Study On Mesomechanical Model Of Coarse Aggregate Skeleton Of Stone Matrix Asphalt

Asphalt mixture is viewed as a composite material of multi-phases consisting of rigid solids separated by continuum of viscous cement. Due to mechanical properties of viscoelastic plasticity of asphalt binders, response of the mixtures to load is basically nonlinear and very sensitive to the temperature. On the other hand, the mechanical property of the asphalt mixture depends also upon the interlock of the particles characterized by the shape and gradation of the aggregates used in the mixture. Importance of the aggregate composition can not be overemphasized since the volume fraction of aggregates is normally around 90% of the total volume of the mixture in practice.Arrangement of the skeleton of the coarse aggregate of the asphalt mixture is neither completely in order nor absolutely in random. The model for the description of aggregate distribution in asphalt mixture should have random attribute thereby. Based on Monte-Carlo method, the model for the simulation of aggregates distribution is developed in this paper. While the aggregates can be positioned randomly within the mixture in this model, relocation process under gravitational stability criterion is carried out. To build the skeleton, the coarse aggregates generated by Monte-Carlo method are filled in the given space governed by the rule of compatibility. Following this procedure, skeleton of SMA16 is simulated and analyzed. The simulation results confirmed that the model is applicable to describe the location of the aggregates in the mixture according to the given grading curve and boundary restrictions. Also in this work, the computer image process is employed in the dissertation to visualize micro structure of the simulated mix.Mechanism of the grinding, contacting and wedging among aggregates is very complex. A mapped distribution model of aggregates mixtures have been studied using discrete element method. A micromechanical model which can be applied to discrete particles is proposed and a computer program is developed on the platform of VC++ accordingly. Numerical experiments on packing of mono-sized and binary mixtures of spherical particles confirm the applicability of the code. To solve the problem of computing time consuming in particle contact search, a novel algorithm is incorporated in the program using the technique of "irregular search window method". This paves way for practical simulation for aggregate skeleton of the asphalt mixture since the mixture normally has a wide particle size distribution which is very difficult to be handled by the traditional algorithm of neighboring element search of "regular window method". Coupled with random packing algorithm, the program achieved the goal of re-locating aggregates of SMA in gravity field.Another feature about the simulation philosophy in this dissertation is the use of taping mechanism to reflect vibration during packing. Particles are relocated during artificial vibration so that the mix gets denser due to the void fillings among particles. Comparing it with physical and numerical experiments, it is found that results obtained from this method match reasonably well with packing experiments.The packing experiments pounding with a pestle of a single group of aggregates and three grades (upper limit, median and lower limit grade of SMA16) of aggregates have been carried out. Experiment results show that VCA_drc formed by the single group of aggregates have little discrepancy. VCA_drc formed by the three grades are almost the same. Because of filling among several groups, VCA_drc formed by each grade of aggregates are all lower than VCA_drc formed by each single group of aggregates.Micro-structure of a single group of aggregates and SMA16 (three grades) are simulated using the program after the packing experiments. VCA_drc measured by experiment is close to VCA_Drc calculated by the program. Moreover, coordination number which is an important micro-structure parameter and impracticable to measure in the tests have been analyzed statistically. The following conclusions can be drawn from analyses: (1). There is little discrepancy in coordination numbers formed by the four groups of aggregates. Compared with mono-sized aggregates, curve of distributed coordination number of a single group of aggregates shifts to the larger direction because of filling among aggregates. The mean coordination number increases also. (2). Distribution of the coordination number formed by coarse aggregates skeleton of SMA covers much wider than that made up of single group of aggregates. The coordination number in SMA could be as high as 23. (3). Frequency of the coordination number equal to 3 in the coarse aggregates skeleton of SMA is relatively high. This shows that there is significant number of small particles in the mix that are not the component of the mix skeleton but the void fillings under the shelter of big aggregates. This illustrates again the complicity of load transmission within the frame of SMA. Property of in-homogeneity of SMA in lab tests should therefore be accounted for cautiously and the tested samples of the mixtures should be fabricated in proper sizes to be compatible to the composition of the aggregates.