From the June 2, 2014 PRECAST Magazine post Causes and Fixes for SCC Bug Holes, by John Pelicone
Like a persistent mosquito, one question has plagued precast concrete producers for years: “How can I eliminate bug holes?” In the past, this question was much harder to answer, because concrete was placed at a stiffer consistency that required excessive vibration. And excessive vibration sometimes caused more bug holes. After the introduction of self-consolidating concrete (SCC), bug holes(ii) became a less common occurrence. Yet, as a recent online industry discussion revealed, this perturbing problem is still with us.
"Two types of release agents
- Chemically reactive agents: When a chemically reactive form release agent is used, a nonviolent chemical reaction takes place when fatty acids react with free lime on the surface of fresh concrete. This reaction results in the formation of a metallic soap, a slippery material that allows air bubbles to rise along the vertical surface. This “soapy” film also prevents the hardened concrete from adhering to the forms during stripping.
- Barrier release agents: Thicker coatings on forms are typical of the older barrier-type materials, like heavyweight used motor oil, vegetable oils, diesel fuel and kerosene. Barrier type release agents are less expensive than chemically reactive agents, but they are not generally recommended for reducing SCC bug holes."
"Bug Hole voids are formed during placement. Small pockets of air or water are trapped against the form. The problem increases with the height of the lift. Vibration may not be adequate or well spaced. The mix may be sticky.
- Primarily caused by the way concrete is placed and compacted
- Entrapped air not removed by vibration, air bubbles move to the form
- Improper application of Form Release agent or wrong type
SOLUTION I PREVENTION: Avoiding Bug Holes
- Work the voids at the form face up and out of each lift
- Let the vibrator drop through the lift, then vibrate upward
- Don't overvibrate at the center of the wall
- Move the vibrator as close to the form as possible
- Add upward external vibration if necessary
- Reduce the height of each lift to make void removal easier
- Aggregate - consult ready mix producer and review aggregate size and shape
- Reduce sand content
- Use low slump concrete"
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