concrete advice req

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hello guys. i have had my trench fill footings filled this morning for my cons but i have a slight concern regarding the slump used. as they were pumping the concrete from the volumetric truck it seemed a bit wet to me so i quieried it with the concrete guy, he said it had to be that wet to be able to pump it.

i picked up a shovel load and it did not hold its shape and just ran of the shovel. am i worried about nothing or would i have a problem later?
i realise this is all academic now :unsure:


also, another question. the cons floor area has a concrete patio sub base on it, it is about 50mm thick. i have taken the tiles of. question is: can i put down sand, dpm and insulation and use 150mm of screed on top of this?

thanks and sorry for the long post
 
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Oh no, you got the slump wrong! :eek:

Only kidding.

Believe me, i've seen it pumped in like water and set like bell metal. You try breaking the stuff up in a months time.

If you were building a dam or a bridge caisson, then mebbe slump would be a factor.

We are forever telling them to add MORE water so you are lucky.

Yes, you can leave the sub base down so long as there has been no weird contamination of the base.
 
almost made my heart stop :eek:

thanks for that mate, i didn't fancy doing more digging as my garden has a massive mount of earth on one side and bricks on the other. missus is not happy at the mo lol.

i am thinking of getting mixamate in to do the screed with they're combined screed pump at some point.

many thanks for the reply, i can stop thinking about a crumbling foundation now :D
 
just another quick question. how long should i leave the foundation to dry before i can start building on it?

cheers
 
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thanks stuart, i thought i might have to leave it for a month.
 
Hi,

I have a similar worry, I hope the O/P doesn't mind me extending his thread.

I'm having a largish greenhouse base installed with a dwarf wall. The foundations were filled with concrete which came in a huge lorry that mixed the stuff on delivery. I used half a barrow load left over to fill a few holes in the drive and it set like er, concrete.

When the base was installed on Friday (four days ago) the same lorry came but the mix was very wet, almost sloppy. It was levelled by the concrete delivery man using a large tool shaped like an upside-down T but with two handles. He sort of tamped (well, sloshed really) it down from one end to the other. The concrete was pretty much self-levelling.

After the mix settled there was a fair amount of water standing on the surface, I don't know how much. but perhaps 5 or 10 mm. It has set now.

Now as he'd done this before, and as he had a tool to do this, I assume that this is normal practice. I haven't walked on the base yet, but I scratched it with my fingernail tonight and a film of soft dust came away. I'm petrified that this will be another dusty concrete floor (i had to buy tiles for the garage after that was done!) but the builder assured me that it wouldn't be, and the mix was suitable for floors. Is it likely that the water on the surface has weakened the mix, and is it likely to be dusty?

I'll sweep it after seven days, but I'd like a few opinions before Friday!

Thanks.
 
For maximum strength concrete needs to be almost semi-dry when mixed. The optimum amount of water would be so little the mix would be far too difficult to work. So concrete is always a lot wetter than is ideal. And it needs to be a lot wetter still when it's pumped. As you can imagine, it needs to be very wet just to flow through the pump. Also, carriers usually include an additive (superplasticiser) to pumped concrete to increase flow even more. This doesn't really make it wetter - but it makes it look a lot wetter. For the average extension foundation, or floor base, this makes very little difference in the scheme of things. You are pouring a concrete with a design strength of about 20N and even if you lost 20% or 25% strength for it being too wet that still gives you a strength of 15N or so, which is plenty for the average extension foundation or floor. A much more important consideration is sub-base preparation and drying time. You don't want concrete to dry out too quickly. Keep it covered with poly for as long as possible and, if you can, soak it with a hose pipe on dry windy days. Also don't let it freeze.
 
Irrespective of concrete strength the finished floor surface will depend on how the concrete has been trowelled.

In order to get a decent finish using concrete the installer needs to powerfloat the concrete a good while after the concrete has been laid. In fact many hours after. This is a very specific design , is not easy to do and is stipulated at the design stage.

The other method is to keep the concrete a few inches down from ffl, lay a rough tamped concrete slab then lay a smooth screed finish on top of the concrete.

The two methods are quite different, the former requiring a bit of concrete know-how on the part of the installer.
 

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