Urgent - Concrete pouring for support slab: volumne or weight ?

[Lucag]

Hello everyone,

i's a sunny day in London, and I have decided to pour my concrete slab.
It is a a circular slab, slightly bigger than my brick base.
I have created the wooden support for the pouring.

Slab size: 9cm thick, 140cm diameters.

My calculation ( based on the standard concrete recipe, and assuming that the ballast bag is 1 part sand and 2 part stones ) :
-----
2 cement bag ( 25kg )
11 ballast bag ( 40Kg )
17 litre of water

My mixer is very small and I have no idea on how to proceed now...

As suggested in this forum, I would like to use the 1:5 cement / ballast ratio.
But is it WEIGHT or VOLUME ?

How do I prepare the small batches by volume or weight ?

[Gulf]

The ratio is measured by volume.

[david s]

Standard “government mix” is 3 parts aggregate 2 parts sand, 1 part cement by volume. High end mixes are done by dry weight of ingredients. Density should be 2.4 kg/litre

[bentedesco]

By the way, I love the round base, nicely done Lucag !

[Lucag]

Thank you everyone. When I was done with some preparation rain started so I had to postpone the concrete making to next week. I'll go with 1:5 as volume then, very excited to conclude that phase and start building.

The round base looks good so far, I hope I will be able to integrate the dome to the structure in a way that looks good!

[wotavidone]

Standard “government mix” is 3 parts aggregate 2 parts sand, 1 part cement by volume. High end mixes are done by dry weight of ingredients. Density should be 2.4 kg/litre

Good ol' 3:2:1 "gummint" mix. Laid a fair bit of that in my time.
That is 3 volumes of gravel, glued together by a 1:2 paste of cement and sand, and was considered about the right strength for most government contract work.

A note of caution:
When you mix 1 part by volume of sand with 2 parts by volume of gravel, you don't get three volumes of sand/gravel mix.
The sand fills in the gaps to a certain extent, rather than add to the overall volume, so two parts gravel by volume plus 1 part sand might only give you 2.5 volumes total.

If that "ballast" is 1 part sand, 2 parts gravel, then 5 volumes of ballast might be the equivalent of 4 parts gravel, 2 parts sand if they were individually measured out.
Mix that with one part cement and you would have 4:2:1 - not "gummint" mix.
However, a few manufacturers websites say this should give 20MPa concrete - should be just right.





.

[mongota]

[My calculation ( based on the standard concrete recipe, and assuming that the ballast bag is 1 part sand and 2 part stones ) :
-----
2 cement bag ( 25kg )
11 ballast bag ( 40Kg )
17 litre of water

My mixer is very small and I have no idea on how to proceed now...

As suggested in this forum, I would like to use the 1:5 cement / ballast ratio.
But is it WEIGHT or VOLUME ?

How do I prepare the small batches by volume or weight ?

You have 50kg of cement.
You have 440kg of ballast bags.

If the ballast is, as you wrote, "1 part sand and 2 parts stone", the 440kg of ballast will be about 147kg of sand and 293 kg of stone.

50kg cement
147kg sand
293kg stone

Roughly, a 1:3:6 mix as I see it.

Personally, I'd not use that mix for an oven hearth slab. Again, my personal opinion.

If this was my build, I'd want a mix stronger than that.

Edit to add: as the others have written, measure your small batches out by volume. By the shovel load if you can keep them fairly even, if not, you can use a bucket.

[Lucag]

Thank you for all the answers... unfortunately now I am more confused than before.
I would really appreciate if someone can help me with the calculation, for my peace of mind. I can repay with a Pizza in south London, when ready.


I want to make a round slab: 140cm diameter, 9 / 10 cm thick.


I have:
25Kg generic portland cement ( several bags ):
http://www.builderdepot.co.uk/hanson...ment-25kg.html

40Kg of this ballast ( 11 bags but I can buy more ):
http://www.builderdepot.co.uk/20mm-b...-40kg-bag.html
The ballast does not specify the gravel / sand ration unfortunately, I have just assumed it's 2 : 1 gravel / sand

My mixer is very cheap and small, so I will need to make several batches.
I will use a bucket to precisely measure the materials by volume.
Is possible I would like to measure water as well because I do not have much experience in this things.

Can you confirm me i can go with 1:5 cement / ballast ?

Or do you suggest me to change the ratio ?

[david s]

Maybe you should determine the sand aggregate ratio. You could do this by first thoroughly dry mixing one bag of the ballast in your mixer and then measuring accurately say 2 litres of it, separate the sand and aggregate my sieving then measuring the resulting proportions.
My own mix that was originally worked out by weight and then converted to volume is 2.2 (7 mm basalt) aggregate, 2.1 sand, 1 OPC, 0.68 litres water This makes a very strong concrete but I also use some additives like water reducer, fibres and chemical admixture. I suspect you need to increase your cement ratio a bit. The volume method is easier because the sand and aggregate water content will often vary enormously particularly when materials are stored outside.and this interferes with the weight calculation.

[mongota]

Thank you for all the answers... unfortunately now I am more confused than before.
I would really appreciate if someone can help me with the calculation, for my peace of mind. I can repay with a Pizza in south London, when ready.


I want to make a round slab: 140cm diameter, 9 / 10 cm thick.


I have:
25Kg generic portland cement ( several bags ):
http://www.builderdepot.co.uk/hanson...ment-25kg.html

40Kg of this ballast ( 11 bags but I can buy more ):
http://www.builderdepot.co.uk/20mm-b...-40kg-bag.html
The ballast does not specify the gravel / sand ration unfortunately, I have just assumed it's 2 : 1 gravel / sand

My mixer is very cheap and small, so I will need to make several batches.
I will use a bucket to precisely measure the materials by volume.
Is possible I would like to measure water as well because I do not have much experience in this things.

Can you confirm me i can go with 1:5 cement / ballast ?

Or do you suggest me to change the ratio ?

If you're going to mix a 1:5 volume ratio of cement to ballast instead of by your on-hand batch weights, and the ballast is 1:2 sand to stone, you'll end up with a 1:1.67:3.33 mix. That would be fine for a slab.

One bucket of cement to every five buckets of ballast.

Water can vary based on how much moisture is in the materials, as well as the material proportions. You'll get a feel for the water, the key is that as you are getting a feel for it, don't add too much all at once. It's best to have to add more to loosen the mix than to have to add more dry ingredients to stiffen up a sloppy wet mix.

A couple of posts ago wotavidone wrote about ingredient consolidation. Essentially, if you have a bucket of stone, a bucket of sand, and a bucket of portland, and mix them all together, you won't get three buckets of dry mix. The coarse sand will fill in the gaps between the stone, and the fine cement will fill in the gaps between the coarse sand. Your advantage is that the stone and sand are already consolidated in your ballast bags, however, additional consolidation will occur when the mix comes together in the barrel mixer.

Your slab has a 70cm, or 0.7m radius, and will be 10cm, or 0.1m deep.
3.14 x .7 x .7 x .1 = 0.154cum. Concrete weighs around 3400kg/cum, so your slab will weigh around 525kg if I've done my metric math correctly. If you still have 50kg of cement and 440kg or ballast, you may want a few more bags of material on hand so you don't come up short, or in case you lose a load to a spill, or have an issue with your form. You want to be able to recover from any mishaps right then and there.

I'm not sure if your form is complete, and it's hard for me to see how well it is secured to the base in your previous photo, or how the circular form is secured to itself so it is circular.

But consider adding some blocking around the outside of your circular form. I doubt all that is holding the form edges together is tape. But I'd add extra reinforcement outside of the ring at those taped joints as well. When you're done pouring, you'll want to vibrate the mix, or the form itself, to help the mix settle and eliminate any voids in the mix. That vibrating and consolidating the concrete in the form can put a lot of lateral pressure on that form. The last thing you want is a form blowout, or for the form to lift up off the base. Secure it well.

Hope this helps more than confuses!

Best, Mongo

[Lucag]

Hello Mong, this really helps, thank you very much!
My form is pretty secured with several wooden block screwed in all around the shape. I’ll add a photo soon.

The only failure point now seems seems to be the ratio of the ballast, which I don’t know exactly until I measure it myself. I have contacted the company asking for clarification but I imagine if it’s not written in the packaging they don’t know exactly, or it’s not a constant value.

[fox]

Hi Lucag, do the bags you have actually say “ballast” or do they say “aggregate”

Balast is not a common term word used in my area for mixing concrete, I have been a builder for most of my life and generally speaking max strength concrete It mixed 6-1 .... that is one part cement six parts aggregate. That is a standard concrete construction mix that offerer maximum strength and the least chance of cracking under normal circumstances.
Aggregate come already mixed in the correct ratios, ready to mix straight with cement.
5-1 will be fine in your relatively small area but 6-1 is the standard (at least in my part of the U.K. )

[Lucag]

1:6 it is then, thank you very much!

[david s]

You still should work out your proportions of sand to aggregate. For my money 6:1 is not strong enough for this application. Here is a guide for various concrete mixes.

http://structx.com/Material_Properties_011.html

[wotavidone]

Mix it 5:1, take a close look at it when it is mixed and if it looks lean add a little more cement.

https://www.allmix.co.uk/much-ballas...-mix-concrete/

"Aggregate or ballast is typically composed of chunks of rock such as limestone or coarse gravel together with other ingredients such as sand. Recycled aggregates are often employed, while the material can also be pre-manufactured in a blast furnace. Some applications require a finish or specific look, and to achieve this it is possible to add glass or decorative stone into the mixture – this is known as “exposed aggregate”.

To create concrete of specific hardness, the following general rules are often applied. High strength concrete would be mixed using 1 part cement to 5 parts ballast – this affords the sort of water-tight concrete used for building slabs or concrete panels. Mixing 1 part cement to 6 parts ballast creates concrete suitable for driveways, paths and patios, and increasing the ratio to 1:8 gives foundation-strength concrete. It is of course possible to add other materials to increase the strength of the concrete – these often include steel reinforcing bars (“rebar”)."


BTW, I think Mongo is mistaken. Most sources say concrete is 2400 kg per cubic metre, not 3400 kg.
So you should have enough materials, as long as "several bags of cement" = more than 2 bags.