It is commonly accepted that there is a size effect on the nominal resistances of quasi-brittle materials such as cementitious materials. This effect must be taken into account in the design of the ultimate behaviour of concrete structures in order to avoid damage and crack openings. These parameters are frequently used to study the behaviour of concrete and to characterize the durability of structures. Different theories exist in the literature to describe the size effect. Among them, we find the deterministic theory of Bazant where fracture energy is considered independent of the size and it is assumed that at peak load, the crack length is proportional to the size of the specimen. In this work, attention is paid to investigate experimentally and numerically, the relationship between crack openings and length, and the size of the specimens. In the experimental study, RILEM size effect method is adopted to test geometrically similar concrete beams of various sizes. Two types of concrete mixes are used with varying aggregate sizes in order to further investigate the effect of aggregate size on fracture behaviour. Digital Image Correlation (DIC) technique is adopted in this study to measure the cracking in concrete on the surface of the concrete specimen. It is established as a robust and highly precise tool for fracture measurements such as crack opening and crack length. The results have shown a significant size effect on the process of crack propagation. Furthermore, as the aggregate size increases the size effect becomes more critical. A case study of the size effect on crack openings and crack spacing in reinforced concrete beams is also presented. From serviceability point of view, it is observed that the Eurocode 2 underestimates the crack openings. In the numerical study, the concrete beams are simulated using a nonlocal damage model. The internal length and other model parameters are calibrated by an inverse calibration technique using an automatic procedure. Crack opening profiles are extracted through post-treatment procedures. The mechanical behaviour and the crack propagation are finally analysed, reflecting a similar size effect as detected by the experimental results.
