by Dan Empfield
8.7.02 (www.slowtwitch.com)
There is the easy way to do something, and the hard way. I've chosen the latter. I don't mind it. In a perverse way choosing the difficult route is often more fun.
I'm laying my own slab. An Endless Pool ought to go on a flat surface, and since I'm sticking mine outside I've got to make the surface. The easy way is to hire somebody to come in and do it for you. I'm doing it myself, not because I'm a champion slab layer, but because I just feel like doing it myself. So sue me.
PREPARING THE SLAB
LAYING THE SLAB
FINAL WORD ON CONCRETE
PREPARING THE SLAB
I went to Harbor Freight Tools and bought a portable cement mixer for $250. It looks like a grown up model and holds 3.5 cubic feet. That means I've only got to make, ahem, 40 or 50 loads.
The pool is 9' x 16' inside, a foot longer and wider outside, and I'm making the slab 12' x 18'.
I know I can't pour it all in one fell swoop, so I've put forms up that split the job into six equal parts. The forms have been leveled and trued, and prior to that I watered my "dirt" for a few days to pack it down. I'm going to lay a thin layer of gravel, and then put 6" x 6" wire mesh down just underneath the forms. This will act more or less like rebar. The mesh will span the panels, linking them together, so that they don't move independent of each other. I'll leave the two-by-fours in.
At least that's the plan.
It would probably cost me about $600 to have somebody come in and lay this slab, forms and all, and it would be done in an afternoon. I'll probably spend about half that in material costs alone (not counting the mixer). It is therefore a better idea to farm this out.
So why am I doing it myself? Because I've never laid a slab, and I want to do it once before I die. It's something I should've done a long time ago. They should've given a class in slab laying when I was in high school. There were lots of things I should've learned when I was young. "Let not the sun go down upon your wrath" was great to know, but "Let not the sun go down upon your dirty dishes" would've been more helpful to a young single man. I'm making up for lost time.
So tomorrow (and perhaps the day after) I'll lay my slab, eat dinner, do the dishes, and then write about the experience here on Slowtwitch.
LAYING THE SLAB
If there was ever a justification for my uttering the statement, "do what I say, not what I do," this is it. I can truly say with no hesitation or reservation, do not lay your own slab for your Endless Pool.
Oh, if you want to prepare the place for it, and perhaps even build the forms, fine, but pouring the concrete? No way. Why do I say this? Because by the time you drive back and forth to the Home Depot with your truck and schlep 130 90-pound bags of concrete to your home—which weighs in total 6 tons, and will therefore take several trips unless you have a very big truck—you'll discover you haven't saved very much money over having the guy out with his big truck full of enough pre-mixed concrete for three such slabs.
Not that I regret pouring our own. I wanted to learn the world inhabited by the concrete layer. Plus I have several smaller concrete jobs to do around the house, and need to know how it all works. And, finally, and most importantly, I want to be able to answer any question asked by any Slowtwitcher with the authority of having gone there beforehand.
It took me a day to assemble the concrete mixer I bought (a Chinese-made job with instructions like, "Handle with carefully"). Then I went to Home Depot to buy the concrete. They've got many types there, and what I decided on was your basic 3000psi ready-mix with the lime and the sand and the gravel all included. Just add water. It's pretty much like making a huge Krusteaz pancake. Or in my case six huge ones, because my 12' x 18' slab was sectioned into six equal 6' x 6' panes.
Keep in mind Monty and I had never done this, at least on this scale. So, for the first panel, we threw in three bags of concrete, added the water, and then turned on the mixer. We watched for several minutes as the water sloshed around on top and the concrete stayed concrete on the bottom. After an hour of prying the concrete off the bottom of the mixer with a shovel, we got the concrete mixed and out into the panel. We needed six additional loads that size in order to finish the panel off.
For the second load—since our initial tactic didn't work—we put the water in first, and then the three bags of concrete, but with basically the same result.
By the third load we'd abandoned the idea of tryhing to measure the water, and just shot the water into the mixer with the hose. We hit on the idea of putting the water in little by little, and shoveling the concrete in a shovelful at a time, and this kept the mixer from getting clogged up. We basically eyeballed it, and poured the concrete when we thought we were ready to make a good "pancake."
If you were to look at our slab, with its six panes, you might think of a schoolhouse chart of the "descent of man," with an ape on one end of the chart and modern man on the other, and six intermediate steps inbetween. You can tell which was our first (ape) pane, followed by our Peking pane, our Homo Erectus pane, and so forth. Fortunately, few people will ever seen our slab, because in short order it's going to have an Endless Pool on top of it.
On a technical note, I decided to run our 6" x 6" mesh inside each individual slab, and drill holes through the inside panel separaters—which are made of pressure treated two-by-fours—through which run three-foot pieces of 3/8" rebar. These rebar pieces act as pins, theoretically, to keep the slabs from moving independently of each other.
I suppose there is one justification for laying a concrete slab yourself (if your Endless Pool installation requires a slab at all). That would be to do it as a father-son project. It is definitely a two-man job, both in terms of using a mixer, and in running a long two-by-four over each pane to smooth the concrete out (you then ought to use a float or a trowel to finally smooth it out). I think if I had a teenage son it would be a good thing for him to learn to do with "pop" (though after doing this job I doubt "my son" would think it was such a grand bonding project).
One fortuitous aspect of building my slab in six equal panes is that I used several different types of concrete, to see which I liked best. In all cases I used the kind in which you just add water (as opposed to buying cement, sand and gravel individually).
But I thought I'd try out a few different brands, types, and retailers. I bought from both Lowes and to Home Depot. I used concrete made by both Riverside and Quikrete. In truth, I didn't find too much difference. But there were a couple things I'll mention.
Concrete mix generally comes in either 60-pound bags or 90-pound bags. I built half the panes out of 90-pounders, because I figured that—me being a real man and all—I ought to be using the heavy bags. I then found, though, that the job went significantly faster if I used the 60-pound bags. It took me a third less time to do the work if I was tossing the lighter bags around.
In fact, with smaller bags—60 pounds or less (40 pounders would've been perfect, and I understand that's what gets sold in some parts of the U.S.)—I could just dump the contents of the bag into the mixer, without having to resort to using a shovel. Big time saver. The trick is to start with a good deal of water already in the mixer, and dump the contents of the bag in slowly. It helps to have a second person spraying a mist of water into the mixer as you do this, to keep the dust and fumes down. I always wore a paper mask when mixing concrete, and the "better" the concrete the worse the dust was, since the portland cement particles are so fine. This also makes higher spec concrete trickier to mix—you might have to stop your mixer occasionally to unstick (from the sides of the mixer) the fine cement particles.
For structural applications—like house building—you'd need to use 3500psi concrete or better. I found, though, that for me and for my Endless Pool slab the cheapest non-structural Quikrete was fine. In California that is model #1015. It's only sold in California, at the behest of Home Depot, Quikrete's biggest customer. I didn't get the impression that Quikrete is happiest about that, I think it would rather Home Depot just buy the #1101, Quikrete's flagship product (just the sense I got). But Home Depot prefers a garden-variety concrete product for its California stores, and when Home Depot talks, vendors listen.
I know all this because I spoke with Greg Gibbel, Quikrete's sales manager for all of California and Arizona. In other parts of the country it's probably best to use the concrete that Quikrete sells in its "yellow" bag, and that's the famous #1101. Home Depot sells it for $1.99 for a 90-pound bag out here where I am, which is cheap. But Home Depot won't buy #1101 in 60-pound bags (where I live).
As far as I can tell, the difference in strength is just in how much cement is in the concrete mix. Cement is the "binder" in concrete and the more cement the stronger the concrete will be (in ratio to concrete's "filler" which is gravel and sand, concrete's other two constituents). As an example, one part cement to two parts sand and three parts gravel is considered a high-strength product, and some of the lower strength concrete I used had a sparser ratio, perhaps 1:3:5.
Cement—called "portland cement" in this application—is made from limestone, clay or shale, sand, and iron ore, in a process that requires a lot of crushing and grinding and a lot of heat (almost 3000 degrees F). Portland cement was invented in England in 1824. It was so-named because it resembled stone that came from the Isle of Portland. Concrete is the most commonly used building material in the world, and I find it fascinating that a product invented less than 180 years ago has had such a momentus impact in the world in which we live. It's inventor was a builder in Leeds, England, named Joseph Aspdin. Historically speaking a fairly overlooked guy, I'd say.
When water is added to cement, a chemical reaction occurs, and keeps occuring for years. Cement that is five years old will be four times harder than it was the day after it was poured, and twice as hard as it was a week after it was poured.
Steel in the form of metal bars (rebar) or wire mesh is used because while concrete is hard, and has great strength in compression, it has little tensile strength. So it cracks. Steel has great tensile strength, so combining the two elements creates a strong combination.
Mortar is also made of cement, but without the aggregates (like gravel), It's just cement, sand and lime.
There are other types of specialty concrete. You can add a higher-grade aggregate if you want—replace the gravel, for example, with pieces of steel. You can add other fillers that add strength and workability. But you don't need that for your Endless Pool slab.
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