Asphalt is like no other road construction material in that it lasts indefinitely and is 100% recyclable. While this is a widely touted fact, many people do not fully understand just how green the Asphalt is ‘green’ for many reasons. Firstly, asphalt is not a throwaway product. Whether asphalt roadways are one year old or 60 years old, the asphalt is able to be milled, or removed, from the road, ground, and black stuff is integrated right back into the new mix. This procedure saves on oil costs, reduces virgin aggregate needs, and creates a cycle in which asphalt is never dumped in a land fill or discarded. It can always be reused. This is good not only for the consumer, but for the environment too.
Asphalt is green not just because it uses old asphalt in the production of new, but because other waste products are able to be integrated into the mix. Shingles and ground tire rubber are able to be integrated into asphalt. This reduces the virgin material need while providing an incredibly useful home for materials that would otherwise find themselves in landfills. As an industry, asphalt producers, engineers, and stake holders continually push the envelope and find more ways to improve our product and help the environment.
Using recycled asphalt pavement (RAP) in the production of new pavements is one of the greatest developments in the history of asphalt paving. Asphalt is unique in that old asphalt roadways are not waste. Inversely, old asphalt is one of the most valuable materials available to asphalt contractors as it can be used in the production of new mix. This cuts construction costs because less virgin aggregate and virgin binder is needed when RAP is used.
RAP IT UP
Reclaimed asphalt (RAP) comes from old asphalt roadways. Asphalt is milled off the road and this material is taken to the asphalt plant. The RAP is integrated into the new mix by heating and mixing it with the freshly produced asphalt in the asphalt plant which reactivates the asphalt binder. This reactivated RAP mixes with virgin aggregates and binder to make a recycled mix. RAP mixes are carefully engineered to ensure proper proportions and blending.
Utilizing RAP has become commonplace in our industry with almost 100 million tons of RAP is reused each year throughout the United States. Missouri uses roughly 1.5 million tons each year! This saves on fuels, production costs, and makes a durable mix.
benefits of asphalt made with RAP
- reduced virgin binder content
- stiffer mix
- high crushed content
- reduced rutting
challenges associated with using RAP
- slight fluctuations in PBE (Percent of effective binder)
- reduced workability
- may require a binder grade change
- moisture in stockpiled RAP
- proper stockpile management techniques are essential when dealing with RAP
Just as reclaimed asphalt is integrated into new asphalt, so are shingles. Nearly 10 million tons of shingles are disposed of each year with nearly 1.5 million tons be reused in asphalt. Shingle waste is generated from either manufacturer waste or tear offs from roofs. These would typically go to landfills, but with asphalt, shingles are able to get new life. Shingles are ground and screened to a uniform size and then added into plant mixed asphalt. Integration of shingles into mix allows for a reduction in the amount of virgin binder which reduces costs while creating an incredibly sustainable mix.
Shingles are useful in asphalt production because they are made up of about 20% liquid asphalt by weight giving them their strong economic value in asphalt production. While incredibly useful, shingle asphalt is typically significantly harder than the liquid asphalts used in road asphalt. Therefore, there is a limit on the amount of shingles that can be integrated into asphalt. Generally, this is around 5%.
benefits of asphalt made with RAS
- stiffer mix
- reduced rutting
- reduced cracking
- lower virgin binder content
- higher density achieved with less compaction effort
challenges with utilizing RAS
- the binder grade used for asphalt production when integrating shingles must be of special consideration. Different grades are often needed depending on the qualities of the RAS utilized
- shortened working time; because the mixes are stiffer, the workability is reduced slightly
- shingle grinding and storage can be problematic for some
- sometimes moisture retention increases with the use of RAS
Warm mix asphalt is a relatively new technology that has taken the asphalt industry by storm in recent years. Warm mix asphalt is a hybrid of sorts, combining all the qualities of traditional hot mix asphalt but drastically cutting the temperature of the product. On average, warm mix can shave anywhere from 50-100 degrees off production temperatures. This means while traditional hot mix averages around 320 degrees, WMA averages around 240 degrees. This reduction results in less fuel consumption, lower emissions, and a reduced carbon footprint. On average, contractors report energy savings of almost 25% during Warm Mix production. In addition to reducing fuel consumption, Warm Mix Asphalt has the added benefit of being easier to compact than traditional hot mix. Warm Mix Asphalt also allows contractors to extend their season into colder temperatures because warm mix does not loose heat like traditional hot mix.
Warm mix asphalt was pioneered in Germany in the 1990’s and underwent significant testing and analysis in European markets. The National Asphalt Pavement Association became aware of this research and brought the proven technology to the United States.
WARM MIX COMES TO THE U.S.!
The first public demonstration of the technology took place in Nashville in 2004. Since then, WMA has undergone extensive testing in the United States and is now widely recognized as a viable product and used often. In 2011, 78 million tons of warm mix asphalt was used throughout the United States. This is a 300% increase from 2009-2010. To date, 47 state departments of transportation along with The Federal Highway Administration (FHWA) support the use of WMA. The FHWA has even recognized WMA in their “Every Day Counts” program which pushes for green materials. Missouri is proud to use warm mix throughout the state. In 2012, over 350,000 tons of warm mix asphalt were used throughout the state of Missouri on DOT projects. Routes such as US 63 and I-70 were paved with Warm Mix Asphalt. Travelers don’t even realize a difference because once on the road, there is little to no variation between WMA and traditional Hot Mix Asphalt. The product offers the same smooth and quiet ride associated with asphalt and has equivalent performance. WMA is expected to continue to increase and maybe even become the standard means of mix production in the United States.
How do we cool it down?
Over twenty technologies are available to assist in producing warm mix asphalt. These range from mechanical components including foamers retrofitted on asphalt plants to chemical and organic additives. Foamers are one of the most common technologies used to produce WMA. Foamers consist of a device that injects a small amount of water into the liquid asphalt binder before it is mixed with the aggregate. When the water comes in contact with the hot asphalt, it vaporizes and expands to over 1500 times its liquid volume. This cools the binder while expanding it by five to ten percent in turn reducing the viscosity allowing for optimal aggregate coating. Another foaming process involves adding a powder substance which is foamed with hot water and binder. This uses the same principle of expanding the volume while reducing temperature. The powder is water logged and introduced to the hot binder which vaporizes the moisture, reduces the viscosity and temperature of the asphalt, while maintaining optimal workability of the mix.
Chemical additives are the second most common aid in producing WMA. Chemicals are added to the liquid asphalt or binder and allow for reduced mixing temperatures. Chemical additives can either lower the viscosity of binder at working temperatures and then harden once placed or lower the surface tension of the binder improving aggregate coverage. In essence, such additives render the liquid asphalt as fluid as traditional hot asphalt without being heated to over 300 degrees to achieve such the desired viscosity. The additives allow for such a reduction and create WMA. Contractors throughout the state of Missouri are making use of Warm Mix technology and appreciating its benefits.
benefits of warm mix asphalt
- allows for easier compaction
- allows for paving in colder temperatures than traditional hot mix
- longer haul distances
- reduced emissions
- reduced fuel consumption
- higher percentages of recycled material can be integrated
- longer construction season
- an estimated 30 million gallons of fuel are saved through WMA production
Porous, or permeable, pavements are yet another innovation that sets asphalt apart from its counterpart. Porous pavements are just as they sound- they have an open graded surface that allows storm water to pass through the asphalt in turn removing water from the roadway or parking lot. Such mixes are credited with keeping ground water cleaner, creating a safer surface for pedestrians, and helping to control run off. These pavements, often indistinguishable from other asphalt, are used mostly for parking lots or other light traffic areas. Porous pavements allow water to drain through the pavement surface into a stone recharge bed and infiltrate into the soils below the pavement. This effectually controls the rate of runoff because the water does not rush off the hard pavement, rather seeps into the rock beneath where it will slowly filter through the rock in a controlled fashion. This is incredibly good for communities because such a system actually replenishes water tables and aquifers rather than forcing rainfall into storm sewers.
Porous mixes are great because they not only help the community, but they do not require any special tools to produce. Porous asphalt combines a course graded rock bed beneath the asphalt to collect the water and an open graded surface mix that allows the water to penetrate. Typically the rock bed consists of anywhere between 18-36 inches of stone. This means contractors geared for the production of asphalt can construct a porous pavement with just a little engineering and few adjustments.
These pavements last a long time and continue to drain effectively for decades. The surfaces of these pavements wear extremely well and even after twenty-five years in place show little distress or damage. They continually prove a cost effective choice for areas with strict run off and ground water regulations.