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Poured concrete foundations are among the most widely used of the several types, largely because of their strength and solidity. They are particularly good in areas of severe weather or high ground moisture content, and serve equally well for low crawl-space or the full- basement types of foundations. From a practical standpoint, making poured concrete foundations demands that ready-mix concrete is available within a reasonable distance and can be trucked to the building site. If the service is not available or your location is such that the huge, extremely heavy mixer trucks can't get to your site, you might want to consider another type of foundation. The alternative to ready-mix concrete is to mix up a good many cubic yards by hand or with a small power mixer, and this is a big, big job.
The first requirement for a poured concrete foundation is a concrete footing or footer. The bottom of this footing should rest a minimum of 18 inches below the original grade level or 24 inches below a new finish grade level made up with fill dirt. Footings should always rest upon or be dug into undisturbed native soil, and never poured on loose fill dirt. Where the use of some fill under parts of the footing is unavoidable, the fill must be soaked and compacted mechanically in shallow layers, built up as necessary, and tested for density of compaction until suitably solid results are obtained. In areas of severe winter weather, the bottom of the footing should rest below the approximate average frost-line depth, which in many places means a depth of 4 feet or more.
The general rule for footing sizes is to make the depth of the footing the same as the thickness of the foundation wall, and the width of the footing twice the thickness of the foundation wall. This is suitable for small- and medium-size houses and for many log houses. However, because of the extra weight that large log houses (or houses made of large logs) impose upon foundations, tripling the wall thickness to arrive at a footing width to provide extra flotation might be necessary. Very heavy log houses made of massive logs often require even larger footings, and the foundation walls are also likely to be thicker than the usual 8 inches.
The footing forms can be prepared in two ways. One is to cut a fairly wide working trench, then build up forms of planks, ply wood, or special premade concrete form sections, properly braced and staked to hold them rigidly in place and level upon the floor of the excavation (Fig. 4-5). The tops of the forms must be level in all directions.
The second method can be used where the building site is relatively flat and the un disturbed native soil is dense and tight. This involves first digging a square-sided, flat- bottomed trench of the correct width and depth, and exactly following the line of the foundation (Fig. 4-6). This can be done with a narrow bucket backhoe and then trimmed and smoothed with a pick and shovel, or the whole job can be done by hand. This system, by the way, works fairly well for small projects, but is less effective and can mean more work than the usual forming methods.
When the form is finished, lay in two or three parallel No. 4 (½-inch) reinforcing bars, set so they are about 2 inches in from the side of the footing and about at mid-depth. (This is a standard arrangement, but a fully engineered footing for a particular house might be done differently). The rods can be perched on small rocks, chunks of brick, or scraps of wood; pull these props out as the concrete is being poured. Some workers like to wet the forms and the soil down with a hose just before pouring so the moisture in the fresh concrete mix is not sucked out into the surrounding materials.
Pour the form full (or to the correct depth in the trench) of concrete, mixed in a ratio of 1 part portland cement to 2¾ parts sand to 4 parts gravel. This is a commonly used mix that works well for footings, but other ratios are sometimes used. As the pouring proceeds, rough-level the top surface with a shovel or rake, taking care not to knock dirt down into the fresh mix. With the trench form you must level the surface continuously as you go along, smoothing as necessary and pushing excess mix ahead. With the plank form, strike excess concrete off by scraping along the form top with a chunk of board or 2 x 4.
As the top surface is leveled and smoothed, press lengths of 2 x 4 down into the wet mix until the top is flush with the concrete, and re-level the surface as necessary. After the concrete has begun to cure but before it has completely hardened, remove the 2 x 4s, taking care not to dislodge any of the concrete. This will leave a keyway that will help bond the foundation wall to the footing (Fig. 4-7).
Allow the footing to cure for a least five days for best results, and preferably for seven days. As it cures, keep the concrete surface damp by misting occasionally with a hose, by covering with wet burlap, or by sealing the moisture in with a strip of construction plastic. This process allows the concrete to cure to maximum strength without drying. At the end of the curing period, strip away the wood forms. The trench form needs no further attention.
The next step is to construct the forms for the walls. Professional concrete workers use a premade forming system that can be reused time and again; you can do the same by renting or borrowing the necessary components and supplies. Most owner/builders, though, construct their own one-time forms, then use the materials later for other purposes. These forms can be made from sheets of ½-inch or ¾-inch exterior-grade plywood, supported by studs and wales plus staked braces where necessary (Fig: 4-8).
Don’t skimp on studs, because fresh concrete will exert a tremendous amount of pres sure on the form walls. Two wales are usually sufficient, depending on the size and frequency of the studs, for a low wall. For full basement walls, several wales should be used along with extra bracing. The shoes on each side of the form bottom lie flat atop the footing so that the longitudinal centerlines of both footing and form match up. The shoes can be nailed to the still-green footing top—right into the concrete—with ordinary common or box nails, or with double-headed scaffold nails for easier removal. Provided that you don’t use too many nails, this will not harm the footing.
Combination tie/spreaders, a variety of which are available at lumber and hardware supply outlets, should be inserted through the forms as they are built. These serve to hold the form walls in place until the concrete cures, so that the form walls can neither shift inward and out of place while still empty, nor bulge out ward as the concrete is poured. After the concrete has cured, the breakaway heads of these tie/spreaders are knocked off with a hammer, and the remainder stays embedded in the concrete. Some wall designs require that reinforcing rod or mesh be embedded in the concrete; these materials are placed according to plan be fore the tie/spreaders are installed.
Once the forms are erected and double- checked for rigidity, accurate dimensions, and proper level, the concrete can be poured. The same concrete mix ratio can be used here as in the footings, provided that normal ground moisture levels at the building site are low. However, in many areas of the country where precipitation and consequent ground water content is moderate to high, a more watertight mix is preferable for full-basement foundations. A mix consisting of 1 part portland cement, 2¼ parts sand, 3 parts gravel, and water not to exceed 6 gallons per sack of cement (including the moisture in the sand) is a good one to use. Top the forms up full and screed the surface (scrape off the extra) with a block of wood. Em bed a series of anchor bolts deep into the concrete about a foot from each corner and at intervals of about 4 feet elsewhere, making sure that the threaded portion sticks up far enough to go fully through the sill plate or sill.
For best results, all these concrete pouring operations should be done in temperatures between 40 degrees F and 75 degrees F, and a relative humidity of around 40 percent. The upper temperature limit can be extended to 85 degrees F with no problems if the winds are calm and the air is humid. Cold temperatures or hot, dry, and windy conditions bring about problems in concrete work, and special precautions must be taken to avoid damage or improper curing.
Cover the exposed concrete to retard evaporation, or moist cure with frequent misting, or cover with damp hay or burlap for about three days. Allow the concrete to cure for at least 5 days and preferably longer. Strip the forms away carefully and set all the material aside for later use in the building. Plywood, for instance, can be reused as subflooring or roof sheathing, while the studs and wales can be put to work as blocking, supports, hidden studs, or braces.
If openings are needed through the concrete walls, they are provided for as the forms are built. Full-height openings are made simply by stopping the forms at the appropriate points and closing off the ends. Smaller openings such as for windows or vents are made by blocking out a suitable opening in the form it self. A window opening, for example, could be made by building a rectangle of nominal 2-inch stock, with a couple of braces within the rectangle if it's very large. The frame is inserted into the form and nailed into position. Lintels or headers across the top of the window opening for strengthening can be of steel, stone, wood or other materials, set into place and se cured. Then the concrete is poured all around the frame, with precautions made to ensure that there are no air voids next to it, especially underneath.
Small openings, chases, or sleeves such as might be needed for running pipes or wires through from the outside, can be made by inserting short lengths of plastic or steel pipe of appropriate diameter through the form. These can be cut flush to the wall surfaces later. Notches for bolt pockets or door sills located at the top of the wall can be formed simply by impressing pieces of wood cut to the required shape and dimensions into the wet concrete after the pouring has been completed, and removing them before the concrete sets up hard. Pockets or saddles for joists or girders can be made in the same way. Just box out an appropriately shaped area at the required locations in the forms so that the concrete can't fill those spots.
Full basements are usually provided with poured concrete floors. This job can be done as soon as the foundation is finished, or delayed until the shell is built, provided that there is at least one suitable opening through which a concrete chute can be run. Delayed floor pouring does have one advantage: all the pouring, curing, and finishing work is done in an area completely protected from the weather, so the job is easier and the results often better. Either way, the earth floor must be cleaned up and rough-leveled.
An expansion joint is usually installed around the perimeter of the foundation to separate the wall from the floor and reduce the possibilities of cracking from expansion. The concrete is often poured directly onto the dampened subsoil of the excavation, though in some cases a layer of sand or gravel about 2 inches deep is spread first. Reinforcing mesh is best included in the floor slab, about an inch from the slab bottom. The floor slab is generally about 4 inches thick. A plastic vapor barrier and insulation might also be called for; if so, this is laid before the mesh and sometimes before the sand cushion.
After pouring, the concrete is leveled and screeded flat and excess concrete removed. Just as the concrete begins to cure, a few hours after pouring, the surface is finished with a hand float. For a very smooth surface, subsequent finishing can be done with a steel trowel. The slab should be moist-cured by misting with water or covering with construction plastic, or both, for about three days. It can then be walked on, but should be allowed to gain strength for several more days before being subjected to heavy traffic.
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