| Durability is the key to keep the concrete structure in normal service,and in cold regions,freeze-thaw cycle damage seriously affects the durability of concrete structures.At present,the research on concrete freeze-thaw cycle is mainly based on experimental method,and the finite element model of freeze-thaw cycle based on pore structure degradation has not been realized;meanwhile,the capillary ice volume is an important parameter in the simulation of freeze-thaw cycle,but the classical ice volume equation has not been verified experimentally;the existing concrete freeze-thaw cycle model only realizes the simulation of single freeze-thaw of concrete,and does not consider the damage of concrete by multiple freeze-thaw cycles.In this paper,the pore structure deterioration process of mortar was examined in situ by using low-field NMR technology,and the combination of low-temperature bin and low-field NMR was used to modify the classical capillary icing volume equation,combine the random aggregate model with the multi-physics field coupling equation,establish a finite element model of concrete freezethaw cycle close to the real one,and evaluate the frost resistance of concrete and its influencing factors.The essence of freeze-thaw cycle damage of concrete is the process of freeze-thaw damage to its internal mortar.The rapid freeze-thaw damage process of mortar specimens with three water-cement ratios and four air-entraining amounts was studied by low-field nuclear magnetic resonance porosimetry.The macroscopic freeze-thaw damage coefficient of mortar was established by combining the relative dynamic elastic modulus and mass loss using the reconciliation function,which solved the problem that the mass loss and relative dynamic elastic modulus loss were not synchronized.The in situ inspection of mortar pore structure during freeze-thawing by low-field NMR reveals the deterioration process of pore structure during freeze-thawing,and the freeze-thawing deterioration coefficient of pore structure is established according to the change rate of capillary pores of each grade,and the evaluation model of the remaining freeze-thawing cycles of mortar is established by combining the macroscopic freeze-thawing damage coefficient and the pore structure deterioration coefficient.The change process of the ice volume in capillary pores with temperature was monitored in situ by low-temperature low-field NMR technique,and it was found that the calculated results of the classical ice volume equation within 0～-8℃ were close to the measured values,however,when the temperature was lower than-8℃,the theoretical ice volume was larger than the measured ice volume,and the difference between the theoretical and measured values gradually expanded as the temperature decreased,which was due to the morphology of cement hydration products The reason is that the morphology of cement hydration products varies,which affects the smoothness of particularly small capillary pore walls,and the classical ice volume equation does not consider the hysteresis characteristics of ice volume during freezing and melting.Based on the results of the low-field NMR test,the pore structure was classified according to the pore size class,and the variation of the icing rate of each class of capillary pores with temperature was analyzed.Based on the dependence of the solution and the freeze-thaw hysteresis characteristics,more accurate capillary pore icing equations were established for pure water,salt solutions with 4% and 8% concentration during freezing and thawing,respectively.Based on the coupled equations of random aggregate generation and water-thermalforce multiphysics,a concrete freeze-thaw finite element model containing aggregate,interfacial transition zone,mortar and introduced air bubbles is established to realize the simulation of hydrostatic pressure field,temperature field and stress field during the freeze-thaw process of concrete.The blocking effect of the coarse aggregate on the water transport in the mortar during freeze-thaw is analyzed by the seepage field,which leads to the rapid increase of water saturation in the interface transition zone near the aggregate.By varying the effective air-entrainment in the concrete freeze-thaw model,it was analyzed that the introduction of an appropriate amount of air bubbles has a dual protective effect on the concrete during freezing and thawing.The introduction of air bubbles can release the hydrostatic pressure and store water during freezing,and can replenish water to the cement matrix and reduce the negative pressure inside the mortar matrix during thawing.By varying the freeze-thaw rate of the concrete freeze-thaw model,it was obtained that reducing the freeze-thaw cycle rate could reduce the intensity of damage to concrete by freeze-thaw cycles.By varying the pore structure,it was calculated that decreasing the average radius of capillary pores can improve the resistance of concrete to freeze-thaw cycle damage when the total porosity is certain.By comparing the concrete freeze-thaw model established using the classical icing volume equation with the measured icing volume,it was confirmed that using the classical icing volume equation would overestimate the damage to concrete by freeze-thaw cycles.The model predicts the critical number of freeze-thaw cycles and the measured values are compared,and the error is within the acceptable range,which proves that the model established in this paper can not only calculate the stress and hydrostatic pressure distribution in concrete during multiple freeze-thaw cycles,but also be used for the freeze-durability testing and evaluation of concrete structures.The model can be used to calculate the stress and hydrostatic pressure distribution inside the concrete during multiple freeze-thaw cycles,and it can also be used to test and evaluate the frost durability of concrete structures,and the test method used in this study provides a reference for the application of low-field nuclear magnetic resonance in cement-based materials. |
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