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Quality Control for Precast

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.

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, 6x12-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.

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