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What makes a geosynthetic effective in asphalt overlay reinforcement? Introduction to Asphalt Overlay Reinforcement Paved road surfaces must be maintained when they develop significant cracks and potholes. The rehabilitation of cracked roads by simple overlaying, or placing an additional layer of asphalt over the old paved surface, is rarely a durable solution. The cracks in the old pavement eventually propagate through to the new surface. This is called reflective cracking.

In spite of limited resistance to reflective cracking, traditional overlays are still the most common approach to maintaining distressed pavements. Generally, the thicker the overlay the longer it will last. However, thicker overlays are proportionately more expensive.

A geosynthetic can be placed over the distressed pavement prior to the overlay to create an overlay system. The geosynthetic is placed on a liquid asphalt "tack coat" which is sprayed on the old road surface to enhance the bond between the old and new pavements. The resulting geosynthetic interlayer, impregnated with liquid asphalt from the tack coat, can enhance the life of the overlay via stress relief and/or reinforcement.

A stress relieving geosynthetic retards the development of reflective cracks by absorbing the stresses that arise from the damaged pavement. And, since it is impregnated with tack coat, it also prevents seepage through the pavement when the old cracks eventually reflect through.

Reinforcement occurs when a geosynthetic is able to contribute significant tensile strength to the pavement system. The reinforcement attempts to prevent the cracked old pavement from moving under traffic loads and thermal stress by holding the cracks together.

Newly manufactured geocomposites can provide both stress relief and reinforcement.
The benefits of geosynthetic interlayers include:
· Saving 1 to 2 inches of overlay thickness
· Delaying the appearance of reflective cracks
· Lengthening the useful life of the overlay

Paving fabrics are nonwoven geotextiles that have relatively high elongation and low tensile strength. They are commonly used for stress relief. When impregnated with tack coat, the paving fabric allows considerable movement around a crack but nullifies or at least lessens the effect the movements have on the overlay. This type of interlayer also waterproofs the road structure.

The crucial parameters in a stress relieving overlay system are the overlay thickness and the tack coat application rate. The addition of the interlayer allows the overlay thickness to be reduced by as much as 1.3 inches; however, the overlay must be at least 1.5 inches thick. Tack coat application rates vary with temperature and site specific conditions. Too much tack coat can cause the liquid asphalt to bleed up through the overlay; too little will cause the overlay to slip and will not waterproof the structure.

* MARVs ** product specific certification

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Reinforcement

To reinforce, a geosynthetic interlayer has to hold the underlying crack together and dissipate the crack propagation stress along its length. If the interlayer elongates under stress, it will allow the crack to open. Consequently, the interlayer must have high axial stiffness so that little elongation takes place. In order to stop the crack from opening, the interlayer also needs to control horizontal movement of the asphalt. A stiff interface between the interlayer and asphalt especially the underlying pavement is needed to do this. The resistance of the reinforcement and the asphalt to move relative to each other is called pullout. The interlayer should have appropriate strength/stiffness and pullout resistance.

To-date both laboratory simulations and large-scale field trials have been used to evaluate potential overlay reinforcement systems.

A Proposed Alternative To Full-Scale Tests

TRI has recently completed an effort to quantify the stiffness and pullout characteristics of asphalt reinforcement geosynthetics using modified index tests and a beam flexure test. The beam flexure test uses out-of-plane cyclic loading to quantify fatigue resistance, or flexural improvement. The test program used standard asphalt mix, AC 10 and 20 asphalt cement tack coat, and a range of geosynthetics. The measured level of flexural improvement was then correlated to more easily measured and specified index properties. Admittedly, the beam flexure test is NOT a performance test. Yet, it is a reasonable measurement of the improvement provided by asphalt reinforcement, and thus, it is likely to be a reasonable indicator of performance and may provide a bridge of understanding between pure index properties and the geosynthetic-asphalt system being modeled in large-scale tests.

Product Testing And Correlation To Performance

Results from the various tests were compared. The following points summarize the findings of the comparative testing:

1. There is no meaningful correlation between asphalt flexural reinforcement as measured via beam testing and flexibility/rigidity, aperture size, percent open area, ultimate junction strength, ultimate wide width tensile strength/stiffness, or interface friction.

2. Conversely, there appears to be significant and meaningful correlation between geosynthetic flexural reinforcement as measured via beam testing and the following properties:
· Initial Junction stiffness
· Initial Pullout force
· Initial wide-width tensile strength/stiffness

Recommendations

Table 2 summarizes the recommendations resulting from this program.

Table 2. Relevant Properties for Asphalt Reinforcement

Reinforcement Characteristic	Property Measurement	Test Method
Flexural Improvement	Flexural Factor	Beam with Cyclic Loading
Reinforcement Stiffness	Junction Stiffness @ 0.02 in. Elong. / Area per Junction	GRI GG2 - modified
Reinforcement Stiffness	Tensile Stiffness w/o Preload @ 0.1% Strain	ASTM D 4595 - modified
Asphalt/ Geosynthetic Interaction	Pullout Force per Area @ 0.1 in. Elongation	ASTM D 5321 + Demo 82 Calculations

Description of Test Methods

The beam flexure test is briefly described here. The other tests are detailed in Lab Update Vol. 01, No. 16.

Beam Test: A series of both reinforced and unreinforced 3-inch deep asphalt beams are exposed to cyclic loading using loads scaled to simulate field loading. Three load increments are used and cycles to failure (full crack propagation) are recorded. The ratio of cycles to failure for the reinforced to the unreinforced condition is termed the flexural improvement factor.