Abstract Blockage of concrete flow through opening or gap often occurs during placement of self-consolidating concrete due to particle interlock of coarse aggregate particles. Suggestions to reduce the coarse aggregate content in order to minimize such particle interlock have been made by some researchers. However, reduction of the coarse aggregate content would also decrease the particle packing density and, if overdone, might adversely affect the overall performance of the concrete. A better understanding of how particle interlock would occur is desperately needed. Herein, a systematic study on particle interlock and flow of aggregate through opening is reported. It will be shown that the aggregate grading has great effects on the flow of aggregate through opening and that to alleviate particle interlock, the fine aggregate content should be more than sufficient to fill the voids between the coarse aggregate particles so as to provide a certain thickness of excess fine coating the coarse aggregate particles.