Masonry Techniques for Building Concrete Foundations--Slab Foundation

The slab foundation is a top choice when you plan to build a garage or workshop on relatively level ground. The great advantage of a slab is that once it's poured you are ready to put up walls.

If you live in an area where the ground isn’t subject to frost heave and you want a simple 8-foot by 8-foot tool shed in the back yard, you can make the foundation simply by nailing some 2 by 4s together. Once the forms are square and level, brace them every 2 feet with stakes, and fill them with concrete.

For larger and more complex slabs, such as a garage slab, this method is not sufficient. For these slabs, additional construction techniques de scribed below are required, so the slab will withstand frost heaves and support the weight of the building.

84a After the forms are filled with concrete, cut between the concrete and forms as shown, then remove the forms.

84b A slab foundation involves building a form around the perimeter, pouring concrete, and finishing the surface to the desired smoothness.

- Code Requirements -

When building a garage slab, you must submit plans to the building department and obtain a building permit. Here is a checklist of code requirements you generally will have to meet, but you should consult with your building inspector in case local codes differ.

• The concrete slab itself must be 4 inches thick.

• A footing ditch must be dug around the perimeter of the slab. The ditch must be 6 inches wide at the base and 12 inches deep, or below the frost line.

• The footing ditch must contain two layers of reinforcing rod, one about 3 inches from the bottom of the ditch and one about 3 inches from the top of the slab. Where one rod meets another, they must overlap by specific amounts and be wired together. Overlap 1/2” re-rod 24 inches and 5/8” re-rod 30 inches.

• The top of the slab must be 6 inches above the surrounding soil.

• Most areas require a 3” to 4” layer of gravel under the concrete. This keeps the slab off the ground and reduces chances of groundwater soaking through the concrete. If you are laying down gravel, form the slab with 2 by 8 boards, which allows 4 inches of gravel and 4 inches of concrete.

• Although not required, it’s good practice to cover the gravel with 6- mil polyethylene, which keeps moisture in the concrete and thus aids in the curing process. If you don’t use plastic, thoroughly wet the gravel be fore making the pour.

• A layer of 6”-square welded wire must be placed in the slab. The wire should be held off the surface of the gravel by rocks or small blocks so it will be centered in the concrete.

• Anchor bolts must be placed in the concrete to hold the mudsill. Use 1/2”-diameter anchor bolts that are 10 inches long and embed them 7 inches in concrete. Place them 6’ on center and not more than 12 inches away from the end of a mud-sill. Remember not to place bolts in door openings where there will be no mudsill.

- Constructing the Forms -

As with any major foundation, first lay out the site with stakes and twine to roughly position the building, then construct your batter boards and place the perimeter strings. Batter boards for a slab should be built fairly low to the ground so that the string lines will be only a few inches above the top of the form boards. If you are going to have a backhoe dig the footing ditch, place chalk lines on the ground and remove the strings for the backhoe. If you are going to dig the footing ditch yourself, it can be done after the form boards are in place.

Ill.85 Slab foundation (cross section): No mudsill in door openings; Re-rod reinforcing in footing; Splice re-rod by overlapping and tying with wire; 6-mil polyethylene over gravel; 3- to 1/4” layer of gravel beneath the slab; Footing 6 inches wide and at least 12 inches deep; Slab at least 4 inches thick and at least 6 inches above grade. Mudsill; Anchor bolts no more than 6 feet apart and not more than 12 inches from the end of any mudsill.

Form Boards

Usually the ground for your slab will not be perfectly level, but this is not a serious problem. The low spots can be filled in with gravel and additional concrete. Start placing the form boards at the highest corner and work toward the low corner. Using a plumb bob, drop a line from where the strings cross at the corner and place the first form board there, with the inside face just under the string. Drive a stake along the outside of the board, and nail the stake to the form board. Place a level on the form board and stake it at the other end. Continue around the perimeter in this fashion, then begin straightening and bracing the form boards.

Sight down a form board to see if it bows in or out. Pull it into line under the string and stake it. Drive stakes every 18 inches along the forms to keep them from bowing out under the weight of the concrete.

At the low side of the slab forms, there may be a gap of a few inches to nearly a foot under the top form board. Fill this gap with additional boards nailed to the stake, then pile dirt against the outside of the forms.

Footing Ditch

With the forms in place, dig the footing ditch according to your local code requirements. After the ditch is completed, hang the steel as described. With that job complete, you are ready to construct screed guides (for large slabs).

Screed Guides

Garages are often 20 or more feet wide, which means you can't find a board long enough to use as a screed when striking off the concrete. There are two ways to solve this problem. The first is to install permanent dividers in the garage, using either treated or naturally rot-resistant wood. The second solution is to install temporary screed guides that can be used both to level the gravel base for the slab and to strike off the slab itself.

Screed guides are 2 by 4s sup ported on stakes. Permanent guides must be level with the tops of the forms and , if more than one is used, you must space them equally apart.

The guides should be 8 feet to 10 feet apart, depending on the width of the slab. For a slab 24 feet by 24 feet, for example, place the two screed guides at the 8-foot and 1 6-f.oot marks. For temporary screed guides, place one end of the 2 by 4 screed guide on top of the form, level it, and stake it in place. Now, working from the other side, repeat this process so the guides meet at or near the center.

With the screed guides in place, you will need a screed made from 2 by 4s. For temporary guides, nail an “ear” made from a 1 by 2 stake to the end of the screed that will ride on the temporary guide. The other end of the screed rides on top of the exterior form. For center screeds, use ears on both ends of the screed. Temporary screed guides will be removed after the concrete is poured.

Ill.86a Building the forms: Flour marks inside the edge …; Top of form boards are always made level; Fill in gaps beneath form boards with scrap lumber and bank dirt against the form for support.

Ill.86b Screed guides: Temporary screed guide; Footing trench

Gravel

Add the gravel once all the items above are in place. To level the gravel inside the forms, use a screed made from two 2 by 4s held together with plywood cleats. Screeding the gravel the width of the additional 2 by 4 places the gravel 4 inches below the top, of the forms, which is the standard thickness of a garage slab. For thicker slabs, adjust the screed boards accordingly.

Vapor Barriers

A vapor barrier provides protection against water working its way through the concrete slab and helps the concrete cure better. The barrier is made by spreading 6-mil plastic sheets over the leveled gravel. Over lap the sheets by 12 inches. Do your best not to tear the plastic.

Reinforcing Wire

Reinforcing wire is sold in rolls, and you must cut it to fit within your forms. Leave it long enough to reach from form to form, and wire it to the steel in the foundation ditch. Wherever one strip of wire meets another, they should overlap by 6 inches.

The wire should be raised off the ground so it will be in the center of the slab. Place rocks at regular intervals to raise it. Be careful that the wire doesn’t bow up so that it would be exposed on the surface.

Hanging the Steel

Steel reinforcing rods must be placed in the footing ditch in two layers, one about 2 inches from the ground and the other about 2 inches below the surface of the finished slab. Where two reinforcing rods meet, they must overlap by 24 inches for 1/2”- diameter rerod and 30 inches for *- inch rerod. Normally, 1 1/2” re-rod is all that's needed in slab foundations. Use tie wire, available at most hard ware stores, to wire the steel together. To bend the steel to go around a corner, just put one foot where you want the bend to be, then pull up on the remaining length of rod.

Ill.87 Screeding: Gravel smoothed to 1/4 inches from top of form with screed; Plastic sheeting over; Hanging steel: Hold wire mesh above gravel and plastic with stones or dobies.

To hang the steel, drive nails on the inside of the forms, 2 inches below the surface and about 3 feet apart. Using tie wire hung from the nail ends, suspend the bottom layer of steel 2 inches off the ground. Repeat this process for the top layer, hanging it right next to the nails, or resting it on top of the nails if you can. Keep the steel away from the form boards or it will be visible and ineffective.

- Pouring and Finishing the Slab -

Once you are ready for the concrete, be sure a building inspector first approves your work. The inspector will check that: 1) the footing ditch is deep and wide enough; 2) there is the right amount of steel in place and the overlaps at joints are long enough; 3) the gravel base is in place; 4) the reinforcing wire is laid out; and 5) you have the required number of inches (usually 4 inches) between the surface of the gravel and the top of the forms.

When the concrete truck arrives, the driver will hook up extension chutes for the concrete. If your slab is wider than about 18 feet or if the driver can't drive to all sides of the forms, you will have to move concrete to the far corners of the slab by wheelbarrow. It is advisable to have at least two wheelbarrows and a couple of helpers on hand.

Start by pouring the area that's farthest from the transit-mix truck. As you begin filling the footing ditch, watch that the reinforcing rods are not pushed against the forms.

Pour concrete in only one section at a time. When that's done, move to the next area while your helpers screed off the first section. If you place too much concrete to move readily with the strikeoff board, use a shovel to move the concrete. Throw additional concrete into low spots.

When using temporary screed guides, where the strikeoff board must fit between the guides, you can not saw the strikeoff board back and forth much to settle the coarse aggregates. In this case, it's good to have a jitterbug tamper on hand to settle the concrete. Do not overdo it, however, or you will cause too much separation of water and aggregates.

As work progresses, have a helper use a hammer to rap on the outside of the forms. This causes the concrete to settle smoothly against the forms so no air pockets will be visible when the forms are removed.

After the concrete has been poured and screeded, remove the screed guides and stakes. Work the concrete lightly around the stake holes to fill them in.

As the pour proceeds, keep a close eye on the concrete. In hot weather, the first part may be setting too fast while you are still pouring the remainder. After each section has been screeded, smooth it with a bull float.

Ill.88 Pouring and striking off: Have helpers strike off first section as second section is poured. Settle concrete next to form by hitting it with a hammer. Helper shovels concrete into gaps.

Ill.89 Finishing: The initial smoothing is done with a ball float; For a smoother finish use wood floats. For a real slick finish use steel floats.

Once the concrete has set enough to place kneeling boards on it without leaving much impression, begin finish floating the slab. Distribute your weight on kneeling boards made from 3-foot-square pieces of 1½” plywood with 2 by 2s nailed along two edges for handles. Use kneeling boards in pairs, one under your knees and the other under your toes. Go over the surface once with a wood float for a coarse finish, and again with steel floats for a slick finish. For large slabs, use a rented power finisher, as described.

Before the concrete hardens, take a trowel and cut between the forms and the concrete.

- Placing Anchor Bolts -

Place the anchor bolts for the mudsill as soon as the concrete has been screeded and bull floated. Using a tape measure as a guide, place bolts 1½-inches away from the edge of the concrete. Push them down until 3 inches remain above the surface.

Keep the bolts straight up and down and in as straight a line as possible.

Place anchor bolts every 6 feet, and one about 6 inches from the end of each mudsill. That means you should know how long your mudsills will be and place one at each end. Do not place the bolts in any door opening. Also, use your tape mea sure as a guide so you don't put a bolt right where a stud will be placed on the mudsill. However, if that hap pens, just cut a notch in the base of the stud to fit over the anchor bolt.

If you forget an anchor bolt, drill through the mudsill into the concrete and use an expansion bolt there.

Curing Concrete

Cure the concrete as described, preferably by keeping it under a sheet of plastic with all the edges and seams weighted to trap, the moisture. Otherwise, cover the concrete with straw or burlap and keep it constantly damp.

[[ When Interruptions Occur: A transit-mix truck holds about 8 yards of concrete, and if you are using more than that, you should arrange with the dispatcher for a second truck rather than having the first truck go back for a second load. Ideally, this second truck should arrive just as you have finished spreading the first load, but it's hard to time this exactly. If the second truck has to wait on you, it will cost you money. ]]

When you must wait for a second truck to arrive, keep the area where you stopped the pour continuously wet. Then, when you start pouring there again, work over the old and new concrete with a jitterbug tamper and there should be no visible joint.

Keep all of your tools and wheel barrows washed and clean during an interruption or the concrete will harden on the equipment.