Mechanisms of induction of corrosion and the influence of blast furnace slag
Author: Antnio Nogueira
Author: Antnio Nogueira
Along with chlorides and carbonation, there are many factors that influence the mechanism of corrosion of reinforcement, the main being the strength, curing conditions, the chemical composition of pore water and the properties of concrete cover thickness. In these factors, there are local variables, such as the mineralogy of the aggregates, conditions environmental and construction practices that act strongly in the corrosion mechanism (MORRIS et al., 2002). In specifying the service life of reinforced concrete structures, standards of various countries consider that the most important thing is to try to prolong the period of initiation, because it corresponds to the lifetime of the project. Corrosion induced by carbonation The effect of carbonation by the penetration of CO2 and other acid gases such as SO2, NO2 and SO3, in relation to corrosion of reinforcement, is given by reducing the pH of the water pore. The drop in pH at certain levels can cause loss of first passive, then the initiation of corrosion and, over time, severe corrosion of the reinforcing (Castro, Moreno; GENESCO, 2000; CASCUDO, 1997; HELENE, 1993; MEHTA Monteiro, 1994). CO2 enters the pores partially filled with water and then dissolve and react with alkali pore water, forming carbonic acid. After the dissolution in water,first reactions take place with NaOH and KOH, which are more soluble, and then withCa (OH) 2, producing calcium carbonate according to the generalized reaction (BAUER,1995; HELENE, 1993; Houst, WITTMANN, 2002; IHEKWAHA; HOPE, HANSSON,1996; MEHTA Monteiro, 1994; STEFFENS; DINKLE; AHRENS, 2002):
H2O Ca (OH) 2 + CO2 CaCO3 + H2O. The formation of CaCO3 consumes alkalinity and lowers the pH of the pore water of between 8 and 9 (BAUER, 1995, Brown, 2002; HELENE, 1993; MEHTA Monteiro, 1994). If the front carbonation reach the armor, and if sufficient moisture and oxygen are present, the protective passive layer is removed and the corrosion can be installed (BROWN, 2002; HELENE, 1993; IHEKWAHA; HOPE; HANSSON, 1996; Houst, WITTMANN,
2002; NEVILLE, 1997). When Ca (OH) 2 is exhausted, for example, through a secondary reaction withpozzolan or slag by activation of blast furnace, you can also occur carbonation HSC, which leads, in addition to the formation of CaCO3, the formation of silica gel with pores large, greater than 100 ?m, and facilitates continuity of carbonation (OPURO?LU;Fraaije, Bijen, 2006; IHEKWAHA; HOPE; HANSSON, 1996; MUKHERJEE, 2004; NEVILLE, 1997; STEFFENS; DINKLE; AHRENS, 2002): H2O C-S-H + CO2 CaCO3 + H2O + SiO2nH2O, Aluminate hydrates CaCO3 + CO2 + hydrated alumina, Hydrates Ferrite + CO2 CaCO3 + hydrated alumina + iron oxides. As the penetration of gas occurs uniformly by diffusion, forming a front homogeneous penetration of the concrete, the corrosion induced by carbonation is manifested in armor across the board, and, when severe, causes cracks in the concrete that develop parallel to the bars (Brown, 2002; HELENE, 1993; MEHTA Monteiro, 1994). As the CO2 will permeate the pores partially filled with water through diffusion zones are formed at different pHs: a zone with high pH, the non - carbonate, and an area with pH
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