Friday, April 4, 2008

Exploding concrete

I’ve done a bit of building around our place. Twenty years ago, I built a shop. I don’t do the excavation, the pouring of the concrete, the brick work, the sheet rock hanging, tape work or the texture work. I do about everything else. About ten years back, I built 900 square feet of living space onto our home. It included a bedroom and a bath. This was built onto the back of our house. To get this done, I had to remove a patio. The patio was concrete with bricks around that. The bricks had been added on. They were already allowing grass to grow up through the mortar. I used a crowbar and just chiseled them apart. This exposed the edges of the concrete patio, which had been poured as part of the house foundation. That foundation is what is called a “post tension” design. Instead of using a lot of rebar in it, they used these cables. The cables are in a plastic sleeve. They pour the concrete into the form and let it harden around the cables. Then, they use a hydraulic machine to pull on something like 10,000 pounds of pressure. This places the concrete under tension, so when it cracks, it stays together. Remember, concrete is guaranteed to do two things. Turn grey and crack! Knowing that these cables require a professional, I call up a company to come out and advise me about the tension and the cables. Over the phone, they tell me that what they do is show up and saw through the concrete and into the cable. This busts the cable and releases the tension! Once I get the patio jack hammered out of the way, they will come back and apply tension to the cables in the old foundation. I can then use a hammer-drill to drill into the old concrete, set some anchors and use those to use standard re bar construction on the new foundation. So the old foundation will remain as post-tension, and the new foundation is anchored to that with rebar. Sounds like a good plan and I schedule them to come and cut the cables.

So the guy shows up and checks out the exposed patio concrete. He just has a power saw with a large circular cutting blade on it. He eyeballs the concrete and notes where the cables exit. He finds one going across it and three coming out of the house. He takes some chalk and marks the patio where he thinks he will hit and cut the cables. He dons some gloves and safety goggles and advises me to stand clear. It seems that when the cables cut, they sometimes shoot out of the concrete! He said, sometimes by 20 feet! He goes about cutting the first one. The saw makes quick work of the concrete and hits the cable. When it snaps, you hear a dull “thump”. The steel cable shoots out, but only a few feet. He and I grin at each other as he cuts the rest of them. This would make a really cool man-hobby.

After he finishes, he hands me an invoice for just the cutting part. He tells me when I get the patio jack hammered out, to call them and they will come apply the pressure to the cables sticking out of the house.

So I spend the next miserable Friday, using a jack hammer. Those things are WORK! I use some heavy gloves and eye protection. I’m chipping away with a concrete cutter on the jack hammer. I reach the first tension cable and cut around it. The concrete doesn’t stick to the sleeve that the cable is in, so the concrete pretty much just falls away from the cable. This is looking pretty good but it is wearing me out. I get about half way across the concrete in about four hours. I hit a cable and hammer around it. About six inches later, I hit another sleeve for a cable. What? Why is this so close to the last one? They were spaced about two to three feet apart! I get down and look at the concrete from the edge. I come to a very bad conclusion. This next cable was not relieved of its tension! What to do! I’m too tired to be thinking clear, the cable is forty percent exposed already, so I make the numb-brained decision to just keep jacking around it! What is the worst that can happen! I was about to find out. I decide to try and use the jack hammer and cut a hole down to the cable and hope I can cut it with the jack hammer blade. I go about this, trying to cut at an angle to keep from standing too close to this hole. After about 15 minutes, there is an EXPLOSION of concrete, and a loud BOOM. I feel something hit my leg. A minute later, when breath returns to my lungs, I check my leg. I’ve got an inch long gash in it, but it isn’t deep at all. I run inside to look over the rest of me. Mostly, I’m as white as some notebook paper. White from exploded concrete dust all over me. No other bodily damage is found. I head back out and shut off the air compressor running the jack hammer. Lying in the pile of busted concrete is a tangle of inch diameter concrete tension cable. It looks like a rubber band that has been played with for days. It is gnarly, concrete still hanging from it. For about ten feet around the site, the grass is now white with exploded concrete dust. Looking across the yard, there are chunks of busted concrete, some about three inches around that were blown out. All in all, about three FEET of concrete, has vanished into dust and chunks.

I’m one lucky dude!

About a week or two later, after I finish busting and hauling off the concrete, I get the site excavated. Then, I call the tension guy back and have him apply the pressure to the concrete. I mention what I’d done. His eyes get as big as pie plates and he tells me I’m lucky to still be alive.

About a year later, I’m building a two story playhouse / fort for my girls. While I was up on the second floor of the fort, I look over to the roof of my shop. The shop is about 75 feet from the new addition that I built. I see some rocks up on the top of the roof. But there aren't any rocks around that look like that, which the kids might have thrown up onto the roof. I switch the ladder over there and head up. Yep, I got bits of concrete that were blown on TOP of my shop, which is 75 feet away from the blast site, up hill, and the walls are 10 feet tall. And the bits of concrete were blown on TOP of that.

OSHA would be so proud.

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