Precast concrete walls are made in a controlled plant environment where weather can't interfere with the curing process. They're cast with insulation before they arrive at the construction site. All that's needed is a crane to do the heavy lifting.
Precast concrete walls have a tough exterior that holds up to all kinds of weather, but making these structural walls panels is an inside job. By manufacturing them indoors, they can better prepare them for the outdoors.
Production starts with a mix of crushed rock and sand. This mix is limestone based. The mineral mix can be altered for different strengths and finishes. A conveyor delivers the minerals to a huge mixer. They inject cement, water and additives to improve the flow of the concrete, accelerate the cure and give it a brownish hue. Beaters whip the ingredients into a thick gritty mix.
They cast some of the concrete into cylindrical shapes for testing. After a one day cure, they crush the cylinders with a ram. The amount of force it takes to crush the cylinders is a measure of the concrete batch's strength.
Before they cast the wall, a crew arranges giant foam letters to make an imprint in the mold. They position the letters in the center of the casting form and secure them with silicone. A release agent is sprayed onto the casting form and letters.
High strength steel cables are strung throughout the form. With a hydraulic device, they pull each cable to a specific tension.
Then, they bend a rebar rod into a loop. The rebar loops are linked, creating long cages. They place one cage at the top of the form and one at the bottom. Then, a team inserts a sheet of welded wire mesh. It's been cut to fit the size of the form.
They're now ready to pour concrete. A truck dispenses the concrete directly into the form. The concrete flows around the steel reinforcement. The steel will keep the wall strong when the concrete expands and contracts as the weather changes. The team spreads the concrete and levels it with a tool called a 'Screed'.
This first layer of the wall panel is essentially one side of a sandwich. It will be the exterior part of the wall. At the center of the sandwich is this rigid foam insulation. It's studded with protrusions to secure it to the concrete. More steel reinforced concrete completes the wall sandwich. This final layer will face the inside of the building, so the crew takes extra care to ensure that there are no pits or crevices.
After about 12 hours, the wall is solid enough to be handled. They take it outdoors where a worker sandblasts a small section of the exterior side. This removes the surface cement to expose the aggregate, creating a decorative band next to the embossed lettering.
After a three day cure, the precast concrete wall is ready for the construction site. A crew guides it onto footings. They align its design to the one on the next wall. When the two walls match up perfectly, they bolt it down. The concrete continues to cure over time, so these precast walls will stand strong for many years.
Precast Concrete in Google News
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