permeability of pavements and the reduction of water getting into the base. There have been at least 4 studies in the laboratory to evaluate the reduction of water seepage through a pavement when paving fabric is being used. These studies continually showed that paving fabric interlayer systems reduced water seepage by one to three orders of magnitude.

Another study observed that where cracks do reflect through the overlay in asphalt pavements, the paving fabric will stay intact and offer waterproofing. A number of field investigations have shown moisture barrier improvements when a paving fabric system is used. A study in Texas in 1989, observed the performance of paving fabric in several locations and compared to control sections. One section located near Amarillo, five different paving fabrics as well as control sections for comparison were installed. After the rain, sections containing fabric showed less pumping was involved than control sections. This seems to imply that the subgrade support was better in paving fabric sections due to the lower base and subgrade moisture contents than in the control sections. Another study was performed in Oklahoma in 1996 to evaluate the effectiveness of drainable bases and edge drain systems. Five pavement sections were monitored for three years. The five sections of pavement had varying degrees of permeable bases and had differences in edge drain systems.

In the sections with free-draining bases, the flow-through from the pavement and out of the edge drains was up to 80% of the rainfall but normally between 20 to 40%. One of the pavement sections had a break and seat PCC pavement with a broken section averaging around 4 to 12-inches in size. Over the broken and seated concrete, a leveling course was in place followed by a Petromat system and surface course. The edge drains in this section of the road showed almost no response to rain. In 1997, the state of Oklahoma returned to this site to find out why water was not draining from the pavement. During their investigation, they cored through the paving fabric system to the top of the break and the seat base layer. A seepage flow test was run by pumping water into the hole to find out if it would flow to the edge drain system. The water flowed and the break and seat base was found to have an AASHTO drainage capacity of “Good”.

Because the base was drainable, the most probable reason why water was not flowing from the pavement after a rain, the paving fabric system was restricting the infiltration from reaching the base layer. When properly installed, paving fabric interlayer systems will keep the water from the base which should come in at least as Good to excellent AASHTO drainage classifications since there is limited water-dwelling time in the pavement base. That said, it's reasonable to apply a structural credit, normally used for improved drainage where a Petromat paving fabric system is being used.

At the Oklahoma DOT, it was decided that the incorporation of the Propex system underneath an overlay was more cost-effective to keep water out of the road base than installing road edge drains. The GPR, or ground-penetrating radar, was employed to detect the presence of moisture underneath the pavement with and without paving fabric membrane systems during an investigation in 1997, examining 2 roads in North Carolina. Each road had sections with and without the paving fabric membrane system. Microwave signals penetrated the pavement and the reflectance and absorption of these microwaves were monitored to indicate moisture. The results from the tests on both roads showed much higher moisture levels in the road base and subgrade in the sections without the paving fabric interlayer system. In another finding from these studies was the sensitivity of the paving fabric system with proper installation. In general, Propex paving fabric, a tack coat application rate of 0.25 gallons per square yard is recommended. This suggests that approximately 0.05 gallons per square yard will be absorbed by the existing pavement and new overlay. If a tack coat rate is only 0.20 gallons per square yard is applied to the pavement, the outcome of the tests will show that the fabric would be able to absorb only 0.15 gallons per square yard.

The state or quality of the material or membrane tests that were performed on the sections cut from the asphalt absorption tests is shown in Figure 1. Asphalt tack coat numbers in Figure 1 include the amount of asphalt absorbed into the paving fabric during these tests along with 0.05 gallons per square yard, which is usually required to bond the interlayer to the pavement layers. The tests showed that very little improvement in waterproofing can be expected until the tack coat is at levels over 0.21 gallons per square yard. With the tack coat levels over 0,.23 to 0.24 gallons per square yard, the paving fabric will start to reach the quality of the material or membrane of 10-5 mm/sec or less which will improve the waterproofing of the pavement.

Summing It Up

Many past studies have continually shown that the infiltration of rain through pavements is the main source of water entering pavement bases and subgrades. Quickly removing or preventing water from entering the base and subgrade, significant structural benefits can be applied and will substantially increase the pavement and overlay design life.

Both laboratory and field pavement cores show that having the presence of a correctly installed paving fabric interlayer system will reduce the permeability of the pavement by one to three orders of magnitude.

Pavement base drainage improvement is the only practicable option for rehabilitation if pavement bases have a permeability that is greater than 10-1 to 10-2 cm/sec. Since improvements to drainage is not usually a reasonable option, placement of paving fabric moisture barriers should be taken into consideration. Reducing the infiltration by one or more orders of magnitude, the system will be an effective, efficient moisture barrier to improve the performance of the pavement. In order to provide a continuous moisture barrier, a sufficient quantity of asphalt cement tack coat must be used to saturate the paving fabric and bond the interlayer system approximately 0.23 to 0.25 gallons per square yard.

Fewer quantities of asphalt cement will weaken the waterproofing effect. The tack coat must be uniformly applied. Quality control in field installation is critical for the performance of the paving fabric. Using Petromat in both new projects as well as rehabilitation projects and keeping with a good installation practice, substantial improvements in pavement's performance can be achieved from the moisture barrier function. Propex fabric interlayer systems should be designed into the new pavement structure to decrease water access into the base from the start. This is particularly true if an open, free-draining base layer and edge drain system is not included in the initial design. Including a paving fabric system will lead to better drainage coefficients as in the AASHTO pavement design methodology.

1. Marienfeld, M.L. and Baker, T.L., Paving Fabric Interlayer System as a Pavement Moisture Barrier, Transportation Research Board Paper # 981112, 1998.
2. Cedergren, H.R., Drainage of Highway and Airfield Pave- ments, John Wiley and Sons, Inc., New York, 1974.
3. AASHTO Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials

4. Ridgeway, H.H., Pavement Subsurface Drainage Systems, National Cooperative Highway Research Program Synthesis of Highway Practice 96, TRY, National Research Council, Washington, D.C., 1982