Fireplace Renovation: Part 6 – Pouring a Concrete Hearth

Our mold was ready and we were excited. The day we were working towards was finally here! Today we mixed and poured our concrete.

Prep

To prep the area, we taped strong painter’s plastic to the melamine form to make for easy cleanup. We also had a bucket of water and sponge on hand for any major disasters (which we didn’t have!). We also made sure that the mold area was very clean (vacuumed, wiped with a sponge) and the only tools or materials near our work site to avoid any stress.

We also cut our 2×4 to size, making sure that it wasn’t too big, but that it was long enough to reach everywhere it had to. I did a dry run of the screeding step to make sure.

We also locked up poor Gus to make sure that he didn’t get curious during the pour…

We also pre-cut the mesh wire that we used as our reinforcement. Depending on the project, mesh or rebar can be used to reinforce the concrete. The stuff we got came in a roll, so after cutting it, I placed the bags of concrete on top to try to flatten it, which mostly worked.

Finally, we made this super handy list of what had to be done and stuck it to the wall for reference. The list helped us remember everything that had to be done.

Mixing the Concrete

With all of our prep work done, we were ready to start mixing the concrete.

We were excited. Really excited.

According to this useful youtube video, it’s best to just put the sack of concrete right in the machine. So Alex did.

It gets pretty dusty.

We used our amazing new garden hose to add water. The first bag mixed really well in the mixer. For some reason, the second and third bags started creating tiny concrete balls that kept rolling into larger concrete balls. We had to keep stopping the machine to break up the bits and keep adding water. At first we thought it was because we didn’t add enough water at the beginning. But we tried to correct for this for the third bag and it didn’t seem to matter. It’s quite possible that the balls would have eventually broken apart with more water, but we kept breaking them up in case that didn’t happen. As newbies, we were pretty nervous about chancing anything.

The mixer itself has a few different tilt settings. We generally started with a pretty upright position and then moved it down slightly once it started mixing. The lower the drum, the better it mixes. But there’s a fine line between the right tilt and having your concrete end up on the ground.

We were very happy with the mixer. After mixing small quantities of grout and concrete filler over the last couple of days, I couldn’t imagine having to mix this amount of concrete with anything but this machine. We were lucky to have access to one via our tool library, but I hear rental places have them as well.

I don’t have pictures of the next few steps because it was pretty intense and our hands were covered in concrete. The mixture was fairly oatmeal-y and could be loosely clumped in our hands. Alex pour the concrete from the mix into a bucket, and I scooped the concrete out of the bucket into the mold. After using about half the bag, we managed to fill about half the mold. I thought we might be able to get away with a single bag at this point!

Once a decent layer of concrete was in the mold, I added the mesh wire that I had pre-cut. It was still a bit bent and I was worried that it wouldn’t flatten, but it worked out.

Pretty snazzy, eh?

After installing the mesh, I kept filling the mold with concrete. As I added more, I worked it down into the mold so that it would fall under the mesh and fill in any spaces. Basically, this was my first unofficial screed attempt. By pressing on the concrete and moving it back and forth with my hands, it started falling into place and becoming more compact. I was wrong before – the second half of the bag would definitely not be enough to finish the job!

It took two and a half bags of concrete, which was pretty much what my concrete calculator told me. I didn’t believe it at first, but filling the last 10% of the mold seemed to take more concrete than the first 90%.

Once the mold was really full (like, slightly overflowing full), we starting to screed. This was tough. I had sweat falling from my head onto the concrete. So my sweat is literally in this project.

The inside of the firebox was particularly challenging, but my extra melamine strip really helped keep the 2×4 level. I’m really proud of that innovation! I’m benefiting from it in the photo below:

Once the screeding was done, we left it to dry.

The surface wasn’t smooth, but it was in pretty good shape.

After we stopped screeding, I knocked the sides with the hammer to let any air bubbles escape. I didn’t see any, but maybe we just did such a good job that there weren’t any?

Alex then went to return the mixer. By foot. Weirder things have happened in Parkdale.

After about 45 minutes, I had to float the concrete. I don’t have any photos of this step because Alex went to meet his mother and I was covered in concrete. 

The floating step was interesting and I’m not sure how to describe it. After watching the Buddy Rhodes video for concrete countertops, I expected the concrete to be pretty hard. But it wasn’t, and the concrete was sticking to the float. So I waited some more, thinking that it wasn’t dry enough.

I kept waiting.

I kept trying to float.

I kept waiting.

Eventually I decided that maybe I should skip the floating all together and skip right to troweling. Finally I was able to get the concrete nice and smooth! A crazy thing happens with concrete. As soon as you work it with a trowel, a cream in the concrete starts to rise to the surface. This cream can be spread around with the trowel and create an amazingly smooth finish.

The only problem was that two parts of the concrete were already too dry and I couldn’t get the cream to rise to the top. I tried to mix more concrete and patch it, but that didn’t seem to help. Actually, it really only made it worse. I have no idea what will come of this non-smooth spot, but hopefully I can patch it with something else or smooth it out with some sand paper.

The two dry parts are on the front piece on the left and right, right up against the brick. I’d be less worried if it was in the firebox, but this part will be seen and not hidden by a stove.

Once I was happy with the finish (minus those two spots), I left it alone. It looks nice from a far at least!

Then I tented it. It’s important to tent so that the water doesn’t evaporate too quickly and compromise the strength of the concrete. Covering the concrete with plastic might be more important with countertops, but I wasn’t risking it.

Now we wait a few days.