Laying concrete floor over old foundations?

[julianf]

The summer project is to rebuid the garage.

Stage one is to make some hard standing (trailer, and occasional car, when it needs works, but not every day parking)

There are old wall foundations where i want to cast the slab.

The ground here is very stable. The lane behind the houses has 3" direct onto the soil.

Ive dug out to an average of 8" or so currently, but im wondering what thickness i can get away with over the old foundations? Ie how much do i need to take them down by?

Theyre not going anywhere - theyve been there since the early 70s, and, as i say, the ground here is stable (well draining, very stony soil - you cant dig it with a spade, you need a pick)

I can lay mesh over the top of them too.



Basically, what im asking is do i need to worry about them / take them down to the full slab thickness, or are they almost a bonus, given that, as i say, they wont move (but obviously the area around them wont be aged in the same way, so may?)

Thank you.

 

[mike os]

you will need at least 100mm over, preferably 150mm, and may still crack over the wall

 

[brightspark]

lower the wall a bit . the concrete raft in my garrage is only 100 mm and its never cracked

 

[J.B. Welding]

It'll be a sudden change in the thickness that makes the slab crack, it's best try to maintain a constant thickness throughout. But if your putting in mesh anyway and don't mind the odd crack then digging out founds to fill the trench back in again does sound like thankless work-how thick will the slab be and how much cover do you have over the old foundations?

 

[colnerov]

Hi, With the foundations and undisturbed stable ground and reasonable thickness overall it will be OK. Too much disturbance will create a weakness.

 

[julianf]

I think ill need to take a photo, but ill try and describe in the mean while -

Original setup -

Concrete sectional garage (1970s), sat about 1m back from the boundary (to allow turning into it from a narrow lane)
Garage sat on a couple of courses of 7n type blocks, possibly on a trench of concrete below.
Area between the land and the garage entrance covered in concrete
Garage floor itselt - soil


Proposed setup -

Rotate garage by 90 degrees (thereby having same footprint, but using overgrown allys either side)
Set garage back from lane by 2.4m, so allowing for a car parking space in front.


Findings -

Concrete in front of garage was about 3" thick (if that) laid over a very thin (like an inch, if that) layer of pea shingle.
Once that concrete was taken up, and a lot of the soil removed, old (pre garage) walls exposed - lime / stone bounry wall and some old structure foundations also. These all fall under the proposed 2.4m wide parking space.
The two courses of 70s foundations protrude into the 2.4m strip by aprox 1.5m, and are currently higher than the surface of the space will be - so will need to be cut back, but by what depth, i dont know.


Additional info -

Kerb will be laid along the boundry, using the old exposed (lime/stone) boundry wall foundations as foundation for the kerb. The kerb stones will be cut to meet the wall, rather than the other way around, so as not to overly disturb the well settled wall foundations.



The level of the garage floor will be higher than the level of the parking space. In an ideal world, i know it would not be, but we're talking far more digging than i want to get involved with. Im thinking that, for the hard standing, ill take out one, or both courses of the 70s 7n blocks...

But for the garage, i have two options -

I either dig out the soil base by as much as i can be bothered, and then take out one course of the block, and lay the new base to the same height as the top of the blocks would have been.

Or, a simpler job would be to dig out the soil a bit, fill with hardcore, flush to the top of the 70s foundations, and put the slab over the lot. This would be *way* less work, but raise the garage by 4" or 6" (ie however thick the slab is)

This option would not be as good, but it would be a whole world easier.



Summery -

Im trying to work out how much i need to take the 1970s and ancient walls down by on the hard standing.
and
What i can get away with on the garage

 

[julianf]

This is the garage, from the back -

This is the plan of how it was -

It has these two usless bits down its sides -

and

And, so, as the garage needs work anyhow, im taking it down, and rotating it, so ill get this -

That means that the garage will stay the same size, but ill gain about a foot more garden, and a 2.4m wide parking space at the end.


This is a photo from the lane, before i started -

The yellow line shows my boundry.

The surface in the above photo is about 2" thick, laid on a 1/2" layer of pea shingle (or at least the bit on my property was).

Im currently laying kerb along the back, which will be level (its level in the photo, but only the one at the end is concreted in)

Ill use this to bring the level of the concrete to. I fully realise that, at the low end, there will be a good drop, but if you come up to the top of the lane, and then drive in at an angle, the bump wont be too much (i can smooth it with a temporary ramp, if needed)

As i wanted a flat (just enough fall for drainage) surface, this was my best option.

Yes, i know its a tip - ive been dumping sacks of hardcore there from other areas of the house.

In the photo above you can see some of the old stone walls that were covered by the concrete that ive cut away, and also the newer foundation wall to the garage (ive shortened the garage in that photo, so as i can work on the hard standing first)

Here is another shot showing the same. The old bit of wall comming towards the camera was covered by the concrete, which came up to the edge of the newer blocks -

So, ill lay the hard standing level with the top of the kerb stones, and ill have to take out some of those garage foundations, and cut away some of the old concrete path, but im wondering how much.

And then, when ive done this, am i going to take out one course of the garage foundations the whole way around, and the old path (that currently runs along the side of the garage, shown in one of the earlier photos), or lay the new slab right over the top of all of them?

 

[J.B. Welding]

Or, a simpler job would be to dig out the soil a bit, fill with hardcore, flush to the top of the 70s foundations, and put the slab over the lot. This would be *way* less work, but raise the garage by 4" or 6" (ie however thick the slab is)

4" of concrete on well wackered down hardcore it should be fine or dig it down a bit more to lower the level but keep the idea of an even slab on soild ground you'll be fine.

 

[DecK]

For the hard standing you would only need to take the existing 1970s walls down a few inches to cover with hardcore. You would probably get away with filling the hardcore level with them and whackering it if it's only ever going to take the weight of a car or such. For the garage you can do whichever of the 2 ideas you mentioned but digging out the topsoil and soft subsoil would be best. What kind of walls are going in the garage. You never mentioned about foundations and I wouldn't be keen on building block or cast walls on a 4 or 6 inch slab. Best route in that scenario would be to allow for a 200mm or more deepening on the perimeter of the floor slab to act as a foundation for walls.

 

[julianf]

The stucture will be the same as what's there. Its ugly as sin, but its cheap (as i have it anyhow!). Ill take it all down (as i have the front), stack it on the hard standing, do the floor, then re-assemble it.

I had not thought in much detail about the foundation for the garage yet - what i would probably do is work out where the walls were to sit, and dig deeper at those points. Or, possibly, replicate what was done originally, and use the walls to tamp the base off. That seems like more work / expense though, so ill probably just dig deeper trenches around the edge.

...but that can be worked out later. Stage one has to be the hard standing, else i have no space to do anything else (youve seen the photos!)

The 70s foundation wall, at present, finishes about 3" above the level of the hard standing (ie the kerb) so its got to come down a bit.

Ill take another photo tomorrow that shows levels better.

Thank you all for the help so far - its appreciated.

 

[DAPPH]

If youbn haven't managed to doit just yet here's my take on it all .

Hire a medium demo hammer & transformer for a weekend .

Prior to the hire . Clear a patch for loosening . stack the materials you want to keep on it once it's been freed up and then work the rest of the areas in the same manner .. Yes I know it's double handling but sometimes things like hard work have to happen for things to progress.

You could then make the basic raft in two sections over a couple of week ends , using a few lengths of 15 mm re-bar , polystyrene for spacing and some board drilled for the re-bars to make the concrete form to keep the slab joints in alignment . Lay the materials on the set slab/s a week or more later when it's part cured ( 10 days is ideal for a base ) Then dig the foundations out of the loosened soil etc and cast them all in one go . Finally build the shed squared up etc and then fillet up the differences at least 50 mm above the mean slab average inside and out using some PVA bonding adhesive in the mix to get it to stick and help keep .
the water out . Finally used a jointing compound to seal the space between the two slabs if you have one .

If your using bricks & bust up stone .concrete as hard core instead of reinforcing wire mesh , hose them clean over several sessions to take the all soil off them and then smash them up exactly where they are going to stay into 1/4's with a lump hammer .. There will be plenty of little bits for the infilling , then use a heavy vibrating plate to set them down in the soil a bit .. Don't try and make them dead smooth as the difference in thickness and different angles at which the concrete sticks to the rubble faces is what gives a rubble made bed base a tremendous strength , so long as you use a decent consistent concrete mix at a reasonable thickness of say 4 to 5 inches thick .

A 3 inch thick is usually the spec for a path or a driveway ( not usually the actual place the car will park up over night etc. ) for a medium car that is not going over the foot way .
If your going to end up playing in the garage the extra thickness usually pays off in the long run. You can then waterproof the slab with any of the reasonable slab coating membrane substances and put down a 50 mm floated off screed to get a real good flat floor if you have the money & the inclination for it .

If you're able perhaps run a couple of service pipes in via the slab onto opposite corners inside the planned walled area and block them off with foam sponge ..in later years you may want to put the washing machine & dryer out there as well as having water & electricity via a heavy duty SWA cable able to drag in say 40 amps or more through a protective underground run rather than an aerialized cable hung off a cattinery wire