3D concrete printing is an additive manufacturing method that uses a printing robot to build 3D concrete elements. In the last decade, several 3D concrete printing techniques have been developed and extensive research has been conducted on suitable concrete mixes, the printing process and the material and mechanical properties of the finished product. However, certain challenges have remained unresolved. Two of these challenges are the low tensile strength and the limited ductility of the printed concrete. As concrete is a brittle material that is weak in tension, traditional structural concrete elements make use of steel reinforcement to ensure tensile strength. The incorporated steel rebars are designed to take over the tensile stress when the concrete cracks. With the linear elastic and eventually plastic behaviour of steel in tension, the steel provides a ductile behaviour to the rein- forced concrete. This ductile behaviour is required to comply with modern building regulations, as it ensures structural integrity and safety by timely warning in case of overload. However, for printed concrete the incorporation of steel rebar reinforcement is not evident.
The use of Strain-Hardening Cementitious Composites (SHCC) in the field of digital concrete printing is one of the many possible ways to incorporate reinforcement into printed structural concrete elements. However, just like regular printable concrete, the bond between subsequently printed layers has shown to be a point of concern. The research presented in this master thesis focusses on the interlayer bond of these printable SHCC’s and more specifically on developing improvement methods for this interlayer bond. To better comprehend the origins of problem a thorough literature study was performed on the following topics: printing facilities, 3d printing compatible mix designs, (printable) SHCC, anisotropy and research on related interlayer bond improvement methods. On the basis hereof an experimental research was designed and conducted on various methods unrelated to the SHCC mixture and their effect on the interlayer bond. The methods can be divided in three groups: nozzle types, surface modification by brushes and incorporation of vertical steel reinforcement. In total 7 parameters were designed and tested: three nozzles, two brushes and two steel reinforcements. Tensile interlayer bond tests were performed to assess the influence of the parameters on the interlayer bond strength. To make sure that the overall strain hardening capacities of the material were not negatively influenced by the interlayer bond improvement methods the research also included compressive tests in two directions and uni-axial tensile tests parallel to the printing direction. Finally, the research was extended with a concise CT scan analysis on porosity and reinforcement bonding. Results show that the type of nozzle has significant influence on all mechanical properties. Sur- face modification by brushes tend to result in SHCC elements that have higher interlayer bond, but a reduced strain-hardening capacity. Specimens with interlayer steel reinforcement completely lose strain-hardening capacity in printing direction. In general, it is concluded that there is potential in designing methods to improve the interlayer bond of printed SHCC.