Like a persistent mosquito, one question has plagued precast concrete producers for years: “How can I eliminate bug holes?"
In the past, this question was much harder to answer, because concrete was placed at a stiffer consistency that required excessive vibration. And excessive vibration sometimes caused more bug holes. After the introduction of self-consolidating concrete (SCC), bug holes(ii) became a less common occurrence. Yet, as a recent online industry discussion revealed, this perturbing problem is still with us.
Let’s focus on production’s tail end
The first thing we all learn about SCC is that it’s a tricky devil to work with. There is no room for error, consistency and control are king, and problems, like bug holes, can have more than one cause.
Most online commentators agree that there are three main causes of bug holes:
1. Improper selection and application of form release agents
2. Problems with SCC mix design (cement, water content, viscosity, admixtures)
3. How SCC is placed in the form
Rather than trying to cover all possible sources of bug holes, I decided, like the online commenters, to focus on form work, placement and form release agents. Attempting to cover complex SCC mix-design issues would be too unwieldy for one article(iii).
National Precast Concrete Association / Precast Magazines / Precast Inc. Magazine / 2014 – May-June / Causes & Fixes for SCC Bug Holes / Causes and Fixes for SCC Bug Holes
By John Pelicone, June 2, 2014
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.
A problem with the heavier, thicker barrier agents is that the flowing SCC may actually push the release agent down the vertical face of the form, thereby enfolding or entrapping air pockets that lead to surface bug holes.
(The article continued with comments from industry leaders.)
Bob Waterloo, technical sales manager, Hill & Griffith Co., Indiana (hillandgriffith.com)
“Training. I’m a huge proponent of training workers on proper application. ‘Thinner is better’ is what I advise plant workers during my training sessions.
“Here’s an analogy: Think about waxing your car. You put on a thin coat and then buff it out; it’s the same with release agents. In fact, the coating should be thinner than a wax finish on a car.
“But do workers always take the time to mop or rub down forms after spraying? Labor is a major expense for all precasters, and the person prepping forms may not follow proper application methods if he knows the forms are needed in production ASAP.”
So in conclusion, let’s summarize the industry’s consensus from for preventing SCC bug holes in three points:
1. Use the thinnest application possible of a quality form-release agent, using superior sprayer equipment (pumpers maintained at proper pressure and sprayer tips/nozzles in good condition);
2. Maintain proper SCC placement from one pour location in the form, remembering that a slower placement rate allows entrapped air to escape; and
3. Maintain forms in clean condition.
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