Damage Of Cememt Stabilized Macadam Base Material Subjected To Load And Freeze-thaw Action

The cement stabilized macadam base material(CSMB material)is the most widely used semi-rigid foundation materials in pavement engineering.The complicated load action and the freeze-thaw cycle action subjected to CSMB material are the main reasons to induce pavement deterioration and damage.Considering the variability of freeze-thaw action and the complexity of load action,the deterioration and damage of asphalt pavement induced by freeze-thaw damage of CSMB material are happening with increasing frequency in practical engineering.Tracing its root,the freeze-thaw and dynamic vehicle coupling effect are not comprehensively taken into account in the study of freeze-thaw damage of CSMB material,which lead to fail to analyze the freeze-thaw damage mechanism of CSMB material comprehensively and objectively,unable to evaluate the damage characteristics and to predict the fatigue life accurately.Based on the previous statements,considering the load and freeze-thaw action together,the freeze-thaw damage characteristics,the freezing deterioration mechanism and fatigue life prediction of CSMB material were systematic studied to establish the freeze-thaw damage evaluation methods and evaluation criteria of CSMB material in the terms of the freeze-thaw and dynamic vehicle coupling effect,life prediction equation for freeze-thaw damage of CSMB material was proposed eventually,so as to provide scientific basis for evaluating the damage characteristics of CSMB material reasonably.Based on the previous research,a scientific design scheme of indoor freeze-thaw evaluation of CSMB material was designed.The unconfined compressive strength,flexural strength,dynamic compressive modulus of CSMB material were determined as the evaluation index,the variation of unconfined compressive strength,flexural strength and dynamic compressive modulus with freeze-thaw cycles,the amount of cement and emulsified asphalt was explored,the fatigue equations of freeze-thaw damage of CSMB material under different freeze-thaw cycles were established.The results showed that: with the increase of freeze-thaw cycles,the unconfined compressive strength,dynamic compression resilience modulus,flexural tensile strength of CSMB material gradually decreased,faster in the early cycles,after 15 times of freeze-thaw cycles attenuation tended to be stable.Accordingly,the index-the loss rate of dynamic compression resilience modulus after 15 freeze-thaw cycles was recommended to evaluate the frost resistance of CSMB material.CSMB material with 5.0% of cement content and 2.5% of emulsified asphalt showed better mechanical properties and frost resistance.Meanwhile,the residual fatigue life of CSMB material decreased with the increase of damage degree,showing the tendency that the fatigue life declined faster in the early freeze-thaw cycles and tended to stable at late stage of freeze-thaw cycles.In the light of the meso-and micro-scopic freeze-thaw damage mechanism of CSMB material,the X-ray industrial CT were conducted to investigate the meso-to micro-structure of CSMB material under the action of freeze-thaw cycles.The variations of void fraction,pore area,pore number and mean pore diameter of CSMB material with different freeze-thaw cycles are analyzed.By applying micro-analysis methods such as MIP and SEM,freeze-thaw damage mechanism was investigated on CSMB material with different composition based on the microstructure.The results concluded that mesoporosity of CSMB material increased with the increase of freeze-thaw cycles,the location of freeze-thaw damage was mainly concentrated in the edge and around the primary voids,the damage and cracks were formulated as the voids expanded gradually after multiple freezing and thawing cycles.The pore structure distribution of cementing material after hydration was significantly affected by the action of freeze-thaw cycles,mainly embodied in the range of harmful pore,while the size of the pore larger than 200 nm.From the observation of MIP and SEM,the conclusion was made that a superior structure will be formed with fewer harmful pores for CSMB material with high cement dosage and emulsified asphalt can enhance the frost resistance of CSMB material effectively.The frost heave stress and erosion effect resulted in the change of cement paste structure and composition and induced damage.The strategies by adjusting void structure,optimizing cement material and adding emulsified asphalt to improve frost resistance of CSMB material was finally recommended.Based on the ABAQUS finite element platform,a mechanical behavior analysis model of a special small size pavement structure was developed,the mechanical behavior of CSMB base layer was analyzed under the action of dynamic load and freeze-thaw cycles.Based on the multi-physics simulation software-COMSOL,a multivariate Weibull distribution model to predict residual life of CSMB material was constructed by employing Manson-Coffin theory model,the fatigue life of the small size pavement structure was accurately simulated and forecasted.The results showed that simple support condition,the rubber layer thickness and modulus had a severe impact on mechanical behavior of whole structure.Under the action of the standard axle load,the fatigue was more susceptible to induce at CSMB layer rather than asphalt layer.Considering overloading and freezing and thawing effects,fatigue damage of CSMB layer was more easily to induce,and it was also an important reason causing a sharp decline of pavement structure performance.Based on MMLS3-a 1/3 scale vehicle load simulation system made in South Africa,a fully controlled load and freeze-thaw test system which was suitable for the simulation of cement stabilized macadam base course was designed.Based on the test system,the mechanical responses of small-scale pavement structures under the action of load and freeze-thaw were monitored.The influences of load and freeze-thaw action on the mechanical response behavior of small size pavement structure were discussed.The evaluation method of load and freeze-thaw damage of CSMB material was put forward.The results indicated that the fully controlled load and freeze-thaw test system with a complete set of pavement structure information monitoring and acquisition system and precise temperature control system could simulate freeze-thaw process of pavement structure more accurately and evaluate the mechanical behavior and damage characteristics of CSMB material by considering load and freeze-thaw action comprehensively.At last,based on the accelerated loading test results of small scale pavement structure,the fatigue life prediction equation of CSMB material recommended in current specification was optimized and modified in this thesis,the relationships between the damage of CSMB material properties/its structural properties and freeze-thaw cycles were established.Considering the influence of the bottom tensile strain index,a coupling damage factor was constructed.Through introducing the coupling damage factor into fatigue equation,a new modified life prediction equation of cement stabilized macadam base structure subjected to load and freeze-thaw action was deduced,which provided a theoretical basis for the engineering application of CSMB material.