Recycled Materials in Vertical Moisture Barriers

Damage to U.S. roadways from expansive soils exceeds $10 billion annually. In roadway sub grades that contain clay or shale, an influx of water can cause the soil to shrink and swell, with accompanying pavement damage.

Vertical moisture barriers can control this destructive movement of the sub grade soil by reducing the influx of water. Current practice is to construct the moisture barriers using impermeable fabrics and granular backfill.

But several recycled materials - such as polypropylene and polyethylene plastics, and fly ash - can potentially be used to construct vertical moisture barriers. If successful, the barriers would reduce pavement distortion caused by moisture, while simultaneously making good use of large volumes of recycled materials.

Objectives

The Texas Tech University Departments of Civil Engineering and Chemical Engineering and The University of Texas at El Paso Center for Geotechnical and Highway Materials Research conducted Study 0-1354, "Recycled Materials in Vertical Moisture Barriers," for the Texas Department of Transportation (TxDOT), the Federal Highway Administration (FHWA), and the Texas Commission on Environmental Quality (TCEQ) to produce a rational design procedure and draft guidelines, specifications, and/or standards for statewide use of recycled materials in vertical moisture barriers.

This objective was accomplished by: 1) interpreting information acquired from a technical literature review to determine which, if any, recycled materials have been employed in the construction of vertical moisture barriers; 2) evaluation of the relative success of such applications; and 3) determination if any films or other materials constructed from recycled materials have the potential to be used in vertical moisture barrier applications. Limited laboratory testing was also performed to evaluate the potential applications identified in the technical literature search.

Findings

Two recycled-content materials were evaluated in this study: fly ash and plastic (polypropylene and polyethylene).

Three principal properties were evaluated with respect to recycling fly ash in a vertical moisture barrier application: strength (ability to resist cracking), permeability (degree of imperviousness to penetrating water), and environmental concerns (degree of heavy metal leaching from the in-place fly ash-sand-cement mortar).

Based on initial test results it is probably feasible to use fly ash/sand/cement mortar as a barrier backfill material. Tests showed that the best combination is probably 7 percent cement by dry weight, a 3:1 fly ash/sand ratio, and a water/cement ratio of 3.00. The mix showed maximum compressive strength (unconfined), which will help in reducing shear failure of the backfill material.

Based on the results of limited laboratory tests and small-scale extrusion tests, high-density polyethylene (HDPE) also has considerable promise as a recyclable material used to construct vertical moisture barriers. HDPE can be purchased in flake form, or it can be produced into flake form in the field. The flakes can be melted and then extruded in the field directly into the vertical moisture barrier trench. Because of the minimum trench width produced by currently available trenching equipment (8 to 12 inches), both the availability and cost of HDPE may limit its applicability in vertical moisture barriers.

Implementation

This study was constrained by limited performance time and limited laboratory testing and evaluation. No field testing was performed. Consequently, the following tests or testing programs should be conducted to learn more about the use of the recycled materials considered in this study.

1. Fly ash/sand/cement: Evaluate the potential for leaching of heavy metals, besides barium, typically contained in fly ash.
2. High-density Polyethylene (HDPE):
   1. Determine the number of heating-and-cooling cycles that recycled HDPE can be subjected to before it reaches an unacceptable level of flexibility/rigidity and loses its impervious properties.
   2. Locate a source of ground-up automobile tires less than 12.3 mm in maximum dimension, and conduct laboratory tests of a mixture of HDPE and ground-up tire particles. Evaluate the mixture for its value as a vertical moisture barrier material.

The contents of this summary are reported in detail in Texas Tech University Department of Civil Engineering and The University of Texas at El Paso Center for Geotechnical and Highway Materials Research Center Research Report 1354-1, "Recycled Materials in Vertical Moisture Barriers," Warren K. Wray, Vivek Tandon, Miguel Picornell, Raghu S. Narayan, and Natasha J. Mikel, Preliminary Report Dated - November 1995. This summary does not necessarily reflect the official views of the FHWA, TCEQ, or TxDOT.