We did it! We made it out of the ground! Last week felt like my birthday – full of presents and extremely rewarding. Not only did we pour the slab, but we also built our first two walls. Let the vertical [games] begin!
However, before I get carried away let me back up and start with the slab.
I recently had a meeting with a client who wanted to connect his studio and main house to make room for his expanding family. While listing his desires (efficiency and affordability being paramount) he nonchalantly described pouring the slab as: “paving” the dirt between the two structures.
I wish it were that easy. (It has taken us a month just to get out of the ground!)
I guess I feel the need to drive this one home: concrete cannot simply be poured on the ground. No site on earth is perfectly level, and you need a level pad on which to build. (Have you ever lived in a house with sloping floors? I have, it causes vertigo.) It is also much easier to order the correct amount of concrete for a level building pad. A guessing game ensues with uneven ground; too much concrete and you can’t get your money back, too little and you’re screwed. It’s best to be level and right on the money.
In case you (or the aforementioned client) were wondering why the slab is so important, following is a list of its virtues:
-Transfers building loads to the ground.
-Anchors a building against lateral forces, such as wind.
-Isolates the building from frost and soil expansion (moisture).
-Raises the building above ground, providing a dry comfortable living space.
-Slows down heat loss from conditioned space.
-Houses plumbing and electrical systems.
-Provides a square and level substructure for framing.
Slab-on-grade is literally just that. There are other floor system options, like crawl spaces, but slab-on-grade is the primary method used in New Mexico. It requires both plumbing and electrical trades do their rough-in before the slab is poured, whereas a crawl space allows for less precision in the rough-in – pipes and conduit are in the crawl space cavity and can be easily changed (not buried in 4” of concrete.) Crawl spaces seem preferable, but they are not compatible with concrete slabs.
Placing the Mud
The first step in “placing the mud” (a.k.a. pouring the slab) requires setting the height of the screed stakes and pipes. The pipes rest on the stakes and provide a guide for the screed (2×6 lumber), which levels the surface of the slab once poured.
Next, the layer of crusher fines below the slab must also be hydrated, so that it does not wick water away too quickly from the slab, which can cause cracking. Fortunately it rained overnight, so we didn’t have to wet the ground before pouring the slab.
Before pouring it is necessary to specify the “slump” of concrete mixture, which determines the amount of water added, and therefore the workability and how fast the concrete will set. I still don’t fully understand the water to concrete ratio – it depends on the mix (additives, type of cement, etc.) However, the actual method of measuring slump is pretty straightforward.
We poured the slabs over two days, and did so because we had five different finished floor elevations throughout the house. Monday we started with the patio (-6” B.F.F. (below finished floor)) the studio (+2” A.F.F.) and the carport footer (-24” B.F.F.). Wednesday we poured the slab in the main house (+0” A.F.F.) and the concrete stem wall in the carport (+18” A.F.F.)
Monday and Wednesday treated us well, providing ideal weather conditions: slight humidity, no wind, and overcast skies, all of which helped to delay slab dehydration. We started work earlier on those days, 6:45am, to beat the noon heat. I am telling you, the concrete gods were on our side last week.
It is imperative while pouring that the concrete crew rolls deep and moves fast. The concrete is coming out of the hose at 140 cubic yards per hour… FAST. Everyone is needed to push and pull concrete to far reaching corners.
On Tuesday Gil and the guys built the formwork for the carport stem wall. I wanted a smooth surface on the bench and stem wall, so we lined the formwork with 1/8” Masonite. I also wanted the bench and stem wall to have a beveled edge in order to minimize concrete chipping, and increase comfort while seated. We ripped ¾” triangular wood strips and nailed them to the top of the wood forms – later to be removed with the formwork – and Voila! Bevel.
Before pouring the concrete in the forms, it is important to grease the formwork, making it is easier to remove them. We used a petroleum-based oil, in a spray can, and gave everything a good coating.
After we poured the concrete in the carport formwork, we then used a Sawzall, without the blade (a little trick the locals use instead of renting a concrete vibrator), to vibrate aggregate away from the walls, enhancing both the smooth and uniform concrete finish.
To jazz things up, we cut out the year MMXV (2015) in wood and glued it to the Masonite, creating a bas-relief in the bench. That way, in 10,000 years, when archeologists are digging up this ancient site, it will have a time stamp. Move over Pompeii!
Finishing began immediately after the concrete was laid. The first step in finishing is screeding, which creates a relatively level surface.
There were still uneven areas in the slab, which the bull float eventually leveled out. Martin, the owner of Maes Electric, is the supreme bull float operator. I don’t think there is anything he can’t do (he does earthwork, electrical, and concrete). I asked him if he could do plumbing, and he said “No, the only thing I knows is that shit runs down hill!” (He must have heard that joke from my dad.) I have a feeling he was being modest. In fact, he doesn’t normally work with Gil and the guys, but he is so good at the bull float we brought him on for the day.
Next, we hand troweled, smoothing out the surface with metal concrete trowels. In order to get to the middle of the slab, the guys had to kneel on kneeboards.
The last step in finishing is the power trowel, which Martin was also the master of. The metal blades on the power trowel must be adjusted to the perfect angle, or you will gouge out chunks of slab – it requires skill.
We ended up with three concrete finishes throughout the house. The patio received a broom finish, which is good for traction when left exposed, or as a substrate for further finishes – tile or stone. The studio has a smooth hand troweled finish because it will remain exposed. The rest of the house was finished with the power trowel; it will eventually have wood flooring covering it.
It takes 28 days for concrete to completely cure, but within an hour it is walkable. After the concrete slab was finished, we put down a layer of 6mil Poly – opaque plastic sheets, 20’ wide x 100’ long. We weighed it down with CMU block and extra rebar, and then doused it with water from the hose. The poly remains for as long as possible to protect the slab from the elements.
After the concrete cured overnight we cut control lines with a skill saw, ¼” deep, at locations throughout the house that were most susceptible to cracking.
Masonry and Framing
We took Friday the 3rd off for the holiday, which was disappointing (only fore me). By Thursday the CMU wall for the fireplace was laid and the first wall was raised. I wanted to keep going!
This week will consist of framing, framing, framing!