Influence of Carbonation on the Chloride Diffusion in Concrete

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1 Influence of Carbonation on the Chloride Diffusion in Concrete Sang-Hwa Jung 1,a, Young-Jun Choi 1,b, Bong-Chun Lee 1,c 1 Construction Material Research Center, Korea Institute of Construction Materials, , Seochodong, Seocho-gu, Seoul, Korea, a jsh2593@kicm.re.kr, b knujuni@hanmail.net, c leebc@kicm.re.kr ABSTRACT Recently, the corrosion of concrete structures has received great attention related with the deterioration of sea-side structures, such as new airport, bridges, and nuclear power plants. In this regards, many studies have been done on the chloride attack in concrete structures. However, those studies were confined mostly to the single deterioration due to chloride only, although actual environment is rather of combined type. The purpose of the present study is, therefore, to explore the influences of carbonation to chloride attack in concrete structures. The test results indicate that the chloride penetration is more pronounced than the case of single chloride attack when the carbonation process is combined with the chloride attack. It is supposed that the chloride ion concentration of carbonation region is higher than the sound region because of the separation of fixed salts. Though the use of fly ash pronounces the chloride ion concentration in surface, amounts of chloride ion penetration into deep region decreases with the use of fly ash. The present study allows more realistic assessment of durability for such concrete structures which are subjected to combined attacks of both chlorides and carbonation but the future studies for combined environment will assure the precise assessment. KEYWORDS: Chloride ion diffusion, carbonation, freezing-thawing, fly ash. 1. INTRODUCTION Owing to its remarkable durability, concrete is the most used construction material for infrastructures protecting the residential, activities and industrial spaces. However, concrete structures are also subjected to chemical and physical deterioration provoked by harsh environmental conditions since their construction due to their inherent characteristics and exposure environment. 1,2) Reports were done since 1960s on the shortening of the lifetime of reinforced concrete structures due to the degradation of concrete itself or the corrosion of reinforcement. Accordingly, stress was given on the necessity to secure quantitatively durable lifetime of reinforced concrete structures and corresponding researches were actively implemented in 1980s and 1990s. 3-5) Since actual structures are more likely to experience deterioration due to the combination of more than 2 factors rather than the action of a unique factor, researches in advanced countries recently attempted actively to investigate the penetration of chloride ion under the combined action of freezingthawing and salt attack, the chemical reaction of hydrates in order to predict the composite degradation provoked by salt attack and carbonation, and the penetration depth of chloride ion and carbonation depth in environment subjected simultaneously to salt attack and carbonation. 6) Because concrete nuclear power plant (NPP) structures are located in coastal areas, the most important degradation among the multiple deterioration processes can be seen to be the corrosion of reinforcement due to the penetration of chloride ion. Following, this study performs chloride ion penetration test using materials used in existing NPP concrete structures to examine the deterioration process of durability due to the diffusion of chloride ion. Especially, investigation of the diffusion 1191

2 characteristics of chloride ion under single environment together with the investigation of the variations of the characteristics of chloride ion diffusion in the case of carbonated concrete are carried out experimentally to clarify the penetration pattern of chloride ion in concrete under composite environment combining wet-dry cycling, carbonation and salt attack. 2. TEST SUMMARY 2.1 Materials and test variables This study uses type 1 and type 5 cements (specific weight 3.15) produced in Korea and fly ash (specific weight 2.1) produced in the Boryeong thermoelectric power plant, and performs tests with the type of cement (type 1, type 5, substitution rate of 20% by fly ah) and the water-cement ratio (38%, 42%) as main test variables. The reason for the tests to focus on type 5 cement is because this type of cement is used for the NPP in Korea, and the use of fly ash is conducted by the fact that large quantities are already used in the domestic ready-mixed concrete. Following, tests were performed by mixing fly ash to type 5 cement by substitution of 20% of the whole volume of cement. The aggregates are Figure 1. Test series identification crushed aggregates exhibiting specific weight of 2.63 and fineness modulus of 6.7 and river sand with specific weight of 2.55 and fineness modulus of 2.6. Table 1 summarizes the chemical composition of the cements and fly ash used in this study. The target slump of the mix design is 15±2.5cm and the target air content is 4.5±1.5% for which AE agent has been introduced with a quantity corresponding to 0.15% of the unit weight of cement. Table 1. Chemical components of cement (%) Cement SiO type 2 Al 2 O 3 Fe 2 O 3 CaO MgO Na 2 O K 2 O SO 3 Type Type Fly ash F- CaO Ig. loss C 3 S C 2 S C 4 AF C 3 A Table 2. Mixture proportions for test series Test series Cement Type W/B (%) Cement Fly ash Water Fine agg. Coarse agg. H1FA00 I H5FA00 V H5FA20 V N5FA00 V Testing method In order to examine the effect of carbonation on the diffusivity of chloride ion in concrete, the specimens have been cyclically immersed during 1 week in a 5% NaCl solution maintained at 25 C 1192

3 and subjected to accelerated carbonation during 1 week under concentration of 10% of CO 2, temperature of 40 C and relative humidity of 60%, before being tested according to the methods prescribed by ASTM C and ASTM C to analyze the penetrated quantities of acidsoluble and water-soluble chloride ions according to the immersion duration (5, 15, 30, 56 weeks) and the penetration depth from the surface. 7,8) In addition, for comparison with the above mentioned experimental results, the analysis of the penetrated quantity has also been performed following the same testing methods for a specimen cyclically immersed during 1 week in a 5% NaCl solution and exposed to the air during 1 week. For the latter, the analysis was conducted at 4, 12, 28 and 52 weeks after the start of immersion. The periods at which both analyses were carried out are thus slightly different. The test determining the effect of carbonation on the diffusion of chloride ion was executed after 28 days of water curing for all the specimens. The standard specimens are cylindrical specimens with length of 10cm and diameter of 10cm and coated with impermeable epoxy on their surface to secure one-directional penetration of the chloride ions. 3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1 Compressive strength tests Table 3 arranges the experimental compressive strengths by age for each test variable. The design strength (H series: 38MPa, N series: 28MPa) was satisfied for all the mix designs. Table 3. Test results of compressive strength Series Compressive strength (MPa) 28 days 90 days H1FA H5FA H5FA N5FA Penetrating amount of chloride ion by depth Figure 3 plots the degree of penetration of water-soluble chloride ion according to time for the specimens that have been cyclically subjected to immersion during 1 week in a 5% NaCl solution and accelerated carbonation during 1 week under concentration of 10% of CO 2. In this case, the penetrating amount of chloride ion is seen to increase at the whole with longer immersion duration. Similar penetration trends appeared for both acid-soluble and water-soluble chloride ions. According to Figures 3, the largest amount of chloride ion penetrating in concrete and the corresponding variations according to time and depth were observed for mix design H1FA00. Comparatively, mix design H5FA00 is showing increase of the chloride content for depth exceeding 2cm according to time. The chloride content according to time for mix design N5FA00 appears to be significantly larger than mix designs H1FA00 and H5FA00. Such trend can be explained by relatively large difference in the structure of concrete due to large difference in the design strength despite of comparatively similar water-cement ratio. On the other hand, mix designs H1FA20 and H5FA20 mixed with fly ash are exhibiting very different chloride ion penetration patterns compared to the mix designs without fly ash. Similarly to N5FA00, the chloride content according to time appears to be larger at depth of 1 to 2cm from the surface than that within depth of 1cm. The chloride content at depth exceeding 2cm is seen to be significantly smaller compared to the specimens without fly ash. This reveals the differences in the characteristics according to the structure and carbonation of concrete in the specimens mixed with fly ash. 1193

4 (a) Type I, w/c 38%, fly ash 0% (c) Type V, w/c 38%, fly ash 20% (b) Type V, w/c 38%, fly ash 0% (d) Type V, w/c 42%, fly ash 0% Figure 3. Chloride profiles for various mixtures under cyclic one-week in chloride solution and oneweek in carbonation(case of water soluble) Figures 4 compare the chloride contents by penetration depth for the specimens subjected to wetdry cycling, the specimens subjected to cyclic immersion in NaCl solution during 1 week and the specimens subjected to cyclic accelerated carbonation during 1 week. Figures 4 reveal that the chloride content in concrete according to time is larger for the specimens subjected to accelerated carbonation than the ones that have received simple wet-dry cycling. Moreover, specimen H5FA00 that has been tested during more than 30 weeks is exhibiting smooth variations of the chloride ion content by depth. In concern with depth within 1cm from the surface, H5FA00 is presenting smaller quantities of chloride ion compared to the specimen subjected to wet-dry cycling. This seems to confirm previous results which reported that, when carbonation of concrete is processing, the concentration of chloride ion increases due to the crystallization of the chloride ion in the pore solution through fixation as Friedel s salt in the cement hydrates. 1) In other words, when the concentration of free chloride ions increases in the pore solution of the carbonated area according to the progress of carbonation, the gradient of the concentration of free chloride ions increases at the interface between the carbonated and non-carbonated zones and therefore contributes to the migrationof the chloride ions toward the non-carbonated zone. This migration results finally in chloride content larger in the non-carbonated zone than in the carbonated zone. 1194

5 (a) Type V, w/c 38%, fly ash 0% (5 weeks) (c) Type V, w/c 38%, fly ash 0% (30 weeks) (b) Type V, w/c 38%, fly ash 0% (15 weeks) (d) Type V, w/c 38%, fly ash 0% (56 weeks) Figure 4. Comparison of chloride profiles under cyclic one-week in chloride solution and one-week in carbonation with chloride profiles under cyclic one-week in chloride solution and one-week in dry condition (case of acid soluble) 4. CONCLUSION This study performed immersion test in a 5% NaCl solution considering the type of cement, the eventual admixing of fly ash and the water-cement ratio as major test variables in order to examine the characteristics of chloride ion penetration in NPP concrete structures. Especially, tests were performed for the case where carbonation is exerting composite action so as to secure basic experimental data concerning composite deterioration due to the effects of these variables. The main results derived from this study are summarized hereafter. 1) The penetration of chloride ion in concrete is accelerated when carbonation is exerting composite action. This can be explained by the growing diffusivity of chloride ion provoked by increased concentration gradient of chloride ion due to the crystallization of Friedel s salt in the carbonated zone. In such case, the concentration of chloride ion in the area within 1cm from the surface tends to be lower than the noncarbonated specimen under identical conditions. 2) The admixing of fly ash is extremely efficient in preventing the penetration of chloride ion under single condition. However, when carbonation is exerting composite action, the concentration of chloride ion tends to be extremely high until a definite depth, but the concentration of chloride ion decreases suddenly beyond a certain limit depth, which minimizes its effect on the corrosion of reinforcement according to the admixing of fly ash even under combined action of the carbonation. 5. ANKNOWLEDGMENT This research was supported by a grant(06-cit-a02: Standardization Research for Construction Materials) from Construction Infrastructure Technology Program funded by Ministry of Construction & Transportation of Korean government. 1195

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