Friday, July 31, 2009

Final section of parking deck poured

View of the parking deck from the entrance

The concrete mixture used in the parking garage is a made up of cement, sand, aggregate (small stones) and water. The concrete comes from the pump truck at the street, and is carried through a telescopic boom over to where the pour is happening. The final section of the boom is a rubber hose, seen below. A laborer controls the hose and pours the concrete into the rebar of the deck. The end of hose cannot be more then 4 feet above the placement, because the cement will separate from the aggregate.

With the concrete poured in place, a rake crew working alongside a cement finisher using a hand screed, which is an aluminum 2x4, move the cement to a rough grade. Next, a cement finisher using a power screed (seen below) and laborers with shovels and rakes bring the cement to a finish grade. The power screed had a small engine that vibrates the unit, which helps the stone aggregate settle down into the pour as the cement rises up, smoothing the surface.

With the concrete graded, it begins to set pretty quickly. As seen below, workers can already walk on top of the surface. A cement finisher uses a power float, also called a whirly bird, to pull the moisture up to the surface to prepare the area for the broom finish. Before this machine came around, this portion of the prep required multiple workers using hand trowels. The process is much more efficient thanks to the power float, it can manage a much larger area while maintaining flatness.

Finally, a cement finisher uses a bull float for one last pass to bring saturated concrete to the surface, making the area smooth. Pushing out with the bull float down, the moisture saturated concrete is brought to the surface. At approximately 20 feet out, he twists the pole, which lowers a broom attached to the end of the float. Pulling back towards his body, the broom runs along the surface of the concrete. The broom finish gives a level of coarseness, allowing for traction for both vehicles and pedestrians.

The surface is then sprayed with water to slow down the curing process. The entire area is then covered with burlap bags, which are also sprayed with water. The purpose of this is to keep the concrete from curing too quickly.

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