Lake Austin Build Along: Foundation Pour

Our Lake Austin build is progressing nicely and its foundation was recently poured. Adrian, our construction manager, was kind enough to write the following blog entry and share his photographs from the pour. 

The foundation pour is one of the most exciting days on the project. It starts early, about 6 am or sooner depending on the neighborhood restrictions. Our superintendent meets the concrete truck at the job and gets him positioned in the best location.
Double pour
We'll specify where the trucks will wash out their drums after they unload. Thought should be paid to this so that trucks can be efficiently moved around the job and so that the spoils from the wash out are not driven through and are easy to dispose of after the pour. About 30 minutes after the pump truck is scheduled to start, the concrete trucks should start arriving. Spacing the concrete truck's deliveries is a critical element of the pour. If the spacing is too short, the trucks will get backed up and concrete may start hardening in the drum before they can get it to the pump hopper.
If the spacing is too long, there will be down time between trucks, and the crew will be waiting on concrete. Spacing should be such that 1 truck is waiting so that as one empties another can move in and keep the flow of concrete smooth and consistent.
rebar 2
The first thing we do when the pour starts is to fill all of the interior and exterior beams. Passes are made down each beam to fill them to the top.
more pour
On the following pass, the top of the foundation is filled. The whole time the crew is moving the pump boom around, there will be a crew member following with an immersion vibrator to vibrate the concrete as it is poured. This vibrating is critical to make sure the concrete flowing into and fills the forms completely. It helps remove air pockets and allows the concrete to flow into all the nooks and crannies. Over vibrating is bad, for most concrete mixes, over-vibration creates the problem of segregation in which the denser aggregates settle to the bottom while the lighter cement paste tends to move upwards.

Estimating the  volume of concrete which will be needed is a skill that is honed through experience. A volume estimate is calculated and an order placed with the concrete plant. However, after every truck has emptied it is wise to recalculate the quantity needed and fine tune the amount until the foundation is full. You do not want to be short, but you also don't want 4 trucks waiting after you finish.

Once all of the concrete has been placed in the forms then it is time to start finishing. First a pass is made with a tamper (sometimes called a Jitterbug) to push the coarse aggregate below the concrete surface and consolidate the concrete. Most tamping is done with the finisher standing in the wet concrete, although there are roller tampers that can be used from outside the forms. Tamping should only be performed on low slump concrete. With high slump concrete, the coarse aggregates sink naturally and tamping can cause segregation of the aggregates.

A large metal or wood board is used to screed the top of the concrete. This screeding process helps compact and consolidate the concrete, and begins the smoothing and leveling of the top of the concrete. Once the surface has been screeded, the concrete is floated. This involves using a special trowel called a float. Floats can be a small hand held trowel for edges and detail work, or a large trowel called a bull float for working large areas of the concrete surface. The surface is floated to further compact the concrete, even out any depressions or high areas, and create a smooth finish on the surface. At the same time early finishing takes place, joints and edges are worked into the concrete with special hand tools.

The concrete will be left to rest until the surface begins to firm up. Once firm, steel troweling is performed to create a smooth, hard and uniform finish across the concrete surface. Steel troweling can take place by having contractors "skate" across the surface on knee boards troweling small areas at a time, or with larger trowels on poles. Then larger gas trowel machines, or whirlybirds, are used to give the top of the foundation a nice polished finish.
While the finishing is taking place, a crew member should place all of the frame anchor bolts around the perimeter of the foundation for the exterior walls to be attached to.
The form boards will be removed as the concrete starts to set. This allows the vertical planes of these areas to be finished and the holes left from the forms to be filled and finished.
After all of the finishing has been completed the slab can be left for 12-24 hours and then all of the forms can be removed. After all of the forms have been removed and the site graded and cleaned of all foundation work, frame stage can begin.
"We have been extremely happy with every process of the construction of our home with Jenkins Custom Homes."

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