Concrete Flatwork (Columns?), Sept 22, 2015

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:

formwork,TO, Baier 139

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:

formwork,TO, Baier 143

formwork,TO, Baier 147

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!)

formwork,TO, Baier 149

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.

formwork,TO, Baier 151

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:

formwork,TO, Baier 154

formwork,TO, Baier 157

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.

formwork,TO, Baier 159

formwork,TO, Baier 160

I quickly catch up to the work going on at the end of the form:

formwork,TO, Baier 163

formwork,TO, Baier 164

Once the initial float is finished, it is time to insert the foundation bolts before the concrete is too hard.

formwork,TO, Baier 169

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:

formwork,TO, Baier 171

Because, I cannot reach to the centre, we have designed a place to hook on a ladder to act as a scaffold:

formwork,TO, Baier 173

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.

formwork,TO, Baier 175

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!


About jhcarpentry

I am a teacher in the Construction Carpentry Program at the Chateauguay Valley Career Education Centre (CVCEC) for the New Frontiers School board. This blog is a way for me to connect with my students.
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4 Responses to Concrete Flatwork (Columns?), Sept 22, 2015

  1. Zsolt says:

    Looks like nice work! Would you feel comfortable doing the slab for an entire house if you had to?

    • jhcarpentry says:

      Thanks!! Everything about concrete is difficult and hard work! Lots can go wrong and you will not find out until the forms are off and the concrete is set. I never minded doing smallish jobs, but for the big ones, I would hire out with a guarantee that I can count on. On the other hand, it is always cheaper doing it by yourself….just try to get some experience under your belt first!

      • Zsolt says:

        If you need some help some help framing those walls I’d love to help if it falls in my free time. Let me know!

      • jhcarpentry says:

        Thanks, but that project was signed, sealed and delivered about 4 years ago (including everything but the painting). I appreciate the offer though and if you want to give some free labour, I can probably think of some other project!!

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