Research On The Fatigue Damage Mechanism Of Fiber Asphalt Concrete And Its Mesostructure

The maintenance of asphalt pavements,which currently make up several highways in China,has become a complex task at this stage of construction and load network development.Durability,especially fatigue durability,has become a major engineering challenge.Fibers are currently used as an important additive to improve the fatigue durability of pavement materials.However,theoretical research on the inner mechanism,particularly between fiber and asphalt-like materials,need to be further investigated.In this study,improvements to the viscoelastic properties and anti-fatigue damage mechanism are explored.Then,the fiber effect on asphalt concrete is determined by means of mesostructure modeling based on the multi-scale method,finite element simulation,and laboratory test verification.The first part of the study focused on the scale of the asphalt mortar.A random 3D distribution algorithm of the fiber is proposed and a fiber numerical model is generated using MATLAB programming to be imported to the ABAQUS finite element software.Then,a finite element composite model of the fiber-reinforced asphalt mortar,which corresponds to the fiber and asphalt mortar matrix phase,is established for flexural-tensile rheological simulation.The numerical analysis of the fiber-reinforced asphalt mortar model considered different factors,such as orientation,content,aspect ratio,and fiber type.The fiber can effectively absorb the stress of the mortar matrix.Results indicate that the flexural-tensile rheological value of the asphalt mortar is reduced to the level of reinforcement effect.Moreover,the flexural-tensile rheological values are reduced with the increase in fiber content,aspect ratio,and fiber distribution along the horizontal direction of the mortar specimens.The result of the basalt fiber is less than that of the steel fiber.In particular,the rheological values of the mortar with 0.3%content of basalt fiber or steel fiber at 3600s are reduced by 61.7%or 44.0%compared with that of the control,respectively.Finally,the relationship equation between fiber factors(content,aspect ratio,and modulus)from sensitivity analysis and the parameters of the Burgers constitutive model for the asphalt mortar is established.The second part of the study focused on the mixture scale.A typical 2D mesostructure consisting of coarse aggregate and asphalt mortar is obtained using image slice scanning and digital image processing technology.Then,the creep of the mesostructure specimen is simulated in ABAQUS.Results of simulation showed that the stress field of the mesostructure specimen exhibited an apparent stress concentration in contrast to the stress field of the homogeneous specimen with gradient variations.Furthermore,in contrast to the microstructure specimen control,the modulus of adding fiber into the mortar in the microstructure specimens increased.Thus,the mixture modulus also increased and its resistance deformation capacity is enhanced when the creep deformations of the asphalt mixture with 0.3%content of basalt fiber or steel fiber at 1800s are reduced by 36.1%or 19.3%,respectively.A numerical analysis of the influencing factors(fiber and coarse aggregate modulus)is also conducted,and results show a non-significant effect of the fiber modulus and coarse aggregate modulus on the viscoelastic parameters.Thus,the modulus effects of fiber and coarse aggregate can be ignored,which further imply that the viscoelastic deformation between the mortar and mixture scale is positively related considering the influence of other fiber factors.On the basis of these results,the quantitative relationship between the mortar and mixture is obtained,and the relationship equation between the fiber factors(fiber content and aspect ratio)and viscoelastic parameters of the mixture is established.In the last part of the study,fatigue tests in stress control mode are initially conducted on the asphalt mixture specimens with 0.3%content of fiber and control.Results show that adding fiber can significantly improve the fatigue life of specimens.The fatigue life of mixture specimens with basalt fiber and steel fiber under 4kN loading increased by 63.4%and 25.0%,respectively.Then,the parameters of the relationship equation between the fiber factors and viscoelastic properties of the mixture are considered,and the evolution equation of the viscoelastic deformation under cyclic loading for the fatigue specimens is deduced.Results indicate that the deformation value of the fiber mixture is less than that of the control,which is similar to the deformation result of the fatigue test.Subsequently,viscoelastic deformation and damage are coupled,and the damage evolution model of the fatigue specimen is established.The damage of the fiber mixture specimen is reduced in contrast to the control given the same cyclic numbers.The values of the basalt fiber mixture and steel fiber mixture in 6,000 cycles are reduced by 47.8%and 26.2%in contrast to the control,respectively.Thus,the fatigue damage evolution is prolonged and fatigue life is increased.Findings also reveal the capability of the inner mechanism of the fiber to improve fatigue performance by reducing the viscoelastic deformation of asphalt mixture materials.Furthermore,the evolution of viscoelastic deformation is analyzed by theoretical derivation,and the slope characteristics of the evolution equation to describe the effect of fiber factors on fatigue life is proposed,i.e.,the fatigue equation is modified in relation to fiber factors.Then,the effect of different fiber factors on fatigue damage is studied.Results show that increases in fiber content and aspect ratio are both conducive to the reduction of fatigue damage under uniformly dispersing fiber conditions.In particular,the damage value of 0.3%and 0.2%contents of basalt fiber mixture are reduced by 20.7%and 13.7%in 10,000 cycles with 2MPa stress level in contrast to 0.1%of fiber content,respectively.Furthermore,the 50,40,and 30 fiber aspect ratios are reduced by 8.3%,4.3%,and 2.0%in contrast to the 20 fiber aspect ratio,respectively.This study initially explores the mesostructure of the fiber-reinforced asphalt concrete composite.Then,the internal mechanism of the fiber-reinforced mortar is investigated by the multiscale method.The viscoelastic deformation of the asphalt concrete is reduced.This result reveals the advantageous mechanism of the fiber-reinforced asphalt concrete to counteract fatigue damage.This study therefore offers a significant guide for the rational usage of fibers in asphalt concrete.