Bond investigation of iron-based shape memory alloy bars embedded in UHPFRC – Comparison between testing methods

Roughly 50 % of the existing bridges in Switzerland are now over 50 years old. Following a common trend, in most industrialised countries, many bridges are approaching or have reached their design lifespan. Thus, awareness towards retrofitting demands of existing bridges is increasing. In this context, a new prestressed strengthening technique is pro-posed, where a layer of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) reinforced with Iron-based Shape Memory Alloy (Fe-SMA) bars is cast on top of an existing bridge deck. Besides increasing the deck bending resistance under hogging moments, the innovative method can reduce transverse bending moments in the bridge webs and significantly improve the structure's serviceability thanks to the Fe-SMA prestressing capabilities. As part of the study of the proposed system, and to address the existing gap in knowledge on the bond behaviour between UHPFRC and Fe-SMA, two series of tests were conducted: (i) short pull-out tests (adapted RILEM tests) instrumented with Digital Image Correlation (DIC), and (ii) pull-out tests with longer embedment lengths instrumented with Fibre-Optical (FO) sensors and DIC. For comparison, two types of steel reinforcing bars (B500B and Fe-SMA) were tested. The main results from both pull-out test campaigns are discussed, highlighting the advantages and drawbacks of each of these approaches. The FO sensors used in the long pull-out tests provided new insights into the bond behaviour between UHPFRC and reinforcement steel. Post-processing of the long pull-out tests results revealed average peak bond shear stresses of 28 MPa and 23 MPa for B500B and Fe-SMA steel reinforcing bars, respectively. Conversely, the short pull-out tests yielded unrealistically high bond shear strength values, in some cases overestimating the maximum bond capacity by more than a factor of two. While the bond strengths observed in the long pull-out test align well with the fib Model Code predictions, they remained significantly lower than recommended in existing standards, such as the Swiss technical specification SIA 2052. • UHPFRC is combined with steel reinforcing bars (B500B and Fe-SMA) for bridge strengthening. • Short and long embedment length pull-out tests are performed to investigate bond behaviour and compare testing methods. • DFOS and DIC provided new insights into steel-UHPFRC complex bond mechanisms. • Short RILEM-type pull-out tests yielded unrealistically high bond shear strength. • Results can support refined models for estimating crack spacing, crack widths, deflections, and tension stiffening effects.