Life-Cycle Environmental and Economic Assessment of Using Recycled Materials for Asphalt Pavements

Principal Investigator:

Arpad Horvath

Assistant Professor

Department of Civil and Environmental Engineering

University of California, Berkeley

215B McLaughlin Hall

Berkeley, CA 94720-1712

Tel.: (510) 642-7300

Fax: (510) 643-8919

horvath@ce.berkeley.edu

Funded by UCTC Year 14 Research Grant

Overview of the Research and Tasks

This research synthesized the state-of-the-art of asphalt pavement recycling in the U.S. Various data sources with volumes and rates of recycled aggregates have been compiled. Different recycling technologies and the practices applied by State Departments of Transportation in the U.S. have been surveyed. A literature review of the costs associated with the major recycling technologies has also been carried out.  The research results have been embedded in a computer tool that can be used to analyze energy consumption and environmental emissions associated with constructing and recycling asphalt pavements. Recycling of pavements represents an important opportunity to save the mining and use of virgin materials, and divert construction and demolition away from landfills. However, this research finds that energy savings and pollution reduction depend on the types of processes and transportation distances associated with the production of virgin aggregates versus the process steps involved in recycling pavements into reusable aggregates.

Key words: pavements, asphalt, life-cycle assessment, environment, energy, recycling, life-cycle costs.

Key Findings and Recommendations

Scale and Economics of Recycling Roadway Materials

We have surveyed the literature on secondary asphalt and other waste materials and byproducts usage. The asphalt pavement recycling rate in the U.S. (80%) is the second highest of all major commodities surveyed. By volume, about 100 million tons of asphalt pavements are recycled annually. Only reclaimed concrete comes close to this volume. There are significant potentials with the use of other materials in roadways. For example, 100 million tons of coal fly ash are generated each year, and their use in concrete and asphalt pavements, as well as embankments and flowable fill is expanding.

The feasibility of recycling is driven by the transportation between the material source and the construction site (table 1). Therefore, not only the choice of a recycling technology and its costs affect the economic performance of road construction and maintenance but also the transportation distances.

Table 1: Cost Breakdown of Different Layers in a Roadway[i]

In many cases, recycling can imply significant costs savings. For example, for cold-in-place asphalt recycling, savings of 30% to 60% are not atypical over the hot-mix asphalt process[ii]. In addition, a single lane requirement for this process minimizes road user inconvenience, and reduces user cost. For recycled concrete aggregates (RCA), operating costs range from $1.60 to $6.00 per ton, averaging $3.04[iii]. For recycled asphalt, they range from $1.30 to $3.65 per ton, averaging $2.29. Selling prices for RCA range from $0.75 to $15 per ton, averaging $4.93; they compete with virgin aggregates with a price range from $2.85 to $19 per ton, averaging $6.64. Still, transportation costs can significantly change the profitability of recycled aggregates, depending on the distance of the quarry to the job site. In areas where local aggregate shortages are being experienced (e.g., in Texas and California), using recycled materials competes favorably with virgin materials transported in from a large distance.

Environmental Effects of Substituting Recycled for Virgin materials.

Literature has been surveyed and data have been collected on the material inputs of pavement construction, estimated energy use and emissions associated with mining aggregates, producing roadway materials, constructing, maintaining, and recycling asphalt pavements (including various equipments and materials). Environmental effects (energy consumption, carbon dioxide (CO₂), sulfur dioxide (SO₂), nitrogen oxides (NO_x), particulate matter less than 10µm of diameter (PM-10), and carbon monoxide (CO)) have been traced through the related life-cycles and supply chains using life-cycle assessment (LCA) methods.

Table 2 shows energy consumption estimated for the operation of various recycling equipment. It is evident that some recycling technologies are more energy intensive than the others.

Table 2. Energy demand of Recycling Operations (for 1-4 only energy associated with equipment used is reported)[iv]

The energy consumption of the equipment used in different recycling processes is only one of the components in the comparative assessment of recycled versus virgin materials. An LCA approach should also involve the investigation of the energy consumption during mining, manufacturing, transportation, maintenance, and final disposal of materials.

Asphalt pavements should be analyzed as “perpetual pavements,” with regularly scheduled maintenance actions and a predetermined planning period. Recycling is beneficial if the source of virgin aggregates is far away from the site where they are used. Energy use of and emissions from equipment used to process and produce recycled aggregates are minimal compared to the impacts from production and delivery of virgin aggregates.

Develop a Computer-based Decision Support Tool for Asphalt Pavement Recycling

We have developed a computer-based decision-support tool (in MS Excel) to analyze the use of various recycled materials in asphalt pavement and road base construction, and compare the life-cycle economic and environmental outcomes associated with the use of virgin and recycled materials in roadways.

The tool calculates the environmental effects from initial construction and maintenance of pavements based on the use of various materials, their corresponding transportation distances, and the use of equipment for a particular process.  Energy consumption, and emissions of carbon dioxide (CO₂), sulfur dioxide (SO₂), nitrogen oxides (NO_x), particulate matter less than 10µm of diameter (PM-10), and carbon monoxide (CO) are quantified. The tool draws on LCA procedures that capture impacts from every material and energy input during the service life of the pavement. The method evaluates the performance of structures considering all stages of their service-life, including raw materials manufacturing, construction, maintenance, and end-of-life.

In order to assess the effects from the use of aggregates, the tool estimates emissions from transportation of the materials used in hot mix asphalt, the operation of the mixing plant, and the production of virgin and recycled aggregates. The tool is designed to assess and compare both on-site and off-site recycling technologies. Final results are given both in the form of both tables and graphs, which allow a clear portrayal of emissions over the life-cycle of the analyzed roadway.

The LCA of materials used in pavements is based largely on an input-output analysis-based approach that describes the entire U.S. economy by roughly 500 sectors in a square matrix. The matrix has one row and one column for each sector, and each intersection between a row and a column represents the economic transactions in dollars between the two respective sectors. Thus the matrix represents total sales from one sector to others, purchases from one sector, or the amount of purchases from one sector to produce a dollar of output for itself. All relationships embedded in the input-output table are linear, so effects of a $1,000 purchase from a sector is tenfold greater than the effects of a $100 purchase from the same sector.[v] Because EIO-LCA emission factors are available in metric tons per dollar of sector output, the present tool uses average national prices in $/metric ton for each material in the U.S.[vi] to calculate emissions per mass of material used. For instance, air emissions from asphalt manufacturing are calculated with the sector “Asphalt paving mixtures and blocks”, and an average price of $29/metric ton; for concrete, the sector is “Ready-mixed concrete”, and the price $29/metric ton; for steel, the sector is “Blast furnaces and steel mills”, and the price $300/metric ton; finally, for aggregates, the sector is “Sand and gravel”, and the price $6/metric ton.

The tool also assesses impacts from the construction and maintenance phases of pavements using process-based LCA. The inventory of different steps of a given maintenance activity is carried out with the identification of the equipment used in each one. As a result, emissions from the use of equipment and manufacturing of construction materials are captured.

In contrast to infrastructure facilities, which despite long service lives become obsolete and need to be demolished[vii], the emphasis with pavements is to extend the lifetime of the aging infrastructure[viii]. Pavements are perpetually reconstructed, and their life-cycles run over decades. Therefore, the tool allows the assessment of a sequence of multiple cycles,  materials manufacturing, construction, maintenance, reconstruction, materials manufacturing, maintenance, etc. The tool can be customized by the user so that roads are analyzed over a given time period (planning period) in which several maintenance or reconstruction activities might occur. At the end of the period, the road is still functional[1]. By playing different scenarios, the user can compare different options for roadway planning, and find out which scenario is environmentally less damaging. For example, the user can compare alternative pavement designs over the same functional period, impacts associated with specific manufacturing, construction, and maintenance options for each alternative, etc.

When starting with the tool, the user describes the design of the pavement to be built or retrofitted: length, width, depth and type of the different layers, including the base. Multiple designs may be specified at this stage. The serviceability of the pavement or duration of the analytical period is also chosen in the first step. Roads can be specified to utilize various percentages of recycled materials or byproducts. The tool then calculates the air emissions and energy consumption associated with the manufacturing of all materials (asphalt for the top layers, but also aggregates for the base). Such emissions factors are estimated for the whole process, from the quarries to the asphalt plants, and include the entire supply chain. The tool yields emission and energy factors for the construction processes and the transportation of the materials to the construction site. The user also selects the distance between the construction site and the asphalt plant.

Finally, the tool presents emission and energy consumption factors for the typical maintenance treatments available: cold in-place recycling (CIR), hot in-place recycling (HIR), hot mix asphalt (HMA) overlay, full depth reclamation, etc. Each one is customized to the needs of the user: depth of treatment, depth of new overlay, emulsion amount, life expectancy before next maintenance, and percentage of recycled material if applicable.

In the cost calculation sheet, the user enters the life-cycle costs of the specified construction and maintenance scenario(s), and the tool calculates net present value and annual emissions to facilitate equitable comparisons.

Interpret and Assess Uncertainty of the Results

This task permeates all the above tasks. The purpose of the decision-making tool is to carry out comparative analyses of different construction and maintenance options for asphalt pavements. The tool was developed in a spreadsheet and allows for sensitivity analyses and the customization of various variables according to the judgment of the analyst. The transparency in the tool allows for a better understanding of the effectiveness of single parameters in the final results, and contributes to the assessment of uncertainties. The tool has been reviewed by a group of experts to assure the choice of representative parameters.

Papers to Date:

A technical report has been prepared, and papers are being prepared on the research results for targeted publication in Environmental Science & Technology and J. of Construction Engineering and Management.

Conference Presentations:

• Horvath, A. (2003), “Decision-Support Tool for Life-cycle Assessment of Pavements.” Transportation Research Board Summer Workshop “Beneficial Use, Sustainability, and Pollution Prevention in Transportation Infrastructure,” Portsmouth, NH.
• “Life-Cycle Assessment of Pavements.” Beneficial Use of Recycled Materials for Transportation Applications Conference, Washington, DC (November 2001)
• “Environmental Assessment of Asphalt versus Concrete Pavements.” 2^nd International Symposium on Transportation Infrastructure Management, Berkeley, CA (October 2001)

References