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Concrete Casting News from the Hill and Griffith Company

Concrete Bug Hole Prevention

Posted by Hill and Griffith Company on Feb 9, 2016 3:29:38 PM

From the Portland Cement Association site,

bugholes_th.jpg"Bugholes are surface voids that result from the migration of entrapped air (and to a lesser extent water) to the fresh concrete-form interface. These surface defects manifest themselves mostly in vertical surfaces."

All Grifcote products are chemically formulated to help reduce the formation of bug holes. Reactive fatty acid and or methyl esters react with free lime on the surface of the concrete to form a metallic soap that eases separation from the form. Bug holes are minimized due to the metallic soap formed after the casting process that minimizes adherence of air to vertical sidewalls, allowing free air to rise more easily to the surface. The release characteristics are enhanced with the formation of the metallic soap, which also minimizes cleaning of forms after stripping.

From the June 2, 2014 PRECAST Magazine post by John Pelicone, 

"Two types of release agents

  1. 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.
  2. 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."

In summary, this is a good PDF from CalPortland,

"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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