A Mechanism Study On The Effect Of Pore Structure Defects On The Mechanical Properties Of 3D Printed Coarse Aggregate Concrete

3D printed concrete construction technology,as the key to the development process of intelligent construction in the construction industry,has rapidly promoted the deep integration of the global construction industry in the field of "information" and "intelligence" and has become the core component of the future intelligent construction field of the construction industry.Compared with traditional concrete construction,3D printing concrete construction technology has significant advantages such as no formwork support,significantly reducing labor costs and shortening the construction cycle.However,the current widespread application of 3D printed concrete for fine aggregate concrete did not achieve coarse aggregate incorporation,and the lack of coarse aggregate led to a material system with a high proportion of cement and sand,resulting in a 3D printed concrete structure with poor stress performance,high drying shrinkage,shortening the service life of the structure and other issues limiting the application and development of the technology.Therefore,the application of 3D printed coarse aggregate concrete(3DPCAC)has become the key to solving this problem.However,the application of 3DPCAC has also given rise to new problems,most notably the formation of pore structure defects at the interlayer and interstrip interfaces of 3DPCAC that are significantly different from those of cast concrete and 3D printed fine aggregate concrete(3DPFAC),which have a significant impact on its mechanical properties.However,few related studies have been reported.In this paper,we focus on the unique pore structure defects of 3DPCAC based on the improvement of 3DPCAC buildability,investigate the influence mechanism of 3DPCAC interlayer bonding performance,basic mechanical properties and bond-slip properties with steel rebar,and aim to reveal the basic force behavior characteristics of 3D printed concrete structures and provide theoretical support for the design and construction applications of 3D printed concrete structures.The main research contents and results are as follows:(1)The effects of different setting times on the rheological properties of 3DPCAC were investigated.Based on the optimal design of 3DPCAC material components with recycled coarse aggregate(RCA),the effects of RCA replacement ratios on the timevarying growth pattern of yield stress and shear modulus of 3DPCAC were studied,and the evolution mechanism of rheological behavior of 3DPCAC subjected to shear and the differences with 3DPFAC were revealed.The constructability of 3DPCAC was evaluated in terms of strength,stiffness and stability,and a 3DPCAC constructability failure model was established.(2)The morphological characteristics of the 3DPCAC surface,interlayer and interstrip interfaces,and the printed component surface as a whole were analyzed and compared with 3DPFAC.Focusing on 3DPCAC pore structure defects,X-ray computed tomography(X-CT)tests were used to analyze the effects and differences of different printing layer heights and interlayer printing intervals on pore structure parameters such as porosity distribution,pore three-dimensional spatial distribution and pore orientation characteristics of 3DPCAC and 3DPFAC.The mechanisms of differential effects of different construction methods of 3D printing and casting on the pore structure defect parameters were elucidated,revealing the formation and geometric configuration evolution mechanisms of 3DPCAC pore structure defects and the force behavior characteristics of pores under different loading directions.(3)Focusing on the pore structure defect characteristics of the 3DPCAC interlayer interface,the effects of the layer height and interlayer printing interval as well as different RCA replacement ratios on the bond strength of the 3DPCAC and 3DPFAC interlayer interface were compared and analyzed.Based on the characterization of the debonding interface morphology by digital image correlation tests,the interlayer interface cracking behavior and damage morphology were analyzed,the 3DPCAC interlayer interface bonding system was established,and the mechanism of the influence of pore structure defects on the bonding performance of the 3DPCAC interlayer interface was revealed.The 3DPCAC interlayer interface bond strength calculation equation was proposed,and the interlayer bond strength calculation model based on printing time interval was established.(4)Focusing on the characteristics of 3DPCAC pore structure defects,the effects of different loading directions,RCA replacement ratios,ages and construction methods on the compressive,flexural and splitting tensile properties of 3DPCAC and 3DPFAC were comparatively investigated.The damage process and damage morphology of 3DPCAC were analyzed,the 3DPCAC multiple partition-interface model was established,and the mechanism of the influence of pore structure defects on the cracking and damage behavior and anisotropic mechanical properties of 3DPCAC was revealed.The anisotropic mechanical property evaluation method of 3DPCAC was proposed,and the suggested calculation formula for the basic mechanical strength of 3DPCAC was established.The research results can provide important theoretical support for the design and application of 3D printed concrete structures.(5)The 3DPCAC-rebar bond slip properties of 3DPCAC strips and rebar under the influence of different arrangement directions,construction forms and RCA replacement ratios were investigated,and comparative analysis with 3DPFAC was carried out.The characteristics of the concrete-rebar bond interface composition were analyzed by X-CT and Scanning Electron Microscope(SEM)tests,the bond interface-partitioning model of3 DPCAC and rebar was established,the influence of pore defect distribution on the3DPCAC-rebar bond interface was discussed,the influence mechanism of the bond interface on 3DPCAC-rebar bond slip damage was revealed,and the optimal placement method parallel to the 3DPCAC printing plane layer was proposed.Finally,a bond-slip calculation model of 3DPCAC and rebar considering the pore structure defects at the bond interface was established,and the prediction of the bond-slip damage process of3 DPCAC and rebar was realized.