Breedon High Strength Cement Mix Ratio?

[Antony11]

Hello,

I am building a garden room 7.4m x 3.7m garden room on 15, 20cm x 40cm x 40cm concrete footings that I have made formwork for. I will also be putting 8mm stainless steel rebar in them as they are quite thick at 20cm. Everything is set in place, levelled, placed on Mot type 1 and ready to be filled.

I was going to use Blue Circle or Breedons C40 pre mixed bagged concrete, but decided to go for Breedons high strength 52.5N cement instead, to mix with my own ballast as there was a price difference of about £250 between pre mix C40 and mixing my own "high strength concrete".

I called Breedon twice to ask about mix ratios for this product for my application and in all honesty I had more chance of getting blood from a stone than getting an answer from the person I got put through to. She just seemed keen to get me off the phone and not willing to enquire further. Just all our info is on our website type of thing!

I also asked at builders depot where I bought it from and nobody I spoke to really knew.

The back of the packet doesn't really state anything for mix ratios either.

Here is a link to the product:

https://www.builderdepot.co.uk/breedon-high-strength-cement-25kg

So I have come here in the hope to seek some advice...

So if anyone has any solid advice about cement to ballast ratio for my application with this type of product I am all ears.. well eyes so to speak.. ha

Thanks,

Antony

 

[jeds]

Trouble is, concrete mixes are normally prescribed for known grades of aggregate. For example; 1:3:3 cement:sand:aggregate. But you are planning to use ballast, which is not a known grade of aggregate. Different ballast in different parts of the Country - and from different manufacturer's - will not be the same animal.

However, in your case, 1:5 will do it.

 

[^woody^]

You know you can actually make concrete too strong and cause it to crack. There is no need for high strength cement for foundations and certainly not rebar, and the actual placing and tamping of it can be more important that the ratio, or can even affect the end result.

You can go up to 8:1

 

[Antony11]

Hello Jeds & Woody,

Thanks for your replies.

I was initially going to go with a 1:4 mix of ballast:cement, but judging by your replies so far it seems like an average would be around 1:6 ratio.

I did see quite a lot of charts on google saying basically a 1:1 ratio of ballast:cement to make it like a M40 mix which is basically C40 I think, which led to a bit of confusion...

I attached a picture for the type of chart I mean.

So with regards to the actual type of "high strength" cement I am using I guess I should just treat it like normal cement with normal cement ratios?

Thanks,

Antony

 

[mikeey84]

M40 mix which is basically C40

I think a C40 is more like an M30.

As Woody has already said, for foundation work, it really doesn't need to be that strong, as the forces acting on it are pretty mild

 

[Antony11]

Thanks for your reply Mikey, well after hearing this I am glad I didn't fork out an extra £250 for the pre mixed C40!

So as an example if I go with 50kg (fairly dry) ballast, 10kg cement. Any ideas how much water? One thing the cement does state is to keep the water content as low as possible.

Thanks

 

[mikeey84]

The fact its going into damp ground means you can mix it on the dryer side, especially as you could cover it with some plastic to help it not dry too quickly.

It's difficult to say how much, as you really can't say exactly. Just chuck a bit in, mix it, and chuck a bit more in, but not too much

 

[Antony11]

Thanks, although the ground is fairly dry at the moment. I suppose I will just have to see how its looking in the mixer

 

[^woody^]

You need the mix to be fairly wet and sloppy for ease of placing, tamping and leveling.

 

[Antony11]

Thanks woody, I know what you mean otherwise air pockets and gaps will form. I will start with 10% water content and work my way up till it looks right

 

[lostinthelight]

I dont know how much concrete youve mixed but I use the first couple of mixes to gauge the amount of water per mix so for further mixes you can get most of the water in the drum first to help keep the drum clean

 

[^woody^]

Thanks woody, I know what you mean otherwise air pockets and gaps will form. I will start with 10% water content and work my way up till it looks right

It's not like fine engineering or chemistry, so approximate quantities will be fine.

Your ballast is going to be wet anyway, so additional water required is an unknown so don't bother gauging it. Add the water until the mix is "right" - best example may be like thick porridge and not like soup.

For foundations you want the mix to flow and have a bit of self levelling quality, so it runs sideways and then when you use a rake and pull it up and down quickly in the poured concrete, the concrete spreads and tries to level itself. This vibration also toughens it by removing air.

 

[Antony11]

I dont know how much concrete youve mixed but I use the first couple of mixes to gauge the amount of water per mix so for further mixes you can get most of the water in the drum first to help keep the drum clean

Thanks for your reply lostinthelight,

The only real mixing I have done was a 5inch 11m2 slab by myself in one very LONG day! Lol

I will probably gauge by doing something along those lines.

Am I right in thinking you should add water to the drum first before cement and ballast to stop it from sticking?

Thanks

 

[Antony11]

It's not like fine engineering or chemistry, so approximate quantities will be fine.

Your ballast is going to be wet anyway, so additional water required is an unknown so don't bother gauging it. Add the water until the mix is "right" - best example may be like thick porridge and not like soup.

For foundations you want the mix to flow and have a bit of self levelling quality, so it runs sideways and then when you use a rake and pull it up and down quickly in the poured concrete, the concrete spreads and tries to level itself. This vibration also toughens it by removing air.

Thanks for this helpful info woody, I will keep it in mind when mixing next week

 

[lostinthelight]

Yes, add water keeping a little back then small amount of ballast which scours the drum then cement and when well mixed to slurry add rest of ballast and rest of water as required.
I have mixed hundreds of tons of compo,concrete,etc and this is by far and away the best way for a consistant mix in the shortest times that I have found over 60 odd years of mixing..never too old to learn though if anyone comes up with anything better