Freeze-thaw Deterioration And Prevention Of Bolted Parts Of Assembled Country Houses In Highland Areas

The bolted connection form of prefabricated buildings has gained popularity in rural areas with high elevation due to its advantages of clear force transmission and fast construction.However,when constructing prefabricated buildings in cold regions,the potential damage caused by freeze-thaw cycles to the concrete and bolted connection parts should be considered.Unfortunately,the current research has not yet explored the deterioration characteristics of bolted connection nodes after freeze-thaw cycles,nor has it investigated the mechanism of freeze-thaw damage in concrete containing bolt hole forms.To clarify the factors that influence the freeze-thaw deterioration of bolted nodes,this study focuses on single-bolt connection nodes and concrete with holes.The impact of freeze-thaw cycles on the performance of concrete with holes and bolted connections is investigated by comprehensively considering the entire freeze-thaw process and bolt tension changes in the specimens.Additionally,SEM and fiber optic sensing techniques are employed to propose an effective method to mitigate the freeze-thaw deterioration of bolted connections.The study consists of the following main contents:(1)The damage development characteristics of concrete with holes subjected to freeze-thaw cycles are systematically studied.The process of water freezing in the holes and the theoretical freeze swelling force are analyzed.The mechanism of freeze swelling cracking of concrete with holes is discussed based on the test results.Finally,prevention and control measures are initially proposed based on the mitigation of freeze swelling force in the holes.The results reveal that the process of water-ice phase change volume expansion is a significant factor affecting the cracking of perforated concrete.Sufficient water and the "ice plug effect" at the end of the hole provide the necessary conditions for the generation of freezing and swelling forces.Although traditional polyurethane control material can extend the number of freeze-thaw cycles to about 150,its durability leads to late concrete cracking.(2)A rapid freeze-thaw test using fiber optic sensing technology is conducted to track and monitor the bolt connection parts.The change in bolt tensioning force with freeze-thaw cycles is studied from the perspectives of bolt preload and concrete material.The test results show that during the freeze-thaw cycle,the tension force is significantly correlated with temperature,and concrete cracking is the main factor leading to the loss of tension force of the specimen.The loss of tension force not only increases with the decrease of the initial preload force but also correlates with the concrete properties.Based on the test results,the risk of specimen cracking is further analyzed by combining with the theoretical frost expansion calculation model of the specimen.It is pointed out that reducing the hole distance,improving the tensile strength of concrete,and reducing the frost expansion in the hole can effectively reduce the risk of freeze-thaw deterioration of bolted nodes.(3)Based on the prevention and control concept of reducing the freezing and swelling force in the hole,the study proposes the prevention and control measure of arranging silicone rubber sleeves outside the bolt.The effectiveness of the prevention and control measures is compared and analyzed from the perspectives of the number of freeze-thaw cycles,the preload of the bolt,and the elastic modulus of the material.The test results show that the freeze-thaw damage of concrete is the main factor leading to bolt tension loss under the prevention and control measures.Plain specimens show freeze-thaw cracking after 150 freeze-thaw cycles,while highland specimens do not even crack.The selection of materials with small modulus of elasticity and good durability can effectively prolong the service life of bolt connection nodes.