Joining garage floor slab - self level over old and new?

[dangti6]

My garage is accessible via the house and naturally fairly full of stuff, so I have had it extended with the intention to knock the existing outside wall down when the garage door has been added, which means the slab would be the last task. 

 

In hindsight maybe I should have built a partition running along the inside of the wall so it could be knocked down sooner, but it is what it is now.

 

 

I will lap a membrane up against the existing slab and under the skirt that runs along the DPC of the new walls. I'm allowing for ~4" of concrete.

 

Initially I'd intended to get someone in to lay and trowel the floor 'nicely' to match the existing as best as possible as I thought it may be beyond me to get a nice smooth finish on my own as I'm not particularly familiar with trowelling concrete.

 

I am now wondering whether I could do it myself in a way where I am more likely to get a better result - perhaps tamp it as best I can and then self level over the entire floor for a seamless finish between old and new. What I would pay in self levelling compound would surely come in less than the labour element of having the slab laid, and may give me an overall better finish.

 

My concern with the self levelling is the front edge threshold for driving over. Perhaps add a rubber seal to ramp over it so it doesn't crumble, or is it as firm as concrete?

 

What would you do in my shoes? Opinions and other options to consider would be appreciated.

 

  

[dangti6]

Currently leaning towards giving it a punt myself, but still not at that stage yet so no doubt I’ll sway either way multiple times until I need to do something.

 

I have taken down the end of the wall and noticed that the existing slab is floating in this corner.

 

 

 

I’m assuming that’s just where the hardcore base slid down in to the front trench dug along the original door opening. I’ll know more as I knock the rest of the wall down when I finally have the garage door installed.

 

Rather than cut that section out, would you  push and whacked the hardcore against the void to fill it or perhaps allow the new concrete to spill under there to make it up? Don’t want it to crack when a car drives over it.

 

The mind wanders when you have time to overthink rather than just get on with a job! 

[markc]

Fill the void with a fairly wet mix, well poked to get it into the voids and crevices

[dangti6]

I have since removed the existing wall.

 

 

I am using A142 6mm steel mesh within the new slab. 
 

Would you drill and resin fix some rebar sticks in to the side of the existing slab to tie it in or leave it independent? It’s only about 90mm thick at the thickest point.

 

I have noticed that along the edge there are further areas where there are voids - at the far end the hardcore beneath appears to have sunk down about 50/75mm. It’s dropped here about a foot inward. Clay soil settlement perhaps.

 

 

The mortar frogs will be knocked off.

 

I’m tempted to lay the DPM flat under/against the existing slab rather than running up the side of it to avoid creases and get a flusher joint. Or will the weight of the concrete when tamped force and creases together enough to not be a concern? 

[markc]

As the edge of the old slab is thin and has voids i would excavate under it a little way, lay the DPM under the old slab, Dill and fix shear keys (rods in resin will do fine) to prevent the slabs settling or becoming uneven, and then pour the slab so the concrete flows under the existing to support the edge.