| 3D printing on cement-based materials is one of the hot topics in the field of civil engineering materials.In this project,to solve the problems of cold joints and cavities usually presenting in the 3D-printed body of cement-based materials,improvement design on the nozzle part of the existing 3D printers was carried out,namely,adding a portal constraint box below the nozzle.And the influence of constraint conditions on the defect state and mechanical properties in the printing body was studied through X-CT detection and mechanical properties test.The following results were obtained.(1)The effects of water-cement ratio,delay time,and thixotropic agent and its addition amount on the thixotropy of mortar were studied.Combined with the results of jumping table and embryogenic strength tests,the constituent ratio of a9 mortar was determined to be the standard ratio for subsequent 3D printing,namely,the lime sand ratio was 1:1.2,the content of high-efficiency polycarboxylate water-reducing agent 0.3%,the water-cement ratio0.34,and the content of nano-clay addition 0.3%.The compressive strength of the casting sample with this constituent ratio is 54.3 MPa,and the flexural strength 13.67 MPa,the splitting strength 2.16 MPa.(2)Improvement on the nozzle part of the existing 3D printer was designed,namely,adding a portal constraint box below the nozzle that was manufactured by light curing 3D printing technology.3D printing experiment with this box shows that this improvement design significantly changes the flow state during the mortar deposition process and effectively solves the cavity problem in the 3D printed body of cement-based material.Adding the portal constraint box can reduce the holes in the 3D printed body,and improve its splitting strength and flexural strength by 6% and 34%,respectively,compared with the sample without the portal constraint box.Further setting the square grooving on constrain plate of the box increases the interface area of printing layers,which can absorb more fracture energy that greatly increases the interface strength.As a result,the splitting strength and flexural strength are increased by 89% and 126%,respectively,compared with the sample without the portal constraint box.These exceed the relevant strength data of the casting sample.(3)The fiber net was laid between the printing layers,which can reduce the porosity at the interface.This can its flexural strength increased by 116% compared with the sample with no portal constraint frame,but the splitting strength decreased.(4)Combined with numerical simulation,the design of portal constraint box was further improved,namely,increasing the width of the box and reducing the height.This produces very good effect of 3D printing.This improvement design considerably raises the strength of the printed sample.The splitting strength and flexural strength are increased by not set box door shape constraint samples increased by 86% and 113% respectively,compared with the sample without the portal constraint box.These also exceed the relevant strength data of the casting sample.The shape of the grooving has considerable influence on the strength of the printed samples.Triangular groove can make the splitting strength and flexural strength increased by 97% and 123% respectively compared with the sample without the portal constraint box and semicircular grooving by 106% and 143% respectively.The sample with the highest comprehensive performance is k8 sample with semicircular groove on the improved portal constraint box.k6 sample with the improved portal constraint box has the highest compression strength.These results provide valuable experimental evidence for improving the performance of 3D-printed cement-based materials. |
