Study On Design?preparation And Performace Of 3D Printed Cement-based Materials

3D printing,integrated by digital engineering,intelligent manufacturing and material engineering,etc.is referred to as a rapid prototyping process that sequentially deposits materials in a layer-by-layer manner to build the expected product.Unlike conventional construction methods,3D printing has the ability of free-form construction and highly automatic operation,it can reduce significantly the manufacturing costs of complicated objects,as well as contribute to higher construction efficiency,less intensive labor and less waste production.As the leading technology to upgrade the construction industry,the mix composition and preparation process of printable concrete“ink”are critical to successful printing.In cement-based material,the mixture of mineral admixtures and organic admixtures is chosen and incorporated to achieve instantaneous coagulation/colloid structure upon the removal of shear stress,which is very key to obtain a stable geometry of 3D printed cement-based material(3DPC).This research initially depicts the design and manufacture of an industrial-grade type 3D printed concrete equipment system.Then,the development of 3DPC with a good bleeding and stable shape retention via the addition of mineral admixtures and organic admixtures was presented.The effect of various key factors on the rheological properties and printability of 3DPC was systematically investigated by experiment.Meanwhile,the closed relationship between 3DPC layered structure and harden properties was elaborated by macro-scope test and modern analysis techniques with the combination of model proposed.In addition,the influence of the 3DPC layered structure on durability was also firstly analyzed in-depth in this work.Several important research results are as follows:(1)Design and manufacture of industrial-grade 3D printed concrete equipmentBased on the principle of extrusion-based 3DPC,an industrial-grade type concrete3D printing equipment system with a build volume of 1.5×1.5×1.5 m~3 was designed and manufactured independently.In printing system,the walking speed of nozzle,the extrusion rate of screw twisted and the stirring rate of blades can be coordinated and controlled all the time based on the ideal of separating the control module of printing process and the XYZ axis motion.Finally,a progressively operation process was formed:model design and digital processing system?concrete 3D printer?3D object for extrusion-based 3DPC.(2)Mix proportion design,preparation and rheological properties of 3DPCIn this work,according to the results of the rheological test,hydration heat evolution and printing process,the influence of mixtures of mineral admixtures,organic admixtures and cement on the buildability,rheological properties and the formation of coagulation/colloid structure of 3DPC was investigated,the partnership amongst various constituents in 3DPC was proposed as well.At the same time,the effect of the key factors such as water to binder ratio,aggregate to binder ratio,kinds and content of organic admixtures,etc.on the rheological properties of 3DPC was emphatically studied,the rheological model of 3DPC that can be used to reveal the principle of its excellent printability was established.(3)Printability of 3DPCIn this section,the influence of water to binder ratio,aggregate to binder ratio and polypropylene fiber,kinds and content of organic admixtures,etc.on the printability of3DPC was systematically investigated,which provided the basis for the establishment of evaluation system on the printing performance of 3DPC.In the meantime,with the combination of early age compressive strength of the 3DPC cylinder and Pascal liquid(solid)pressure theory,a prediction model on the height of 3DPC sequentially deposits materials was proposed,also verified by experiment results,which can be used to ensure the quality,size,and aesthetics of 3DPC buildings during printing process.(4)Mechanical properties of 3DPCThe effect of curing age and preparation methods on the mechanical strength of3DPC was studied,the anisotropy mechanism of 3DPC layered structured was revealed by using X-ray computed tomography(X-CT):it is found that the orientated voids are connected or even continuous between the layers(adjacent layer),as well as the interface zones between the aggregates,fibres and paste were orientated,besides,an effective method was proposed to reduce the anisotropy of 3DPC.Thereafter,the effect of curing age,stacking interval time and numbers of stacked layer on the bond performance between the layers(upper and bottom layers),the closed relationship between voids distribution and bond strength was quantified with accordance of mercury intrusion porosity(MIP).Moreover,based on the results of X-CT,a void evolution model between the layers was established.(5)Durability of 3DPCThe shrinkage,anti-frost,chloride penetration and sulfate corrosion resistant were carefully studied,and the effect of 3DPC layered structure on the evolution law of durability was also revealed.Furthermore,based on the theories of capillary adsorption,the process of water transport at different interlayer zones of 3DPC was in situ continuously visually tracked by the technique of X-CT combined with Cs enhancing.According to the water transport process,a quantitative relationship between the capillary coefficient and voids distribution at the interlayer was revealed.These results will provide a strong scientific bases to design and prepare 3D printing concrete.