Tuesday, April 24, 2007

Concrete countertops - a reflection on process

We are well on our way to having concrete counter tops! They are out of the mold and looking fabulous so far. We hope to have them finished and installed by Sunday, so this post will detail the first part of the process and I'll have follow up next week.
So here it goes, the good, bad and stupid:

1) We first consulted with www.concreteexchange.com and researched a lot of Cheng's books. We ultimately ended up using his mix as well, which we found very easy to work with. With so much going on, we didn't want to mess with recipe of additives to the sacked concrete. We knew we wanted dark grey solid color counter tops, something with an effect similar to soapstone. this eliminated any grinding on the surface and need to consider decorative inserts or aggregate.

2) Building the molds was very strait-forward for us. Accurate measurements, solid construction and wood are all strong points so we didn't run into any problems here. We followed a number of the suggestions made by Cheng, some of which may or may not have been necessary, but I like directions. The material to make the molds cost approx $75, as we did not have any melamine on hand and we needed rebar, wire mesh, silicone, etc.

3)A level area to set the molds for the pour is critical, so that you end up with level counter tops and not wedges. Since we were pouring all three counter tops at once, we needed a fair amount of space under cover. None of our work benches in the garage were large enough, so Matt built a level platform on the garage floor. The platform worked well, but was only a couple of inches off the ground. Ideally, it would be much higher, so you a) wouldn't have to work all crouched done, and b) wouldn't have to lift the ridiculously heavy counter tops off of the ground.

4) Based on my calculations, we were going to need 5.9 cubic feet of concrete. So, we ordered 6 cubic feet of charcoal mix. The directions specifically indicated that we should not use air entrained sacked concrete, and we had a little difficulty locating the right stuff. We eventually ended up having to go with 4000 psi instead of 5000 psi. We got the concrete, rented a mixer, and got ready to pour.

This is were things went a little askew...
First, we mixed the first batch a little too dry. We were both so worried about adding too much water and running the risk of cracking that we didn't add enough. This made the concrete very difficult to vibrate into place. I mean it was really dry. We even added more than the directions on the mix initially called for, but it still wasn't enough. We decided to mix the second batch a little two wet and figured that the two batches would mix with all of the vibrating, and the problem would be solved (which it was!).

Unfortunately, even though everything I read had said make sure you have extra concrete and mix, I cut it too close. We ran short, which really sucked. Once we distributed all of the mixed concrete evenly amongst the molds, they were all about 1/4 inch short, so we weren't able to screed the tops. Needless to say, I whacked myself on the forehead several times.

But, after a week, they popped right out of the molds looking pretty fabulous.

Normally, you need to dis-assemble the mold any pry it away from the counter top, but I guess because our mix was so dry, ours just popper right out. So our mold are intact and can be stored to use again should we decide now or later that we want to try again.

Next up...

I'll be spending this week grinding the underside of the counters because they are really rough and a little uneven since we were not able to screed. Once that is done We will apply a slurry to all of the surfaces to fill in the air pockets and then seal and wax them once they are in place. The sealer and wax will bring them back closer to the color they were when wet - a dark dark grey, almost black.


donutboy said...

Great write up of the whole process. I actually ended up making a second batch due to my first batch being too thin (not enough concrete in the mold, because I made the same mistake) If you poured 2" countertops then you will be totally fine. I poured 1.75 on my second run and they are SOLID!

The only thing that I have come across on mine is the wear down of the wax in high traffic areas. Right beside the sink and over on the prep side. But we just mix up another batch of bees wax, fire up the buffer and wah-lah!

You are going to love your countertops, and be so proud of yourself when people don't believe that you guys made em yourself!

About Me said...

Thanks for writing this up. I'm considering doing this too. I like the looks of it - and it fits my lack o' budget