Innovative Seismic Strengthening Techniques to Be Used in RC Beams’ Critical Zones

The seismic performance of a structural frame system can be enhanced by strengthening the RC beams’ critical zones. In this paper is presented an experimental study on the improvement of the beam behaviour, subjected to an alternative cyclic procedure which considers the gravity loads reflecting the real demands on the beams’ critical zone. Two strengthening solutions are presented: unbounded post-tension (PT) tendon strengthening to increase resistance and limit residual deformations, and unbounded post-tensioning with jacketing of the RC beam with unidirectional fibre mat reinforced grout (UFRG) to limit compression damage, improving section confinement, thus delaying concrete crushing and buckling of longitudinal reinforcement. The original UFRG material was developed within this study, to apply as a small thickness jacketing material for strengthening RC structures. The main idea was that the steadiness provided by preplacing continuous fibre mats into the mould reduced the fibres’ segregation tendency during the high-performance grout pouring and allowed for the optimisation of their percentage and alignment, attaining a higher tensile strength. The experimental response of the tested Specimens is presented and evaluated through performance parameters that are properly discussed and adjusted to the alternative cyclic procedure. Finally, theoretical predictions are presented, and an adjustable multilinear model is proposed to estimate the strengthening solution’s response.