The effectiveness of the same volume of steel and synthetic macro fibres on impact resistance and microstructure associated mechanical behaviour is studied for concretes made with two different coarse aggregates-gravel and incinerator bottom ash (IBA). The microstructure-strength relationships were analyzed via electron microscopy and X-ray computed tomography (CT). The analysis indicated that increased paste strength was not so significant for the mix properties of plain IBA concrete as extensive cracks through the aggregate took place likely due to a reduced grain-grain interlocking and inter-particle friction as a result of the distinct pore structure, and fibre orientation in gravel concrete produced a line of weakness for failure to take place along. The results also show that the increase with the steel fibre inclusion in the post-peak stress, absorbed energy values, impact life for IBA concrete is much higher than those for gravel concrete. Overall, the research suggests that fibre-reinforced IBA concrete would be very promising as a multifunctional construction material. (C) 2011 Elsevier Ltd. All rights reserved.