Due To Shrinkage And Creep Affect The Long-term Crack Width Of Reinforced Concrete Structures Calculated

The control of long-term crack in reinforced concrete is important to guarantee structural durability and is a key design index in requirement of serviceability limited state, in which the accurate prediction of crack width is the fundamental. The present theories on crack mainly focus on analysis of crack under short-term loads which long-term crack is estimated by multiplying an experimental based amplified coefficient, which is difficult to predict crack width precisely for newly constructed reinforced concrete structures with new material by new technology and various environment. There is no agreement with the mechanism and effect coefficients for crack around the world even today, and still no theory on calculating long-term crack has been accepted broadly. In this paper, after the progress of the mechanism of increase of crack width including the effect coefficients is summed up, the models of long-term crack for reinforced concrete bent members based on newly established short-term model and Code-provided model are put forward, respectively, which will prompt the system of crack control in reinforced concrete members.First of all, the methods of crack width control and crack width calculation in codes around the world are generalized, and a train of thought to control crack width by limiting the tensile stress in reinforcement in our country is presented while referring the method adopted in American codes which can avoid tedious procedure calculation complicated with many parameters.Secondly, based on the age-adjusted effective modulus method, the stress redistribution in cracked and uncraced cross sections in reinforced concrete members caused by creep and shrinkage is analyzed by introducing internal force equilibrium and deformation compatibility, then the formulas to calculate long-term cracking flexural moment in reinforced concrete members considering creep and shrinkage are established.Thirdly, a new model of crack in reinforced concrete members when loads are applied is put forward which can be used to establish the long-term crack model to calculate crack width considering the effect of creep and shrinkage in compressive concrete and tensile concrete between cracks by introducing the redistribution of stress, and the practical formulae are deduced to calculate long-term crack width by adjusting some of parameters according with experimental results. Additionally, the practical formulae to calculate long-term crack width based on model of short-term crack provided in code GB50010-2002 are obtained considering the effect of creep and shrinkage, also the practical formulae are deduced when the experimental data are used to adjust the values of some parameters.Finally, both the established formulae to calculate long-term crack width are developed by Fortran Language which are used to analyze the samples adopted in tests accomplished by Nanjing Institute of Engineering. Comparison between theoretical values and experimental results shows that the presented methods hold acceptable precision, and these methods are convenient to consider the effect due to variations of creep and shrinkage and characteristics of reinforcement arrangement, showing that theses methods can be applied much comprehensive than those provided in present codes.