PSI - Issue 33

Fabrizio Pittau et al. / Procedia Structural Integrity 33 (2021) 630–645 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The impermeability towards liquids and vapors of the concrete is an important property due to its significant connection with the durability of the reinforced concrete (r.c.) art works. It is well known that concrete with high porosity can show degradation in time. Permeability may indirectly influence the durability of concrete structures by governing the penetration rate of aggressive agents responsible for degradation under a pressure gradient, or directly in the case of structures with tightness role Titi et al. (2016). Hence, there is a clear motivation to investigate the variation of the permeability of concrete, particularly during live loads. In addition, in industrial plants, the impermeability of concrete can be advantageously exploited to improve durability and to build reservoir or facilities containing gas and vapours generated in chemical processes. For these reasons concrete tightness became a key parameter to decrease the maintenance cost of a facility. If the application in the field of water permeability is widely investigated in the technology of dams and reservoir, as well as during archaeological excavations of masonry structures under earth pressure Sassu et al. (2013), gas permeability is still an open field. In this contest, reinforced concrete exhibit some significant limits as highlighted in Puppio et al. (2018) Stochino et al. (2018)c. Bridges, viaduct and art work in general needs of strategic inspection strategy Pucci et al. (2019) Puppio, Vagaggini, et al. (2019) Stochino et al. (2018)a and effective diagnosis instruments Coni et al. (2021) Stochino et al. (2018)b. The main causes of concrete permeability can be found in its porosity and its proper rheological behavior. The porosity and the shrinkage are two significant elements in concrete technology. The shrinkage is strictly related to the mix design, to the maturation process, the presence of restraints, the stiffness and dimension of the aggregates, the geometry of the elements and the reinforcement ratio. The porosity is connected to the mix design and to the water to cement ratio. The direct consequence of the shrinkage is the crack formation. The cracks can be seen as some infinitesimal joints, or releases, to the shrinkage effects. The average dimension of this crack can be evaluated as reported in BS EN 1992-1-1 (2004) D.M. 17/01/2018 (2018) furnish useful tools at this purpose. An interesting quick method to evaluate concrete permeability has been established by Lu et al. (2000) thanks to the application of critical voltage to thin concrete specimen. The duration of the single test, excluded the preparation, is of about 5 minutes. The procedure appears in good agreement with the water penetration tests standardized in ASTM C1202 ASTM C1202-19, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration (2019). The estimation of the porosity can be carried out with ultra-sonic testing. In Vergara et al. (2014) two approaches are discussed, both based on the velocity of acoustic propagation and on structural noise. The main advantage of this non-destructive technique is the short application time. This can be useful in mortar characterization in addition to the measure of porosity. The permeability, in case of severe conditions due to possible extreme scenarios, especially in nuclear plant, is discussed by Dandachy et al. (2017). The concrete permeability of cylindric specimens in the two direction (cilindric and radial) is analysed considering different temperatures. In this work the radial permeability is found greater than the axial one because of radial cracking. The influence of tensile cracking and aggregate size on concrete permeability is studied by Choinska et al. (2018). Some cylindrical specimens with different aggregate dimensions, subjected to indirect tension through Brazilian tests, are injected with nitrogen to study their own permeability. As expected, his study refers an increasing of concrete permeability with the diameter of the aggregates as well as with the cracking increase. Long-term concrete shrinkage effects on the performance of r.c. structures are analysed by Dey et al. (2021). In this work 14 tensile tests on samples with different reinforcement ratio are analysed with five-year aging. The effect of concrete permeability in case of recycled aggregate instead of natural ones is discussed by Soberón (2001). The recycled aggregate shows an increase of porosity and of the water depth of penetration Pani, Francesconi, Rombi, Mistretta, et al. (2020) Pani, Francesconi, Rombi, Stochino, et al. (2020). The use of additive to increase the r.c. performance is quite standard nowadays. For instance, the addition of waterproofing additive Xypex C-1000 is discussed in Nataadmadja & Runtuwene (2018). This study highlights an increase of water permeability and a decrease of concrete strength depending on the mixture dosage.