How To Make Concrete Slabs

Concrete slabs for the DIY homeowner can save a lot of money over hiring a concrete contractor. Small to medium slabs for storage sheds, sidewalks, air conditioner pads and the like are well within the capabilities of the DIY type person.

All you need is a little basic knowledge on how to design, setup and build a concrete form. A form is what wet concrete is usually poured into to hold the desired shape until it dries or sets up. The graphic below shows the basic design of a build up form constructed of lumber. The components consist of 2 by lumber and 1 by lumber. The stakes are usually 1x2's and the form sides 2x4's or 2x6's. The stakes should be set about every 24-36 inches and driven a least 10 inches into the ground...if the soil is very sandy or otherwise unstable...drive the stakes deeper. If a form collapses during a pour it is not good.

The angled stakes are optional on slabs 3 1/2 inches thick or less, but should be used on thicker pours. Concrete is very heavy and may force the form to collapse if not properly braced.

 Building your form using the form design is simple enough, but there is a little more to getting the form set up square. Small slab forms can be squared up and then staked, but larger forms need to be layed out before building the form. If the slab will be any larger than say 6 feet by 6 feet, then you might want to do a layout where the slab will be poured before you set the form walls in place.

The site where your slab is to be poured should be close to level, If not, then the form will have to be built higher on one side than the other. Concrete will usually flow and be some what self leveling depending on the water/mix ratio...so, if your form is not level it will be hard to get a level slab...if not nearly impossible.

The best way to set up a form on a larger pour is to lay it out with "batter boards". This set up allow you to level your forms and also square up the forms. The graphic below shows a slab laid out with "batter boards" using string(red line) to make a square or rectangle. The area indicated by the big X with arrows on the ends of the lines would be the area of the slab. Batter boards allow you to move the strings at (F) and (E) until you achieve a squared shape. The string is adjusted at least two corners and you check for square by measuring diagonally (D-B) and (A-C). The diagonal measurement should be equal to achieve a squared layout.

In the picture below (graphic 3) shows a built batter board at one corner of a proposed slab area. It does not matter how far away from the slab corner you build the batter boards...but usually about 2 feet is enough to allow working room. A rectangular slab layout takes four of these assemblys...one at each corner

 The red lines represent the strings that will align with the form and the actual edge and corner of the slab. The strings are wrapped around nails and the nails are adjusted back and forth (white Horizontal arrows at B and C in graphic 3)) until a square measurement at A-C and B-D is achieved(bird's eye veiw). The batter boards should be built so the top edge (measurement A) is at the desired height of the slab. The horizontal members should be leveled with a carpenters  level .

Place the first batter board set at the highest grade corner of your slab site...this first set will be the master set and all the batter boards will be leveled to this first set. Use a water level, string level or transit set to set the height of the next batter board set. Work your way around the slab area until you have all four corner batter board sets aligned and level, always using the first set as a master to set the hieght of the other batter board sets.

After you get all four batter board sets built and the slab area squared up, it's time to build the actual form.

Build the form so it looks similar to graphic 4. Begin by driving a couple of stakes (B) along the string line but leaving enough room for the form lumber(A), usually 2x4's or 2x6's. between the stake and the string line. The distance between these first two stakes should be approximately the length of your form lumber but do not put the stakes right on the ends of (A), leave room enough for a splice so, if you have to add another piece(A) to complete one side of the form. If you have to make a joint in the middle of a side reinforce it on the outside with a short piece nailed on.

The stake height does not have to be exactly on the string line, but shouldn't be driven to much below the string line. Nail  your first piece of lumber to the first stake, then nail to the second stake bring the lumber to the height of the string line. Dont' worry too much about the lateral lineup (E) this can be adjusted when you put in the bracing stakes. The bracing stakes are installed after the form area is completely outlined with form lumber and stakes(A+B) and the space filled in--if any--below (A). Trim the corners of the form lumber to from a butt joint for the form corners at the string line where the two strings cross and form the corner (see graphic 3).

The space below (A) has to be filled in with lumber or if very slight--1 inch or less--with packed fill dirt on the outside of the form. This is to keep the wet concrete from running out under the form. Complete this step before you drive the bracing stakes (C). After you get the basic form (A+B) bulit completely around the area begin driving the bracing stakes(C) in at approximately a 45 degree angle. Move the form(A+B) in or out (E) aligning the edge of (A) to the string line...continue this all the way around until it is fully braced and the edge of (A) is aligned with the string at (D).

NOTE:  You can build a curved concrete form by substituting a flexible material such as 1/4 inch or 3/8 inch plywood for (A). Composite siding material  may also used. You may have to add more form supports(A+B+C) to maintain the desired radius.  Tip: Plywood is more flexible when cut across the outer veneer grain.