Home DIY

Ideally the pvc pipe should be surrounded by pea shingle to protect it and stop it cracking when you get the whacker plate on the type 1.

I've done a fair bit of formwork for concrete in my time and while what you are suggesting is a good idea it is not easy to acheive in practice. Concrete doesn't easily flow (unless you get a poker in it as its being poured) the problem is if you do get a poker in it the formwork will want to float on top of the muck. If you don't use a poker the formwork won't rise up but it will be hard to get the concrete under it and the risk of leaving voids will be very high if you leave voids the whole slab can potentially be compromised. On a civil engineering site channels like that would normally be acheived by pairing an experienced formwork chippy and an experienced concrete finisher working together, the formwork would be made with an open bottom and held in place with steel wire and the concrete finisher would be able to make the bottom to the right depth.

Other problems that you have will be striking the shutter after the concrete has cured, this can be problematic even with foil coated shuttering ply and the world's best releasing agent - red diesel (there is a reason why concrete workers are known in the trades as muck-savages). Also ensuring that the channel is completely inaccessable from the outside of the building - if its not it WILL become a rat run, I know this because back when I was a supervisor on big commercial rennovtion projects my least favourite job was superising labourers in hazmat suits digging out years of chewed up stuff, rotten food and of course rat piss and shit - unless its completely sealed the little shits will get in.

If I were doing what you are trying to do at my house I'd do it one of two ways:

1. Pour the slab and hire a floor saw (basically an angle grinder version of a plunge saw) or chaser (the same but it cuts out a channel) and cut a channel when the concrete is still green. By green I mean just hardened - the chemical process that makes concrete go from liquid-y to hard takes time and after it is set it will continue to get harder for a couple of months, during the inital stage of this hardening process the concrete is darker, some (presumably colour blind) people say its green in colour at this stage but importantly its much easier to cut. Once the channel has been cut fill it with your pipes and then back-fill with either concrete or sharp sand and cement.
   
2. Pour the slab to ~ 60mm below the height you want and put a screed over the top to bring the floor up to the desired height. There is an added bonus with doing this you will get a MUCH better surface for laying your floor if its done really well you may even get away with not putting self levelling on it.