Corrosion fatigue of reinforced concrete
This work is concerned with the corrosion fatigue characteristics of full-scale reinforced concrete beams partially submerged in 3.5% NaCl solution or in tapwater of low dissolved salt content. The test beams were subjected to constant amplitude fatigue loading in uni-directional or reverse bending at slow cycle rate of 0.17 Hz and various load levels. The test programme had two stages: Stage I, carried out at relatively high load levels, represented a study of the fatigue-failure phenomenon but also provided guides for the more detailed study undertaken in Stage II which was devoted to low load conditions under which the main steel deterioration process was corrosion. In the latter stage, attention was focused on the monitoring of a number of electrochemical parameters including the corrosion rate. Late in the programme, electrochemical noise technique was also examined. The phenomenon of concrete crack blocking, previously reported in seawater environment, was observed in both test environments in this work. This phenomenon was closely examined and the mechanisms of the formation of deposits and its effects were described. A hypothesis was proposed for the structural behaviour of reinforced concrete beams during cyclic loading in aqueous environment. Failure normally occurred by the fracture of one of the main tensile bars due to fatigue, often followed immediately by yield of the remaining bar and beams collapse. Fracture surfaces were examined under SEM.Corrosion rate measurements involved formidable difficulties which had to be overcome to obtain accurate measurements. For instance, the current interruption technique for the estimation of the IR-drop was developed and established as the most appropriate method for concrete beams with complex reinforcement configuration. Extensive polarisation measurements indicated clear effect of the test condition on the technical variables involved in various measuring techniques (viz potentiodynamic and potentiostatic techniques). Based on these observations a criterion has been proposed to determine the appropriate variables necessary for the accurate determination of the polarisation resistance Rp. The work has demonstrated that the corrosion behaviour of reinforced concrete sustaining dynamic loading is extremely complex, and short term indications could not be used safely for long term predictions. Based on corrosion rate measurements and the actual corrosion pattern observed upon completion of the tests, a concept of a change in corrosion mechanism from a microcell process of relatively low corrosion rates to a macrocell process at much accelerated high rates is introduced. The prevailing mechanism depends on time of exposure, load level and reinforcement details. Results from long running fatigue tests in seawater from concurrent research were incorporated which also support this concept.