# Concrete slab calculation

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Hello I wonder how many bags of cement do I need for a 1m x 1m x 0.15m concrete slab ratio 1.2.3 (Cement/sand/aggregates) Thanks in advance Lloyd

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Assume water/cement ratio of 0.45 BY WEIGHT for a slab mix, so the water volume will be about 1.4 parts by volume, or 1.4 water: 1 cement: 2 sand: 3 coarse aggregates, 7.4 parts total.

For 0.15 m^3, you need about 0.15/7.4 or about 0.021 m^3 of cement. That will be about 66 KG of cement, or a little more than one 50-kg sack (a common size). The US standard bag is 94 lbs (43 KG), you would still need two bags.

Note that you may wish to adjust the cement and sand fraction based on the nature of the coarse aggregate. If the coarse aggregate is very angular and relatively uniform in size, it can be very difficult to work with especially if you are mixing by hand. Under those circumstances you might consider 1 cement: 2.5 sand: 2.5 coarse aggregate or even 1:2:2.

Dave

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Hi Dave, thank you very much for the reply, the other thing i forgot to ask in my question above is, will that ratio (1cement. 2sand .3aggregates) can give me a 3500 psi? so for a 50 square meter (10 L x 5 W) with 0.15 meters high, I will need approximately 100 bags (43kg), thank you

( 2018-09-04 19:31:25 +0100 )edit

Hi Lloyd. Most mixes in the books have twice the amount of aggregate as sand. Peter Stern in Field Engineering gives 326 kg of cement for 1m3 of 1:2:4 mix and 536 kg for 1m3 of 1:1:2 mix, so for your case 50 kg and 80 kg respectively. Best regards

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thank you!

( 2018-09-04 19:31:39 +0100 )edit

Hi Lloyd,

Thanks for reaching out. I've passed along your question to some of our experts and one of them suggested utilizing this site here: https://source4me.co.uk/calculate_con... which provides information for a ratio of 1:2:4 which is the normal ratio for unreinforced mass concrete. You would need to make some adjustments for the 1:2:3 ratio, however.

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thank you!

( 2018-09-04 19:31:48 +0100 )edit

Hi Lloyd,

I would say a good mix to use by volume would be 1 part cement / 2.5 parts sand / 3 parts coarse aggregate. The coarse aggregate being graded from 20mm to 5mm in size. For 1 cubic meter, this would equate to approximately, 350kg cement, 875kg sand, 1050kg coarse aggregate (20-5mm) and 180 litres of water. So for 0.15 of a cubic metre you would need 52.5kg cement, 131.25kg sand, 157.50kg aggregate (20-5mm size) and 27 litres of water. Cement usually comes in 25kg bags these days so it should be OK to use 2 x 25kg bags (50kg) for a 0.15m3 batch. Try not to make the mix any wetter than is needed to ensure full compaction as this will reduce the strength. You may need to add a little more or a little less water from the target 27 litres. This mix should achieve a strength of about 30 MPa / 4000 psi. Best regards, Les

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Here's some additional information from one of our experts, Chris Nixon:

Here are some figures from a simple mix batch calculator I developed a few years ago for site calcs.

I concur with others the 1:2:4 ratio is more normal.

Additionally the volumes of component materials are NOT additive, as the cement and sand / fine aggregate fill voids between the course aggregate (typically a multiplier is used to get the estimated mixed volume).

You could reduce the following water contents by max 20% if you have really good quality mechanical vibration (brings W/C down to about 0.45) but this may not be suitable for small quantities site hand mixed, and manual compaction.

The nearest mix I know to Lloyd’s 1:2:3 mix would be a 1:1.5: 3 mix with W/C = 0.52, which is a fairly dense impermeable concrete for tanks etc at about 200 kg/cm2. A 1:1.5: 3 mix:
Typically 1bag cem

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Hi Lloyd. While the mix ratios are important for strength, so is the quality of the sand, aggregate and cement. Dirty sand, weak aggregate and low quality cement will all weaken a mix. The quantity of water is also very important, see comments above. Peter Stern suggesty 1;2:4 mixes for slabs without giving any strength. You mention 3,500 psi but do not say whether this is working or ultimate strength - and I assume compressive load... Please advise

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