| Cement stabilized macadam base asphalt pavement is a typical type of pavement structure in China.Due to the requirement of bearing capacity,the thickness of base is usually larger than the maximum rolling thickness required by construction specifications,which makes the cement stabilized can't be paved at once.With the development of high-power paving and rolling equipment in recent years,many owner and contractor hope to increase the rolling thickness of cement stabilized macadam base,so the construction period can be shortened and the integrity of base can be enhanced.However,the application of large-thickness(more than 20cm for one roller compaction)cement stabilized macadam base doesn't work well in some highway projects.The high-power compaction machine over-crushes the surface and upper part of the large-thickness base with compaction degree exceeding 102%.A large amount of coarse aggregates are crushed and the skeleton is damaged,It satisfies the overall compaction degree of the base,but causes great damage to the upper part of the base,resulting in defects in the overall mechanical properties of the thick base.On the other hand,“The technology conditions for construction of highway pavement base”published in 2015 proposed new requirements for the composition design,paving compaction and interlayer treatment of cement stabilized macadam mixture.Meanwhile,a higher standard was put forward for the raw materials,like the coarse aggregate content,aggregate crushing value and the needle flake content.Moreover,gap-graded interlock structure is recommended for the design of cement stabilized macadam base;In addition,the test procedures and evaluation criteria of this kind of cement stabilized macadam mixture with dense skeleton are basically suspension cement stabilized macadam mixture,which is not reasonable.As a base course of asphalt pavement,the cement stabilized macadam base course with large thickness should not be stressed on compactness,no need for excessive rolling,under the consolidation of cement paste,depending on the fine skeleton structure of the mixture,the bearing capacity of the thick base course can be realized,whether the skeleton structure is good or not should be taken as the key point of gradation evaluation of the mixture,to a certain extent,it should take into account both its voidage and compactness.How to design the cement stabilized macadam mixture with excellent skeleton structure and without large tonnage and overpressure of high-power machines is the precondition for the one-time paving and rolling construction with large thickness.However,the main problems are:how to evaluate what gradation of cement stabilized macadam mixture has excellent skeleton structure;There is no cement stabilized macadam mixture gradation for the construction of large thickness;There is no specific design method for the target mix ratio of cement stabilized macadam mixture for the construction of large thickness,and there is no suitable evaluation standard for indoor test.Besides,the construction quality and surface condition of base are usually neglected by the constructer,since the defects are covered by the upper asphalt layer.Thus,the poor interlayer conditions between base and surface are always ignored,resulting in bad pavement performance and service life of the whole pavement structure.Therefore,the treatment of base-surface layer also needs special study.In order to solve the problems existing in the application of large-thickness cement stabilized macadam base,four stages of studies will be conducted.First,according to the gradation design method of graded mixing,the discrete element method will be used to optimize the design method of large-thickness cement stabilized macadam mixture.The stability of skeleton structure will be evaluated from the views of skeleton,nodes and voids within the mixture.Second,the determination method of maximum dry density,interior vibration molding parameters and the evaluation standard of skeleton stability will be put forward to the construction of large-thickness cement stabilized macadam base through a series of laboratory mechanical tests.Third,overpressure is not required in the construction of large-thickness base,that is,under normal compaction,the problem of overpressure failure is solved by relying on the structural performance of cement stabilized macadam mixture.Therefore,the discrete element modeling method is adopted to simulate the vibration pressure of the cement stabilized macadam base with a stable skeleton,and the specific parameters are proposed to guide the practical engineering application of the cement stabilized macadam base with a stable skeleton.Finally,the finite element method was adopted to simulate the interlayer condition between base and surface.A high-performance prime coat material was developed for the treatment between base and surface.The condition between surface and base was classified.Different treatment methods were proposed for different interlayer conditions.Several conclusions can be made from this study:(1)According to the classification of blending gradation design method,the discrete element modeling from skeleton,gap,voids and detailed study of the mesoscopic structure of the cement stabilized macadam mixture,this study put forward a series of skeleton stability evaluation and the control index,such as stress reduction ratio,suspended coarse aggregate content,"unsuspended"coarse aggregate average coordination number,the number of the contact points of the stress concentration,the average stress concentration contact increment,reducing the average void fraction,average coefficient of variation of void fraction.Three kinds of gradations(G4?G7?G11)with excellent skeletal stability were designed by using the stability evaluation index of micro-structure to be applied to large-thickness base.(2)The evaluation index of micro-structure was used to define the skeleton stabilized cement-stabilized macadam mixture,and the cumulative difference of sieve hole pass rate was used as the evaluation index of gradation decay of the mixture.And then,confirm the indoor forming method and the method of the cement stabilized macadam mixture which face the construction of large thickness:the vibration-forming method,the vibration molding parameter is set as follows:vibration frequency is 30HZ;Amplitude is 1.5mm.Static pressure is less than 100KPa;The excitation force is 5000~7000N;The eccentricity Angle is 60~o;The shaking time is 120s.Propose the evaluation index of skeleton stability of cement stabilized macadam mixture for large-thickness construction:CBR;According to the strength evaluation index,the evaluation standard of the indoor test of the cement stabilized macadam mixture for construction of large thickness is proposed as follows:the CBR value of the skeleton stability is not less than 450%;The unconfined compressive strength at 7 days is not less than 5MPa;The bending strength of 28 days is no less than 1.2MPa.(3)The construction vibrating compaction time of the G4?G7 and G11 skeleton stabilized cement macadam mixture with large thickness should not be less than 120s or more than 180s.The three-level(G4?G7?G11)skeleton stabilized cement stabilized macadam mixture should not be used for one-time spread and rolling over 35cm when it is used as the base of large thickness.Vibration compaction after 120s,stable skeleton type(G4?G7?G11)of the cement stabilized macadam mixture,the upper and lower levels of segregation is about5%,the middle segregation is about 2%,the degree of segregation was significantly less than the suspended dense type cement stabilized macadam mixture,select the skeleton stability gradation of cement stabilized macadam used in construction of big thickness can effectively reduce the segregation of construction.After dynamic water scour,the scour resistance of skeleton stabilized macadam base(G4?G7?G11)with large thickness of cement stabilized macadam base is significantly better than that of suspension compacted macadam base.(4)Based on the basic concept of"special treatment of special sections"and the analysis of shear stress,the work state classification between layers of the base surface is proposed.Based on single factor,the work state between layers of the base surface is divided into three levels:"1","2"and"special".The quality control index is proposed.The measures of interlayer permeability and roughening treatment in different sections are proposed.The treatment measures of"1"level are as follows:emulsified asphalt permeable layer+top surface structure depth of base is equal to 1.2mm;The treatment measures of"2"level are as follows:the permeable layer of highly permeable emulsified asphalt+the structural depth of the top surface of the base is greater than or equal to 1.2mm;The treatment measures of"special"level are as follows:build-in prime coat+top surface structure depth of base is greater than or equal to 1.2mm.The significance of this study is:design big thickness skeleton stable cement stabilized macadam mixture gradation,complete its target mixture ratio design,put forward suitable for stable skeleton type of cement stabilized macadam mixture indoor test methods and evaluation standard,simulate vibration pressure implementation work to provide technical support for practical application,solve the problem of bonding between base-surface layer upon layer.It lays a good foundation for gradually increasing the construction thickness of cement stabilized macadam base. |
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