Study On The Influence Of Aggregate Strength And Shape On The Performance Of Asphalt Mixture

The strength and shape of aggregates significantly affect the durability,tensile strength,and stiffness of the asphalt pavements.The aggregate must have the appropriate shape and strength for the high-performance pavement because it directly influences the performance of the asphalt pavements.The strength and shape of aggregates have a considerable impact on the Internal Skeleton Structure（ISS）of asphalt mixtures,which in response has an impact on the service life of asphalt pavement.The ISS is a significant factor in determining the load transfer route in an asphalt mixture.The Coarse Aggregate Void Filling（CAVF）design method was used to design the gradation and guarantee the dense skeleton structure.Three different aggregates,Granite,Diabase,and Limestone with different strengths and shapes were used to improve the performance of the asphalt mixture.Two-dimensional（2D）cross-sectional image analysis by Image Analysis and Processing System（i PAS）software was performed to acquire the ISS.The CCD digital camera was used to obtain the internal structure of the asphalt mixture combine it with digital image processing techniques to analyse the fine-scale characteristics of the asphalt mixture and construct a simplified skeleton based on the coarse aggregate skeleton contact parameters.The internal stress paths of the mixture were identified based on the Finite Element Model（FEM）and skeleton parameters and the intrinsic correlation between the stress path characteristics and the macroscopic mechanical properties was explored based on the generated force chain network.The Marshall,Hamburg wheel tracking and three-point bending beam tests were carried out to check the performance of asphalt mixture and to evaluate the relationship between skeleton indices with mechanical properties.Furthermore,the grey relational analysis was done to understand the influence of strength and shape parameters individually and combine the skeleton structure and mechanical performance parameters of the asphalt mixture.Results indicated that the CAVF design method could obtain the mineral gradations with the same volume combinations for the three aggregates.Granite has the maximum Marshall stability value of 8.1 k N,Diabase has the lowest value 7.01 k N,and Limestone has 7.2 k N.Granite and Limestone are weak against strength parameters,but they have more Marshall stability values than Diabase.Granite has the highest Dynamic Stability（DS）value,9600 passes/mm;Limestone has the lowest DS value of 6897 passes/mm;Diabase has the DS value of 7092 passes/mm.On the other hand,Granite material shows the lowest rutting depth of 1.28 mm,while Limestone has the highest rut depth of 1.49 mm.The linear correlation analysis results demonstrate a close linear relationship between shape parameters and mechanical properties,which indicates that shape is more dominant on the performance of asphalt mixture.Granite had a more angular shape and made excellent interlocking between aggregate particles,resulting in an excellent mechanical performance of the asphalt mixture.The correlation coefficient R2 of RI for Marshall stability,Dynamic Stability and bending stiffness modulus at 10℃ is 0.9046,0.8621 and 0.9426,respectively.The results indicate that RI has a significant influence on the performance of the asphalt mixture.There is a weak linear relationship of strength parameters with mechanical properties because Diabase is three times stronger than Granite and Limestone.However,Diabase had lower mechanical performance,proving that strength had a minimal influence on the mechanical performance of the asphalt mixture.Based on contact structure characterization,Granite shows more number of contact points（249）and contact length Lsum（722 mm）than Limestone and Diabase materials because of excellent interlocking between aggregate particles due to angular shape.The value of correlation coefficients for the shape parameters（R2=0.9749）is higher than the strength parameters（R2=0.8046）,which indicates that shape parameters,including aggregate angularity significantly influence the skeleton stability of the asphalt mixture.The length of most contact lines is 1-3 mm for Granite and Limestone but in the range of3-5 mm for Diabase.The weaker the aggregate particles,the shorter the contact length intervals.Granite and Limestone produce shorter contact lengths intervals（0.1-1 mm and1-3 mm）because they are three times weak in strength than Diabase,therefore,aggregate particles break down during the compaction process and resulting in shorter contact lengths.Strength and shape significantly influence the contact length intervals but do not influence the contact orientations.The linear relationships between single skeleton indices and mechanical properties are not very significant.The Internal Skeleton Index（ISI）was calculated and had a significant relationship with dynamic stability and flexural stiffness modulus.An aggregate search and contact point calculation program was developed based on MATLAB.The results show that the self-researched contact point calculation program can effectively locate the aggregate particles and calculate the skeleton parameters.The Abaqus contact force extraction program based on MATLAB can accurately extract the stress state in the target region within the mixture.The force chain analysis shows that the number of strong and weak force chains is influenced by the type of load and the location of action,indicating that the asphalt mixture is a stress-dependent material.Isolated force chains are not effective and the mastic properties should be ensured in the mix design to resist the effects of isolated force chains.Granite had a higher grey relational grade value than Diabase and Limestone,proving that Granite had excellent mechanical performance based on the strength and shape parameters.The Granite material has the maximum value（0.763）of grey relational grade then Diabase（0.467）and Limestone（0.517）based on the impact of shape on the road performance tests and internal skeleton structure properties.Consequently,Granite has excellent performance because of its shape properties.The lithology of the mineral aggregate needs to be considered.However,the test results indicate that the influence of aggregate shape and strength on the performance of asphalt mixture is more critical than lithology.When the aggregate shape and strength parameters are fairly good,the influence of lithology can be ignored to some extent.The results obtained in this study could be used to improve the utilization efficiency of aggregate.