Manufacturing Precast Products
This is an important aspect of precasting: getting used to the idea that you are a manufacturer rather than a contractor. As a manufacturer, you will need to set up work flow procedures, configure your production area, and establish quality control methods. Of course, you do those things on a construction job site too, but since a job site is a temporary operation, you may be content to work around small inconveniences that in a precasting operation could cost you time and money.
Material handling is an important part of the operation-we talked about that a little in relation to materials. But you also have to be able to handle molds and the finished product. Lifting systems need to be safe, easy to use, and not damage the finished product. Some proprietary systems have been developed to minimize your space requirements, such as Verti-Crete. Forklifts and overhead cranes are the most common material handling equipment. Used equipment is often a good buy when you are starting a new operation since you can usually get it immediately and at a better price than buying new.
Often, lifting inserts are cast into the concrete products. Select inserts that are simple to install and use. There are many commercially manufactured inserts available made from reinforcing steel, steel cable (prestressing strand), or plastic. For precast products that will be outdoors, steel should be galvanized to prevent corrosion. An advantage of commercially manufactured insets is that they have been designed for their rated load. Home-made lifting inserts are cheaper but if one fails, falling precast concrete is very dangerous. Some precast products may lend themselves to the use of special clamps or slings designed just for that product.
Size your mixer to match the size of your castings and the type of concrete you plan to cast.
Molds can be a big expense in a precast operation. Steel molds are available from companies like Del Zotto or Norwalk Precast Molds. These are more commonly used for large precast items, like septic tanks or burial vaults. Molds of expanded polystyrene work well, but will only get a couple of uses-one source for polystyrene baluster or column molds is Hotwire Direct. Most decorative molds are custom made using melamine and rubber-see Concrete Molds for more on that. You can also get someone else to make custom molds for you-such as Architectural Precast Inc. Molds can incorporate form liners and various insets and blockouts-remember that the geometry must allow stripping.
Coat your molds with a form release agent prior to casting concrete. Be careful with block outs, since some materials can react with some form release agents. The best release agents are reactive releases that form a metallic soap to aid in mold removal. Some plastic or melamine molds may be slick enough to demold without a release agent.
Proper consolidation of your precast products is the only way to get good surface finishes and the required strength and durability. In precasting it is much easier to test and perfect your consolidation methods than it is on a job site. Any concrete mix will typically have about 20% entrapped air coming out of the mixer. Consolidation gets that air out, increasing the concrete density which improves bond, increases strength, and reduces permeability-it also eliminates surface voids and bubbles.
A good way to consolidate precast, especially smaller products, is with vibrating tables. The better vibrating tables eliminate any dead spots and can consolidate the concrete in seconds. Some even come with a foot pedal to turn the vibration on, and with controls to adjust both vibration frequency and amplitude to fine tune it for a specific product. Other vibrators used are internal pencil vibrators (stingers) and form vibration (external vibrators mounted on the outside of the forms).
Curing the precast products is critical to getting the quality you want. Protect your precast products from the sun and wind during curing and make sure they have adequate moisture. One good curing method is simply to leave the concrete in the forms. But if you must strip, especially in hot dry weather, maintain high humidity in the curing area.
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