Installing a concrete slab outside the entrance to your shed is the perfect way to make the entry into your shed. Concrete pads are also used for many reasons, the most common of which are outside a home door way, at the bottom of deck stairs or under an air conditioner. Most concrete pads are small enough to mix the concrete by hand from pre mixed bags. Larger areas are usually done by ordering pre mixed concrete.
There are several ways that concrete is sold:
Ready Mixed: Ready mixed concrete come in a concrete truck, this is convenient but there are usually minimums that are large enough to pour a 10 by 10 foot patio. You can order less but there will be a surcharge.
Bagged: Bagged concrete comes in 60 or 80 pound bags. You mix bagged concrete at your house.
Mobile Mix On Site: This concrete comes on a truck but it is not mixed until they are at your house. The truck mixes exactly the amount of concrete that you need and then dispenses it where you need it.
Keeping things safe when working with concrete is important. The acids in cement are irritating to the skin and eyes. This is a brief outline of the safety precautions you should take when working with concrete.
Concrete is measured in cubic feet and yards. Bagged concrete is sold by weight and cubic feet. Below is a table to get you close to how much concrete you will need. If you want an exact measurement the math is as follows:
Set The Elevation: After you figure out where you will pour the pad and how large it will be you need to determine the elevation you want the top of the concrete to be at. The concrete should be 3 1/2 inches to 4 inches thick and should have a few inches of gravel under it. The gravel is for drainage under the pad. Gravel is also alot cheaper than concrete so fill the bottom of the form with gravel.
Build The Form: The form holds the sides of the concrete in place while it dries. Build the forms by using 2x4 wood cut to length and nailed or screwed together to form a box that will hold the concrete while it hardens. Make sure the corners are square. If the sides of the pad are the same length you can check for square by measuring from diagnal corners until you have the same measurement. Use steel or wood stakes to hold the sides. Drive a nail thru the stake into the side of the form board to hold it at the proper elevation. Remember to leave a little slope from one side of the pad to the other so water will run off it. About 1/8" to 1/4" per foot works nicely. (on a 4' pad that slope is 1/2" to 1" in the whole 4').
Set The Slope: Concrete should always have a little slope so water does not stand on it. Usually 1/8" to 1/4" per foot. If it is a landing you should slope it away from the stairs.
Before mixing concrete make sure that you have all your tools ready and enough materials to finish, have a few more bags of concrete than you think you will need, you will not have time to run to the store once you start mixing the concrete and pouring the pad. it seems that you always need more concrete than you think.
Concrete dries and hardens faster in hot or windy weather than cooler and calm weather so every time you pour concrete the timing of the whole process will be slightly different.
Follow the instructions on the bags that the concrete comes in to mix the concrete. Put only enough water in to get a thick mixture heavy "oatmeal" like texture, you should be able to pile it up about 8 inches high in the mixing container.
Place the concrete in the form on the side farthest from where you are mixing it and fill the form with subsequent batches working your way closer to the side you are mixing on. This is so you don't have to walk thru the concrete.
Screed: After pouring enough concrete into the forms to make about 2' of surface, take your "screed" board and while using a sawing motion from side to side pull it across the top of the concrete making sure to keep both ends on the form. Screeding gets the top of the concrete very close to the finished elevation. When screeding you should have a small pile of concrete in front of the board. If there are holes or dips in the top of the pad you need to put a handfull or shovel full of mud in them and screed the area again until the top is flat. Finish filling the form and screed the whole surface making sure that the top is flat with not dips or bumps.
Immediately after striking the concrete you will go over the surface with a darby, on larger concrete slabs you can use a bull float.
The rubbing of the concrete brings some of the cement cream to the surface and fine tunes the levelness of the surface. Rub the surface back and forth and you will see the cream come to the surface. Once you bring the cream to the surface in an area you can move onto the next area. Keep moving across the surface this way making sure to smooth the new area back to the previous, trying not to leave ridges from the ends of the trowel.
This is done immediately after screeding. This step flattens the surface and pushes rocks down in away from the surface. There will be many open spots on the surface. Don't seal the surface by putting an angle on the darby as you go over the surface. Sealing the surface is when there is no holes and the surface is smoother.
Wait until water leaves the surface. During this step water will come to the surface and pool. It is important to not work on the surface of the concrete during this phase because it causes damage like blistering, crazing or dusting that will show up later on. When this water is gone you are ready to edge the edges.
Immediately after going over the surface with the mag trowel you should smooth the edges of the pad with the edger trowel. Put the curved edge of the trowel against the wood form board and pull it back and forth along the edge. Keep the front edge up off the surface about 1/2" so it does not dig into the surface but don't raise it so high that the back edge scrapes the surface.
Holes In The Edge: You will have holes show up along the edge and sometimes rocks will be pulled out by the edger which make even larger holes. These are easily fixed by taking the cream from the surface. Use the edge of the trowel to scrape some cream from the surface and then put it in/on the hole and make another pass with the edger.
This is the first pass with the edger.
This is how to scrape cream from the surface with the edger.
This is the finished edge, ready to be broomed. (after the mag fixes the main surface one more time)
Step 5 Trowel The Surface With A Mag Trowel
The Magnesium float is used to put a "Sweat" finish on the surface. Rub the surface with the mag trowel in an arching motion. Work from the far side of the slab back towards yourself blending the secon area you troweled into the first. You will feel the surface of the concrete has hardened but as you rub you will see cream and moisture come to the surface. Use the trowel to smooth out the cream.
step 6 Apply The Final Finish
Broom: Brooming the surface of the concrete is the most common way to finish exterior concrete pads. It is simple to do and provides a nice non skid surface. A broom finish is applied after the Magnesium Float work is complete. You need to wait for the perfect time to pull the broom across the surface of the concrete. If you do it too soon the broom bristles will dig too far into the surface and pull small stones up and pull too much cream off the surface. If you do it too late the broom bristles will not be able to mark the surface, the concrete will be too hard. So you need to watch it closely, and about every 15 minutes you can test a small area until it is ready.
This image shows what a perfectly broomed concrete pad surface looks like.
Steel Trowel: The steel trowel has a finer edge than the magnesium trowel. When it is pulled and pushed across the cream that you brought up in the last step it smooths it just like frosting a cake. You can create different patterns depending on the motion of the trowel. Rub the surface until it is smooth and then leave it. Work across the pad blending the new area back into the old. Try to not over work the surface. If the cream is too wet you will not be able to get a good finish. If the surface is too wet you should wait until it dries a bit and then trowel it.
step 7 Cure The Concrete
Properly curing concrete is a very important step that is often overlooked when pouring concrete. The chemical reaction in concrete creates heat and quickly dries the concrete. When moisture leaves concrete too quickly it weakens the finished concrete. Keeping the surface of the concrete moist for several days after pouring slows the reaction down and increases the hardness of the concrete.
After the surface of concrete is hard enough to stand on, usually the next day after it is poured, it is recommended to lightly spray the surface with water and then cover the concrete with plastic sheeting. Let this sit for several days.
Another way that concrete finishers keep the moisture in concrete after pouring is apply a concrete sealer after the surface is finished. Concrete sealers are sold at most home stores and concrete supply stores.
"I'm declaring victory over the shed project. Thanks for the great plans and for answering my various e-mails along the way." John
You guys are awesome! Thank you very much!" Denise