Q: We are having a problem with bug holes on a 45-degree sloped formed surface. We have tried everything we can think of to try to cut down on the immense amount of finishing work that we are doing. So far we have tried cleaning the skin on the metal form to perfection, trewax, Cresset 880 at all applications of thin and thick, 50/50 oils, a vibrating screed that runs on the top of the form, 1-inch vibrators,1.5-inch vibrators, different vibration methods close to the form and in the center of the form, with still no luck on removing the air from the 45-degree angle on a 24-inch octagonal pile. The bottom of the pile seems to be producing extremely good results. Is there a solution to our problem? Our form is in medium condition but obviously has the potential to produce a pile with no bug holes or fewer than what we are currently achieving, which is about 30 bug holes approximately 1/4 to 1/2 inch in size over the span of a foot. We have tried various mix designs as well but have eliminated this as a cause since the bottom part of the pile looks so good.
A.: Filling forms at an angle is about the most difficult situation in terms of bug-hole problems. There is no surefire answer, but these things might help:
Try a gap-graded mix with careful control of the sand content (the less the better from the standpoint of bug holes). This seemed to work well at the Pulitzer Museum.
Try pumping the mix in from the bottom of the form. There wouldn't be air mixed in from the placement process. A fluid mix would be best.
The best solution might be self-consolidating concrete. (See "Self-Compacting Concrete" Concrete Construction, January 2002, p. 44.)
Try high frequency (>100 Hz), low amplitude vibration, which generally results in better consolidation and better surfaces (fewer bug holes) for more plastic consistencies.
Plastic or more flowable mixtures may be consolidated by rodding. Spading is sometimes used at formed surfaces. A flat tool is repeatedly inserted and withdrawn adjacent to the form. Coarse particles are shoved away from the form, facilitating movement of air voids and water pockets toward the top surface, thereby reducing the number and size of bug holes.
Another procedure that has given good results in precast work involves continuously placing ribbons of concrete 2 to 4 inches (50 to 100 mm) thick, accompanied by continuous vibration. It can produce surfaces nearly free of bug holes.
ACI 309.1 and 309.2 recommend revibration, particularly for the wetter mixtures, in eliminating water gain under reinforcing bars, reducing bug holes, especially in the upper portion of deep lifts, all of which will increase the strength of the concrete. Revibration is most effective when done at the latest time possible when the vibrator head will still readily penetrate the concrete.