Once ice forms in highly saturated concrete material, internal tensile stress will be generated and causes damage to the material, which is a serious problem for concrete structures in cold and wet regions. On one hand, each component (porous body, ice and liquid) should satisfy the compatibility of stress and strain, which has been discussed by the poromechanical theories. On the other hand, if some empty voids exist, the hydraulic pressure will release when liquid water escapes from the expanded area according to Darcy's law. Recent closed freeze-thaw tests on the saturated mortar showed a consistent tendency: as the number of freeze-thaw cycles (FTC) increases, the deformation changes from the expansion to the contraction. In order to make clear the physical and mechanical changes during this process, a more comprehensive hydraulic model is developed, which combines both the mechanisms mentioned above. The estimated strain behavior by this model is in a good agreement with experimental measurements, and also, it has good potential and is more flexible to be applied to different cases such as different saturation degrees and cooling rates. The permeability change can be also considered in this model as a reflection of frost damage level.
This paper presents the experimental methods and findings in obtaining the strain behavior of mortar during freezing and thawing cycles (FTC) at the meso-scale under fully saturated condition and the coefficient of thermal expansion (CTE) and elastic modulus of mortar after FTC tests. A heat-cool cycle test comparing an oven-dried and undried mortar was also performed prior to the FTC test, confirming that oven drying at 105°C does not affect the CTE of samples, thereby ensuring the reliability of the results. During FTC with constant moisture content, a limitation in the increase in tensile strain was observed and this tensile strain decreased until contraction was observed. The contraction is attributed to the removal of gel pore water arising from negative pressures. Due primarily to the absence of available water supply, the displaced pore water cannot be refilled, which results in contraction at the end of the FTC. More importantly, the results show that after FTC, the CTE of frost damaged mortar increases while its elastic modulus decreases, primarily owing to microcracking when frost damage sets in. Microcracks act as broken bridges that can detach the aggregate from the hardened cement paste and in effect reduces the thermal restraints that each part (fine aggregate and cement paste) exerts on the other. The hardened cement paste can then expand/contract more freely under temperature variation, and thus can significantly affect (increase) the CTE of the whole composite (mortar). Further, stress transfer in the material is prevented due to microcracking resulting in elastic modulus reduction.
Frost damage mechanism under freezing and thawing cycles is an important issue for service life evaluation of concrete structures in cold regions. In order to simulate the frost damage mechanism, this paper presents a simulation method in meso-scale for coupled mechanical and transfer analysis in which Rigid Body Spring Model (RBSM) is applied. This method can simulate the coupled heat and moisture transfer in mortar, and also the ice formation process based on thermodynamic equilibrium. In addition, a degradation constitutive model is proposed to describe the deformation behavior under several Freezing and Thawing Cycles (FTCs). To evaluate the effectiveness of this method, the simulation results are compared with experimental data of the strain behavior under FTCs and found in satisfactory agreement with the experimental data.
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