Saturday, December 24, 2011

Stemwalls and Rat Slabs - Part II


So, helping me get the stem-walls completed and working the concrete during the pour, again, is Joe Strickland from Skyline Development and his crew, Bob and Nick.

In come the pump truck and the concrete truck:

The pour started with the front corner and slowly progressed around the house. The major issue with pouring concrete walls is the pressure that builds up at the corners. It's entirely a matter of hydraulic pressure and the corners will face the greatest loads. So the guys were incredibly careful when pouring the corners.

Here you can see a good view of the concrete pump filling the cavity in the ICF. There is both horizontal and vertical rebar within the cavity.

Once the stemwall pour was complete, we quickly finished preparing the crawlspace for the rat slab. The dirt had already been graded and the next step was to lay down a 6mil plastic barrier that would help limit moisture migration into and out of the slab.


While the guys were handling the rat slab, I started setting the various bolts. There were two types of bolts, hold downs, which needed to be set in a specific way so that they looped around the rebar in the stem-wall, and anchor bolts, which only needed to be set in the concrete, but without any special placement. Here you can see the guys finishing up the rat slab and you can see the anchor bolts set into the stemwall.

And the result is incredible. I now have a smooth rat slab, a strong stem-wall and I can begin working on the next phase of the project (rough framing).

Stemwalls and Rat Slabs - Part I

As you'll note, it's been over a month since the last post. I (naively) thought that the stem wall would go rather quickly. And, realistically this *could* have been the case.

But I had a few decisions to make, and once the decisions were made, I had to finish up the work to make it happen.

The big decision was around interior of the crawlspace, and whether I should insulate the floor of the crawlspace now and cover it with a thin layer of concrete. (This thin layer of concrete is called a "rat slab" because it's not thick enough to be structural, it's just thick enough to keep rodents from tunneling up through it.)

The advantage of pouring the rat slab at the same time as the stemwall, is that I wouldn't have to have the pump truck come out a second time, and the pump truck costs roughly $600 per visit. The disadvantage is that I haven't picked out the type of insulation I want to use to insulate the floor. Normally, when insulating a floor like this, you place the insulation underneath the concrete (see above).

In the end, I decided I would go ahead and pour a rat slab when I pour the stemwall, but I wouldn't insulate underneath. The reason I think this will work is that I'll simply place the foam insulation above the rat slab and then lay down a plywood subfloor above to protect the foam.

Normally, this wouldn't work very well, but because the crawlspace will not be heavily used, the plywood should do a decent enough job of distributing the load.

Now that I've made the decision to pour the rat slab, I also had to figure out how I would support the house during the pour. Currently the house is supported on the interior with eight steel posts sitting on 1/4" steel plate. Unfortunately these supports are on the interior where the rat slab needs to be poured.

So I consulted with my architect and confirmed that I could support the beams with some of the left-over 2" pin pile pipes. I then used a laser level to cut the various pipes to the perfect length to bring the house to level.

Above, you can see the pipes after they were cut and welded to 1/4 plates on the bottom to help distribute the load. I also took the precaution of filling the pipes with concrete so they wouldn't be hollow, on the assumption that this would help strengthen the pipes.

I then jacked up the house a little bit, positioned the pipes underneath the beams and welded the pipes to the beams. I then welded the pipes to the vertical rebar, so that they would be supported laterally and keep the house from moving. And as you can see in the picture below, the posts were positioned within the stem-wall.

So I used a relatively "cheap" rotating laser when performing the measurements. According to the laser, the footings were sloped by about an inch over the length of the house. That didn't seem extreme to me, but when Joe the contractor came out, he was surprised. He was very exacting when he leveled the footings, and used his *very* expensive and *very* accurate laser. As soon as he said this, my shoulders slumped because I realised my laser was likely off and I had cut the pipes to the wrong lengths.

This led to him using his really awesome laser to help me figure out how far off each of the beams was from level. I then had to figure out how much I would need to shim each beam in order to bring the beams and house to perfectly level. This took about two hours, but I can't complain too much, because now the house should be level within 1/8". And because the foundation is sitting on pin piles, there will be virtually no settling. It is a little surreal to think that 50 years from now, this house will still be level within 1/4".

Now comes the ICF! ICF stands for Insulated Concrete Forms. The idea is that you used thick Styrofoam for the forms, and then after you fill the forms with concrete and the concrete cures, you leave the forms in place. The Styrofoam then acts as insulation. An added bonus is that they are pretty quick to setup as well!

The easiest way to think of ICF, is as large, hollow LEGO blocks. You stack them up and overlap the edges for strength.

Up next, the concrete trucks show up again and it'll be time for mud!