Concrete structures with stay-in-place flexible formworks and integrated textile reinforcement: An exploration of the design space and mechanical behaviour

Abstract

The reduction of the concrete volume used in the construction sector – triggered by the urgent demands on sustainability – has become one of the critical drivers for developing new composite materials and structural typologies. This thesis presents a novel approach for stay-in-place fexible formworks with integrated textile reinforcement based on the KnitCrete technology, reducing the environmental footprint of concrete structures through structurally informed geometries and slender dimensions, enabled by the use of weft-knitted fabrics and non-corrosive high-strength fbrous materials. The frst part of the thesis explores the possibilities arising from knitted textiles – including the feasibility of creating doubly curved geometries and introducing continuous rovings and spatial ribs – and revisits conventional reinforcement types and their suitability for complex geometries, proposing two directions for lean reinforcement strategies: (i) use of high-strength fbrous materials for the manufacturing of weft-knitted textiles and (ii) guiding conventional reinforcement (i.e. deformed steel bars or post-tensioning tendons) with integrated features within the fexible formworks. The second part of the thesis aims at characterising the mechanical behaviour of weft-knitted textile reinforced concrete regarding the (i) strength, (ii) stifness, (iii) bond, and (iv) deformation capacity. To this end, several experimental campaigns are conducted to prove the feasibility of the manufacturing procedure and examine the structural response under various loading conditions, including uniaxial tension, bending, and shear. The investigations focus on various knitting patterns, textile materials, coating types, and the infuence of shear connectors to enhance the bond conditions and the addition of short fbres to the concrete to improve the post-cracking behaviour. Furthermore, the combination of brittle and ductile reinforcement materials and the optimisation of the geometry by means of thinwalled cross-sections are studied. The load-deformation and failure behaviour is evaluated using refned measuring techniques, i.e. digital image correlation and distributed fbre optical sensing, which allow assessing the mean strains in the reinforcement and the crack kinematics. Analytical methods following the Tension Chord Model, which considers the stress transfer between the reinforcement and the concrete based on mechanically consistent assumptions, allow the adaptation to the specifc geometry of the weft-knitted textile reinforcement, the back-calculation of bond shear stresses, and the consideration of the short fbres in the concrete. The resulting predictions are validated with the experimental results. The load-bearing capacity of concrete beams with integrated transverse textile reinforcement is analysed regarding various contributions from the concrete and the high-strength rovings to the shear transfer across the governing crack and modelled using numerical simulations based on the Compatible Stress Field Method. The third part of the thesis discusses potential structural applications based on the fndings on the mechanical behaviour obtained from the experimental investigations. The implications of using brittle reinforcement materials are addressed, and various means to implement ductility in the global structural response are discussed, eventually proposing a classifcation framework for safe and reliable design principles to achieve an adequate post-cracking behaviour. The thesis concludes with a case study examining the structural behaviour of doubly curved concrete shells using non-linear fnite element analyses, highlighting the potential of the developed reinforcement approach for structures with complex geometries.