Study On Water Absorption Property And Thermal Insulation Performance Under Freeze-Thaw Cycle Of Tunnel Insulation Materials In Cold Region

Laying thermal insulation layer is one of the common thermal insulation measures to prevent frost damage of tunnels in cold regions.The performance of thermal insulation materials directly determines the effect of thermal insulation layer.At present,tunnel leakage water is relatively common,and it is possible that the air humidity in the tunnel is too high in rainy season.However,the study of tunnel insulation layer in cold region does not consider the influence of external moisture on insulation materials.Therefore,it is of practical significance to study the water absorption characteristics of tunnel thermal insulation materials and their thermal insulation properties under freeze-thaw cycles.In this paper,polyphenolic and polyurethane heat preservation materials commonly used in tunnels are taken as research objects.The hygroscopicity and immersion characteristics of heat preservation materials under different water absorption modes are studied through laboratory tests.On this basis,the thermal conductivity of thermal insulation materials under different water-containing conditions was predicted and analyzed by means of experiment and theory.The distribution characteristics of pore water in thermal insulation material were obtained by CT technology.The calculation model of equivalent thermal conductivity of single-side water-absorbing thermal insulation material was established,and the variation rule of equivalent thermal conductivity during freezing process was analyzed.Through the freeze-thaw cycle test and micro-scanning test,the evolution law of thermal insulation performance and the deterioration process of microporous structure of thermal insulation materials were obtained.The main work is as follows:(1)Through water absorption experiments,the hygroscopicity and immersion characteristics of thermal insulation materials are studied.The hygroscopicity time curves of thermal insulation materials under humid environment,non-pressure drip infiltration and pressure immersion are obtained.The optimum isothermal hygroscopicity model of thermal insulation materials is selected,and the actual water content of thermal insulation materials under unfavorable conditions is obtained.(2)In order to predict and analyze the thermal conductivity of thermal insulation materials under different water content conditions,the practical relationship between thermal conductivity of thermal insulation materials and water content and ice content is obtained by using quasi-steady state method and Transient plane heat source technology,and the corresponding fitting model is established.In theory,the series model and parallel model under three-phase state are combined and the theoretical calculation is carried out.In this paper,a theoretical prediction model which is more in line with engineering practice is proposed,and the purpose of prediction and analysis is achieved.(3)Three-dimensional reconstruction of single-sided water-absorbing thermal insulation material was carried out by CT scanning technology.The water absorption process was water immersion from the outside layer to the inside layer,and then the thermal insulation material was equivalent to three-layer composite material consisting of dry layer,water-absorbing unfrozen layer and water-absorbing frozen layer.Combining with the calculation method of equivalent thermal conductivity of multi-layer composite material,the calculation model of equivalent thermal conductivity was established.The variation law of equivalent thermal conductivity of polyphenolic insulation material during freezing process was obtained.(4)Through freeze-thaw cycle test,the evolution of water absorption,thermal conductivity and compressive strength of thermal insulation materials with the number of freeze-thaw cycles was analyzed.It was found that the main reason for the decrease of thermal insulation performance was the increase of water absorption.By means of SEM scanning technology,the change process of micro-pore structure of thermal insulation material before and after freezing-thawing was compared and analyzed.It was proved that the damage of micro-closed pore structure of thermal insulation material was the basic reason for the increase of water absorption rate and the decrease of thermal insulation effect of thermal insulation material.The relationship between the thickness of insulation board and the number of times of freezing resistance is established,and the corrected value of the thickness of insulation layer is given.