Capacity Without Ductility: A Quantitative Synthesis of Flexural Strengthening Evidence for High-Strength Concrete Beams

Abstract

While high-strength concrete can increase its compressive strength without a corresponding increase in tensile strength, there are four strategies for strengthening concrete: ultra-high-performance concrete (UHPC) overlays, fibre-reinforced polymer (FRP) systems, matrix fibres, and recycled constituents. They are tested separately against their respective control on different specimen geometry and cannot be compared with published results. This review aims to consolidate 23 studies found between 2020 and 2026 that can be reproduced in the database search. Results were normalized against the capacity of each study's own control, presented as capacity gain ratio and ductility retention ratio, as well as presented in terms of a strength-ductility trade-off index (STI), which is the fractional change in the capacity gain ratio and ductility retention ratio product. A capacity gain can be calculated in 13 studies: The range is wide from 26.4% to 36.6%, with the mean being 31.0%, without discriminating between systems. There are only three studies that report both quantities and the index rearranges these quantities. The hybrid FRP sheets are ranked 1st for capacity and -0.245 for STI and stainless reinforcement are ranked 1st for STI and -0.750 for capacity, respectively. The reason the sample is three is the second finding. Fifteen of the 23 studies report on ultimate load, six on other outcomes, none on deflection at yield, and four studies on the interface between which overlay systems transfer their gain. So, a minimum reporting set is suggested. If either deflection at first yield or ultimate load would be required, the comparable number of samples would be tripled, and if both deflections would be required, it would be tripled again, without casting another specimen.