Characterization of Failure and Permanent Deformation Behaviour of Asphalt Concrete

Abstract

Asphalt concrete is a viscoelastic material consisting of aggregates, filler and bitumen. The response of asphalt concrete is highly dependent on temperature, loading rate and confining pressure. Permanent deformation is one of the most important distresses developing during the flexible pavement service life. The total deformation which is visible at the pavement surface is the sum of the deformation that developed in each and every layer. In this thesis however attention will only be paid to permanent deformation of the asphalt layers. The main goal of this research was to investigate and better understand the permanent deformation behaviour of asphalt mixtures at 50°C which is a temperature that regularly occurs in asphalt wearing courses in the Netherlands and which therefore is applied in the Dutch standard for testing the resistance to permanent deformation of wearing courses. The thesis is divided in two parts. The first part focuses on sample preparation, testing procedures and fundamental properties of dense asphalt concrete (DAC) and porous asphalt concrete (PAC) mixtures and skeletons. In the second part the focus is on the prediction of permanent deformation. In the first part of this research special attention was paid to the following aspects: Effects of end constraints on test results Friction between the ends of the specimen and the top and bottom loading platens introduces extra confinement at the top and bottom of the specimen. In this research an extensive monotonic compressive testing program was performed on DAC and PAC mixtures under two different end contact conditions being full friction and reduced friction. “Full friction” was achieved by gluing the specimen to the top and bottom loading platens. “Reduced friction” was obtained by using a sandwich-shaped friction reduction system which consisted of two thin rubber sheets and vacuum grease in between. The results show that in the case of “full friction” the failure stress is overestimated and the displacement at failure is underestimated. The results also show that in the case of uniaxial testing without confinement and when using the friction reduction system, a deformation correction is needed to obtain the true deformation of the specimen. When confining pressure is applied deformations due to the friction reduction system can be ignored. The stress-strain behaviour of asphalt mixtures The permanent deformation behaviour of asphalt mixtures is highly dependent on temperature, stress conditions and number of load repetitions. A better understanding of the stress-strain behaviour of asphalt mixtures is beneficial for a better understanding of the permanent deformation. Therefore an extensive monotonic compressive test program was conducted on DAC and PAC at 40°C and 50°C with 3 various confining pressures and 5 different loading rates. The test results showed that the stress-strain behaviour of DAC significantly depends on temperature, strain rate and confinement. The results also showed that at high temperatures the PAC mixture behaves much alike a granular material with little cohesion. In this case the skeleton of PAC plays a significant role in the mechanical behaviour and this behaviour is highly dependent on the level of confinement. Behaviour of aggregate skeletons Permanent deformation develops at elevated temperatures. At elevated temperatures, the contribution of the aggregate skeleton becomes crucial. For this reason monotonic compressive tests were conducted on DAC and PAC skeletons at two strain rates and two confining pressure levels. The stress-strain behaviour of the DAC and PAC skeleton were compared with the stress-strain behaviour of both mixtures. The results implied that the bituminous mastic in DAC acts as a binder and contributes to the behaviour of the DAC asphalt mixture. The results also showed that the PAC aggregate skeleton shows typical elastoplastic behaviour regardless of the strain rates. In the second part of this thesis repeated load triaxial tests to study the development of permanent deformation were performed on the DAC mixture at 50°C and the following questions related to permanent deformation of the DAC mixture were discussed. Scatter observed in permanent deformation results A power function was used to model the obtained permanent deformation. A large scatter in the model parameters was observed even at the same stress ratio for selected test specimens. The possible relation between the scatter on one hand and the air voids content and resilient modulus of specimens on the other was studied. CT scanning was used to investigate the internal structure of the intact and tested specimens. The results showed that there is no clear relationship between air voids and the parameters describing the development of the permanent deformation with increasing number of load repetitions. The results also showed that the model parameters were stress dependent and a strong relationship was found between the model parameters and the resilient modulus after 1000 load repetitions. The CT scan results showed that different failure modes took place in the permanent deformation tests and that the internal structure of specimens is important for the development of permanent deformation. It is believed that part of the scatter in the test results can be explained by the variation in internal structure. Influence of loading pattern on permanent deformation The influence of different loading pulses and rest times on the permanent deformation of DAC was also investigated by performing Triaxial Repeated Load Permanent Deformation (TRLPD) test. The tests were performed at two different loading patterns, being 0.2 s load + 1.8 s rest and 0.4 s load + 0.6 s rest, and at two confinement levels of 150 kPa with a stress ratio 0.43 and 100 kPa with a stress ratio 0.3. The results showed that the stress ratio has a significant influence on the permanent deformation while the loading time has little influence on the development of permanent deformation. At the same stress level it seems that the longer loading time does not result in larger permanent deformation. Evolution of the resilient modulus in relation to the number of load repetitions In order to obtain a better understanding of the relationship between permanent strain and resilient strain which is developed in the DAC test specimens during the repeated load triaxial tests, the evolution of the resilient modulus of DAC with the number of load repetitions was investigated and modelled. The results showed that the resilient modulus of DAC reduced during the first load repetitions and tended to take a constant value after 1000 load repetitions. Evolution of Burgers’ model parameters in relation to the number of load repetitions The measured total strain was decomposed into elastic strain, delayed elastic strain and viscous permanent strain and modelled by means of the Burgers’ model. The evolution of Burgers’ model parameters obtained at two different loading patterns was investigated in this study. It was found that the value of the parameter representing the dashpot in series, used for modelling the permanent deformation, increased with increasing number of load repetitions and tended to be constant after thousands of load repetitions. This value however strongly decreased when dilation of the specimen occurred. The loading pattern had a significant influence on the value of this viscous parameter Shake down limit in permanent deformation of DAC In order to explore the existence of a shakedown limit for the tested DAC mixture, five representative permanent deformation tests were analyzed. From this analysis it appeared that below a stress ratio of 0.3 (the ratio of applied vertical stress to vertical stress at failure at the same confinement level) only a limited amount of permanent deformation developed after a large number of load repetitions. The stress ratio of 0.3 is proposed to be the shake down limit at 50°C for the test conditions and DAC mixture used in this research. Permanent deformation modeling based on Dissipated Energy Concept Most of the permanent deformation prediction models are simply relating permanent deformation to stress conditions and the number of load repetitions. In this study a permanent deformation model was developed based on dissipated energy. It was shown that the initial dissipate energy, the applied stress level and the number of load repetitions explain very well the permanent deformation development.