How to Pour a Concrete Slab for a Shed?

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Pouring concrete slab in the right way is considerably crucial to construct strong and durable slab that makes a good foundation for a shed. Concrete slab keeps the shed off the ground reducing the transfer of moisture from the ground into the shed material and its contents to prevent mold and rot.

Pouring concrete requires adequate planning, preparation, delivering materials, and following suitable procedures to carry out the work properly and achieve the expected results. The concrete slab for a shed would not conform with requirements of the project unless necessary measures such as curing is not considered after the completion of the concrete pouring process.

1. Planning

At this stage, required materials and necessary tools are prepared and delivered to construction site.

2. Prepare Construction Site

Specify the size of the slab and remove any bush, tree, rocks or obstructions from the construction site. It is advised to clear an area greater than that of the slab to make pouring operation easier.

3. Mark Slab Base Location

Specify first corner of the slab from any known points, and use this point to determine the remaining corners. After that, measure the diagonal distances from each pair of opposite corners and adjust one until both are the same. Then, run string-line around the corner stakes at a desired height.

4. Excavation for Shed Foundation

The area that needs to be excavated for the foundation is slightly larger than the area of the shed to account for formwork accommodation. The depth of excavation, which is measured from string line, is based on the thickness of the slab, extension of the slab above ground, thickness of gravel layer at bottom of the foundation.

If the gravel thickness is 15cm, slab thickness is 10cm, and slab extension above the ground is designed to be 10cm, then the depth of excavation would be 20cm. Levelness of the excavation area shall be check during the digging work. Added to that, loose materials are required to be removed.

5. Fix and Level Formwork

After the completion of excavation work, a formwork is fixed and levelled, and its diagonal shall be checked. It needs to be strong to make sure that the concrete pressure is contained. No gaps in formwork are acceptable since it allows concrete to escape. Frequently, wood formwork is used.

6. Place and Compact Gravel Layer

Gravel layer can be placed after the formwork is fixed and levelled. In order to achieve good compaction degree, it is recommended to place and compact gravel in three layers.

Suitable compaction machine such as vibrating plate can be used to apply compaction effort. Gravel layer levelness shall be examined, in addition to its thickness which can be checked by measuring from the top of the form to the top surface of gravel layer.

Gravel layer thickness around the perimeter of the slab is decreased in order to have greater concrete thickness. The width of the thickened concrete around the perimeter of the slab would be 30cm.

7. Install Rebar

Steel bars or steel mesh are installed to overcome the brittleness of concrete. Sometimes rigid insulation is placed before steel bar installation. The rebar needs to be suspended in the middle of the concrete to be effective. Rebar chairs are wired to the grid to lift it off the ground.

An alternative to the grid or mesh is using concrete with synthetic fibers added to it. The fibers are mixed into the concrete to increase the tensile strength and to help prevent cracking.

Finally, the thickened concrete around the perimeter of the slab shall be reinforced with two additional rebars preferably No.13 bar size or greater.

8. Pour and Compact Concrete

After reinforcement placement and preparation of necessary tools, concrete pouring process can be carried out.

The location of openings and doors shall be specified on the outside of the formwork. The level and diagonal of the forms shall be checked.

It is advised to slightly moisten steel bars before concreting to make sure that concrete encircles the bar easily and improves steel-concrete bond. Lastly, the concrete shall be compacted properly using suitable means. 

9. Level and Finish Concrete

The concrete should be slightly above the forms. Use a shovel or rake to spread the concrete around and move it into low areas.

Place anchor bolts into concrete during screeding and smoothen the area around bolts. Smoothen the concrete surface after screeding process is finished.

After the smoothening is done, finishing work begins. Concrete finishes should be applied when the top of the concrete is semi-dry. After slab levelling, the water rises to the top of concrete surface which gives a glossy look to the concrete. After the slight evaporation of water, finishing process can be commenced.

The time it takes for this to happen varies on the temperature, and how much sunlight is exposed to the concrete. If concrete is poured in hot weather condition, then finishing shall be done quickly. However, if the slab is constructed in cold weather, it might take a while till concrete is ready for finishing work.

Edge the concrete where it meets the forms because it reduces the chance of chipping and makes it easier to lay the bottom plate flat.

10. Concrete Curing

Based on the weather condition, select suitable curing regime to make sure that concrete achieves the designated strength. Commonly, the curing process begins 24 hours after the pouring of concrete.