A Comprehensive Study Of Recycled Concrete Aggregates As A Drainable Base Layer For Pavements
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The use of recycled materials in construction is becoming increasingly popular due to the shortage of raw materials. Coupled with the environmental concerns surrounding the quarrying of natural minerals, waste materials are being considered as viable replacement sources. By recycling waste materials, available landfill space is increased and costs of transportation are reduced. This dissertation study was conducted to evaluate the use of recycled concrete aggregates as flexible base as per the requirements of the Texas Department of Transportation (2004). Two sources of recycled aggregates and one source of natural limestone aggregate were collected from the Dallas/Fort Worth Area and used in the present study. The selected materials were evaluated in laboratory conditions, modeled in the use as base materials for flexible and rigid pavement structures, and compared in cost for their use in construction. Laboratory testing was conducted to assess the physical, mechanical, and flow properties of the materials. The physical properties testing provided characterization of the materials that were used to ensure specification requirements. The major testing program was conducted to determine the small strain shear modulus (Gmax) using bender elements. In doing so, the Gmax results were used to estimate the modulus of elasticity of the materials. Unconfined compressive strengths (UCS) were also determined for the materials. Both Gmax and UCS were measured at varying moisture contents at 7-day and 28-day curing periods. The moisture contents represented a dry, optimum, and wet of optimum conditions related to 95% optimum dry density conditions. Results from the study show UCS to be highest at optimum moisture content-dry density condition and to increase with curing time. Results from the study also show Gmax to decrease with an increase in moisture content and increase with curing time. Permeability of all three materials met the requirements for drainage of base courses. The recycled aggregates demonstrated a decrease in permeability with an increase in moisture content. Flexible and rigid pavement tables were prepared for varying traffic and materials conditions. The determined modulus of elasticity was employed into the design of the pavement structures. Based on the design of pavement structures for flexible pavements, base course layer thickness using recycled aggregates were larger in poor subgrade conditions and high traffic volume conditions. The rigid pavement design structures showed no effects due to base course layer thickness. The cost analysis for the use of recycled materials concurs that savings can be expected during the construction. Maintenance and rehabilitation costs were not included in the analysis as both types of materials showed similar properties and hence are expected to perform similarly. The use of recycled aggregates can produce positive impacts on engineering, economic, and social environments.