Carbonation can be defined as the chemical reaction between carbon dioxide present in the atmosphere and hydration products such as calcium hydroxide (CH) and the CSH gel phase, which results in the formation of calcium carbonate (Ann 2010). COâ‚‚ is present in the atmosphere which dissolved in water to form carbonic acid and reacts with CH to form CaCOâ‚ƒ. Carbonation of concrete results in a slightly increased strength and a reduced permeability, possibly because water is released by the decomposition of CH on carbonation, aids the process of hydration and CaCOâ‚ƒ is deposited in the voids within the cement paste (Neville 2001). However, carbonation reduces the hydroxide concentration in the pore solution destroying the passivity of the embedded reinforcement bars making the bars vulnerable to corrosion (Chang 2006).
2.2 FACTORS AFFECTING CARBONATION
The factors can either speed up or slow down carbonation process and these factors can be internal or external. Several investigations had been carried out to measure carbonation depths applying different factors in their experiments. The carbonation process indicates that factors controlling carbonation are the diffusivity of COâ‚‚ and the reactivity of the concrete with COâ‚‚. The diffusivity of COâ‚‚ depends on the pore system of hardened concrete and the exposure condition (Wang & Lee 2009). Houst & Witmann (2002) stated that numerous factors can influence the rate of carbonation:
Water to Cement (w/c) ratio
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Lo & Lee (2002) stated that the carbonation depth increases with w/c ratio and the age of concrete. Further studies made by Lo et al (2009) proved that the increase in the carbonation depth with the increase in the w/c ratio due to the increase in the porosity of the concrete. Since the permeability of concrete is governed by the w/c ratio, concrete with a high w/c will be more prone to carbonation (Neville 2001). Khan & Lynsdale (2002) stated that the transport of COâ‚‚ into concrete depends on its permeation characteristic which according to Basheer et al (2001) is influenced by pore size distribution and continuity. Papadakis (2000) reported that as the w/c ratio is lowered, the pore system becomes finer and less connected which will lead to very low transport rates. Other authors (Houst & Wittmann 2002) stated that higher w/c ratios lead to higher carbonate contents in which the carbonate content depends on the content of hydration products, which depends itself on w/c. However, as the permeation of concrete decreases, its durability performance, in terms of physicochemical degradation, increases (Basheer et al 2001).
Curing time and conditions
There are two aspects of curing conditions that control the rate of carbonation. The first one is the type of curing condition and the other one is time of curing. Balayssac et al (1995) reported that the test results for carbonation have shown that curing conditions, and especially curing time, have a large effect on the durability of the concrete. Lo & Lee (2002) found that the rate of carbonation in air-cure concrete is greater than concrete cured in water. The difference in rate is influenced by the hydration speed of concrete cured in water and air. Air-cured concrete has larger pores and more interconnected channels and thus permits a greater COâ‚‚ diffusion, which in turn increase the rate of carbonation. In summary the diffusion of gas into concrete depends on porosity and the curing conditions thus play an important part in controlling the formation of pore microstructure (Lo & Lee 2002). According to Atis (2003) longer initial curing period of PFA incorporated concrete has shown better carbonation performance. This was due to better refinement that was achieved through enhanced moist curing. The carbonation depth for all concrete mixes under accelerated curing (high temperature) is higher than that of concrete under normal curing (room temperature) due to the fact that larger interpores in the cement paste (Lo et al 2009)
Type of cement
Several investigations have proved that OPC is better resistant in carbonation compare with concrete incorporating mineral admixtures. Pozzolanic concretes have long been considered to be less capable of resisting carbonation than OP and therefore have also shorter induction period for carbonation (Simsomphon & Franke 2007). Several researchers such as Atis (2009), Papadakis (2000) & Khan (2002) have concluded that the addition of PFA physically reduces the concentration of the carbonatable constituents of calcium hydroxide (CH) and CSH of the cement, which causes higher rate of carbonation. The effect of pozzolanic replacement on carbonation of concrete concerns both binding capacity of COâ‚‚ and porosity. It was previously reported that carbonation is strongly dependent on the degree of porosity which is the path for the carbon dioxide and water to transport in concrete, and the content of calcium hydroxide which reacts with carbon dioxide and water to form carbonation of concrete. To refine the pore structure, pozzolanic materials such as pulverised fuel ash has been used, but they adversely accelerated the rate of carbonation (Malami et al 1994). However Papadakis (2000) reported that in the case fly ash is introduced as fine aggregate replacement, the carbonation rate is reduced due to decrease in porosity of the concrete.
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According to Khan & Lynsdale (2002), the depth of carbonation significantly and linearly increased with an increase in PFA content. The author concluded that for every 10% PFA content, the carbonation depth increased approximately by 0.3mm. According to Lo et al (2009) up to 25% replacement level was marginal but at 40 and 50% replacement levels of PFA, the carbonation depth increased significantly. Papadakis (2000) stated that the carbonation depth decreases as aggregate replacement by FA (fly ash also known as pulverished fly ash) increases but increases as cement replacement by FA increases. The author also stated that as FA reduces the amount of CH, the rate of carbonation increases and this is because not only CH is carbonated, but also the C-S-H, which is the main product of pozzolanic reaction. Thus for aggregate replacement by pozzolanic materials the total amount of carbonatable constituents remains almost the same and moreover the porosity decreases resulting in lower carbonation rates. For cement replacement by pozzolanic materials the total amount of carbonatable constituents decreases due to decrease in total CaO, resulting in higher carbonation rates (Papadakis 2000).
st hogh replacement levels of PFA (>25%) the reduction in CH due to the reduced cement content prevails over the pore refinement effect, which resulta in the significant increase of carbonation (Lo et al 2009)
The risk of carbonation is more severe in area where COâ‚‚ concentration is very high (K.Y.Ann 2010). L.Basheer et al (2001) stated that the carbonation rate is controlled by the ingress of carbon dioxide into pore system by diffusion, with a concentration gradient of carbon dioxide acting as a driving force. According to Simsomphon & Franke (2007) when using an accelerating COâ‚‚ concentration of 3%, the difference in the rate of carbonation rate is about a factor of 10, which means that the carbonation rate in natural exposure is estimated to be approximately 10 times slower than the accelerated tests (Sisomphon & Franke 2007).
Carbonation of concrete is less likely to occur under a condition of low and high humidity because under low humidity concrete does not react with COâ‚‚ as there is insufficient water for it to dissolve and form carbonic acid while under high humidity penetration of COâ‚‚ into saturated concrete is difficult. The diffusion of carbon dioxide into concrete depends on the pressure differential when the concentration of carbon dioxide outside the concrete is high (Parrott 1987 & McCurrich LH 1985). According to Kwon & Song (2010) carbonic reaction and COâ‚‚ diffusion coefficient are so sensitive to saturation of pores, carbonation depth is significantly affected by relative humidity. Optimum conditions for the carbonation reaction process are the humidity range 60-70% (Y. Lo & H.M Lee 2002). According to Roy et al (1999) as the humidity level increases from 52% to 75% there is a significant increase in carbonation depth with increasing humidity level. There is then a decrease in carbonation as the relative humidity increases from 75% to 84% (S.K Roy et al 1999)
Investigation on the effect of temperature towards carbonation is limited. Ruxia (2010) concluded that the carbonation depth increased with increased in temperature. This is due to the faster spreading COâ‚‚ molecules in the pore solution of concrete when environment temperature increases, enhancing the speed of carbonation (Ruxia 2010). According to Song (2006) as temperature goes up, the solubility of COâ‚‚ is decreased and acid formation is also decreased, but diffusivity of COâ‚‚ is increased due to increased activity energy.
Presence of damaged zones and cracks
Cracks in early-aged concrete are unavoidable due to heat of hydration, drying shrinkage and improper curing of concrete. These cracks become a main path for COâ‚‚ penetration inside concrete so that the carbonation is accelerated in cracked concrete (Song 2006). The author concluded that as the crack width increases the carbonation depth also increases. The fact that, as the crack width increases, atmospheric COâ‚‚ enters through the cracks and it deteriorates the concrete in terms of carbonation by forming CaCOâ‚ƒ and reducing the alkalinity of the concrete, which reduces the durability of concrete. Wider surface crack widths indeed allow faster diffusion of COâ‚‚ than the smaller ones (Saraswathy & Song 2007). Although it allows faster diffusion of COâ‚‚, but the strength of the concrete increases due to formation of CaCOâ‚ƒ which decrease the porosity and increase the overall volume of concrete (Kim 2009).
2.3 Carbonation coefficient
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The diffusion of COâ‚‚ always takes place through concrete sections, which are already carbonated. It is usually assumed that the reaction from progresses after all alkaline material has been transformed. Therefore, the carbonation rate is diffusion-controlled and the diffusion coefficient for COâ‚‚ in carbonated concrete is the characteristic transport coefficient (L.Basheer et al 2001). According to Roy et al (1999) the carbonation rate constant K (mm/year â°Ë™âµ) can be calculated using square-root-t-law with the following equation:
X = K.âˆšt
Where x = depth of carbonation (mm) at time t (years) and K = constant
From the empirical analysis, it was found that the rate of carbonation is proportional to the square root of time of exposure (K.Y. Ann 2010) which has been very widely used in predicting further carbonation and the carbonation-free service life of concrete structures. According to Sisomphon and Franke (2007) other approach for determining carbonation coefficient is done according to the law of diffusion where K can be derived from the integration of Fick's first law of diffusion and finally yields
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Where D is a diffusion coefficient (m2/s), câ‚ is COâ‚‚ concentration in the environment and Câ‚€ represents an amount of COâ‚‚ (g/mÂ³) required to react with alkali phases contained in a unit of a sample.
2.4 Method accessing carbonation
Kritsada Sisomphon (2007) suggested that acceleration carbonation is an essential test in the carbonation study. Carbon dioxide concentration the atmosphere is 0.03% which makes it impossible to get results within short period of time. By increasing the C0â‚‚ concentration by a factor of 10 to become 3% COâ‚‚ concentration, the study of carbonation can be shorten from 10 years under normal condition to only few weeks (Kritsada Sisomphon 2007). The acceleration carbonation test is normally carried out in a controlled tank using 5% COâ‚‚ 60% relative humidity and 25Â± 5â°C. (P.H.R Borges et al 2010). The most common and cheapest method used for determining carbonation depth is by spraying a phenolphthalein indicator solution onto the surface of a split freshly concrete sample (Chen Feng Chang 2004). This kind of test gives immediate results and is an indication of the useful life remaining in existing structures (S.K. Roy 1999). According to Lo & Lee (2002) this techniques also useful as it gives a continuous carbonation front in a visual form. The solution is a colourless acid-base indicator, which turns purple when the pH is above 9, indicating carbonation zone (Y.Lo 2001). According to Cheng feng-Chang, when the pH in pore solution is 9.0, the degree of carbonation is 50%. However using this kind of technique would give less accurate results because the indicator is not sensitive to pH changes and also a darkly coloured aggregate leads to a visual illusion in the phenolphthalein indicator method and causes difficulty in differentiating the colour changes at the carbonation front (Y. Lo & H.M Lee 2002). According to Parrott (1987) carbon dioxide could react at depths greater than those indicated by a phenolphthalein indicator. Borges et al (2010) suggested that averages of 5 measurements are taken for carbonation depth.
2.5 pH in carbonated concrete
The alkalinity of the concrete is due to the hydration reactions which produce an alkaline compound, calcium hydroxide (CH) keeping the pH in the concrete high up to pH 13. However this is not the case when COâ‚‚ from the atmosphere diffuses through the unsaturated concrete pores dissolves in the pore water and then reacts with carbonatable solutes (M.A Peter 2008). The precipitation of calcium carbonate as shown in the following equation reduces the pH level of concrete (Lo & Lee 2002):
Ca(OH)2 + C O2 â†’ CaCO3 +H2O
Chang & Chen (2006) stated that the pH of the pore solution in concrete changes with the degree of carbonation. When the pH of the pore solution is less than 7.5, the degree of carbonation of the specimen is 100%. When the PH value of the pore solution is between 7.5 and 9.0, the degree of carbonation is 50-100%. When the pH of the pore solution is 9.0-11.5, the degree of carbonation is 0-50%. When the pH of the pore solution exceeds 11.5, the specimen is not carbonated (Chang & Chen 2006).
2.6 Carbonation of CH and CSH
Carbonation of concrete involves a physiochemical reaction between atmospheric carbon dioxide and the calcium hydroxide generated in cement hydration. The precipitation of CaCOâ‚ƒ as shown in the following equation reduces the pH level of concrete:
Ca(OH)â‚‚ + COâ‚‚ ïƒ CaCOâ‚ƒ + Hâ‚‚O (Y. Lo & H.M Lee 2002)
Carbonation is a common type of attack in cement paste and concretes. The process begins when CO2 penetrates into the cement matrix, dissolving in the pore solution to produce carbonic acid which reacts with CH, C-S-H and the hydrated calcium aluminates and ferro-aluminates to precipitate as various forms of CaCO3, silica gel and hydrated aluminium and iron oxides. Generally an initial reduction of porosity is expected because CH is the first phase attacked and the volume of the carbonates (calcite) formed is 11-12% greater than the volume of CH. Therefore, it is frequent to find an increase in the weight of carbonated samples as well as lower porosity and higher compressive strength at early ages of carbonation. This is the case for low porosity OPC pastes, where calcite formation decreases the overall porosity, preventing further diffusion of CO2 and reducing the carbonation attack. However, when the porosity in pastes is sufficiently high to permit constant CO2 diffusion, the CH is further depleted and the interlayer calcium from CSH also reacts with carbon dioxide. The removal of interlayer Ca2+ ions creates an excess of negative charges, which are balanced through subsequent formation of Si-OH groups. Condensation of neighbouring Si-OH groups to Si-O-Si linkages then forms silica gel. This condensation increases the mean silicate chain length and forms bridges between neighbouring regions, thus pulling them closer together leading to shrinkage. As a result, CO2 attack causes polymerisation of silicate chains in CSH which may cause a volumetric decrease (shrinkage) and cracking, coarsening the porosity. There is evidence in the literature that CO2 simultaneously reacts with CSH and that the decalcification of CSH takes place prior to the formation of silica gel, the latter starting at the later stages of carbonation CH may be initially more rapid than that of CSH gel, but the situation soon reverses because of the formation of a layer of CaCO3 micro crystals at the surface of CH. In blended cement paste, where the amount of CH is reduced due to pozzolanic reaction, carbonation of CSH seems to be even more dependent on the permeability of the pastes. Rapid decalcification of CSH is expected in highly permeable of the pastes, accompanied by carbonation shrinkage, which is accelerated when CaO.SiO2 molar ratio (C/S) is reduced below 1.2. On the other hand, carbonation of the low C/S ration CSH might not be a risk when the blended pastes have low permeability to hinder the CO2 ingress (P.H.R Borges 2010)
2.7 Strength of concrete in carbonation
Roy et al (1999) concluded that there is relationship between the rate of carbonation and the strength of the concrete with the carbonation depth being inversely proportional to strength. Chi (2002) stated that the compressive strength and splitting strength of carbonated concrete at the age of 28 days are slightly higher than those of concrete without carbonation. According to Basheer et al (2001) the strength of the concrete is influenced by the total volume of pores. Strength and hardness of concrete increases due to carbonation reduces the porosity of concrete and because the carbonation product, CaCOâ‚ƒ occupies a greater volume than Ca(OH)â‚‚ (Kim 2009).