Laying a concrete slab adjacent to existing.. Help!

Xcellor

I have an existing path concrete slab and I am laying another slab directly beside it. I want to but the new slab against and give an expansion gap... I bought the following
https://www.goodwins.ie/products/Expansion-Joint-Strip-for-Concrete-5Mtr-PVC.html

But I don't think it's right for the job as it seems to need concrete from both sides.

What im considering doing is putting a length of timber then using a curved trowel to give a round edge to the new slab. But then when the concrete is cured it's gonna be hard to get that piece of timber out.

I have a form built which attached to the existing slab presently.

The solution must be very straight forward but most tutorials are around new concrete slabs rather than butting up against existing...

TheBoyConor

What you need there is to glue a strip of joint foam along the edge of the existing slab. Like this stuff.https://www.goodwins.ie/products/Joint-Expandafoam-100x10mm-25-Metre-Roll.html?filter_set[]=1,52

You'd only really need this if you are putting down a significant area of slab like more than 4x4m, like a yard or something. If it is only a footpath or a very small area then there really isn't a need for it as any movement will be miniscule.

Another word of caution from experience - don't bother with that edging trowel business on the joint between the new and existing concrete. It actually looks way better if the concrete is just finished up flush to the existing edge, nice and tidy. After a bit of weathering it blends in nice.
Those edging trowels when used on joints between slabs just end up leaving you with big dirty joints that stand out and gather dirt that ultimately allows weeds to grow out after a few years.
Edging trowel should only be used on the outside edges of the concrete.

Xcellor

TheBoyConor wrote: »

What you need there is to glue a strip of joint foam along the edge of the existing slab. Like this stuff.https://www.goodwins.ie/products/Joint-Expandafoam-100x10mm-25-Metre-Roll.html?filter_set[]=1,52

You'd only really need this if you are putting down a significant area of slab like more than 4x4m, like a yard or something. If it is only a footpath or a very small area then there really isn't a need for it as any movement will be miniscule.

Another word of caution from experience - don't bother with that edging trowel business on the joint between the new and existing concrete. It actually looks way better if the concrete is just finished up flush to the existing edge, nice and tidy. After a bit of weathering it blends in nice.
Those edging trowels when used on joints between slabs just end up leaving you with big dirty joints that stand out and gather dirt that ultimately allows weeds to grow out after a few years.
Edging trowel should only be used on the outside edges of the concrete.

The existing slab is a 1meter wide path, the new slab is 2.7 x 5 so large enough... I wanted a small channel as i'd be worried about water pooling if it's put flush to the new slab...

TheBoyConor

Is the 2.7m side against the existing path?
I wouldn't bother with an foam joint for a slab that size to be honest if the edge of the existing slap is fairly regular. the other free edge of the slab will accommodate any thermal movement. However if the other side was up against a wall or something that would restrain it against movement, then you would need an expansion joint.

If you're worried about water pooling just lay the slab with a fall to the free edge so that all the water runs off. Don't put in a channel unless absolutely necessary as it will only accumulate shít and dirt. A channel and any crap that it accumulates is a trip/slip hazard that is best eliminated if possible.

Xcellor

TheBoyConor wrote: »

Is the 2.7m side against the existing path?
I wouldn't bother with an foam joint for a slab that size to be honest if the edge of the existing slap is fairly regular. the other free edge of the slab will accommodate any thermal movement. However if the other side was up against a wall or something that would restrain it against movement, then you would need an expansion joint.

If you're worried about water pooling just lay the slab with a fall to the free edge so that all the water runs off. Don't put in a channel unless absolutely necessary as it will only accumulate shít and dirt. A channel and any crap that it accumulates is a trip/slip hazard that is best eliminated if possible.

Here is a pic, easier than explaining! The longer side is butting up against the existing path.. what do you think?

TheBoyConor

No need of any expansion joint on that slab.

Just make sure you compact the base very, very thoroughly. This cannot be stressed enough. Failure to compact the base sufficiently is the ruination of many a yard or road. Did you hire a good heavy whacker? It's well worth it. Spend a good hour or more running it around the base.

Lay it with a 1 in 50 fall to the edge to throw off the water.
Backfill to the edge of the slab with free draining stone or crushed rock to allow the water to soak away into the soil

Xcellor

TheBoyConor wrote: »

No need of any expansion joint on that slab.

Just make sure you compact the base very, very thoroughly. Did you hire a whacker?

Will get the whacker on Friday...

What about the water pooling though? there is no drainage on the path.. I was thinking the expansion gap would at least give some way for water to move ?

TheBoyConor

No sure the expansion joint wouldn't really allow anything through.
As long as there is a continuous fall across the slab water won't pool on it.

Just make sure to use the stone backfill at the free edge where it falls to. If you backfill with topsoil there you may will get ponding as the water has nowhere to go. The stone backfill should allow all but heaviest of downpours to soak away through the soil.

TheBoyConor

Another word of caution. In this summer weather make sure you keep the concrete wetted as it cures otherwise it will start cracking on you. Once the concrete is finished and is has set enough that you can gently place your hand on it without making an impression, you should start spraying a mist of water over the surface to keep it wet. Do this every hour or 2 depending on the head of the day.

Xcellor

TheBoyConor wrote: »

Another word of caution. In this summer weather make sure you keep the concrete wetted as it cures otherwise it will start cracking on you. Once the concrete is finished and is has set enough that you can gently place your hand on it without making an impression, you should start spraying a mist of water over the surface to keep it wet. Do this every hour or 2 depending on the head of the day.

Thanks for the heads up.

I was planning on putting some polythene between the concrete and subbase after doing the whacking.. this will slow down curing I've read as moisture can only go up so I would expect the drying out to take longer.

I'm still a bit worried about no expansion gap, just dont want to end up with the existing foot path buckling as it's stuck between the house and the new slab.

I'm overthinking things ...