Facts About Concrete Slabs

A CONCRETE SLAB with perimeter footings poured at the same time provides a floor and support foundations for walls. En­tire houses are built this way, often with copper water pipes im­bedded in the slab to provide radiant heating for the home. A base of this type is often used when a room is added to an existing home or when a small extra building or garage is needed. Sometimes the footings are poured first (mostly when the footings must be higher than the floor level), but more frequently footings and slab are poured at the same time.

An additional room, a garage or even an entire house can be built on concrete slabs, and it's easier than you think

Size of footing—height above grade, depth below grade, thick­ness—depends on local regulations. It's important to check on this. Actually, the matter will come up anyway if you're adding a room or building a garage because you'll need a building permit. With the permit you can get the information you need to do the job right.

The dimensions and shape of the new foundation must be care­fully laid out before you can start on forms. This is done with lines and stakes. Don't rely on your eye alone even if you are putting up just three walls (for a room in which the fourth wall will be supplied by the house, for example). It is impossible to judge at first sight whether the lines of a new slab are at right angles to existing walls. Being off an inch or two will give you much trouble later on. It's easier to do it right the first time.

If you are putting down a slab for a separate building, a garage for example, first stake out a base line, measuring either in from the property line or out from a house wall. Use a stake at each end to mark two corners of the new building and a taut line between to indicate one side. To be sure that coiners will be square, make and use the large wooden right triangle suggested in the drawing.

Forms for slab 5 or 6 in. above grade are easiest to build (left). This slab is for room being added against rear wall of house. The preparatory work is not difficult, but be sure to get help with the concrete.

The ground should be worked so that the slab will be uniform throughout (r.). Fill under slabs is often regulated by local build­ing codes, so check with your local building inspec­tor before you start work.

Make a large right angle to work with as shown in the drawing at right. This will speed up laying out the job. You can also es­tablish a right angle with line as described in text.

Underground drains can be placed before pouring slab (left). This opening will take downspout from the roof. The joints will be sealed with paper.

Forms for low slabs are not fancy, do not need much bracing (1., below). Form is 10 in. high, slab 24 in. Trench in the ground is a form extension.

In slab construction, rough plumbing is installed with forms (below). Accurate plans and careful work are required to locate the pipes correctly.

If you choose, you can do without the square by following the procedure de­scribed below.

Construct a right triangle with line. Form the first leg by placing a third stake exactly 6 ft. away from one of the stakes which marks a corner. Connect these two stakes with a line. To be sure the corner is square, tie a second line to the third stake and cut it to measure exactly 10 ft. Now, move the third stake until the 10-ft. line just touches the line which marks the es­tablished side of the new building at a point 8 ft. away from the corner stake. What you have done at this point is to set up a triangle with sides that measure 6 ft., 8 ft. and 10 ft.—a right triangle. Re­peat the procedure at each of the corners or use the wooden square if you decide to make one. For a final check on the layout, measure across from opposite corners (the two diagonals). If they are equal, you know that the corners are square.

Batter board setup is shown above. As a final check for squareness, look to see that diagonal A equals diagonal B. Vertical lines where the lines intersect mark corners of the slab.

The method of determining corner points of slab is shown in the drawing at right A plumb bob is dropped from the points where lines intersect.

Typical slab is poured on solid soil with trench, out­side boards the only forming. Note methods of reinforcing. Wire mesh is placed before the pour but lifted during pouring.

Batter boards (see page 61) are often used to establish a layout. Lines are stretched between opposite sides so that they cross exactly at corners and in the direction established by the right triangle. Plumb lines, dropped from the point where the lines intersect, establish the corners. Usually, stakes are driven into the ground under the intersection points, and nails are used in the stakes to mark the exact cor­ner points.

For levelness, establish the height of one corner stake so that it represents the height of the new slab. If the slab is for a room that is to be tacked onto a house, establish the height point against the existing wall. From this point you can stretch lines from corner to corner, and use a line level to es­tablish the correct height of the remaining three corner stakes. When using the line level, be sure to stretch the line as tightly as possible. When you complete these steps, you will have established the form of the building, made sure the corners are level and that the new slab will be level.

Forms and Reinforcement

How much forming you do depends on the site. If the slab is raised above the ground, the outside forms (which repre­sent the outside face of the footings) will have to be high and adequately braced. Since you'll require only a 4- to 6-in. thick­ness in the slab itself, a situation where the slab will be raised calls for fill in under the slab area. Whatever local regulations specify on the subject, be sure that any fresh fill is solidly tamped.

The ideal situation is one where the slab will be elevated about 6 in. above the ex­isting grade. Where the ground is level, the outside form boards can be quite simple since you can trench the ground to provide footing forms.

Most situations fall somewhere between the two, with the slab raised more than 6 in. above grade but not so high as to re­quire extensive forming. At any rate, when a slab and footings are poured simultane­ously, the only forming you must do is on the outside. The inside face form for the footing is supplied by trenching or by the fill used under the slab area.

Be sure the bottom of any trench is smooth and firm; tamp if necessary. Trench sides should be smooth and perpendicular. The break from slab base down into the footing trench should be nicely rounded off.

All steel reinforcement is placed after the forms are complete. Be sure the steel is clean and free of rust scale and other coatings. It should be placed in such a way that it forms a continuous line through the footings. Usually, two lines of steel are called for. To keep these in place, cut shorter pieces of rod and drive them into the bottom of the trench. Drive them until they are at least 4 in. below the ultimate level of the slab, then tie the steel to them with wire.

Code in this area required that all pipes in or under concrete be wrapped with roofing paper. Note temporary blocks for holding pipes In place.

Always bend the steel to turn a corner, making the bend so it will be centered in the footing pour. Where you must connect rod-ends tomake a continuous line, be sure to lap them at least 24 in. and tie them firmly together with wire.

Slabs that project from an existing foun­dation can be tied to it in one of two ways. Use a carbide-tipped bit to drill holes in the foundation. In these, set long bolts in expansion sleeves to which you can wire-tie the steel in the new slab, or simply press the ends of the rods themselves into the holes in the oldfoundation.

Steel mesh is used to reinforce the slab area. This is cut to size and placed on the ground. Later, during the pour, it is lifted slightly to center it in the slab thickness. Cut the mesh long enough so that it can be bent over to drop into the footing trench. Sometimes the ends of the wire mesh are wire-tied to the reinforcement steel in the footings.

Be prepared when concrete comes. Be sure all form work is complete and inspected. Plan steps of job before the truck arrives. Get enough help.

Two methods of tying new slab into an existing foundation are shown below. Wire tie steel to expansion bolts, or set the steel into drilled holes.

CUBIC YARDS OF CONCRETE REQUIRED IN SLABS
Square Feet Slab Thickness
In Slab 4        5 6    8
50 .62             77 .93           1.2
100      1.2     1.5    1.9       2.5
200      2.5     3.1     3.7       4.9
300      3.7     4.7     5.6       7.4
400      4.9     6.2     74        9.8
500      6.2     7.2     9.3       12.4

Pouring the Concrete

Most often, concrete for a job like this is required in sufficient quantity to justify a trip by transit-mix truck. Ideally, the truck should be able to maneuver around the site so that its chute can be utilized to deposit the concrete where needed. Often this is not possible and some auxiliary muscle work and a wheelbarrow are re­quired. Don't try to do the job alone. Trade a day's work with a neighbor or hire someone to work with you. Even if you decide against doing the concrete work yourself, you will still save a large amount of money by doing the preparatory part.

Typical slab and footing pour in moderate climate area (left). Slab, on well-tamped fill, is 6 in. above the grade. Note anchor bolts for sill.

One man pouring, one raking. Pour concrete to minimize amount of raking necessary to spread con­crete. Pour a little high so the strike board will level.

On large slabs, run straight 2x4 across middle and set it level with temporary braces. This gives you something to work to in the center of the pour.

Pour the concrete into the footings first, using a 2x4 or a 2x6 to work it down so that it fills the forms completely. Don't overdo the tamping—you'll be pushing the larger aggregates to the bottom and caus­ing the fine materials and the cement to rise to the surface.

Tapping along the outside of the forms with a hammer will help settle the concrete and provide a smooth surface on the outside face of the footings.

As the footings fill, start dropping the concrete for the slab. One man can pour the concrete, another move it around with a rake. As the slab fills, hook the wire mesh with a rake and lift it enough for some aggregate to get under it and support it.

Tamp the entire surface slightly with a rake and then level the surface with a strike board.

Lilt wire mesh reinforcement with rake so ag­gregates can get under it. It should rest midway in the slab. Too high is even worse than too low.

Initial finishing is with wood float. Final touch for smooth floor is done with steel float. Use steel float after the concrete has pretty well set.

Initial leveling can be done at any point, from any point, but the final striking is done with the strike board resting on opposite sides of the forms.

Follow up man with the steel trowel to fill in any air holes remaining and smooth them down. A rougher surface may be desired for outdoor floor

Before the concrete sets too much, insert the anchor bolts that will be used later to lock the sills in place. Twist these a little as you press them in to help settle the concrete around them. It may prove helpful to nail temporary supports to the forms to hold the anchor bolts ver­tical until the concrete hardens. Some craftsmen prefer "floating" the sills in place while the concrete is wet. To do this, cut the sills to the lengths required and drill holes for the anchor bolts. Insert the anchor bolts through the holes and set the nuts so that the threaded end of the bolt projects through the sill about 1 in. Posi­tion the sill so it makes firm contact with the wet concrete and press the anchor bolts down into place.

Usually, anchor bolts are required to be on 6-ft. centers, but be sure that you also insert them at corners and on each side of door openings.

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