Concrete pad foundations are used to support large pieces of machinery or tanks and, in some areas, can be used as building foundations as well. Unlike other types of foundations, the weight of the overhead structure is dispersed horizontally throughout the surface of the pad instead of vertically. This allows for a pad to be thinner and cost less to construct. Once the concrete forms are prepared, it is easy to pour a concrete pad foundation. Be sure to do it when the heat of the day is not extreme or the concrete could dry (cure) before it can be levelled and finished.
Preparing the concrete pad form
Measure and mark the dimensions of the concrete pad on the ground using a tape measure and orange paint.
Drive a wood stake at each corner and tie a string between the stakes, so the string forms an outline of the pad edges. Check to make sure the string is exactly where the edges should be, as it will be used to line up the concrete forms. Adjust the string if necessary.
Drive wooden stakes into the ground every 1.2 m (4 feet) along the string. Do not drive the stakes all the way in, but leave enough of the stake above ground to be equal to the height (thickness) of the pad to be poured.
Nail plywood strips to the stakes to form the side forms of the concrete pad. Cut the plywood, so the height is equal to the planned thickness of the pad. Check the sides with the string line to make sure they are straight.
Brace the side forms by driving a stake at an angle, slanting from the top of the plywood form back into the ground (outside of the pour areas) at every vertical stake. Nail the bracing stake to the vertical stake to hold it in place.
Run a string line between the form sides across the pour area and measure down from the string to the ground to check that the thickness of the slab will be even. Add or remove dirt with a shovel and rake to adjust the ground level until the entire pour area is graded. A graded area is that which has an even depth.
Compact the bottom of the pour area with a hand tamp or tamping machine.
Install any reinforcement specified in the building plans.
Pouring the concrete
Pour concrete in one of the corner areas first. Do not pour the concrete directly against the form. Start the concrete a few feet away from the corner and use shovels to fill the corner area. This will "set" the corner, preventing the form from moving during the pour.
Fill the pad area with concrete. Work from edge to edge bringing the concrete level to slightly higher then the planned thickness of the finished pad. Use shovels and screed boards to move and level the concrete. A screed board is a straight piece of 5 by 10 cm (2 by 4 inch) wood that is drawn across the surface of the concrete to level it.
Check the level by holding a string from edge to edge of the wood form and measuring down to the concrete. Add or remove concrete as necessary using a shovel and a screed board.
Finish the concrete with a magnesium float. Wait until water begins to rise to the surface of the concrete and "float" the concrete. Small pads can be floated with hand-held floats that are "swept" lightly across the surface in an arc to smooth the surface of the concrete.
Use long-handled bull floats to finish large pads. Stand outside of the form and place the float (an oval shape piece of metal) on the concrete surface and then push the float across the surface with the handle. The handle on these floats can sometimes be 6 m (20 feet) long. Pull the float back and then push it out again, on a slight angle, to move the float to a new position on the surface.
Apply a broom finish to the concrete to increase traction on the pad when it is cured by lightly dragging a standard push broom across the surface to create small grooves. Swirled patterns can also be made using a hand-held broom when applying the finish.
Lightly spray water over the ground before pouring the concrete; this will help to prevent the water from the concrete being absorbed into the earth and making the mix too dry.
Concrete is toxic and can cause skin burns. Clean any concrete off of exposed skin immediately to prevent injury.
Tips and warnings
- Lightly spray water over the ground before pouring the concrete; this will help to prevent the water from the concrete being absorbed into the earth and making the mix too dry.
- Concrete is toxic and can cause skin burns. Clean any concrete off of exposed skin immediately to prevent injury.
Things you need
- Tape measure
- Orange marking spray paint
- Wood survey stakes
- String line
- Tamper (hand or machine)
- 5 by 10 cm (2 by 4 inch) board 1.2 m (4 feet) long minimum
- Magnesium float
- Long-handled bull float (optional)
- Push broom (if desired)