concrete floor for cellar questions

[scooty]

Hi all,
I am looking for some advice please. I am a plasterer.
I am converting an existing cellar into a usable room, or store room. The cellar is about 3 meters by 4 meters. I have plasterboarded and skimmed the ceiling, now i need to dig down to put in a concrete floor. I have started to dig and i am digging up mostlly like cobble stones with a fine sand and some earth all mixed together. The house is victorian.I have dug down about 9 inches , i planned to put in a 4 or 5 inch concrete floor and so i will have a little more head room to the ceiling. As i dig i have come to the bottom course of bricks that havnt really got much of a foundation under them so i plan to concrete to the first course of bricks.now i have realised i have a few problems. As i dig the cellar is getting very full with rubble so i think about concreting half of the cellar first but i will still have to carry out all of the rubble in order to bring in the new aggregate and cement to do the concreting. This is where i would like some advice from the experts.i am thinking ,
1. because i have dug out mostlly small cobbles and sand with a bit of earth mixed in, could i actually use this mixture with cement to make the concrete and tip it in where i dug it out from? This would solve a lot or problems.or will i have to go and buy new aggregate?
2. i am thinking that maybe i will have to concrete the cellar in maybe 4 sections, i would like to know about any problems this would cause as i dont know if it is ok to do this.IE. concrete a quater of the cellar, then maybe after a few days another quater etc.I am wondering about any problems where the concrete joints will be.
Thanks for any advice
Scooty

 

[JerryM]

Much depends on room’s purpose – a proper room to be lived in or an improved cellar

The bld regs spec dig out is 15” (6 hardcore, 1 sand, dpm, 2 insulation, 4 conc, 2 screed). Trouble is old foundations are often exposed before this and then you need to be very careful not to disturb. If it’s an improved cellar then 4” conc is more than enough.

Conc floor mix needs to be 1:2.5:3.5 cement:sharp sand:aggregate. I think you could use the cobble to offset the aggregate. Concreting sand is not like soft building sand and the strength of the concrete would be reduced. Suggest use rubble as hardcore and the sand to blind.

There is no reason why you can’t concrete in quarters. My only though would be a possible damp path but this would not apply if DPM is underneath. You would have to keep off each quarter for at least 3 days though – conc takes 28 days to get near its full strength.

Other thoughts:
1) You will need at least 1 builder’s skip probably 2 - Surprisingly rubble is a big part of putting in a new solid floor. There is a lot of it and you really need a skip to get rid of it. Get it all out to start with before you start concreting
2) You will need to buy new aggregate (it’s not that expensive in te bags)
3) You will need to hire a cement mixer and get a least 4 buckets for mixing
4) If digging below the foundation would consider digging out/concreting alternate 1m strips of concrete around the perimeter before digging out the centre
5) You will need to treat the walls to stop the damp
6) Would recommend depending on the use to submit the work on a building notice – that way you would get a completion cert in the event you sell.

 

[scooty]

Yo Jerry ,
thanks very much for your reply. you have given me some sound advice.The cellar will just be used as an extra room ( storage, play room , work shop)I am only converting the cellar because it is there and i have a lot of the knowledge to do the work myself.Also it should put some value on the house.
Well today by chance an ex girlfriends father came to my house to give me a Christmas card, he is a council building inspecter.I was able to pick his brains and get some good advice. He looked at the aggregate that i have been digging up and says it is good enough to use to mix concrete for the new floor , but to also buy some more bags of aggregate to mix in . ie. 1 cement, 3 of old aggregate and 2 new aggregate.He recommended putting 2 inches of scalpings down and whacker plating them and then lay 4 inches of concrete. I think i will concrete the floor in 2 sets. i can dig out half and shovel the rubble to the other half of the cellar and then concrete, like Jerry says leave at least 3 days and then i can dig out the other half and shovel the aggregate onto the new concrete. One thing i am told to be careful of is not to use a petrol concrete mixer because the carbon monoxide fumes in the cellar with no air circulation could Kill.
once again , thanks for the reply
Good luck to all and Merry Christmas
scooty

 

[JerryM]

glad to know you've got VIP friends.

i don't think the scalpings (presume crusher run) would be essential for the intended use.

if you do go ahead with the scalpings i would go for a sledge hammer and bit of sweat rather than wacker plate in a cellar (ear defenders otherwise). i would also try to mix outside and tip the wheel barrow directly in then rake to where you want it (hopefully there is an existing opening ie coal or take up internal floor board).

also try to put a DPM in below the concrete. you would need to put a thin scrape of sand over any scalpings to stop piercing DPM (ideally 1").

on mixers i got my electric off ebay at a decent price compared to hiring. would not normally recommend ebay but the mixers now my pride and joy.

 

[scooty]

Thanks again Jerry for the reply.
I agree , i don't think i will bother with the scalpings for the cellar floor, the advice from the building control man is good but i suspect he is being a bit fussy. In my case its not really practical to mix outside the cellar. I am going to concrete in 2 halfs and i am digging out and shovelling the aggregate from the one half of the floor onto the one half of the cellar. I have a nice big pile. i do agree that i should put down a layer of sand onto the floor before the dpm goes down.
The question i would like to ask is , how many bags of sand would i need to cover the cellar floor which is about 3 metres x 4 metres.
Thanks.
And I wish all a Merry Christmas and a Happy New Year
regards
Scooty

 

[JerryM]

i recon you will need about 20 (25kg bags) of sand ie 1/2 te of sand. i would use your existing supply "sand with a bit of earth mixed in" rather than buy new.

if you do need to buy new and it's approaching 1/2 te then i would get a 1/2te bag delivered as it's works out better value (my cut off is upto around 300kg or 14 bags which i would buy & collect as bags)

the sand calc is based on: area of 3x4m @ 25mm gives 0.3m^3 and applying sand at density of 1670kg/m^3

i also have an old wood lintel (7 x 3 x 50 inches). anything similar would be ideal for final tamping down.

agree on the scalpings. for info on a similar job (damp proofing under a wooden floor) i used 2" of concrete which turned out strong enough (no cracking). it really depends on how settled the sub base is.

best wishes.

 

[scooty]

Hi all,
I thought i would post an update about my cellar floor.
After digging up half of the cellar floor , the aggrgate that was already in the cellar looked good enough to make concrete with.So i find myself with half a cellar floor and a big mound of aggregate. Then i bring in bags of cement and a concrete mixer and buckets of water. I find that there is not much room to move. Anyway i concrete half of the cellar floor about 5 inches deep. In one corner of the cellar the concrete sunk down about 1 inch, but when i concreted i had very little light and could not see very well the level that i laid the concrete.I left the concrete to set for 2 days, then i moved the mixer onto the new concrete and then dug out the other half of the floor and shovelled the aggregate onto the new concrete, i then concreted the second half of the floor.
I will say that i really dont want to do this job again in a hurry as it is a lot of very hard work i found. I am still left with about 20 spare bags of aggregate to get rid of. I am now very glad and pleased that the job is done and i have a new floor and am left with about 4 inches of headheight from my head to the ceiling , about 6 foot in all.
i am now looking for some advice about plastering the walls.I am a plasterer by trade but i am still not sure of the best way to do the job.these are my thoughts
Do I
1.Black jack the walls , and then use a carlite bonding coat and skim. ( this way i can just render the walls bit by bit when i have spare time)
2.use sand and cement with a waterproofer and render the walls and skim( this means i then have to hire a cement mixer and so would need to get the rendering done in about 2 days , otherwise it will work out more costlly )
3. Should i buy the waterproof Limelight plaster and render the walls and skim. ( i dont know the cost of the limelight)
Any advice iswell appreciated.
regards
Scooty

 

[JerryM]

Pleased it's not going too bad. i know exactly how much effort it will have taken.

onto the walls. i am not familiar with black jack.

ideally you need to get some form of moisture barrier directly onto the brickwork - tanking. if you have access to a damp meter would take a few readings to see what the dampness is like. if not stick a square piece of plastic onto the wall sealed with tape around the edge and leave for 24hrs. if it contains water when you remove then i would say best to put some form of tanking on. i am conscious of the cellars intended use and don't want to go ott. the trouble is that cellars have -ve pressure ie the water in the brick is trying to force any barrier off (as opposed to say swimming pools when it's +ve pressure). i will have a think about it (i am thinking a dpm sheet held temp on nails bedded onto the wall using a thin skim of render). i don't know of anything off the self. i would use what's called K11 but it’s a trade product.

after the membrane then i feel it must be your option No 2 ie render with a waterproofer in it. the browning just isn’t strong enough. I feel you should be able to complete in the 2 days once the area is clear of the remaining aggregate. if you need the spec then just say.

just a thought depending on mixer hire cost it may be worth trying to buy one. i got mine for off ebay (would not normally recommend) for a good price compared to hiring (i think people buy new for an extension then want to get rid when jobs done). i also think it would be useful in the plastering work ie bay windows etc.

 

[JerryM]

thought about the dpm sheet but can’t get it to work unless adopting battens and plasterboard but reluctant on cost and performance.

i've used b i t c h u m e n damp proof paint in the past for penetrating damp, mainly upstairs where K11 would be ott. i've had a look at the spec sht for Liquapruf (bought mine from wickes) (http://www.sheffins.co.uk/Literature/Liquapruf.pdf)

it's not high spec for the application and certainly no good for living accommodation but given the intended use (storage/workshop/play area) i think it would be a cheep but adequate solution.

would also say that you could use browning on top as render is to stop salts getting into the plaster (b i t c h u m will also stop). if you do use, the trick is to get a key on the last coat quickly before it dries out by chucking grit sand at it.

one other thought is to protect your ground floor joist ends going into the walls (particularly if you intend closing any wall vents).

ideally jacking up using acro just enough to slide a piece of dpc underneath and up the sides. if there is not enough give in the skirtings then abandon and use a paint/spray wood rot preservative (the downside with this is that you can't be sure it soak's in enough). why i specifically mention is that once the tanking is in place the water will no longer be able to evaporate on the surface of the wall in the cellar. this water will try to go elsewhere due to the pressure and up is the most likely direction towards the joists. you could also just wait and see by taking damp readings after the tanking has been in place for a few weeks. anything above 20% in wood will lead to rot after 8 weeks.