| 抄録 |
Concrete is fragile to freeze-thaw cycles (FTCs) in cold and wet regions, especially with the presence of de-icing agents (eg. sodium chloride). While surface scaling spoils the appearance, more importantly, the internal frost damage threats to the safety of concrete structures. Although it is well known that the sodium chloride (NaCl) can accelerate the frost damage, the internal damage mechanisms due to the combined effects of FTCs and NaCl attack still have not reached an agreement. On one hand, chloride ions may have reactions with hydrated products to form new salts (eg. Friedel’s salt). The salts fill the mortar voids and the degree of frost damage is reduced because of lesser water content and porosity. On the other hand, the expansion due to salt crystallization induces internal pressures, subsequently, resulting in micro cracks of mortar. The calcium leaching could be accelerated due to the existing of NaCl, which deteriorates the mechanical performance and increases the porosity. Besides, with the salt presence, the freezing point of concrete is decreased, the ice content reduction eases the pressures in pores, but the sudden ice formation may be enough to cause serious damage. Therefore, the amounts of ice and crystallized salt with freezing temperatures are essential to understanding the salt frost damage mechanism. If the phase changes are known, it is possible to model the combined effects. In order to predict and evaluate the mechanical performance of concrete suffered from the combined effects, this study conducted the following steps: (1) estimation of ice content in mortar saturated with deionized (DI) water during FTC based on electrical measurements, which includes the combined effects of moisture and temperature to the electrical properties of mortar; (2) estimation of ice amount in mortar saturated with NaCl solutions during FTCs based on electrical measurements, which includes the combined effects of moisture, temperature, microstructure, pore solution concentration to the electrical properties of mortar. (3) moisture transport property change due to NaCl attack; (4) mechanical property and chemical components change before and after FTCs; (5) mechanical property prediction model for the combined effects. Since NaCl concentrations are not uniform along with the depth, the pressures in pores could be very different. In order to understand the role of NaCl in frost damage quantitatively, the solution concentration in pores is better to be uniform. Therefore, the mortar specimen in meso-scale was deployed in this study. In this case, the mechanical behavior of each layer could be obtained, which could be applied for a specific position of bulk specimen. Electrical measurement is a useful technique, which was widely used to study the durability problems of cementitious materials. Because electrical conductivity of mortar is determined by the moisture content, temperature, microstructural characteristics and pore solution compositions, they were coupled to estimate the ice content in mortar saturated with NaCl solutions during FTCs. Firstly, by introducing the relationship between activation energy for conductance and saturation degree, the combined effects of moisture and temperature to electrical conductivity of specimen was clarified. Sequentially, an ice content estimation model was proposed, test results of bulk cylinder specimens saturated with DI water were applied to verify the model. After that, the electrical response of meso-scale specimen saturated with NaCl solutions during FTCs was investigated. A damage index was developed to evaluate the frost damage degree based on the electrical resistivity at reference temperature and activation energy, the effect of microstructural change was clarified as well. Then the ice content estimation model was extended to the cases of NaCl solutions. Besides, since the pore solution concentration was considered in the model, the crystallized salt with FTCs can also be calculated. In addition, the results from experimentation were compared with theoretical analysis, which shows acceptable agreement. After NaCl attack, the pore characteristics will be altered due to the calcium leaching and salt crystallization. To understand the salt effect to meso-scale specimens, the absorption of NaCl solutions and reabsorption of DI water tests were conducted. As found, when exposed to low concentration NaCl solution, the diffusivity of mortar would increase due to faster calcium leaching. However, for high concentration NaCl solution, it would be reduced since pores filled by crystallized salt have dominant effect in this case. In addition, after immersed in NaCl solutions for 7 days, the meso-scale specimens were sealed and exposed to FTCs. Afterwards, three-point bending test and TG/DTA test were applied to evaluate the mechanical performance and chemical components alterations. According to the mechanical and chemical test results, before FTCs, the flexural strength and elastic modulus fluctuated with the NaCl concentration, which was mainly due to Portlandite leaching during the immersing process. After FTCs exposure, for specimens with higher water-to-cement ratio (w/c), the frost damage was severer and the damage developed gradually with increase of FTCs. For specimens saturated with deionized water and intermediate concentration solution (5%wt NaCl), the flexural strength and elastic modulus dropped to less than 60% after 30 FTCs, while there was almost no damage for specimens with w/c=0.3 (high strength and stiffness). For specimens exposed to high concentration solution (20%wt NaCl), over than 20% mechanical reduction after 30 FTCs was observed in the case of w/c=0.7. The salt can contribute to the frost damage, but the chemical results imply that the formation of Friedel’s salt is less likely to cause destructive crystallization pressure. Finally, the mechanical performance of specimen exposed to FTCs and NaCl solution was predicted based on the ice amount, which has acceptable agreement with the experimental results.
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