Long-Term Field Performance of Warm Mix Asphalt Technologies (2017)

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1 Long-Term Field Performance of Warm Mix Asphalt Technologies An increased awareness of sustainability in the pavement industry has encouraged the use of warm mix asphalt (WMA) technologies. Compared with conventional hot mix asphalt (HMA) that requires a high production temperature, WMA has several benefits, such as saving fuel and energy, reducing greenhouse gas emissions, and improving the work environment. However, truly sustainable pavements should not compromise field performance. Therefore, evaluation of the long-term field performance of WMA has become critical for WMA users. NCHRP Project 9-49A focuses on the long-term field performance of several WMA tech- nologies. The 28 WMA projects selected in this study, including 5 newly constructed projects (i.e., constructed during the course of the study) and 23 in-service projects, cover different service ages, traffic volumes, pavement structures, WMA technologies, and four climate zones across the United States. Each project has a HMA control pavement and one or more WMA pavements. The main objectives of this study include the following: • Evaluate long-term field performance of WMA by comparing distresses (transverse crack- ing, wheel-path longitudinal cracking, and rutting) between HMA and WMA pavements. • Determine significant determinants for each type of distress by comparing the HMA and WMA field performance of various mixtures and asphalt engineering properties. • Propose best practices regarding the use of WMA technologies. The major findings from NCHRP Project 9-49A include the following: • Pavements containing various WMA technologies exhibited long-term field perfor- mance comparable with that of the companion HMA pavement in terms of transverse cracking, wheel-path longitudinal cracking, and rutting. A companion HMA pavement refers to the HMA pavement that shares similar pavement structure, climate, and traf- fic conditions with the WMA pavement. No moisture-related distress was found in the field for either the HMA or the WMA pavements. • The short-term performance (determined by their transverse cracking, wheel-path longi- tudinal cracking, and rutting) of the three WMA technologies (chemical additive, organic additive, and foaming) is similar. However, in the longer term, the effect of aging on organic WMA technology is more prominent. The chemical and foaming WMA pave- ments appear to have better resistance to transverse cracking and wheel-path longitudinal cracking than the organic modified WMA pavements in the long term. • The mixture’s fracture work density value obtained from the indirect tensile (IDT) test performed at the low testing temperature (14°F) was found to be a significant deter- minant for transverse cracking. A higher fracture work density value correlates with less transverse cracking. S U M M A R Y

2• The mixture’s indirect tensile strength obtained from the IDT test at the intermediate temperature (68°F) was found to be a significant determinant for wheel-path longitudi- nal cracking. A higher indirect tensile strength correlates with more wheel-path longitu- dinal cracking, within the range of the indirect tensile strength values measured during this project. • The mixture’s rutting resistance index (RRI) value obtained from the Hamburg wheel tracking test was found to be a significant determinant for rutting. A higher RRI value correlates with a lower rut depth value in the field. • Other material properties were also proposed as potential significant determinants for WMA and HMA pavement performance. They include mix E* (14°F), mix indirect ten- sile strength (68°F), mix creep compliance (32°F), and mix vertical failure deformation (68°F) values for transverse cracking; mix vertical failure deformation (68°F), horizontal failure strain (68°F) from the IDT test, and mix creep compliance (14°F) values for wheel- path longitudinal cracking; and mix E* (86°F), creep compliance (86°F), binder PG, and binder Jnr for rutting. • The current Pavement ME Design program has limited capability in predicting the long- term performance of WMA and HMA pavements, especially with regard to transverse and longitudinal cracking. As an alternative to the Pavement ME Design program, statistical- based predictive models were developed to identify critical factors that have direct cor- relations with field cracking (initiation and propagation) and rutting for both WMA and HMA pavements. These factors usually include material properties, pavement age, pave- ment structure, climate, and traffic conditions, but they vary for different distress types.