We are building a house using a lightweight concrete product called AAC (Aerated Autoclaved Concrete). All walls, interior and exterior are built in AAC, so it made sense to create our stairs out of concrete as well. However, because of the complex, somewhat circular design, each step was a unique size and shape. The distance between floors that this staircase spanned was 13', so, to simplify the concrete pour, we divided the steps into 3 stages and built structural walls at these points. First, we poured the landing with about 5 steps, then the cut up angled portion, and finally the three circular shaped stairs at the bottom of the landing.
Next, we created the wooden form that the concrete would be poured into. Because of the shape of the area that the stairs were fitting into, the form under each step was not necessarily level from one side of the wall to the other. We decided that the most attractive way to create this form was to use two triangle shapes for each step. These triangle shapes were cut from plywood and nailed to the 2x6’s already attached to the wall. Then, a 2x4 was screwed into place under each seam, and braced with vertical boards to the floor.
The engineering called for #5 rebar to be placed in a grid 3/4" from the bottom of the concrete stairs. The rebar was tied in a grid of approximately 5" squares bent to follow the shape of the stairs.
We proceeded by screwing ~8" 2x8's to the wall to hold in place the boards used to form the run height. In this case, the run was 7 3/8", so, 2x10's or plywood were cut to size and nailed into place. These pieces were also braced horizontally & to each other from above.
At this point, we were ready for inspections by the engineer & the county. Once approved, we began the concrete pour. We made our own concrete mix using a 4 cubic yard batch plant and mixing 1/2 yard at a time. This amount was used because of the size of the concrete bucket and the weight limit of the crane lifting it into place. This also was a good amount of concrete to work with at one time. We began pouring at the lowest step, using a concrete vibrator to assure the mix was working its way into all corners and around the grid of rebar. This same procedure was followed for each step as we continued to work our way up the 17 steps. While we waited for the concrete to set up enough to add more to higher steps, it gave us time to work lower steps to a finish.
The first section that we poured was the simplest. It was the landing with five stairs. We did not have to build a form under most of this area, we simply filled it in with scrap pieces of AAC material, leaving room for about 6" to 8" of concrete. We placed the grid of rebar in place, formed the rise of the five steps, poured a slab and then the steps.
The final section was the circular shaped stairs at the bottom. We formed the circular shape by cutting a template shape into two 4x8 sheets of plywood, drilling holes through the wood and into the footer below, and screwing the boards into place. We then bent a 1/4" piece of plywood (cut to the proper rise height) into the template shape and secured it to the plywood. We added a grid of rebar and poured the first step. Each successive step was done the same way. The third step was a continuation of the landing, so the excess rebar from the first concrete pour was used to form the final grid.
These 3 sections of cast in place stairs used apporximately 5 cubic yards of concrete (20,000 pounds).
Stay tuned, I will be adding more information about this project soon.