THE CURE FOR CONCRETE

Friday the concrete was poured for the main floor of Underhill House.

Doug and I set the alarm for 5, but when we got to the house site at 7:15, the pour was already underway.  They needed a pumper truck to get the concrete high enough and far enough out to reach the uphill corner of the house, and this kind of specialty rig is almost impossible to book on short notice .  The only open time slot was the very first one of the day.

That was a good time for us because the concrete was poured over a metal base screwed tight to the whole tree floor joists below.  Whole Trees has had success with corrugated metal roofing as decking to support concrete 2nd-story floors, but metal heats up quickly in the sun, and a hot surface causes concrete  to cure too fast.  No problem – Mike Flynn was pouring the floor while the decking was still covered with dew.  In fact, a bit of rain had fallen overnight, so the deck was in good shape to receive its new concrete cap.

Our main floor was poured partially over a basement and partially slab-on-grade.  The entire floor will be heated by six separate loops of PEX tubing embedded in the concrete.  Two of those PEX loops were stapled down to 4 inch thick Styrofoam under the slab-on-grade portion of the roof, and the other four were tied to a wire mesh laid out on top of the metal decking.  These last four loops over the basement  required special care to avoid a potentially uneven heat distribution between the floors of our house.

We also have PEX tubing in our basement floor, and Andy DeRocher from Full Spectrum Solar cautioned us that the tubing in the upstairs floor could end up heating the basement from above, making it too warm and leaving the upstairs out in the cold, relatively speaking.  Andy suggested that we insulate the basement ceiling, and that as little as R2 insulation would make a big difference.

The “typical” Whole Trees basement ceiling under a concrete upstairs floor looks like this: There is a main whole tree beam holding up whole tree joists that are spaced two feet apart on center. Hemp cloth is rolled out down the length of each joist cavity, pulled taught and stapled to the joists. This creates the finished ceiling between the joists as seen from below in the basement. A four foot wide roll of Relectix insulation is then rolled out above the hemp, and the metal roofing is screwed down as the base for the concrete.

Although Reflectix can have a pretty good R value if it faces an air gap and can reflect back radiant heat, there’s no air gap in this application, so Doug asked that a second layer of ¼ inch thick Styrofoam insulation be laid out between the Reflectix and the metal decking.

That R2 insulation on the bottom side will send most of the heat upstairs.

Because the forecast called for 90+ degrees on Sunday, , Mike added a compound to slow down the concrete curing time.

The best cure is a slow cure, and the best way to ensure a slow cure is to keep a freshly poured floor cool and wet.

The concrete cured slowly all day Friday. They couldn’t finish power-troweling till almost 3 p.m.  About 7:00pm several members of Mike’s team returned to score the concrete to minimize cracking    The PEX tubing is not very far below the surface, and everyone was concerned that scoring the concrete might cut through a piece of tubing.  Andy had left the entire PEX system pressurized to give us a way of checking for an unwanted punctures.  We all breathed a big sigh of relief when the job was complete with the pressure intact.

A hard rain was predicted for Friday night, and one of the carpenters stayed late to help us drag a tarp up on the roof and cover the opening for the stairs to keep the basement dry.  We left Friday night pretty tired, but happy that the pour had gone well.

Because we were entering the three-day Memorial Day weekend, it fell to Doug and me to protect the concrete from excessive drying and too-fast curing.

We came out Saturday morning after a very intense rain had pounded the area.  All around the site, the unprotected earth was rutted from the runoff, and the ground was a slippery, clay-mud mess.

The tarp we had set up over the stairwell opening had let a lot of water into the basement, and the concrete surface was already  drying out.  We had to work fast.

The Whole Trees team uses discarded billboard plastic for tarps.  We had three of them.  They are about 15×50 feet of tough plastic sheeting.  They were dirty from being left in the clay and the rain and they are also very heavy.

We got them up on the floor and spent several hours spreading them over the floor after spraying it down with the backpack sprayers we use when we are doing prairie burns.  Because we won’t have an operating well for another few weeks, we used water hauled out from town.  With the heat and the wind, the concrete was drying almost as fast as we could wet it, but finally we had it all covered and weighted down, and we added a few screws around the edges.

We came back again Sunday during the one-day heatwave and worked from 11:30 till 1:30 re-wetting and readjusting the tarps and making sure they were screwed down all the way around the edges.  The concrete stayed nice and wet except where the tarp edges had been blown up and air had gusted underneath.  I went over the tarps and duct taped every tear and puncture I could find.  We were taking no chances.

We came back Monday, and the system was holding pretty well.  Just a few spots that Dragging those tarps into place made me feel like a fisherman hauling in my nets, and it gave me a lot of respect for what fishermen do.  Between hauling those heavy tarps and pumping the sprayer for hours, I am really feeling it in my arms today.

But we are confident we did the right thing.  Mike told us it would be best if we covered the concrete for at least a few days, and Doug’s research confirmed this.

According to Concrete Network, keeping concrete moist for the first seven days after pouring can increase its strength by up to 50%.  Even if it’s kept moist for just three days, you get 80% of the benefit.

Concrete is not the greenest material in the world, but it has its value as a way to hold heat and conserve fuel, and it can be very strong and durable.  To make it as green as possible, we want to ensure that it cures slowly and serves the people who use this home for generations to come.

I hope it will.  We did what we could to help this weekend.

Advertisements

4 replies

    • Hi Lorijo,
      It was definitely a fun stage. Yesterday we had a picnic lunch in the basement with our daughters as rain poured in buckets (literally into buckets – there was some leakage over the hole for the stairway), and we felt like lucky fish.

    • Hi Joanna,
      On the way out this morning we were thinking of the ways our house is a hybrid, and one of those ways is it’s kind of a hybrid owner built. While we are not building this all ourselves, we are lucky enough to be able to get quite involved. Doug gets summer vacation from teaching biology at UW-Platteville, and I have been trying to shift my freelance writing schedule around to free me up as much as possible during the building process.

      We feel very fortunate to have the time and opportunity to plug into this buiding process.

      At the moment we are just helping around the edges, but we will be fully plugged in with the straw bale and plastering process and much of the finishing work.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s