The project was aimed at developing and characterizing a bio-asphalt based on swine manure and crumb rubber (CR) for highway construction application. The study focused on producing the bio-binder (BB) through thermochemical liquefaction of swine manure to the BB and blending the BB with crumb rubber by means of a shear mixer. The modification of asphalt binder with CR is mainly controlled by the exchange equilibrium between the components of asphalt binder and CR. The light components of asphalt binder easily penetrate into rubber particles, while some polymer chains from CR move into the asphalt binder matrix. These phenomena increase the size of rubber particles (swelling) up to three times their original size, leading to a significant increase in the resulting asphalt’s viscosity. Swelling and increased viscosity not only promote separation of the modified matrix into two distinguishable phases, but also increase the difficulty of pumping and application of rubberized asphalt. Promoting interactions between rubber polymers and asphalt aromatic compounds by adding BB could alleviate the aforementioned issues while enhancing overall performance. This NCHRP IDEA project investigated the effectiveness of treating rubber with an amide-enriched BB to facilitate breakage of disulfide bonds in rubber polymers, while promoting interactions between free radicals and the amide groups of BB. Accordingly, the physiochemical and rheological properties of rubberized asphalt before and after bio-modification at both binder and mixture level were studied.
The results of the study showed that crossover temperatures of the bio-modified rubber (BMR) asphalt are consistently lower when compared with those of crumb-rubber-modified (CRM) asphalt specimens, even after short- and long-term aging, indicating that BMR is more ductile than CRM. In addition, bio-modification of rubber asphalt was seen to increase the non-recoverable creep compliance (Jnr) and reduce the percent recovery. However, the inclusion of 15% BB appeared to show improved results compared with CRM for unaged binders, as well as PAV aged binders with and without conditioning. Compared with the CRM sample, the results indicate that BMR has an improved ability to relax stress, making the stress accumulation lower than CRM. For the unaged samples, the CRM showed higher fracture energy results after both 0- and 12-hour conditioning at ˗18ºC compared with the corresponding BMR samples. However, after PAV aging, the BMR showed significantly higher fracture energy results at both 0- and 12-hour conditioning. It was further observed that the introduction of bio-modified binder into crumb rubber in this study led to increased workability as shown by the compaction resistance ratio.
An examination of specimens after moisture conditioning showed that CRM had the lowest tensile strength ratio, followed by the neat binder, while the BMR had the highest tensile strength ratio. None of the CRM and BMR showed any sign of stripping inflection. At low temperature, the results of the disk shape compact tension (DCT) test indicates that the BMR displayed a more ductile behavior than the CRM as evidenced by the larger area under the load-displacement curve for BMR than that of the CRM. At low temperature, the indirect tensile test (IDT) results showed that the BMR samples have nearly similar creep compliance behavior to the neat samples. In addition, BMR asphalt showed enhanced workability by reducing the required compaction energy, while improving low-temperature cracking properties by increasing the fracture energy.
The results of the study show that application of BMR could be a promising method to promote the use of crumb rubber in asphalt while enhancing rubber modified asphalts’ low temperature properties and workability. In addition, surface activation of the rubber particles using an amide-enriched bio-modifier could enhance rubber-asphalt interactions while increasing its resistance to moisture damage.