Good Vibrations

by Kim Basham, PhD PE FACI
Created: June 25, 2012

Avoid repair costs and aesthetic disputes in concrete forming projects by minimizing consolidation-related surface blemishes.

Two stages of consolidation: leveling and de-aeration. Due to the oscillatory energy waves, entrapped air bubbles are driven to the form face and move upward to escape.
Two stages of consolidation: leveling and de-aeration. Due to the oscillatory energy waves, entrapped air bubbles are driven to the form face and move upward to escape.
Surface air voids or bugholes are acceptable for vertical cast-in-place concrete walls and columns unless voids exceeds the maximum allowed size.
Surface air voids or bugholes are acceptable for vertical cast-in-place concrete walls and columns unless voids exceeds the maximum allowed size.
Lift lines are typically caused by color variations between two success lifts of concrete. Lift lines can be minimized by completely knitting successive lifts of concrete together.
Lift lines are typically caused by color variations between two success lifts of concrete. Lift lines can be minimized by completely knitting successive lifts of concrete together.
When a concrete placement delay occurs, try to avoid a cold joint by keeping the top of the placed lift plastic by periodically revibrating. When concreting resumes, place a short lift and...
When a concrete placement delay occurs, try to avoid a cold joint by keeping the top of the placed lift plastic by periodically revibrating. When concreting resumes, place a short lift and thoroughly vibrate the two lifts together.
Avoid honeycombing by using appropriate concrete mixtures with sufficient workability, proper placement and consolidation techniques.
Avoid honeycombing by using appropriate concrete mixtures with sufficient workability, proper placement and consolidation techniques.

Cold joints

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.