This paper presents the results of an experimental study on the effect of progressive replacement of natural aggregate with recycled concrete aggregate (RCA) on bearing capacity, deformability and crack development in reinforced concrete slabs subjected to emergency loading. The study included four series of slabs in which the proportion of RCA was 0%, 20%, 50% and 100% by weight to natural coarse aggregate, respectively. The specimens were made with CEM I 42.5 R cement and the concrete mixtures had a constant w/c ratio 0.51. Measurements of ultimate bearing capacity, deflections, crack development and regression analysis of the force-deflection relationship were carried out. The results showed that replacing the natural aggregate with RCA to a level of 20% did not result in a significant decrease in bearing capacity or excessive deflections. In contrast, increasing the proportion of RCA to 50% and 100% led to a systematic reduction in load carrying capacity (by 6.4% and 12.5%, respectively) and an increase in the deformability of the elements, especially in the final loading phases. More intensive crack development was also found at higher RCA contents, which is related to the lower stiffness and higher porosity of the recycled concrete. The results obtained indicate that concrete with RCA can be used in structural elements provided that the proportion of secondary aggregate is controlled and that the technology for producing this type of concrete is further improved.