Tekieli, Katarzyna
(2019)
A novel hollow cylinder apparatus to investigate shearing characteristic of asphalt.
PhD thesis, University of Nottingham.
Abstract
The design and testing of asphalt to minimise wheel track rutting in highways and the prediction of its development with time remain areas requiring improved technology. Simple tests such as wheel tracking and repeated load compression are, at best, approximate and rely on empirical calibration against field performance. In the US, the Superpave Shear Test (SST) was developed during the Strategic Highway Research Program (SHRP) but, while this has proved useful, question marks remain about its reliability for fundamental research.
Permanent deformation (rutting) is one form of bituminous pavement distress that affects a road performance significantly and it may lead to premature pavement failure. Research shows that rutting is generally associated with a shear deformation in the upper asphalt layers of pavement structures and better characterisation of the shear deformation of asphalt mixtures is required in order to improve rutting predictions. In order to investigate this phenomenon, a large, innovative hollow cylinder apparatus has been designed and commissioned at the University of Nottingham and this thesis describes the development process. The equipment used a hollow cylinder as the test specimen and applied repeated torsion and axial stress together with a confining stress, if required. The test philosophy is that an element of the cylinder wall can be subjected to the combination of shear and normal stresses that arise in the pavement under a moving wheel load.
The results of the literature review stressed the importance of specimen geometry and end boundary conditions. It was demonstrated that a specimen that reflects the asphalt mixture in the top 50mm to 100mm of a pavement should be used. Therefore, the large size specimen was adopted in order to test asphalt with realistic sized aggregate (10mm maximum).
The new HCA proved to be a versatile shear testing device, that can follow stress paths involving stress invariants which are functions of the principal stresses but allow the stress conditions and corresponding strains to be expressed in a way which is independent of direction. The flexibility of stress path application afforded by the HCA is such that a wide range of possibilities arise for future research.
The test programme allowed initial exploration of the relative effects of stress ratio (shear/normal stress) and shear stress level. The results indicated that permanent deformation is affected by load magnitude. It was found that for a high stress ratio, the results appear to be unreliable. At this state of stress, the mixture became unstable and the load magnitude does not appear to influence the permanent strain in a similar trend. The HCA results provided considerable evidence for the dilation phenomenon.
Comparison of data from the HCA, Repeated Load Axial Test (RLAT) and SST, revealed discrepancies between the RLAT, SST and HCA tests and it showed the HCA test is a more reliable tool to investigate asphalt permanent deformation performance than the RLAT and SST but it is too complex for routine use.
From the results presented, it can be seen that the new HCA equipment behaved in a consistent manner and the results obtained during this research appeared to be generally acceptable. The device showed the potential to correctly reproduce the insitu stress regime and, hence, allow more appropriate relationships to be developed between the applied stresses and the resulting deformations.
The resulting high quality data on shear and volumetric strains, particularly permanent strains, will assist with the development of theoretical methods for pavement analysis of rut development in future research and act as an accurate reference to assess the performance of simpler test equipment.
Key words: asphalt, shear testing, permanent deformation, laboratory testing
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