Mechanical Behavior Of 3D Printed ECC And Failure Mechanism Analysis Under Tensile Or Bending Loads

3D Concrete Printing（3DCP）technology is a new construction method,which is intelligent and mechanized,with the advantages of low-carbon-emission,low-cost and short-construction-period,emerging in recent years.The major obstacle for implementation of 3DCP is the need for steel reinforcement and the challenges this presents to the 3DP process.Engineered Cementitious Composites（ECC）is a new construction material,which hold promise to attain structural integrity,durability,reliability and robustness without steel reinforcement,designed based on the theory of micromechanics.But its mechanical stability depends on the dispersion and orientational distribution of fibers.Therefore,the difficulty of reinforcing 3DCP may can be solved by the self-reinforced performance of ECC,and the fiber orientation pursuit of ECC is well matched with the constraint effect of 3DCP nozzle.In this paper,the mechanical behavior research and mechanism analysis of printable ECC（3DP-ECC）are carried out in depth.The main works and achievements are as follows:（1）.A new rapid assessment method of printability of cementitious composites was established based on Bingham model.By this method,the printable range of rheological parameters can be calculated theoretically for various printing scene.And then the rheological parameters can be converted into two easy-measured workability parameters.As a result,the printability of a mixture can be estimated efficiently,since the method avoid the rigorous test method or repeated trial and error.Then,the method was used to evaluate the effect of each component on printability quantitatively.Finally,the optimal printable ECC which satisfied both workability and mechanical requirements was achieved.The table flow printability range is 150 mm<d<180 mm,the slump printability range is 45 mm<s<70.5 mm,and the open time is about 18 mins.Meanwhile,the ultimate tensile strength is 3.17 MPa and the ultimate tensile strain can reach 4.22%.（2）.The split tensile test and double-sided shear test showed that the layer bond strength of 3DP-ECC decreased with the increase of interval time.On the other hand,the cube uniaxial compression test showed that the compressive strength of 3DP-ECC was improved and performed obvious anisotropy:f_z>f_x>f_y>f_cast.（3）.A theoretical calculation method was developed to quantitative characterize the constraint effect of 3DP process on the orientation of fibers according to various sizes of nozzle and fibers.The fiber orientation probability density function p（θ）was calculated by this method,and then it was embedded into the ECC tensile model to simulate the performance of 3DP-ECC.Both experimental study and theoretical simulation results showed that the printing process can significantly improve the tensile properties of ECC,and the improvement effect increased with the nozzle diameter decreased.The ultimate strength and ultimate strain of 3DP-ECC were 18.6%and 39.4%higher than those of cast specimen,respectively.（4）.Four-point bending test results showed that the layered structure of 3DP-ECC can change the crack propagation mode and improve the bending toughness.Three typical crack propagation modes of ECC beams under bending load were found and summarized:bending crack penetration,bending crack penetration-deflection circularly and shearing crack deflection.The mode change resulted that the bending toughness rose and fell with the increase of the interval time.In addition,two bending crack propagation criterias to assessed the relative tendency of a bending crack to deflect or penetrate were established based on fracture mechanics theoretically,and the toughening mechanism of 3DP-ECC beam formed by"multi-layer and multi-crack mode"was analyzed,which was verified by microscopic pore analysis and macroscopic fracture toughness test results.