Theoretical Investigation To Cracking Prediction Of Early-age Concrete

The cracking of concrete is so often that it impacts the presentation and durability of concrete structures, it reduces the structures' service life and makes the building uneconomic because of reconstructing. It is not conform to the sustainable development. Detecting and forecast the cracking time and the trend of concrete is the premise to take effective measures to reduce or avoid cracking. And only understanding the cracking mechanism, the development of cracks, that can take effective engineering measures from the design and construction aspects to avoid or reduce the cracks in the structure of construction hazards.In this dissertation, the crack prediction of early age concrete has been studied by using strain and Energy conservation principle. The main original work includes: Firstly, on the basis of initiation fracture toughness theory, a new method of estimating values of the Initiation fracture toughness has been brought forward in this dissertation. Functions in time domain of Initiation fracture toughness, Initiation crack tip opening displacement and Young's modulus have been obtained by using of data of fracture test.Secondly, the influencing factors of shrink and creep have been generalized in this dissertation. Simultaneity, the mechanics of shrink and creep have been studied. Functions in time domain of shrink strain and specific creep of early age concrete have been obtained in this dissertation.Finally, using Energy conservation principle, energy equation of early age concrete ring has been established. R-equation and G-equation of early age concrete have been amended through theoretical derivation. The function in time domain of maximum allowable tension strain has been obtained by using the function in timedomain of fracture parameters. Simultaneity, The function in time domain of reality tension strain has been obtained by using the function in time domain of shrink, specific creep and Young's modulus. The cracking Age of early age concrete has been predicted by comparing reality tension strain with maximum allowable tension strain. Lastly, this dissertation has analyzed result of prediction of different strength concrete. Through analysis and study, the main conclusions are as follows: the ultimate tensile strain is increased with the intensity levels, For the concrete of C30 and C50,the ultimate tensile strain is in the range of 2.2×10^-4-2.8×10^-4 . In the same ageperiod, the tension is increased with the intensity levels. However, under the same environmental conditions, the high-strength concrete is more easily to crack than the normal-strength concrete. In fact, the restrained shrinkage strain is small than the free shrinkage, this is because that the creep release some contraction stress, thus delay the time of cracks. From this point of view, the creep was favorable.