Research On Design Of 3D Printable Concrete Based On Rheology And Its Basic Properties

3D printable concrete(3DPC)technology possesses many advantages like high level of informatization,automation,and flexible design for construction.It has attracted wide attention worldwide and has been successfully used in a number of projects.However,3DPC technology is still in initial stage,and there is a lack of scientific and clear mix design method to guide the design of printable concrete.From the perspective of rheology,this research conducted a systematic study on the quantitative relationship between the rheological property and compositions of 3DPC.The main research contents and conclusions are shown as follows:(1)Based on the analysis of the rheological theory for concrete,3DPC are regarded as a two-phase material consisting of paste phase and aggregate phase.The quantitative relationship between the static yield stress and components of concrete was investigated.The mixture design method for printable concrete based on the key parameters of flowability and excess paste thickness was proposed.The results show that for 3DPC with given buildability,there is a linear relationship between the flowability and aggregate content.Based on this relationship,the mixture of printable concrete with similar rheological behavior can be designed by using the flowability of paste as key parameter.At the same time,the relationship between the ratio of the static yield stress of concrete and paste and the excess paste thickness complies with the modified Coussot model.Since there exists a quantitative relationship between the flowability and the yield stress of paste,the flowability can be used to take place of the static yield stress of paste,along with the excess paste thickness,as two crucial parameters to predict the static yield stress of concrete and guide the design of printable concrete based on the modified Coussot model.(2)Based on the fiber factor theory,the effect of polypropylene(PP)fiber on the static yield stress of 3DPC was investigated by taking PP fiber as the third phase in3 DPC mixture.The results suggest that fiber factor can reflect the influence of PP fiber content and length on the static yield stress of PP fiber reinforced cement-based material.There is a relationship between the static yield stress of PP fiber reinforced concrete and the relative volume fraction of aggregate and fiber,which can provide a reference for the design of 3D printable PP fiber reinforced concrete.(3)Solid waste brick powder was adopted as supplementary cementitious materials to prepare printable concrete.The effect of solid waste brick powder on the rheological properties,printing performance,mechanical properties and microstructure of 3DPC was studied,and the feasibility of using solid waste brick powder to prepare printable concrete was discussed.The results suggest that the particle size of brick powder that is governed by grinding time is the key factor affecting the properties of3 D printable brick powder concrete.The solid waste brick powder with mean particle size of 27.4 μm,can directly replace part of cement within 30wt% in 3DPC mixture to prepare printable brick powder concrete.The solid waste brick powder with 30 min grinding time has no obvious negative impact on the print quality,mechanical properties and pore structure of concrete within suitable substitution percentage,and thus possesses potential application in the future.(4)The two-phase design method for 3DPC with coarse aggregate based on printable mortar mixed with coarse aggregate was proposed.The quantitative relationship between the static yield stress and plastic viscosity of concrete and the content of coarse aggregate was investigated,and the influence of coarse aggregate on the printability of concrete was analyzed based on rheological properties.The results show that the quantitative relationship between the static yield stress and plastic viscosity of concrete and the volume content of coarse aggregate can be established by using Coussot and Krieger-Dougherty equations separately.The mixture of 3DPC with coarse aggregate can be designed based on printable mortar to reduce the difficulty of mixture design for printable concrete with coarse aggregate.However,when the content of coarse aggregate is too high,the plastic viscosity of 3DPC with coarse aggregate is too large,which is the main reason why the concrete with high content of coarse aggregate cannot be printed.Therefore,for 3DPC with coarse aggregate,the printability cannot be evaluated solely by yield stress,plastic viscosity should be considered as well.