Today’s buzzwords are sustainability, green construction, carbon footprint and greenhouse-gas emissions. The asphalt paving industry is well aware of these terms and their implications.
It is well recognized that materials like reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) and technologies like warm-mix asphalt (WMA) can reduce the industry’s carbon footprint. But what is the magnitude of the reduction in greenhouse-gas emissions and where do those savings occur?
Looking at the industry from a raw material extraction from the ground all the way to placement on the roadway through rehabilitation and maintenance, we can begin to understand the industry’s carbon footprint and where savings can occur. This idea of looking at the entire process is referred to as a life-cycle assessment. The specific aspects that are considered in the following are the extraction and refining of crude oil into asphalt binder, aggregate extraction and processing, transportation of raw materials, plant production, transportation of the hot-mix asphalt (HMA), and HMA placement.
Emission Reduction Opportunities
RAP is the most recycled product in the U.S. at 100 million tons. RAP can come from a number of different sources like roadway millings, broken asphalt and take backs during production. Generally RAP is processed to control material variability and obtain a maximum aggregate size. The asphalt binder content of RAP is in the range of 4 to 5% asphalt and is dependent on the RAP source and the degree to which the RAP is processed. The amount of RAP that may be introduced into new HMA is based on the agency and can be as high as 50%. RAP has the tendency to have a higher moisture content than virgin aggregate, and as a result production temperatures are generally higher than with conventional HMA in order to remove this moisture through convective heating.
There are two types of asphalt shingles: post-industrial and post-consumer. Post-industrial asphalt shingles are rejected asphalt shingles or shingle tabs that are discarded in the manufacturing process of new asphalt shingles. Post-consumer asphalt shingles is scrap derived from re-roofing projects whereby the old shingle layers are removed to prepare the roof surface for new shingles or other roofing materials.
Savings associated with RAS is highly dependent upon type. Asphalt binder content for shingles can range between 18% and 40%, which can significantly reduce the virgin asphalt binder demand of an HMA mixture. Although these asphalt binder contents are typical of solvent extractions, the effective contribution to the mix will be less. At this time, 14 state standard specifications or special provisions allow for up to 5% manufactured and/or tear-off shingles in HMA. RAS is introduced through the RAP collar and provides both asphalt binder and aggregate to the mixture. Plant temperatures may be increased in an effort to activate a larger percentage of the asphalt binder in the RAS.
WMA reduces the mixing temperature in comparison with that of HMA and can allow for higher percentages of recycled material like RAP and RAS to be used. This technology is in its infancy, so information on energy and emissions reductions is limited. Temperature reductions are largely technology dependent with the greatest reductions being recognized with the chemical and wax packages. These technologies constitute the greatest savings at the hot plant in terms of temperature reduction and fuel consumption. However, this does not consider the manufacture and transportation of the WMA technology. The temperature reductions for foaming WMA technologies are less than compared to chemical and wax technologies, but have the added benefit of only consuming water, which requires very little additional energy input. Because there are differences in the manufacture and transportation of the various WMA technologies, the environmental impact associated with each is not similar and must be evaluated on a technology-by-technology basis.
Asphalt burning
Based on review of available information, extraction and refining of crude oil into a ton of asphalt binder can consume between 600,000 and 4.2 million Btu of energy, and the amount of emissions range from 280 to 675 lb of CO2 equivalent (CO2e). The environmental impact associated with the production of asphalt binder will be dependent upon the crude oil source and technology employed to recover the asphalt binder. In terms of aggregate extraction and production, between 3 and 20 lb of CO2e are generated per ton of material. The amount of energy to process a ton of aggregate is dependent upon aggregate source, extraction (pit, quarry or dredge), crusher configuration, crushed product composition, support vehicles and stockpile management.
The environmental impact associated with HMA material transportation is largely dependent upon the distance over which it must be moved. When raw materials sources are great distances from the HMA production site, the environmental impact can be reduced by selecting modes of transportation that capitalize on the movement of large quantities of material and are fuel efficient. For comparison purposes, a semi-truck can move one ton of material on a gallon of diesel 155 miles, whereas by rail the same amount of material can be moved 536 miles. Regardless of the mode of transportation, diesel fuel will most likely be consumed, and the burning of one gallon will generate approximately 22.5 lb of CO2e.
The amount of energy to produce one ton of HMA is dependent upon the plant type, moisture content of the aggregate, the fuel source and a number of other factors. Typical energy consumption at the hot plant is between 250,000 and 320,000 Btu/ton of HMA. On an equivalent energy basis, if residual fuel were used to produce a ton of HMA, it would generate between 42.9 and 54.9 lb of CO2e (152,000 Btu/gallon and 26.1 lb of CO2e per gallon). If, however, the contractor were to use diesel, between 40.5 and 51.8 lb of CO2e would be generated (139,000 Btu/gallon and 22.5 lb of CO2e per gallon). Most fuels are selected based on cost and fuel value, however, emissions are quickly becoming a more important factor. Cleaner fuels will reduce the overall environmental impact by the hot plant.
During HMA placement, a variety of equipment and vehicles are employed. Each piece has a unique production rate, fuel-type requirement and horsepower, which influence the amount of fuel consumed and in turn the amount of emissions generated. Because each project is different, the equipment composition can change, and the environmental impact analysis would have to be done on a case-by-case basis. In a typical case there will be a paver, breakdown, intermediate and finish rollers along with a number of support vehicles, which on average generate about 7 lb of CO2e per ton of HMA.
Environmental savings
The environmental advantages of these technologies and materials should be evaluated in the same manner that roadway engineers use life-cycle cost analysis (LCCA) to identify the most economical option among construction, rehabilitation and maintenance alternatives. By coupling environmental impact and the price of construction into one analysis, contractors and agencies can identify material, construction and rehabilitation alternatives that meet the public’s desire for protecting the environment while considering their budgets.
Recycled materials like RAP and RAS derive their environmental benefit upstream of the hot plant. Both RAP and RAS contribute asphalt binder and aggregate to the HMA. This contribution reduces the amount of crude oil that must be extracted from the ground and aggregate from the bank. The environmental savings is largely based on the asphalt binder content of the RAP and/or RAS and its effective contribution to the mix and to a lesser extent the aggregate content. Asphalt binder content for RAP ranges from 4 to 5%, whereas for tear-off asphalt shingles the typical range is 32 to 40%. However, effective contribution is likely much less due to the stiffness of the asphalt binder. Usage of RAP and/or RAS results in very little change in plant emissions, although higher production temperatures are required and has been confirmed by the stack-emission factors reported in Chapter 11 of EPA AP 42.
All told, there is about 105 lb of CO2e produced to turn all of the raw materials into a ton of conventional HMA (0% RAP). The savings associated with RAP and RAS at typical asphalt binder contents in relation to conventional HMA is shown in Table 1. These savings can be further perpetuated through the life of the pavement if the HMA is recycled at the end of its service life whether it is in the form of RAP or by other processes like cold in-place recycling or hot in-place recycling.
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Every industry generates emissions, and the asphalt paving industry is no different. The green activities that are pursued today like RAP, RAS and WMA only serve to reduce the environmental impact, but we should understand the impact in relation to other industries and activities. For instance, in 2007 there were approximately 104 million head of cattle in the U.S. The average cow generates about 2,800 lb of CO2e per year, which led to an annual production of ~146 million tons of CO2e. By comparison, the asphalt paving industry produced about 500 million tons of HMA in 2007. That means the overall environmental impact is 28.9 million tons of CO2e (105 lb of CO2e per ton), far less than that of the U.S. cattle industry. The asphalt paving industry must understand its carbon footprint and the footprints of other industries so that the benefits of recycled materials like RAP and RAS and technologies like WMA can be fully appreciated.
