Optimizing Composition Of Cement-treated Aggregates Based On Weakening Interface

With excellent bearing capacity and low cast,semi-rigid materials act as the main base layer in China.At present,the longest semi-rigid base asphalt pavements have been paved in China.As a result,reflective cracking,caused by the semi-rigid base shrinkage,becomes one of the main pavement distress types in China.To minimize reflective cracking,from the point of view of energy evolution,this paper investigated the formation mechanism of reflective cracking in the semi-rigid base asphalt pavement,established the mixture design method of large stone base layer filled with cement stabilized macadam(F-LSBC),and studied the performance systematically.At first,the stress state of pavement structures was studied during the base layer shrinkage,and subsequently verified by a finite element model.Accordingy,the energy evolution mechanism of pavement structures was established during the base layer shrinkage.The results show that,when the base layer is in the linear elastic state,the base layer shrinkage is completely restricted by the base layer end constraint.When the base layer loses the strength,the shrinkage is restricted by the interlayer constraint and the surface layer end constraint.When the base layer is damage,the shrinkage is restricted by the above three kinds of constraints.Corresponding to the above stress state,during the base layer is shrinkge,the inner energy of the outside moist air is input into the base layer,making the base layer accumulate elastic strain energy.With the base layer shrinkage,the microcrack damage occurs gradually.The accumulated energy is partly dissipated as surface layer energy and partly stored in the base layer,namely energy dissipation and energy storage.Under the action of interlayer restraint,part of the energy stored in the base layer is released into the surface,and the other part is left in the base layer,namely energy release and energy surplus.Based on this energy evolution mechanism,a method for calculating the maximum value of surface layer energy is proposed,and the smaller the value is,the more difficult it is to produce reflective cracking.Furthermore,the maximum surface layer energy can be reduced by weakening the interface transition zone(ITZ)of coarse aggregates.Meanwhile,the compression fracture characteristics of the base material show that increasing the internal friction angle to ensure the bearing capacity of the base layer is necessary.According to the concept of weakening the ITZ,the skeleton dense F-LSBC was constructed,in which coarse and fine aggregates were designed separately.Firstly,the coarse aggregatess and fine aggregates were tamped and filled to determine the coarse aggregate size and fine aggregate gradation.Then,the compaction test was carried out on fillers(fine aggregate,cement and water)with different cement contents to determine the mass proportions.Subsequently,the compaction and filling test was performed on coarse aggregates and the filler to determine the mass proportion to further decide the mineral aggregate gradation.To select the optimal cement content,filler specimens and F-LSBC specimens with different cement contents were made,and the unconfined compression test and the split test were carried out.The results show that when the particle size of coarse aggregates is 19?26.5mm,and the maximum particle size of fine aggregates is 2.36 mm,the voids in mineral aggregates(VMA)is the minimum,and the mineral aggregates were in the state of skeleton dense.Although the cement content is different,the mass of the filler is always 0.3 times of coarse aggregates.When the optimal cement content of the filler is 30%,both the unconfined compressive strength and the split strength of the F-LSBC turn.Moreover,the coarse aggregate skeleton provides the strength,and the filler supports the coarse aggregate skeleton.Based on the above research,the mix design method of F-LSBC is proposed.By using the mix proportion design method,the F-LSBC specimens were prepared to systematically explore the macro and micro performance.For comparison,the corresponding performance of the traditional cement treated base(CTB)was also investigated.At first,through the nanoindentation test,the micro mechanical and microstructure characteristics of the ITZ were investigated.Then the macroscopic properties of the F-LSBC and the CTB were studied by the unconfined compression test,the splitting test,the bending test,the fatigue test,the dry shrinkage test,and the splitting loading and unloading test.The results suggest the ITZ elastic modulus of the FLSBC is 60%?75% of the CTB,and the hardness is around 55%;the porosity is approximately 1.1times;the ITZ thickness of F-LSBC is 55?90 ?m while the CTB is roughly 40 ?m.Then,the macro performance of the F-LSBC and the CTB is studied.Furthermore,the unconfined compressive strength,the indirect tensile strength,the bending strength,and the fatigue performance of the FLSBC are all worse than the CTB,while the drying shrinkage of the F-LSBC is better.Specifically,the unconfined compressive strength of the F-LSBC is about 30%?40% of the CTB,and the splitting strength and flexural strength are about 40%?45%.Meanwhile,the accumulated dry shrinkage strain and average dry shrinkage coefficient are 81.36% and 86.62% of the CTB,respectively.Furtheromre,when the F-LSBC base layer is used,the energy in the surface layer is less than 40% of that of when the CTB course being used.In addition,the cluster analysis is recommended to deal with the data of nanoindentation test.Compared with deconvolution analysis,the cluster analysis is more efficient and easy to separate the microscopic mechanical properties of the phases,which is helpful for further study.To apply the F-LSBC into pavement structures,a finite element model of the F-LSBC upper base layer asphalt pavement structure is built by using the ABAQUS.Through analyzing the mechanical response of the pavement structures S1?S7,the S4 is finally recommended.For the S4,the F-LSBC upper base layer,the CTB middle base layer,and the lime-fly ash aggregates subbase are all 20 cm.At this point,all surface layers and the F-LSBC upper base layer are subjected to compression while the CTB lower base layer and the lime-fly ash subbase layer are subjected to tension at about 50% and 75% of their allowable tensile stresses,respectively.