So I did get an “official” request from a loyal reader: he wants me to comment on floating a slab on grade, sometimes referred to as “flatwork”.
I showed you the formwork and double ties a few posts ago, and mentioned that to finish the job, my partner and I floated a slab on top of the walls once the soil had been compacted within the concrete walls. Now I will illustrate the steps to you.
Once the soil was compacted, we drilled and inserted rebar in a grid pattern 24″O.C. and tied concrete trellis to the grid. This was the reinforcing for the concrete floor to follow:
You can see the form around the concrete wall structure that will define the dimensions and height of the slab. Once the truck arrived, we started to place the concrete:
As you can see, there is lots of muscle be used here…not a task for the faint of heart, especially as the water/cement ratio was low, the slump less than 3 inches and the consistency fairly thick. (Please note that I am using the incorrect shovel: it should be a flat-mouthed one to avoid promoting segregation – my bad!)
Once the level had been achieved for the first 3 or so feet of the length of the pour, we began to “screed” the surface, or level it to the top of the forms. This is done with a sawing motion, which creates the level, acting like a plow to push extra concrete ahead of the screed, and it also helps to consolidate the concrete in place.
The surface is fairly rough after this step, but it is important to let the water rise to the surface of the fresh concrete and let it evaporate. Using a trowel too early in the process will result in a fragile surface that will not withstand the wear and tear of life.
We move across the surface of the slab:
As we get to the end, and there is less and less space for all the people in the form, I left them to it and returned to the start of the pour….Using my adjustable magnesium bull float, I began to trowel the surface, thereby forcing down the larger particle and causing the “cream” to rise. It is this that will make the durable and hard finish.
I quickly catch up to the work going on at the end of the form:
Once the initial float is finished, it is time to insert the foundation bolts before the concrete is too hard.
Then, as this is a small form, I begin to trowel the final finish with a steel trowel, working from the edge in and around the edges and bolts:
Because, I cannot reach to the centre, we have designed a place to hook on a ladder to act as a scaffold:
After a few polishes with a little time to harden between (very dependant on temperature, humidity and possible additives), the slab is ready to be covered with plastic to retain humidity while it cures.
Ta-da!! Done….at least until we come back to build the walls and roof, install the windows and doors, insulate, do the exterior and interior finish and have a celebratory beer….that is another story!