PSI - Issue 17

S. Tsouli et al. / Procedia Structural Integrity 17 (2019) 268–275 S. Tsouli, Lekatou A.G., C. Nikolaidis, S. Kleftakis / Structural Integrity Procedia 00 (2019) 000 – 000

270

3

al. (2016), Shi et al. (2012)). FA is a powdery by-product of solid fuels combustion in the thermoelectric power generating plants. The beneficial effect of FA on the corrosion performance of reinforced concrete has been shown in several studies. It is ascribed to the pozzolanic reaction of SiO 2 with Ca(OH) 2 to form calcium-silicate-hydrate (C-S-H) with binding properties (Mehta et al. (1987)). C-S-H is more resistant than Ca(OH) 2 to the penetration of aggressive species, such as sulphates and chlorides (Nie et al. (2014)). The formation of C-S-H decreases the hydration heat release, the drying shrinkage and the porosity of concrete (Chousidis et al. (2016), Shi et al. (2012)). Moreover, in the presence of chlorides, additional reaction products may fill the capillary voids of concrete and therefore reduce its permeability, such as calcium chloroaluminates (Chousidis et al. (2016), Yue et al. (2018)). Preliminary effort by the authors has shown the positive effect of FA (up to a certain percentage) on the electrochemical behavior of 316L in mildly acidic environments containing Ca(OH) 2 (Tsouli et al. (2018) a ). The present study deals with the effect of FA on the corrosion performance of 316L rebars subjected to two different modes of accelerated testing: a) Electrochemical degradation in a slightly alkaline solution simulating corroded concrete that has allowed direct access of the AR to the steel reinforcement and b) mechanical degradation of rebars embedded in concrete cubes due to salt spraying. This work has been motivated by the limited literature on the electrochemical corrosion and structural integrity of concrete stainless steel reinforcement in environmentally polluted and coastal areas and is oriented towards applications relative to restoration works of ancient monument and historical buildings.

Nomenclature E corr

corrosion potential

E a/c tr

anodic-to-cathodic transition potential

E b

breakdown potential corrosion current density

i corr

2. Experimental

Rebars of 316L steel (Fe, 0.022% C, 17.31% Cr, 10.08% Ni, 2.02%Mo, 0.54% Si, 1.75%Mn, 0.0032% P, 0.0001% S) and 304L (Fe, 0.03% C, 18.00% Cr, 8.00% Ni, 1.00% Si, 2.00% Mn, 0.0045% P, 0.03% S) were employed. An alkaline fly ash (FA) from the Hellenic Public Power Corporation lignite mines in the Region of Western Macedonia, Greece was used as a corrosion inhibitor. The FA mainly consisted of CaO, SiO 2 , Al 2 O 3 , SO 3 , Fe 2 O 3 and MgO (Tsouli et al. (2018) a ). Cubes of ordinary Portland cement (OPC) mixed with pulverized FA and reinforced with 316L or 304L (for comparison reasons) rebars of 6 mm diameter and 12 cm length were also employed in this work. 316L rebar specimens of 6 mm diameter and 2.5 cm length (with their cutting edges mount in epoxy and further masked with PTFE) were subjected to cyclic potentiodynamic polarization testing in order to assess the localized corrosion resistance (at least 3 replicate runs for each specimen). The three electrode galvanic cell set-up used in this work has previously been described (Tsouli et al. (2018) b , Tsouli et al. (2018) c ). The corrosion current densities were calculated by Tafel extrapolation, conforming to several criteria previously described (Lekatou et al. (2016)). A positive hysteresis loop (i.e. a loop formed by anodic reverse scans corresponding to lower current densities than the respective current densities of the forward scans) manifested high resistance to localized corrosion (ASTM G61 86).The electrolyte contained 1.8 g/l of a mixture of Ca(OH) 2 and FA at different contents (0 - 25 wt.% of the dry mixture) and an AR simulating solution (H 2 SO 4 : 0.032, HNO 3 : 0.015, Na 2 SO 4 : 0.032, NaNO 3 : 0.021, NaCl: 0.084, (NH 4 ) 2 SO 4 : 0.046 in g/l H 2 O, pH=3.1). The initial pH of the electrolyte was 7.70-7.76 irrespectively of the FA content. Since an alkaline solution of saturated Ca(OH) 2 (1.8g/l of H 2 O, pH  12.6) simulates the solution remaining in the pores of concrete during the hydration process (Veleva et al. (2002)), a slightly alkaline electrolyte simulates corroded concrete that has allowed direct access of the AR to the steel reinforcement (Fan et al. (2013)). Concrete cubes reinforced with 316L and 304L rebars were subjected to salt spray testing for 2 m in a Vötch chamber (4 replicate runs for each specimen, ASTM B117-97, 5 wt.% NaCl, 35 o C). The stainless steel / concrete junction of the free surface was coated by an epoxy glue. Uniaxial tensile testing of the rebars was conducted before and after 2 months of salt spraying in a Galdabini 100 kN Universal Testing Machine (ASTM E8/E8M-09).