Unless you’ve been living in a cave for the past 20 years, you know that the United States has a major solid waste disposal problem. Every community, no matter how large or small, is concerned about landfill space and the scarcity of it.
The United States generates 4.5 billion tons (4.1 billion t) of waste per year, or 15.4 tons (14 t) per capita — that’s for each man, woman and child in this country.
Of all the waste we generate, mining and mineral waste makes up 41 percent of the total. Domestic waste is only approximately 4 percent; industrial is 7 percent and the remaining 48 percent is agricultural waste that we generate as a nation.
EPA reported that municipal solid waste (MSW) increased 50 percent from 1980 to 2003. This represents 4.5 lbs. (2 kg) per person per day with only l lb. (0.4 kg) per person per day being recycled.
All these waste materials come from a supply of raw materials, from manufactured goods, and from distributing finished products and consumer products. So there are a lot of different avenues through which waste products get into the overall waste stream.
Where does all the waste go? Some of it goes to a landfill, some to incineration, and some to recycling.
Recycling by EPA’s definition refers to the waste product going back to its original use, like asphalt going back into asphalt. Reuse refers to a waste material being used in another product, such as tires into rubber molding products.
The hot-mix asphalt (HMA) industry has been recycling its own product for approximately 40 years. RAP, or reclaimed asphalt pavement, is the recycled asphalt product that is routinely used. The HMA industry produces more than 99.2 million tons (90 million t) of RAP annually with more than 80 million tons (73 million t) that are recycled.
According to EPA’s data, this amount of recycling is more than all other MSW combined. Obviously a significant percentage of RAP that is reclaimed does get recycled.
By recycling HMA, we save landfill space, and we’re recycling a resource that has value both from the aggregate perspective and from the asphalt cement perspective.
The HMA industry has been asked to incorporate a wide variety of waste materials into highway projects — everything from toilets to tires.
These materials include:
• blast furnace slag,
• coal bottom ash,
• coal boiler slag,
• foundry sand,
• mineral processing wastes,
• municipal solid waste,
• combustor ash,
• nonferrous slag,
• roofing shingle scrap,
• scrap tires,
• steel slag, and
• waste glass.
Many of these products (such as foundry sand, shingles and tires) are very similar to materials already in use in HMA and present little concern.
Other products (such as combustor ash and glass) may present some issues with which the contractor must contend in order to make the material useable.
Some of the concerns industry has in terms of waste fall into the three Es — engineering, environmental and economic.
In the engineering sense, we want to make sure that all materials going into hot-mix asphalt produce good quality hot mix. When we use virgin materials we have very tight specifications that must be met — and that shouldn’t be different when we use waste materials.
The waste material should meet appropriate specification requirements as well as the virgin materials do. The word “appropriate” is important because conventional aggregate requirements may not provide the necessary evaluation of the material.
So we’ve got to engineer whatever waste material that is being proposed for it to be used properly. We’ve got to understand the properties of the total material, and we can’t use products that are going to diminish the quality of the final hot-mix asphalt.
With the advent and now common use of Superpave and SMA mixes, the evaluation of the final HMA product is even more important. The mixture produced is required to meet high standards of engineering quality to achieve long-term durability and performance.
The focus of this article is to raise issues to be addressed in the process of deciding whether to use, and how to use, any waste product in hot mix asphalt. For some of these waste products, many questions remain to be answered.
Engineering Concerns
In the engineering sense, there are concerns about special requirements for waste transportation. Any potential hazard of the material needs to be understood. As part of their site management responsibility, contractors must be sensitive to containment or runoff control of potentially hazardous materials.
Over time, contractors don’t want to have anything on their site that’s going to deteriorate, degrade or leach out in some way, or create a by-product that is detrimental. And we certainly don’t want anything in our plant operations that will endanger personal health and safety of our employees or the public.
On the processing side, will it be necessary to add special equipment to handle the waste? Will there be residual waste from the processing?
When using these waste products in hot-mix asphalt we’ve got to look at the test methods. We have to look at mix design procedures, and obviously quality control/quality assurance issues are part of every hot-mix asphalt that is produced.
On the supply side of the waste, probably the biggest issue for the contracting community is the consistency of the supply. As an example, for a 200-tons (180 t) per hour plant, 10 tons (9 t) of waste material is needed if the waste is used at 5 percent of the total. That could be more than 100 tons (90 t) of the waste material per day.
If a consistent material and a consistent supply can be obtained, the HMA contractor can usually produce a quality mix. But any variations in the properties of the waste, what those variations are, and how to get appropriate quantities for the project need to be clearly evaluated.
On the performance side, assurances are needed that HMA made with any waste material is equal to or better than the conventional HMA — and that once the mix with the waste material in it is paved, it is recyclable.
These concerns have proven to be unfounded and if the performance is good then the material is recyclable. Experience over the past 40 years has shown that if the performance is good, then the material is recyclable.
Environmental Concerns
On the environmental side, safety is the foremost concern. Clearly the top issue for safety is worker health. Is something being introduced into the workplace that is dangerous to workers? Each employer must issue an MSDS, or Material Safety Data Sheet, for every product. And we must understand what fumes and gaseous emissions may be generated, and take steps to control such emissions if necessary.
As an industry, we need to define what a waste is, and make sure we know who has ownership of the waste at every given point in the processing stream. That’s important from a liability standpoint.
In the public relations area of the environmental issue, there are two sides to the coin.
On the positive side, we can take pride in what the hot mix asphalt industry is doing to help society’s problem of waste management. On the other side, we want to look at the impact of waste handling for the surrounding community. What will local residents’ opinion be of bringing a “waste” into your contractor yard? Will that damage your public relations with the community?
Economic Concerns
As for economics, we’ve got to look at equipment, performance, political issues and lastly, disposal options. With respect to equipment, modifications or additional equipment may be needed to process and handle the material, and assure that the hot mix operation complies with environmental regulations.
We also need to recognize and account for changes in production rates that may be incurred as a result of using waste materials in our hot mix operation. Any increase in cost needs to be offset by improved performance or incentives for using the waste material.
Because the HMA industry uses large volumes of materials groups will sometimes have the urge to mandate the use of a waste material in HMA. This can be a dangerous practice especially when the material affects the costs, performance and ability to recycle the HMA.
Assuming that only costs are increased, the Departments of Transportation (DOT) probably should not bear all of the additional costs of waste usage. If waste is a societal problem then maybe other departments within the governments ought to be looking at the problem. It is unrealistic to expect contractors, DOTs, or owners to be burdened with the cost of paying to recycle products generated by other industries.
Groups interested in recycling a product first need to look at all options to recycling, not just the HMA industry.
When it is determined that the HMA industry is a viable option for recycling the material, which has met engineering, environmental, health and safety requirements, a better option to mandates is to provide incentives to use the material.
Incentives could be monetary to offset any increased costs. These monetary incentives should be paid by the industry that generates the waste material and not the final user of the HMA.
Another incentive may be a credit in the mix type selection process. By using incentives, the marketplace will determine the true value of recycling the product. We’ve got to understand this issue and know who’s going to pay those potential additional costs.
Conclusion
In summary, the key issues are engineering, economic and environmental. We do need evaluation protocols for these waste materials. Each waste stream product requires an evaluation program to understand the particular waste’s impact on HMA.
The hot-mix asphalt industry is a very large user of materials, certainly one of the largest in the nation. The industry has substantial public funding.
Because of the versatility of our hot-mix product, we can use some of society’s waste. The key issue is that the waste product must be evaluated in an appropriate manner to ensure that use of the waste does not damage the quality of the hot-mix asphalt product.
(Dale Decker is president of his own consulting engineering practice and provides professional services in materials and mix design, QC/QA, training, and technology transfer.)
(Reprinted by permission of National Asphalt Pavement Association from its “HMAT” magazine, January/February 2007.)
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