Concrete Slabs

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• About Concrete Slab Construction: How to pour a proper slab.

- John P. Bridge

I can recall the first time I became aware of “slabs.” I was growing up in the Pacific Northwest, and my older brother purchased a new home, built on a slab instead of a foundation with a wood-framed sub-floor above it. Boy, this was high-tech stuff.

Slabs, of course, had been used in the southwestern portion of the country for years, but I didn’t know that. It was a new thing, and it seemed to me a good idea. I was about 13, and most things are good ideas at that age.

Well, between 40 and 50 years later (I’ll let you figure that out.), I’ve changed my mind a little, and I’ve decided that “slab-on-grade” is probably not the best way to go when building a new home. I think the old fashioned way (foundation and wood sub-floor, maybe a basement) was probably better, though more expensive. I know in many parts of the Northeast, as well as in the Midwest, basements are still a part of most homes.

Still, though, slabs are a reality — a done deal as it were, and that’s what I came here to talk about. In Texas, where I have made my home for nearly three decades, slab-on-grade is what you get, unless you can afford a high-end custom home. And then you might still end up with a concrete slab.

And since we’re stuck with the situation, let’s at least construct the best concrete slab possible.

A concrete slab can be done in a number of ways. Much depends on what sort of soil is beneath it. In some cases (in southern Arizona, for instance) you can simply scrape the brush away and dump the concrete on the ground. Smoothing it a little on the surface makes it easier for subsequent craftsmen to do their job.

In every other part of the country, however, some sort of “footing” must be provided (even in Arizona, if the truth be known). There has to be a mass of concrete around the edge of the slab to support the outside (bearing) walls of the house. Additionally, some of the inside walls will have to be supported by footings. That is, unless prefabricated “trusses” are to be used in roof framing, in which case the entire weight of the structure rests on the perimeter (outside) walls.

That last paragraph was a mouthful for a guy who is supposed to be a writer. Let’s just say that in the absence of a foundation wall, a “footing” is still essential.

In the Houston area the soil is expansive, meaning it is in flux constantly depending on the amount of rainfall we receive. When the soil is damp it expands, and when it dries out it contracts. Thus it doesn’t present a very good substrate for a concrete slab, which depends on support from underneath.

Besides the problem of expansive soil, we have a “subsidence” problem as well. Subsidence is caused by the use of wells for a good portion of our domestic water needs. Water is pumped out of underground aquifers faster than it can be replaced. Our entire area is settling, some regions at the rate of nearly half an inch per year.

To counteract poor soil conditions and subsidence in our area, tract housing builders have come up with a technique called “post-tensioning.” A post-tension slab is built with beams (thickened concrete) running through it at right angles to one another, as well as a continuous beam around its perimeter. A sheathed cable is laid through each interior beam before the slab is poured. After the slab has cured, the ends of the cables are pulled taught with a tensioning device and tied off. This puts a slight camber in the slab, giving it spring. Supposedly, this will hold the slab together, even if the soil beneath it shifts.

It is worth noting, however, that builders of moderate to high-end custom homes will have nothing to do with post-tension slabs, indicating that post-tensioning may not be all that great. Here are some reasons why.

Re-bar is seldom used throughout a post-tension slab. Consequently, although post-tension slabs will generally hold together, they have a higher propensity for cracking than do slabs built with plenty of re-bar.

Additionally, the perimeter “beam” of a post-tension slab does not go very deep into the ground, and the slab is more likely to shift laterally than one built by more conventional means. It can more or less slide around on top of your lot, causing additional settling and cracking.

When my brother and I (and a whole bunch of other builders) were building houses out West years ago, we always dug a footing and ran re-bars around the perimeter of the structure, usually four strands of #4 re-bar, or even #5 in some cases. From the footing we would string #4 re-bars across the future slab, about two feet apart running in both directions, and tie them in on the other side. The concrete would then be poured to a thickness of about four inches, which is standard in housing construction.

Besides using an abundance of re-bar and digging the footing deep enough to prevent shifting, we always placed “control joints” in the slabs, even in tract house slabs. Control joints consist of a programmed weak area of the slab, a controlled crack that will occur under a long interior wall. When the slab cracks, as it inevitably will, it cracks where the cracking poses no problem. For some reason, Houston builders seldom use control joints.

There are builders, though, who construct proper slabs in our area, but they are always builders of the more-expensive homes. The reason I sub-contract exclusively to one builder only is because he spends a great deal of time perfecting the various aspects of the structures he builds, including the concrete slabs. The builder’s name is Gary Bumpass (Bumpass Builders, Inc. of Houston), and when Gary builds a slab it is six inches thick instead of four, and it contains two layers (strata) of intersecting number four re-bars instead of one. The slab usually rests on a very substantial beam, which is supported by a network of underground piers. As slabs go, this is about as strong as they get.

But even at that, no flooring material (except carpet) goes directly onto the slab in a Bumpass home. The slab is poured below finish floor level, and flooring installers (including myself) build back up to the desired grade. Tile and marble floors, for instance, ride on a reinforced mortar substrate that is not attached to the slab. When and if the slab cracks, the tile and marble above it will not. Wood floors are laid upon wood “screeds” installed between the floor boards and the slab.

I may have digressed again.

The remaining two factors governing the quality of slab floors are workmanship and the quality of the concrete being poured. In tract house building in our area, good workmanship is virtually nonexistent. It’s hard to think of more to say along that line, but generally, the slabs are not tamped to eliminate voids when poured, they are not properly leveled, and they are finished in a very poor manner. I face (and somehow overcome) these problems every time I tile a floor in an existing house.

The concrete itself should have a minimum strength of 3,000 pounds per square inch. This means that if it were possible to apply that much pressure to one square inch of the finished slab, the concrete would not crumble. Seldom if ever is this strength attained in tract house construction. Why? Well, primarily because no one checks. In areas outside the city of Houston there are no building inspections at all on residential construction. The builder himself is the chief inspector of his projects. Talk about the fox guarding the hen house.

Many times when concrete leaves the batch plant its strength is around 3,000 PSI, but it doesn’t remain that way. If, for example, it takes the driver much longer than half an hour to get the mix off his truck, the original strength has been lessened considerably. This is true particularly during our hot summer seasons. The concrete can begin to set in the truck. It then becomes a “hot” load.

If too much water is added to the mix, the strength is decreased drastically. Water is usually added at the site right before the mud is poured, sometimes because the mix is “hot.” Sayonara 3,000 PSI. Hope for 1200 to 1500 instead.

The last thing affecting ready-mixed concrete is the quality of aggregate used in the mix. We have a shortage of rock around here. It all must be hauled in from the Texas Hill Country, quite a few miles to the west. Consequently, crushed construction rubble is often used in place of rock. The rubble is not as strong. I have even seen oyster shells used as aggregate.

If concrete sand has not been properly washed, dirt is introduced into the mix. Dirt, I’m sorry to say, is not strong at all.

Summing this up, it must be said that many of the drawbacks to post-tension slabs have to do with inferior products and workmanship. I think the slabs would perform much better if done correctly. No production home builder, however, is going to do this.

I must also point out that post-tension structures have become the norm in commercial and industrial construction. These projects have no connection, however, with the subject at hand.

Unfortunately, unless you are building a custom home, you have little or no control over the construction of your slab. Tract builders only operate in one fashion. What you see is what you get. If you are fortunate enough to be able to afford a custom home, make sure you get the best slab possible. Then again, why live with a concrete slab at all?

When I build my next home, it will not have a concrete slab under it. It will instead be built well up off the ground over a substantial foundation wall supported by ample footings in the ground. The sub-floors will be wood, either solid joists or wood trusses. There will be at least a three-foot crawl space under the house. Allow me to enumerate the advantages of this type of structure.

The floor will remain level, even if the foundation shifts or sinks a little. You can simply shim the sub-floor up off the foundation in places where it has subsided (loosening the anchor bolts and then tightening them back down).

You can still have ceramic tile or stone floors by simply depressing those areas when the floor is framed. The difference is then made up with reinforced mortar, making an absolutely lovely substrate for the stone or tile surface. These floors will never crack. Or, you can install a tiling membrane over well-engineered wood floors and tile directly to them. Schluter Systems and the Noble Company are two companies that make tiling membranes.

You can run all your plumbing under the house instead of through the attic. Broken pipes will not cause flooding, and soil (sewer) pipes are accessible — they can contain clean-outs, making them easy to un-clog should the need arise. Having them under the house makes the pipes very easy to insulate.

Speaking of insulation, you can insulate the underside of your floors. Wood sub-floors are warmer than concrete slab floors.

You can run electrical wiring under the house, shortening the lengths of “home runs” and making circuits more efficient. You can have floor outlets in your living area (or anywhere) without the hassles associated with concrete slab construction.

Finally, you can have those great raised wood-planked (or concrete) porches that people had years ago. You can sit out back and watch the sunset, and you can sit out front and keep track of your neighbors (nonchalantly, of course).

More about post-tensioning from Sun Coast Post Tension.

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