Vibrated Short Rebar Insertion - The Effect of Integration Time on the Resulting Bond Quality

Additive manufacturing is gaining popularity in the construction industry due to its advantages in producing complex concrete components. However, current techniques often focus on unreinforced concrete, which poses a challenge for structural elements, that usually require reinforcement. A promising solution is the integration of short rebars, where concrete layers are first printed and then reinforced with inserted rebars. Thus, the bond quality of the inserted rebar is highly dependent on the rheological properties of the printed concrete. This study investigates if vibrating the short rebar during insertion can improve the bond zone by locally fluidising the surrounding concrete. Rebars are inserted into fine-grained 3D printing concrete both with and without vibration. To study the effect of the concrete structural build-up, i.e. rheology, rebars are integrated at three different time steps (10, 30 and 60 min). The yield stress at each integration time is measured using penetrometer tests. The bond strength is evaluated by pull-out tests according to RILEM RC 6. For reference, conventional specimens are produced in moulds. The results show a decrease in bond strength with increasing integration time for all specimens. A negative correlation between the concrete yield stress and the resulting bond strength is observed. However, the vibration of the rebar significantly improves the bond strength, increasing it at early integration times to the bond quality of the cast reference specimen. Even at an integration time of 60 min, vibration increases bond strength by 166% compared to direct insertion without any vibration.