Time-varying Law Of Early Age Concrete Structure Properties

Because the cement hydration process has not been completed at early age, the concrete strength and bond properties with the changing age of concrete, concrete structure properties are constantly changing, early age concrete structure and concrete structure during normal use are essentially different. The force characteristics of early age concrete structural elements is systemly studied and time-varying resistance model is established, which has important theoretical and practical value to reinforced concrete construction security analysis and control and life cycle analysis of reinforced concrete structures.There are three main problems to be solved for the bearing capacity calculation of early age reinforced concrete structures: the time-varying law of concrete strength changing with age; the establishment of bond - slip constitutive of early age fly ash concrete; the calculation method of the early age reinforced concrete structures bearing. Thesis, this paper around the "material properties - bond performance - capacity performance" of the main line, using on-site investigations, experimental research and numerical simulation method, the system of the early age of reinforced concrete beam and the bearing capacity of eccentrically compressed time-varying law. The main research work are as follows:Taking the typical multi-storey and high-rise residential buildings in Xi’an and Xianyang city and their periphery county city as research objects, more than 1000 residences built in recent four years are investigated, the actual data measured are obtained such date as concrete and steel strength as well as the dimension parameters of all kinds of RC members. According to the statistics analysis, probability distributions and statistics parameters of the resistance of building materials and structural members are studied. At the same time, some influencing factor of material function and dimension parameters are analyzed.Adopting combine methods of experimental studies and field investigations, the increase of the early strength of fly ash concrete and the impact of curing time on the increase law is studied, the increase law differences of the cubic concrete compressive strength, the same condition curing strength and the actual structural strength is compared, and the increase law differences of the different type component strength is compared.The relationship among compressive strength, rebound values and the carbonation depth within 90 days after pouring is analyzed, the carbonization development law of the fly ash concretes in early 300 days under the natural condition is established through the experiment, the rebound regression equation of different fly ash content is established, which provided a scientific basis for the rebound method used in early age fly ash concrete strength testing.According to various advantages and disadvantages of bonding performance test method to determine the early age properties of fly ash concrete bond test program. Through the pull-out test on bonding properties of fly ash concrete time-varying with the age rule is analyzed, combined with fly ash concrete beam test of the main factors affecting bond behavior (strength of concrete, fly ash, steel type and diameter, test methods, etc.) were systematically studied to establish the early age of fly ash concrete ultimate bond stress formula, regression analysis, the early age of fly ash concrete bond-slip constitutive relation. The finite element analysis model built on the pullout test and beam test carried out numerical simulation, finite element analysis and experimental results agree well.On the basis of finite element analysis of early age flexural members, the force characteristics of early age reinforced concrete flexural member and compression members are analyzed. For the force characteristics of early age reinforced concrete beam, the use of limit equilibrium theory, the capacity calculation model of early age RC flexural member is established. Through the test verification, the calculation model is in good agreement with the experimental results. On this basis, the bearing capacity time-varying law of early age reinforced concrete flexural strength and compression members is analyzed.