Geosynthetic products, defined herein as fabrics, grids, composites, or membranes, have been used by TxDOT since the mid-1980s. The primary purpose of incorporating geosynthetics in the upper pavement structure is to reduce reflective cracking in HMA overlays and to resist moisture intrusion into the underlying pavement structure. Geosynthetics can be part of an overall rehabilitation strategy that will, as a minimum, include the placement of a new wearing/surface course of hot-mix asphalt (HMA). TxDOT investigated these products in 2001 as part of research project 0-1777. One of the products from this research was the publication of . This document discusses the advantages of using geosynthetics in HMA applications, guidance on the selection of materials, cost considerations, pavement design, as well as construction considerations. One concern that the geosynthetic users should keep in mind is future rehabilitations, as any anticipated milling of HMA layers must avoid RAP contamination and possible fouling of milling equipment.

The main function of a geogrid in an HMA application is to retard the occurrence of reflective cracking. In evaluating the appropriateness of use, cracking in the existing structure should be limited to cases in which the crack faulting does not fluctuate significantly with traffic loading and crack width does not fluctuate significantly with temperature differentials. The pavement should be structurally sound, with existing cracks limited to less than 3/8 in. width. Hence, low to moderate levels of alligator cracking or random cracking may benefit from application of grids in HMA; whereas, widely spaced thermal cracking or underlying rocking/faulted Portland cement concrete (PCC) slabs will probably not benefit. It is necessary to repair localized, highly distressed/weak areas and apply a level-up course of HMA. Where rutting exceeding 1/2 in. exists, milling prior to applying the level-up should be considered. A minimum 2.0-in. surfacing course is recommended. Installation of this type of product has proven to be problematic and will result in premature failure (fatiguing) of the surfacing overlay where a lack of bonding (surface to grid to level-up) occurs. It is highly recommended that the manufacturer’s installation procedures be strictly followed and that a manufacturer’s representative be present during the planning and construction process.

These products provide a moisture barrier in addition to varying degrees of resistance to reflective cracking. Application guidelines are similar to those recommended above for the geogrid. The impermeable qualities of these products can be a double-edged sword in that they prevent trapped moisture within the structure from transpiring out. This may result in debonding of HMA layers and/or stripping of HMA layers below the product, especially if the lower mixes are moisture-susceptible. Also, if the surfacing overlay is permeable and surface moisture cannot readily escape the section laterally (mill and inlay technique is especially prone), stripping of the surface mix may also occur. It is incumbent upon users of these products to ensure laboratory testing is performed to determine HMA stripping susceptibility of existing mixes (highway cores) and the proposed level-up and overlay mixes.