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How to set up and pour a concrete slab

It’s Week 2 of the One Room Challenge and we are off to a great start.

We have six weeks to transform our small tiled patio into a large tiled patio and the first step of pouring the concrete slab is behind us.

Preparing the sub-grade

First, we (Michael) had to excavate a bunch of dirt. For Houston, the specification for a 4-inch slab is dig 5 inches into the sub-grade, compact the sub-grade, and fill and level with 1 inch of sand. I got this information from my neighbor, who is a landscape architect. Most other places in the US need 4 inches of compacted gravel under the slab — way more work. I had some low spots elsewhere on the property that I used the excavated dirt to fill in. Alternatively we could have paid to have it hauled away with a “dumpster-in-a-bag” service, which is less intense than a real dumpster, and probably way easier to fill.

The next step was compacting the sub-grade with a manual tamper. Our soil here in Houston is nearly 100% unconsolidated clays and clay-shales, so this is easiest to do when the soil is slightly damp. Note here that our French drain will be running through the slab, and will have a minimum of 2-inches of concrete over it.

Making the concrete form

After compacting the sub-grade we made a wood form shored up by back-filling soil around the outside perimeter. The forming was made by 1×6 nominal boards (0.75×5.5 actual). The slope was set along the top of the two shorter sides, and pitched down at a 1% grade (0.6 degrees). I obsessively checked this, since I really don’t want water flowing towards the house. The long side of the form (~20-feet) on the fence side was broken up into three roughly equal sections. The front was sloped towards the street (like 0.2 degrees), the middle was level, and the back was sloped towards the back of the house (again, like 0.2 degrees).

We reinforced the corners to make sure they stayed square with some bracing blocks screwed into the top. We also notched out the form around the French drain.

Anchoring the slab

The slab specification we were following called for 18-inch long by 3/4-inch diameter steel dowels to be placed 18-inches on-center (o.c) inside holes drilled into the existing slab at a height of ~2-inches. This ensures that the house-slab and the new-slab will shift together as one unit. This was very difficult without a hammer drill, but still possible with a standard drill and a hammer and old chisel. I started these holes with a 3/8-inch carbide-tipped drill bit, and then used a 3/4-inch carbide-tipped drill bit to drill the holes to final dimension. The latter also employed periodic hammer-and-chiseling into the hole to break up the aggregate that bogged down the drill. The holes were just a bit large for the dowels, so I placed a small piece of rubber along with the dowel and hammered them in together to get a tight fit.

The expansion joint (required every ~250 square feet) needed to be notched to fit over the dowels. If I were more careful I could have drilled each hole individually for each dowel, but I opted for a the quicker option.

Leveling and laying the rebar

With the dowels and forming in place we were ready for sand. I laid down long pieces of scrap and screeded sand to a consistent level across the pit.

We bought #4 rebar in lengths of 20-feet and 10-feet. Luckily we have an angle grinder, so using a metal cut-off blade made quick work cutting the rebar down to size.

In a similar process to our concrete counter-tops we used plastic stands to hold the rebar at the correct height in the middle of the slab. The rebar just snaps into place and a grid can be made pretty quickly. I spaced the long rebar at 14-inches o.c. (closer then spec, so stronger) and the short pieces 18-inches o.c. (matched spec).

After clipping the rebar grid together we went around and reinforced each intersection with wire ties. This just makes sure the rebar can flex together as a unit and not slide relative to itself since the stands don’t grip all that tightly.

Pouring the slab

We rented a mixer and recruited our neighbor (landscape architect) to help us out. The mixer was very big, and under-powered (also looking at the stock photo ours was definitely broken — rollers for the mixing drum aren’t there), so we ended up mixing in batches of 320 pounds plus water, though it was rated for 600 pounds. Our first batch was a bit dry, really it should look like oatmeal, and ours was more like wet granola.

While my neighbor and I mixed, Grace was hard at work leveling with a screed board and hand-floating using a magnesium float (and taking all the pictures).

Working the surface with a float pulls the finer particles and water to the surface creating a smooth slurry (the concrete “crème”). It wasn’t strictly necessary for our application since we will be tiling over this slab. Our slab ended up fairly level, but we will probably skim-coat over the obvious low spots with Portland cement. Any remaining difference will be made up with the 1/4-inch of mortar we put down for the tile.

After the pour we waited 24 hours before light use (doggos walking on it) and 48 before we stepped on it. Full cure for a slab is 28 days, but we will be sealing and starting tile after 12-14 days. We definitely recommend hiring this large of a project out, as we didn’t really save all that much by doing it ourselves — but we did learn a lot along the way!

Make sure to check back next week and also follow along with everyone else participating in the challenge!

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