(This week's post is an educational article by Bill Rostine from "Pre-Cast Concrete News." )
Concrete formwork is one of the biggest investments a precast or prestress plant has. Therefore, taking care of the forms extends form life and protects a valuable investment. This makes economic sense and contributes to a healthy bottom line.
Care of concrete forms starts with the day-to-day use. Problems on steel forms can occur with inadequate cleaning or with excessive use of wire brushes or sandblasting. Form surfaces can also be damaged by internal vibrators.
Another important aspect of form care is the selection and proper application of a good release agent. Proper application of release agents is necessary for economy and for producing the best product possible and for minimizing form clean up.
There are three main types of release agents.
The first is the barrier type. These products, as the name implies, provide a barrier between the concrete and the form allowing it to strip. The original, modern-day, form oils were barrier types-diesel fuel, greases, used motor oil, etc. These allowed for a good release, but negatively affected product quality by causing surface voids, staining, and over all poor product appearance. They are heavier in nature and are harder to apply due to their higher viscosity.
The second type of release agent is the reactive type. These are chemically active and contain compounds that react with the free lime in the concrete to produce a soap-like film between the form and the concrete. This type of release agency is the most widely used. They are easily applied in a thin film. Because they can be applied in a thin film by spraying, wiping, or brushing, you can produce stain-free, void-free concrete surfaces. Reactive type release agents applied in a thin film allow the form to strip cleaner which saves on labor costs related to form cleaning and extends the life of the form.
The third type of form release agent is a combination of the other two types.
Coating manufacturers and coating formulations, including those of release agents, were affected by the Clean Air Act of 1990 passed by Congress. This law allowed the Environmental Protection Agency to set into motion a process to limit nationwide the amount of Volatile Organic Compound (VOC) that a coating should contain. VOC’s are thought to react with sunlight and cause ground level ozone or smog. The original law called for testing or different VOC’s to see how much, if any, reactive capability a given VOC had with sunlight and the formation of smog. When this testing wasn’t done properly many coating manufacturers (mainly California paint companies and other concerned parties) took the EPA to court to fight the law. After several court fights, lawsuits and rulings during the1990’s, the EPA prevailed and the law limiting VOC’s was allowed to take effect. In September of 1999 release agent manufactures and concrete producers were required to make and use limited VOC products. Some companies, including Hill and Griffith, saw this coming years in advance and were already producing VOC compliant products. All new products were formulated based on the new EPA law. Some states, such as California have stricter rules than that passed nationally.
There are four main application methods-spraying, wiping, mopping or brushing and dipping. Spraying is probably the most common and efficient method of application. Care should be taken to avoid over application for the reason mentioned earlier. An extremely thin film of release agent can be applied with proper equipment and technique. Working application pressures of 35 to 50 psi are best. Pump unit sprayers or centralized systems with air pressure regulators give a good consistent pressure and work well. Higher pressures put more airborne particles in the air throughout the plant and can be harmful to personnel in the plant. Lower pressures allow for over application, puddling in the form, and waste. A flat fan spray nozzle of .5 gpm maximum size will work with many release agents. Better yet is a flat fan spay nozzle of .1 or .2 gmp spraying a good thin release agent. Many of these thin, chemically active release agents are more expensive per gallon, but with coverage rates at 2000-2500 sq. ft. per gallon the applied cost is much less than a cheaper, less effective release agent. A second type of application is wiping on the release agent. This can be done with a rag or sponge with excellent results. Architectural precasters like this method because over application is eliminated. Some burial vault manufactures use a sponge for application because they clean the form with every application of release agent. A third type of application is by mopping or brushing. When using this method over application can be a problem. As with wiping, the application device must be wrung out in order to achieve the desired results. If puddles occur, they should be wiped up. A fourth method of application is dipping. Dipping systems are fast, labor efficient, and assure total coverage of the form. Dip tanks collect the excess release agent that drains off the form. Over application is probable, but this type of application works well in some operations. Whatever method is used for applying the release agent care should be taken to keep it off reinforcing steel.
Another important aspect of form care is protection of forms in storage. Precast concrete producers, especially those with diverse product lines, occasionally take forms out of service between jobs. The investment in forms needs to be protected from rust and corrosion. Some producers user grease, diesel fuel, or release agent and apply it to the forms. Application with these products needs to be done irregularly with limited protection of the forms. A better choice is a good rust preventative. When shopping for the best product, there are four things to consider.
First is quality of protection, second is length of protection, third is ease of application, and fourth is ease of removal.
Formwork is a major investment for precast concrete producers. Utmost care should be taken to insure product quality and exend the life of these valuable assets. Money spent taking care of forms now can save hundreds or thousands of dollars in the future and make a concrete business more profitable.