Physical And Mechanical Properties And Bond Performance With Steel Bars Of 3D Printing Recycled Concrete

3D printing technology is an additive manufacturing technology based on a three-dimensional digital model that builds a three-dimensional entity by stacking materials layer by layer.The application of 3D printing concrete technology to the construction field has some advantages that traditional in-situ concrete cannot match.Incorporating solid construction waste into 3D printing concrete and adopting the development model of "digital construction and green construction" can not only take advantage of the advantages of 3D printing technology,but also utilize solid waste and reduce the consumption of natural resources,which is a promising development direction of 3D printing concrete.The current study replaces cement with the recycled powder produced during demolishing waste concrete,and replaces natural river sand with the recycled sand separated from engineering muck.The printability,mechanical properties after hardening and bonding performance with steel bars of 3D printing concrete mixed with recycled powder and recycled sand were evaluated.The main research work and conclusions of this study are as follows:1、The printability tests of 3D printing concrete mixed with recycled powder and recycled sand were conducted.The effects of the replacement ratio of recycled powder and recycled sand on the extrudability,fluidity,buildability and open time were evaluated.The appropriate replacement ratio of recycled powder and recycled sand was obtained,and a reasonable range of fluidity of 3D printing concrete with recycled powder and recycled sand to meet printability was proposed.The results show that:(1)With an increase of replacement ratio of recycled powder,the extrudability,fluidity and open time of fresh concrete decreased,while the buildability increased first and then decreased.The printability of fresh concrete was well when the replacement ratio of recycled powder was below 10% and the replacement ratio of recycled sand had little effect on the printability of fresh cooncrete.(2)For the 3D printing concrete mixed with recycled powder and recycled sand,the printability of fresh concrete was well when the fluidity and slump of fresh concrete were in the range of 150～180 mm and 25～55 mm respectively.2、The compression tests of in-situ concrete with different replacement ratio of recycled powder and recycled sand were carried out.After obtaining an appropriate replacement ratio of recycled powder and recycled sand,the compressive,split tensile and flexural strength tests of 3D printing concrete with 10% replacement ratio of recycled powder and 100% replacement ration of recycled sand were conducted,and the results were compared with the corresponding strength of in-situ concrete.The results show that:(1)The compressive strength of concrete was increased first and then decreased as the replacement ratio of recycled powder increased,and the compressive strength of concrete with less than 10% replacement of recycled powder and 100% replacement ratio of recycled sand was not significantly reduced.(2)The compressive strength of 3D printing concrete with recycled powder and recycled sand was greatly discrete and significantly anisotropic.The 28-day compressive strength along the printing strip direction(x direction)of 3D printing concrete was higher than that of in-situ concrete by 33%,while the compressive strength in the other directions(y and z direction)of the former was slightly lower or equivalent to the latter.However,the split tensile strength in the three directions of 3D printing concrete was comparable to that of in-situ concrete,while the flexural strength in y and z directions of the former was about twice that of the latter.3、The center pull tests were carried out on 21 steel bar-3D printing concrete and 2 steel bar-in-situ concrete bond specimens.The effects of concrete casting method,addition of recycled powder and recycled sand,types of steel bar,anchoring length of steel bar,printing direction and whether the steel bar is pressed and embedded on the bonding performance between steel bar and 3D printing concrete were investigated,and the effectiveness of the methods such as welding studs on the surface of steel bar and sprinkling fibers at the interface between steel bar and concrete on improving the bonding performance was studied.The results show that:(1)The bond strength of 3D printing concrete was not always lower than that of insitu concrete specimens,but the bond strength of 3D printing specimens was more discrete.(2)Compared with 3D printing concrete specimens without recycled materials,the bonding strength of the specimens incorporating 10% recycled powder and 100% recycled sand had a certain reduction.(3)Under the conditions of the present study,the steel bar in the specimens with a bond length of 7d and 9d(d is the diameter of the steel bar)yielded before being pulled out.(4)Welding studs on the surface of the steel bar could greatly increase the bond strength and reduce the relative slippage between the steel bar and the 3D printing concrete,but the sprinkling of fibers at the steel bar and concrete interface would reduce the interface bond strength.