# Forum Home Renovation Concreting  How do I stop concrete sticking to formwork?

## aaronjthompson

I am putting in some new drainage pipes for a bathroom renovation and have had to cut a trench in a concrete floor to do so, which I now need to concrete up again once the new PVC drainpipes are in place. 
To do so I will seal each side of the trench with concrete plugs and then bricks flush with both sides of the respective walls each side and once this has set place concrete in between - rather than risking a single cement job that may leak away. 
However I will have to use some sort of removable formwork to do so. My question is then how do I stop the concrete sticking to it so that I can remove the formwork and so that the next stage of the cement job has a reasonable bond with the first, although not essential as the final pour is wall to wall as both plugs will sit entirely under brickwork? 
Any suggestions would be appreciated!

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## cyclic

> My question is then how do I stop the concrete sticking to it so that I can remove the formwork and so that the next stage of the cement job has a reasonable bond with the first, although not essential as the final pour is wall to wall as both plugs will sit entirely under brickwork? 
> Any suggestions would be appreciated!

  Oil the formwork with old (or new) sump oil, or, if you wish, a mixture of sump oil and diesel fuel mixed 50-50.

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## Belair_Boy

> how do I stop the concrete sticking to it so that I can remove the formwork

  I have never had and problem with concrete sticking to formwork but I generally use formply that has a good smooth face.
The only problems have been when concrete gets around the sides and wedges the formply in place.  A release agent (as suggested by cyclic) may make things easier but wont help much if the concrete gets around the edges.   

> so that the next stage of the cement job has a reasonable bond with the first

  Getting the two concrete pours to bond will be very difficult and I suggest you incorporate a key joint, dowels or other method if you need the two sections to be mechanically linked.  Any small shrinkage gap may allow termites to enter so this should be considered as well. 
Personally I would pour the concrete in one go and not be too worried about a little leakage at the end of the trench.  Good formwork will take care of all but a little seepage. 
Good luck

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## Whitey66

Cooking oil works fine too, and is not as messy.

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