In the present study the damage mechanism of Alkali-Silica-Reaction (ASR) in concrete bearing reactive volcanic rock is investigated. A combined numerical and experimental research is presented. The mechanism of ASR is investigated on a concrete microbar specimen by using various microscopy techniques. A meso mechanical model based on lattice theories is used as a starting point. Variables in the model are the properties of the concrete components on the meso-level, like mechanical properties of cement paste, aggregates, dissolved aggregate, bond properties and finally the properties of the gel. It is found that current numerical model is able to simulate ASR cracks similar to the experimental observations.