waterproofing timber formwork for complicated concrete shape

[tonyjeffs]

I'm making some ornamental exposed aggregate concrete blocks to match those in our 80 year old garden wall. I'm very pleased with my wooden mould - designed to make 10 blocks at once. So that it doesn't get wrecked by the water and concrete, and remains reusable, I've varnished it with polyurethene, and lined it with blue damp proof membrane.

Would polyurethene varnish on its own protect the wood? The dpm plastic is a pain - it flaps about and makes it hard so see where the screws are going when assembling the mould, and if it gets creased, it shows in the blocks. It has the advantage though of allowing easy release from the mould.

Any ideas on a better way to water-protect the formwork?

Secondly, I think there is such a thing as a washer with teeth around the edges, which I could nail over a screw hole so as to ensure that the screw can only go in the correct place. Anyone know what these things are called so I can tell the store what I want to buy?

Thanks

tony

[dom]

Sound like you're overdoing it. Commercially, a mould release agent is used. A light smear of engine oil will do the job though.

Check out

for builders metalwork.

[Tim S]

[e-mail] coughed up some electrons that declared:

One of my (American) concreter's books suggest spraying on a mix of diesel and sump oil. Not that I'm suggesting the OP do that, of course not...

Cheers

Tim

[RW]

Just brush ordinary diesel onto the wood before pouring the cement.

Silicon spray is a more expensive version producing the same result

[Andy Dingley]

I tend to use shellac, because it is easy to repair the worn surface (shellac bonds well to old shellac). Varnish is strong but not so sticky, so you can find it flaking off.

I also prefer wax (liquid emulsion of wax in wate, as sold to wood turners) rather than oil. I've had one job where things came out with an oily rainbow sheen and it didn't shift for years afterwards!

T nuts. They're intended to take an M6 screw or similar, but they also work fine as a hammer-in hole reinforcer for dowel pins - they're cheap enough too. You might even find that a machine screw is a better choice than a woodscrew anyway. If you're using MDF, then use a screw- in threaded insert rather than a T nut.

[stuart noble]

Vaseline, or anything from the mineral oil family e.g. paraffin wax. Good waterproofer and a good release agent

[Tanner-'op]

Tony

Have a look at:

the following excerpt is taken from there. You could try googling for your nearest supply of Release Agent, Mould Oil or Shuttering Oil.

==============

Release Agents

All formwork, whether is be steel roadform or timber shuttering, needs to be coated with a Release Agent (Mould Oil) before any concrete is placed. The purpose of a Release Agent is to prevent the concrete bonding to the formwork and then 'scabbing' when the formwork is removed. They also help ensure the formwork stays clean and free from concrete, so that it can easily be re-used elsewhere.

There are many products on the market that can be used as Release Agents for formwork, but the most commonly used is known as 'Soap Oil'. This liquid is painted onto the formwork in advance of the concrete being poured and it dries, leaving behind a soapy or waxy residue that has no deleterious effect on the concrete. Where the finish of the concrete is not important, some contractors will save themselves a few bob by using engine oil or diesel in place of a proper release agent; while there is nothing technically wrong with this practice, it's not exactly environmentally friendly and should be avoided whenever possible.

Tanner-'op

[tonyjeffs]

Thanks everyone for all the info. I'll try the diezel, or a proprietory mould release agent. And get some T-nuts.

Knowing the terms alone helps!

Cheers

Tony

[Graeme]

In message , Tanner-'op writes

posted for a long time. Reading the site explains why :-(

[Cicero]

================================== I think you should go for the 'soap oil' option as the other suggested release agents (diesel etc.) might interfere with the mortar bonding when you come to build your wall.

Cic.