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Mechanisms of induction of corrosion and the influence of blast furnace slag
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 (ÇOPURO?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 <9, designated area or carbonated front carbonation (CASCUDO, 1997; HELENE, 1993; IHEKWAHA; HOPE; HANSSON, 1996; MEHTA Monteiro, 1994).
They are identified by means of a pH indicator, usually colorless, whose turning is around 9, so areas of partial carbonation in which the pH varies from 9 to 11.5 can not be detected by this indicator.
Considering that the pH 11.5 of the armor is already despassivada, in many studies the depth of carbonation the concrete may be underestimated (CHANG, CHEN, 2006).
Their use techniques ATG (thermogravimetric analysis), XRD (X-ray diffraction) and FTIR (infrared spectroscopy, Fourier Transform) have been identified three areas in carbonated concrete: the first, which was close to the outer surface, in which the pH of the pore water was between 7.5 and 9, and the degree of carbonatação12 was 50 to 100%, the second call ahead of carbonation, which was a transition zone in which the pH of the pore water was from 9 to 11.5, and the degree of carbonation was 0 to 50%, finally, the third area that was not carbonated (CHANG, CHEN, 2006; Houst, WITTMANN, 2002; SÆTTER; Vitaliano, 2005).
Such techniques (TGA, XRD and FTIR) showed that the depth of carbonation front was two times greater than that determined by phenolphthalein, although considered that reliable indicator for the analysis of corrosion of reinforcement (CHANG, CHEN, 2006).
The apparent resistivity of the concrete RESI registered on the meter (as defined in Chapters 3 and 4) is equivalent to the resistivity of the mixed zones with 3 different concentrations of CaCO3.
CO2 enters by diffusion mechanism and the depth of carbonation increases, initially at a higher rate, progressing more slowly over time in a curve exponential, tending asymptotically to a maximum depth (Bashir; KROPP; CLELAND, 2001; BAUER, 1995; HELENE, 1993).
The K for CO2 depends on his concentration and characteristics of concrete and the environment, such as type of cement, the water / cementitious material, conditions of healing, degree of hydration, cement consumption (C), relative humidity, equilibrium moisture any cycles of wetting and drying, temperature, alkali content, degree of carbonation, electrical resistivity, the presence of degraded areas, cracks, among others (Andrade, 2005; CASCUDO, 1997; HELENE, 1993; HELENE, 2004; Houst, WITTMANN, 2002; IHEKWAHA; HOPE; HANSSON, 1996; JUNG, YOON, SOHN, 2003; STEFFENS; DINKLE; AHRENS, 2002).
One of the most significant is the relative humidity, which range between 50 and 65% shows the highest rate of carbonation (Brown, 2002; CASCUDO, 1997).
The coefficient of carbonation has a good relationship with the total absorption of the concrete, and its speed can be slow in the concrete with a lower pore volume communicable, depending on the thickness of concrete cover thickness (BAUER, 1995, Brown, 2002; FIGUEIREDO; NEPOMUCENO, 2004).
The ability to protect the concrete cover thickness in a given environment may several failures due to lack of an adequate study of strength and durability: the concrete with higher water absorption, due to a low ratio water / material cementitious, may cause faster diffusion of carbon dioxide and become more susceptible to carbonation; low the water / cementitious material can cause shrinkage on drying autógena13, or concrete can show high capillary absorption, due to decreased pore diameter (Brown, 2002; CASCUDO, 1997; Houst, WITTMANN, 2002; MEHTA Monteiro, 1994).
It is known that slag of blast furnace has the effect of reducing the resistivity of the matrix cement and transport properties, by reducing the diameter and network connectivity pore. If the reductions in transport properties and overcome the conductivity reduction of hydroxides, the carbonation will be reduced overall. This effect may increase period initiation 14 corrosion and decrease the corrosion rate in the period propagação15.
Concrete wet cure undergo efficient have lower rates of carbonation that cured in air (HELENE, 1993, Castro et al., 2004, Dias, 2000), and less concrete the water / binder result in less carbonation than the more the water / binder (BAUER, 1995, Castro et al., 2004, Coelho, 2002; HELENE, 1993 and Taylor, 1992; Houst, WITTMANN, 2002).
In addition, changes in temperature interfere with the kinetics of reactions and, over the height structural parts or building may be different concentrations of CO2 and fog
saline (Houst, WITTMANN, 2002). This can cause potential difference along structure, which can facilitate the onset of corrosion of reinforcement.
Also, as cycles of wetting-drying conditions are common in environmental air free, if the external parts of buildings, the carbonation front can move relatively fast (HELENE, 1993; Houst, WITTMANN, 2002).
With increasing temperature, moisture content is being reduced, providing more space for the diffusion of gas, and increasing the diffusivity of CO2. Although concrete dry or exposed to dry indoor environment, have their maximum diffusivity of CO2, the lack of water can prevent the reaction of carbonation, since it is a necessary means aqueous to be the dissolution of CO 2 and Ca (OH) 2 (BAUER, 1995; GRUBE; KERKHOFF, 2004; Houst, WITTMANN, 2002; IHEKWAHA; HOPE; HANSSON, 1996; MEHTA Monteiro, 1994; STEFFENS; DINKLE; AHRENS, 2002).
Some authors believe that a smaller amount of Ca (OH) 2 in concrete with slag blast furnace results in lowering the pH and, consequently, the time required for passivation reinforcement induced by carbonation, which consumes alkalinity, would also be reduced (FRANKE; SISOMPHON, 2004).
However, a smaller amount of Ca (OH) 2 does not necessarily mean that they are found very low values of pH. Found pH values above 12.78 concrete with slag from blast furnace (at levels of substitution of 50% and 70%) and never less than 11.5 (COSTA; GASTALDINI; ISAIA, 2002; GASTALDINI; ISAIA; Zanella, 1999).
The alkalinity of pore solution seems to depend on the basicity of slag blast furnace used in concrete, but even at pH 11.5, the diffusivity of CO2 and electrical conductivity are difficult as a dense matrix of concrete with additions and with a low water / cementitious material (BAUER, HELENE, 1993).
The effects of carbonation in the concrete appear to vary depending on the content of items and stage of deterioration of the structure. In little carbonated, if there is clogging by calcium carbonate layer nearest the surface, the rate of carbonation seems to be slower.
In concrete with advanced stage of carbonation, if dissolution of calcium hydroxide and compounds carbonatáveis, may experience swelling, size and diameter of pores in the deeper layers and subsequent loss of resistance. The hydration products occupy less volume than the products of carbonation, which include increasing mass of water.
Thus, the compressibility of hydrated cement paste increases temporarily, causing the so-called retraction by carbonation (BAUER, 1995; CASCUDO, 1997; HELENE, 1993; Houst; Wittmann, 2002; MEHTA Monteiro, 1994; RICHARDSON, 1998).
Because of the carbonation, the folder or Portland cement concrete without additions create a surface layer of dense, rich in calcium carbonate, which forms a barrier to dissemination, limiting the access of CO2, while the volume, size and diameter of pores in layers or internal non-carbonated can be larger. Thus, the sorption (absorption rate) and the rate of carbonation in the outer layer are smaller than the inner layer. This leads to the slow progress of carbonation, which decreases with time (BIER, 1987 apud Bakharev, Sanjayan; CHENG, 2001, Dias, 2000; HELENE, 1993).
It has been found in studies of De Ceukelaire and Van Nieuwenburg (1993 apud DIAS, 2000) that the carbonation of Portland cement concrete without additions was an increase of solid volume around 11% in the conversion of Ca (OH) 2 to CaCO3, and mass around 35% based on molecular weight, it was felt that an expansion in the volume and consequently, a reduction in total porosity.
The carbonation in concrete without additions, although slow, may cause some changes physico-chemical and electrochemical processes, such as increased resistance to compression, the mass specific modulus of deformation, and hardness, the reduction in transport properties, as absorption, exchangeable with age, the permeability, the diffusion of chlorides; increase in electrical resistivity, decrease the potential for corrosion and difficulty in electrochemical chloride extraction (DIAS, 2000; HELENE, 1993; IHEKWAHA; HOPE; HANSSON, 1996), and less able to protect the front cover thickness agents aggressive, as sulfates (Verbeck, 1958 apud CASTRO, Moreno; GENESCO, 2000) and chlorides (DIHR et al., 1993 apud MONTEIRO; NEPOMUCENO, 1996).
In concrete with slag, the carbonation of calcium hydroxide and other compounds, depending on their contents, initially leads to a decrease in pore volume in Because the filling, and sequentially, there is an increase after the formation of silica or porous silica gel (ÇOPURO?LU; Fraaije; Bijen, 2006, Dias, 2000; Ngala, PAGE, 1997; MUKHERJEE, 2004).
The porous silica or silica gel, which is the product of reaction of calcium silicate hydrate with CO2, is more open microporosity (CEUKELAIRE, Van Nieuwenburg, 1993 apud DIAS, 2000). Therefore, the reduction in volume and pore diameter because of the carbonation in the advanced stage may not occur in concrete with slag from blast furnace.
Bauer (1995) found an increase of mass due to the carbonation, more significant with cement mortar with 24% slag (CP II-E) than with cement mortar without slag (CP II-F).
The lowest levels of Ca (OH) 2 in the concrete with mineral addition, due to the replacement of Portland cement, an opportunity for CO2 attack faster the CSH, as the Ca (OH) 2 is depleted more rapidly, thus the advance of carbonation and the increase pore volume of larger diameter may be faster in concrete admixtures, depending on other properties of concrete.
Thus, the increase in the rate of carbonation in concrete with high levels of slag (Bourguignon, 2004, Coelho, 2002; ISAIA; vaguette; GASTALDINI, 2001; MUKHERJEE, 2004) is due to two reasons: the lowest content of calcium hydroxide (BAUER, 1995; BELIE et al., 1996) and other compounds available to react with CO2 atmospheric (Papadakis, 2000), and the consequent change in the distribution of porosity in fraction of the actual folder itself due to carbonation (MUKHERJEE, 2004), which accelerates further the mechanism.
The redistribution of the porosity depends on the diffusion coefficient and the degree of carbonation, and especially affects the transition zones in cement pastes rich slag (ÇOPURO?LU; Fraaije; Bijen, 2006; STEFFENS; DINKLE; AHRENS, 2002). A more rapid deterioration of the transition even easier penetration of agents aggressive, which leads to a shortening of the inception phase induced by carbonation, and increases the risk of corrosion (Papadakis, 2000).
The ionic diffusivity may increase by an order of magnitude if the carbonated concrete
leaching suffer enough to create a microstructure with pores of larger volume (BENTZ; garbocz, 1992 quoted in CHEN, THOMAS; Jennings, 2006).
After dissolution of CH, different results can occur in cement pastes, the most obvious are the increase in total porosity and subsequent loss of resistance (CHEN, THOMAS; Jennings, 2006; HEUKAMP, ULM, GERMAINE, 2001; Ngala, PAGE, 1997).
Often, the leaching of concrete is the result of an attack fluid (pure water or water with very low pH compared to the pore water of concrete), and induces the hydrolysis of composed of hydrated cement paste, due to the diffusion of aggressive agents transported from the surface of SWNT into the concrete, and dissemination of products dissolved in the interior of the concrete to the surface. This leads to an increase important in volume and diameter of pores of cement paste, something that brings such effect of increasing the coefficients of mass transport (Burlion, BERNARD; CHEN, 2006).
A higher total porosity probably produced by certain strengths of concrete with higher levels of items can contribute to the diffusion of CO2, because that influences the rate carbonation is not just the CO2, but also its distribution inside the concrete, which is increased by greater distribution of porosity, pore although more refined, and the lower relative amount of Ca (OH) 2 available for carbonates.
An example of successful dosage is decreased carbonation in performance service to some European studies, which used slag content of 85%. Therefore requires more knowledge in this area (MUKHERJEE, 2004).
Castro et al. (2004) studied the carbonation in concrete with the addition of blast - oven at levels of 65% by mass of cementitious material, found that the refinement microstructure, caused by the slag of blast furnace to the concrete with a low water / cementitious material, can overcome the problem of alleged reduction in alkalinity (innconsequence of reducing the amount of calcium hydroxide from the proportioning).
From the above, the deleterious effect of carbonation can be minimized by studies of strength and durability, in order to achieve durable concrete denser microstructure or discontinuous porosity, which is obtained, among other factors, with the adoption of low the water / cementitious material (FIGUEIREDO; NEPOMUCENO, 2004; HELENE, 1993).
About the Author
António Nogueira is a Civil Engineer and expert in Concrete. Visit Projetos de Engenharia and see the calculo estrutural
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