Evaluation Method Of Freezing-thawing Deterioration Of Brick Wall Based On Freezing Point Depression Theory

Bricks are the main components of traditional buildings in China,and brick masonry,especially brick walls,are important load-bearing or enclosure components in ancient buildings and antique buildings.Freeze-thaw damage is one of the main threats to traditional brick buildings under influence of natural environment.Parts with high water content,such as wall bases and near drains,are more susceptible to frost damage.Freeze-thaw cycles can cause deterioration such as spalling,chalking and cracking on the surface of brick walls,and more seriously can cause stress concentration and imbalance in the wall base,even leading to structural safety problems such as crooked flashing and collapse of the wall.The purpose of this research is to clarify the differences in freeze-thaw cycles and ice content of brick walls under different meteorological environmental conditions,and to establish a quantitative correlation between temperature,water content,number of freeze-thaw cycles and freeze-thaw deterioration.This research firstly compares the current status of research on the mechanism,prediction and evaluation methods of freeze-thaw deterioration of porous building materials through literature research.Secondly,based on the theories related to the coupled heat and moisture transfer of porous building materials and the freezing point depression in porous media,a water and heat migration model for brick walls considering the freeze-thaw process is established;the model is used to explore the heat and moisture properties of bricks such as thermal conductivity,isothermal moisture absorption curve and water permeability coefficient,as well as the effect of the initial moisture content of bricks on the freeze-thaw process.Thirdly,freeze-thaw cycle experiments are conducted to investigate the effect of saturation and other factors on the freeze-thaw damage of bricks.Finally,the distribution and fluctuation of temperature and ice content of brick walls under different meteorological conditions are simulated and predicted and freeze-thaw deterioration is assessed.Based on the above study,the main conclusions of this paper are as follows:(1)There are obvious differences between the gas-liquid two-phase heat and moisture coupled transfer model and the gas-liquid-solid three-phase heat and moisture coupled transfer model that considers the freeze-thaw process for predicting the temperature and moisture distribution patterns of brick walls.The three-phase heat and moisture coupled transfer model can specifically describe the freeze-thaw process of moisture with temperature change.The number of freeze-thaw cycles using the occurrence and melting of ice content to judge the wall will generally be more than the number of freeze-thaw cycles obtained by the cross-count method with 0°C as the base.In the same location,the number of freeze-thaw cycles obtained by the two criteria can differ by a factor of up to three.(2)The heat and moisture properties of brick significantly affect the distribution of moisture,temperature and ice content within the brick wall.Under the same environment and through the same time,brick walls with higher water retention capacity freeze to a lesser extent,have lower icing content,and have a relatively lower risk of freeze-thaw damage.Brick walls need to meet temperature and moisture conditions for freezing to occur.When the water content of a brick wall is reduced to a certain level,the wall will not freeze even if the internal temperature of the brick wall is below 0°C.(3)The experimental results of freeze-thaw damage of bricks show that the degree of freeze-thaw damage of blue bricks is obviously related to the moisture content and pore structure of the specimens.The saturated brick specimens undergo 5～10 cycles at-10℃～20℃ to show damage,and the decrease of relative ultrasonic pulse time mainly occurs in the first 5 freeze-thaw cycles;while when the saturation of brick specimens decreases to 80%,more than 40 cycles are needed to show obvious freeze-thaw damage.In the case of similar porosity,the more the percentage of large pore size pores,the more likely the brick specimens are to show cosmetic damage and the greater the decrease in relative ultrasonic pulse time.(4)The simulation results show that under the meteorological conditions in Nanjing,for bricks with moisture saturation of 88% or even more(pore size mainly distributed in 0.1～100 μm),they will experience about 19 freeze-thaw cycles per year at maximum,and the degree of freeze-thaw deterioration(relative dynamic elastic modulus decrease)reaches 90% after 3 years.(5)The fluctuation patterns of temperature,moisture and ice content of brick walls with different orientations are affected by the amount of solar radiation,with the western wall showing the most drastic changes in icing volume during the day,while the northern wall has low temperature and high icing volume,but low ice volume fluctuations.The fluctuation of ice volume implies the frequent occurrence of freezethaw process.Therefore,the west wall may suffer the most severe freeze-thaw degradation,while the north wall is less severe.(6)With the same initial moisture content of the brick wall,the lower the temperature on the outside of the room,the greater the extent of brick wall icing,and the location of the peak icing volume will be closer to the indoor side.The amount of ice fluctuates in an "n" shape during the day,and the number of freezethaw cycles can be judged according to the change in the amount of ice.The higher relative humidity of the air on the interior side will provide moisture for the freezing process of the brick wall,while the insulation on the interior side will block the indoor air from heating the wall,making the overall temperature of the brick wall lower and accelerating the freezing of the brick wall.Therefore,to reduce the occurrence of freezing and thawing of brick walls,the interior side should be kept dry and hot,and water barrier materials can be installed.Clarifying the relationship between the surrounding environment,the heat and humidity properties of bricks and the freeze-thaw deterioration of bricks is conducive to the effective prediction and scientific evaluation of the risk of freeze-thaw deterioration of brick walls,and to promote the preventive conservation of traditional architectural heritage.In this research,a heat and moisture transfer model for brick walls considering the freeze-thaw cycle process is implemented to predict the internal temperature,moisture content and ice content of the walls,providing a scientific basis for fully understanding the freeze-thaw process and the risk of frost damage deterioration of brick masonry of traditional buildings in different regions of China.The combination of experiments and simulations to achieve the assessment of freeze-thaw process and frost deterioration risk of brick walls has good application value.Words: more than 58,000;Figures: 81,Table: 13.