Filling a trench - thickness issue.

Galvanised chicken wire in a couple of layers is what I would use but I'd also make the concrete a stronger mix with more cement and finer aggregate but others may give you better advice
I helped a bloke make a concrete canoe once and that was only 5mm thick [ sorry I do not know that particular recipe but it had no water in it only straight Bondcrete as the liquid ingredient] but the stronger the mix the thinner you can go

If is only foot traffic then pretty much plain sand and cement mix would be fine (i.e.: mortar without lime). Adding more cement or additives is not going to make any difference - if it is 30mm thick then it is likely to crack if there are point loads, but a footfall across it is not likely to do any damage. I assume the pipework is PVC - so the flexing of that under the thin concrete is a weak point as are the two edges where it meets the existing slab, but again once it has set and hardened - more than a month then it should be fine. You didn't say how wide it is - I am assuming around 100mm wide and with the filler mix going down the sides of the pipe so that will give an arch shape that will help prevent cracking anyway. You could add the chicken wire or even a strip of 3mm reo mesh, but for foot traffic only I wouldn't bother.

Or you could use one of the Ardit products. I wouldn't bother with any particular waterproofing - any capillary action from that small surface area when the rest of the slab is sitting on plastic will be tiny. Usual story though - make sure all external drainage runs away for the house/ basement walls.

Pictures really Really help
You may need to do other stuff like taping the edges and people can't help without having lots of information
Pictures are the best information you can give to help people help you
If you have broken a waterproof barrier it needs to be made good
How you should do that ????

I'm filling in a trench in a concrete floor (in a basement) that has had some new pipework put in it - the only issue is that in a couple of places the concrete is only going to be about 3cm thick over the top of the pipe. The room is only going to be used for foot traffic - ie it's not going to have cars driving over it etc, and will be tiled over the top of the concrete. The original concrete was about 10cm thick.

And then

The original floor was made up of hardcore at the bottom, then a plastic sheet, then approx 100mm of concrete. I've cut a 150mm wide trench through the concrete, through the plastic sheet and down into the hardcore. I've put in some pipes and filled the trench with sand up to the level of the base of the old concrete. My next step is to fill the top 100mm of the trench with concrete

So is the pipe in the ground under the concrete or is the pipe inside the new concrete? If inside the concrete you have to pack the pipe with something that allows movement in the concrete without breaking the pipe. Plastic under the concret or no plastic seems the least of your concerns if you have a pipe 30mm from the top.

A picture or a small drawing may clarify what the problem may be.

Originally Posted by mxh

Are we at cross purposes here?

The original floor was made up of hardcore at the bottom, then a plastic sheet, then approx 100mm of concrete. I've cut a 150mm wide trench through the concrete, through the plastic sheet and down into the hardcore. I've put in some pipes and filled the trench with sand up to the level of the base of the old concrete. My next step is to fill the top 100mm of the trench with concrete. My question is - does there need to be any plastic sheet between the sand and the concrete that I'm just about to lay? Obviously I'm not going to get a complete waterproof layer under the concrete as I've cut a 150mm wide hole in the original sheet along the length of the trench, but is it best practice to put something down?

Definitely have been - I don't know about others, but I read from your opening post that you were going to have only a 30mm thickness of concrete over plastic pipe. If I read it correctly the thickness of the replacement concrete will be 100mm so that is not a problem and all you were really asking is should you put some plastic down in some attempt to replace the section of original plastic you removed as you cut through and laid the pipes. IMO it is not critical - you could lay some down on the sand running a little up the sides and that means that there would only be a small area where any water might rise up. If it were me I'd be putting some plastic down and refill with concrete, but I don't think it is going to matter one way or another in the situation you described.