Here at Pioneer Builders Inc, we self-perform framing, siding, foundation, and some flatwork (as needed), but last week we decided to change gears due to COVID-19. Instead of waiting on a subcontractor to fit us into their schedule, we decided to form and pour the front porch on a spec house we've been building so we could finish the siding. We have experience in this, so we were able to pivot. I called and ordered concrete and the next day we set up the porch.
Since we are framers first, we tend to be a little unorthodox in our forming. This porch is 38 feet long, so we used two straight 20-foot-long 2x10s to form across the front, and picked up an I-Joist from the lumberyard’s “boneyard” to reinforce and straighten them. We often use I-Joists to help straighten things, because they are light, dead straight, and very easy to use. I would have preferred to use a longer joist, but remember that this was all last-minute.
One point to note is that this front porch was back filled and compacted in July 2019, while we actually formed and poured the porch in March 2020. Before the pour, we had already installed four 3-foot-diameter-by-48-inch-deep sonotubes for the porch roof supports (but that is way overkill). Since our frost line is 12 inches here, our engineer doesn’t require steel reinforcement for a concrete pad, though I did run a row of rebar around the perimeter and dowel it into the foundation. This detail has worked well for us for 40+ years.
We’ve learned from expert concrete contractors like Gaetano at NE Construction Ltd that bracing is extremely important when pouring a slab, so we used ripped 2x6 stakes and kickers to brace the porch. Everything was fastened with screws, so we could easily disassemble the forms and reuse the fasteners.
Because this was going to be an exposed aggregate porch, we ordered a concrete mix with pea gravel. We poured/placed the concrete as normal, using just enough bull float to flatten the top. After sealing the surface of the concrete with a fresno - a large, long-handled trowel with a tempered steel blade - we edged the slab and stripped the forms. We poured at 11 a.m. in about 50° temperatures, and finished at about 12:30 because I was a little short on mud. The slab was stripped and sprayed by about 2:30 pm.
Exposing the Aggregate
Once we were happy with the finish, we sprayed the top of the slab with Coca-Cola, using a pump sprayer to distribute the Coke. This is a cheap and consistent way to get sugar on the top of the concrete to retard the curing process. Then we let the slab cure overnight.
In years past, the process we used to expose the aggregate was to wait long enough for the concrete to set up before getting on it and using a garden hose and broom to lightly hose the top layer. But I've found that using a combination of Coke (to slow the curing process) and a pressure washer (to expose the aggregate) works better for a few reasons. One is that I can leave and come back the next day fresh. I can also adjust the pressure (and amount of exposed aggregate) depending on the tip I use.
On this project, I uses a nozzle that isn’t all that powerful and is a little more gentle than the yellow tip I’ve used in the past. You don’t want to spray too close the top or you can blow rocks out. With the tip I’m using here, I keep the wand about 90° to the concrete while removing the top layer. A low angle can also blow rocks out. I worked across the porch, spraying until the water was clear. After taking a short break, I moved back across the slab, repeating the process until satisfied with the results.
That’s really all there is too it. Coke acts as an easily sourced retardant that is cheap and consistent. Exposed aggregate is common where I live in the Pacific Northwest and is a durable finish. If, after the job is done, you want to remove a little more cement, you can brush muriatic acid over the surface and then seal it.
