Size effects in shear force design of concrete beams

Sammanfattning: Shear failures in reinforced concrete beams are complex problems that depend on many different mechanisms, such as aggregate interlock and dowel effect. This makes the prediction of shear failures difficult as they are not strictly tied to the material and depend on other factors including the size of the beam. In particular the height of the concrete cross section plays a role in the amount of shear stress the beam can carry. This effect is appropriately named the size effect, and is not well defined for smaller concrete beams, which is why they are studied in this master thesis. The main objective of this report is to determine if the size effect is present in smaller reinforced concrete beams as well as examining the accuracy of the Eurocode formulas when determining shear failures for smaller concrete beams. The study consists of a theoretical part where the Eurocode is studied along with the mechanisms of shear failure as well as a practical part where concrete beams were cast and tested. In the laboratory beams the conditions, aside from height, were held as constant as possible in order to limit the other factors, thus giving a more accurate representation of the size effect. Both beams with and without shear reinforcement were studied. The experimental results showed that a size effect is present in the smaller reinforced concrete beams, both shear reinforced and unreinforced, as the stresses at failure decreased in the beams as the height increased. Because the other known factors for shear failure were kept mostly constant, the conclusion is that the size effect contributed to the variation of stress at failure for the different beams. Regarding the Eurocode formulas, differences could be seen between the calculated theoretical shear capacity and the laboratory shear capacity. The theoretical values for shear reinforced concrete beams were closer to the laboratory values than the unreinforced beams, though the reason for this is unknown. The theoretical values were always on the safe side for all the tested beams.