Research On Behavior Of Recycled Cement Concrete Aggregate In Asphalt Concrete

Recycled aggregate asphalt concrete(RAAC)containing waste cement concrete aggregate,which can undertake traffic tasks,consume construction waste and reduce ore resources,is of great benefit to reducing industrial energy consumption and promoting the "carbon neutral" process.Nevertheless,existing researches almost focus on the relationship between recycled aggregate content and performances of RAAC,ignoring the influence of attached cement mortar.Due to the fact that additional cement mortar-asphalt mortar interface and old aggregate core-asphalt mortar interface appearing in asphalt concrete,a simple index of recycled aggregate content can’t reflect the effect of recycled aggregate on performances of RAAC,leading to the contradictions of related research conclusions.Therefore,based on multi-scale technical methods,this article conducted a comprehensive analysis of interface damage between recycled aggregate-asphalt mortar,RAAC fracture,and RAAC rheological behavior during mixing-compaction-service process,for putting forwarding an evaluation system of recycled aggregate reinforcement and RAAC performance improvement/repair.Firstly,a multi-scale material analysis of recycled aggregate was carried out to discover the correlation between the physical mechanics,morphology,and mineral composition.The performance difference between recycled aggregate and natural aggregate was clarified based on macroscopic performance test.The distributions of recycled aggregate morphology,attached cement mortar,original pore were statistically analyzed by means of a 3D image analysis using industrial CT scanning,image processing and visual reconstruction technology.Meanwhile,the microscopic morphology and mineral composition acquired from scanning electron microscope(SEM)and X-ray diffractometer(XRD)was quantified using differential fractal dimension algorithm and Rietveld full spectrum refined fitting.Based on these,the Pearson correlation analysis of recycled aggregate’s multi-scale properties was performed.Secondly,the behavior characteristics of the interface transition zone(ITZ)between aggregate and asphalt mortar in RAAC were explored using multi-scale technique.The changing law of interface transition zone’s morphology and mechanical characteristics was obtained using SEM and nanoindentation tests.The finite element model based on 3D virtual recycled aggregate was employed to discovery the formation and developing law of the mesodamage at the interface.Meanwhile,the influence mechanism of recycled aggregate morphology and composition on meso-damage at the interface was quanlified using genetic algorithm and BP neural network.Thirdly,a multi-scale material analysis of RAAC was performed to explore the internal causes as well as crack formation and propagation law in RAAC.The semicircular bending test(SCB)at low temperature was employed to investigate the effects of composition,damage state,environmental conditions on the macro-fracture characteristics of RAAC.The digital image correlation technique was introduced to capture the process of meso-crack formation and propagation,for revealing the path selection mechanism of crack propagation in RAAC.Meanwhile,the SCB test was simulated using a two-dimensional FEM model with different cohesive zone model(CZM)interfaces.After that,the Levenberg-Marquardt global optimization method was used to establish a unified prediction equation between the macrofracture index,the meso-fracture index and the characteristics of recycled aggregates.Fourthly,the rheological behavior of RAAC during mixing-compaction-service process was investigated based on recycled aggregate morphology and composition.The mixing torque test was performed to explore the influences of recycled aggregate characteristics,design and construction factors on rheological behavior of the RAAC mixing.With the aid of Stribeck curve theory,the gyratory compaction test and two-dimensional slice scanning technique were both introduced to figure out the influence mechanism of the recycled aggregate on the rheological behavior of RAAC compaction.By means of uniaxial penetration test,uniaxial compression test and industrial CT scanning,the meso-structure damage mechanism of the high-temperature rheological failure in RAAC was analysed.Lastly,based on the cause analysis of performance fluctuation in RAAC,an evaluation system of recycled aggregate reinforcement and RAAC performance improvement/repair were established.The evaluation contents,methods and indice of recycled aggregate reinforcement were first proposed according to previous research results and the relevant specifications.To meet application requirements of asphalt concrete,the bending beam test and freeze-thaw splitting test were employed to evaluate the RAAC performance improvement/repair.They can provide theoretical basis for the decision making of the recycled aggregate reinforcement strategy,and ensure the optimum rehabilitation of RAAC.