Preparation And Performance Research Of Ultra-high Performance Concrete Based On 3D Printin

In recent years,3D printed concrete has become a hot topic in the construction field because of its large degree of freedom in design,high construction efficiency,energy conservation and environmental protection.However,the traditional concrete materials used in 3D printing have defects such as low strength and poor toughness,which makes the development of 3D printing concrete technology face a huge challenge.Ultra high performance concrete(UHPC)based on close packing theory has good mechanical properties and durability,in which steel fiber can effectively improve its toughness.Therefore,it is very necessary to study UHPC that meets the requirements of 3D printing construction.The main research contents and conclusions are as follows.Study the benchmark mix proportion of 3D printing ultra-high performance concrete.By measuring the standard consistency and minimum basic water demand of the mixing system,the analysis shows that the best ratio of UHPC cementitious material powder mixing system is cement: Fly Ash: mineral powder: Silica Fume: Gypsum = 0.65:0.1:0.1:0.1:0.05.The effects of mineral admixtures,proportioning parameters and admixtures on the flow and mechanical properties of UHPC were studied by orthogonal test,and the benchmark mix proportion of UHPC was obtained by range analysis.The results show that silica fume has the most significant positive effect on the fluidity and mechanical properties of UHPC in mineral admixtures,and steel fiber plays a leading role in both early strength and late strength.In order to prepare 3D printing ultra-high performance concrete with high early strength to meet the needs of 3D printing technology for continuous printing,sulphoaluminate cement was used to replace some Portland cement,and the effects of different replacement amounts on the setting time,mechanical properties and flow properties of UHPC were studied.The results show that the best substitution amount of sulphoaluminate cement for Portland cement is 20%.The performance test of the obtained optimal mix proportion shows that: the jumping table fluidity is 197 mm;The initial setting time is 23 min;The compressive strength at 3 h and 28 d is 20.08 MPa and 149.35 MPa respectively;The flexural strength at 3 h and 28 d is 6.5 MPa and 29.78 MPa respectively,indicating that the strength of UHPC develops stably,and the fluidity and initial setting time can also meet the requirements of 3D printing.According to the process characteristics of 3D printing technology,the effects of the interval time between the lower layers of the optimal mix ratio and the ambient temperature on the mechanical properties of UHPC are studied.The results show that with the increase of the interval time between the layers,the 28 day compressive strength,flexural strength and bond strength of the specimens are significantly reduced.When the interval time is 5 ～ 15 min,the freshness and bond strength of UHPC reach the best balance state;The ambient temperature has a great influence on the early internal microstructure of UHPC.The increase of temperature is conducive to the hydration of cementitious materials,improves the integrity of the interface between gel and aggregate,and improves the early compressive strength of the specimen.At the same ambient temperature,the addition of steel fiber can not significantly change the micro morphology of ultra-high performance concrete,indicating that the increase of flexural strength and compressive strength of concrete by adding steel fiber is mainly due to the physical role of steel fiber.Through the freeze-thaw cycle test,the freeze-thaw resistance of 3D printed UHPC is tested.The results show that the freeze-thaw cycle has little effect on the appearance and quality of the specimen,and the ability of 3D printed specimen to resist the freeze-thaw cycle is slightly inferior to the cast specimen,and the overall difference is not significant;Through the scanning electron microscope analysis of the influence of forming method on the freezing resistance of UHPC,it can be seen that the reason why the 3D printed specimen has good freezing resistance is that the bridging effect of steel fiber effectively resists the formation of micro cracks in the structure.