PSI - Issue 78

Dario De Domenico et al. / Procedia Structural Integrity 78 (2026) 65–72

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1. Introduction Many bridges built over 50 years ago feature prestressed concrete (PC) girders with pre- or post-tensioned tendons (Cosenza and Losanno, 2021). Corrosion-induced deterioration of prestressing steel, especially in harsh environments, poses serious risks to structural integrity (Gandelli et al., 2024). To address this, controlled lab simulations are essential for developing reliable assessment and monitoring techniques (De Domenico et al., 2022). This study presents early results from an experimental campaign at the University of Messina, investigating dynamic behavior of 12 PC girders with simulated prestress loss. Each beam (30 × 25 cm cross-section, 800 cm length, 700 cm span) includes six 0.5" pre-tensioned strands — five bottom, one top. Nine specimens are bonded; three use unbonded tendons in plastic ducts. Damage is introduced via soffit access holes at varied locations (one third, one-quarter, mid-span) to study modal sensitivity. Steel blocks and a transverse beam simulate bridge-like loading. Using Operational Modal Analysis (OMA), acceleration data is processed to identify modal parameters. This contribution highlights representative results, including changes in natural frequencies, mode shapes, and modal curvatures between undamaged and damaged 2. Plan of the tests and objectives of the experimental campaign The goal of this experimental campaign is to evaluate the performance of OMA methods in detecting and quantifying damage in PC bridge girders under service load conditions. To achieve this, a series of laboratory tests are conducted on PC beam specimens with purposely induced damage to their prestressing strands. These controlled experiments are designed to enhance knowledge of how dynamic monitoring techniques can be used to identify structural deterioration. The experimental activities are carried out at the Structural Engineering Laboratory within the Department of Engineering at the University of Messina, Italy. The facility features a structural testing frame with a working area of 9.0 m × 7.0 m and a vertical clearance of 5.0 m. It is equipped with hydraulic jacks capable of applying loads ranging from 250 kN to 1000 kN. A control system allows for the application of customized loading protocols, operating in either force-controlled or displacement-controlled modes. The twelve pre-tensioned prestressed concrete (PC) beams were produced at a prestressing facility operated by SICEP S.p.A. in Belpasso, Sicily. The plant includes four parallel casting beds, each 100 meters long, with strand positioning masks allowing a 5.0 cm spacing both vertically and horizontally. The minimum strand diameter used is 0.5" (cross-sectional area = 93 mm²), and concrete of class C45/55 ( =55 MPa) is typically employed. The 7-wire prestressing strands used have an ultimate tensile strength of 1860 MPa and a 1% proof stress of 1670 MPa. Strand release occurs approximately 16 hours after casting, provided that the average cube strength of the concrete has reached at least 35 MPa. To take advantage of the existing casting setup, each PC beam was designed with dimensions of 30 cm × 25 cm and a total length of 8.0 meters, corresponding to a clear span of 7.0 meters. This configuration led to the production of 12 beams within a single 100- meter casting bed. The beam’s front and cross -sectional views are shown in Fig. 1. The reinforcement layout includes five prestressing strands positioned along the bottom and one at the top (with a concrete cover of 5 cm), supplemented by four ϕ 10 mm bars at the corners (with 3 cm cover) and ϕ 8 stirrups spaced at 30 cm intervals. The load-bearing capacity of the PC beam, tested under simply supported conditions with a clear span of 7.0 meters, is governed by the ultimate bending moment at mid-span. This ultimate moment was calculated using a fiber-section approach in SAP2000 (Computers and Structures Inc., 2024). The cross section was discretized into 25 × 10 fibers, and an axial force of 725 kN was applied to simulate the prestress, corresponding to an initial stress of 1300MPa per strand. The analysis used a mean concrete compressive strength = 53 MPa, an average steel yield strength = 500 MPa, and mean prestressing strengths 1 = 1840 MPa. Based on these assumptions, the calculated mean ultimate limit state (ULS) mid-span bending moment is , =150 kNm. Based on the computed ultimate bending moment, the corresponding ultimate load is approximately 100 kN, as determined through a pushdown analysis on the main girder. This results in an estimated shear force of about 50 kN. The shear resistance, evaluated using the Eurocode 2 truss model (European Committee for Standardization, 2005), is approximately 83 kN, thus confirming that the beam is governed by flexural failure. Specifically, failure occurs due to concrete crushing in the compression zone at ultimate conditions. For the case of PC beams with unbonded prestressing strands, the sectional analysis was carried out without including the strands, due to the absence of strain