Investigation Into The Mechanical And Durable Interlayer Bonding Performances Of 3D Printing Concrete

As a formless,fast and flexible advanced construction technology,3D concrete printing(3DCP)has been successfully applied in the field of construction.However,the inherent forming process of layer-by-layer stacking makes the 3DP concrete structure have regular interlayer weak surfaces,resulting in the meso-heterogeneity of the material,weakening the mechanical bearing capacity and durability of the structure and restricting the utility and application of this technology.It is of great significance for promoting the engineering popularization and application of 3DP intelligent construction to study the formation mechanism of interlayer weak surfaces of 3DP concrete structures,explore the bonding mechanism between layers,develop interface strengthening materials,and improve the continuity and durability of 3DP concrete.In this paper,firstly,the research progress and achievements of 3DP concrete interlayer weak surface are summarized,the formation of 3DP concrete interlayer weak surface is analyzed,and the mechanical influence factors of interlayer bonding are clarified.Polymer modified mortar was prepared to improve the weak bonding of the interlayer interface.The effect of polymer modified mortar on improving interlaminar bonding properties of printed interface was tested experimentally,the effects of interval time and interfacial bleeding on interlaminar tensile strength and shear strength were analyzed,and the durability of 3DP concrete was tested experimentally.the effect of interlayer interface on carbonization resistance and frost resistance is analyzed.At the same time,the interlaminar bond behavior of 3DP concrete is simulated by density functional theory(DFT)and molecular dynamics(MD)method,and the mechanism of interlaminar improvement is revealed.The results show that:(1)the formation of weak interface between concrete layers is mainly due to 3DP technology,geometry,physics,materials and other factors.Micro-focus CT and SEM micro-test showed the microstructure and morphology of the interlaminar weak surface;(2)under different interlayer intervals,polymer modified mortar can obviously improve the interlaminar bond properties of printed concrete materials,and the effect of epoxy resin mortar on improving interlaminar tensile properties is obviously better than that of neoprene mortar.The polymer modified mortar with 8% epoxy resin content is the optimal mix ratio of the interfacial reinforcement material;(3)the carbonation resistance and frost resistance of the printed structure are weakened by the weak surface of the interlayer.When the carbonization time is 28 days,the carbonization depth at the interlaminar weak surface of the printing block is 226.04 % deeper than the inherent carbonization depth of the printed material.After 150 freeze-thaw cycles,the mass loss rate of the printed block is 188.96 % higher than that of the cast block;(4)the molecular dynamics simulation results show that the electrostatic interaction(Coulomb force)between epoxy resin and calcium ions from hydrated calcium silicate(C-S-H)counteracts the weakening effect of water molecules(surface water)on the interlaminar bond strength,thus significantly improving the interfacial mechanical properties of printed concrete materials.