PSI - Issue 64

Pascual Saura Gómez et al. / Procedia Structural Integrity 64 (2024) 2125–2132 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. The resistivity (  ) was measured at the same time, as corrosion potential, at regular intervals of 30 cm along the entire length of the strands. 3. The corrosion rate ( I corr ) was measured employing the Current Modulated Confinement Method, see Andrade et al. (2012). The interpretation of the corrosion rate measurements was performed based on the UNE 112072 standard. The electrochemical technique used to measure the corrosion rate was the polarisation resistance (R p ) method. The value of R p is related to the corrosion rate , I corr , by means of a constant called B by Stern, see Feliu et al. (1998), and Stern & Geary (1957). 2.3. Chlorides content The chloride content was measured along the length of the beams in different positions: drilled cores were extracted in different positions and identified with letters A, B, and C to refer, respectively, to the left end, middle point, and right end of the beams. In some cases, a fourth point D and even a fifth point E were taken for more data to check the validity of the results. In areas where the reinforcement was left completely uncovered, such as at the ends of the beam and on the resulting exposed portions next to the main reinforcement, concrete dust was extracted using a 20 mm diameter drill bit. After extracting the concrete dust from each defined point and identifying the sample in a plastic bag with the corresponding beam number and letter (e.g. 1a), the presence of chlorides and sulphates were measured by using an Olympus portable XRF analyser, see Chinchón-Payá at al. (2021 a ), and Chinchón-Payá et al. (2021 b ). This technique uses X-ray fluorescence (XRF) to identify the different elements in the sample based on the binding energy between the orbital layers that characterise it. Firstly, the concrete powder was placed in a tube covered with a transparent plastic film. Secondly, the X-rays were directed at the sample and finally the data were collected by using a computer application to determine the levels of different elements present. Two readings for each sample were then used to calculate the mean value. The drilling procedure in the central area of the beam, where concrete cover was intact, was then repeated. The drilling was carried out from the external surface of the beam, going deeper until reaching the area close to the reinforcement from its known location (50 mm of cover). 3. Results This Section shows the results obtained from the previously described methodology for the PC beams. Three points on each beam were selected (A, located on the left lateral side, B, located at the middle point, and C, located on the right lateral side), and identified depending on their distance from the beams’ ends for potential ( E corr ), resistivity (  ) and corrosion rate ( I corr ), while the chloride content was measured at five points on each beam, as described in 2.3. The impact of crack opening will be analysed in the subsequent discussion. The Fig. 4(a) shows the corrosion potentials measured in the uncracked and cracked zones of the beams, with also the indication of the limits set by regulations for determining the probability of corrosion development. Values higher than -274 mV indicate a low probability of corrosion (shown in green in Fig. 4(a)), whereas values lower than -424 mV (shown in red in Fig. 4(a)) mean high probability of corrosion. It is important to note that nearly all corrosion potentials measured in areas with cracks fall within the high probability zone for corrosion, whereas more positive value can be generally recognized for the uncracked points. The Fig. 4(b) shows resistivity measurements in uncracked and cracked zones of the beams. It is worth noticing that values below 100 k Ω cm (shown in red in Fig. 4(b)) were considered to imply a high risk of corrosion, while values above 100 k Ω cm imply a low risk of corrosion, as stated by Garzon et al. (2014), Polder (2000), and Sanchez et al. (2017). The presence of cracks reduces the measured resistivity when the concrete is close to saturation, which is a well-known factor that can lead to corrosion of the reinforcement. The Fig. 4(c) displays the corrosion rate measured in uncracked and cracked zones of the beams. A corrosion rate ( I corr ) equal to 0.1 μA/cm² (shown in green in Fig. 4(c)) indicates active corrosion, while values above of 0.2 μA/cm² imply higher risk of corrosion. The presence of cracks coincides with generally higher corrosion rate values, indicating a greater loss of section in the active reinforcement per unit of time. The Fig. 4(d) displays the chloride content [Cl - ] in concrete measured at the level of the reinforcement in uncracked and cracked zones of the beams, which is much higher than the thresholds of the standard for prestressing steel, i.e. 0.3 % by weight of cement (shown in red in Fig. 4(d)). As can be seen, the cracking of the cover allows chlorides to penetrate faster, resulting in higher concentrations.