Blow holes are individual, generally rounded, cavities on vertical surfaces of concrete, generally less than 10 mm across. They are caused by air in the concrete being trapped against the form face, sometimes due to insufficient vibration. Some blow holes are almost inevitable unless a permeable formwork material is used. The occurrence of blow holes can be minimised by the use of a suitable release agent on the surface of the formwork and the use of adequate vibration. In addition the concrete should have adequate workability. Blow holes are a cosmetic problem only and will not affect the long-term performance of the concrete structure.