| Since the invention of Portland cement, concrete structure has been the most used structure form in infrastructures. As widely known, reinforced concrete (RC) structure embraces the advantages of concrete, costs less, and it has become the primary choice of structure design in civil engineering, with its wide range of usage. However, most of the time people focus on calculation of load capacity and ignore the durability of concrete structures, leading to early functional loss of structures. Research on durability of RC structure is very urgent and important.Corrosion of steel bars induced by penetration of chloride in the concrete cover is a frequent cause of degradation of RC structures exposed to marine environments or to the use of de-icing salt. Chloride lead to a local breakdown of the protective oxide film on the reinforcement in alkaline concrete, so that a subsequent localized corrosion attack takes place.Performance based approaches to the durability of RC structures have recently been proposed by the International Federation of Concrete, which are aimed at the design of concrete structures in order to guarantee the stability and serviceability requirements throughout the design service life with an adequate level of reliability. These approaches require the estimation of the initiation time of corrosion. This can be done only if an accurate estimation of both the chloride penetration in time and the chloride threshold level (CTL) is available. A great amount of studies are published regarding the penetration of chloride ions as a function of concrete composition. Conversely, there is a lack of knowledge regarding the chloride threshold level, especially under temperature's influence. Many studies have focused on the CTL, but with different presentations, conditions, and detection methods, CTL varies largely. To ensure the reliability of RC structures, standards of many countries adopt a quite conservative value.In this dissertation, author mainly cling to the idea that --corrosion potential exceeding pitting corrosion is the sufficient and essential condition for depassivation of reinforcement. Experiments are done under high temperature (50℃), in synthetic pore solution with different pH values, using potentiostatic method. Mean values and variation coefficients of Cl- content at the reinforcement corrosion initiation and CTL values with 95% acceptance under different pHs, different potentials are suggested. Comparison has been done between CTL under high temperature and results suggested by Chen Qing under low temperature (5℃) and normal temperature (25℃). It shows that: when pH is less than 11.6, the influence of temperature to CTL is less evident, while pH is greater than 11.6, temperature plays an important part in CTL; when potential is greater than -300mV SCE, temperature's influence is less evident, on the other hand, when potential is less than -300mV SCE, temperature's influence on CTL is more obvious. If temperature rises from 5℃to 50℃, CTL will decrease by 88%.Since temperature is not a constant value, it is time- dependant, this dissertation has also designed an experiment to study the relationship between steel corrosion and temperature fluctuation, which aims to investigate how steel reacts after staying in high temperature for some time and corrodes. Experiment shows that, in a certain Cl- content, even steel keep passive under normal temperature, after staying in high temperature for no matter 2 hours or 24 hours, depassivation of steels is irreversible. It illustrates that CTL does not correspond to the only one environment condition; it is relevant to the history of environment conditions, which has real important meaning in deciding CTL values.
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