Determination Of Glass Transition Temperature For Asphalt Binders And Analysis Of Quasi-brittle Behavior For Asphalt Mixtures

Asphalt is a typical viscoelastic material.The essential physical basis behind its viscoelastic behavior is that the chain lengths of the molecules constituting hydrocarbons and their derivatives are larger than those of the regular small molecules,and the particular relaxation motion of the molecular chains lead to the temperature and rate sensitive characteristics of asphalt.In terms of different molecular motions,the behavior of asphalt can be divided into different states:viscous state,viscoelastic state,and glassy state.The glass transition temperature is a characteristic temperature at which reversible changes towards the rigid and brittle glassy state occur in asphalt.At this temperature,the physical quantities,e.g.,the specific heat,specific volume,modulus,dielectric loss,change dramatically.Due to the freeze of the relaxation motion,the deformation of the material induced by external forces cannot be relaxed in time,easily leading to cracking.Low temperature cracking is the main distress for asphalt pavement structures in cold regions.The research on the glass transition temperature,Tg,is of great importance to prediction of thermal stress as well as identification of low temperature brittleness within asphalt binders and mixtures.The traditional approach to study the glass transition temperature is the dilatometric method based on free volume theory.This method is less used due to its complex operation.The dynamic mechanical test measuring the dynamic modulus and phase angle(or equivalently storage modulus and loss modulus)under oscillation loading is another method for determining the glass transition temperature.The indicator of the glass transition temperature is the peak value of the loss modulus.The dynamic modulus from this method is closely associated with asphalt performance and the peak value is easy to identify;therefore,it has gained increasing attention.However,the conventional temperature sweep test only determines the glass transition temperature at one frequency.In order to determine the glass transition temperature at any frequency and further analysis the quasi-brittle fracture behavior of asphalt mixture,the following research efforts were made in this dissertation:(1)The glass transition temperature is a characteristic temperature corresponding to the peak value of the loss modulus master curve.In the past research,the construction of the master curves at higher frequencies was usually disregarded and the correlation between the glass transition temperature and dynamic shear modulus master curve of asphalt binder was rarely studied.The present study developed a method that determines the glass transition temperature through viscoelastic parameters and the Williams-Landel-Ferry(WLF)equation coefficients based on the master curve of the Modified Havriliak-Negami(MHN)model.The advantage of this method is that it develops the analytical relationship between the glass transition temperature of asphalt binder and the MHN complex modulus model parameters.By means of the measurements of fifteen asphalt binders,the master curves of the MHN complex modulus model were constructed and the glass transition temperature was determined.The results were compared with those from the traditional method,demonstrating the effectiveness of the presented method.(2)The dynamic modulus master curve can not only reflect asphalt performance under practical conditions but also reflect the change in constituents sensitively.Taking into account this advantage,the weight-average molecular weights and molecular weight distributions(MWD)for the fifteen asphalt binders were measured by using gel permeation chromatography(GPC)test,and the correlations between these two quantities and the viscoelastic relaxation spectra and characteristic angular frequencies were developed,respectively.The correlations facilitate the understanding of the microscopic constitutive mechanisms behind the complex macroscopic performance of asphalt binder.(3)Asphalt exists within asphalt mixture in the form of asphalt mastic.To further characterize the viscoelastic behavior of asphalt mastic,the dynamic shear modulus master curves of asphalt mastic with filler volume fractions ranging from 0 to 41.2%were constructed using the MHN complex modulus model,and the changing trend of the master curves and viscoelastic parameters with the volume fraction of fillers was analyzed.With the increase of the volume fraction of fillers,the glass transition temperature represented by the loss modulus peak value shifted to high temperatures.The change of the glass transition temperature verified the presence of the interface layers between asphalt and fillers.Further,according to the changing trend,a phenomenological predictive model was developed and it would facilitate the characterization of the linear viscoelastic behavior of asphalt mastic with different volume fractions of fillers(4)The dependence of temperature and loading rate poses a challenge to the strength and fracture parameter analysis of asphalt mixture at low temperatures.Once the glass transition temperature was used to define the brittle behavior of asphalt under this temperature,the fracture behavior of asphalt mixture satisfied the quasi-brittle assumption.Based on the two-parameter model following the quasi-brittle fracture mechanics,the changing trends of the flexural-tensile strength of the three-point bending beam specimen,flexural-tensile strength of the semi-circular bending beam specimen,and indirect tensile strength were compared,which provided a theoretical foundation for the reasonable selection of the asphalt mixture strengths and fracture parameters from the three test modes under low temperatures.