Experimental Study On Creep Characteristics Of Recycled Asphalt Pavement As Fills

Beneficial reuse of solid wastes has not only disposed of high-volume wastes,but also reduced the consumption of natural resource as well as decreased environmental pollution.This study proposes to use the recycled asphalt pavement(RAP)as backfill material in view of the current situation that RAP has a large recoverable volume,low utilization rate and narrow utilization scope.Through experimental research and theoretical analysis,the basic physical and mechanical properties of RAP as backfills are studied.Considering the viscosity and temperature sensitivity of asphalt in RAP,the direct shear creep tests at different compaction temperature,vertical stress and shear stress level were carried out using the stress-controlled direct shear apparatus modified from the common direct shear apparatus.The creep deformation characteristics and the creep rupture mechanism of RAP were explored,and a creep model was thereafter established to consider the effect of compaction temperature,all of which can provide the theoretical basis design recommendations for the beneficial reuse of RAP.Main works are summarized as below:1)Determination of basic properties of RAP and formulation of testing programeA bulk of RAP samples were taken from local road maintenance company to conduct conventional laboratory tests.Basic properties of RAP can be obtained such as asphalt content,specific gravity,grain size distribution characteristics and compaction characteristics,etc.The applicability of RAP as backfills is analyzed by comparing it with traditional backfill such as sand and stones.Through modifying the conventional strain controlled direct shear apparatus,the shear box size was re-designed concerning the specimen particle effect,with upper and lower half shear box are with 100 mm in length,100 mm in width and 25 mm in height.The direct shear strength parameters can be obtained by the strain controlled loading system.In order to perform direct shear creep test on RAP,constant horizontal shear stress can be applied by modifying the strain controlled loading system to stress controlled loading system through setting up a fixed pulley system to the shear box.A detailed strain controlled direct shear test and stress controlled direct shear creep test program is thereafter made Considering the influences of compaction temperature,vertical stress and shear stress level.2)Experimental study on creep properties of RAP at room temperatureDirect shear tests were conducted using modified strain controlled direct shear apparatus on compacted RAP specimens at room temperature with four different vertical stresses(50,75,100 and 200kPa)to get the shear strength parameters.Direct shear creep tests were thereafter performed using modified stress controlled direct shear apparatus on compacted RAP specimens at room temperature with four different vertical stresses(50,75,100 and 200kPa)and different shear stress levels(?=0.9,0.85,0.8,0.7,0.6,0.4),where ? is definded as the ratio of applied horizontal shear stress to the maximum shear stress obtained in the direct shear test.The variation of shear strain and shear strain rate with elapsed time under different vertical stresses were investigated and the mechanism of creep rupture is then revealed.Based on the threeparameter creep model of soil given by Singh-Mitchell,a three-parameter model reflecting the creep characteristics of RAP is established.The validity of the model proposed in this study is verified by comparing the calculated results and the experimental results.3)Experimental study on the direct shear creep response of RAP under different compaction temperaturesFor simulating the effect of different season temperatures on RAP as backfills in the construction process,direct shear tests were conducted using modified strain controlled direct shear apparatus on compacted RAP specimens at two compaction temperatures(0℃ and 50℃)under four different vertical stresses(50,75,100 and 200kPa).The effect of compaction temperature on the direct shear strength behavior and its mechanism are explored based on the analysis of testing results.to get the shear strength parameters.Direct shear creep tests were thereafter performed using modified stress controlled direct shear apparatus on compacted RAP specimens at two compaction temperatures(0℃ and 50℃)under four different vertical stresses(50,75,100 and 200kPa)and different shear stress levels(?=0.9,0.85,0.8,0.7,0.6,0.4),where ? was determined as ratio of the applied constant shear stress to the maximum shear stress obtained under the same vertical stress in direct shear test at room temperature.The shear stress levels for compacted RAP specimens at temperature of 0℃ are ?=0.9,0.85,0.8,0.75,0.7,0.6,0.4,and at temperature of 50℃ are ?=0.95,0.9,0.85,0.8,0.6,0.4,respectively.By comparing the creep characteristics at room temperature,the effect of temperature on creep properties of RAP and its mechanism were revealed.4)Creep model for RAP considering the effect of compaction temperature.The variation of shear strain and shear strain rate with elapsed time were compared under the same vertical stress and shear stress level at different compaction temperatures(0 ℃,20 ℃ and 50 ℃)to investigate effect of temperature on creep characteristics of RAP and the mechanism.Based on the three-parameter creep model established at room temperature and the quantitative variation of three model parameters with the compaction temperature,the creep model is extended to a modified three-parameter creep model considering the effect of compaction temperature.The validity of the model is verified by comparing the measured and model predicted results.It will help to provide a design guidance for practical application of RAP as backfills.