Mechanical behaviour of textile reinforced 3D printed concrete structures
Author: Francesco Moiana
Language: English
Abstract
The current production of curved concrete shells presents some critical issues in terms of productivity. In fact, the whole construction process is characterized by a high level of required time and manpower, and several inefficiencies are related to the formwork, especially in series production, because if the geometry of the tank varies, new formworks are needed, and this may result in additional stocking spaces and in additional depreciation costs. All these reasons have been pushing some companies to re-think the whole construction process of such kind of structures. A possible solution that can help to solve all these production issues can be represented by 3D concrete printing, with several benefits in terms of time, manpower and productivity. This thesis analysed the feasibility of a 3D printed curved concrete wall with carbon fibre textile reinforcement. This solution allows fast implementation on site and requires less labour than convetional steel reinforcement, especially since it does not require the use of formwork.
A 3D printed cross section is not homogeneous due to the high number of cold joints between filaments. Moreover, carbon fibre textile reinforcement is a pretty new material and no studies are available for the specific combination between these two different construction techniques. In particular, shell structures of various under tension, shear and bending loading were analysed.
Stress analyses were carried out by following Timoshenko’s shell theory, and it revealed that a bidirectional reinforcement is needed to cope with bending moment and direct tension. Since there are no specific design criteria for this particular composite material, most of the pre-design methodologies were taken from the approaches for steel reinforced concrete, making some modifications from case to case in order to consider carbon reinforcement instead of steel. Serviceability and ultimate limit state verifications were performed, focusing respectively on the ultimate limit strength of the tanks and on their attitude towards crack width. The main uncertainties are related to a possible reduction in terms of shear strength due to the printing process. For this reason, a shear strength reduction coefficient was introduced in order to consider a possible reduction in terms of shear strength due to the printing process. Furthermore, uncertainties arose also for the bond strength between concrete and textile reinforcement, which has important consequences on crack width and on serviceability limit state verifications. Currently, no analytical methods are available to calculate the bond strength between concrete and carbon fibre, which is dependent on the geometry of the textile and texture properties. Thus, a bond strength value from the academic literature was taken.
Pre-design results have shown that the limiting parameter for the design of curved concrete wall is the amount of textile reinforcement that can be embedded in the cover concrete. Moreover, if the 3D printing drastically affects shear strength (and thus is very low), the cross section required to fulfil shear verification may be too thick and this may represent a problem for printing. An accurate campaign of laboratory tests is needed in order to verify the validity of the assumption made in this project.