Hang Ten
Great moments in building history: Never forget those basic physics principles
Concrete is hardly an exciting topic for most people, but as a contractor, I feel a little anxious whenever a concrete truck is scheduled. So many things can go wrong on a pour.
Years ago, I was hired to add a kitchen and full basement onto an 18th-century frame and stone house on a bank of the Raritan River in central New Jersey. My crew and I had successfully dug the new basement alongside the antique fieldstone foundation without damaging any part of the old structure.
A week later, the block work was done with new 8-ft. high walls next to but not touching the stone cellar of the old house, and we were set to pour the concrete slab for the new basement. That pour would wrap up the first phase of the project: the foundation and masonry work.
The homeowner, a software engineer, was interested enough in the project to delay going to work so that he could watch the concrete pour. We had no problem placing the concrete for the slab, but when we were done, there was still about 2 cu. yd. of concrete left in the truck.
Rather than waste this material, the owner suggested filling the narrow cavity between the old foundation and the new wall. To do this, we added water to the mix to make “chicken soup,” a thinned mixture that would flow into the 2-in. crevice between the old and new walls.
I climbed on top of the 8-ft. high block wall to make sure that the wet mix went in properly. As the crack filled up, I agitated the mix, prodding it with a piece of rebar to eliminate voids.
Looking back on it, I wish I had remembered a basic physics principle I learned in college: The pressure at the bottom of a dam is proportional to the height of the dam squared. At that point, though, I was just glad to be using the extra concrete and to be giving the foundation extra reinforcement. By the time the concrete had risen to 71⁄2 ft. high, it had created enormous pressure at the base. The bottom of the wall exploded, carrying me on a 2-ton wave of wet concrete and tumbling cinder blocks.
The truck driver, the homeowner and my crew watched in horror as I disappeared from view, falling to the newly poured concrete floor in a masonry cascade. When they looked into the basement, I was up to my armpits in rubble and wet concrete. Somehow I had managed to remain vertical for the entire ride, emerging with only a few flesh wounds and my scarred pride. With concrete hardening by the minute, we faced the prospect of cleaning up, needing to act quickly to dispose of the mess before it became a permanent feature of the new addition.
We lost a week from the schedule as we rebuilt the walls, and the second time around, I decided to leave the harmless 2-in. airspace between the old and new foundation walls.
—Paul E. Mitchell, Frenchtown, NJ
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