What is the difference between entrained air and entrapped air?
According to the ACI publication "Cement and Concrete Technology," entrained air is 'Imicroscopic air bubbles intentionally incorporated in mortar or concrete during mixing." The purpose of the bubbles is usually to provide better durability to freezing and thawing by providing relief from buildup of excessive hydraulic pressures within the concrete at the time of freezing. They are also used sometimes to improve workability. Entrapped air, according to the same publication, is "air voids in concrete which are not purposely entrained." A small amount of entrapped air can usually be expected and, unless it produces unwanted bugholes, is not objectionable.