Study the use of reclaimed asphalt pavement (RAP) with foamed bitumen in Hong Kong

Abstract

In order to fulfill highway's sustainable development, it is necessary to utilize more and more reclaimed asphalt pavement (RAP) materials in the new road construction and road maintenance. This study was launched under this circumstance. The primary objectives of this research were to investigate properties of the cold-recycled RAP mixes stabilized by foamed bitumen (also called foamed asphalt mixes, or FA mixes) in the laboratory. There are six main parts in this dissertation. Chapter 2 introduces a literature review of the cold recycling by foamed bitumen. Chapter 3 presents the study of the bitumen foamabilities under 140 combined testing conditions for Shell 60 bitumen and Shell 100 bitumen, including the foaming properties and decay functions. Water content and bitumen's viscosity significantly affect the decay line and foaming property of the bitumen. The combination of 5 bar pressure, 170 C and 1.7% water content is determined as the optimum foaming condition (OFC) for Shell 60 bitumen. The OFC of Shell 100 is the combination of 3% water content, 5 bar pressure and 160 C. Chapter 4 discusses FA mixes' mix design. Two bitumens (Shell 60 and Shell 100) and two RAP materials (RAP#1 and RAP#2) were selected, and four RAP contents were considered in mix design. It is found that the optimum moisture condition (OMC) of FA mix decreases as RAP content increases. The design bitumen contents (DBCs) of the FA mixes were determined as 3.5% for the lower RAP content (0% and 20%), and 3.0% for the higher RAP content (40% and 60%) based on the soaked indirect tensile strength (ITS) test. In Chapter 5, permanent deformation of FA mixes was evaluated in laboratory by repeated load axial creep (dynamic creep) test. As well as the ultimate strain, the creep strain slope (CSS) and secant creep stiffness modulus (SCSM) were initially used to investigate the susceptibility of FA mixes to permanent deformation. High bitumen grade may help FA mixes to reduce their susceptibilities to permanent deformation. Ageing of RAP and RAP content insignificantly affect susceptibilities of FA mixes to permanent deformation. Susceptibilities and creep strengths of FA mixes are better than those of the hot asphalt mixes. Fatigue properties of FA mixes were analyzed and discussed in more detail in Chapter 6. The indirect tensile fatigue test (ITFT) was firstly adopted to test FA mixes in this study. Fatigue lives between FA mixes and the hot-rolled mixes were compared, characteristic of the fatigue failure and materials' effects on fatigue properties were analyzed, nonlinear characteristic of FA mix was discussed. Fatigue lives at 100 microstrains (Nf100) of FA mixes are far smaller than those of the hot-rolled mixes. FA mixes show an apparently violent facture at failure in the ITFT, substantially different from the fatigue failure characteristics of the hot-rolled mixes. The higher viscosity bitumen is more advantageous to Nf100 of FA mix with/without RAP material than the lower-viscosity bitumen. Moisture susceptibilities of FA mixes were firstly and comprehensively studied in Chapter 7. The soaked ITS test, the soaked dynamic creep test, the soaked ITFT test, and the freeze-thaw ITFT test were conducted in order to investigate and evaluate the moisture susceptibilities of FA mixes to permanent deformation, ITS and fatigue under the moisture conditions. All FA mixes meet the requirement of the moisture susceptibility to ITS in terms of Lottman's criterion. Under the soaked condition, harder bitumen (Shell 60) is statistically superior to the softer bitumen (Shell 100) in reducing the moisture susceptibility of FA mixes to permanent deformation. For the soaked FA mixes, when RAP content is small, the high penetration-grade bitumen is advantageous to fatigue lives of FA mixes; on the contrary, the low penetration-grade bitumen is advantageous to FA mixes' fatigue lives. Under the soaked or the freeze-thaw conditions, FA mixes containing 60% RAP#2 have poor moisture susceptibility to fatigue. From tests and evaluations of permanent deformation, fatigue and moisture susceptibility of FA mixes presented in this study, it can be found FA mixes stabilized by the harder bitumen (e.g. Shell 60) and containing the less aged RAP materials (e.g. RAP#1) have better engineering properties and can be recommended as road base layer under the rainy condition in southern China. Group D has the best overall properties, which contains 60% RAP#1 and is stabilized with 3% Shell 60.