Uneven Concrete Pad

[physco]

I have just had a 4" deep concrete pad poured by a local builder for a hot tub. As the pad will be supporting a hot tub which ways about 1900Kg it needs to be flat and level. The builder was awre that it would be used for a hot tub and as such that it needed to be flat.

Unfortunately it is neither level or more importantly flat. There are variations in level / flatness of as much as 10mm over 2metres making it unsuitabel for a hot tub. If I placed teh tun on this surface I would risk distorting or cracking it ince filled with water. The builder is coming around tomorrow to take a look, but I want to have a good feel of what can be done before he arrives. From what I can see there are 2 choices:-

1.) Self leveling external compound such as Ardex K301 which can be poured thinly.
2.) A thicker layer of screed.

I dont want a thick layer of screed as this will result in the hot tub neing at the wrong level. I am therefore wondering what is the thinest layer of screed that could be used externally that wont break up over time. From what I gather screed is normally around 50mm thick.

I am also wondering if there are any other suitable leveling compounds that are cheaper than Ardex K301 as that is about £60 per bag an I am guesing that woudl require about 3 bags ie about £180 which I think teh builder is not going to swallow - especially as the whole job was only £900.

Would reqlly appreciate some ideas on what is the best long term solution to this.

Thanks

 

[r896neo]

If the slab is going to be exposed to the weather you are in diffs as at that thickness unless you use a high performance cement or epoxy type screed it may not bond properly and lift at the first sign off frost. As you have found the ardex/ epoxy type are mega bucks.

I assume its not that big? Apart from a thicker 60mm ish screed as you said, the only other real option to keep the height is to break it up while it's still softish and re-do it. Of course that is likely to cost the builder even more than the screed.

Such a small area should not be difficult to get right for even a competent diyer.

 

[physco]

Appart from finding the time to do it, one of the main reasons I got a professional builder to do it was because I knew it needed to be flat and I have not poured a slab before. In hindsight it seems that as always if you want something done properly you have do do it yourself. I am maybe unlucky, but every time a decide to get a professional in it is always a disaster.

I guess I will have to see what the builder says on Monday, but from what you have said it sounds like I am right that its either mega bucks Ardex, or 50mm or so of screed.

If we went for 50mm of screed, would there be anything specific needed in the mix as it will be exposed to the elements?

Also, would a real thin layer (say 10mm at deap points and as little as 3mm at high points) of Ardex K301 do the job or is that likely to lift in teh frost also????

Thanks

 

[physco]

Further to my last post. I assume it is reasonable to expect a flat and level base when that is what you sepcify and highlight that it is a hot tub going on it?

10mm over 2metres of non flatness seems like a lot to me - The floor in our garage is absolutely flat and that is a crappy new build.

I just want to make sure I do not getted fobbed off.

Will provide an update as to how things pan out. Fingers crossed the builder accepts it needs rectfying and comes good. Seems like decent chap.

 

[r896neo]

Can't say about the ardex, you'd need to check with them. When you say 10mm over 2 metres do you mean the level or that is the depth of some of the low spots?

 

[physco]

I actually mean both:-

At one end of the slab it is flat but is off level by 10mm over 2 metres. It looks like the shutting was not level.

At the other end of the slab, there is also a drop of about 10mm over 2 metres, but it is not flat ie the concrete curves (like a small hill) for the high point to low point.

To me it looks like the problem is that the shuuting was not put in level and so the concrete is both not level and not flat - its twisted if that makes sense.

 

[theoldun]

What’s the size of the slab? 2100x2100? If so, wish we could find a few people to pay us 900 notes for slab that size.
Standard tolerance in screeds is 10mm in 3 metres. 10mm out of level in 2 metres is no big deal as your boat will still float as water is level. Your problem is that it appears you have a couple of camels in the slab. As we have never fitted a hot tub we can not say whether the humps will distort or crack the tub. Would not think so, but then again who knows.
Screeds. A bonded screed can go down to 10mm in thickness , as long as it is bonded chemically (epoxy) or laid with SBR/PVA bond and SBR/PVA modified screed mix. If the screed is to be cement bonded then 30mm should be the minimum depth.
This means you want 20mm at your low point and levelled out accordingly.
Would suggest SBR Modified 3 grit sand not to course, 1 dust, polypropylene fibres and your SBR.
Give slab a real good drink before priming with SBR and hit the primer with the screed before primer dries.
Don’t use PVA. Google SBR and read the instructions. Can not be bothered to write any more.
Best of luck oldun

 

[physco]

Thanks for the advice - very useful.

One last question, considering the slab is external for a heavy hot tub so will get weathered, splashed with spa water that contains bromine or chlorine and will be in freazing winter temps. what would the best solution be - Epoxy bonded or SBR bonded? The reason I ask is I dont want a soloution that only lasts a couple of years - it needs to be permanent.

And another 'last question'. The buidler came out today and was appologetic and will come and sort it out, but from what he was saying I think he intendends to blend the screed covering the low points into the existing concrette at teh high points if that makes sense ie a depth of 10mm on low points and no coverage at all on the high points.

From what I gather, this is not a good idea as in some places it will be VERY VERY thin. I gather the whold base would need screeding with at least 10mm depthe everywhere.

Also wondering how long teh original base needs to be left to dry - looking at the Ardex K301 stuff they sugeest 6 weeks before a thin screed is poured.


Sorry for all of teh questions, I just want to make sure I am left with a long term fix - not something that breaks up over time.

Thanks

 

[freddiemercurystwin]

I just want to make sure I am left with a long term fix - not something that breaks up over time.

Fair enough but does beg the question though how will you ever know and where do you think it will go anyway?

Reminds me of a neighbour opposite who's rear garden I could see into, around 4 years ago maybe we had a long hot summer down this way, I kid yee not the very last day of this magnificent hot spell he had a dirty great hot tub craned in. He's maybe used it a dozen times in the last couple of years! Always makes me chuckle to see it!

He's also got a dirty great roofing slate resting just above the bottom row of slates (held by nothing but friction) that came loose higher up that's just waiting to fall on his head (I have mentioned this to him BTW) but he's hoping friction will hold it there permanently! Maybe it will land in his hot tub!

 

[r896neo]

A cement screed will be better layed as soon as possible after the concrete. A n epoxy type may need to dry out for the primer to take properly, you'd need to check the MI.

 

[physco]

Well the builder came back last Friday and was very appologetic. He spent a couple of hours trying to smooth out the uneveness with a very wet mix with PVA. He blended the uneven area in with the flat area with a very thin layer in places.

I was a littler nervous after the posts on here ie about using PVA instead of SBR and also the depth of some of the 'screed' is literally 2mm thick where it blends in. But he assured me it would be ok and also that it was flat.

Anyway, back out with my level teh next day only to find it is still totally uneven. Dropped him a text to highlight my annoyance and asked him to call me - he didnt. Will call him tomorrow to ask him to come round again and sort out.

Very ****ed off. How hard can it be do pour a flat slab should haev done it mysefl.

£900 for an uneven 3m x 3m concrete slab. I feel robbed at the moment.

 

[Norcon]

Jack hammer it all out and start again.

 

[physco]

That would be my preference. Will see what the builders suggests next.

 

[physco]

Well I now suspect the builder is ignoring my calls. In a week of trying to call him I have and one text from him saying he would come round tomorrow and didnt show up.

I guess my ususal view of if you want something done properly do it your self was correct again.

Will keep trying to contact him, then I guess its time to take legal action and do the work myself......

I feel robbed. He may aswell of taken £900 from my wallet and left me with nothing. In actual fact that would have been better, as now I may aslo need to remove the mess of a slab he has poured before pouring one properly mysefl.

 

[physco]

To sort this problem out ie an uneven concrete pad, without digging the whole bloody thing out I am considering laying some 50mm x 900mm x 600mm concrete pavers in the area where the Hot tub will be placed. The complete concrete pad is 3m x 3m but the tub is only 2m x 2m. I dont want to raise the level of the whole slab as we intend to surround the tub with decorative stone.

What would be the best way of bonding the concrete pavers to the existing concrete pad. It needs to be very durable, VERY flat and take the weight of an 1800Kg hot tub.

I really dont fancy leveling the entire concrete slab with a screed etc as that would raise the level of the whole thing and I am also thinking it might be easier and cheaper to simply level things out using some big concrete pavers.

Any advise appreciated