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How is concrete made?


Mixing and Pouring Concrete
A construction worker pours some concrete from a rotating drum mixer.
A construction worker pours some concrete from a rotating drum mixer.
Alistair Berg/Lifesize/Getty Images

­After all the required dry materials have been assembled, they typically go into one of two destinations: a bag or a mixer. Ready-mix concretes are premixed bags of dry concrete ingredients that only require water and mixing. These are ideal for small projects, and manufacturers tend to offer mixtures that are ideal for various specialized home projects. Users can generally add water in a mixing ­pail and apply the concrete with a shovel.

If you're looking to pour a lot of concrete though, premixed bags get expensive really fast -- to say nothing of the packaging waste. For larger projects, some construction companies mix concrete on-site. Otherwise, mixing takes place at the concrete plant, and the batch is subsequently transferred to the job site in a rotating drum mixer. These devices keep the concrete from setting by constantly tumbling the mixture around and are usually either truck or trailer mounted. However, this hot potato act can't go on indefinitely. Most batches of concrete need to be discharged 90 minutes or 300 revolutions after the addition of water. After that point, the batch experiences slump loss, which refers to decreased workability. If construction crews know in advance that time will be an issue, certain admixtures can make it possible for concrete to travel longer in drum mixers.

Once at the job site, it's time to pour the concrete into prearranged forms. These wooden molds are carefully measured to meet the necessary slab specifications. In the case of reinforced concrete, the form will contain steel rods or mesh. Then the concrete can be compressed and smoothed over as needed. Temperature and moisture are both important factors in proper concrete drying. As the concrete dries, it inevitably shrinks. To prevent this process from taking place unevenly and potentially warping the finished slab, it's important to keep the surface of the concrete damp to slow the shrinkage uniformly.

Extremely low or high temperatures can also pose a problem. Extreme heat will cause rapid curing, while chilly temperatures can draw the process out and produce weaker concrete. As such, summer construction crews typically work during the cooler portion of the day and use warming additives during cold weather. Calcium chloride, for instance, will speed up the chemical curing process, which produces heat. A good sealant will help protect the concrete once the slab has fully dried.

Just as you can avoid baking mishaps by simply buying a prebaked pie, you can also purchase various sizes and shapes of precast slabs directly from the concrete plant. Sorry, mobsters -- they're typically not available in boot size.

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