Assessment of environmental factor effects on fatigue and healing performance of asphalt mixtures

Oxidative ageing and moisture damage can be an example of time-dependent environmental conditions affecting the durability of bituminous mixtures simultaneously. Ageing may excessively increase the stiffness modulus and moisture damage may excessively decrease it causing different types of distresses such as cracking and rutting. Consequently, this thesis set out to investigate and understand the individual and combined effects of moisture damage and/or ageing on fatigue and healing performance of asphalt mixtures. The experimental programme to achieve this aim was applied to asphalt mixtures exposed to moisture conditioning and/or ageing as following: Part One: Measuring the indirect tensile stiffness modulus (ITSM) and dynamic complex modulus |E*| using the Nottingham Asphalt Tester (NAT) and Four-Point Bending (4-PB) machine respectively. The Ageing Index (AI), Index of Retained Stiffness Modulus (IRSM), and |E*|index were determined to assess the effect of these environmental conditions. Part Two: Characterising the fatigue behaviour using the 4-PB test and the developed indirect tensile fatigue test (ITFT) in the constant strain method. The traditional approach (Nf50), ASTM D-7460 method, and Arizona State University (ASU) method were applied to analyse these fatigue testing results. Part Three: Assessing the healing performance using the ITFT and 4-PB test. The Healing Index (HI) and Healing Ratio (HR) were determined to evaluate the effect of rest periods. Limestone aggregate from Tunstead quarry was used in a continuously graded dense bitumen macadam (DBM) asphalt mixture. Different air voids content, testing temperatures, binder penetration grades, binder content and time of loading were also investigated. In addition, the British Board of Agrement (BBA-Thin Surfacing: A.2) and BS EN 12697-12: 2008 procedures were individually used to induce the moisture damage in asphalt mixtures. AASHTO P-2002 was also used for simulating the short and long-term oven ageing (STOA and LTOA). The results of IRSM and |E*|index indicated that no significant moisture damage issue has arisen from using both water sensitivity methods, even when the moisture conditioning was applied to specimens after ageing. Limestone aggregate may play the important role in these results as a non-susceptible aggregate to moisture damage. In addition, the results of modified ITFT showed a general behaviour that, the stiffer the asphalt mixture the better the fatigue performance. This stiffening might motivate the elastic properties of the bitumen to dominate and increase the ability of asphalt mixtures to recover their original shape during the 0.4-second rest period following the 0.1-second load application. Therefore this might generally result in a longer fatigue life. It was also found by using the 4-PB test that moisture conditioning and ageing reduced slightly the fatigue life of asphalt mixtures. However, both have less effect at 10 than at 20oC. The fatigue life of aged mixtures has improved at 10 and 20oC when aged asphalt mixtures have been exposed to moisture conditioning. This conditioning could increase the flexibility of aged specimens and the increased flexibility might increase the fatigue life. Furthermore, it was possible to apply uniform and non-uniform rest periods between the repeated loadings by means of 4-PB and ITFT. The modified ITFT showed an increase in the number of cycles to failure due to applying random rest periods regardless of mixture circumstances. Additionally, the moisture conditioning of control mixtures had less effect on the potential of damage recovery than ageing and ageing combined with moisture as determined by the 4-PB healing tests. Moisture conditioning could increase the flexibility of control mixtures and this flexibility might contribute to more recovery. Healing tests of 100 loading cycles (10 seconds) followed by 100 rest cycles (10 seconds) induced a slight damage and therefore showed the best healing indices at all circumstances. It may indicate that the less the fatigue damage, the greater the opportunity to heal the cracking during the applied rest periods. A statistical model was developed based on the ITFT results to predict the number of cycles to failure by incorporating the combined effects of ageing and moisture conditions applied to asphalt mixtures of different parameters. Finally, it is recommended as a future work to assess the combined influence of moisture damage and ageing using different types of aggregate.