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:
- Improper selection and application of form release agents
- Problems with SCC mix design (cement, water content, viscosity, admixtures)
- 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).
Two types of release agents
- 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.
- 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.
International input on SCC bug holes
The following is advice from industry experts on bug hole causes and remedies.
Juan Manuel Pereira, Concrete Quality Software, Spain (concrete-quality.com)
“Generally, oils will give the worst finish, especially if applied in excess. Use a wax-based mold-release agent in a thin layer (like polishing furniture), which gives excellent results. They are more expensive, but, when you do the math, the cost per square foot is negligible. Maintaining good form condition is also important.”
Jeff Bishop, precast division manager, Nox-Crete, Nebraska (noxcrete.com)
“Too often, the form release application equipment is inferior, the sprayer has a faulty tip, or the equipment fails to maintain the minimum pump pressure to adequately apply a thin coat on the form surface. Not many plant workers will make frequent stops to check or correct the pressure after they have started spraying. We spent three years developing quality pumping and spraying equipment that ensures a consistent, fine spray.
“Sometimes with truck deliveries, the discharge chute gets ahead of the initial hydraulic head formed by flowing SCC; this can cause entrapped air. You have to make sure you are adding concrete to concrete in such a way that the head is maintained. As the SCC flows down the form and up the side of the form, entrapped air is pushed forward, up and out.”
Cecil Wilson, plant manager, Metromont Corp., South Carolina (metromont.com)
“There are three reasons for SCC bug holes – it’s 1/3 form release agent, 1/3 the mix and 1/3 how the concrete was placed. All three potential causes need to be checked systematically, one at a time, so you can pinpoint the problem. It is imperative to pour SCC so that entrapped air has the opportunity to escape(iv).”
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.”
John Stewart, global business development manager, Ecoratio, The Release Agent Co., Great Britain (ecoratio.com)
“The main thing is to use a good release agent. Where possible, revert to a top-class release agent rather than a mold oil. This will help surface air escape quickly rather than being retained by a thick oil.”
Alexis Borderon, Reval Stainless Steel & Concrete, Italy (reval-stainless-steel.com)
“We have tried coconut oil, fat, oil-free release agents – all kinds of miracle products from world-class salesmen. I certainly learned that whatever product you use, it must be applied in a thin layer, concrete must be poured close to the form, and never let fresh SCC be poured from a height! We never vibrate.”
Sam Strong, president, Strong Products LLC, Michigan (strongproductsllc.com)
“Release agents generally average $7 to $10 per gallon, but specialized applications may call for a more expensive product. A precaster’s cost and time concerns can result in a poor choice of release agent. A cheaper price may look good but can lead to more labor cost down the road after you have product staining and bug holes. Truth is, product reps rarely see forms being wiped-down or mopped according to manufacturer’s instructions, especially at the smaller precast operations.”
Brian Robinson, continuous improvement manager, Humes Pipeline, New Zealand (humes.co.nz)
“You should try a few different mold release oils – diesel-based, bio oil-based, etc. Suppliers should bust your door down to provide some free samples; stay away from your current so-called ‘cheap’ option. A good release agent that works will end up a similar price – if applied sparingly (correctly), it will result in better quality products. It’s a process of elimination; using different release agents will help you determine if the mix has any issues.
“You’ve got to place from one point in the mold and let it flow. We noticed the SCC places better in the mold using about a 150 mm-diameter opening – not a wide-mouth opening. Also, if the concrete needs to fall a considerable height, a tremie pipe or sleeve works well. This prevents segregation and removal of release agent by falling concrete (which also affects the finish) and reduces bug holes. We never need to mechanically vibrate (we outlaw this) but large, deep tanks may need a small amount of rodding (12 mm-diameter ) reinforcing bar, rodded up and down a dozen times at the opposite side of the mold (from where the concrete is placed) to prevent a discolored seam.”
Greg Stratis, manager, Shea Concrete Products, Massachusetts (sheaconcrete.com)
“We don’t use external vibration for our SCC products – you can quickly over-vibrate the mix. We use SCC on 95% of our product. A lot of our products are vaults with reinforced 3-in. or 6-in. walls. Our SCC design is 5,000 psi with 26-in. spread.
“We are able to get a smooth finish without vibration. In my opinion, if bug holes are present on our product using this mix, it is usually due to too much form oil or a change in our oil consistency. But generally, I would say that the cleanliness of the form and the quality of the release agent used are critical to preventing finish bug holes in the concrete surface.”
Claudio Subacchi, director of R & D, Hawkeye Pedershaab, Iowa (hawkeyepedershaab.com)
“To make sure it is not your release agent, try to make a pour after waxing the surfaces. Sometimes fatty acids (probably the form release agent) generate some bug holes. For wax, we do not use an oil-based one and avoid ones that have beeswax, because they have sugar residues that can potentially cause other kinds of problems. We use a very thin layer of carnauba oil and let it dry, and then cast. If the bug holes go away, then it is your release agent; if not, ask your admixture supplier for a defoaming agent.
“Typically on large architectural surfaces, you do not want to have the SCC travel more than half a meter on a 40 mm-thick pour. If that happens, you may not have bug holes, but you get a shadow from the release agent. See my video for a demonstration(v).”
Todd Leber, chief inspector, Nebco Inc., Lincoln, Nebraska (nebcoinc.com)
“We have had success using two form release agents casting SCC. The key is to apply it as thin as possible. Contact your local distributor to find the right material for your application.”
So in conclusion, let’s summarize the industry’s consensus from for preventing SCC bug holes in three points:
- 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);
- Maintain proper SCC placement from one pour location in the form, remembering that a slower placement rate allows entrapped air to escape; and
- Maintain forms in clean condition.
John Pelicone is a private consultant for Big River Industries Inc. and has worked in concrete materials, testing and sales in the precast and prestressed industry for more than 40 years. Contact him at [email protected] or (770) 682-9896.
ii According to PCA (Portland Cement Association), “The ever-increasing use of structural concrete as an architectural building material has catapulted quality in surface appearance to a prominent position within the concrete construction industry. One of the primary influences affecting the surface aesthetics of concrete is bug holes. Bug holes are surface voids that result from the migration of entrapped air (and to a lesser extent water) to the fresh concrete-form interface. These surface defects manifest themselves mostly in vertical surfaces.” http://www.cement.org
iii See: SCC Part I (characteristics, aggregates and equipment) and Part II (troubleshooting and test methods) in Precast Inc. magazine, January-February and March-April 2014 for detailed information on mix design and control of free water.
iv Best Practices Guidelines for Self-Consolidating Concrete, prepared by the Ready Mixed Concrete Association of Ontario, January 2009, page 10. RMCAO recommends: “For large vertical elements care should be taken not to fill the formwork too rapidly. The placement rate should be slowed to the point that there is sufficient time for the entrapped air to rise to the concrete surface. Since air movement can only take place when the SCC is itself moving into the formwork, slowing the placement rate may assist in removing unwanted air pockets at the formed face of the concrete.”