One of our customers is having a problem with excessive bugholes on the surfaces of walls that are at a 45-degree angle. We have offered every idea we can think of in trying to help this customer to reduce the immense amount of after-the-pour finishing work required for the walls to meet the inspector's quality check.
Before the project started, the contractor first cleaned the surface of the metal form in an attempt to return it to its original surface quality. After the first pour and his discovery of the bughole problem, the foreman experimented with form oils of several different types and makes. Even after all of the prep work and experimentation, the contractor is still getting about 25-30 bugholes approximately 1/4-1/2 inch in size per foot.
We've been working on this as well. I have tried supplying various mix designs. The current mix seems to have eliminated any bugholes at the bottom of the form, but they keep emerging at the top.
During the pouring process, the contractor uses a vibrating screed to finish the top of the form and several different vibration methods, but he has had no luck on removing the air from the 45-degree angle on a 24-inch octagon pile.
Filling forms at an angle is about the most difficult situation in terms of bughole problems. There is no surefire answer, but we've come up with a couple of suggestions.
Start by educating your customer on how to determine the quality of the concrete surface finish. Guide for Surface Finish of Formed Concrete, which is available in the World of Concrete Bookstore (www.wocbookstore.com), provides opportunities to rate the finished quality of a vertical surface. The guide, developed with help from the American Society for Concrete Construction, includes photos with which an inspector can objectively determine the quality of the surface. This can take the guesswork out of what ôacceptableö means.
Then we suggest reviewing the mix design. Check that you're providing a gap-graded mix. (In very simple terms, a properly gap-graded mixture is one where there's a sufficient amount of 3/8-inch pea gravel or chips.) This holds the sand and coarse aggregate portions of the mix together. Reducing the sand percentage of the coarse-to-fine aggregate ratio also reduces the chance of bugholes.Many producers have been developing self-compacting concrete mixes. From one mix-design perspective, the self-compacting mix is just an enhancement of a properly designed gap-graded mix. But many producers also offer a mix that includes a new type of admixture known as a viscosity-modifying admixture (VMA). Many producers, especially precasters who work with heavily reinforced elements, report good results in both final surface texture and workability with VMAs.You might also advise the customer to take another look at placing techniques. One of our staffÆs problem solvers suggested filling the form by pumping the concrete from the bottom. This would reduce the entrapment of air during the pour.
Another suggestion, from ACI 309, Guide for Consolidation of Concrete, is a procedure that has provided surfaces nearly free of bugholes in precast work: continuously placing ribbons of concrete 2-4 inches (50-100 mm) thick while continuously vibrating it.
ACI 309.1 and 309.2 recommend revibration, particularly for wetter mixtures, to eliminate water gain under reinforcing bars and reduce bugholes (especially in the upper portion of deep lifts), all of which will increase the strength of the concrete. Revibration is most effective when done as late as possible when the vibrator head will still readily penetrate the concrete.
The customer could also try high-frequency, low-amplitude vibration, which generally results in better consolidation and better surfaces (fewer bugholes) for more plastic consistencies. (High frequency means greater than 100 Hz.)
More flowable mixtures may be consolidated by rodding. Spading is sometimes used for 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 bugholes in the formed concrete surface.