Creating Nice Concrete Floors

These are my notes on creating nice residential concrete floors. In my primary residence, I put in about 1500 sq-ft of concrete floors in the lower level. I used a 6-inch slab on crushed stone with 1/2 inch PEX tubing for hydronic heating. I’m pretty happy with these floors, although not wild about the results I got in finishing/sealing them. I am in the process of building a second home in which all three levels will have concrete floors. In principle concrete is (a) very inexpensive, (b) a wonderful means of installing hydronic heating, and (c) attractive. But, I’ve found that there is all kinds of confusing information about how to achieve these aims. Here is what I’ve learned based on experience, research, talking to concrete contractors, and my own experiments.

Formulation

Concrete is formed by mixing water, portland cement, sand, and gravel. A chemical reaction occurs between the cement and the water that results in a rock-like material with very high compressive strength. This material ends up in a mixture with sand and gravel, which forms a very strong, very inexpensive composite material. How strong? It depends on the mix, but typically several thousand pounds per square inch compressive strength.  In flooring applications, concrete is almost always reinforced with either wire mesh or rebar, which theoretically provides tensile strength as well. Concrete gets stronger with age…for decades. Old concrete is super strong stuff. (The corollary is that if you want to cut or drill concrete, do it when it is young.)

There is something called “lightweight concrete” which entrains air in the mix. It’s not that light. It also has risks of delamination of the top layer if steel troweled. I wouldn’t bother.

Cost

Concrete is an inexpensive raw material. It is sold in units of a cubic yard (usually just called a “yard”). A yard is a lot of concrete. (A typical concrete truck holds 5-10 cubic yards.) By definition, a yard of concrete is 3ft x 3ft x 3ft = 27 cu-ft. So, a yard of concrete is equivalent to these floor areas:

54 sq-ft 6 inches thick

65 sq-ft 5 inches thick

81 sq-ft 4 inches thick

108 sq-ft 3 inches thick

162 sq-ft 2 inches thick

If your floor is 1000 sq-ft and 3 inches thick, you’ll therefore need 1000/108=9.25 yards of concrete.

The material itself costs about $100-$130/yard depending on local conditions. This is the price for a truck to deliver several yards to the site. Let’s assume $130/yard. At that price, the raw material for your 1000 sq-ft floor is going to run about $1200, or $1.20/sq-ft.

The cost to “place” the concrete is about $1.00-1.50/sq-ft for interior floors. (It’s about a third less for driveways, walks, etc.) This includes getting the concrete from the truck to the floor, leveling it, and steel troweling it to form a smooth surface (done several hours after the initial “pour”).

You’ll also pay about $750-1500 (lump sum) for a pump truck if your concrete can not be delivered through a chute from the back of the truck, as will be the case for most upper floors. The pump truck literally pumps the concrete through a large hose to the floor location.

Finally, you’ll pay for the rebar or wire mesh that reinforces the floor. This will cost another $0.25-0.50/sq-ft depending on how much reinforcement there is.

So, to get a smooth freshly poured concrete floor in place will cost about $2.00/sq-ft plus the cost of the material and the cost of the pump truck. All in, a 1000 sq-ft lower level floor with a 5inch slab will cost about $5000. That same area with a 3 inch slab (say on an upper level) will cost about $4000. (Both scenarios include $1000 for pumping.) This does not include placing any crushed stone under the slab, installing a vapor barrier, or installing insulation, as you will probably do for lower-level slabs. It also does not include placing in the slab any plastic tubing for hydronic heating, which you’ll want to do if you live in a climate requiring heat.

These prices are for Park City, Utah, a resort community outside of Salt Lake City. Prices are similar where I live outside of Philadelphia. Prices may be lower in some rural locations. They’ll be a lot higher in Manhattan.

On a lower level which will have a slab poured anyway, a finished concrete floor is the cheapest floor you can do. On an upper level, there are less expensive options. Specifically, you can install decent carpet for less money, especially when you consider that the floor framing (e.g., floor joists) will be somewhat more expensive because of the extra weight of a concrete floor. However, if you are certain you will use hydronic (i.e., “radiant”) heating in your floors, then there is no less expensive option than concrete. (This is because the alternative ways to get plastic tubing under floors for hydronic heating are all about as expensive as just putting in a concrete floor.) Cost can be a primarily motivation, but honestly, I just like the look and feel of warm gray concrete. It looks great with colorful rugs. It is a nice neutral background for almost any decorating style. It is really low maintenance to boot. It feels very nice in bare feet during heating season.

Reinforcement

Some argue that rebar and wire mesh are equally effective in reinforcing concrete if properly installed (e.g., Fine Homebuilding article “Rebar vs. Welded-Wire Mesh,” 2009). However, one respected contractor declared to me “wire mesh does nothing.” I’ve demolished a few concrete floors for other projects, and have found the mesh at the bottom of the slab, nowhere near the center of the slab where it belongs. My opinion is that either mesh or rebar can work as reinforcement, but that wire mesh is more easily installed incorrectly, and so rebar is the safer method. I also believe that in most upper-floor residential slab applications, it pretty much doesn’t even matter if the concrete is reinforced. Generally, the structural engineer does not assume any strength or stiffness from the concrete floor…it’s basically just a layer of concrete sitting on the subfloor. In my opinion, cracks will form no matter what you do…so that isn’t the issue. So, in sum, I would use rebar for slab-on-grade (e.g., the basement floor) and use whatever the contractor wants to use for the upper floors.

Slab thickness

In lower levels (e.g., basements) your slab will typically be 4 inchs to 6 inches thick. Mine will be 5 inches thick just because that’s the minimum my “flat work” contractor recommends.

When  you use concrete in an upper level you have to support it with the floor structure. It’s heavy, about 40 lbs/sq-ft for a 3 inch slab. So, you can’t generally make it 5 inches thick. Probably the thinnest feasible slab is 1.5 inches. Probably the thickest you would make it is 3 inches thick due to weight considerations. My brother, who is also an obsessive engineer, has determined that 2.25 inches is perfect. My architects argue that 3 inches is the right thickness to minimize cracking. (I doubt anyone has studied cracking as function of slab thickness in any controlled way, and I personally doubt it matters much on that dimension.) I’m going with 3 inches based on the preferences of the architect, the contractor, and to be absolutely sure I’ve got plenty of coverage over the heating tubing.

You also need to think about what happens at the edges of the floor. In general, you pour the floor after the walls have been framed, but before the drywall has been installed. You pour it right over the sub-floor, usually oriented strand board (OSB) or plywood. In most cases you need to add additional  layers to the “plate” (the “2-by” lumber that supports your walls). This is because you need something to attach the bottom of the dry wall to after you pour the floor. So the wall plate has to stick up above the concrete at least an inch or so. So, if your slab is 1.5 inches thick, you’ll double the wall plate (two layers of 2×4s or 2×6s…you know 2×4s are just 1.5inches thick, right?) so that the concrete comes up to the level of the first 2×4 and the dry wall and baseboard can be fastened to the second one. In my case, since I’m using a 3 inch slab, I’m going to use a triple plate; i.e., there will be three layers of 2×4s at the bottom of the walls. The slab will cover the first two, and the top one will be the nailer for the baseboard and dry wall. Note that the wall plate is super cheap…don’t worry about that.

Protecting the Surface

In most residential construction, a concrete floor will be covered by carpet, tile, or wood. Thus, the trades usually make a mess of the floors without any concerns. Paint will splatter, drywall mud will cake surfaces, there will be chips and dings from dropped objects, etc. This is all bad if the concrete will be the finished floor. I spent a couple of days scraping and scrubbing with a scotchbrite pad on my first floor, and thus learned my lesson the hard way. You should protect the floor shortly after it is poured with Ramboard or Masonite. I’m going to try Ramboard, which seems like a nice material for this purpose. It is plastic laminated to a thin hardboard and it comes on a roll. You cut and tape it to fit the entire floor area. I’ll leave it down for the entire construction project after the floors are poured.

Insulation/Vapor Barrier

On a lower level (e.g., basement or garage) you need to insulate the slab. Theoretically you only need to insulate the edges of the slab. I believe code requires something like R10 for four feet in from the edge. The reason you only need to insulate the edges is that once you have warmed up the earth under your house to a depth of about 12 feet or so you won’t lose any more heat to the ground in the straight down direction. You will continue to lose it out the edges and to the sides if you don’t insulate the edges. This is a big deal, arguably the most important few hundred dollars you’ll spend on your project.

Do not believe a heating contractor who tells you that a reflective insulating blanket is all you need. That blanket is pretty much a waste of your money. I’m not going to go into the details other than to say that there is no radiant heat transfer in a system with no air gap. Thus, there is absolutely no reason to embed a layer of reflective foil between the crushed stone and concrete on a lower-level slab. Having said that, the stuff is a good vapor barrier, which you do need. But, you can buy vapor barrier much less expensively than the reflective/quilted blanket product.

Despite what you might imagine intuitively, concrete is a terrible vapor barrier. Water vapor passes right through it. So, you absolutely must have a great vapor barrier under your lower-level slab (not an issue for upper levels). This means some kind fo thick poly film. There are several vapor barrier products ranging from just thick polyethylene to woven/laminated polyesters. I don’t think the specifics matter much as long as the barrier is continuous. (This means any tears, voids, gaps, etc. should be taped up before the pour.)

Many people argue that you should insulate the floor joists below your heated slab on an upper floor. They argue that this ensures the heat will go “up” not down. They also argue that this allows you to maintain better temperature control between floors. I really don’t buy either argument. I believe the heat transfer between a bare concrete floor and the air above it is dramatically better than between that slab and the room below…that path requires conduction through the sub-floor, convection through the air space in the floor joist cavity, conduction across the cieling material, and then convection downward to the room. I’m just not convinced you’ll get very much heat transfer in the downward direction. Of course if the lower space is unheated, you should definitely insulate. But, if you are just trying to thermally separate two heated rooms, I doubt it matters much. (I could be convinced otherwise by evidence…but not by the lame reasoning I’ve heard so far.)

Heating

As I’ve already suggested, you pretty much want to use in-floor hydronic heating with concrete floors. In most cases, you’ll want to use 1/2 inch PEX tubing. There are a lot of internet resources for how to do this. I won’t repeat them here.

Different tints in concrete tiles.

Achieving Desired Color

The natural color of most concrete is light gray. This is because most portland cement is very gray, and most gravel and sand is gray enough that the color of the cement dominates. Portland cement can also be white, which may be used in special applications like countertops. In some parts of the world the sand is a distinctive color like red or black, in which case the concrete takes on the hue of the sand. Most of the time your concrete will be light gray naturally.

There are a bunch of ways to get a color other than gray. First, the obvious. You can paint it. This is very common in industrial plants, and is a common approach to garage floors. Paint is a simple technology, is inexpensive, and can achieve pretty much any color you want. It’s main disadvantage is that without expensive faux-painting techniques you’ll end up with a very uniform color, which is not very interesting visually. You’ll also need to repaint periodically when the paint wears through (which it will do). Still this is a pretty good option, particularly for an old concrete floor, where you need to clean up some surface defects. (Incidentally, most concrete “sealers” are just acrylic or epoxy paints with no pigment added…so you are probably going to be painting your floor anyway if you apply a sealer.)

Acid stains are applied to bare concrete. Metal salts in the stains react with the surface to leave a color on the surface. For example, the copper-based stains are greenish, the iron-based stains reddish, etc. The nice thing about the acid stains is that they look fairly natural. The colors are mostly muted, the distribution of color is mottled, and the patterns are soft. The disadvantage is that the color palette is quite limited (greenish, brownish, yellowish). In the Church House project, I experimented with a bunch of acid stain options under my stair on a surface that would be hidden in the finished house. I liked the effect, but I really wanted more of a gray than a brown/yellow. You can’t get a dark gray with the acid stains. If you are interested in the brownish/yellowish colors, then by all means experiment with the acid stains. They are easy to use and the colors are quite integral to the top surface of the concrete.

Dye-based stains are simply applied to color the surface. These are available in pretty much any tone.

You can also “color the mud” directly. To get a nice dark natural-looking gray, I like 10-15 lbs carbon-black pigment per cubic yard of concrete. The pigment I experimented with is the “230 Pigment” from Direct Colors, Inc. There are lots of colors available (e.g., orange, blue, etc.) so you can get pretty much whatever you’d like. Of course in most cases you are starting with a gray material and so getting a light pink or something is unlikely. (In a small area, say for a hearth or countertop, you can use white portland cement, which lets you achieve light colors.) My plan for the Park City house is to color the mud with a carbon-black pigment. This is an affordable way to get a nice dark gray that is integral to the material.

No matter how you color the concrete, you still need to seal it.

Finishing/Sealing

There is a lot of mysticism around sealers. I’ve come to conclude that the best overall approach is a simple clear acrylic sealer. Basically, you are painting the floor with a clear acrylic paint. Acrylic sealers are mostly water born coatings and they are available in lots of different sheens (e.g., satin, semi-gloss, gloss) They come in lots of different solids ratios (what fraction of the mix is the acrylic polymer versus the water). The acrylic sealers that come in “jugs” are really just low-solids mixes. Those that come in “paint cans” are higher solids mixes. Your goal is to get a nice uniform layer of the acrylic onto the surface. The advantage of the lower-solids formulations is that they dry a bit more slowly (more water to evaporate) and are less viscous. As a result, you may be able to apply them more uniformly and they may flow into the cracks and interstices more readily. I have found that if you are trying to coat a warm concrete floor in a low-humidity environment, the coating dries almost immediately on contact, which can be a challenge.

One caveat with acrylic coatings. Fresh concrete is highly alkaline (high pH) and some acrylics don’t like highly alkaline surfaces. On my first try, the coating literally disintegrated into granules after drying. I ended up sweeping and vacuuming the granules and starting over. I first sprayed the floor with diluted muriatic acid (resulting in fizzing) and then washed the floor with a mop. After that treatment, the coating adhered very well. I don’t think this is a problem for all acrylic formulations, but I haven’t tried them all. It’s hard to believe that a clear acrylic sealer intended for concrete (e.g., Behr clear garage floor paint) has this problem, although they do warn that the concrete must be at least 30 days old before applying the sealer.

I have been very happy with the Behr products (sold through Home Depot). They do very well in the Consumer Reports tests and they are good values. There are three products I’ve used: Behr Wet Look Sealer (in a jug); Behr concrete primer (in a paint can); and Behr garage floor paint (in a paint can). As best I can tell, all are basically water-born acrylics.  I have not tested the primer to know whether it is necessary or valuable. Most primers are just thin versions of the top coat. The prices are similar, so there is probably no harm in using the primer as the first coat.

I’ve also experimented with oil finishes. I’ve tried tung oil finishes (including Danco and Sutherland and Welles formulations). Honestly, I wouldn’t use these. They are slow to dry, smell bad, and are expensive. Elsewhere, in my post on interior wood finishes, I discuss the extent to which these are really “green.” I also found that they did not resist water as well as the acrylics. If you use a satin acrylic you are going to get a very similar look to an oil finish, but with much less fuss. I believe it is a myth that oil finishes penetrate better. They are just a coating like any other coating. The oil polymerizes in the presence of air, resulting in a layer of “plastic”  just like with an acrylic finish.

I’ve also used water-based polyurethane coatings (e.g., Minwax water-based polyurethane wood finish). I was not happy with the results. I have had peeling problems with those floors. I have not tried more conventional oil-based polyurethane floor finishes on concrete. I would expect them to be a bit more durable than acrylics. However, acrylics are so standard in floor coatings, and so easy to apply and re-apply, that I lean towards that approach. (I have a different opinion for wood, where I believe the effect on the appearance of the grain is better for polyurethanes.)

Epoxy coatings have become popular for garage floors. Most true epoxies are thermosetting polymers, meaning they are mixtures of two compounds, which react to form a hard polymer. These can be applied in thick layers and are extremely tough. They are also expensive and a bit tricky to apply. I’ve not used them myself. I believe the clear epoxy coatings are what you see in many Whole Foods supermarket floors, usually applied over acid stained surfaces.

There are mysterious “tile and stone sealers” sold in jugs for application to tile. The one I tried is 4 Care brand (TileLab) tile and grout sealer. It looks and smells like water. (I didn’t taste it.) I tested it on concrete and on a marble tile. Water doesn’t bead, it doesn’t seem to resist staining. It doesn’t change the appearance. The MSDS lists as the active ingredient perfluoroalkyl methacrylic copolymer in the range of 1-5% total weight. (So it basically is water.)  That compound is a water-repelling material used on fabrics and other surfaces, but there isn’t much of it in that jug and it doesn’t seem to do anything when applied to concrete. Maybe there are tile and grout sealers that actually do something, but this particular product does not seem an appropriate sealer for concrete floors.

Finally, there are a whole class of penetrating concrete sealers called silicates. (There are lots more mysterious sealers like silicones, siliconates, and silanes…from what I can discern, the best for smooth floors are the silicates.) These are very low viscosity liquids that penetrate the concrete and react with the CaOH in the material. This results in a permanent integral seal. The reaction can go as deep as several inches. It apparently increases hardness and resistance to liquids. The silicates do not, however, impart any sheen to the surface. I do not know the extent to which they resist dusting, either. I believe that a polished concrete floor (e.g., a Home Depot floor) is first treated with a silicate before polishing with a diamond pad, which gives it some sheen. Conventional acrylic coatings adhere over silicates, so one approach might be to first apply a silicate and then apply an acrylic coating. I’ve not tried this, and my sense is that it is overkill for a residential floor.

Casting a concrete tile using the bottom of a plastic bin.

Experimenting

If you are really into this stuff, and enjoy experimenting, I recommend you buy a couple of bags of premixed dry concrete at Home Depot (about $6-7…but really heavy… 60-80 lbs.) and play around a bit. I made a mold by slicing off the bottom of a plastic bin roughly 19in x 14in x 2in. I used that mold to make big concrete tiles. You can mix the concrete right in the mold, trowel the surface smooth, let it sit for a couple of days and pop it out of the mold. You can then play around with finishes on the bottom surface, which should be really smooth from the plastic on the mold. I used this method to test different concentrations of pigment and to test different sealers.

I made a bunch of tiles to try different sealing methods and different coloring techniques.

Tags: concrete, concrete floors, flatwork, floors, sealing concrete