Precast Concrete Quality Control
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Precast Concrete Quality Control
If you are going to manufacture precast products in your shop, you should consider doing your own quality control. Large precast operations have big labs and develop intricate quality procedures, testing all aspects of the concrete both before and after casting and developing extensive QC tools like fishbone charts and scatter diagrams and Pareto charts. You may not need to get to that level, but some basic testing and analysis can save you money both in production costs and higher quality precast products.
Starting with good materials is the first step. All aggregate is not acceptable for use in concrete. If there are organic materials, shale, chert, or other soft materials in your aggregate, you can’t produce good concrete. Reactive aggregates can lead to alkali-silica reaction that can destroy your concrete. The best defense may be simply to require that the aggregate meets ASTM C 33, “Standard Specification for Concrete Aggregates.” It’s your aggregate supplier’s responsibility to assure that you are getting good aggregate.
Triparish Testing and Inspection
Sampling the fresh concrete is critical and the simplest test is slump. Although not always the best measure, slump does indicate workability, which Adam Neville, in his book “Properties of Concrete,” defines as “the amount of useful internal work necessary to produce full compaction.” Slump can vary dramatically, though, when admixtures are used. The best use for slump is to give you some indication of whether the mix will flow easily into the forms and to monitor different batches of the same mix design. All else equal, different slumps indicate different amounts of water, but slump variations can also indicate changes in entrained air content, aggregate moisture, or temperature.
The temperature of your concrete is another important thing to test. Cold concrete can set very slowly and will mean waiting longer to strip the molds. Hot concrete gains strength faster, but the ultimate strength will be lower. Higher temperatures can also reduce air entrainment and workability. Hot concrete can lead to cracks as thermal gradients develop.
Entrained air content is something you’ll have to control for concrete that is to be used outside where it will be exposed to freeze-thaw cycles. Without any air entraining agent, well-consolidated concrete will have an air content of about 2% (this will be entrapped air, not entrained). For concrete that is to be used outside, you will want an air content around 5 to 6%. This is achieved using an admixture called an air entraining agent-basically a soap that makes the concrete froth when mixed. But you need to be careful to control the air content since too much can reduce strength. Air content can be difficult to control and can change from temperature changes, changes in admixtures, and different handling methods. Air content is measured with either a pressure meter or a volumetric meter (also called a roll-a-meter). You can also monitor air content pretty easily just by running unit weight tests (ASTM C 138).
Finally, you may also want to monitor the strength of your concrete. For cast-in-place concrete, 6×12-inch cylinders are standard. You would probably want to send those cylinders to a lab for testing. You can also make 2-inch cubes, following ASTM C 31. Either of these will tell you the compressive strength of the concrete, which is an important indicator. Checking your concrete’s compressive strength at various ages, may help you decide when the precast product can be put into service and might also allow you to fine tune your mixes.
3 Rules for Successful Precasting
There are three rules for successful precasting:
- Weight/size-the weight and size of the unit cast must be something that you have the equipment to handle and move and it can’t be too big to move into place at the job site.
- Pull-out-the shape of the precast unit must be such that it can be stripped from the mold or the molds must have the flexibility to handle the shape.
- Repetition-the more times you will make the same unit, the more effective precasting is as a manufacturing process, although some custom jobs also lend themselves to precasting, especially countertops and furniture.
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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