Here is part 2 of my concrete counter top build.  In Part 1, I covered some background and basic preparation.  Here, we build the forms and pour the concrete.

Counter Top Forms

Cross section of the 1/4 bullnose form. Form will cover up to 5/8″ of substrate; in my case I had 1/2″ Hardie plus 1/2″ MDF, so I needed to conceal the exposed MDF edge with band edging. The form produces a 1.5″ overhang on the counter top.

With forms, there are (at least) two approaches; build your own forms with e.g. melamine or buy pre-made forms. Building your own is an option for both cast in place (CIP) and precast (see Part 1), while most pre-made forms are really an option for CIP. I did a precast with my own forms for a utility sink in the garage several years ago. It turned out pretty well, and would have no qualms about doing the same thing. However, in a CIP, one needs a base and, depending on your forms, the edge of the base will be exposed. So you either need to be careful about building your forms – you can’t just build a simple box – or get pre-made forms. I opted for the later. I’ve settled on Z-forms from Concrete Countertop Solutions. These are plastic forms that come in a variety of edge profiles that you attach to the base with pan head screws. These run about $150 for 30 linear feet and $250 for 60 linear feet; shipping nearly doubles the cost though! Fortunately, I found a local distributor that carried the forms so avoided the shipping- as I’m doing several more pours after this one, I got 60 linear feet of the quarter bull-nose. If you go with pre-made forms, definitely look for local distribution. An attractive feature of the forms is the design of the overhang which extends down to cover the edge of your substrate. After the concrete sets, the edging is snapped off at a built in ‘weak’ spot in the form underneath the overhang with the end result being that your counter top sits in front of the cabinet face and looks ‘seamless’. In my case, owing to the mistake in the prep with the MDF and the fact that form will only cover 5/8 inch below the base of the counter top, the MDF edge remained exposed and I had to edge band it. The form is attached directly to the substrate with pan head screws. Note the back edging against the wall; this edge does not get removed and as a ~1/4 in lip to provide support when screeding (see Pour). It’s obviously designed for someone with more skill than I as I could not effectively use it for screeding.

Here the forms are installed, mesh and rebar in place. On the right, I’ve covered the cabinet and wall with plastic to mitigate the inevitable mess that is incoming. Floor has been covered with ram board as well.

Corners on ‘like’ forms are easily cut on a miter saw – Concrete Countertop Solutions sells a wooden jig to hold the forms ‘perfectly’, but unless I was doing 10’s of cuts per day every day, it’s not really worth it – easy enough to hold in place. In contrast, corners where your edge form meet the back form will require a bit of snipping of the back form to merge them – extend your edge form all the way to the wall and cut sections of the back so it overlaps well with the edge form. On exposed corners, you’ll want to re-enforce with tape – I used duct tape – and caulk. But only caulk the OUTSIDE of the form – any caulk on the inside of the form will imprint on the finished surface and you don’t want that. Or maybe you do, I’m not your supervisor. But it will be ugly. Where the edge form meets the back form, you can caulk and tape your improvised seams (the edge form should extend all the way to the wall) from the inside as it will all be covered by the concrete when you’re done. Note that the forms will generally add about 1.75 in to your cabinet height so plan accordingly for the final height. If you build your own form, you can customize a bit, but I don’t think you’d want to pour anything less than 1+ inches or it’s going to be prone to cracking.

Depending on the size of your pour, you’ll want to re-enforce the concrete with a a mesh and/or rebar. In my case, I used a 6 in steel mesh for most of the counter top. For this counter top, I have 90 degree corner which is weak/stress point, prone to cracking diagonally back towards the wall corner. Therefore, I put some rebar across the corner to stabilize it. The counter top did crack very slightly as it cured; but it was shallow and stopped… where the first piece of rebar was. With the right fill and stain, I’m calling it an aesthetic touch. You could do two lifts here – e.g. pour a first later of concrete, then lay the mesh and rebar and then pour the remaining concrete. I just cut the mesh such that I had ends I could bend down to hold in place above the HardieBacker substrate and tied the rebar to the mesh with steel wire. Note here the tape on the form corners; You really want to have these corners stabilized and sealed before pouring. In addition, now is the time to cover everything. You want ram board or equivalent on the floor (unless you’re pouring on an unfinished floor or you’re replacing the floor later) and a heavy duty plastic covering every exposed surface on the cabinets. I also covered the walls to minimize potential splatter on my newly painted walls.

The Pour

Now it’s pour time; presumably, you’ve picked the concrete. In principle, you can use standard concrete mix. You’ll have large aggregate (pebbles/chunks of rock) and it will be a bit harder to work with and you may need to do more grinding on the larger aggregate. There are also counter top specific mixes – I used this on a small vanity and it works well. But I couldn’t find any in stock, so I ended up using sand topping mix. Concrete Countertop Solutions also makes a specific mix for counter tops; looks interesting, very small (no?) aggregate, but the shipping (and cost itself) is ridiculous and simple sand mix yields good results at a fraction of the price. There are calculators to figure out how many bags you’ll need for a given size and thickness; they seem pretty accurate, but I make sure to buy at least 2 bags more than they recommend – if they say 4.5 bags, I’ll buy 7. It’s much easier to return an unused bag than come up short on pour day. So the process on pour day is pretty straightforward – mix the concrete, pour it into the form, mix next bag, pour into form, rinse, lather repeat. I found it better to mix with paddle mixer in single bag batches – a 10 gallon bucket is best here rather than the standard 5 gal Home Despot/Lowes buckets. Here’s where it’s very useful to have two people; one can spread the poured concrete while the other goes and mixes the next batch. I did do two lifts here; poured about 1 in deep with the first few mixes. That made it easier to focus on getting it down into the base of the edge forms and flow around the rebar/mesh.

Trying to make a flowable concrete was my major mistake on this pour. I wanted to minimize air pockets and voids in the form edges and easy flowing concrete is easier to work with. There are additives that are supposed to increase flowability without adding water – Concrete Solutions makes Liqui-Crete, but shipping was too much and I couldn’t find it at the local distributor; when you combine the cost of the concrete with this at one box per bag, it rapidly gets unreasonable for – as I’ve found – minimal benefit.  I used Counter-Flo, but that didn’t really add to the flowability. So I checked a few youtube videos, and saw people adding up to 7-8 quarts of water to make the concrete flowable, even on concrete I know specifies no more than 4.5 quarts. Since I’d never expect youtube to lie, I went with flowable and added ~6 quarts of water per bag.  Big mistake, almost botched the entire effort. Also note that my test pour (bench) was done with the additive and recommended water added – that was harder to work with than I wanted, and I deviated from my test pour to try and ‘fix’ that. If you’re going to experiment, do another test, don’t experiment on your final work! With that much water (6 quarts in my case), the bleed water (water that rises to the top of the pour, ‘cream’ that allows a smooth finish) was so watery as to essentially rinse the (small) aggregate into a sand, removing all the cementitious material; I was basically screeding water. That’s going to lead to a powdery, soft layer in the concrete. Speaking of screeding – that’s the process of ‘rubbing’ a long flat board across the surface to the poured concrete to level it. Basically, you over fill your form very slightly, take a long flat, in my case, piece of 2×4 and resting it on either edge of the form, pull it across to remove the excess, leaving flat surface behind. Generally, you want to draw the screed board across the surface with a sawing motion. However, up against a wall like this pour was, the very narrow back form sections do not provide support to ‘saw’, especially with the soft watery concrete I had f’d up into. The screed board would slip off the edge and I’d end up screeding below the level of the form. If you’re doing an island (or doing a pre-cast) that’s not up against a wall, and do a proper concrete consistency, this will not be a problem. In my case, it just meant a lot of fussing around and manual by-eye adjustment and attention to detail, making the process more tedious than it needed to be. Once you got the basic pour and screed done, start vibrating the edges; in fact you should be vibrating the edges as you pour, at least taping with the end of the trowel you are spreading the concrete with. This is necessary to remove air pockets on the form edges. With the pour complete, start in earnest with the vibrating. When you think you’ve done enough, make another two passes. And then 3 more. I used my random orbit sander on the lowest setting. I stopped at “you think you’ve done enough” stage and still had some air holes on the edge. This whole stage is attention to detail and there’s not really a formula. You have you float in one hand, a pail with extra concrete (usually stuff you’ve screeded off), and you’re vibrating and floating the surface. If vibrating lowers a section a bit, add concrete and work it in with the float. You have to be careful here – always run the float at a slight angle, elevated in the direction your are moving the float – you don’t want to dig the front edge of the float into the concrete or your going to spend a bunch of time fixing the imperfections you just introduced. For me, this was 10-15 minutes with the float just babying the surface – and lamenting the issues I knew I was going to have given how much rinsed aggregate there was in the bleed water. DO NOT OVER WATER YOUR CONCRETE!!!!!

All the concrete is poured, screeded, floated, and vibrated. It is mess, but the ram board did a great job protecting my new tile.

Once you’ve floated and vibrated to your hearts content, just walk away. I find it hard to leave it as this stage, it’s definitely a case of trying to do to much. You should have a pretty smooth surface at this point and it’s time to let the concrete do it’s thing a set up a bit. Check over the next 20-30 minutes; when it’s set up enough that it’s firm to the touch (you don’t really leave a mark when you touch it lightly with your finger), break out the steel trowel. While it was important at the floating stage to be careful and always run at an angle in the direction of travel, here it is critically important – it will be much more difficult to recover (impossible?) if you dig an edge into the surface. For the steel trowel, I’d strongly suggest one with very rounded edges; sharp corners are too easy to dig into the surface with a momentary lack of focus. You want to run the trowel over the surface at a shallow angel with firm pressure. If you are not hearing the ‘scrapping’ sound, you are probably too early. Make another cup of coffee and wait a bit more. I like to work it with long circular arcs. The concrete that is. During this process you can get the surface very smooth – not me since I was still beset by my mistake on the pour with too much water – but in previous pours I’ve gotten a very good finish at this stage. Careful work up to this point will reward you when you get to the grinding and polishing stage, believe me.

Speaking of finishing – that’s what we’ll cover in the final installment.  Try to contain your excitement.