Cold joints are different than lift or layer lines. Lift lines occur when successive lifts of concrete are not completely knitted together. In many cases, lift lines occur because of color variations between loads of concrete and do not indicate there is a joint or discontinuity in the concrete. In contrast, a cold joint is a discontinuity between two lifts of concrete. They occur when, due to a concrete placement delay, the earlier lift of concrete hardens sufficiently to preclude knitting and bonding of the two lifts. Except for the unsightly appearance, cold joints are typically not a concern unless the concrete is unreinforced or an environmental tank.
Surface air voids
Commonly called bugholes, surface air voids are small, regular or irregular cavities, usually less than 58 inch in diameter, caused by entrapment of air bubbles in the surface of formed concrete. Bugholes are normal for vertical cast-in-place concrete and not considered a defect unless voids exceed the maximum size specified by the contract documents.
Minimize bugholes by using: smooth, impermeable formwork; the thinnest coat possible of an appropriate release agent; limited lift thicknesses; high-frequency vibrators; and proper vibrating procedures with sufficient periods of vibration to de-aerate the concrete.
Contractors will save time and money when formwork consolidation it done right. The key to avoiding repair and aesthetic disputes is both knowing why surface blemishes occur and how to properly vibrate the concrete.
References: ACI 309R-05 Guide for Consolidation of Concrete, American Concrete Institute, www.concrete.orgACI 309.2R-98 Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces, American Concrete Institute, www.concrete.org
Kim Basham is president of KB Engineering LLC, which provides engineering and scientific services to the concrete industry. Basham also teaches seminars and workshops dealing with all aspects of concrete technology, construction and troubleshooting. He can be reached via email at KBasham@KBEngLLC.com.