By Dwight Walker, P.E
Highway agencies have recognized the benefits of using modified asphalts to reduce the amount and severity of pavement distresses and to increase service life.
The primary benefit of using these high-performance asphalts is improved rutting resistance, with less thermal (cold-temperature) cracking and overall improved mixture durability being secondary benefits. Additionally, some modified binders provide improved stripping (moisture damage) resistance.
Narratively, many agencies estimate that an additional four to six years of pavement life from a pavement constructed using a modified asphalt binder is a reasonable expectation.
Agencies are always striving to lengthen the service life of pavements. The traffic demands placed on our pavements are significantly more severe than in the past.
The AASHO Road Test (1958-1962) is still widely used for pavement design. That research was based on a legal load limit of 73,280 pounds. In 1982, the load limit was raised to 80,000 pounds – which does not seem like a huge increase. But that 10 percent load-level increase corresponds to 40 to 50 percent more stress to the pavement.
Another change that brought greater demands to the pavement was the introduction of radial truck tires. Bias-ply tires typically had a pavement contact pressure of about 75 pounds per square inch (psi). Radial truck tires apply about 125 psi of pressure to today’s pavements.
An even greater compounding factor is the growth in truck traffic volume. Our highway system is filled with trucks. Modified binders help to meet this challenge.
Is it modified?
There is a concept in the Superpave binder system called the “Useful Temperature Interval (UTI),”which is simply the spread between the PG high and low temperature classifications. The UTI “Rule of 92” gives an indication of whether a binder is modified. If the UTI is less than 92 degrees Celsius, the asphalt binder is probably not modified. Most binders with a UTI value of 92 are modified, but a few crude sources may meet the value without modification. If the UTI is greater than 92, the binder is probably modified. For example, the commonly used PG 76-22 grade has a UTI value of 98 degrees Celsius (76°-(-22°)) and is a modified binder.
Types of modifiers
Modified asphalts are often generically called “polymer modified asphalts.” Polymers are probably the most common type of modification, but today’s modified asphalts may be produced in several ways. According to the Asphalt Institute’s “(MS-4) Asphalt Handbook”, modifiers and additives being used to boost performance include polymers, chemical modifiers, extenders, oxidants and antioxidants, hydrocarbons and anti-stripping additives.
“Polymers” cover a broad range of modifiers, with elastomers (rubbers or elastics) and plastomers (plastics) being the most commonly used types. Styrene-butadiene rubber (SBR) and styrene-butadiene-styrene (SBS) are frequently used elastomers. Research conducted as part of the National Cooperative Highway Research Program found that SBS is the most often used modifier. These modifiers are used to reduce rutting and to improve fatigue and thermal cracking resistance.
Crumb rubber is an elastomer made from ground tires. Several technologies are in place for using ground tire rubber. This material is used primarily to address rutting.
Plastomers are used to improve the high temperature (rutting) properties of modified materials. Low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) are examples of plastomers used in asphalt modification.
The most commonly used chemical modifier is polyphosphoric acid (PPA). This modifier may be used in combination with polymers to increase the high-temperature stiffness.
Other modifiers that may be used include asphalt binder extenders (primarily sulfur) and hydrocarbon materials. Hydrocarbons can produce either hardening or softening effects. Materials can be added to increase rutting resistance. Softening agents or rejuvenators are used to lower the viscosity of aged asphalt binders in mixes containing recycled asphalt pavement (RAP).
AI modified asphalt study
The positive performance of modified asphalts is commonly recognized and demonstrated by lab testing, but there has been little available documentation of improved field performance. A study undertaken by the Asphalt Institute cited here is an example of one effort to document the performance benefits. Although, this study addressed polymer-modified asphalts only, it provides documentation for the performance benefits of this type of modifier and how other types could be evaluated and documented.
The affiliate committee of the Asphalt Institute initiated a study to quantify the performance benefits of polymer modified asphalt pavements and overlays using field data from controlled experiments across the U.S. and Canada. The principal investigator of this study was Harold Von Quintas with Applied Research Associates. The full report of this research, “Quantification of the Effects of Polymer-Modified Asphalt for Reducing Pavement Distress,” is available from the Asphalt Institute as Engineering Report (ER) 215. An abbreviated version, Informational Series (IS) 215 is also available.
According to the report, “The purpose of this study is to use mechanistic-empirical (M-E) distress prediction models for comparing the performance of different test sections to quantify the improvement in pavement life or reduction in surface distress by using PMA, as compared to conventional unmodified HMA mixtures.”
The distresses included in the study’s performance predictions and comparisons were rutting and fatigue cracking. Transverse cracking was also included. Thermal cracking and stripping comparisons were not included because too little data was available.
The report summary concludes that ”… the use of PMA mixtures result in less cracking and rutting – extending the service life of flexible pavements and overlays …” The study found that PMA wearing and binder (intermediate) mixtures exhibited about half the cracking and about 40 percent of the rutting measured on comparison projects. It was found that “…PMA mixtures provide on the average about a 25 percent or a 2- to 10-year increase in service life …” Additionally, it was concluded that PMA could reduce maintenance costs and the number of maintenance operations.
Use as needed
While the benefits of using modified asphalts are widely acknowledged, not all asphalt mixes or treatments need to be modified. Each application should be evaluated to determine if the traffic loading, anticipated service life, environmental conditions and desired performance justify the use of modifiers. Modified asphalts can be a good investment.