PSI - Issue 46

Roberto Serpieri et al. / Procedia Structural Integrity 46 (2023) 112–118 R. Serpieri, A. Bossio, G. Faella, G. Frunzio, M. Guadagnuolo / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Reinforced concrete is the most used building material since the last century, especially for long-span beams. The European (EU) attention towards RC degradation is raised because of the recent collapses of several long-span structures, some of which are ascribable, or possibly relatable, to degradation. In particular, the following list collects the long-span structures (all in ordinary and prestressed RC, except the last one in steel) which have been subject to collapses or failures that occurred during the last five years in EU: Polcevera bridge (Italy, 14/08/2018), Albiano Magra highway viaduct (Italy, 08/04/2020), Camerano overpass (Italy, 09/03/2017), Troja footbridge (Czechia, 02/12/2017) and Pont de Mirepoix-sur-Tarn (France, 18/10/2019). Reliability in the diagnosis of degradation and prediction of serviceability life for RC buildings and infrastructuress is today understood to be not only relevant for decisions in transportation and economic policies but also in ecological programs since cement production contributes up to approximately 8% of C0 2 emissions from human activities (Olivier et al., 2015). Suitable practices for diagnosis of degradation in R/C structures, and prediction of their serviceability life, should be identified finding compromise, and discernment, between requirements that can be also diverging, such as the need to minimize test costs and invasiveness, on the one hand, and the need to achieve sufficient diagnostic reliability in the test campaign, on the other hand. Similarly, a choice must be often made between consolidated engineering practices and more innovative test methodologies, or between assessment criteria arising from the evidence of large-scale field observations over a class of structural typologies and the need to take due account of the peculiarity of an RC structure. Synthesis of best available practices for degradation diagnosis from 160 years of engineering and scientific knowledge on RC structures is, however, not simple. To show, in a brief and schematic sketch, the existence of conflicting perspectives between past and present on fundamental topics in the assessment of RC structures degradation, some sharply controversial statements are pinpointed hereafter from the scientific literature concerning: 1) corrosion protection and initiation time, 2) expected engineering service life, 3) role of carbonation in the degradation process, 4) degree of reliability recognized to on-site electrochemical test methods for corrosion measurement . Corrosion protection and initiation time. ‐ The 1970 National Cooperative Highway Research Program report (NCHRP 90) (Moore et al., 1970) states: “ It is now known that corrosion of steel in concrete marine structures can be minimized, if not completely prevented, through use of concrete that is largely impermeable ” and that “ corrosion-free service of 50 to 100 years is not only possible for such bridges, but also highly probable ”. Direct observations by Houston et al. (1972) showed that complete corrosion protection is ensured even against the very severe exposure to daily spraying a 3% NaCl solution during more than two years, if adequate water/cement (w/c) ratio, concrete consistency, and cover depth is granted. The perspective of NCHRP 90 was substantially confirmed in the 1975 edition of the renowned Italian treatise by Santarella (1975) on the technics and applications of RC. ‐ The method for estimating the corrosion initiation time, recommended as “ absolutely simplest and most reliable ” in the highly cited watershed thesis by Tuutti (1982) on the corrosion of steel in concrete, indicates, for structures not sheltered from rain, a number as low as 20 years even in presence of common parameters such as cover 2 cm, w/c=0.65, and absence of chlorides. Corrosion is reported to be “ a headache problem in practical engineering ” by Tang et al. (2015).  Service life ‐ The scientific advice of NCHRP 90 in 1970 indicated an expected service life of highway prestressed concrete bridges greater than 75 years. ‐ The thesis by Tuutti (1982), which well exemplifies more recent perspectives, proposes to compute service life as the sum of corrosion initiation time plus corrosion propagation time. The propagation time is advised to be computed as low as 15-20 years, for CO 2 initiated corrosion, and as low as 5-10 years for Cl initiated corrosion. For structures with prestressing reinforcement, the prediction scheme of these recommendations leads to computing an expected service life that is even lower than 20 years.