PSI - Issue 62

Giulia Rossini et al. / Procedia Structural Integrity 62 (2024) 347–354

348

2

Giulia Rossini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

1. Introduction The effective residual prestressing force after short and long term losses is a crucial factor determining the serviceability and safety of prestressed concrete (PRC) structures. Accurate assessment of these residual forces is essential, especially in cases where external evidence of structural damage or deterioration is limited (Martinello, 2021). Analytical models can be employed for this evaluation, but conflicting opinions in the literature regarding their reliability exist. Moreover, uncertainties in analytical models, which may differ from experimental results, are even increased by the fact that empirical methods for in-situ evaluation of prestressing systems are limited and their reliability and applicability still need further demonstration. In order to choose an operating procedure, key considerations include the accuracy and reliability of measurements, the invasiveness of the test, ease of execution, and the associated costs. A commonly utilized method for evaluating residual prestress stresses is based on 'tension release', which involves estimating the current prestressed state of PRC girders from the prestress strain release acting on the concrete. One of the earliest methods proposed in the literature is drilling, which involves observing changes in strain near a relatively small hole created in a concrete element. This approach has found application in various studies, and several authors (ASTM E837-13a, 2013; Azizinamini et al., 1996; Chang et al., 2009; Kesavan et al., 2005; Owens, 1993; Trautner et al., 2011) have proposed different geometric configurations, including variations in hole diameter, depth, and strain gauge placement relative to the hole. However, the complexity of the method arises from the need to introduce calibration coefficients. In addition, when the existing stress levels are low, the measurable quantities can be extremely minimal, resulting in frequent errors. The effectiveness of this method has been discussed in the literature (Khaled, 1999; Sánchez-Beitia & Schueremans, 2009). To overcome the limits of the Hole Drilling method, the core trepanning method was developed. This method requires placing strain gauges in the center of the core, instead of placing them radially outside of it (Kesavan et al., 2005). In this configuration, the strain gauge can capture the entire stress release resulting from the core drilling, facilitating data analysis. An advantage of this method is that the extracted cores can later be tested in the laboratory to determine the concrete's elastic modulus. However, a limitation of these methods is related to the size of the holes: tests with different diameters are reported in the literature, but it is crucial to emphasize that dense reinforcement is present in prestressed concrete (PRC) structures, which makes difficult to create a hole without intercepting reinforcement. Another stress release method is the Saw-Cut method, which operates very similarly to the Hole Drilling and Core Trepanning methods. However, in this approach, stress release occurs when two cuts are made in the intrados of the beam. These cuts isolate the concrete block from the acting forces, while stress or strain changes are measured in the section between the two cuts performed (Bagge et al., 2017). Kraľovanec et al. (2021) applied this method in situ to an existing 60-year-old bridge. They made four cuts 120 mm apart, made at the soffit of the lower flange. Two cuts were 23 mm deep, while the other two were 31 mm deep, achieving 86% stress release. It is evident, however, that achieving complete tension release depends not only on the depth of the cuts, but also on the relative distance between the two cuts. The closer the cuts are, the less depth is required to completely isolate the concrete block. This aspect can be advantageous in the case of old PRC bridges still in operation, where the concrete cover is extremely small, limiting the maximum depth of the saw cuts. In particular, Kraľovanec et al. (2021) argue that there is a linear association between the depth of saw cuts and stress release. Another important advantage of the saw-cut method highlighted by Kraľovanec (2021) is the low influence of the initial prestress level on the results: the percentage change in stress or strain after the application of saw cuts is almost the same in all specimens tested by the authors despite different initial prestress levels. A significant challenge associated with the drilling and coring method is the difficulty of conducting tests at maximum stress points due to reinforcement congestion. The Saw-Cut method aims to solve this problem by confining the operations within the thickness of the concrete cover, typically around 30-35 mm. However, this is not always guaranteed, especially in precast elements and at the intrados of the girders. In response to these considerations, Martinello (2021) proposed an innovative approach: stress release tests using blunt pyramidal specimens. The specimen has a depth of only 25÷30 mm and thus affects only the concrete cover thickness, consequently this method can be applied even in heavily reinforced areas. In addition, the blunt pyramidal shape facilitates the extraction of the specimen, providing complete tension release. Lupoi & De Benedetti (2021) also