PSI - Issue 84

Available online at www.sciencedirect.com

ScienceDirect

Procedia Structural Integrity 84 (2026) 337–343

III Fabre Conference: Existing Bridges, Viaducts, and Tunnels: Research, Innovation, and Applications Laboratory-Based Evaluation of Flat-Jack Test Accuracy for Assessing Residual Prestress in Post-Tensioned Concrete Elements D. Rossi a *, C. Pettorruso a , L. Zoccolini a , V. Quaglini a , G. Raoli b , V. Carusi b , A. Forte b

a Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano 20133, Italy b GR Consulting Srl, Via Sardegna 9, San Giuliano Milanese, 20098, Italy

© 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the Conference Keywords: Flat-Jack Test; Post-Tensioned Concrete; Residual Prestress; Non-Destructive Testing; Experimental Campaign Abstract This study evaluates the potential of the flat-jack test (FJT) as a method for assessing residual prestress in post-tensioned (PT) concrete elements. Experimental investigations were carried out on three concrete slabs subjected to known axial stress conditions in a controlled laboratory setting. Simplified sample geometry was adopted to minimize external uncertainties and isolate the performance of the technique itself. Two flat-jack sizes were tested to examine their influence on measurement accuracy and repeatability. A dedicated calibration procedure was developed to simulate realistic field conditions and enhance the reliability of the results. The findings demonstrate that the FJT can provide reasonably accurate stress estimations, with performance influenced by the size of the flat-jack. Larger flat-jack provided more consistent results, while greater variability was observed with the smaller device. The study also places the method within a broader context by comparing its performance in concrete and masonry and discussing its position relative to other residual stress assessment techniques. Although the methodology is not yet sufficiently mature for widespread practical application, it shows strong potential for evaluating residual prestress in PT structures. The results clarify both the capabilities and limitations of the FJT in this context, offering valuable insights for the development of standardized testing protocols. Furthermore, this work lays the foundation for future experimental campaigns on full-scale specimens and, ultimately, on in-service structures, with the goal of advancing reliable and minimally invasive stress assessment methods for critical infrastructure.

* Corresponding author. Tel.: +02.2399.4248. E-mail address: dalila.rossi@polimi.it

2452-3216 © 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the Conference 10.1016/j.prostr.2026.06.044