The shear capacity of concrete members is a major challenge of structural concrete research through the years. Many theoretical models have been developed and various experiments have been performed, focusing on the accurate prediction of the shear behavior. Many of the available theoretical models assume that a large contribution of the shear capacity is transferred through cracks by a mechanism often recognized as aggregate interlock. At the same time, due to the increase in the complexity of the structures and the development of the concrete technology, the mechanical properties of concrete have improved significantly. This fact leads to the need for modification of the existing models or the development of new ones, accommodated to the improved materials. Since, the aggregate interlock plays a significant role in the development of the shear capacity, the present research proposes a new numerical methodology for the calculation of the aggregate interlock in high strength concrete in which aggregates break, based on the widely recognized model proposed by Walraven and the results of direct surface roughness measurements. The crack surfaces of concrete cylindrical specimens drilled from a 70 years old existing concrete bridge and newly casted cubic specimens generated by splitting tensile tests were measured by a laser scanner. Moreover, the surface of a reinforced deep beam after flexural shear failure was measured as well. The measured crack surfaces were used to implement the plasticity based aggregate interlock model proposed by Walraven with an algorithm which was validated with Walraven’s theoretical model, using a so-called mesostructural model. The output of the analysis gave suggestions on the adjustment of the available aggregate interlock model for high strength concrete. The proposed model is then implemented into a shear test on a 1.2 m concrete beam, which has a concrete strength larger than 70 MPa and the aggregate interlock seems to influence significantly the shear resistance of a cracked section. Based on the observation of the surface roughness of a crack, the thesis further proposed that with a sufficiently large crack face, the localized variation in the crack surface is averaged out. Thus, the surface can be used to develop a master curve for the given concrete type. In the last part of the study, two improvement suggestions are given regarding the Critical Shear Displacement Theory. The one point is relevant to the simplification of the crack profile that can be changed from a straight line into a more inclined and the second point is related to the correction factor considering the fracture of the aggregates, that should be dependent on the crack width.

In the Netherlands, yearly 33 Mton new concrete (roughly 15 billion m3) is producewhile 12 Mton concrete waste becomes available. Recycling of the yearly 12 Mton concrete waste results in roughly 7 Mton coarse recycled concrete aggregate (CRCA) and 5 Mton fine recycled concrete aggregates (fRCA). Currently, fRCA is not used in structural concrete, but only for low grade applications. This research aims to investigate the effect of recycled concrete aggregates (RCA) incorporation on the performance of structural concrete mixtures. Three streams of fRCA were considered in this research and two deliveries of each source were examined. River sand was replaced by 25% fRCA, which was found to be the optimal amount that does not require cement increase. Both deliveries were used in order to identify potential differences. Additional concrete mixtures with simultaneous replacement of river sand by 25% fRCA and river gravel by 30% CRCA were evaluated. Parallel work was conducted during an 8months internship at TNO, where optimization of mortar mixtures with full replacement of river sand by fRCA was investigated. This was performed by considering the water demand of solid constituents (binder and fRCA) and packing of fRCA. Optical microscopy was used for closer inspection of the microstructure of fRCA. Fresh properties of studied mixtures were measured. Assessment of the mechanical performance of concrete mixtures was expressed by means of compressive strength and elastic modulus. Long term behavior was also investigated by shrinkage deformation measurements up to 95 days after casting. Results show that substantial rise in cement content is necessary for obtaining the desirable flow and high levels of compressive strength, when full replacement of fRCA is performed. Regarding concrete mixtures with 25% fRCA, increase in air content was measured. Entrapped air is strongly dependent on the porosity of adhered cement paste on RCA, content of 00.25 mm fraction as well as agglomeration and presence of impurities in as received fRCA. This affects negatively their mechanical performance. Additional porosity resulting from CRCA incorporation was responsible for further reduction in mechanical properties of concrete mixtures. Mixtures with RCA showed three times higher mass loss at the age of 95 days.