, Former Professor, Soil, Water & Climate
Because of the cost of virgin aggregates, federal and state agencies are encouraging the recycling of waste materials including materials in old pavements. This study assessed the suitability of several recycled materials relative to virgin aggregates for use as base and sub base materials. These materials included reclaimed concrete, recycled asphalt pavement, asphalt shingles, shredded tires, foundry sand, fly ash, and municipal solid waste (MSW) bottom ash. The study aimed at characterizing water retention, hydraulic conductivity, shear strength, resilient modulus, and leaching characteristics of several recycled materials used in roadbed construction. The research methodology consisted of measuring all relevant properties on samples of both pure materials and mixtures containing recyclable materials, packed to Proctor density at optimum water content. Regression relationships developed in this study showed whether hydraulic and mechanical properties of the mixtures could be predicted from their corresponding measurements of pure materials and other easily measurable properties such as particle size distribution, density, and amount of recyclable material. Leaching experiments evaluated the extent of heavy metal and other contaminants losses as a function of cumulative infiltration. Hydraulic, mechanical, and leaching characteristics were used to delineate the suitability of recycled materials in pavement construction. Except for slightly higher fine content in some RAP-aggregate mixtures, particle size distribution of all mixtures was within the Mn/DOT specification band for Class 5 materials. Water retention (pore size distribution), hydraulic conductivity, resilient modulus, and shear strength measurements were generally similar to that of 100% aggregates (with the exception of foundry sand). Heavy metal concentrations in the leachate were also generally less than the EPA drinking water standards. Researchers concluded that reclaimed asphalt pavement (RAP), fly ash (FA), reclaimed concrete material (RCM) mixtures would be good substitutes of virgin aggregates as base and subbase materials.