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Water in concrete


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Found this attachment today.

I hope this is useful for beginners, experienced builders and professionals alike. The former to learn, and the latter to show it to people who don't believe you (including 90% of groundworkers).


Water content in concrete is a precise science.

Concrete hardens by chemical reaction, not 'drying'.

Many groundworkers add extra water to concrete to make spreading easier, not knowing that it is bad for the concrete.

They seem to think that Engineers, other professionals and the concrete companies know less than they do.


All concrete suppliers have small print about not adding water without permission. Here is something much clearer.


So see the attached very plainly stated leaflet from Hansen Concrete. I think it needs to go on the wall of every site hut.


Hanson too much water leaflet.png

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Agree and disagree - if I ask for a specific slump on the spec then I expect it to turn up like that, and if it doesn’t meet spec (ie it’s too stiff) then that suggests I should send an 8 yarder back to the batching plant ..? That would be an interesting discussion … 


Its also marketing bo**ocks words - they don’t actually have answers so they use the “as much as” and “potential” “possibly” and “up to” which means they don’t actually know. Given the slump is pretty much the standard water/cement ratio, on a standard gen/c25 mix going into floors and foundations you would hardly notice the difference. On a bridge it may be a different matter. 


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7 hours ago, saveasteading said:

Water content in concrete is a precise science.

There is a similar problem with composite plastics.

Many a time I have heard 'we added loads of hardener and it went off (hardened) just fine'.

This is a real problem when doing flat roofs as the pigments change the depth that UV radiation can penetrate.  So you can end up with a partially cured base layer, with excess liquid or gaseous styrene monomer trapped in it, but a fully cured top layer.

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10 hours ago, PeterW said:

if I ask for a specific slump on the spec then I expect it to turn up like that

And so you should. I don't think they are disagreeing with that, and you would be entitled to send it back if it is wrong. 

The batching plant will even monitor the dampness of the sand so that this is already included in the mix allowance.


10 hours ago, PeterW said:

they don’t actually know.

I don't agree. This has been studied at length and is a fairly precise science when you consider that it is made of of broken up ground from various sources.

I have worked in the lab on a big project (all of 3 days as CPD) and sampled/tested using slump and cube tests. It is remarkably predictable and adding water is a bad thing.

Yes this was for bridges, but quality is quality, and the same control was applied to mass bases, kerb bases and everything else.


10 hours ago, PeterW said:

It's also marketing

No it is quality.  All concrete companies have the exclusion clauses re added water, but they are hard to find. I admire this up-front statement. 


I don't envy the worker with the shovel, but that is not my job, and it is theirs. For £4/m3 extra you can add a plasticiser and everyone is happy, except the buyer.


Have I ever authorised additional water? Yes, but only a small amount and for mass footings.

How often have I stopped the addition? Many times. They had to sweat a bit but managed fine.



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The Official Secrets Act prevents me from saying where (no I'm not bs'ing) but I worked on a building where there was a concrete lab on site. Water content was a huge consideration and they made hundreds of test cubes then tested them to destruction. Biggest issue was getting the right aggregates that would bond through the special reinforcing mesh that doubled as an RF screen. 100mm thick glass and so on. Amazing place. 

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5 hours ago, Mr Punter said:

Perhaps not even an effect:

Interesting. I still mean what I mean, but perhaps need another fancy term for it.


Groundworker (digger driver) thinking he knows about concrete.....

what would an Architect know about it?

of course you add water, everybody does it

no need to compact it when it is wet enough

no need to cover it as it has plenty of water and needs longer to dry.


any more?

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