Inherent Anisotropy Of Field Cores Of Newly Constructed Asphalt Pavements

Asphalt mixtures have been widely used in pavement construction because of their excellent road performance.The true mechanical properties of asphalt mixtures provide the basis for the design of pavement and the analysis of actual pavement structure response.Road design and pavement structure response analysis are generally based on the mechanical properties of the lab-mixed-lab-compacted specimens and assume that asphalt mixtures are isotropic,which cannot truly reflect the performance of asphalt pavements.Investigation of the basic mechanical properties of field cores with consideration of inherent anisotropy can provide a solid foundation for the pavement design and pavement structure response analysis.In this paper,the field cores obtained from the Dawu section in the newly built Macheng-Zhuxi highway are considered as the research specimens,whose basic viscoelastic properties and inherent anisotropy are analyzed and studied.Firstly,the uniaxial compressive creep test was used to determine the basic viscoelastic parameters(creep compliance,relaxation modulus and complex modulus)of the core samples measured in two directions(compaction direction and horizontal direction that is perpendicular to the compaction direction).In this paper,uniaxial compression creep tests were carried out at three temperatures(5,20 and 35oC),and the creep compliance under each temperature was calculated based on the measured stress and strain data.Based on the linear viscoelastic theory,the relaxation modulus and complex modulus at each temperature were calculated according to the relationship among creep compliance,relaxation modulus and complex modulus.Then the creep compliance master curve,relaxation modulus master curve and complex modulus master curve were constructed based on the time temperature superposition principle,which reflect the creep properties,relaxation properties and dynamic properties of the field-cores in a wider range of time and frequency.This method provides an effective way for the analysis of the basic viscoelastic properties of the filed-cores.Secondly,the digital image processing technology is used to study the apparent characteristics of the aggregate and the microstructure of the core samples,in which the AIMS is used to quantify the sphericity,angle and surface texture of the aggregate,and the digital camera and IPP software is used to analyzed the microstructure of the core samplbaid.The results show that the aggregates from the same source have some differences in the apparent characteristics.The uniformity coefficient of asphalt mixture and the deviation ratio of aggregate distance can be used to evaluate the homogeneity of asphalt mixture better.What'mare,the long axis direction of the flat and large aggregates tends to tilt horizontally during the process of compactionThen,the anisotropic degree is used to quantify the direction difference of the mechanical properties of the core samples,and the influence of temperature,loading time and loading frequency on the anisotropy is analyzed.The results show that the degree of anisotropy increased with the elevation of temperature and loading time,and decreased with elevation of loading frequency.It was found that the Sigmoidal model was able to describe the variation of the anisotropy degree with time and frequency.Finally,the influence of apparent characteristics,distribution and size of aggregate on anisotropy was analyzed by the grey correlation analysis method,and the mathematical model between the microstructure parameters and the anisotropy degree is established.The study shows that the degree of correlation between the different size aggregate and the anisotropy of asphalt mixture is different,and it should be considered separately in the analysis.The influence of the large size and flat aggregate on the anisotropy is greater.Besides,the influence of the single factor on the inherent anisotropy of the asphalt mixture is not obvious,and the inherent anisotropy of asphalt mixture results from the combined effects of various microscopic factors.