Multi-scale assessment of the performance-related effects of hydrated lime replacement on SMA mixtures

To gain a deeper insight as to the combined effects of hydrated lime (HL) on asphalt mixtures, a performance-related assessment using a multi-scale approach is thoroughly presented in this dissertation. The experimental work covers an extensive and coherent set of materials, test methods and analytical tools to identify the effects of HL on the mixture response to the main pavement distresses affecting road infrastructures worldwide, which include, rutting, cracking, ageing and moisture-induced damage. In consequence, one of the contributions of this research is producing a comprehensive discussion on the effects of HL through a broad and advanced laboratory characterisation, including bituminous mastics and mixtures, allowing for a deeper understanding of the benefits and limits associated with the use of HL as a partial replacement of Limestone Filler (LF) in SMA mixtures. Furthermore, this research provides insights related to the chemistry and microstructure changes induced by HL on the bituminous mastics, along with detailed comparisons on the rheology of the different material combinations. To accomplish the main objective of the research study, the experimental work included three parts with specific objectives that include: Part 1. Assess the characteristics of the bitumen and mastics. This part was conducted to determine the basic characteristics of the materials by using conventional and advanced test methods, which included: penetration, Ring & Ball softening point, Brookfield rotational viscosity, DSR-master curves. These test methods allowed for studying the stiffening mechanism of HL-replacement by studying the mechanical and rheological changes induced to the base bitumen by the filler to be used as inputs in the rheological and micromechanics-based models of Part 2. Part 2. Study the performance-related impacts of HL-replacement on the bitumen and mastic phase. This was completed through a full-laboratory characterisation of the bitumen and mastics through rheological test methods (Dynamic Shear Rheometer, DSR; Bending Beam Rheometer, BBR; Double-Edged Notch Test, DENT); rheological models (Mooney, Krieger-Dougherty, Chong, generalised Einstein model, and Nielsen model); micromechanics-based model; microstructural characterisation of the bitumen-filler interactions (Cryogenic Scanning-Electron Microscopy, Cryo-SEM); and chemistry-ageing (Fourier-Transform Infrared Spectroscopy, FTIR). The fatigue performance was studied through the SHRP parameter, the Linear Amplitude Sweep (LAS) test; rutting based on Multiple Stress Creep and Recovery (MSCR) test; fracture in terms of the DENT; the ageing kinetics and chemical changes of the mastics induced by HL-replacement was studied from the FTIR functional groups that are typical of bitumen oxidation. Part 3. Study the performance-related impact of HL-replacement on the asphalt mixture phase. Based on the results from Part 1, a set of asphalt mixtures were manufactured following the UK specifications for typical 10-mm SMA for surface wearing courses (BS EN 13108-5/ PD 6691:2007). The mixtures were subjected to specific test methods focusing on the main pavement distresses: rutting (Repeated-Load Axial Tester, RLAT), cracking (Indirect Tensile Fatigue Test, ITFT; Semi-Circular Bending Test, SCB), ageing and moisture-damage (Indirect Tensile Stiffness Modulus, ITSM; and Saturated Ageing Tensile Stiffness, SATS). The mechanical characterisation of rutting, fatigue and stiffness was completed using the Nottingham Asphalt Tester (NAT machine). The main findings from this research work include a better understanding of the bitumen-filler interactions induced by HL-replacement at the mastic level through the application of rheological and micromechanics-based models and fundamental theories of suspensions. The link of stiffening mechanism with performance-related characteristics of the mastics and the mixtures is also provided based on the permanent deformation response, cracking resistance (i.e., load and non-load related), moisture-induced damage and ageing. The use of chemical-based and simple test methods allowed to better understand the mechanism of HL-replacement on the modification of bitumen in terms of the chemical changes in main bitumen functionalities as well as those linked with oxidative ageing. Consequently, it was possible to validate various theoretical postulations regarding the mechanisms of HL modification in bituminous mixtures, which adds further and updated information to the body of knowledge regarding the benefits and limitations of HL-replacement in SMA mixtures. Overall, the results from this research work have the potential to be used by researchers/practitioners for research initiatives aiming to explore HL use with additional material combinations and mixture designs, which is highly encouraged to foster the understanding of this active filler type. Furthermore, recommendations for HL-replacement ratios in SMA mixtures were derived considering the performance-related characterisation of the mastics and mixtures. Finally, a set of relevant and specific recommendations for future research is offered as part of the outcome from this investigative work