Q.: We've got problems with bugholes in an 8-inch-thick architectural concrete wall that we're building. It's 4 to 6 feet high and the surface looks fine, except in the top 1 foot where lots of bugholes have formed. The air-entrained concrete is colored with an integral coloring agent and is being placed against a plastic form liner to produce a textured effect. We can't vibrate close to the form liner because we're afraid we'll hit it and mar the surface. The concrete is being placed at a 4- to 5-inch slump. We don't know the air content. What can be done to prevent the bugholes?
A.: Try inserting the vibrator at closer intervals and leaving it in the concrete longer. Hand spading concrete near the liner also may help. World of Concrete seminar speaker Ralph Spannenberg suggests several additional steps that may help. An accumulation of bugholes at the top of a wall indicates that they may be traveling upward as the concrete bleeds. Try using a lower slump concrete to reduce bleeding.
Also check the concrete air content. Most integral coloring agents contain water reducers that also may entrain air. If both coloring and air-entraining admixtures are being added at the plant, bugholes may be caused by too much air in the mix. Adjust the air by cutting the dosage of air-entraining admixture. Try to hit close to the lower specification limit for air content.
Finally, you might try vibrating the top 1 foot of concrete using a form vibrator. After topping out the forms and vibrating the concrete internally, hold the form vibrator against the studs or walers near the top of the wall.