Measurement On Dielectric Constant And Porosity Of New Asphalt Pavement Based On Method Of Constant Thickness

With the rapid development of China's highway asphalt pavement,quality testing and control methods have been gradually regularized.In general,the commonly-used instruments for non-destructive testing on density distribution of road surfaces are Pavement Quality Indicator(PQI)and 3 dimensional ground penetrate radar(3D-GPR).Based on a large amount of data,this study targets to corporate PQI and 3D-GPR to measure the dielectric constant and porosity of newly-built asphalt roads.After the asphalt road is paved,gradation segregation in certain areas results in the changes of the maximum theoretical density.However,engineers usually adopt the maximum theoretical density of the designed gradation without considering the effect of gradation segregation,which causes potential difficulties in the new asphalt pavement quality control.Therefore,this paper first corrects the maximum theoretical density of the area where the gradation segregation occurs,and then applies the amended maximum theoretical density to building the relationship between PQI and 3D-GPR.The dielectric constant directly obtained from PQI and 3D-GPR is based on the movement characteristics of electromagnetic wave in the material with different dielectric properties.Then the maximum theoretical density and the calculated porosity based on the measured density by nuclear density meter can be calculated.Therefore,this study employs two reasonable assumptions for convenience: 1)the application of nuclear density instrument is mature and the measured density is accurate;2)the accuracy of the current calculation of porosity has been validated.To correct the maximum theoretical density,this study designs a series of Marshall specimens to concern influencing factors including coarse/fine aggregate density ratio,amount of ore powder,asphalt ratio and etc.Then the grey correlation method helps to sequence the degree of these influencing factors.In the meantime,the indoor combustion screening test was conducted on the paved asphalt mixture taken from the pavement on site at different to determine the differences between the segregate region's gradation and the design gradation.The maximum theoretical density of the segregation region is thus finally corrected according to the influencing degree and gradation difference of each influencing factor.The necessity of the correction of the maximum theoretical density depends on the variation of porosity.In this paper,the great difference between respective porosity calculated based on maximum theoretical density of the designed,borehole and indoor correction implies the significance in correcting the maximum theoretical density.In details,the difference account largely of the designed change of the designed porosity amplitude(design porosity is 3% to 6%,the range of change is 3%).The study illustrates the slight difference between the corrected maximum theoretical density and the corresponding measured value from the core.The negligible percentage in the change range of the designed porosity occupied by the void difference from the corrected value verifies the accuracy the corrected method.In the practice,the 3D-GPR is used to scan the asphalt surface layer to obtain the three-dimensional image of the road surface.Thus the two-way travel time can be calculated from the time-amplitude curve.With the assumption of a certain thickness according to the thickness of the surface layer's cores,the dielectric constant can be back-calculated by the two-way travel time and the assumed thickness.This approach is called the constant thickness method.Moreover,the porosity is calculated by density measured by PQI and the maximum theoretical density by indoor correction.Finally,the dielectric constant-porosity model is established by the dielectric constant obtained by the back-calculation and the porosity obtained by the conversion.After applying the proposed the dielectric constant-porosity model,the predicted porosity can be calculated.The good agreement between predicted value and the measured value validates the proposed dielectric constant-porosity.Overall,it is convenient and accurate to measure the dielectric constant and porosity by using 3D-GPR based on the constant thickness method,which overcomes the limitation that the 3D-GPR can't measure porosity of asphalt pavement accurately and effectively.