Adding strip of concrete to existing slab

[markhambly]

Hi - I have had a look at the great advice regarding adding new concrete to old but have a question about my project.

I want to fit a well-sealing garage door in my new house. The previous owner was a builder and built the garage with a stout concrete floor about 20cm deep but sadly the slab ends at the internal surface of the brickwork so the existing garage door hangs in mid-air rather than being based on the floor.

I want to extend the slab but would only need to add 7-10cm depth to take it to the front of the wall were I could use it for the bottom strip of a side-hung door. It can't really go further than this for a variety of reasons.

Seems simple enough, but that is a fairly thin strip of concrete (2.2mx20cmx7-10cm). Are there any special considerations when trying to join it with the old slab. I thought that perhaps using some rebars drilled into the old slab would do the job but should I be leaving in any sort of gap/join to allow for moment/expansion.

I've never done this before so would gladly take any expert advice! Mant thanks.

[harry]

It won't be possible to join it to the existing slab. It will always be "adjacent". All you can do is dig out down to some firm ground and lay the new bit alongside the old. But you need reinforcing bars and it needs to be at least 300mm deep to prevent it breaking. Leave for a month before driving a car over it.

[markhambly]

Thanks Harry

There is no issue with cars as there is currently a step up to the garage (it's not used as a garage). Foot traffic only.

Regarding reinforcing bars do you mean drilled into the old slab to straddle the join or would you leave them completely separate? Or do you mean running through the extra strip? I suppose I could make a simple frame anchored in the existing slab extending into the new area.

One last question, would you line it with a DPM and if so would you let this run into the join?

Mark

[Tim Lamb]

In message , markhambly writes

This may be overkill for your requirements.

I put a 2" high rain stop across the stable doors here which has withstood 16 hand horses on a daily basis.

My technique was to clean the contact area with a stiff wire brush (run at low speed in an angle grinder) and then secure some folded 2" galvanised mesh reinforcing with stainless screws plugged into the original concrete. A stiff mix concrete was trowelled into a dome shape and appears to have bonded well with the original.

[harry]

Because it is a long thin bit of concrete you will need rebars running almost from end to end. Two should do it, one near the top and one near the bottom. They should be covered in 50mm minimum concrete everywhere. Otherwise it is likely to break. Even if you don't drive cars over it. Though you/someone might in the future.