A grade (or gradient) is the measure of the height of a slope divided by the length of the slope and expressed as a percentage. For example a 1 per cent grade would be a 1.2 cm (1/2 inch) slope height over a distance of 1.2 m (48 inches), so 0.012 divided by 1.2 equals 0.01 which expressed as a percentage is 1 per cent. Pouring concrete on a grade allows you to set a slope of a specific height over a specific distance so that you have an even ramp or walkway up a hill as a result.
Measure the height and distance of your slope to figure the grade you wish to pour. The steeper the grade you wish to pour, the less water you will mix into your concrete. Consult the manufacturer's specifications to determine the range of stiffness your concrete can handle (usually expressed as the "slump" of the concrete). Do not exceed the manufacturer's recommendation for grade or slump.
Stake out the area for your concrete. Use the level to set the slope of your stakes. Tie the string line to the stakes at the angle of your grade. A quick way to set the grade will be to figure the drop over the length of your forming two-by-fours. If you have 2.4 m (eight foot) boards and you have a 3 per cent grade, you can figure the drop over 2.4 m (eight feet) by converting to centimetres (multiplying by 100) which is 240 cm (96 inches). Then multiply your length by the percentage of the grade, so 240 times 0.03 equals a height of 7.2 cm (2.83 inches). Cut a piece of scrap two-by-four to extend 12.2 (4.83 inches) or the closest you can get. Then nail it to a 2.4 m (8 foot) two-by-four at one end so that only 7.2 cm (2.83 inches) extends below the bottom of the board. Now if you hold the level on the board it will show a 3 per cent grade.
Dig out the area where you will be pouring concrete and build your forms to match your staked-out grade. Test the slope of your grade as you nail together the two-by-fours. If you require reinforcing screen or rebar, lay it in the forms now.
Mix the concrete with as little slump (as stiff as possible) as you can per your manufacturer's specifications. You can always add more water if necessary. Start mixing with less water and add it in as you need more.
Pour the concrete into the forms, starting at the top and pushing it down to the bottom with the shovel. Fill the forms completely. If you think you won't be able to fill the forms with one batch of mixed concrete, get a friend to help you mix more concrete at once so you don't have partially filled forms. If the mixing is too much for you to handle manually, you can order mixed concrete to be trucked in.
Screed the concrete from the bottom of the grade up to the top. Screeding is the process of smoothing out the top of the concrete with your long two-by-four. Drag it across the top of the form, pushing any excess concrete as you go. You want to finish with a nice smooth surface that exactly matches the slope of your forms. Once you get to the top, if you have any excess concrete you can pull it off and discard it.
Hammer the sides of the forms with firm strokes to drive out any air bubbles in the concrete. Work your way around the forms to evenly drive out the air.
Texture the surface of the concrete with a brush while it is still wet. Then let it set for at least 24 hours or longer if you are in a cool or humid environment before removing the forms.
Things you need
- Concrete mix (pre-mixed will include the sand and aggregate necessary)
- Chalk line
- 5 cm x 10 cm (2 x 4 inch) "two-by-four" boards (for forms)
- 1 long 5 cm x 10 cm (2 x 4 inch) "two-by-four" board
- Wheel barrow
- Box-nosed shovel
- Reinforcing screen or rebar
- Wooden stakes
- String line