Recasting a concrete floor.

[benners66]

This may seem like a very straight forward question, but I’m at a loss at the best way to recast a concrete floor ensuring it is level.

The floor in my kitchen has “dropped & cracked” by approx 35-40mm on one side of the room, leaving a slope.
I want to break this whole floor out (about 5.5m x 2.8m) & relay/recast it as there is also a plinth from the Aga (now removed!).
Can anyone help with the correct method.
What thickness of base / slab etc, DPM requirements, screeds etc.
How can I ensure the floor is level? Will I be ok mixing & pouring myself (with some help of course) for this sort of area??

Sorry for all the questions but I’ve never had to do anything like this, but happy to do it with the correct information. I have searched back for an identical post, but with 114 pages, I ran out of time!

Any help on this will be gratefully received.

Ben

 

[noseall]

do you wish to lay the floor to current insulation standards?

whenever i lay concrete floors i allow for a 75mm screed. i find it less hassle than trying to powerfloat a decent finish to raw concrete.

floor spec as follows:

dig out floor and spoil to a depth of 400mm plus.

lay 100mm mot stone and compact.

blind mot with 25mm sand.

lay 1200 gauge polythene, turning it up the sides.

lay 125mm jablite insulation (or 75mm celotex, kingspan).

cover jablite insulation with 500g polythene.

lay and tamp level 100mm concrete.

lay 75mm screed (once tamped concrete has hardened).

 

[JerryM]

it's quite a big job and u need to be sure a repair etc won't do

if you need to go ahead then best to do through building control as u will get a completion cert for selling on.

it's quite a simple job but hard work and u will need to hire a concrete mixer or get readymixed. i believe most recommend a cut off at 2 cubm but i mix my own always as i feel i've more control than a lot of conc needing pouring all at once.

the depth required is quite frightening though:

the spec i used for my last job was (in inches):
1. Dig Out existing – depth 15” (+ ½” for tiles)
2. Lay hardcore 6” (heavy stuff ie upto say 4” then 2” crush & run or ballast
3. Building Sand Blind 1”
4. DPM (taken up walls above DPC)
5. Insulation 2” (Jab Floor)
6. Concrete slab 4” (mix 1:2.5:3.5, cement : conc sand : aggregate, max water : cement of 0.5 : 1 by weight). Water content is v.important.
7. Sand & cement Screed 1 to 2”

 

[benners66]

Thanks very much noseall & JerryM.

I didn't realise it was such a deep job, but then thats why I'm on here to get the right advice.

I would never be happy with a repair job, so it will be a replace!


Excuse my ignorance, but could a latex (or similar) self leveling screed be used instead of the 1"-3" sand/cement screed to ensure a final level finish?

Thanks again!!

 

[Shytalkz]

You need to find out why the slab has dropped. Most probably it's inadequate compaction of the fill material (does your site slope and this has taken place where the ground level to floor level is greatest?); but it could also be degradation of organic material, due to inadequate stripping of the site.

If your property is greater than 20 years old and the settlement has arisen due to fill compaction, then it would be reasonable to assume that just about all the movement that will take place has done so; not necessarily the case with organic materials, though.

If it is fill compaction, there is no need to take the whole floor up: you can just remove the screed and re-lay to suit the skirtings, or use a latex self-levelling screed, which you might have to do in two hits, depending on the variation in level.

Even if it was material degradation, an alternative to removal would be to use a specialist company to pin-pile it using 100 dia steel casings installed with a Grundomat driver, which are then grout-filled; the hole through the slab is wedge shaped to stop the slab slipping down the pile. You would need to re-screed and redecorate, as the operation is a bit messy.

If the soils under the slab are inappropriate for a ground-bearing slab (eg organic or very loose voided sub-base material or virgin soils), then digging it up and replacing it will just mean that you're back there again at some point doing the self same repair. If you didn't want to mini-pile, then you would need the use a reinforced concrete slab, supported by dwarf walls built off the existing foundation against the perimeter walls of the room/s you're dealing with.

To see what the cause is, break out a small area of slab say 300x300 and have a dig around underneath first. If it is just fill compaction and you intend to keep the slab, remember to reinstate any dpm that you've had to cut through.

 

[benners66]

The house is nearly 50 years old, so as suggested, It could possibly be a compaction issue.

I will do as you said shytalkz, and break a small area out & see.

Thanks for the advice!! No doubt as my DIY projects get more adventurous (you should see the state of this place!! Formica city, even on the ceilings!!) I will be on here more & more

Ben

 

[JerryM]

Excuse my ignorance, but could a latex (or similar) self levelling screed be used instead of the 1"-3" sand/cement screed to ensure a final level finish?

u will need to use at least 1" inch of screed. you can't get a good flat level surface with concrete (not without specialist tools) or control the height that well. the latex is normally laid to 5mm and although more than 1 layer can be applied it is expensive to apply more than 1 coat (relative). definitely work back from you finished floor level to leave room for 1 coat of latex and your soft finishings ie laminate etc.

shytalkz is spot on suggesting breaking out an area to investigate. it's not normal for a slab to drop so much on one side only.