Rubberized Asphalt


With more research on tire pavements, new compositions can be created to improve the properties of the material. Construction methods also have room for improvement, along with methods of application. In the rubberized asphalt process the tire pieces are blended together with hot paving grade asphalt cement. This blend is used as a binder in the pavement. Eighteen to twenty five percent of the binder consists of the recycled tire. The tire makes the binder tougher and more flexible than it would be on its own. The tire also makes the binder more viscous which allows a higher proportion of binder to be used in the HMA. Previous testing has shown that the addition of the tire will also improve the resistance of the pavement to temperature and cracking. While rubberized asphalt may initially cost more, it makes up for its increased cost in resisting oxidation and weathering up to three times longer.

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Types

Asphalt Rubber

This specific version of rubberized asphalt is composed of vulcanized rubber, paving grade asphalt, and other additives. The rubber is mixed in with the asphalt cement until it begins to expand and then is placed. This type of rubberized asphalt is mixed on site. Asphalt-Rubber has been in use for about a decade longer than the terminal blend asphalt.

Terminal Blend

The terminal blend version of rubberized asphalt uses finely ground rubber. This type of rubberized asphalt contains 15-20 percent of this crumb rubber modifier. The terminal blend asphalt is mixed at the refinery rather than on site. The crumb rubber is mixed with liquid asphalt that is highly modified to make the right consistency overlay, and is also mixed with course aggregate. The finely ground rubber pictured below supposedly allows for less rubber to be used, as it is more efficiently mixed in. Also, with this method the required mixing temperatures are not as high.
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History

Rubberized asphalt was created in the 1960’s in Arizona. Since then, California, Texas, Florida, Arizona, and even Canada have implemented rubberized asphalt on their roadways for decades with great success. Colorado is now trying to get a portion of this success but is first creating test sections so that they can monitor the success in their specific climate. Sustainability and economics are a huge focus in engineering, and recycling piles like the one pictured below will allow engineers to reuse unwanted materials and save the new materials for later use that would have been wasted.
However, rubberized asphalt still has potential for design improvements.

The ASTM is making history on the subject of rubberized asphalt with the release from August 2008 which brings to light the number of annually discarded tires. This growing number of discarded tires has led to a proposed ASTM specification for their use. They created the WK20240 and called it the Specification for Performance Graded Tire Rubber-Modified Asphalt Binder, as stated in the ASTM newsroom reference in the list of references at the end of this paper. The ASTM looks to be as sustainable as possible and this new specification has helped push recycling of tires that would normally just be discarded. The specification also sets ten percent to be the minimum amount of finely ground rubber to be included.

The Intermodal Surface Transportation Efficiency Act of 1993 was passed to push agencies to use up the piles of scrap tires all over the nation. The increase in use of rubberized asphalt lead to a general interest in ways that pavement performance could be increased in comparison with typical asphalt concrete pavements.

The Federal Highway Administration has approved the Surface Transportation Efficiency Act which pushes the use of ground tire in federally funded roadways. This act requires that five percent of roads paved in 1994 use the scrap tire. This law is extremely dated, but the law also requires the percentage of required rubberized asphalt cement pavements to increase which makes it much more relevant to today.

Advantages

Cost Effective

One reason rubberized asphalt is considered to be cost effective is that up to fifty percent less is required to produce the same amount of standard asphalt overlays. Half of the amount being used results in a very beneficial cost savings.

Rubberized asphalt requires less maintenance than conventional asphalt overlays. Less maintenance can cause a surprising amount of cost savings. A big reason that less maintenance is required is that asphalt concrete is much more resistant to cracking, as will be discussed in the next section of this paper.

Another reason that rubberized asphalt can be cost effective is that it lasts longer than standard asphalt concrete overlays. Along with retarding aging, the addition of the tire in the pavement also retards the oxidation of the binder. Oxidation is prevented because the rubber makes the binder more viscous, which will create a thicker film on the aggregate. This thick film allows the concrete to resist weathering up to three times as long as normal asphalt concrete. It is extremely important to prevent oxidation in pavements because this will also make the pavement less brittle and less likely to crack, as is discussed in more detail in the following section.

Durable

Rubberized asphalt is more durable than standard asphalt concrete overlays because it is much more resistant to cracking. For this reason, it can last up to twice as long as conventional asphalt. In addition, it is typical for old roadways to be preserved with chip seals, overlays, etc. to prolong the life of the roadway. Overlays on existing failing asphalt pavements are desirable because the rubberized asphalt does not have problems with reflective cracking. Also the tire rubber resists the brittleness and softening that occurs in typical asphalt concrete. This is a very important aspect of the asphalt concrete to overcome. This means that thermal cracking is not an issue in rubberized asphalt like it is in typical asphalt concrete.

The discarded tires contain antioxidants that can retard the aging process of the roadway. Along these lines, the asphalt rubber mix also coats the aggregate more thoroughly than standard asphalt binder which will also slow down the aging process. Another big advantage that rubberized asphalt has is that it is much more skid resistant. This would be ideal for areas that may have cold temperatures, snow, and ice. Another similar advantage is that the rubberized asphalt has been proven through research to improve rutting resistance.

Safe

Rubberized asphalt is smoother than other standard types of overlays. While typical overlain asphalt concrete roads are not normally very dangerous, there are sometimes problem areas in roadways which can cause potential danger. A smooth road promotes the comfort of the rider, which many consider to be an important influencing factor in overall safety on the road. With a smoother road there is less likely to be cracking which may lead to potholes and other dangers.

Many times runoff contains harmful roadway contaminants. Rubberized asphalt filters the storm water to help prevent contaminants from draining into the water table. Along these lines, there is not as much water coming off of the tires on the car in front of you which can be extremely dangerous. Similarly, the possibility of potential hydroplaning is greatly reduced. Also, when there has been a great, sudden rainstorm there is often a glare on the road which makes it extremely hard to see the road markings. This can be a very hazardous situation, but because the rubberized asphalt is porous, the water will be drained from the surface much faster and the lane markings will be much more visible.

The rubberized asphalt road lasting up to twice as long also contributes to its safety. With the road lasting this much longer, less construction will be required and driver delays will be avoided. This is a great advantage because construction usually causes a lot of confusion and can create an increase in accidents. If the terminal blend rubberized asphalt were used the construction delays would be shorter because the tire rubber would mix in much faster on site than it would with the other type of rubberized asphalt.

On a few different occasions large stockpiles of tires have ignited from reasons such as heat or nearby grass fires. This is extremely dangerous for many reasons, some of which include: harmful and toxic gas emissions, high costs of putting the fires out, etc. It is very important that we use up the tires from these landfills so that situations like this do not occur. Below is a picture from the Madera California fire:
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Quiet

One advantage that may not be as commonly known is that the rubberized asphalt maintains its dark black color (shown below) longer than standard overlays; this means that the paint on the road remains visible for a longer period of its life-cycle. Additionally, since the finish on the road is not worn away as fast it remains like new and keeps its ability to absorb noise, resulting in noise reduction on the road itself.
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The noise heard inside and outside of cars is greatly reduced when rubberized asphalt is used. Normally if one stood on the side of a highway, very loud shouting would be required to carry on a conversation, but research has shown that rubberized asphalt reduces the noise so drastically that normal tone conversations can be had when standing on the side of the roadway. Research has shown that the noise level reduction can be up to 10 decibels. This is up to ninety percent reduction, although fifty to seventy five is more typical. This is a great advantage to engineers who consider noise when planning areas around roadways with heavy traffic since noise is considered a type of pollution.

An interesting angle to the noise reduction of rubberized asphalt is that the aggregate size controls the magnitude of noise reduction. It seems that there is research to be done in this area, as major improvements can be made. Studies in the future will likely improve upon this idea and further reduce traffic noise.

Environmentally Friendly

Research has shown that just California tire waste produces 40 million tires in one year. Only seventy five percent of these tires are saved from going to the landfill. Research also shows that a one mile lane of rubberized asphalt takes approximately two thousand tires to complete. It is hard to comprehend the number of tires that would be reused if all states saved all of the scrap tires from going to the landfill, and the land usage that would be saved.

Disadvantages

One big disadvantage is that the rubberized asphalt has to be applied at specific temperatures. Typically it is expected to be 65 degrees or above to properly apply it. In warm climates, this is usually not a problem and this method can be applied year round. However, in some climates this can limit the months in a year that it is acceptable to pave roadways; this can potentially cause great delays in projects. However, the rubberized asphalt can withhold heat longer than conventional asphalt concrete, which will help construction crews to work longer and place the pavement at night in warmer climates. This can be a huge cost savings if it helps the construction to be completed on schedule.

Rubberized asphalt is sticky when applied so it is required for the roadway to be sprayed with water. The amount of water required to cool the asphalt down could be very costly and make the asphalt much less economical. If the terminal blend rubberized asphalt was used this would not be as much of a problem because the mixing temperatures are lower.

To make the addition of the rubber work, a large amount of additives are required to chemically balance the binder. This is a disadvantage to the process as far as cost; additives can be very expensive. However, when one considers the advantages this does not seem like such a bad compromise. If the terminal blend rubberized asphalt is used and not as much rubber is required, fewer additives are required. This would help to decrease the effect of this disadvantage.

Another concern with adding tires to concrete asphalt is that the roadway will no longer be recyclable. Materials engineers typically try to reuse old, torn out asphalt. Perhaps a better option would be to use the residue created from turning the rubber into gasoline as an asphalt additive.

The biggest disadvantage of rubberized asphalt is that adding the tires can increase the cost up to one hundred percent. Studies have shown that the rubber asphalt needs to last only three months longer than the typical concrete asphalt to make up for the extra cost. As discussed above, the rubberized asphalt can be applied in thinner layers which will greatly lessen this extra cost.

References

Pages

http://www.calrecycle.ca.gov/tires/rac/
http://www.astmnewsroom.org/default.aspx?pageid=1466
http://www.springsgov.com/Page.aspx?NavID=1645
http://findarticles.com/p/articles/mi_m1200/is_n10_v141/ai_12033403/
http://www.azdot.gov/quietroads/what_is_rubberized_asphalt.asp
http://www.asphaltmagazine.com/singlenews.asp?item_ID=2305&comm=0&list_code_int=mag01-int
http://www.graniterock.com/technical_notes/asphalt_rubber.html
http://www.betterroads.com/applications-and-innovations-9/

Pictures

http://www.cityofmadera.org/web/guest/waste-tires
http://www.asphaltrubber.org/ari/index.html
http://maxcdn.nexternal.com/tjb/images/hoellen3.jpg
http://www.enoughroomforspace.org/project_pages/view/360

Work Created by Tiffany