Bond of Reinforcement-Cable in 3D Printed Concrete (2020-07)¶

The use of high strength steel cables directly entrained into printed concrete during the printing process, has previously been introduced as a method to provide reinforcement to objects being manufactured through a layer-extrusion based 3D concrete printing process. The bond between the cable and the cementitious mortar is a crucial parameter for the structural performance of such reinforcement, and was hence subject of a detailed study presented in this paper. The bond performance was studied in direct and flexural pull-out tests on cast and printed specimens and further analyzed by microscopic analysis of the bond surface. Two effects were identified that significantly decrease the bond strength. Firstly, chemical reactions create a spongy interface ofpoor strength. Secondly, the flow ofmortar around the cable tends to create a cavity underneath the cable which reduces the effective bond surface. Mortar viscosity, nozzle design and filament pressure, were thus identified as important parameters for the bond quality. The average bond quality seems to reduce with embedment length. As a consequence, cable breakage was not achieved, in spite of considerable embedment lengths that were tested. Likely, this was caused by the cumulative probability of critical defects along the increasing embedment length, in combination with a nonconstant shear distribution. All test series showed significant scatter. It was concluded that, although this reinforcement method is promising as it can potentially provide sufficient post-cracking strength, the bond quality must be improved considerably both in terms of average strength and reduction of scatter.