Investigation Of Concrete Thermal Stress Characteristic

In Xiaolangdi lining structure engineering,at the temperature variation,analyse and sort the observation data of vibrating wire strain meter and reinforced meter embedded in the lining structure, The study found that in construction period, the state of stress and strain in different positions of the lining section are differen, the constraint of observation instruments located in the inner side of lining structure is bigger than the constrain outside of lining tunnel. the pressure stress of inner side is bigger than the outside of lining tunnel. visibly, in addition to the temperature change, constrained state of the structure also have significant influence on thermal stress of concrete.in order to research the relationship between the constraint degree and thermal stress, a parameter called constraint coefficient was put forward based on the constraint condition analysis of concrete str uctures, which means the constraint degree of concrete.Based on the operational principle of vibrating wire strain meter,conduct a preliminary study about the relationship between observation data of vibrating wire strain meter and constraint degree of concrete as well as thermal stress under the action of temperature, and a formula for determining concrete thermal stress based on elastic modulus, temperature change and the constraint degree was derived. Thermal stress tests conducted on the basis of theoretical analysis, the different intensity of elastic constraint were simulated by adjusting the thickness of the elastic elements and applying different initial stresses on the specimens which are called elastic element method and the initial stress method. In the test,the electrical ribbon heater wrapped the lateral surface of cylindrical concrete specimen which was sealed with a layer of polyurethane foam with excellent heat insulating property.Applying a uniform temperature field to the specimen by controlling the power to the electrical ribbon heater, the temperature and strain of concrete controlled by temperature sensor and vibrating wire strain gauge which have been buried in the center of the specimen ahead of time, At the same time, the deformation measured by the dial gauges,and the thermal stresses measured by the pressure sensor.The results showed that:(1) The heat insulation device can supply an uniform temperature field to the concrete specimens, it has a small volume and easy to operate on a servo testing machine in laboratory.And this device can be reused.(2) Elastic element method and the proposed initial stress method can effectively simulate the elastic constraints, achieved the desired effect.(3) A parameter called constraint coefficient was put forward which means the constraint degree of concrete, and Kr has ties with the thickness of elastic element and the value of initial stress, a formula for determining the constraint degree of concrete was derived based on the observation data of vibrating wire strain meter, Kr =1-(e +b DT) / aDT.(4) Present the mathematical relation between the constraint degree and the thermal stress, s T=- FT Ec Kra DT.At the same temperature variation, concrete thermal stress is closely related to the constraint degree of concrete specime ns, the stronger the constraint degree, the greater the thermal stress.(5) In the state of fixed and elastic restraint, the thermal stress produced in the specimen is linearly dependent on the temperature rising, moreover, the measured thermal stresses and theoretical values are in good agreement.