Influence Of Damage Evolution On Concrete Strength

This research is based on the project, Full Span Life Analysis of Concrete Structures(50278038), sponsored by NSFC, and is the pre-work of another NSFC project, Concrete Damage Estimation Based on the Detecting and Monitoring Signals.Under formidable natural conditions, such as freeze-thaw and fire, the changing of micro-structures of concrete material will induce the degradation of mechanical properties and the safety of concrete structures. Present concrete strength theories are mainly based on the experimental results of load test of material, while the influence of the changing of micro-structures is not considered. Therefore, they are not suitable for strength analysis of deteriorated concrete.Researches on the strength of deteriorated concrete mainly orient on engineering objects. That is, variation of concrete strength is studied in different laboratory conditions which simulate different natural situations. The curve graphs, which reflect the relationship of experimental results and influencing factors, such as temperature, freeze-thaw times and strain ratio, are plotted. The curves are proposed to reflect the direct function relation based on the curve characteristics and trends, and parameters fitted on the experimental results are supposed to treat as the concrete strength under that situation. In some sense, those research outcomes based on this method originated from face is limited, for they did not reach the essence of deteriorated concrete variation.According to damage mechanics, variation of microstructures are expressed by macro-variables through the inner variables, and the traditional problems which approach from the material science are avoided. Therefore, inner damage variables which have clear mechanical meaning and represent the variation of the concrete microstructure are selected to couple into the present strength theory, and development rules of concrete strength under environmental factors and loads are disclosed. The works are mainly as following.1)The space measuring question of concrete is proposed. The transition from damage factors and damage functions in common mechanical sense are realized with the nondestructive technology. Therefore modern testing electromechanical signals in macro-view can be applied to reflect variation of concrete micro-structures.2)Based on damage mechanics and concrete strength theories, failure plane is fitted according to the experimental results of modern detection technology. It describes the shrinking rule of failure plane with regarding to the increase of damage.3)Uni-axial constitutive relation of deteriorated concrete is also set up to reflect the influence of damage.4)The abrupt deteriorating point, the cut-off point of the gradual failure process of concrete, is determined.5)The freeze-thaw action is applied as the experiment method inducing the damage development. The deteriorating rule of concrete failure plane is studied, and published results of bi-axial or tri-axial experiments under freeze-thaw cycles and high temperature are compared with the calculation results according to the function in this paper.Based on modern material testing methods and theoretical analysis, development rule of damaged concrete strength are studied with the damage variables according to damage mechanics. Not only the research results can be applied to determine bearing capacity of present concrete structures, but also there is obvious theoretical and practical significance to the quantitative estimation of present structures and prediction of residual lives.