Experimental And Theoretical Research On Damage Assessment Of Reinforced Concrete Member After Fire

The bearing capacity of concrete structure will be inevitably affected after fire. Since the residual strength of concrete relates to various factors and the damage degree of concrete member changes in the direction of depth, it is really a responsible and tough mission to give an assessment on the damage of structure exposed to fire. A reliable assessment ought to include the following contents: accurate estimation of the past fire condition (the heating curve), the variation of concrete's strength in the direction of depth and the residual bearing capacity of structural members. Therefore, there are the demands for studying on concrete's material, detection technology and computing method for the structural member's residual capacity. At present, there are few avaliable methods can directly detect the mechanical capacity of concrete in the direction of depth and no algorithm has been approached to estimate the fire condition in a quantitative way. In addition, most researchers in china carried out experiments of residual bearing capacity on simply support concrete members, which obviously had differences from the practical situation. Moreover, this kind of test can not fully reflect the effect of damaged concrete on the bearing capacity of members.The study in this thesis includes the following aspects:1. The couple effect of maximum temperature and duration time (T&t) on the properties of concrete has been studied. The concrete specimens had been heated under 21 kinds of T&t and the properties of specimens, including axial compression strength, splitting strength, Young's modulus, stress-strain curves, ignition loss and etc, have been tested after heating. Empirical equations had been fitted based on the test data.2. The fundamental research of "layer by layer method" has been carried out through two series of experiments. In the first one, 32 different T&t had been applied to the concrete slices, which were tested for their splitting strength after cooling. In the second, 7 pairs of big specimens were exposed to fire (single side) with different exposure time. After that, specimens were cut into slices in the direction of depth, and the slices were split to demonstratethe variation of damage in the direction of depth.3. Based on the theories of statistic and damage mechanic, the tensile failure of fire-damaged concrete is simulated by model of spring, whose failure is believed to be coincident with the probability distribution of "weibull". The stress-strain relation of tension is deduced by a theoretical, but not a fitting method. What's more, "closure of micro cracks under load" is adopted to explain the three features of fire-damaged concrete under compression load. Another form of "weibull" distribution is adopted to describe the process of crack-closure. Therefore, the relation of compressive stress-strain is deduced by the interaction of two kinds of "weibull" distribution. A...