concrete discolouration

Metric Tensor wrote: »

GGBS is a cement replacement though - not a sand replacement. Unless I'm misunderstanding your post?

I've seen a fair bit of concrete with relatively high GGBS content but have never seen it turn black.

When it is used correctly and with correct mix. GGBS can also be used as a fine particle replacement as well as a cement replacement. If suppliers are struggling to source sand, whcih they have been this last year, a few of them are throwing in sand substitutes. I have had several issues with blackening due to this issues. It took a while to get to the bottom of it, but sand shortage was the answer. My mix designs didnt allow for this alternative, so I was effectively safe, but it caused both blackening and uncharacteristic surface cracking in heavily reinforced structures. Im not saying this is the reason in this instance, but just throwing it in there as a possibility.

Metric Tensor wrote: »

Interesting. Thanks for the detailed reply.

What sort of percentages are we talking in terms of trouble?

Never found out quantities at all. The supplier and client came to some arrangement to leave it in, which suited me grand. If I was going to be implicated in any claims, I would have been taking cores and getting them analysed to determine these quantities. I would imagine percentages were high. On one, the surface of the concrete dried out too fast. The concrete supplier started trying to argue with the client that the cracking was due to inadequate reinforcement. This was a floor slab with H25 bars at 175mm crs top & btm. Massive amounts of reinforcement. As soon as the client came to me with that, it was a very quick and easy clarification. Met on site with supplier, folded within first few minutes and admitted their sand pit was depleted and they used GGBS. I wouldnt be surprised if there was no less that 10% of fines as sand to be honest.
Sand is the second most used natural resource on Earth, after water. Every Local Authority and EPA etc are protecting any natural occuring sand pits as areas of Scenic Amenity etc. Something has to give. Its becoming a huge issue.

Metric Tensor wrote: »

Great stuff - thanks for that. Every day's a school day.

If the slab was so heavily reinforced it's clearly important. Any worries about strength or longevity? How were the cube results?

cubes were all good. no strength concerns. longevity, Ive no concerns, but they had to pour a floor hardener/ sealer over it.

Metric Tensor wrote: »

Surface hardener makes good sense.

The blackening is interesting. A head scratcher at the start I'd imagine! The type of thing you'd like to see out of interest .... just not on one of your own jobs!

Yeah exactly. importance of notes covering general mix requirement and as much as possible. If I had no notes in relation to it, I wouldve been in a fight.

Apologies for jumping , but it seems you guys in here know about concrete.

I got 2m3 of concrete poured just this morning, i shovelled raked and levelled myself.

Should i water this concrete? Ive read that i should, and that you shouldnt?

Thanks in advance.