Post-tensioned concrete produced with the Flexible Mould

Abstract

Nowadays, precast technology is mainly used for the production of concrete shell elements. Computer Numerically Controlled (CNC) milling techniques is the main production method of single or double-curved concrete elements, creating astonishing shapes with high accuracy. However, these techniques are also accompanied with high costs and large material waste. Skilled carpentry or curved steel formwork are other techniques used for the production of concrete shells. An alternative to the aforementioned techniques is the “Flexible Mould” method being developed at Delft University of Technology. This method is based on mass customization of the produced panels, due to the adaptability of the formwork's shape according to the specified geometry. The concept of the “Flexible Mould” will be used as a starting point for the current thesis.

Concrete shells are mainly used for decorative reasons such as claddings and facade elements. Their structural capacity has still to be improved so that they can be applied as load-bearing elements and avoid sub-optimal or even “bad” designs. Several researches are being conducted in order to analyze and improve the load bearing capacity of concrete shells produced with the adjustable formwork technique. Applying conventional steel reinforcement to take up possible occurring tensile stresses in concrete shells produced with the Flexible Mould, has been proved to be possible only with limited diameters due to construction requirements.

In the first part of the thesis, a research was conducted on possible improvements in the production of concrete shells with the flexible mould, so that they could be considered feasible to be applied in real life structures. Having conducted a literature review on production parameters related to the “Flexible Mould” concept, Computer Aided Design (CAD) was used as the main tool to create the digital geometry of double-curved concrete elements. Dimensional limitations imposed from the existing test set-up of the adjustable formwork at TU Delft laboratories, as well as the inherent geometry of shell elements, had to be taken into account. As a next step towards the implementation of double-curved concrete elements in real life structures, non-load-bearing laboratory tests took place. These tests were performed for illustrating reasons with sand to be the main material used instead of concrete. During these tests, a steel wire mesh diamond shaped was applied as the elastic layer of the flexible mould, being a different material in comparison with the previous studies on the flexible mould. Grid distance between the actuators was the main parameter investigated during this part of the research.

In the second part of the thesis, a study was conducted on possible improvements in the flexural tensile capacity of concrete shells via prestressing steel reinforcement. CAD design and Finite Element Modeling (FEM) were the main tools used. Due to shell's complex geometry, the research was conducted first for simple geometrical shapes under linear static analyses. Two and three dimensional elements were investigated being modeled as plane stress elements and solids respectively. Linear or single-curved prestressed concrete elements were analyzed with regard to their deflection and stress field. The number of concrete elements as well as the influence of an intermediate element in-between them simulating a joint, was an additional investigated parameter.

Subsequently, a four-point bending that will take place at TU Delft, was simulated with Finite Element Modeling (FEM), investigating its structural response under both linear and nonlinear static analyses. The created (FEM) comprises of two centrally prestressed concrete elements connected with a mortar joint in between them. No bond interaction between the post-tensioning steel reinforcement and the concrete is incorporated in the FE model. The type and size of the applied reinforcement is investigated in this model. The main goal of this part was to analyze the structure's response for loads exceeding the cracking load of the concrete, taking into account a brittle concrete failure at joint's location.

In the final part of the thesis, a post-tensioned arch structure segmented in a series of concrete elements and joints, was first designed with CAD software and then analyzed under a linear static analysis via FEM. Although more complex, a three-dimensional (3D) modeling was conducted, aiming at describing the three dimensional state of stress and deflection field of the arch. Based on a draft CAD model, several geometrical parameters had to be adjusted in order to meet construction requirements of the final concrete shell structure. Modeling and meshing requirements imposed from the Finite Element Analysis (FEA) software were also taken into consideration for the final shape of the arch.

The current thesis is a part of a research program that is being performed at TU Delft, aiming at developing the Flexible Mould with regard to construction and structural application of concrete shells in real life structures.