Damage Indicator Selection And Durability Of Concrete Subjected To External Loads

Mechanical loading is one of the most important factors inducing concrete cracking, which is intimately related to the degradation of concrete durability. For concrete with loading damage of high level, quantitative investigation on the crack geometry always is the best way to provide insight into its impact on the chlorides transport properties. When the damage level is low, hardly observed microcracks dominate the morphological changes, as a result, adopting macro indicators is a more convenient way to characterize the damage of concrete on this level. On basis of experimental studies, this paper focuses on the damage indicator selection and the investigation of chlorides transportation in damaged concrete. The main work in this paper can be summarized as follows:(1) Firstly, cracks were prefabricated by applying high level compressive loads. Then, the crack parameters were extracted using image analysis technique, and the chloride ion permeability was measured through RCM test. The crack geometry was quantified both perpendicular and parallel to the chloride migration direction by crack density, width, orientation and tortuosity. The effects of each crack parameters on the chloride transportation were studied. Finally, a comprehensive index, effective crack density, was put forward to reflect the influence of cracks on the chloride permeability. In addition, the crack width threshold values(150pm and400μm), which significantly influence the chloride permeability, were obtained on basis of the analysis of experimental results.(2) Experiments were carried out to study the chloride permeability of concrete subjected to long-term sustained compressive loads, and the influence of concrete creep on the chloride transportation. The applicability of three macroscopic damage indicators, stress level, creep strain and the ultrasonic velocity, were discussed. The time-sensitivity and coupling mechanisms of effective damage, creep deformation and hydrated self-healing effects on concrete under long-term sustained loading were discussed.(3) Experiments were conducted to investigate the chloride permeability and the steel corrosion of reinforced concrete subjected to long-term sustained four-point bending loads. RCT test and anode-ladder method were used respectively to determine the chloride transportation and the steel corrosion state. The effects of stress states on the chloride transport properties and the rebar corrosion were analyzed and the critical chlorides concentration(0.52%by weight of cement) for corrosion initiation of steel bar was obtained through the discussion of experimental results.