コンクリート中の鉄筋の腐食に及ぼすひびわれ幅の影響

概要

論文の詳細を見る
1. Introduction Almost every code of concrete structural design relates permissible crack width to exposure conditions, indicating the general agreement about the influence of crack on corrosion. The relationship between crack width and corrosion, however, has not been clarified. The locations of steel corrosion normally coincided with tips of flexural cracks revealing the indisputable effect of crack on corrosion initiation. It also appeared that steel was not corroded at every crack. In addition, the effect of crack on corrosion propagation is remarkably perplexed. With the same surface crack width, the deterioration of steel corrosion can range from zero to high degree of corrosion. The study aims to clarify the mechanism of corrosion induced by crack and the relationship between crack width and corrosion by using an electrochemical technique. 2. Experimental Procedures The Specimens and loading arrangement are shown in Fig. 1. The designation and steel stress of the specimens are tabulated in Table 1. The steel reinforcement was one D 13 millscale bar. The mix proportion and strength properties of concrete are shown in Table 2. Two levels of flexural crack width were introduced and sustained under tensile steel stresses of 2000 and 3000 kg/cm^2. Steel corrosion was accelerated by subjecting the specimens to repeated cycles of one day wetting in 65℃, 3.1 % NaCl solution and one day drying under room environment. Exposure periods are also shown in Table 1. During the exposure test, Half cell potential (E_c) and Polarization resistance (R_p) of steel were periodically measured against a Ag/AgCl reference electrode and a platinum counter electrode. The specimens were split at the end of their exposure periods to observe corrosion situations such as corrosion form, corroded length and rusted area. 3. Results and Discussion 3.1 Locations of Crack and Rust Fig. 2 shows the locations of crack and rust. All rusted areas were associated with crack tips as they were reported. The aggressive substances essential for corrosion processes, such as Cl^-, H_2O, O_2 etc., can undoubtly penetrate through crack easier than through sound cover. However, it was also found that, steel did not corrode at every crack as presented in the literature 3.2 The electrochemical characteristics Fig. 3 shows the typical distribution of E_c and R_p, measured at various exposure ages, along four locations of four specimens. After a period of exposure, in each specimen, one of the four locations remarkably showed the higher reduction in Ec and Rp values comparing to other three locations. Corrosion was certainly found under these spots (see specimens S (3, 2) and L (1, 1)). Fig. 4 shows the typical E_c and R_p time curves. The Ec negative shift revealed the depassivating period after the amount of absorbed chloride ions had exceeded the critical value of C1^-/OH^<-20>). During the depassivating, although all points of the specimens altogether shifted to more negative potentials, only R_p of the corroding spots declined continuously. R_p of the uncorroded spots slightly increased with time. For specimens with more than one corroded spot, the corroding order of the corroded spots can be determined by their R_p characteristics (see specimens S (4, 1) and L (4, 1)). 3.3 The electrochemical characteristics and corrosion situations The comparison of average electrochemical values of corroding and uncorroded spots, measured at various locations, is shown in Fig. 5. Both corroding and uncorroded spots had the same trend of E_c with time. The actual E_c of the uncorroded spots might be defected by the presence of E_c of the corroding spots. In contrast with E_c time curve, the corroding and uncorroded spots had clearly different trend of R_p with time. The R_p of the corroding spots continuously shifted to more negative values, whilst the R_p of the uncorroded spots tended to increase gradually. The active and passive states of corrosion can be undoubtly distinguished by Rptime curve, regardless of the measuring locations. Fig. 2 also shows the locations of longitudinal crack and their first appearing time. The locations showing lower R_p were cracked prior to the locations showing higher R_p (see also Fig. 4). The result is agreeable to the theory that Rp reversely relates to corrosion rate and thus the degree of corrosion. 3.4. The Mechanism of Crack Induced Corrosion The first corroded spot of each specimen all coincided with one of the cracks in that specimen. These cracks were termed 'major cracks' while the other were refered as 'minor cracks'. Fig. 6 reveals the major and minor cracks of each specimen. There is a tendency that the widest crack became the major crack of that specimen. Fig. 7, plotted from the data presented by Makita et al, also shows the same trend. Fig. 8 shows the comparison of average E_c and R_p time curves of major and minor cracks of rank C. After depassivation, R_p of the minor crack was all time higher than that of the major crack. The comparison of average E_c and R_p of steel at major cracks, minor cracks and uncracked covers is shown in Fig. 9. For major cracks, E_c and R_p of the wider cracks started to decline prior to those of the narrower cracks. The Rp of the minor cracks took longer time to start declining and fell down disorderly comparing with that of the major cracks of the same ranks. Metal dissolution of steel at the major cracks resulted in the establishment of macrocell corrosion in those specimens. The macrocell current delayed the corrosion initiation of steel at cathodic sites, which included the minor cracks. Steel at some minor cracks was corroded because Cl^- had accumulated enough to overcome the protection. However, the high Cl^- concentration, O_2 depletion and pH drop around steel at major cracks still facilitated iron dissolution at the cracks. 3.5 The effect of crack width Fig. 10 shows the relationship between crack width and rusted area. For major cracks, there are clear tendencies that the rusted area increases as the crack width and the exposure period increase. The average values of electrochemical characteristics of major cracks, those were wider and narrower than 0.10 mm, were compared in Fig. 11. Both Ec and Rp of steel at the wider crack started to decrease earlier than those of steel at the narrower crack. In the latter time, there was no difference in values of E_c and R_p of steel at both levels of crack width. These reveal that crack width has an effect on corrosion at the beginning state. 4. Conclusions 1) The characteristics of half cell potential (E_c) of corroding and uncorroded spots were similar to each other. The polarization resistance (R_p) of the corroding spots tended to decrease with time while that of the uncorroded spots increased gradually (see Figs. 4 and 5). In addition, the locations showing lower R_p were longitudinally cracked, by corrosion products, earlier (see Figs. 2 and 4). The R_p characteristics can be used to predict corrosion situations. 2) Steel at one of the cracks, major crack, in each specimen was corroded prior to the other locations resulting in the establishment of macrocell corrosion (see Fig. 3). Corrosion at a major crack delayed and paralyzed corrosion at the other cracks or minor cracks of that specimen (see Figs. 2, 8,9 and 10). There is a clear tendency that the widest crack of each specimen became the major crack of that specimen (see Figs. 6 and 7). 3) At the beginning of the accelerated test, the corrosion rate of steel at a wider major crack was higher than steel at a narrower major crack. At the latter period, there was no difference between corrosion rate of steel at the two levels of crack width (see Fig. 11).
社団法人日本建築学会の論文
1989-03-30

コンクリート中の鉄筋の腐食に及ぼすひびわれ幅の影響

スポンサーリンク

概要

著者

関連論文

スポンサーリンク