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How To Lay Concrete For Your Storage Shed Floor

How To Pour Concrete and Finish Concrete

how to lay concrete

The easiest way to lay concrete is to have someone else do it for you, but if you are determined to do it yourself this article will teach you all you need to know about how to pour and finish a concrete slab. Concrete slabs are often used for a shed floor. When pouring concrete, like many things, preparation is very important. Take a few days to set up the concrete pour and make sure everything is ready for the concrete truck to pull up to the location of your backyard shed and pour concrete.

Another critical factor when learning how to finish concrete is to pay attention to the weather. A key element in finishing concrete is how quickly the moisture leaves the concrete. If it is too hot or too windy the moisture will try to leave the concrete too quickly which will make it difficult to properly finish the concrete. When concrete sets up too quickly you will often not get your desired surface finish and more importantly the concrete will not have the proper strength which results in premature failure of the concrete slab.

brief how to

In this article, you will find information about:

  1. Determine The Size Of The Concrete Slab
  2. Prepare The Ground
  3. Set Up The Concrete Slab Forms
  4. Calculate Concrete Yardage
  5. How To Order Concrete
  6. Pour The Concrete
  7. Consolidate The Concrete
  8. Strike The Surface
  9. Level The Concrete
  10. Wait For The Surface To Stop Bleeding
  11. Edge The Concrete
  12. Joint The Concrete
  13. Trowel The Concrete
  14. Broom The Concrete
  15. Cure The Concrete
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tools and materials
  • 2x4x the length needed to form the edges of the concrete slab
  • Nails, Duplex nails work best because they are more easily removed
  • Steel Stakes to hold the form boards
  • Pencil
  • Level
  • Circular saw
  • Shovel, flat nosed shovel works best to move concrete with
  • Magnesium Trowel
  • Edger Trowel
  • Joint tool, 3/4" for 3 1/2" thick concrete slab

step 1 Determine The Size Of The Shed Floor Concrete Slab

The size of the shed floor area where you intend to lay concrete will determine how the forms are set up and the order in which you pour concrete. If you have a larger shed, like 12x20 or a driveway in front of your shed, you may want to pour the slab in two pours. This allows time to finish the concrete. If the slab you are going to lay has a dimension greater than 10 feet it should have control joints evenly spaced on the shed floor.

step 2 Prepare The Ground

Preparing the ground where you will build the shed concrete floor is critical to preventing cracked concrete. Although there are no guarantees that concrete will not crack you can greatly reduce the chances of cracking by properly preparing the ground under the shed floor and foundation.

Compacting The Ground: Compacting the ground under the concrete slab is critical to preventing a cracked shed floor. Compacting is done using a plate compactor that can be rented from most rental stores. It is turned on and pushed around similar to a lawn mower. It is about the size of a lawn mower.

Install Gravel: Install 2 to 4 inches of gravel where you will pour the shed concrete floor. Installing gravel under the concrete slab helps in two ways. First, it makes it easier to level the ground under the concrete which saves on the amount of concrete you will purchase, gravel is much cheaper than concrete. Second, it allows water to drain away from under the shed floor. When concrete is laid directly on top of dirt and the dirt becomes water soaked it can expand or settle, both situations may cause concrete to crack or worse to cause the shed floor to move. Expanding and settling are also a problem when temperatures are below freezing and gravel does not expand and contract with temperature as much as dirt does.

step 3 Set Up The Concrete Slab Forms

Install Form Boards: Use 2x4 wood for the form boards to hold the concrete while you lay the concrete. For shed floors the form boards will be the same dimension as the shed floor. Make sure the forms have the desired slope that you want from the finished slab because you will be following the form boards to level the top of the concrete. If you are building a garage shed you will slope the slab slightly towards the doorway. Use metal or wood stakes to set the height of the form boards and hold them in place while laying the concrete. After the concrete is laid and set up you will remove the form boards.

Footings For The Slab Perimeter: Most sheds need a small footing under the perimeter walls where the most weight is. This footing is often poured at the same time as the shed floor, this is called a monolithic pour. Use a larger form board so it can go down into the footing hole.

Spray form oil: or diesel fuel on the form boards to keep them from sucking moisture out of the concrete and help them release from the concrete.

step 4 Calculate Concrete Yardage

When ordering concrete they will ask you how much you want and since concrete is fairly expensive you will want to be close in the quantity. Concrete is ordered in cubic yards so you will figure out how many cubic inches you need and then convert the amount to cubic yards. There are 46656 cubic inches in a cubic yard.

  • Find The Cubic Inches: Multiply the length and the width of the shed slab in inches and then multiply the answer by the depth of the concrete in inches. The answer is the cubic inches of concrete needed to lay your slab.
  • Convert Cubic Inches To Cubic Yards: Divide the cubic inches by the cubic yard inches number (46656)
  • Answer: The answer is the amount of cubic yards you need. If your answer is a fraction like .5 then you will need less than a yard of concrete.

Example: 10' x 20' x 3.5" concrete pad.

  • Find The Square Inches: In inches 10' is 120 inches and 20 ' is 240 inches so 120 x 240 is 14,400.
  • Find The Cubic Inches: Our slab is 3 1/2" thick. 14400 x 3.5 = 108,400 cubic inches
  • Convert Cubic Inches To Cubic Yards: 50,400 / 46656 = 2.3 cubic yards.

step 5 How To Order Concrete

Concrete is ordered from a concrete batch plant. Some pre-planning and coordination with the batch plant helps to have your concrete order run smoothly. There are two people that can help in ordering concrete, the Concrete Salesman and the Batch Plant Dispatcher.

Concrete Salesman: Most concrete batch plants have salesmen who are very experienced at pouring concrete. They usually run around and talk to contractors who are ordering 10 trucks of concrete a day and keep the relationship happy by solving the problems that come up. These salesmen will come to your job and help you get ready to pour the concrete. They will notice issues such as

  • Concrete truck access to where you are building the shed or driveway slab.
  • Mix of concrete (recommended strength of concrete)
  • Time of concrete pour
  • If the concrete forms are set up properly to hold the concrete being poured
  • Where the concrete truck can clean out its shoot.
  • What additives you will need in the concrete, calcium, hot water etc.

 

Batch Plant Dispatcher: If you feel like you can order the concrete without a salesman you can order directly from the concrete batch plant. You can ask the dispatcher questions about what people have been ordering for different types of concrete pours but they usually do not have a lot of time to talk. The concrete batch plant dispatcher will ask you the following questions:

  • How much concrete you want
  • What mix of concrete. This is expressed in how many bags of cement per yard, the more bags the higher the psi strength of the concrete. read Concrete Slab Thickness for more info but typically 5 bag is 5000 psi and is used for interior concrete and 6 bag is 6000 psi and used for exterior concrete pours.
  • If the weather is cold you will be asked if you want additives to keep the concrete warm, hot water, calcium or other chemicals
  • When you want the concrete delivered

step 6 Pour The Concrete

how to lay concrete

Pouring Concrete: When Pouring concrete you start at the far end of the shed floor and pour the concrete back and forth from side to side as the concrete truck backs out of the shed.

Spreading Concrete: Use a closed end rake or flat nosed shovel to keep the surface of the wet concrete as close to the finished surface level as possible so you won't need to move it again later. Tip: Do not use a yard rake to move the concrete around, the tongs on the rake will separate the rocks from the concrete and cause the concrete mix to be uneven. Use a shovel or a flat concrete rake, show below.

step 7 Consolidate The Concrete

lay concrete rake

 

Consolidating concrete is the process of vibrating concrete so that air and rock pockets are eliminated from the laid concrete. Consolidating also brings the cream of the concrete to the surface and up against the form boards to aid in producing a better finish. Consolidate The Edges: Use a hammer and tap the form boards along all the edges. Consolidate The Surface: Use a concrete tamper, it looks like a walker with wire mesh on the bottom, to tamp the surface of the laid concrete. Walk backwards through the concrete and tamp the surface of the concrete so that the tamper pushes the concrete into your foot prints and covers your tracks while it consolidates the freshly poured concrete. Note: Tamping is sometimes not done on smaller shed floors where you do not need to walk in the mud.

step 8 Strike The Surface Of The Shed Floor

concrete tamper gandy

 

The next step in finishing concrete after consolidating is to strike the surface of the concrete with a very straight board to flatten the surface in line with the form boards. Screeding also vibrates the concrete which helps consolidate or settle the concrete. Use a very straight 2x4 board that is long enough to touch both sides of the form boards at the same time. This board is called a screed. Set the screed board so that it rests on the form boards. Tilt the board slightly towards yourself and with a sawing motion pull the form board toward yourself. Make sure to keep the screed board in constant contact with the form boards so there are no bumps in the concrete surface. Keep a little concrete in front of the screed board at all times to prevent dips in the finished shed concrete slab. If too much concrete piles up in front of the screed board then you will need to shovel it away.

concrete screed strike

step 9 Level The Concrete Shed Floor

concrete bull float

Immediately after screeding you will use a bull float or darby to further level the surface of the concrete. A bull float is a very large trowel, usually attached to a long pole, that is pushed and pulled across the surface of the concrete to make it level. A darby is a long wooden trowel, 24" to 36" long. The bull float or darby does three things:

  1. They further level the surface by taking out the high and low spots.
  2. They push the aggregate deeper into the concrete.
  3. They smooth the surface.

Push and pull the float across the surface of the concrete, leveling the surface as you go. You are not putting a finish on the surface so be careful with these tools to not seal the surface. Keep the tool as flat as possible which keeps the surface of the concrete open. In other words you want the surface to still be flat but rough with lots of tiny holes showing. Stop using the bull float when water comes to the surface.

 

concrete bull float

step 10 Wait For The Surface To Stop Bleeding

After the surfaces is floated the next step is to finish the shed floor is to wait for the surface water leave the surface. This is called bleeding and problems like blistering, crazing, dusting, or delamination may arise if the surface is worked any further during the water bleeding and evaporating process. The NRMCA says "The waiting period depends on the setting and bleeding characteristics of the concrete and the ambient conditions. During the waiting period, protect against evaporation from the concrete surface if conditions are hot, dry or windy. Cover a small test portion of the slab to evaluate if the concrete is still bleeding. General guidance regarding whether the concrete has sufficiently set for final finishing operation is when a footprint of a person standing on the slab is between 1/8" to 1/4" (3 to 6 mm)."

step 11 Edge The Concrete

Use a edger to put a rounded edge on the edges. There are several sizes of edgers with different radius that may be used when you lay concrete. The perimeter of the shed floor should be a smaller radius. You want the edge of the slab to be flat so the bottom plate of the shed walls sit flat.

edge concrete

step 12 Joint The Concrete

concrete joint tool

Anytime you lay a concrete slab or shed floor larger than 10' x 10' it should have control joints installed to control cracking of the concrete slab. Use a edger that penetrates into the surface about 1/4 of the thickness of the slab. For a 3 1/2" concrete slab that would be about 3/4". Measure in from the edges of the to make a parallel line across the slab. Use a string line to snap a line in the concrete where the joint will be. Put a 2x4 board offset from the string line mark so the joint tool will mark the surface while the edge of the tool runs along the 2x4. Draw the joint tool along the board to make a straight line. It may take several passes to get a good clean line. This control joint may also be installed by using a concrete saw the day after you lay the concrete. Just make sure the concrete is hard enough to not be torn by the saw blade. Sawing the joints works well for shed floors that are going to be used for a shop or office situation. Large tools like table saws will not get their legs caught and carpet lays flatter on sawed joints.

step 13 Trowel The Concrete

concrete steel trowel finish

Going over the surface with a trowel will seal the surface by closing any remaining air gaps on the surface. The more times you go over the surface with a trowel the smoother the surface becomes.

Exterior Concrete: For exterior concrete used for driveways and walkways to your shed it is recommended that you go over the surface at least once with a magnesium trowel. Rub the surface back and forth with the trowel until the area is smooth and a little cream has come to the surface. You will see the surface of the concrete begin to seal, look smoother, because the air gaps are being closed by the cream. Do not over work an area, just get the cream to the surface, smooth it and move on to the next area. Work the next area into the previous area so the whole surface you leave behind you is smooth.

Interior Concrete: For a smoother shed floor wait a little bit, depending on temperature and wind, use a 12" long steel trowel and rub the face of the concrete move backward across the floor while troweling in front of you. One pass with the magnesium trowel and two passes with the steel trowel will produce a very smooth finish. It is important to not add water to the surface of the concrete when finishing because this weakens the finished surface. If you have a area that you can't get smooth with the steel trowel you should use the magnesium trowel to rub the surface until cream comes up then use the steel trowel to smooth out the fresh cream. You may also be able to use cream from the extra concrete that you saved on the side.

step 14 Broom The Concrete

Brooming the concrete is done with a special wide flat broom that produces a rough surface that is a very common finish when laying exterior concrete. Dip the bristle ends in water and then starting at the far end of the concrete slab put the broom on the concrete and pull the broom toward you. Attaching the broom to the same pole that was used with the bull float helps to get the broom across the concrete slab.

step 15 Curing The Concrete

Curing the concrete is an important step that is often overlooked. Properly curing the concrete adds increased strength and helps the finished concrete resist cracking and surface problems like spalding, reduced strength and cracking. Curing greatly improves the durability and strength of concrete slabs. Concrete must have water to cure. After about 12 hours you can spray the surface of the concrete with a light mist of water and cover it with plastic to keep the water from evaporating. Extreme temperatures, too hot or too cold are not good for curing concrete. High temperatures and or wind cause the water to evaporate out of the concrete too quickly and reduce its strength. Low temperatures below freezing slow the chemical reaction in the concrete and reduce the strength of the finished concrete. Wind removes heat and water from concrete in cold temperatures so it is important to cover the concrete with plastic and if the temperatures are close to freezing you will need to rent concrete blankets and cover it up.

The form boards around the edge of the shed may be removed the day after the concrete is poured.

References: National Ready Mixed Concrete Association CIP-14

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