UC Davis

Despite proven benefits of rubberized asphalt by the state department of transportation, local governments in California have been slow to adopt this technology. This study investigates the reasons cities and counties in California are not using rubberized asphalt and provides practical guidance for implementation decisions. The primary goals of this study are: (1) to assess the current adoption patterns of rubberized asphalt by local agencies; (2) to compare the field cracking performance of rubberized asphalt materials to conventional and polymer modified asphalt materials under California climate zones and traffic levels of local government roads, considering economic and environmental implications; and (3) support local government decision-making about asphalt mixtures through a practical decision-making and implementation guideline to evaluate, select and implement materials for asphalt pavement projects. Four methods were used in the study: (1) a survey of 31 local agencies in California, (2) asphalt pavement performance modeling using a pavement structural design tool based on load-related cracking, (3) life cycle cost analysis (LCCA), and (4) environmental life cycle assessment (LCA). Four materials were compared in the performance modeling, LCCA, and LCA: conventional hot mix asphalt (HMA), polymer modified hot mix asphalt (HMA-PM), dense graded rubberized hot mix asphalt (RHMA-D, an emerging mix design that has been developed but is not yet used in routing practice by California local agencies or Caltrans), and Gap Graded Rubberized Hot Mix Asphalt (RHMA-G). The framework considers performance measured in terms of load-related fatigue cracking life as a parameter, life cycle costs, and environmental impacts using a 40-year analysis period.In Chapter 4, the results of surveys of local governments are presented and discussed. Survey results showed that 42% of agencies use rubberized asphalt regularly for chip seals, but performance concerns, costs, and contractor availability remain major barriers of adoption for structural applications like overlays. The study models rehabilitation strategies (overlay and mill & fill), applied to existing pavement structures with different underlying conditions across 192 scenarios.Chapter 4 focused on CalME, LCCA, and LCA modeling of 192 scenarios consisting of three climate zones (high mountain, south coast, inland valley), three traffic levels (Traffic Index (TI) – 6,8,10), two rehabilitation strategies (overlay and mill and fill), and two-layer thicknesses (1.5” and 2”). For moderate traffic conditions (TI-6 and TI-8) across all climate zones. CalME modeling of load related cracking revealed that RHMA-G consistently exceeded 20-year design life in all conditions, while conventional HMA failed early in California's Inland Valley climate (lasting only 0.5-7 years). HMA-PM performed well across most cases. Economic analysis over 40 years showed HMA-PM was most cost-effective overall, while RHMA-G provided good value for high-traffic roads. Environmental assessment of embodied carbon (cradle-to-gate) revealed that HMA-PM achieved 26% lower global warming potential (GWP) than conventional HMA, with RHMA-G showing 17% reductions. All modified materials significantly outperformed conventional HMA environmentally. Additionally, economic analysis over 40 years showed that at high traffic conditions (TI-10), rubberized mixes provided the lowest life cycle costs: RHMA-D achieved the lowest NPV in High Mountain and South Coast climates, while RHMA-G was most cost-effective in Inland Valley, compared to conventional HMA due to frequent rehabilitation.Finally in Chapter 5, a practical decision-making framework is presented. The framework evaluates materials based on performance, life cycle costs and environmental impacts. For high traffic conditions (TI-10), RHMA-G provided superior performance with service lives 2-7 times longer than HMA and HMA-PM while reducing global warming potential by 17% compared to HMA. RHMA-G is specifically recommended for Inland Valley climate zones at all traffic levels, where conventional HMA experienced premature failure (5.5-7.5 years versus the required 20-year design life).For moderate traffic applications, material selection depends on specific climate and traffic combinations, with HMA providing the lowest cost at TI 6 in all climate conditions and at TI 8 in high mountain and south coast climates, while HMA-PM is recommended for TI 8 in inland valleys. For high traffic routes (TI-10), rubberized mixes (RHMA-D and RHMA-G) are recommended, with RHMA-D providing the best overall combination of structural performance, cost savings, and environmental benefits. Conventional HMA is not recommended for overlays and mill and fill applications in Inland Valley conditions at all traffic levels and in all climate zones.This study aimed to bridge the gap between technical performance data and practical implementation, providing California local governments with evidence-based decision making tools to select pavement materials based on traffic levels, climate zones, performance, economic constraints, and environmental priorities. The limitations of the study include CalME modeling of load-related fatigue cracking, where age-related cracking is not modeled, which may underestimate the cracking for mixes achieving long service lives. Another limitation includes, conversion of Pavement Condition Index (PCI) values to cracking percentages using engineering judgment, omission of user costs and maintenance activities, and analysis restricted to California climate and cost conditions.