Last night we did it. We finished our test concrete pour – preparatory to pouring our concrete countertops.
I am fascinated with concrete. It is such an interesting material. It arrived in 55 lb. bags. It is a mixture of dry cement powder, aggregate, and in the case of Flowcrete, some other material that adds strength.
We had prepared two forms for this pour, first, a box made of 2 x 4s,
and second, the hole left in our floor from removing the chimney and hearth last year.
First, we calculated the ratio of wet to dry based on the size of a yogurt container for a scoop. We determined a 1 water: 6 flowcrete would yield the right consistency of concrete. The flowcrete had a surprising amount of aggregate in its composition, that is, the stones and gravel the are mixed with the cement to make the concrete.
Next we mixed in small batches in a bucket with our new powerdrill purchased for this purpose. This is an electric powerdrill, with a 1/2″ chuck, to fit the paddle for mixing. It really was a lot like making a cake, starting with the water in the bucket and adding dry until it was the right consistency, mixing between each addition to keep things smooth.
Once we had the right consistency, we continued beating the mix for at least 2 minutes, as per the instructions on the bag. Then we dumped the mix into the form. We had a fairly stiff consistency, slump, and the mix did not slide easily out of the bucket. It had to be helped along by Matt’s handy small rectangular trowel.
Once the concrete was in the form, I screeded it with a nice, straight 2 x 4 on edge. Screeding pushes the fresh mix into the form and ensures the top of the mix is level with the top of the form. Our first form was 3 1/2″ deep and I could see air bubbles getting trapped in the mix as it piled into the form. I used a hammer to knock the form from the sides and the bottom and gradually the bubbles release through the surface, much like pancakes cooking on a griddle.
My next step was to use the mag trowel – this is a non-steel trowel with a slightly curved surface. I used this trowel to wriggle across the surface of the mix and something miraculous happened. As I jiggled the trowell and pushed it across the surface, the lumps of aggregate disappeared below the surface and this frothy smooth film came to the top. They call this bringing the cream to the top. I keep working across the surface, with a jiggling motion and gradually the entire surface became smooth. I was learning to trowel. The angle of the trowel, the speed of the movement, and the amount of pressure applied all effect the kind surface that spreads out behind the trowel.
Once the surface was relatively smooth, I switched to my steel trowel and continued to smooth the surface. There is an art to troweling, depending on what stage the concrete is at in its hardening process. I worked the surface over with the steel trowel for a few minutes and then left it to harden for 1/2 an hour. I had been told to leave the concrete to harden between cycles of troweling so that surface will set up properly.
Meanwhile, we went ahead and mixed the concrete for the hearth, and repeated the process.
We ended the evening with two pieces of concrete, poured, troweled, and smooth.