Today, no general design method for shear strength capacity of RC beams can predict the failure load with a high degree of accuracy. The failure load between two beams cast from the same batch may vary as much as 30 percent. This project aims at investigating factors affecting the shear strength capacity based on an understanding of micro and meso scale material properties. Tests were performed on beams with two different types of aggregate and two different w/c. The crack propagation was monitored during the loading by means of DIC and AE. The results show that the use of natural aggregates or crushed aggregates as finer fractions strongly affect the shear strength capacity. From direct shear tests it was found that the scatter of the shear strength was much higher for the specimens with natural aggregates compared with crushed aggregates. The same tendency was found for the shear crack initiation load for the RC beams. The percentage of fractures propagating through aggregate, paste and the ITZ varied with w/c, type of aggregate and type of failure. Microscopy in combination with DIC and AE measurements makes it possible to determine at what stage different cracks have been formed and their relation to the micro structure.