Investigation into ageing and the effect of hydrated lime on mastics and mixture ageing

Age hardening of bitumen is one of the key factors determining the lifetime of an asphalt pavement. When the bitumen is excessively aged, the asphalt mixture will become brittle and its ability of supporting traffic-induced stresses and strains may significantly reduce, which can easily cause some cracking damage to the bound layer of a pavement. As bitumen is always in contact with mineral fillers in asphalt mixtures, the mechanisms of oxidative ageing of binders are significantly influenced by the physical and chemical interaction between fillers and the bitumen. For a long while, attempts to solve the ageing problem of asphalt binder by various methods, including polymer modification, nano-particle enhancement or functional improvement have been undertaken. Hydrated lime is one of many potential additives used in bitumen to improve the performance of asphalt mixtures. Hydrated lime in hot mix asphalt creates multiple benefits. A considerable amount of information exists in the literature on hydrated lime’s ability to control water sensitivity and its well-accepted ability as an anti-stripping agent to inhibit moisture damage. However, recent studies have shown that lime acts as an active filler and anti-oxidant. These properties create multiple benefits for pavements. Although the majority of research on the use of hydrated lime in asphalt mixtures has been carried out in the USA, the beneficial effects of hydrated lime have also been reported worldwide especially in Europe. In the UK, the use of hydrated lime only started in the early 2000’s and still needs a lot of research into its effect on the traditionally used asphalt materials in the UK pavement industry. This research primarily focuses on an in-depth investigation of bitumen ageing mechanisms and evaluation. Furthermore, this research aims to investigate the advantages of using hydrated lime to mitigate ageing in bitumen mastics and asphalts mixtures as a whole. The full project has been broken down into three parts. The first is a pure bitumen study, the second is a mastic level ageing study and the third is a mixture level ageing investigation. The bitumen ageing study investigated the effect of different ageing methods and parameters on bitumen ageing. In addition, this study focused on the mechanisms and factors affecting bitumen ageing and attempted to develop a better understanding of ageing evaluation methods with different perspectives; basic properties, mechanical and rheological behaviour using a dynamic shear rheometer (DSR) and finally a chemical approach with Fourier transform infrared (FTIR). The second level is the mastics level, which investigated the effect of different fillers on bitumen-filler mastic ageing. Particular attention was paid to study the benefits of using hydrated lime on ageing mitigating in the mastic phase. Furthermore, a detailed study at the mastic level was conducted to investigate the effect of hydrated lime on stiffening and ageing mitigation of bitumen mastics. Different means to evaluate this effect were implemented involving both physical and chemical properties. The evaluation testing was aimed at developing a better understanding of hydrated lime anti-oxidant effects on ageing mitigation. Moreover, special attention was paid to the interactions between hydrated lime and bitumen which affects the ageing of bitumen mastics. The third level was the mixture ageing level. In this study, the effect of hydrated lime replacement of the natural aggregate fillers was investigated. This effect on the asphalt mixtures was investigated by different tests on their mechanical properties such as indirect tensile stiffness modulus (ITSM), indirect tensile strength (ITS) and their fracture properties using the semi-circular bending (SCB) test. Furthermore, the effect of ageing on the recovered bitumen was evaluated. The results and conclusions from the performed studies indicate that hydrated lime slows down bitumen ageing more than granite and limestone fillers. This effect depends mainly on the bitumen type and filler concentration. In addition, the research showed that hydrated lime interacts differently with the different bitumens used in this study. This interaction affects stiffening and ageing properties of mastics and mixtures. Furthermore, results show that different ageing indices used in this project correlated well with each other and also with chemical changes. The detailed results from the FTIR tests on recovered fillers and bitumen from the mastics, showed the ability of hydrated lime to adsorb some of the bitumen components such as carboxylic acid products. This mechanism reduces the formation of ageing products and consequently reduces the age hardening of mastics and asphalt mixtures. Unlike hydrated lime, granite filler does not exhibit this ability to react with bitumen.