Comparison between half-cell potential of reinforced concrete exposed to carbon dioxide and chloride environment
Kitipoom Chansuriyasak 1, Chalermchai Wanichlamlart 2, Pakawat Sancharoen2, Waree Kongprawechnon3 and Somnuk Tangtermsirikul1,2
1
School of Civil Engineering and Technology,
2
Construction and Maintenance Technology Research Center (CONTEC),
3 School of Information, Computer, and Communication Technology, Sirindhorn International Institute of Technology, Thammasat University, Khlong Luang, Pathum Thani, 12120 Thailand.
Received 25 December 2009; Accepted 3 August 2010
Abstract The objective of this study is to investigate the effect of concrete mix proportion and fly ash on half-cell potential (HCP) and corrosion current density (icorr) of steel in concrete exposed to different environments. Reinforced concrete specimens with different fly ash replacement percentages and water to binder ratios (w/b) were studied in this paper. The specimens were subjected to two highly corrosive environments which are chloride and carbon dioxide. HCP and icorr were used to monitor the corrosion process. Results of this study demonstrate that both HCP and icorr indicated the same tendency, especially for corroded specimens after being exposed to chloride. This means that HCP can be used to inspect corrosion of steel due to chloride. In case of carbonation, concrete specimens with fly ash showed more negative potential values than concrete without fly ash. However, chloride exposure test exhibited that specimen with higher fly ash replacement corroded earlier. Moreover, HCP measurement presented different values between concrete exposed to chloride and carbon dioxide. There was an effect of carbonation to increase HCP during the initiation stage. A proper evaluation guideline for steel corrosion due to carbonation needs to be further studied. Keywords: corrosion, half-cell potential, non-destructive test, chloride, carbonation, inspection
1. Introduction Corrosion of reinforcing steel is a major type of deterioration in reinforced concrete structures. Products of corrosion exhibit volume expansion and induce tensile stress in concrete, which ultimately result in cracking and spalling of concrete cover. Due to loss of steel cross-section and covered concrete, there could be a significant reduction in load bearing capacity of the structures. It is well known that
steel reinforcement in concrete is protected from corrosion by a passive film formed due to the high alkalinity of concrete. A passive film of hydrated iron oxide with a thickness of a few atomic layers is created on the steel surface. This passive film is decomposed due to penetration of chloride ions (Cl-) or carbon dioxide (CO2). Mechanisms of corrosion begin when ferrous ions (Fe2+) from anode are dissolved and electrons are set free. These electrons drift through the steel to the cathode, where, together with the generally available water and oxygen, they form hydroxide (OH-). However, many reports (Gu and Beaudoin, 1998; Elsener, 2001; Sancharoen et al., 2009) show different corrosion mechanisms of steel in concrete when subjected to chloride and
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K. Chansuriyasak et al. / Songklanakarin J. Sci. Technol. 32 (5), 461-468, 2010 Corrosion possibility is uncertain when the voltage is in the range of -200 to -350 mV. However, RILEM TC 153-EMC suggests that numerical criteria given by ASTM C 876 lead to misinterpretation because measured values of the potential fluctuate upon various factors such as mix proportion of concrete, type of ingredients, cover thickness of reinforcing steel, moisture content inside the concrete and type of corrosion of reinforcing steel. In addition, as carbonation process leads to an increase of concrete resistivity, HCP values tend to be less negative on both passive and corroding reinforcing steels. When concrete cover thickness is increased, the difference between active and passive potential values is diminished, resulting in a uniform potential value at infinite. Thus, locations of localised corrosion become more difficult to be detected when cover depth is increased (Hansson et al., 2006; Elsener, 2002; Andrade and Alonso, 1996). Moisture condition of concrete also affects resistivity of concrete. Changes in moisture content may lead to a difference of potentials up to 200 mV (Gu and Beaudoin, 1998). It is important to consider not only the difference of moisture condition at a measuring point but also variation along the whole structure. HCP values become more negative as concrete moisture increases (RILEM TC 154-EMC, 2003). Moreover, concrete normally used in construction has large variety in mix proportion as well as type of cementitions materials. The effect of fly ash widely used as partial cement replacement material in Thailand on HCP measurement has not been clarified yet. The main objective of this study is to investigate the effect of different corrosive environments i.e. carbon dioxide and chloride, on half-cell potential and corrosion current density. Concrete specimens were prepared with different cover thickness, percentage of fly ash replacement, and w/b ratio. The effects of these variables on HCP and icorr, used to investigate steel corrosion due to chloride and carbon dioxide, were studied. 2. Experimental program 2.1 Materials and concrete mix proportions Ordinary Portland Cement (Type I) was used in this study. Fly ash type F according to ASTM C618 (type 2b according to Thai Industrial Standard, TIS 2135) was used as a partial cement replacement material for producing fly ash concrete specimens. Chemical compositions and physical properties of the cement and fly ash are given in Table 2. Crushed limestone with a maximum size of 19 mm was used as coarse aggregate. Fine aggregate was river sand with fineness modulus of 3.12. Both aggregates complied with the requirements of ASTM C33. Void content of the compacted mixtures of fine and coarse aggregates was tested according to ASTM C29/C29M. The sand to total aggregate ratio of 0.45, with a minimum void ratio of 0.23, was used in all concrete mix proportions. Nine different concrete mix proportions were prepared for this study as shown in Table 3.
carbon dioxide. In the case of carbonation, anode and cathode areas cannot be localized as they are very small in size and are evenly distributed. On the other hand, corrosion of steel in concrete due to chloride shows much smaller anode area than that of the cathode and both can be far away from each other. Corrosion inspection of steel can be conducted by many different techniques. Non-destructive techniques such as half-cell potential measurement (HCP) and corrosion current density (icorr) are two well known techniques (Pradhanl and Bhattacharjee, 2009). Corrosion of steel can be investigated by HCP measurement according to ASTM C876 via an electrochemical process. If a steel bar is depassivated, electrons flow from the steel bar to the reference half cell (usually a copper rod). At the reference half cell, electrons would be consumed in a reduction reaction, transforming copper ions in the copper sulfate solution into copper atoms deposited on the rod. A voltmeter is connected to the electrical circuit to measure the reference potential as shown in Figure 1. The voltmeter usually indicates a negative value during the measurement. The value of the potential is used as an indicator of the likelihood of corrosion activity as shown in Table 1 (ASTM C876), for a Cu/CuSO4 electrode. If the potential is more than -200 mV, there is a high possibility that no corrosion is occurring at the time of measurement. If the potential is less than -350 mV, there is a high possibility of active corrosion.
Voltmeter
-0.28
Cu /CuSO
-
4
(Sat.)
+
Concrete
Reinforcing Steel
Figure 1. Schematic illustration of the half-cell potential measurement
Table 1. Interpretation guideline of half-cell potential measurement based on ASTM C876 Half-cell potential, mVa >-200 -200 to -350
