The benefits of asphalt rubber are comparable to the benefits witnessed earlier by the concrete industry with the advent of chemical and mineral admixtures. Modifiers allow the designer to customize the final product (the asphalt) with specific performance parameters in mind. Following years of research at both university and federal labs, the properties by which safer, quieter, smoother, and longer lasting pavements are created have been identified.
AR is defined by the American Society for Testing Materials (ASTM) D8-88 as:
"A blend of asphalt cement, reclaimed tires and certain additives in which the rubber component is at least 15% by weight of the total blend and has reacted in the hot asphalt cement to cause the swelling of rubber particles."
Crumb rubber modified asphalt is generally produced by three methods:
- Dry method,
- Terminal blend
- Wet method
The dry method is the introduction of un-reacted crumb rubber during the mixing of the asphalt cement and rock aggregate at the Hot Mix Asphalt (HMA) plant. The terminal blend is produced at the asphalt cement terminal, where a low weight percentage (typically 2%-5%) of crumb rubber is blended with the oil for delivery by conventional tank trucks to the hot mix asphalt plants. Both of these processes are referred to as rubberized asphalt.
By far the greatest performance benefits are realized with the wet method wherein crumb rubber is mixed with asphalt cement in a specialized blending system and allowed to react with the asphalt cement, then mixing the modified asphalt cement binder with the aggregate. The weight percentage is generally in the 18%-22% range and results in a much higher viscosity, allowing the binder to form a superior film around the rock aggregate, and added resilience, among other benefits. When produced with the wet method with rubber content in excess of 15%, the product is referred to as AR.
AR Technology Benefits
AR can be applied as a sprayed-on “chipseal”, as a stress absorbing membrane between paving layers, or as a surface friction course pavement. There are numerous documented cases of roads using AR technology lasting 10-18 years. Benefits of this technology include:
- Significant reduction of traffic noise in urban areas
- Improved surface water drainage, reducing safety hazards of hydroplaning and visual impairing water spray
- Potentially major cost savings through elimination of sound barriers
- Reduction of maintenance costs through improved crack resistance
- Conserves natural resources (friction course can be reduced in thickness, conserving aggregate and asphalt cement)
- Can save initial capital construction costs through cost-effective alternative to road construction
- Improved road structural stability by preventing moisture penetration into road foundation
- Elimination of waste tires through productive recycling
Research has shown that safe pavements are dependent on the friction present between tire and pavement, and also on driver visibility. One solution that the asphalt industry has developed to improve these properties are permeable or open graded friction courses. These pavements enhance safety by allowing the water to drain through the wearing course; this promotes a more rapid removal of the storm water from the pavement surface and thus reduces the risk of hydroplaning.
The design of such pavements requires an open graded aggregate blend and a modified binder with a high viscosity to reduce the effect of draindown. Draindown is the phenomenon by which heated asphalt drips off the aggregate due to the absence of fine aggregate in open graded mixes. This problem is overcome through the addition of fibers in the mix to absorb some of the binder, but also through the use of modified asphalts which exhibit much greater viscosities at elevated temperatures.
Asphalt rubber has a long history of use in open graded mixes. Due to the high viscosities associated with AR it is a natural choice for these pavements. The motives for using asphalt rubber are not confined to high viscosities though; the elastic tendencies of asphalt rubber and decreased temperature susceptibility mean that this type of binder is ideally suited for open graded mixes in both warm and cool climates. Warm climates present the challenge of increased rutting susceptibility due to the softening of the asphalt binder, while cool climates cause the asphalt to become brittle and fracture. For these reasons it is necessary to incorporate an appropriate “glue” to the open graded mix to ensure that it is held together.
The benefits of AR permeable friction courses have been documented in Texas where pioneering work into the field evaluation of pavement safety was undertaken. In one study conducted by TXDOT, an existing CRCP was overlaid with a 1.5 inch AR permeable friction course overlay. The resulting benefits of this work included:
- Improved ride quality over the existing CRCP of 61%
- Increase in excess of 200% for skid resistance
- Significant reduction in major accidents
Pavement safety studies have shown that proper design and implementation of AR permeable friction courses lead to large decreases in wet weather accidents. As seen in another study performed by TXDOT, the addition of an AR permeable friction course led to large decreases in the number of wet weather accidents.
These findings provide evidence suggesting that rapid removal of storm water from the roadway surface provides a safer pavement. In addition to this phenomenon, the removal of storm water from the surface also provides the additional benefit of reduced splash and spray, and therefore increased visibility. As seen in the previously mentioned TXDOT study.
Focusing on the pavement for noise reduction purposes tends to be more effective as studies have shown that the majority of noise produced from a highway is due to the tire/pavement interaction rather than the noise generated by aerodynamic and power train noise.
These findings suggest that if tangible noise reductions are to be achieved, then significant advances are necessary in the field of pavement design. Arizona has witnessed significant improvements in the field of noise reductions. Following years of success in implementing AR pavements, ADOT has also had favorable results in noise emissions using rubberized asphalt. Studies have shown that the use of rubberized asphalt, in certain applications, produces higher quality pavements with reduced noise emissions.
REDUCING NOISE AT THE SOURCE
As mentioned previously, one of the principal methods by which significant noise reduction may be achieved is by reducing the amount of noise from the source. Amundsen and Klaeboe (2005) report that possible methods of reducing noise include:
- Noise reduction due to speed reductions
- Low noise road surfaces
- Reducing noise from vehicles
- Reducing noise from tires
REDUCED NOISE PAVEMENT PRINCIPLES
The mechanisms by which noise is generated in the tire/pavement interaction vary, and will also be dependent on the match between tire characteristics and pavement properties. However, in general the pavement-noise generation relationships presented in Table 3 have been proven valid.
Table 3: Effect of pavement properties on noise emissions.
|Effect on Noise Emissions
|Smooth surfaces are quieter than rough surfaces
|Porous surfaces are quieter than non-porous surfaces
|Elastic surfaces are quieter than non-elastic surfaces
ASPHALT RUBBER AND NOISE REDUCTION
The process of blending ground recycled tires into asphalt binder has been growing in popularity in the US since its initial development in the 1960s. Today, interest is particularly high as it presents an environmentally friendly approach to road construction. Studies have confirmed its applicability for conventional polymer modified asphalt purposes; however, more and more studies are also indicating that this material can successfully be used for the construction of quiet pavements as well.
The use of AR for noise mitigation purposes becomes apparent when examining Table 3. The research has shown that elasticity, porosity, and smoothness all play a role in determining the quietness of a pavement. AR has an excellent track record with regard to these properties, and is therefore considered an excellent material for noise reduction purposes.
A common issue with many pavements is loss of elasticity or resilience of the asphalt through oxidation due to exposure to the elements. This phenomenon tends to adversely affect the asphalt pavement; as temperatures fluctuate, repeated stresses occur in the asphalt due to expansions and contractions of the material, thus causing cracks to appear. As seen in Figure 4, even after 16 years of field life, the asphalt pavement which had previously been prone to cracking exhibited much better properties following the rubberized asphalt overlay.
Before rubberized asphalt inter (SAMI) and overlay.After rubberized asphalt inter (SAMI) and overlay - 16 years performance.
One of the main uses of rubberized asphalt to date has been its application in rubber modified open graded friction courses (R-M OGFC). This type of pavement is typically used as an overlay and will exhibit a higher amount of air voids than conventional dense graded mixes. AR is a popular choice for such applications as it prevents draindown of the binder and permits the necessary levels of adhesions and mix stability to be achieved (ARTS, 2003).
States such as Arizona, California, Florida, Rhode Island, South Carolina, and Texas have all successfully used rubberized asphalt in OGFC applications. One of the main areas in which rubberized asphalt has been used the most has been for porous paving designs.
Smoothness of asphalt pavements is often a function of the number of distresses present on the riding surface. Distresses may include:
- Reflective cracking,
- Longitudinal cracking,
- Thermal cracking, and
- Permanent deformation.
As the occurrence of these pavement failures increases, so too does the amount of noise generated. The addition of crumb rubber to asphalt binder has been documented to improve the pavement’s resistance to the surface distresses mentioned above. Therefore, as rubberized asphalt is generally less susceptible to pavement failures, it provides a smoother ride and, consequently, also a quieter pavement.
Testing conducted at the FHWA accelerated loading facility (ALF) concluded that AR compares favorably compared with other modified asphalt pavements. The study was conducted in an effort to refine the Superpave binder system for modified binders as well and specifically for developing field performance data on hot mix asphalt mixtures with modified asphalt binder.
Cost Effective Pavements
AR pavements have proven cost effective in light of their ability to allow engineers to used thinner asphalt lifts. Following years of research Caltrans developed guidelines for thinner asphalt pavements lifts using AR pavements; these pavements permitted up to 50% reductions in thickness when compared to conventional pavements. Such experiences have led many to believe that with regards to fatigue, thin asphalt rubber overlays perform better than conventional dense graded overlays with unmodified binders. The cost savings associated with such reductions are numerous, specifically in times when asphalt prices are high and inconsistent.
Life cycle analysis has also been used to measure the cost effectiveness of AR. Studies at both Oregon State University and University of Nevada Reno have found that AR is more cost effective than conventional asphalt. The advantages of AR lie in the significant reductions in maintenance rehabilitation necessary when dealing with AR.