Investigation on the effect of supplementary cementitious materials on the critical chloride threshold of steel in concrete

Materials and Structures Investigation on the effect of supplementary cementitious materials on the critical chloride threshold of steel in concrete 0 F. Lollini (&) E. Redaelli L. Bertolini Department of Chemistry, Materials and Chemical Engineering ''Giulio Natta'' , Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan , Italy The critical chloride threshold is a key parameter in the service life design of reinforced concrete structures exposed to chloride-bearing environments. This paper investigates the role of concrete composition, and particularly the effect of supplementary cementing materials, on the chloride threshold. To simulate real exposure conditions, ponding tests were carried out on reinforced concrete specimens with bars in free corrosion conditions and corrosion initiation was detected through corrosion potential and corrosion rate measurements. After two and a half years, the ponding was followed by an ageing period and the initiation of corrosion was further detected with anodic potentiostatic polarisation tests. Results of the tests showed several limitations of the approach based on chloride penetration and monitoring of free corrosion parameters to investigate the chloride threshold. In spite of this, a possible role of natural pozzolan and coal fly ash additions in leading to higher values of the chloride threshold and ground limestone in promoting lower values of the chloride threshold could be observed. Chloride; Concrete; Corrosion; Blended cement; Ponding tests; Potentiostatic tests 1 Introduction A reliable estimation of the chloride threshold (Clth) is a fundamental step in the prediction of the service life of reinforced concrete (RC) structures exposed to marine environments or the action of de-icing salts [ 10, 15, 55 ]. In the literature, several test methods have been proposed to evaluate the critical chloride threshold. Limiting to the tests in concrete, which are expected to be intrinsically more realistic compared to tests in solution, methods proposed by different Authors differ regarding the way concrete is contaminated by chloride ions, corrosion initiation is detected and the chloride content at the depth of the bar is measured. Chlorides can be forced to penetrate into concrete through external sources [ 8, 19, 20, 22, 24, 25, 29, 31, 32, 38, 40, 45–47, 50, 52–54, 56, 57 ], as for instance ponding with chloride-containing solutions [ 32, 46 ], or alternatively can be added to the concrete mix [ 2, 30, 32, 35, 39, 41, 47 ]. The way chlorides are introduced into concrete has several consequences. Tests with chlorides which penetrate from an external source can have excessively long duration, although they are often assumed to better simulate exposure of real structures to marine environments or de-icing salts. Conversely tests with mixed-in chlorides are relatively short, but the microstructure of the interfacial transition zone between steel bar and concrete, as well as the passivation of steel, can be affected by the presence of chloride ions since the early stage of hydration. The initiation of corrosion is often detected by nondestructive monitoring of electrochemical parameters (e.g. corrosion potential and corrosion rate) [ 2, 8, 19, 25, 29, 32, 35, 38, 40, 41, 46, 56 ] and, seldom, by extraction of bars and visual inspection [20] or weight loss [ 52, 53 ]. As far as the measurement of the chloride content is concerned, the analysis of total chloride content is frequently applied in practice. Different criteria for sampling have been proposed: directly from the bar site at the time of corrosion onset [ 3, 19, 28, 31, 41, 50, 54, 57 ] or far from the bar considering the same bar depth [ 25, 39, 40, 45, 52, 53 ] (sometimes even in reference samples exposed to the same exposure conditions, provided similar conditions for chloride penetration occur [38]. Tests can also be different for other aspects such as, for instance, the bar surface conditions, the specimen geometry, etc. Differences in the test procedures used by different Authors is one of the reasons that may lead to a great variation in the values of the critical chloride threshold published in the literature [ 4, 7, 9, 27, 48 ]. This makes the comparison of results of different works quite difficult and, consequently, hinders the evaluation of the role of factors that influence the critical chloride threshold. Nowadays there is a great interest in studying a possible influence of the binder type (and concrete composition, in general) on the critical chloride threshold, due to the availability of a great variety of supplementary cementing materials (SCMs), as also underlined by the activity of specific technical committees (e.g. RILEM TC 235-CTC, www.rilem.org). As a matter of fact, the type of binder may affect the chloride threshold in several ways. It influences the alkalinity of the pore solution, the availability of free chlorides in the pore solution as a result o (...truncated)