Tips for laying down a concrete slab on Perth’s sandy soil. How to get right angles and levels. Concrete cement sand aggregate water ratios.
I’m not a builder but recently I deconstructed a used shed and brought it to our backyard. I decided to go for a concrete slab instead of concrete pavers.
CONCRETE SLAB PLANNING
My first consideration was where to construct the shed’s slab. Under a big shady tree will keep it and shed contents cool. There are no visible signs of tree roots above ground so I hope the roots don’t emerge in the future.
My considerations for planning a concrete slab:
Get corners at right angles. I used a large metal shelf bracket for that. To double-check that I did not have a parallelogram, I measured diagonals and moved the shed frame base over the concrete footings until the diagonals were of equal length.
Get the concrete slab high enough above ground to prevent the floor from flooding. Fortunately, the block of land is on a sandy hill so that is not a problem.
Thirdly to get the slab level I used a clear plastic hose as a U-tube manometer. Water in a container, the hose, will find its level on all sides of the container. The water level on one side of the shed needed to be level with the other side.
Fourthly how strong the slab I’m planning. I planned a lowest cost DIY budget concrete slab in Perth because the force of the load was not planned to be high and because I plan to use expansion gaps, thus reducing the span length of the slab into shorter lengths.
2022 CONCRETE SLAB PRICING PERTH
The price of a concrete slab in Perth in 2022 depends on the size and at what price you can afford for materials with the latter depending on what materials, how much material and where in Perth you buy building supplies. I was hoping to buy some from a building salvage yard but with lack of time I ended up at a Perth hardware store.
$75 was the price in Perth of a trailer load of aggregate (blue metal) and sand.
$113 General Purpose GP grade 10 bags of cement from a hardware store.
$20 for 6 dynabolts to hold the shed to the concert slab above the footings (foundations).
$22 for wire mesh. Re-bar mesh was too expensive and I think chicken wire too weak so I bough cheap 10cm squares of wire mesh, the type I used for hanging passion fruit on the wall of my other metal shed. I also plan to add other bits of steel concrete reinforcement that I can find.
The best ratio of cement, sand and aggrigate for a concrete slab depends on:
- The strength required which depends on the force you plan to load on the floor of your shed.
- The slab strength you can afford. Higher concentrations of cement make for a stronger concrete. I decided to use the recommended best ratio of cement, sand, aggregate of 1: 3 : 3.
- More water makes an easer to flow soup of concrete however too much water in concrete results in a porous, weak concrete.
CONCRETE SLAB PREPARATION
To prepare the bed for the concrete slab construction, I dug a shallow trench for the wall foundations and laid concrete breeze blocks in the trench that I’d found on my property to save on the amount of concrete for the foundations and slab. Where the sand edging was dry and unstable, I used bits of timber that I could find.
A sheet of black plastic was laid on the sandy bed to prevent moisture rising up to the floor in the future but also to stop the sand from sucking water from my concrete mix.
Reinforced steel, my wire mesh and bits of scrap steel were laid out just above the ground on stones that I found.
The best concrete mixing is by a concrete truck but considering my budget slab and the need to get away from this computer for exercise, I opted to mix concrete for my shed slab in Perth by hand in a wheel burrow with a hoe.
SLAB CONCRETE POURING
I mixed the concrete insitu so all I needed to do was pour it straight out of the wheelbarrow I was mixing it in.
I have to go now to pour concrete starting with into the foundations and then as one piece accross the floor of the slab. Concrete slab construction in Perth photos coming soon with this to be continued blog post.