Can someone explain to me about concrete pumping mixes?

[schiz]

Hi,

I'm a bit confused about readymix concrete. My spec says C15 concrete mix (for footings), but everyone says you can't pump a C15 mix, so I've asked them for the nearest equivalent which is pumpable and got three quotes. One readymix supplier (who was the cheapest) suggested what he called a "Gen 3" pump mix, but was unable to give me the ratio that would be used for that, but said it was about equivalent to a C20, therefore over spec.

So I now rang the pumping guy, to confirm with him. He talked very scathingly about the 'Gen 3" specification and said it was rubbish. He told me to make sure I got 300Kg cement per metre cubed for the pump.

Now I'm concerned about the reliability of the suppliers and that the quotes might not be comparing like-for-like. Please let me know the best way of specifying a pump mix so I can get the right thing. Not having much experience I won't know if they've got it wrong come delivery time.

many thanks.

 

[^woody^]

I'm only an engineer-lite, but from what I know concrete dries hard and most mixes will be OK for typical domestic foundations.

If you just ask the supplier for a foundation mix suitable for pumping, then that should cover it.

More cement makes for a smoother mix which flows better

 

[Shytalkz]

Hi,

I'm a bit confused about readymix concrete. My spec says C15 concrete mix (for footings), but everyone says you can't pump a C15 mix, so I've asked them for the nearest equivalent which is pumpable and got three quotes. One readymix supplier (who was the cheapest) suggested what he called a "Gen 3" pump mix, but was unable to give me the ratio that would be used for that, but said it was about equivalent to a C20, therefore over spec.

So I now rang the pumping guy, to confirm with him. He talked very scathingly about the 'Gen 3" specification and said it was rubbish. He told me to make sure I got 300Kg cement per metre cubed for the pump.

Now I'm concerned about the reliability of the suppliers and that the quotes might not be comparing like-for-like. Please let me know the best way of specifying a pump mix so I can get the right thing. Not having much experience I won't know if they've got it wrong come delivery time.

many thanks.

BS8500 recommends a minimum consistence class of S3 for pumped concrete. However, at the risk of sounding all adenoidal, BS8500-1:2006 cl 4.2.2 note 2 states that "Gen concrete...might not be suitable...for methods of placing, such as pumping...", and to revert to the suppliers for advice. Hence the disparity of the advice that you are experiencing.

Gen3 concrete is a C16/20 strength concrete, with a min cement content for 20mm aggregate of 220kg/m3 and S3 slump. So, that could be ok for pumping, if the supplier is happy with that specification, or if you're using trenchfill, upping it to S4.

A min of 300kg/m3 cement content is recommended for RC30/37 and RC32/40 concrete, with S3 slump. Waaaay over what you need.

You don't need a great spec for plain concrete foundations (hence the C15/Gen2 specification), so, if the supplier is happy supplying you - knowing that it's for pumping - with Gen3, I would go for that, but up the slump to S4, whether trenchfill or not and possibly up the cement content to 240kg/m3 per a RC20/25 mix.

Edited for answering the phone and losing the thread of whatever else it was that I was going to say

 

[noseall]

but from what I know concrete dries hard and most mixes will be OK for typical domestic foundations.

couldn't have put it better myself.

If you just ask the supplier for a foundation mix suitable for pumping, then that should cover it.

this is what we do too.

 

[^woody^]

"War and Peace"

LOL - no wonder you guys charge so much to write all that, just to say "ask the supplier for a foundation mix for pumping"

 

[Shytalkz]

Bit o' background doesn't hurt

Should never specify or ask for something, without understanding why it is that you're asking for or specifiying it.

 

[schiz]

Bit o' background doesn't hurt

Should never specify or ask for something, without understanding why it is that you're asking for or specifiying it.

I totally agree, and many thanks for the lengthy reply, but Pavingexpert, he say:

S4 and S5 are likely to be used in specialist applications and advice from a suitably experienced concrete technologist should be sought before specifying concrete in these classes.

He then goes on to say:

When using concrete with high slump values (>150mm [S4 or S5]) there is a risk that the aggregates and cement will settle out. or segregate This is usually countered by the use of various additives to ensure the concrete remains workable and structurally competent

I will check what it'd cost me to keep the pump guy happy, i.e. 300Kg/m3, but if I do use that mix will it cause me any probs?

 

[Shytalkz]

No, it will just be a much, much stronger mix than you want or need and cost you more per m3.

 

[schiz]

No, it will just be a much, much stronger mix than you want or need and cost you more per m3.

For 6 m3, Gen3 (pump) was £493.50, RC40 was £497.00

Not sure if that was the kind of price difference you were thinking about, but for less than a fiver I'm inclined to just pay the extra. I think this must mean when they say Gen3 Pump mix, they mean they've already added extra cement. Either that or the extra £3.50 bought me 480Kg of extra cement...

Thanks for your re-assurance though. Coming here's a bit like seeing the doctor. There's usually nothing wrong, but it really helps to put the mind at rest.

 

[Shytalkz]

Hmm, thought it would be slightly more than that - as you suspect, the Gen3 price is probably with added cement content rather than the basic 220kg/m3. Anyhow, as you say if that's the diff, nothing to worry about - just go for it and keep the mizzo git on the pump happy

 

[schiz]

just go for it and keep the mizzo git on the pump happy

Speaking of mizzo git, I filled the trenches today, and, not content with his extra cement, he asked the readymix driver to add 140 litres of water anyhow (when I was out of earshot). Does 140 litres sound a lot in 6m3? It certainly came out nice and runny. I guess that's some advice for anyone else attempting this: Don't let the readymix driver talk to the pump driver without listening in. I'm just wondering if I can be bothered to give the readymix company a roasting about not asking permission of the one paying the f&*%ing bill before invalidating the warranty on the mix.

 

[noseall]

Speaking of mizzo git, I filled the trenches today, and, not content with his extra cement, he asked the readymix driver to add 140 litres of water anyhow (when I was out of earshot). Does 140 litres sound a lot in 6m3? It certainly came out nice and runny. I guess that's some advice for anyone else attempting this: Don't let the readymix driver talk to the pump driver without listening in. I'm just wondering if I can be bothered to give the readymix company a roasting about not asking permission of the one paying the f&*%ing bill before invalidating the warranty on the mix.

we have only one thing to say to concrete mixer wagon drivers....................WETTER.

their water is very precious and is only meant for cleaning the drum and chute.

 

[rmix]

Hi Schiz,

I was interested to learn about your experience of driver adding water to your readymix concrete without your permission. I recently experienced something similar. I was wondering did you ever get any feedback/clarification about your concerns? Did you ever query/take the issue up with your supplier.?

 

[schiz]

Just to be clear about this, it was the pump operator who requested this without me knowing. How would the driver know who is the recipient of the mix? I certainly don't blame him. The pump operator signed for the delivery when I was inside the house. Noseall may disagree, but I still recon the pump op. was being a c***. He could have at least asked, but with people in the trade there are all kinds of weird rules about what constitutes being a c***, that you and me are never going to learn them all.

The pump operator handed me the signed delivery note only when he was leaving, so it was only as he was walking out that I read the terms and conditions. No problems so far, but who knows?