PSI - Issue 54

João Custódio et al. / Procedia Structural Integrity 54 (2024) 271–278 João Custódio et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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3.2. Swelling phenomena The occurrence of severe cracking in the bridge, mainly in the lower part of piers P6 to P9, led to a study carried out in 2009 to identify the causes. The pattern of cracking varied with the geometry of the concrete member; but was, either, aligned with the reinforcement and the direction of the major stress fields, or was in the form of map cracking. In the study, it was concluded that the cracking was caused by the occurrence of both ASR and DEF. It was also concluded that a moderate residual expansion potential still existed in the sampled concrete. As a result, a plan was prepared and implemented to monitor the concrete moisture content, to determine if the moisture available was sufficient to sustain the deleterious development of ASR and DEF, and to evaluate the effect that a surface protection system could have on the mitigation of ASR and DEF in the structure. To accomplish this, a coating system was selected, taking into account the considerations made in Section 2, characterised in the laboratory (Section 3.3), and applied on an experimental area in the base of the column of pier P8. Its effect on the concrete moisture level is being assessed via relative humidity measurements (Sections 3.4 and 3.5). The results of this study will provide data to support the decision to be made concerning the relevance of the application of a concrete surface protection system to mitigate the deleterious development of ASR and DEF, and to prevent further concrete deterioration due to other mechanisms, e.g., rebar corrosion. 3.3. Coating system characteristics The selected surface protection system consists of two layers: a polymer modified cementitious coating and a water based acrylic coating. Its capacity to bridge cracks, resistance to water penetration, permeability to water vapour, and effectiveness on controlling humidity in concrete, was evaluated at the laboratory on a previous study (Custódio et al., 2022). The main findings can be summarised as follows: • Liquid-water permeability – the results showed that the coating system complies with the requirement set in the standard EN 1504-2 (CEN, 2004) (w < 0.1 kg·m -2 ·h -0,5 ). Thus, it is expected that this system can effectively protect concrete from liquid water ingress. • Water-vapour permeability – the results of the tests performed to the coating system allowed it to be classified as Class II (s D = 9 m), i.e., the coating system is not dense against water-vapour (s D ≥ 50 m) nor permeable to water vapour (s D ≤ 5 m) and demonstrate d that it has a medium water vapour transmission rate (V = 2 g·m -2 ·d -1 ). • Crack-bridging – the results of the tests performed to the coating system, at 23 ºC, allow it to be classified as A3 (the width of the crack bridged was greater than 500 µm and less than1250 µm). Thus, the coating system would be able to maintain protective/barrier properties in the concrete of an affected structure, since it has the capacity to bridge cracks in the concrete substrate, if they do not exceed that level. • Effectiveness of the coating system to control humidity inside concrete – the results obtained show that the coating system allows the concrete to dry, as the relative humidity values decreased once the specimens were placed in an environment with 60 % R.H. However, it appears that the coating does not prevent moisture ingress, as the specimens that were placed in a humid chamber ( ≥ 95 % R.H., i.e., where the surfaces of the specimens are continuously wet), as they showed a continuous increase in relative humidity. Thus, the results obtained so far suggest that this coating system might not be adequate in situations where the concrete is exposed continuously or for very extended periods to high air humidity. Further tests will be performed to confirm this behaviour. 3.4. Coating system performance The in-situ study comprised two stages. Firstly, in February 2019, five pairs of cylindrical holes were drilled in the concrete, with a rotary hammer drill, and in which hollow sleeves were inserted to line the hole until the required depth. The holes were made at the following locations in Pier P8: exterior north (L1) and south (L2) surface of the column, interior north surface of the column (L3), exterior south side vertical surface of the column footing (L4), exterior south upper horizontal surface of the column footing (L5). The two holes of each pair have different depths (5 cm and 15 cm), to allow measuring the R.H. at different distances from the element surface. These holes are sealed, they are only opened when the readings are being made, i.e., when the relative humidity and temperature probe is placed in or removed from the hole. The temperature and relative humidity readings were performed with a handheld