Browsing by Author "Pourbehi, Mohammad Sadegh"
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- ItemNumerical modelling of alkali silica reaction in concrete dams(Stellenbosch : Stellenbosch University, 2018-12) Pourbehi, Mohammad Sadegh; Strasheim, JAvB; Van Zijl, Gideon P. A. G.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Dams are important infrastructure components and an asset for any country. Past earthquakes have highlighted their vulnerability to damage and even failure which can have major socio-economic consequences, losses and other cascading effects (e.g. water supply, power generation and irrigation). Hence, considerable research has been done to evaluating the safety of the aged dams and in some cases to pursue a suitable remedial action and rehabilitation strategy. Alkali Silica Reaction (ASR), a deleterious chemical reaction between siliceous aggregate and cement paste in concrete, causes long term swelling and deterioration of concrete structures such as dams. Several important factors govern the ASR reaction kinetics, of which water and temperature are the most important. Numerical modelling should capture the impact of humidity, temperature, and confining stresses and material degradation during the ASR as the most prominent factors influencing ASR. In general, numerical models for modelling ASR and its effects fall into three categories: (1) macrostructural models concerned with the analysis of structures affected by the reaction; (2) microstructural models which link the chemical reaction to its impact at the material level; (3) mesoscopic models that consider the multi-phases of the aggregate, cement paste, void and ASR gel, whereby anisotropy is explicitly represented. In this study a chemo-damage-plastic model is developed, considering temperature, humidity, non-uniform time-dependent material degradation and 3D stress confinement effects on ASR evolution. The model is validated by modelling the structural behaviour and response of three concrete gravity dams namely Fontana dam, Kleinplaas dam and Koyna dam. Comparing the results with the actual data on macro crack appearance and crest displacement as well as with other numerical models available in literature. The model developed for Kleinplaas dam is calibrated with the measured data and the applicable ASR parameters are identified. An attempt is then made to simulate the past and current swelling behaviour of the dam and the associated damage. The results successfully predicted the displacements and ASR strain rates plus the development of irreversible plastic cracks. While the structural behaviour of ASR affected structures under monotonic and quasi-static loading has been extensively investigated over the last decades, limited research has addressed dynamic loading. The combined effect of old and new cracks under dynamic excitation may cause dam failure. Considering the predicted interaction between ASR and seismic loads, remedial measures can be adopted at the right point in time to safeguard the dam in the event of an earthquake. Fluid-Foundation-Structure Interaction (FFSI) also has received much attention in Finite Element Analysis (FEA) of dams. The SU-ASR (Stellenbosch University ASR FE code) model developed, is used to analyse and predict dynamic behaviour of the Koyna dam, damaged by an earthquake. The combined ASR and seismic action are investigated and comparisons are made through a comprehensive study of the damage development and crest displacement. The seismic response of the deteriorated dam is subsequently analysed based on the state of the structure at the end of the long term ASR analysis. The results show that this combined action can significantly change the dynamic behaviour of typical concrete dams. Finally, a rational prediction of the long term impacts of remedial actions such as the slot cutting technique and the justification of other remedial action based on analytical results, was done. This required the use of specialized software with an adequate constitutive law and time integration scheme. As an example, the slot cutting technique is implemented for a synthetic dam structure using the SU-ASR finite element based computer program code developed in this research. The results of this simulation are discussed and interpreted. The SU-ASR model predicted the behaviour of the dam before and after the slot cutting closure and the subsequent compressive stresses with good approximation.