advice on repairing 'crack' in this concrete slab

From experience thin skims of mortar or concrete are neither robust nor do they adhere well., In all honesty replacing the slab is likely to be the most satisfactory, but epoxy 'mortar' might produce an acceptable repair.

Looking at the pictiurees it looks like the step was cast in place with rather low cement ratio concrete or mortar. I would dig out as much of it as you can and recast it using e.g. wooden edges to form the steps and edges.

A 2:1 sand :cement ratio will give you something that will last for years

I don't understand. You say the slabs are 15cm or so deep. What do you mean by 'deep'? They are very unlikely to be 15cm thick, but equally, unlikely to be only 15mm thick if that was a mistype. Do you mean the 'going', in staircase parlance, i.e. the distance from the front edge of the lower step/slab to the front edge of the next one up?

My approach would be to clean off the grass that's encroaching over the slabs on the LH side, so that you can get a better idea as to what's there. 15cm is a bit small for the width of a slab. Has the someone just laid the slabs and found he has an inconvenient 15cm gap where the last two meet, and just bridged it with coarse concrete, which is now breaking up? I would then make (or buy) a single half-slab to fit under the edge of slab further away from the camera and overlapping the slab nearest the camera, scraping out or adding soil where necessary to keep all the slabs level.

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I find overlapping slabs like this is a very good way of coping with rising pathways.

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Hi Chris I'm not sure where the misunderstanding is ... but the picture is more or less as you show here:

which each slab being approx 15cm in thickness (and perhaps 70cm wide). The first photo was intended to show the 'step' between slabs.

The crack is in the middle of a slab, not between slabs. There is some similar deterioration elsewhere, but this is the worst place.

HTH, J^n

I have tried to explain, it is not a crack as such, but a part where the slab has deteriorated such that there is the appearance of a crack or fissure in the surface of a slab, with the underlying material visible. I don't think this is tricky to understand...

J^n

I don't/didn't understand what you mean by 15cm in thickness. That's a mighty thick slab! 15cm is getting on for 6". I've never seen any that thick. 2", or about 5cm is the thickest I've ever seen, and they were old slabs. They're usually a bit thinner these days, but not as thin as 15mm, so not a mistype on your part either.

It is likely the riser(vertical section of the step) is 15cm but the remainder of the slab is a lot thinner probably only 50mm. I have seen plenty of examples of paths constructed this way. I agree with TNP the only effective repair is to remove the cracked section using an angle grinder to produce a good edge in the sound concrete and hire a breaker to break up the damaged section. Cracks like these usually occur due to ground settlement under the path removing the concrete gives the opportunity to correct that with some hard core before re-casting the slab.

Richard

Paving labs are not normally 15cms or 6 inches thick, more like 5cms.

Think less paving slab, more poured section of path ...

yes, exactly. That is why my first photo showed the edge of one slab, to demonstrate the supposed thickness. I think it very likely they were poured in place. Apologies if 'slab' was a misleading term...

They are clearly not precast, but cast in place with a crappy 5 sand to mortar. Any subsidence will crack a concrete like that as will frost. If there isn't enough cement to full the gaps between the sand it will let in water ... and spall.

It's a piss poor job and needs completely replacing IMHO

I've cast slabs up to 3m! For fundations... It seems he has a set of steps that were cast in place - cheaply.

This is not a paving slab

No support underneath (perhaps laid on eroding earth)? No rebar in the slab?

No f****ng cement in the mix...

No hope!

follow-up question9s), as it looks like I may have the time to do this soon:

2) I can get a suitably-sized bag of 'ballast' - crushed aggregate + sand IIUC - from Wickes pretty cheaply. Would it be worth making the mix from something like that?

Thanks, J^n

Probably.

Unless you are going for a lot of cement, mixing up pre-mixed ballast with cement in a bucket is very lo tech, but it works

Id leave the horticultural sand for horticulturakl projects