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Section 3: Geosynthetics

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3.1 Geosynthetics in Hot-Mix Asphalt (HMA) Applications

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 Geosynthetic Guidelines. 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.

3.1.1. 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 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.

3.1.2. 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 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.

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3.2 Geosynthetics in Pavement Bases (non-HMA Applications)

Geosynthetics are placed in pavement bases to perform one or more of the following functions:

3.2.1 Reinforcement

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. Geogrid has also been used in multi-layered repair of roadway embankment slope failures, and is described in the USDA publication 0577 1204 – SDTDC, “Deep Patch Road Embankment Repair Application Guide,” October 2005 (see Figure 7-3).

Repair of Embankment Failure using Geogrid. (click in image to see full-size image) Anchor: #IVTBFWQPgrtop

Figure 7-3. Repair of Embankment Failure using Geogrid.

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, the Maintenance Division, Pavement Asset Management Section, recommends that its application be viewed as an “insurance policy” rather than a “modulus- multiplier” or structure-reducing product.

3.2.2 Separation

Geosynthetics used for separation have classically been applied to prevent subgrade soil fines 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 retain moisture and significantly reduce the internal friction angle, rendering 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. Geotextile separators act to maintain permeability of the base materials over the life of the section, and they allow the use of more open-graded, free-draining base and subbase materials. Another form of separation is being increasingly explored in Texas where there is a high potential for reflective cracking originating in the subgrade or chemically-bound base/subbase. 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 plasticity index (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., full-depth reclamation/recycling [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.

3.2.3 Filtration

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.

3.2.4 Drainage

Geosynthetics used in pavement drainage have been limited to addressing problematic locations, typically in a reactive manner. Retrofitted pavement edge drains often used when the structure cross section changes transversely (e.g., rigid pavements with flexible pavement shoulders, widening using different structure) is an example of using geosynthetics to expedite lateral drainage of trapped moisture from within the pavement structure. Vertical moisture barriers using an impermeable membrane have been used to prevent moisture ingress through permeable seams from adjacent land into the roadway substructure.

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