Urgent help guys - Concrete Slab

[bunnycd]

Just wondering if anyone could offer any advice I am stressed and cannot sleep because I am worried I found out today that our kitchen floor extension (which has building regs from previous owners!) sits on an old garage slab and 30sqm of the slab is outside. We are in the process of having impressed concrete layed on the outside part of the slab (3 inches thick of concrete) and they are laying on Thurs (yikees!)

I am really worried that if we remove outside slab in the future (say in 10 years time) then this will crack the inside kitchen extension floor. In total there will be 6 inches of concrete including the garage slab.

Do you recommend removing the garage pad?
Do you think that if I get the guys to use a concrete saw around the house before laying the impressed concrete then the kitchen foundations will be ok?
What would you do?

Any advice will be much appreciated

 

[Rob Z]

Hi BunnyCD,

If you are planning on taking out the exterior slab sometime in the future, then it's probably not a bad idea to separate the slabs and place an expansion joint between the two. To prevent the exterior slab from moving relative to the one inside, you could drill into the side of the existing slab and epoxy rebar into the hole, and run this rebar into the new slab. That will tie to the two together so the exterior one can't heave or settle. Your concrete man will know how often to space these sticks of rebar based on the site conditions of your job. For comparison, here in Virginia this project would probably get #4 or #5 rebar doweled in approximately 2 feet on center.

Edited to add: I just thought that rebar sizing in the UK is probably metric, so saying #4 or #5 doesn't do you any good.:smilewinkgrin:

#4 is 1/2" and #5 is 5/8".

I hope this helps.

 

[doug boardley]

welcome back bunny, sorry to hear your concrete slab is still an ongoing concern. As Rob suggests would be my way of tackling it too

 

[Rob Z]

Hi Doug, you're up early and I'm up late.

Just for my info, how is rebar sized in the UK? Is that measured in metric units or still in inches there?

 

[doug boardley]

Hi Rob, I'm actually up a little later than norm (5am is my usual), just offhand I can't remember if rebar is metric or imperial over here:blush5:

 

[doug boardley]

Ajax will know I'm sure

 

[bunnycd]

You lot are up early! I thought I would be on my own at 4:30am.

I don't think he is doing what you advised he is just laying 3 inches of concrete on top of the existing slab and leaving a 3-4 inch gap by the side of the house for the damp proof. is this right?

Positively, I can see a breeze block in the corner of the pad going slightly underneath so I am still in two minds if they have built over the rest of the pad (under the kitchen extention). What do you think here is the picture:

This is a picture of the side of the kitchen extention you can see how close the garage pad is:

The old garage pad has reinforced steel rods in it as well.

If we needed to get rid of the impressed concrete down the line do you think the rods can be cut with a concrete saw?

 

[mikethetile]

Hi Rob

rebar is metric in the uk

welcome back Bunny

are you sure they built an extension using the garage base as a footing, theres no way that building control would pass that and give a completion certificate

if they havent broken the slab out , how can you tell its under the brickwork and not laid against it

the only way to tell is to dig a trial hole down the side of the footing and if it turns out they have then its a call to building control and your insurance company

 

[david campbell]

it does look as if the slab is laid up to the brickwork/blockwork foundation,i would still agree with rob the and cut out a track that way if you remove it in future the vibration won't cause any damage to the internal floor, a sthill saw and a breaker would be ideal,you might find the saw doesn't cut right into it in one go because it is so close to the building and a roadsaw would be way too far out

 

[mikethetile]

those pics didnt download first time I read your post

the slab is up to the wall not the wall on the slab so dont worry about that

as for drainage. not happy about their suggestion as the only way for water to drain will be into the brickwork and may cause rising damp problems

they need to cut the slab away 150mm from the wall and fill with pea shingle, this acts as french drain and will allow rain water to soak away

and yes a cutter will cut the rebar

 

[bunnycd]

Ohhh thanks everyone for your advice.

I think I will ask them to cut around the slab so there is a gap between the wall and the slab and see what they say. Thanks.

 

[Rob Z]

Bunny, I can't see your picture for some reason. I'm not familiar with UK building practices and I still don't know what "breeze block" is , although I think it was Doug that explained that to me once. I'd like to see the pic if you can reload it for me. Thanks.