Numerical simulation of freeze-thaw behavior and fracture behavior of cementitious systems
Deicing salts are applied to the surface of the concrete pavements to melt the ice and snow in an effort to improve the safety conditions for public travel. The solution that is produced (e.g., water-NaCl, CaCl 2, MgCl2) can be absorbed into concrete pores. This solution alters the degree of saturation (i.e., the volume ratio of fluid in the sample as compared to the total maximum volume of fluid that the sample can hold) of concrete pavement, the freezing temperature of the solution within the concrete pores, and may result the damage. First, a numerical model (one–dimensional finite difference model) is developed to describe the freeze-thaw thermal behavior of the mortar containing deicing salt solution. The model is used to predict the temperature and the heat flow for mortar specimens during cooling and heating. Phase transformations associated with the freezing/melting of water/ice or transition of the eutectic solution from liquid to solid are included in this model. The lever rule is used to calculate the quantity of solution that undergoes the phase transformation, thereby simulating the energy released/absorbed during phase transformation. During solidification, undercooling phenomenon is considered in the numerical model. Data from experiments performed using a low-temperature longitudinal guarded comparative calorimeter (LGCC) on mortar specimens is compared with results from the numerical model. Two types of experimental data are used for the comparison. First, data from mortar specimens that were fully saturated (i.e., 100 % degree of saturation) using varying concentrations of sodium chloride (NaCl) solutions (0 %, 5 %, 10 %, and 23.3 % concentrations by mass) were considered. Second, in addition to fully saturated specimens containing sodium chloride solution, partially saturated specimens (i.e., partially saturated specimens with water at degrees of saturation equal to 75 %, 85 %, 95 %, and 100 %) are also considered. Next, a numerical model (two-dimensional finite-element model) is developed to describe the mechanical behavior of cementitious systems at early-ages. This development is influenced by the temperature of the surrounding environment. The effect of aggregates is assessed on maturity-based predictions of early-age flexural strength development. This method is based on the concept that the strength (or property) development is proportional to the extent of chemical reaction (i.e., hydration) that has taken place. It is commonly assumed that the extent of chemical reaction (i.e., the degree of hydration) is a unique function of the product of time and temperature. It is the hypothesis of this work that aggregates can alter the relationship between the maturity and the flexural (or tensile) strength. To verify this hypothesis the effective age (i.e., the degree of hydration) derived from Arrhenius function was related to the flexural strength development of paste, mortar, and concrete specimens. A linear response was detected between the flexural strength respect to the extent of the hydration for the paste specimens, while a bi-linear response respect to the extent of the hydration was observed for both the mortar and concrete specimens. The intersection point (knee point) of this bi-linear response corresponds to the critical time at which the major fraction of the aggregates begins to fail. At the very early ages (i.e., less than the critical time), the flexural behavior of the concrete is dominated by the paste or the bond failure. At the later ages, the flexural strength is governed by aggregate failure. Consequently, a thermo-mechanical model will be employed to simulate the freeze-thaw damage in cementitious systems containing salt solutions. Therefore, sufficient knowledge is acquired to better understand the damage mechanism occurring due to the phase change of salt solution absorbed in to the pores of cementitious systems.
Zavattieri, Purdue University.
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