Thermal And Moisture Stress Analysis Of Bamboo Buildings Based On Heat And Mass Transfer Method Of Porous Medium

With the increase of population and development of construction industry, thedemand of energy and material in construction industry is bigger and bigger. Thedirect and indirect destroying to the natural environment from construction industry isan important reason why the natural environment is destroyed by people. Bamboowood is a kind of new building material. Bamboo wood is gradually becoming thesubstitute material of the traditional building material (concrete, brick, steels, etc.) inbuilding structure. According to the definition of porous media, bamboo wood belongsto typical porous material because of the solid skeleton and pore space that thebamboo wood has. When the environmental temperature and humidity change, theheat and moisture transfer in bamboo wood structure produces the distortion ofexpanding with heat and contracting with cold and expanding with moisture andcontracting with dry. If the distortion is constrained, thermal and moisture stress isproduced in the structure. Though the work mechanism of the transfer of thetemperature and humidity that influence the thermal and moisture stress in the bamboowood structure building is different, they have the same effects on the expanding andcontracting of bamboo wood structure. So, it is an important work in current buildingenergy-saving work to do research on the thermal and moisture stress in bamboo woodstrcuture and to develop and apply this kind of new bamboo structure and reduce andavoid destroying buildings from thermal and moisture stress. On the earlier study ofthermal and moisture stress in building structure, the effect of thermal stress isanalyzed, but the thermal and moisture stress is less studied under coupling effect ofheat and moisture. Interaction effects to thermal and moisture stress from temperatureand humidity fields are not taken into consideration. Due to the lack of theperformance parameters with expanding with moisture and contacting with dry ofporous media, especially the hydroscopic expansion coefficient, there is noappropriate design standard and reliable data in engineering design. This paper startswith properties of porous media in bamboo wood and coupled thermal and moisturetransfer in bamboo structure building. Bamboo plywood wall and bamboo reinforcedconcrete composite structure etc. are the research subjects. The change rules ofthermal and moisture stress influenced by temperature, humidity and expandingproperties of the porous media are revealed in bamboo structure buildings in hot and humid conditions. This provides the basis for further study of porous media likebamboo wood. The study content and result of the paper include:(1) In the paper, the characteristics of porous medium of bamboo structure, thereason of heat and moisture change and the transfer mechanism of heat and moistureare introduced in detail. It emphasis on the deformation mechanism and stresscharacteristic of heat and moisture of bamboo structure under the heat and moisturecoupling transfer. The concept of wet of porous medium is proposed, and itsmathematical differential equation and physical significance are introduced whichprovides the theoretical basis for heat and moisture stress analysis in subsequentchapters.(2) The heat and moisture elastic behavior of porous medium structure in heatand moisture transfer is analyzed through assuming the macrostructure of porousmedium and using micro-element volume per unit length as an object. The analyticalmethod and mathematical model of heat and moisture stress and strain of bamboostructure building are expounded. And furthermore, using the wall of bamboostructure as an object, the variation characteristic of the stress and strain of heat andmoisture is analyzed, and the impact factors on change of heat and moisture stress ofthe wall with bamboo structure are presented, which indicates that the temperature andmoisture content change are the most important factors.(3) The basic physical and mechanical performance parameters such as porosity,effective thermal conductivity coefficient, thermal and humidity expansion coefficientwhich influence the thermal and humidity stress of bamboo structure building areclarified. The effective porosity of building bamboo is tested by absorption osmoticmethod, which points out that the effective porosity of bamboo test specimen is32.1%~33.7%in normal temperature. The concept of humidity expansion coefficientis proposed. And the thermal and humidity expansion coefficient of building bambooare tested by strain method, the results indicate that the thermal expansion coefficientof bamboo test specimen is9.67×10-6℃-1in vertical and22.30×10-6℃-1in transverse,the humidity expansion coefficient is55.28×10-6m/(m·%) in transverse and11.64×10-6m/(m·%) in vertical, the thermal and humidity expansion coefficient intransverse is2.31and4.75times that of the vertical, respectively. The influence ofmoisture content change on thermal performance parameters such as effective thermalconductivity, effective heat capacity, latency coefficient of temperature wave areanalyzed. And furthermore, the correlativity between the total moisture content andthe total thermal resistance of wall is obtained, which demonstrates the basic data of dynamics in physics for analysis of thermal and moisture stress in bamboo woodstructure buildings. At the same time, strain method is adopted to test the moistureexpansion coefficient of bamboo wood. This provides reference for the test methods ofmoisture expansion coefficient for porous medium materials.(4) The change characteristic of thermal and humidity strain of bamboostructure wall under one-dimension steady-state heat and moisture transfer conditionis analyzed and the calculation method of stretching and bending thermal andhumidity stress are expounded, and the differential equation of steady-state thermaland humidity strain and stress of wall is established，which indicates that the thermaland humid stress produced by wall is the D-value of the free expansion calculatedvalue and the test value. And then the characteristic of thermal strain, humidity strainand thermal-humidity strain of wall are studied respectively by using bamboo stripagglutination plank wall as an object. The change rules of thermal and moisture stresswhen the temperature and humidity content of the wall change are analyzed. It pointsout that the thermal strain and the humid strain produce superimposed effect. Thisprovides design basis for steady-state thermal and moisture stress design ofnew-model bamboo wood structure buildings.(5) The change characteristic of stretching and bending thermal and humiditystress of bamboo structure wall is analyzed under one-dimension steady-state heat andmoisture transfer condition, the differential equation of temperature-humidity gradientbetween thermal-humidity stress and wall thickness direction and differentialequation of thermal and humidity stress with time changes are established. The mainfactors of influencing the transient thermal and humidity stress of bamboo structurewall are temperature, humidity, temperature and humidity gradient. Using integratedplank of bamboo as an object, the thermal and humidity strain of wall in typicaltransient thermal and humidity condition including heating, rapidly humidifying onhigh temperature wall, rapidly heating on high moisture wall are studied withexperimental. Meanwhile, using bamboo strip agglutination plank as an object, thestudy on the thermal and humidity strain under natural thermal and humidityenvironment are done continuously with experimental test method. The change rulesof thermal and moisture stress in wall under transient condition are anayzed. Theresults show that thermal and humidity stress change under transient thermal andhumidity condition is a complex and changeful process. This gives guidance for theutilization of transient thermal and moisture stress or the prevention of stressoccurrence in new-model bamboo wood structure buildings. (6) The change characteristic of thermal and humidity stress of bambooreinforcement and concrete interface is analyzed, and the calculation method ofcontact force, normal stress, shear stress of interface and axial normal stress areclarified in bamboo reinforce concrete composite structure. the experimental test onhygral expansion characteristic of cement mortar under normal temperature provesthat the hygral expansion coefficient and moisture content of cement mortar arelinearly correlated, and further, the influence of thermal and humidity stress change onthe thermal and humidity stress of interface and axial is studied by using cylindricalbamboo reinforce concrete component of single bamboo reinforcement as an object,the significant reason why bamboo reinforce concrete structure become invalid isobtained. This sheds light on the design and application administration of bambooreinforce concrete composite structure.