Alkali Silica Reaction (ASR) is a long term chemical reaction causes swelling and damage to the concrete material in the form of cracking and material deterioration. In this research a chemo-thermo-mechanical ASR finite element numerical code is developed to model and analyse this phenomenon in concrete dams. It considers the effects of variables such as temperature, humidity, non-uniform time-dependent material degradation and 3D stress confinement on ASR evolution. 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 the effect of dynamic loads on structures affected by ASR. The combined effect of old and new cracks under dynamic excitation may cause dam failure. The SU-ASR (Stellenbosch University ASR FE code) model developed, is used to analyse and predict dynamic behaviour of the Kleinplaas dam which is located on the Jonkershoek River near Stellenbosch in the Western Cape province of South Africa. The combined ASR and seismic actions based on the state of the structure at the end of the long-term ASR analysis are investigated and comparisons are made through a comprehensive study of the damage development and crest displacement. The development of the cracks in the coupled analysis of the ASR and seismic load is significantly different from the cracks in the dam when only ASR is considered.