Seeking help with required amounts of Sand, Gravel & Cement for small project

TommyKnocker

I am looking to extend a shed base in the back garden from currnet 8' (W) x9' (L) to 9' (W) x 11' (L). I do not plan to remove the existing base, just add on to what is already there. When complete I want to get a steel 10' x 8' shed to hold the lawnmower and assorted garden tools etc.

I have dug out the extra space and to 12" depth and added in 6" of 804 hardcore, which is tamped down. Next step is to pour the concrete, but just being a DIY hobbyist I am not familiar with the ratios for concrete mixes.

I have read on line that I should use a 3:2:1 (Gravel, Sand, Cement) mix, but I am not sure of the quantities of each required. I basically have two slabs to pour.

Slab 1: 9' (L) x 1' (W) x 6" (D) - (3m x .3m x .15m)

Slab 2: 9' (L) x 2' (W) x 6" (D) - (3m x .6m x .15m)

From an online calculator I have been advised that I require .16 m3 for slab 1 and .28 m3 for slab 2, giving a total of .44 m3, rounded up to .5 m3. A second calculator advises that for .5 m3 of cement I need the following

8 x 25kg Cement

.5 tonne Sand

.75 tonne Gravel

I asked in my local builders providers to have these numbers checked, but they advised it was too difficult to work out and all they could do was make a best guess and I got two different amounts from two different people.

I wonder if any of the knowledgeable folks on here could advise if the numbers provided by the calculators I used seem ok or if they are off could they advise what I do need?

Any advice would be greatly appreciated.

Lumen

Those numbers look off to me.

0.5m3 of concrete weighs at most about 1200kg.

Your cement+sand+gravel adds up to 200+500+750 = 1450kg, and then there's the water on top of that.

Using another source for 1 m3 of concrete, scaled by 0.5 (your rounded up volume) gives 75L water + 125kg cement (5 x 25kg bags) + 350kg sand + 600kg aggregates.

standardg60

Half a ton of gravel and four (five if you want to be sure of a strong mix) cement OP.

No need for sand it's already in the gravel.

Half a ton is the colloquial term for half a cube, but they're basically the same thing.

policarp

In the good old days it was 8 measures of gravel to one measure of cement

for foundations.

For lintols, cills and beams it was 6 to one.