Spherical Indentation Technique for Multiscale Characterisation of Asphalt Mixtures
Time: Fri 2021-12-03 14.30
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Video link: https://kth-se.zoom.us/j/63114415370
Subject area: Civil and Architectural Engineering, Building Materials
Doctoral student: Hassan Fadil , Byggnadsmaterial
Opponent: Associate Professor Eshan Dave, University of New Hampshire, Department of Civil and Environmental Engineering
Supervisor: Docent Denis Jelagin, Byggnadsmaterial; Adjungerad Professor Manfred Partl, Byggnadsmaterial; Professor Per-Lennart Larsson, Hållfasthetslära; Dr. Said Safwat, VTI - The Swedish National Road and Transport Research Institute
The viscoelastic properties of asphalt mixtures strongly influence the performance of flexible pavements with respect to their resistance to several common distress modes. Therefore, accurate measurement of these properties and their change during the service life is an important area of ongoing research. Despite considerable progress in this field, certain questions are still not fully resolved. In particular, commonly used experimental methods cannot be applied for the viscoelastic characterisation of thin asphalt layers and asphalt overlays. Moreover, measuring the viscoelastic properties of the downscaled sub-phases of asphalt mixtures, such as mastic or mortar, in the field remains a challenge. Understanding the viscoelastic properties of those sub-phases is crucial for gaining fundamental insight into the mixture performance. In this context, advanced and computationally efficient micromechanical models are also needed in order to establish the quantitative link between the viscoelastic properties of asphalt mixtures and of their sub-phases. This thesis aims to contribute to this important area through the development of new experimental and modelling tools for the multiscale characterisation of asphalt mixtures.
In this thesis, a new micromechanical modelling approach for bitumen-aggregate composites is proposed and used to investigate the mechanical behaviour of mastic, mortar and asphalt mixtures. To achieve computational efficiency, the proposed approach is based on a simplified, computer-generated representation of materials internal structure and utilises periodic boundary conditions to reduce the representative volume element size. Based on the Dynamic Shear Rheometer (DSR) measurements, it is shown that the proposed model can capture the measured viscoelastic behaviour of mastics for the range of loading, temperature and material parameters examined. For the modelling of mortar and asphalt mixtures, the multiscale approach is applied in order to improve computational efficiency. Obtained computational results indicate that the developed approach is capable of capturing the mixtures’ macro-scale viscoelastic properties with reasonable accuracy.
An instrumented indentation test for the viscoelastic characterisation of bitumen and bitumen-aggregate composites, such as mastic, mortar and asphalt mixtures is proposed in this thesis as a new alternative to existing techniques. A new methodology for the indentation testing of linear viscoelastic materials is developed, allowing their characterisation at arbitrary non-decreasing loading. In order to extend the developed method to the multiscale characterisation of bitumen-aggregate composites, the spherical indentation on different types of asphalt mixtures, such as asphalt mortar, mastic asphalt (MA) and asphalt concrete (AC), has been investigated experimentally and through micromechanical modelling. The effect of the indentation test parameters on the measured apparent viscoelastic properties of bitumen-aggregate composites has been evaluated. A particular emphasis is put on the identification of test parameters corresponding to the characterisation of binder-aggregate composites on the macroscale as well as on the individual component scale. The experimental results demonstrate that the developed indentation test can capture the macroscale properties of materials reasonably well, and the obtained results correlate linearly with the properties measured with established test methods. Furthermore, in order to gain better insight into mastic phase properties from the indentation tests performed on MA and AC, a new statistical analysis procedure has been developed for the evaluation of a series of indentation tests. The developed procedure allows identifying clusters of measurements capturing the mastic- and aggregate-dominated responses of the asphalt mixture. The indentation measurements attributed to mastic-dominated response are found to be more sensitive to the temperature and mastic properties as compared to the mean measurements of the indentation test series.
The obtained results indicate that the developed indentation test is a viable alternative to existing viscoelastic characterisation methods, in particular as the test is quasi-non-destructive and can be used to characterise thin asphalt layers. Furthermore, combined with the developed statistical analysis procedure, indentation testing is a promising tool to monitor the changes in the mastic phase of the materials due to ageing, moisture damage or fatigue from the measurements on asphalt mixtures without extracting the binder. The developed micromechanical model can also be used to quantify the effect of changing mastic properties on the asphalt mixture performance. This is particularly true for the strain localisations in the mastic phase and thus the mixture’s damage resistance.