In this video
I want to show you how to calculate the amount of concrete that we're going to need
For this slab on ground now as a couple of things
a little bit complicated about this not only is it L-shaped it also has a
rebate for the Brickwork around the edge and
if you look at the section view
You can see we have a thickened edge or a footing with this 45 degree splaying it to reinforce the corner
so it's going to be a little bit complicated to
Calculate so there's three distinct steps that we need to take to calculate this out
so
Step one well part of it for the first start. I'm not going to worry about the rebate i'm going to consider it as a
complete slab and we'll take the rebate out of it later
So for a start not going to worry about that little bit there, we'll just work it out
as a whole first, then we'll deduct that later
So I'm also going to break up my L-shape into two
recognisable or two simple shapes so two rectangles and
label those Area 1 and Area 2 just so they get confused and
I'm also going to work, split this up into
the slab itself
So the 100mm thick slab and then we'll do the edge beam
Later on, alright, so split that up into those different groups
two different areas on top of the slab the slab itself and the edge beam, so
first thing I want to work out is how much concrete's in this slab the 100mm slab sitting on top so
So we do the two areas so Area 1 is
6m long down this side and
4m wide so
6 x 4 gives us 24 square metres and
Then Area 2 is 3m x 2m
so it gives us 6 square metres so that adds up to a total of
30 square metres in this area so 30 square metres on top of the slab
we know
from down here two slabs 100mm thick so
30 x 0.1 gives us 3 cubic metres of concrete
Just for the top of the slab just for the 100mm on top
not including the rebate. We've got to take that out yet okay, so that's the slab on top
Then what we have to do have to look at this footing now
Working out this
shape is not going to be the easiest thing so what I propose to do is I'm going to turn it into a square
and basically what I have to do is I take this little piece off here and
turn it around and stick it back into that corner there, so I need to work out what distance
this is now because this spray is typically at 45 degrees that means that this forms an isosceles triangle
which means that two sides are the same length
Then we can just work out that half of this distance
Will also be this distance here
so half of 427
will give us
336
so 300 plus
Sorry
236
so 300 plus
236 gives us a total distance across here of
536mm
so I've taken that little bit off there and
effectively in theory stuck it back on there to give us a new footing width of
536mm
so that information I can then identify all my sides where my
footings going to be and
Again as we've done in similar projects before you have to take off the width of the footing off the majority of these sides
so that we're not calculating the corner twice
so for side A 6m less
the width of our footing which is our 536
What we worked out down here
gives us 5.464
Side B The same thing we take off 536 and
again the common theme with any internal corner
we don't need to deduct
anything off that side of those two
trenches that form the internal corner
So for C and d. There's no deduction and
Then e and F again, we deduct the width the footing
536mm
So once we've worked all those that individually we add them up and we get 21.856m all the way
round their footing that's how long our
footing is
So then the depth of our footing is
472mm and the width of our footing is
536
So we can use those dimensions
length x width x depth or
21.856 x 0.563 remember we needed in meters
x 0.472 to give us a total
cubic metres of 5.529 cubic meters just for the footing
So that's the slab worked out and the footing worked out, but we've still got to get rid of this rebate
So the rebates a similar process to what we just did for the edge beam
But this time the rebate is only
150mm wide so instead of
Deducting 536 we're going to deduct 150 and again
the two internal sides, side C and D don't require any deduction
So we take the 6m take off 150mm
3m deduct the 150mm
the 2m and the 3m not required and same in the rest of the way around
so we actually end up with a total length for our rebate of 23.400m
Now the depth of our rebate or the width very that's 150 the depth is 172mm
2 courses of Brickwork
so
length
23.400 x width
which is
150 Or 0.15
x the depth,1.172, gives us a total of 0.604 cubic meters of concrete that would have gone
into that rebate so we actually need to deduct that amount from our previous two totals, so
We can add our 3m
for a slab and our 5.529m cubic metres for our edge beam add them together
deduct
0.604 cubic meters for our rebate and that will give us a total of 7.925
cubic meters of concrete for this slab of course we need to allow for some waste
So if you allowed ten percent waste it will give us
8.718 cubic metres and
of course we then need to round it up to an orderable amount or an increment of point one
So we'd order 8.8 cubic metres of concrete
for this particular job
So there you go that's how we calculate the concrete required for a slab on ground with a rebated edge