CRACKING BEHAVIORS CAUSED BY CORRODING BARS AND RISK ASSESSMENT OF CONCRETE PEELING-OFF

Abstract

The issue on an aged deterioration to a reinforced concrete (RC) building is revealing in recent years. Especially, a falling cover concrete is an urgent subject since it may cause harm to the surrounding people. Number of experimental studies on an aged deterioration have been performed to relate the corrosion cracking appearance on the concrete surface with the corrosion ratio of steel bars. It seems that corrosion cracking propagation also induces cover concrete to peel off. However, there is almost no study on the risk of exfoliation of a cover concrete. Accelerated corrosion tests were conducted to estimate the splitting tensile strength before and after corrosion cracking appearance on the concrete surface and to assess the risk of falling cover concrete. The specimens are concrete cylinders having 150 mm in diameter and 300 mm in length, and a single reinforcement of D16 was installed right in the center of the cylinder. All surfaces except for exposed opposite faces of 50 mm in width were coated by epoxy resin to avoid penetration of NaCl solution. The variable factor is accumulative current density (0, 30, 100, 200 and 400 mA·h/cm2). The specimens were cured for 28 days before accelerated corrosion test. During the accelerated corrosion test, the specimens were placed in the tank containing 3 % of NaCl solution. A direct-constant 10 voltage was given and the current was monitored and recorded using a data logger in 60 minutes increments. The cracks on the concrete surface were visually observed and the crack width at certain locations was frequently measured using a digital microscope having a resolution of 0.01 mm. After accelerated corrosion tests, the corroded steel bars were removed from the surrounding concrete by performing a split tensile test to evaluate a splitting tensile strength on the corrosion cracking surface. A removed bar was soaked into 10% diammonium hydrogen citrate solution. After chemically and mechanically cleaning, the weight loss of corroded rebars was measured. Since a corrosion rate before cracking was slower and increased after cracking, a corrosion ratios was estimated from the accumulative current density and using a bi-linear interpolation having a break point at cracking. The circular cross section was smaller than the rectangular cross section, and the pores to accumulate a corrosion product decreased, as a result, the corrosion crack width of a circular specimen became larger than that of a rectangular specimen. Split tensile test results indicated that the splitting tensile strength before cracking (up to corrosion ratio of 1.7%) nearly didn't deteriorate due to an occasional stain of corrosion product on the concrete surface, however, it gradually decreased after outflowing of corrosion product from a connected crack. It was reduced by 26% and 37% when the corrosion ratio increased to 4% and 10% respectively. A simple model was proposed to assess a retention factor of splitting tensile strength with increasing corrosion. Furthermore, a finite element analysis was conducted on the split tensile specimen to investigate a fracture energy of a cracked surface by corrosion because the corrosion crack did not progress uniformly but was unevenly distributed to the splitting surface. Analytical results showed that the mechanical behavior of the splitting tension test was overwhelmingly dominated by the tensile softening properties of the splitting surface and not only the splitting tensile strength but the crack width should be reduced similarly to obtain a result to be consistent with an experimental strength. Further study is required to improve a prediction of falling concrete from a width of corrosion cracking on the concrete surface due to the sensitivity to tensile softening properties and corrosion level.