Using concrete as an architectural material has brought the quality in surface appearance to an important consideration.
One of the problems affecting the surface aesthetics of concrete are bugholes. Bugholes are surface voids from the migration of mostly entrapped air to the fresh concrete form surface, mostly in the vertical plane.
Photo from Precast Inc. Magazine's May/June 2014 issue.
During setting, the shrinkage of the concrete forces entrapped air voids and excess water out of the mix. Water migrates upward due to density and becomes bleed water. The air bubbles seek pressure equilibrium and when in a vertical form, that's to the interior surface. These bubbles need to be directed vertically to the surface of the concrete form. Bugholes are found in the upper portion of the concrete structure or at angled form surfaces as a result of accumulation of escaping air along the height of the structure. They are primarily an aesthetic problem for exposed surfaces.
The biggest cause of bugholes is improper vibration. Consolidation through vibration, sets the bubbles into motion and sends both entrapped air and excess water to the surface.
Bughole formation can also be caused by the form material and the type of form release used.
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.
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."
Mix design can also contribute to bughole formation. A sticky or stiff mixture that is hard to consolidate can increase surface void formation.
The vibrator should penetrate the previous lift and work the entrapped air towards the form and then up the sides. More vibration is necessary with impermeable forms, to move the air voids to the free surface of the concrete.
Flowing mixtures reduce bughole formation. Concrete that limits excessive fine aggregate, has the proper cement content, and uses admixture for increased flow contributes to bughole reduction. Self-consolidating concrete is becoming increasing popular for precast to improve surface quality.
Bugholes are not detrimental to structural concrete. But, with the increased use of concrete in finished construction, surface quality is important. Through careful selection of materials, quality workmanship, and dutiful supervision, surface voids can be minimized.
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