The corrosion morphology and the mechanical behavior of corroded prestressing strands are investigated in the present study. Nineteen corroded strands are obtained through controlling stress level, corrosion time and chloride ion concentration under artificial climate conditions. A total of 119 corrosion pits are counted to investigate the geometric morphology of corrosion pits. A depth-width ratio parameter is first defined to describe the distribution law of corrosion pits, which obeys the lognormal distribution. A tension test is conducted to investigate the mechanical behavior of corroded strands. The fracture morphology and micro-cracks of corroded steel wires are observed by the scanning electron microscopy. A constitutive model is proposed to predict the stress-strain curve of corroded prestressing strands and verified by the experimental results. Results show that the maximum corrosion loss of strand increases by 4.91% when the stress level changes from 45% to 75% of strand yield strength. The high stress level can promote the propagation of corrosion micro-cracks on strand surface. The propagation of corrosion micro-cracks has a significant effect on ultimate strength of strand, while has a little effect on elastic modulus. The proposed model can give an accurate prediction for the stress-strain of corroded prestressing strand.
Mechanical properties of calcium silicate hydrates (C-S-H) with C/S ratios of 0.75, 1, and 1.33 were examined with nanoindentation after gamma-adsorbed doses of 0.145, 0.280, 0.500, and 0.784 MGy, and were compared with control samples. Young' s modulus and stress relaxation tests showed no apparent trend with irradiation dose. Qualitatively, most of the irradiated samples were found to relax more than their respective controls, but not always in a statistically significant manner. Most of the Young' s modulus irradiated–control pairs showed marginally higher stiffness in the irradiated samples, but overall trends with irradiation dose were not obvious. Creep compliance was obtained for the samples irradiated at the highest dose and their respective controls. Two of the three irradiated samples exhibited less creep than their respective controls, but only one of which was statistically significant. The lack of clear changes in mechanical properties for these samples correlates with separate chemical analyses that showed no loss of interlayer water by exposure to irradiation or changes in the mean silicate chain length. Further research evaluating higher doses (25 and 200 MGy) representative of those received by concrete structures in nuclear power plants at prolonged operation is being carried out to complement the present study.
In this study, a method was developed to estimate accurately the chloride threshold value for corrosion initiation in reinforced concrete. In this method, chlorides are supplied through a part of the specimen surface, and half-cell potential between the rebar and reference electrode embedded in a concrete specimen was measured in every ten minutes. A sudden drop of half-cell potential was observed as the continuation of the measurement. At that time, the specimen was broken and corrosion of rebar on its narrow surface was confirmed. The chloride threshold value can be estimated by determining the chloride concentration at the time of the potential drop. The estimated chloride threshold values for normal Portland cement concrete with cement content between 254 and 446 kg/m3 are in the range of 1.6 to 3.6 kg/m3. This paper is an English translation from the authors’ previous work [Horiguchi, K., Yamaguchi, T., Maruya, T. and Takewaka, K., (2015). “A study on the method of measuring the chloride threshold value of corrosion and on the estimation of the values.” Journal of JSCE, Ser. E2, 71(2), 107-123. (in Japanese)].
During sorption, the microstructural evolutions of two different cement pastes (with water-to-cement ratios of 0.40 and 0.55) are studied via proton-nuclear-magnetic-resonance relaxometry. The water uptake test is performed for samples dried at 105°C under three different temperatures of 20°C, 40°C, and 60°C for the first twenty-six days of sorption. It is observed that the water went first to the larger pores before migrating to the finest ones. This behavior is accelerated with increasing temperature. The rate of water exchange between fine and large pores is estimated and found to increase with temperature for both studied mixtures. The activation energy corresponding to this water movement is calculated and found to be higher for the lowest water-to-cement ratio, owing its finer microstructure. Finally, the activation energy related to the local water transport in re-distribution from large pores to fine pores is calculated and found to be inferior to the experimental results, which can be explained by the dynamic microstructure not being considered in the classical theories.
This paper introduces a novel and simple model for estimation of the shear contribution of the fiber-reinforced polymer (FRP) strengthening system in the FRP-strengthened beams. The model utilizes the bonding-based approach, which considers the shear resisting mechanism of FRP-strengthened beam via the bond behavior between FRP strengthening system and concrete. Herein, the beams strengthened in shear with near-surface mounting (NSM) rods or laminates and embedded through-section (ETS) bars are examined. By utilizing only mechanical consideration, the shear resistances of the NSM-strengthening or ETS-strengthening laminates or bars in the beams are simply derived when several bond factors (i.e. maximum bond stress and slip at peak bond stress) are known without using any empirical coefficients. The reliability of the proposed model is first validated against the test results available in the open literature. The extensive investigation to complement the model validation is then carried out through comparison of the results produced by the experiments and the proposed approach as well as the existing methods. The analyses demonstrate that the bonding-based approach is greatly effective to predict the shear contribution of the FRP strengthening system in the beam. Two examples for calculation of the shear resisting forces of the ETS-FRP and NSM-FRP bars in the FRP-strengthened beams are provided to depict the use of the model.