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Instructions for a garden shed slab:
1. Measure shed floor area, then add 200mm to all sides. 100mm is the drainage gap around the slab edge, and 100mm is the border around the shed sloping down for water run-off.
2. Remove turf, roots and vegetation. Level and compact the dirt.
3. Lay about 50mm gravel bed. Level and compact.
4. Construct formwork (timber box) on a flat area using 90mm x 35mm timber. The inner perimeter is the same as the shed floor area plus the 100mm water run-off border. Diagonals from formwork corners should be equal if the slab floor is rectangular.
5. Place formwork on the gravel and install it level. On the outside of the formwork, hit pegs in every 1.5m and screw to formwork.
6. Ensure formwork has no gaps between bottom edge of timber and the gravel bed.
7. Place about 50mm bed of sand inside formwork. Level and compact.
8. Lay builders' plastic, overlapping any plastic by 200mm and sealing joins.
9. Install reinforcing mesh within the formwork, leaving 50mm gap on the outer edge for concrete to cover the metal completely. Mesh should sit horizontally about half-way or in the upper third of the slab. Use plastic bar chairs to raise it off the plastic.
10. If you will be pouring concrete alone, then install mini-forms down the slab every 1 metre. This is to avoid problems of having to interrupt concrete pours, called "cold joints". Screw a timber piece in, that can be removed when the concrete is set, and replaced with a strip of rubber (expansion joint).
11. Check the weather forecast. Do not pour cement in the rain, as it changes the recipe and weakens the slab. Also avoid dry, hot weather. Overcast days are best.
12. If using a generator, ensure it is full of petrol before starting the cement mixer. Also make sure you have water, lunch and snacks to keep up your energy.
13. Recipe for cement slab, if using "bony" large sharp pieces of aggregate (~2-3cm), is as follows, placing in cement mixer in this order: 2.5 part aggregate, 1 part water, 3 parts sand, let mix for a minute, then add 2 parts cement, add 1/2 part water, then add 1 part sand. Let mix for at least a minute. This order ensures a good mixture, and ratio is for strength.
14. While concrete is mixing for a few minutes, empty previous barrow-load into form. Pour until just higher than formwork. Cut up with shovel to ensure concrete is pushed down into any voids. Avoid pouring aggregate directly at the edges, but push sandier mix to formwork edges.
15. When a full row of concrete is in place, e.g. 3 barrow loads for 3m wide row, then put 1 part aggregate and 1 part water in mixer and let it run. This gives time to screed properly.
16. Timber screed should be 40cm wider than the slab. Screeding: first tamp then saw back and forth. Use a rubber mallet to gently hit top of formwork until bubbles stop emerging.
17. Use steel float within 3-5 minutes of screeding. Should have a jelly-like surface. Float, don't push. Use float on a low angle, smoothing and moving water off. Be efficient and quick and do not over-float as this brings cement to surface and weakens the slab.
18. When water has evaporated from the surface, use concrete edger, pushing down on the outside to create slope for water-run-off.
19. Wait 30-60 minutes after finishing before covering, to prevent the plastic sheet from marking concrete.
20. Next morning gently pour / sprinkle water over concrete and recover. Do this daily for a week, keeping slab covered.
kellyjones00 (593)
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