A REVIEW OF WATER MOVEMENT IN THE HIGHWAY ENVIRONMENT: IMPLICATIONS FOR RECYCLED MATERIALS USE

Significant amounts of recycled materials are being used in structural components of highways such as base, subbase, portland cement and asphalt concrete, and embankments. Past experience with recycled materials in highways has not shown a risk with respect to environmental impact and human health. However, environmental regulatory agencies frequently must assess risk to human health and the environment when conducting beneficial use determinations, particularly for new recycled materials. Contaminant leaching and advective transport are believed to be the primary transport pathways and both of them depend on in situ moisture conditions. The purpose of this study was to describe the state of the knowledge about water movement in the highway environment so that this knowledge may be incorporated into (1) practical leaching and preliminary impact assessments and (2) fate/transport models for use in risk assessment. Measurement techniques for moisture content, pore water pressure, and rainfall were discussed. For measuring in situ water content, time domain reflectometry (TDR) is a growing and promising technique. However, there is not a universal agreement on a TDR calibration equation. Laboratory techniques for measuring saturated hydraulic conductivity center on constant and falling head permeameter tests that should be conducted at low hydraulic gradients, and in addition, laminar and horizontal flow conditions. Hydraulic conductivity data for asphalt concrete, portland cement concrete (PCC), and base/subbase layers displayed wide scatter due to high variability in mixture designs and in measurement techniques. Factors affecting hydraulic conductivity are shape, size, and interconnectivity of air voids, and in PCC, the curing temperature and the type and extent of chemical reactions during hardening. Most of the evidence suggested that addition of recycled materials to PCC decreases the hydraulic conductivity after curing. Water enters pavements despite efforts to prevent it, but the extent of pavement deterioration can be reduced by proper drainage and maintenance. The major water ingress routes are infiltration through the pavement surfaces (through cracks and joints) and shoulders, melting of ice during the freezing/thawing cycles, capillary action, and seasonal changes in the water table. Most important water pathways that are discussed in greater detail in the literature are infiltration through cracks, joints and shoulders and drainage through edge drains. Groundwater level affects moisture conditions in the pavement and the subgrade if it is within approximately six meters from the surface. There is also literature on water pumping, temperature variations, geotechnical properties of base/subbase and subgrade layers as they relate to the water movement in the highway environment. Simplistic and more comprehensive approaches to modeling water movement in the highway environment were reviewed. Among all models presented, none closely matched the purpose of the present study.