Wednesday, March 30, 2011


Our concrete subcontractor poured the foundation footings today. "Footings" is an ambiguous term, since it can mean both the full foundation piece supporting the steel column, or it can mean just the broad pad that supports the vertical piece of the foundation (the pedestal). In our case, we just poured the bottom half, which takes the weight of the column and spreads it out while transferring it to the ground.

The cage of steel rebar inside the concrete handles the tensile forces within the footing, while the concrete takes care of the compressive forces. From a sustainability standpoint, the steel is almost entirely recycled, and the concrete mix includes steel blast furnace slag as a replacement for half the cement that would ordinarily be used. The aggregate and sand that comprise most of the concrete mix are produced locally, to minimize the embodied energy that comes from transporting materials.

One of the steel rebar cages before the pour. The bars in the footing are a smaller diameter (5/8") than the vertical bars in the pedestal (7/8"). I'm sure there's a good reason for that.

One of the footings, after the pour. The concrete will cure (to its full compressive strength of 3,000psi) over the next 28 days, before we load it with structural steel.

The excavations for the footings, the string used to line up the rebar cages, and the temporary wood supports for the rebar have the look of an archeological site. Next step: laying the utilities that pass under the slab, like plumbing lines and electrical conduit. Then we'll set up the anchor bolts for the steel frame, and pour the rest of the foundation. In a couple of weeks, most of this will be covered by the house's concrete floor slab.

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