Honeycomb forms when mortar fails to fill voids between coarse-aggregate particles. The defect may be purely cosmetic or, depending on the location and extent of honeycombing, may be structural and require repair. Voids form when concrete fails to fill areas in a form, typically those under large blockouts, in very deep placements, or that are heavily reinforced. Voids are almost always structural defects requiring repair.
Causes of honeycomb and voids include stiff or unworkable concrete, segregation, congested rebar, insufficient consolidation, and improper placing practices.
Preventive measures include attention to concrete mix proportions and using the proper techniques for forming, rebar placement, and concrete placement.