Chemical Prestressing of Thin Concrete Elements with Carbon Textile Reinforcement

Abstract

Prestressing of concrete with non-metallic reinforcement is an emerging field in structural engineering, as it can significantly contribute to better exploitation of the mechanical properties of such reinforcement and behaviour of concrete structures. Non-metallic reinforcement includes fibre reinforced polymer rebars and tendons, as well as various types of textile reinforcement. Textile reinforcement is particularly challenging as a material used for prestressing because of its geometrical form and structure. Therefore a technology of chemical prestressing, where expansive cement is used to induce stresses instead of mechanical devices, comes as an alternative yet promising method for such types of reinforcement.

The thesis aim was to experimentally assess the behaviour of expansive concrete members with carbon textile reinforcement. Three objectives were undertaken: 1) to prove whether chemical prestressing of concrete elements with textile reinforcement is possible, 2) to quantify the introduced prestressing forces and the influence of prestressing on serviceability limit state with a focus on cracking load, 3) to analyse the influence of chemical prestressing on aspects relevant for the design of structural members and on their performance. A feasibility study was performed to investigate the possibility of prestressing with expansive cement. Expansive strains were measured with standardized methods, as well as with distributed fibre optic sensors in specimens that were subsequently subjected to loading tests. Chemically prestressed concrete elements were tested for uniaxial tensile behaviour, in bond and flexural tests. Their behaviour was compared with this of non-prestressed ones.

Measurements of expansion in time and investigations of the behaviour of tested specimens have confirmed that chemical prestressing of elements with textile reinforcement can be realized. In specimens subjected to tensile tests, concrete prestress estimated from expansion measurements corresponded very well with calculations from tensile tests results, prestressing effect was also observed in beams with steel reinforcement and large scale slab specimens with textile reinforcement. Furthermore, specimens made of expansive concrete shown a significantly higher cracking load in comparison to reference specimens made of concrete without expansive admixture. At the same time, no deterioration of concrete material properties was observed, on the contrary, compressive and tensile strength increased with an addition of expansive admixture and the modulus of elasticity remained on the same level. Finally, an overall performance index was calculated which described the performance of chemically prestressed specimens compared to the reference members.