Techniques Used in Pre-Cast Concrete

Production

• Precast concrete pours a concrete mix into a mold at the factory. These blocks are then dried and shipped on a truck to the job site, where they are dropped into place by a crane. This means that a builder, sending a precast firm the specifications for a foundation can expect strong, identical and accurate concrete blocks that can be put into place with a minimum of inconvenience. This technique creates less waste, less worry and, most of all, less shrinkage in the foundation after the home is built.

Molds

• Precast concrete is wet concrete poured into molds. These molds are built either out of wood or steel to the exact specifications of the builder's order. Steel is the preferred method, although it is normally used for larger pieces, because it can be used repeatedly without losing its integrity. Wood is a little cheaper, but after a few dozen uses, it becomes weak and waterlogged. Sometimes, a silicone solution is used to seal the concrete in place and prevent any leakage out of the mold. For foundations and other heavy duty pieces, these precast firms add fiber reinforced polymers to the concrete once it is poured. Stronger than steel, this is a lightweight sheet that is spread over the product as it is curing in the mold. It can also be placed on the foundation after it has been laid.

Reinforcement

• For building-grade precast, two techniques are used by precast companies. Thinner pieces, if they do not use FRP, will include a much cheaper steel mesh in the center of the mold. The concrete will be poured over and around the mesh, making certain the mesh always sits in the exact middle of the piece being poured. For many foundation pieces for homes, iron bars are part of the mold, and the concrete is poured over and around it. This encases the bars or mesh in the middle of the piece, making it far stronger than concrete itself.

Quality

• Water and air are the enemies of precast concrete. When used for home construction, air pockets can expand when heated in the summer, possibly causing damage. Water can freeze in the cement, then expand as the weather gets warmer. Water and air pockets can be controlled easier in precast then in on-the-job pouring. One way this is done is by a vibrating table. The wet concrete is set on a large table. The vibrating mechanism shakes the air or water pockets out of the concrete before it is poured. The technique is called “consolidation” and it packs the elements of the concrete closer together, expelling any foreign elements. Sometimes, fly ash is added to dry the concrete and make a smoother, more consistent mix to be poured. Finally, a zinc or cadmium coating is applied to the outside once it dries, creating a seal that makes the piece more waterproof and weather-resistant.