Pouring a garage floor

August 20, 2010

You Tube offers a variety of film clips on just about any subject, but when I looked for instructions on how to finish concrete, the supply of information suddenly dried up. After yesterday’s pour of a garage slab at the farm I now understood the reason. With four compulsive photographers on the job, not one thought to pick up a camera. There was just too much grit, too much to do, and too little time to spare for non-essentials.

None of us had poured a floor before. Derek offered to help before heading off to MIT for post-doctoral work. That physics background helped: he turned out to be a smart and willing concrete worker. I suspect Martin, the biologist, had never set foot on a construction site before, but he learned quickly and showed amazing strength and stamina. A veteran of projects with Dad, Charlie brought an eye for detail and a dose of caution to the mix, determined to head off his father’s sometimes-reckless excesses.

The family book on me is that I’m good at measuring and cutting but hopeless with anything sticky. Charlie’s never quite gotten over the time I varnished the transom of the boat with a mop. Concrete was an unknown, the transition from liquid to solid fraught with mystery and conflicting opinion, but all sources agreed that timing is critical.

Building inspector Alahan Kabdasamy insisted that I couldn’t pour a floor by myself, nor with one helper: I would need a full crew. So Charlie rounded up Derek and Martin and the plan came together. It’s one thing to plug away by yourself on a project. It’s quite another to schedule an inspection, a volunteer crew with little time-flexibility, and a truckload of highly-perishable concrete on a day with good weather for a pour.

It worked. The truck pulled in two minutes early, and Charlie and Martin were on time. Derek followed them in the lane. The sky was clear.

The driver quickly sized us up and took charge. He knew what to do. I had straightened a sixteen-foot 2X4 that morning on my jointer. That would be our screed. A small beam laminated up out of 4″ pine boards divided the floor in half and set the grade. We would be able to support the screed on the outside forms and the pine beam to level the concrete, then hopefully remove it at the end of the pour and fill in the gap.

The rebar at the perimeter of the slab sat neatly wired to the little plastic “chairs” I had located in Kingston the day before. For the mesh which covered the bulk of the floor the supports were just a hindrance, though. Pulling the mesh up with a hay hook or garden rake worked much better. I hadn’t anticipated quite how chaotic screeding ten cubic yards of concrete – that’s 40,000 pounds – can be. Alahan was right: we definitely needed all four guys on the crew for this part.

To push across the floor with a bull float, you hold the handle low to plane the large trowel fastened rigidly to a 12′ pole over the rough surface; for the return trip you pull from above your head to plane the other edge of the trowel on the way back. Yikes! Much taller Martin took over and quickly became proficient with the thing. He even had enough energy in reserve to shake the float to work the moisture to the surface as the LaFarge guy recommended.

I placed the anchor bolts and Charlie shaped the edges of the slab. Derek went around the bolts with a hand trowel.

Much debate ensued about when was the right time to start the power trowel: the concrete at the southwest corner was almost two hours wetter than the stuff at the northeast, so when and where do you begin?

Charlie looked horrified as I stepped up onto the slab. Oops. Sank too much. Hasty repairs. Twenty minutes later, though, I only sank about a quarter-inch, and that’s when the Internet advice suggested we start to trowel.

None of us had run one of these things. There are no useful sets of instructions available, either. We put the coarse paddles on it, started it up, and the lads lowered it onto the pad.

A power trowel has to be the most right-brained tool I have ever operated. I couldn’t tell you now how to control it. The more I troweled the less I had to rely on strength, but I don’t know what I did differently. The coarse paddles took a serious toll on the anchor bolts, though. Derek and Martin stayed busy with repairs.

But the machine reduced the sticky concrete to a workable substance I could measure and cut, so I soon had the grade where I wanted it. Internet advice said to continue to trowel until you don’t care any more. That point came quite quickly because we were bushed. The slab was smooth and slightly grainy. Any smoother could be a safety hazard. Into the trailer and back to Elgin the strange inverted helicopter went.

Fortunately by 3:30 a thunderstorm took over and gave me a break from spraying the slab. After supper I rolled on a coat of preservative and the day’s work was done. Whew!

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