Final update:
The solution was to get it screeded. There were 2 options, each added a different amount of height from the highest point:
(A) 50mm. Screed had fibres added for strength
or
(B) 10mm. Screed had both SBR and fibres in it (they called it polymer modified sand cement screed)
We went with (B), but it got sub-contracted and they went 20mm instead. (not sure why)
But they screwed it up and it wasn't level in the corners!!!
So the main contractor came back and added a hard wearing layer just a few mm thick of a self levelling product called Tekcem 375 Fiber.
It's now rock-hard and near perfectly level. Done! (even if looks like a 3 layer cake!
)
Top Tips if you're rescuing a bad concrete base:
1. Ensure the screeder has experience with outdoor screeds (there are various off the shelf products like Sika 30 which may be great, but you don't want to be the guinea pig!)
2. Agree the finish - screed can be quite sandy depending on the mix
3. Ensure your base is really clean and dry before the screed goes on (you want a good bond!)
Top tips for having any concrete shed base poured:
- Agree the tolerance in advance -- that is, how level it will be corner-to-corner measured in mm, whether there will be waves/dips.
-- Everyone has a different idea of what is acceptable! Beware!!!
- Ensure they have the tools to measure the level: An acoustic level or a green laser level are must-haves! If they show up with a red laser level that they can't see in the sun, send them away!
- Agree the finish in advance -- can it be bumpy with bits of gravel sticking up?
- Don't pay until it's dried and you're happy with it.
- Ask them what quality checks they will do and ensure they do them -- beware people who say "trust me it'll be fine
" Everyone thinks they can do a concrete base, but very few can do a large base that is perfectly level and smooth
- Look at screed companies that also do concrete. The screeders are more serious about a quality finish
- If your base is large (anything over 3m by 2m) then
- don't hire anyone hand mixing - it should arrive in a lorry or mixed with a large petrol pump towed behind their van
- ensure they have at least 2 people on the job (ideally 3 or 4!) it's exhausting work and once exhausted mistakes are made
Good luck!
p.s. the above is only my opinion based on my recent experience!