Geosynthetics in HMA Applications
The primary purpose of incorporating geosynthetics in the pavement design process 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 HMAC.
One concern that the geosynthetic users should keep in mind is future rehabilitations as any anticipated milling of HMAC layers must avoid RAP contamination and possible fouling of milling equipment.
Geogrids. 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” 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 PCC slabs will probably not benefit. It is necessary to repair localized highly distressed/weak areas and apply a levelup course of HMAC prior to applying the geogrid.
Where rutting exceeding ½-inch exists, milling prior to applying the level-up should be considered. A minimum 2.0-inch surfacing course over the grid 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 levelup) 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.
Fabrics, composites, and membranes. 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 can not 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 insure laboratory testing is performed to determine HMAC stripping susceptibility of existing mixes (highway cores) and the proposed level-up and overlay mixes.
Geosynthetics in Pavement Bases (non-HMA Applications)
Geosynthetics are placed in pavement bases to perform one or more of the following functions: reinforcement, separation, and filtration. Base reinforcement results from the addition of a geogrid or composite at the bottom or within a base course to increase the structural or load-carrying capacity of a pavement system by the transfer of load to the geosynthetic material.
The primary mechanism associated with this application is lateral restraint or confinement of aggregates in the base. Where very weak subgrades exist, geosynthetics can increase the bearing capacity by forcing the potential bearing capacity failure surface to develop along alternate, higher strength surfaces. Geogrids may also be considered for use in locations where chemical stabilization of the subgrade is not desirable due to possible reaction with sulfates in the subgrade, or not practical because of expedited construction concerns, particularly in urban settings.
There have been assertions that the resultant increase in restraint or confinement should allow for design of thinner structures using these products versus structural designs which do not, however their benefits may only be noticeable over the long term and there appears to be an absence of long-term controlled monitoring. For purposes of geosynthetic reinforcement, CSP M&P recommends that their application be viewed as an “insurance policy” rather than a “modulus- multiplier” or structure-reducing product.
Geosynthetics used for separation have classically been applied to prevent subgrade soil from migrating into the unbound base (or subbase), or to prevent aggregates from an unbound base (or subbase) from migrating into the subgrade. A small amount of fines introduced into the granular base can significantly reduce the internal friction angle and render the flex base weaker. Potential for these circumstances increases where wet, soft subgrades exist. Typically a geocomposite will be used for this application, placed at the subgrade/unbound base interface.
Another form of separation is being increasingly explored where there is a high potential for reflective cracking originating in the subgrade or chemically-bound base. A grid or composite is used to dissipate stresses induced by the opening crack. Longitudinal edge cracking is particularly an issue in areas where moderate to high PI soils are exposed to prolonged cycles of wetting and drying. Geogrids will typically be employed at the subgrade/bound base interface, or if a flex base is placed above a bound base (e.g., FDR projects), the grid may be placed at this location. Grids should be a minimum of 10-ft. wide to reduce the potential for longitudinal cracking due to edge drying. The function of filtration is to allow for in-pavement moisture transfer but restrict movement of soil particles, hence composites or fabrics that are placed for the classical purpose of separation will usually incorporate this function as well.
Original article content and pictures contributed by TxDOT.