Some design requirements for concrete construction, such as seismic post-tensioning, can result in congested areas within the formwork that are difficult to consolidate. Reinforcing steel, embedments, boxouts, prestress ducts, and anchorages all contribute to congestion. Certain form shapes also can make concrete difficult to place and consolidate. Incomplete consolidation in congested areas can lead to structural inadequacy and time-consuming, expensive remedial work. Honeycombed concrete can occur because vibrators can't consolidate concrete around and through the congestion and out to the form face. Incomplete consolidation also can result in excessive amounts of entrapped air, which reduces concrete strength and increases permeability. Successful concreting in highly congested areas requires special design considerations, concrete mixtures, and placement and consolidation techniques.