Recycled Materials in Roadbase

Enormous quantities of waste materials are generated every year in the United States. Research has shown that re-using these materials can reduce both the consumption of virgin materials and the cost of extracting and processing new raw materials. Recycling and re-using waste materials effectively in road construction projects can address public concerns about the vast quantities of useful materials being discarded and wasted.

In the past, the process of selecting which recycled materials were evaluated was somewhat random, and based largely on whether a material had been tried before or had an advocacy group. This research project endeavored to develop a comprehensive and systematic evaluation process that would ensure that the recycled materials with the most promise would be evaluated, saving time and resources.

After verification that a particular recycled material has the fundamental physical, chemical, and thermal properties required for use in a roadway construction project, other economic, societal, and environmental factors should be considered. This study provides a framework for doing so. For example, the benefits resulting from savings in extraction and processing costs for new materials, and the disposal cost savings realized through using recycled materials are two factors, which, when considered, provide a more complete analysis of which recycled materials have the most potential.

Objectives

The Center for Transportation Research (CTR) conducted study 0-1348, "Location and Availability of Waste and Recycled Materials in Texas and Evaluation of Their Utilization Potential in Roadbase," for the Texas Department of Transportation (TxDOT), the Texas Commission on Environmental Quality (TCEQ), and the Federal Highway Administration (FHWA) to determine the quantities of waste and recycled materials (WRMs) available for use in roadbase in TxDOT districts and to classify them by quantity, location, and type.

The objective of this research effort is to develop an evaluation method for WRMs potential use in roadbase construction projects. Sub-objectives of the project include determining the extent of WRMs available in Texas, identifying the rate at which they are generated, and identifying their sources. A final objective of the project is to develop trial specifications that can be used in constructing roadbases containing recycled materials. The waste and recycled material evaluation method described in this report will enable TxDOT to determine the potential for using a particular material in roadbase before that material is subjected to expensive and time-consuming laboratory studies.

Findings

Laboratory studies previously used to evaluate WRMs have not taken into account the socio-economic and environmental benefits of using these materials. The WRM evaluation method developed in this study considers these benefits, in addition to the technical and economic aspects of using WRMs for various engineering applications. An initial screening process was incorporated to eliminate materials with low or no utilization potential, allowing only materials with high potential to undergo extensive technical studies.

The study focused on seven materials used in pavements/roadbase in the United States. These materials include:

1. Reclaimed Asphalt Pavement (RAP): There are several established and viable processes available for recycling asphalt pavements; however, there is a need to standardize the design, construction, testing, performance, and quality of the evaluation processes.
2. Reclaimed Portland Cement Concrete (PCC): Analysis has concluded that, in addition to reducing the waste disposal problem, recycling PCC pavements is technically and economically feasible. However, further research is needed to address the performance problems of recycled PCC pavements and to refine the mix design and construction procedures.
3. Iron Blast Furnace Slag: Iron blast furnace slag has historically been used in the construction of highways. Iron blast furnace slag is defined as "the nonmetallic by-product consisting of silicates and aluminosilicates of lime and other bases." The use of iron blast furnace slag in various highway applications is economical and technically feasible; however, lingering doubts about its environmental impacts need to be investigated further.
4. Steel Slag: A by-product of the steel industry is formed when lime flux reacts with molten iron ore, scrap metal, or other ingredients charged into the steel furnace. During this process, part of the liquid metal becomes entrapped in the slag. Steel slag has been used for highway construction in asphalt mixes, pavement bases, shoulder fills, and ice control. However, one study has identified the leachates from this material as a potential environmental problem.
5. Coal Ashes: Coal burned at power plants for the generation of electricity leaves a residue known as power plant ash, including bottom ash and coal fly ash. Fly ash is used frequently in concrete.
6. Building Rubble: Produced in demolition, building rubble is generally a heterogeneous mixture of concrete, plaster, wood, steel, brick, piping, asphalt cement, glass, etc. Both research and experience in the use of building rubble indicate that it has potential for use as subbase and subgrade/embankment material.
7. Rubber Tires: An estimated 240 million waste tires are discarded annually in the United States. Previous experiments have been conducted using scrap tire pieces in asphalt paving products, subgrades, and embankments.

Implementation

1. After recycled materials have passed the initial technical screening, consideration of socioeconomic factors will provide a more complete cost-benefit analysis.
2. The evaluation method outlined in this report should be further developed and used to screen WRMs. Only then can there be assurance that WRMs with high potential will be systematically evaluated.
3. Draft roadbase specifications are being developed for using RAP, reclaimed portland cement concrete, and steel slag.

The contents of this summary are reported in detail in the Center for Transportation Research Report 1348-1, "Location and Availability of Waste and Recycled Materials in Texas and Evaluation of Their Utilization in Roadbase,", A. Saeed, W.R. Hudson, and P. Anaejionu, Preliminary Report Dated - October 1995. This summary does not necessarily reflect the official views of the TCEQ, FHWA, or TxDOT."