Investigating the viability of bio-additives with rejuvenating properties in 50% RAP HMA mix designs: A multi-faceted approach of analyzing mix performance

Reclaimed asphalt pavement (RAP), though the most recycled material by weight in the U.S. and Europe, is not currently most effectively utilized. The material RAP is readily available due to the vast amount of infrastructure surfaced with flexible pavements. Recycled materials are often desired for reduced material costs in construction, reduced environmental costs, and reduced impacts on non-renewable resources. While RAP has been a commonly used material for decades in the U.S. and other nations, it has been used primarily as base aggregate material, construction site fill, and as a partial aggregate and binder replacement in HMA mixes. Many U.S. state agencies are reluctant to use RAP in significant amounts in HMA mixes with many limiting RAP contents to 15% to 30% depending on the agency and the roadway. Reluctancy to increase the allowable RAP content in new mixes is largely due to the incorporation of a significant amount of aged, stiff binder which, if not properly designed for, can decrease a pavement’s resistance to brittle failure mechanisms and moisture damage. During aging, the ratio of asphaltene to maltene molecules increase which in turn reduces the viscous damping and strain dissipation ability of the pavement. Rejuvenators and specifically engineered non-bituminous bio-binders with rejuvenating properties can both be used to chemically restore the molecular ratios within the aged RAP binder to allow for a greater amount of RAP to be used. This research uses a multi-faceted approach to analyze the performances of two organically-derived rejuvenators and one novel bio-binder in HMA mixes incorporating 50% RAP by mix weight. One rejuvenator used is derived from soybean chemistry and is applied as a binder additive while the other rejuvenator is derived from pine chemistry and is applied directly to the RAP material. The novel bio-binder is a polymer-modified non-bituminous binder made from refined pine chemistry with rejuvenation properties. The first phase of this research was to validate the performance of the novel bio-binder in the laboratory using disc-shaped compact tension (DCT) testing, beam fatigue testing, dynamic modulus and flow number testing, and moisture damage testing. As a control, a mix using the same aggregate gradations, sources, and binder content with a PG 58-28 bituminous neat binder was used. Testing showed that the novel bio-binder with 50% RAP passed all U.S. performance and volumetric criteria for a pavement with a 20-year design load of greater than 10 million but less than 30 million ESALs. Additionally, intermediate and low-temperature test results showed the rejuvenating ability of the bio-binder in restoring the viscous damping and stiffness dissipation properties of the mix at critical temperatures. The second phase of this research was the analysis of mix sensitivity to process control variations such as mixing temperature, curing times, and other variables associated with increasing the mixing scale from a laboratory-mixed scale to a continuous mixing plant operation in the field. Both rejuvenators and the novel bio-binder were assessed for process control and mix scale variation impacts on pavement performance. Performance test results indicated that the bio-binder had little to no sensitivity regarding mix performance to process control variations associated with the change in mix scales. Both rejuvenators showed an increased stiffness with an increase in mix scale and associated increase in mixing temperature, storage time, and increase in aggregate fines content. However, the rejuvenator applied directly to the RAP material showed less stiffness sensitivity to mix scale increase process control variables at both low and intermediate temperatures relative to the rejuvenator used as a binder additive. All three bio-additives reduced the mix stiffness at low and intermediate temperatures below that of a high-performance control mix with just 20% RAP while providing adequate rutting resistance at high temperatures. The three bio-additives met or surpassed laboratory performance test criteria at both the small-scale and large-scale mixing operations for pavements with 20-year design loads of greater than 10 million but less than 30 million ESALs. Mechanistic-empirical software modeling along with in-situ accelerated pavement testing results were used in the third phase of research to analyze how mixes with bio-additives and 50% RAP content perform beyond laboratory-scale testing. The same mix designs were used as the second phase field-produced mixes. Accelerated pavement testing measured both rutting accumulation and cracking as a percentage of surface area. Rutting measurement analysis showed that although the mixes with bio-additives softened the mixes to a degree at high temperatures, the total predicted asphaltic layer rutting was still very similar to the high-performance control. Crack measurements showed that the pavement mixes with bio-additives significantly reduced the amount of early cracking as compared to the control mix. Software predictions were used to project the accelerated pavement test rutting measurements and develop fitted models predicting rutting progression with high-temperature ESALs. Software predictions do not account for aging effects on pavement performance which would have an impact on long-term performance. These models predict that all three mixes with bio-additives will experience less than 10-mm of asphaltic layer rutting with 640,000 cumulative ESALs applied when surface temperatures are in excess of 30°C. In the climate considered, this equates to a total 20-year design traffic load of 9.75 million ESALs. Fatigue cracking predictions showed that lower air void contents significantly increase the mixes’ resistance to both top-down and bottom-up cracking. While all mixes showed exceptional performance against bottom-up fatigue cracking, the three mixes with bio-additives in addition to the control mix were predicted to experience top-down fatigue cracking as the critical design distress