Investigation of the permanent deformation behaviour of hot mix asphalt under controlled confining pressure utilising the modified wheel tracker.

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Theses / Dissertations
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Thesis discipline
Civil Engineering
Degree name
Doctor of Philosophy
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Basu Roy Chowdhury, Abhirup

Permanent deformation or rutting has long been a major mode of failure of the flexible pavements. While researchers have developed multiple tests like the Wheel Tracking Device (WTD), Repeated Load Permanent Deformation (RLPD) test, the Dynamic Modulus test, to characterise this phenomenon in asphalt concrete pavements, little information is available on how the factors affecting permanent deformations interact with each other when it comes to predict mix behaviour or the actual field performance. Moreover, it has been shown that the current design of the Wheel Tracking Device is less useful in providing accurate measurements; the unrealistic confining condition is creating unrealistic boundary condition, which makes it a hindrance to capturing the true permanent deformation behaviour. Hence, the primary objective of this research is to carry out comprehensive investigation on the effect of different confinement conditions on permanent deformation characteristics utilising a newly modified wheel tracker.

In this study, the relationship of the asphalt mixture rutting parameters with the binder rheological characteristics was investigated. While the Superpave high- temperature parameter has been extensively used for binder characterisation, its ability of being a sufficient measure is questionable in particular for polymer modified binder and numerous researches suggest that this parameter solely cannot rank the asphalt binders, when permanent deformation is of concern. Therefore, the Multiple Stress Creep Recovery (MSCR) test was recommended as a more reliable test to characterise the high temperature behaviour of asphalt binder, thus, was performed in this research. Additionally, a combination of both elastic and plastic behaviours in a single parameter designated as the Combined Elastic Plastic (CEP) parameter was investigated in this study. The study shows that CEP parameter is much more reliable than the traditional Performance Grade (PG) high-temperature rutting parameter (G*/sin (δ)) and non-recoverable creep compliance (Jnr) parameters for evaluating the rutting behaviour of asphalt binders, evident from the higher correlations with the

asphalt mix mechanical properties such as dynamic modulus, flow number and modified wheel tracker (MWT) test parameters. The CEP parameter revealed a wider range of values, and the distinction between the softer and stiffer binders based on this parameter is much clearer and comparable with asphalt mixture test results.

This study also aims to establish the direct relationship between the MWT and the dynamic creep test to investigate if the mechanisms of rutting (densification and shear deformation) of the MWT and dynamic creep are comparable or equivalent in both tests. Previous studies could only correlate the rutting parameters from conventional wheel trackers with the Simple Performance Test (SPT) measurements. Therefore, this study aims to investigate if the MWT test and the SPT methods, and thereby the parameters from these tests are correlated with each other in the matter of ranking and screening asphalt concrete mixtures for rutting resistance potential. The study showed that the permanent deformation parameters derived from the MWT were well correlated with the dynamic modulus and dynamic creep test. The MWT showed promising repeatability for the Flow Numbers (FNs).

While several research have been conducted to develop multiple tests for the characterisation of permanent deformation, little information is obtainable from the existing literature on how the factors or the interaction of the factors from these tests affect the permanent deformation behaviour in the field. Therefore, this study also focuses on the ruggedness testing of the factors affecting the permanent deformation behaviour utilising the Modified Wheel Tracker (MWT). The analysis involved five factors, each at two levels. These factors are binder type, voids in total mix (VTM %), nominal maximum aggregate size (NMAS), temperature, and confining pressure. The study utilised half-factorial design in accordance with ASTM E1169 - 20. Experiment with controlled confinement and unconfined mixes shows that “confining pressure” is the most significant factor for all the rutting parameters, with its distinction being the greatest for the creep slope, followed by horizontal deformation, and vertical deformation. The sensitivity analysis points out that minimal change in temperature, VTM, and the confining pressure can considerably increase the variability among the mix replicates. For both FNV and FNH to be within ±25% from the model, the change in VTM (%) and temperature should be controlled within ±10% and ±3% respectively from the target values. The vertical deformation at 2000 cycles to be in the range ±25% from the model, the confining pressure and temperature should be controlled within the range ±25% and ±5%.

The subsequent part of this research aims to establish the relationship between the triaxial test and the Modified Wheel Tracker (MWT) to investigate if the rutting mechanism in these tests with variable confinement are equivalent or comparable to each other. This chapter focuses on the study of the effect of different levels of confinement on the rutting of asphalt mixtures tested in the triaxial setup and the MWT. The lateral confining pressure played an important role in contributing towards the permanent deformation resistance of the mixes, within each mix. The higher confining pressure increased the permanent deformation resistance. The results and analysis provide a reasonable confirmation that the MWT is in agreement with the triaxial test, and can be used for a thorough and routine investigation of the permanent deformation behaviour in asphalt concrete mixtures under different confinement conditions, as observed in-situ.

The final chapter aims at simulating the permanent deformation (rutting) behaviour of asphalt mixtures in both triaxial and the modified wheel tracker (MWT) utilising the Finite Element (FE) modelling, using the general-purpose finite elements software ABAQUS. Additionally, the differences between the laboratory rutting characteristics of HMA using pneumatic and solid wheel tyre in the wheel tracker is explored. The effect of confining pressures on the permanent deformation behaviour was investigated in both the triaxial test and the modified wheel tracker. The triaxial test was conducted with controlled confinement of 15 and 30 kPa, while, the MWT test was conducted on unconfined samples and with a confining pressure of 1.35 kPa along the lateral sides, for each wheel type. As expected, the sample with 30 kPa lateral pressure in triaxial endured a higher cycle number to reach test termination strain of 54000 με or about 8 mm of vertical deformation. This indicates that confinement level has a direct relationship with the permanent deformation behaviour of asphalt mixes, with increase in confinement minimises shear deformation. A better match between the measured and the predicted data was observed for lower confinement level, than higher level of confinement. The MWT results showed that the fully-confined or the conventional setup of the wheel tracker produced lower vertical deformation than that with the unconfined condition and 1.35 kPa lateral pressure, indicating an increased stiffness of the asphalt mix under full confinement conditions and therefore less permanent deformation. Strong correlations were observed for the experimental rut depth at specific cycles between solid and pneumatic wheel for unconfined setup and with lateral pressure. This agreement and the linear regression models should in turn help estimate in-situ rutting in road pavements, where, generally pneumatic wheel tyre is used in the vehicles. Strong correlations were also observed between the experimental and the predicted rut depths at different cycles for individual wheel type and confinement.

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