Tuesday, April 19, 2011

Concrete from Heaven

The concrete pour for the main slab, which will be the floor structure for the kitchen, bedrooms and bathrooms, was delayed a couple of weeks - somewhat on account of rain, but mostly on account of the complexity of the formwork. I designed a brick edging detail (above) around the main slab of the house, which requires an 11 1/2-inch wide and 2 1/2-inch deep notch to be left in the slab all the way around. Oh, except for where there will be columns. Then the notch is only 7 1/2 inches. So my foundation contractor had to first build the main forms for the boundary of the concrete (complex enough given how this part of the house bends around the courtyard), then cantilever additional formwork over this to make the 11 1/2" edge, THEN cut out pieces of those forms wherever there will be a column. Of course unique conditions happen at outside corners, and at inside corners, where there also will be columns. The first photo below shows just the vertical formwork, the next was taken after the edging formwork was built, and the last shows the notch at an outside corner where a column baseplate will sit.

My contractor said that when the building inspector came out to inspect the foundation prior to the pour, he asked, "What's all that stuff on top of the forms?" When the brick edge was explained to him, he just shook his head, said "I'm outta here," and approved the foundation. This scenario happened before regarding the temporary back-up wall to my stone wall (Inspector: "You can't have a 2x4 wall 24 feet high." Contractor: "It's temporary, just to lay the stone to."). Apparently the norm for my inspections is: inspector asks silly question, gets silly answer, approves and leaves. I expect this to continue.

Here's the foundation ready for the pour. The white plastic sheet is a vapor barrier - to keep moisture from coming up from the ground and through the slab. The grid is WWF - "welded wire fabric" reinforcing, to keep the slab from pulling apart when it cracks.

Problem: a concrete truck will not fit down my driveway, what with the curve and the trees. My contractor noticed this immediately upon his first visit to the site, and seemed sure that a small concrete pump would work fine. But he since decided against that pump because of all the special engineering that has to be done to the concrete mix - special aggregate, additives, etc. Even with all that, apparently, the pump still gets clogged and is a pain to clean.

So. He decided to ask my neighbor if we could drive a huge pump truck across his driveway to the side of my property, and pump the concrete through its long arm over the trees. My neighbor said sure, as long as we repair his driveway if it breaks.

Yep, it did. But just a small corner. Mostly we just disturbed an ant colony, which rebuilt itself inside the crack within a day.

Here's a video montage from the day of the pour, including some discussion with the contractor, the concrete pump pushing through the trees, a resident lizard, and the "resurrection" of a tree I saved in the courtyard despite being pretty sure it was dead. Also you should get a good sense of why architect Rem Koolhaas described concrete as having "the consistency of vomit." In general it all looked like jazz to me. Hope you enjoy.

1 comment:

  1. That is one serious concrete pumping truck! I've never seen one like that. When we couldn't fit a truck next to our lots, we would use a pump that you pulled behind a normal pick-up truck, with a lot of hose attached to it. It got heavy dragging a 100ft hose full of concrete slush.

    When you were focusing the camera on the sun coming through the tops of the trees and across the boom of the concrete truck, did you notice the jet trail a ways below the sun. Just saying, kind of puts us in our place.