Asphalt rubber (AR) (usually called ground tire rubber (GTR) modified asphalt or crumb rubber modified  (CRM) asphalt ) originally was designed to fall under the specifications of ASTM D6114 - 97(2002). This type of modified asphalt is consistent with traditional asphalt rubber where a crumb rubber concentration by total weight is necessary. This type of modified asphalt binder has been shown to exhibit excellent properties with regards to safety, noise reduction, and field performance.

ASTM D8-88 defines AR as “a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15% by weight of the total blend and has reacted in hot asphalt cement sufficiently to cause swelling of the rubber particles." Research has shown that the addition of crumb rubber to virgin asphalt produces binders with improved resistance to rutting , fatigue cracking, and thermal cracking, and also reduces reflective cracking at reduced thickness of asphalt overlay.

Research has shown that crumb rubber modification of asphalt binder has many similar effects to polymer modification. The major changes noted by these researchers are seen with the increase in the high temperature stiffness, and these are often seen to exceed levels normally achieved by polymer modification. Similarly it has been shown that crumb rubber modifier also results in a reduction of dependency on temperature and loading frequency. However, it also has been suggested that the main function of crumb rubber is that of interactive filler as crumb rubber remains as particulates even after mixing. As the crumb rubber particles do not dissolve in the asphalt, they have been shown to swell in the asphalt, resulting in effective volumes that are larger than their initial volume.
 

Table 2: Typical wet method rubber gradation

With the development of conventional polymer modified asphalt, new testing procedures were developed by the FHWA. These binder tests were conceived with the purpose of evaluating the binder using “real world” testing and loading conditions. With regards to asphalt rubber, many of these tests are applicable and provide insights into the actual mechanism by which asphalt rubber resists permanent deformation and its increased elasticity.

Using the latest testing methods developed by the FHWA, Ecopath engineers have evaluated the properties of asphalt rubber using the multiple stress creep recovery test. This test was designed specially to evaluate the effect of the polymer modifier, rather than simply testing for the presence of a modifier. Furthermore, using this test it is possible to see how the binder behavior changes as the binder undergoes numerous loading cycles. Figure 1 provides data obtained from the Ecopath offsite laboratory which provides information on how different PMAs respond to multiple stresses.





















Figure 1: MSCR loading for 100 and 3200 Pa at 64^oC

Research conducted by Ecopath suggests that asphalt rubber exhibits greater resistance to deformation as it requires more applied stress than conventional PMAs to strain. As seen in Figure 1, while other PMAs strain more under the same stress, AR tends to strain less. Furthermore, it can also be seen that the elastic recovery of the AR specimen is much greater than the other modified asphalts. It is thought that this considerable elastic recovery is due to the large quantity of rubber (> 20 wt%) in the asphalt. Incorporating such large amounts of rubber into the binder typically results in greater viscosities, but it also tends to transfer some of the elastic properties of the rubber to the asphalt binder.




 

ECOPATH has extensive experience in binder design where emphasis is placed on achieving specified binder in the most cost effective manner. ECOPATH provides unique solutions for every asphalt concern, moreover ECOPATH is dedicated to developing solutions with economic efficiency as a priority. Please contact us for further information on binder design services.