Abstract
Despite the fact that the building stock of coastal areas is known to suffer from durability problems, only in the last decades the scientific community has more systematically linked the phenomena of premature deterioration of the durability of concrete and premature ageing of the building stock with the corrosion of steel reinforcement . In the case of earthquake-prone areas, the combined effect of the coastal environment and seismic actions further enhances the phenomena of structural degradation. However, to date, this effect has not been quantified, which constitutes a shortcoming in the assessment of the structural adequacy of existing reinforced concrete structures.The lack of quantification of corrosion and loading history (due to seismic actions) as factors of deterioration of steel reinforcement and the correlation with the apparent premature degradation of structural elements of existing reinforced concrete structures is the basis of this PhD thesis. In this light, an extens ...
Despite the fact that the building stock of coastal areas is known to suffer from durability problems, only in the last decades the scientific community has more systematically linked the phenomena of premature deterioration of the durability of concrete and premature ageing of the building stock with the corrosion of steel reinforcement . In the case of earthquake-prone areas, the combined effect of the coastal environment and seismic actions further enhances the phenomena of structural degradation. However, to date, this effect has not been quantified, which constitutes a shortcoming in the assessment of the structural adequacy of existing reinforced concrete structures.The lack of quantification of corrosion and loading history (due to seismic actions) as factors of deterioration of steel reinforcement and the correlation with the apparent premature degradation of structural elements of existing reinforced concrete structures is the basis of this PhD thesis. In this light, an extensive study of the consequences of corrosion of steel reinforcement was carried out, which extends along two axes : the mechanical response of corroded steel under cyclic loading and the ways to improve the mechanical behaviour of steel, such as shot blasting followed by the proposal of a dynamic performance index of steel in long terms. The seismic loads applied to structures, and in particular to steel reinforcement, are commonly simulated with low cycle fatigue loads. Consequently, both fatigue and corrosion phenomena degrade the hysteretic stress-strain model of steel in time, resulting in a rapid reduction of the service life of the structural elements of existing (old) structures. Current regulations, however, still treat seismic actions as an individual maximum 'stress' (force, deformation) and not as a sequence of seismic events, therefore ignoring the influence of fatigue damage accumulation and thus leading to uncertainties about the actual energy reserves of steel. In the context of this thesis, the behaviour of two reinforced concrete columns under dynamic stress (repeated loads gradually increasing), before and after corrosion, was investigated. Upon completion of the dynamic loading, the mechanical performance of the corroded reinforced concrete (RC) column, both in terms of strength and ductility, was clearly degraded. Extending the study of the mechanical behaviour of the embedded (corroded) steel reinforcement , its non-linear hysteretic behaviour was investigated by performing analyses of its seismic response. Thereafter, models for predicting the service life of the steel for different levels of corrosion and models for predicting the hysteretic loops of corroded reinforcing bars subjected to cyclic loading were presented. The proposed models simulating the cyclic behaviour of steel (corroded and non-corroded) are in good agreement with the experimental results, both in terms of load-bearing capacity and service life. It is worth mentioning that the (seismic) loading history, brought about a more pronounced degradation in the substructure compared to the corrosive agent (for an average mass loss of 14%-21%). A study of the effect of the microstructure of the steel reinforcement on its mechanical response to both monotonic and cyclic loading was also carried out on reinforcing steel bars of different categories: the traditional Tempcore B steel and a hybrid Dual Phase F. For the experimental study of the corrosive factor, the accelerated electrocorrosion method was adopted by applying a constant current density to the specimens for different exposure times. The set of specimens was subjected to mechanical tensile and low cycle fatigue tests, at two different strain ranges, ± 2.5% and ± 4.0%. The results of the mechanical tests showed a differentiation of the useful life between the two steel grades, which requires further study as the Dual Phase F hybrid steel (although of a lower grade) showed a higher performance in terms of service life. In order to evaluate and compare the two steel grades by incorporating both strength and ductility properties, the concept of a dynamic QF quality index was introduced. The comparison showed agreement of dynamic QF index results with experimental results as the Dual Phase F grade (hybrid steel) showed improved mechanical performance over time. Studying the mechanical performance of steel reinforcement, under monotonic and cyclic loading, the effect of the cleaning process by shot blasting, before and after corrosion, was extensively investigated. The first stage of the specific study focused on the effect of the corundum shot blasting cleaning process in combination with that of (sprayed) zinc-aluminium alloy 15% by weight (Zn85Al15) coating on reinforcing steel bars on the corrosion damage and mechanical response to monotonic loading. The results showed that the hot-dip coating offers satisfactory corrosion resistance and remarkable stability in the mechanical performance of steel reinforcement. Furthermore, it has been demonstrated that the optimum choice of abrasive and the optimum degree of purity in the blasting process improve the ductility properties of steel reinforcement, on the one hand, and ensure a time prolongation in the initiation of corrosion, on the other hand. Following the abovementioned encouraging results of the beneficial effect of shot blasting under monotonic loading and given that the contribution of compressive loads to damage accumulation depends on the deformation range, an extensive experimental study was carried out in a second stage, which is an important step in the study of the influence of shot blasting on steel reinforcement under cyclic loading, in terms of energy. By examining the experimental results of the low cycle fatigue tests at different strain ranges, it has been shown that cleaning by means of the specific process of shot blasting improves the service life of steel reinforcement, which was reflected by the recorded increased values of the number of loading cycles at the imposed strain ranges. Using once again the already proposed dynamic quality index QF , the superiority of the mechanical performance of the pickled specimens over time was confirmed.
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