will metal mesh rust in a concrete slab?

[DIYedboy]

I am building a summerhouse and will sit it on a 4 inch concrete slab that I will put metal mesh in.

In the past I have put a DPM underneath. This time I don't want to as I will paint bitumen on top of the slab.

As there isn't a DPM. will the metal mesh rust and split the slab over time?

 

[joe-90]

Probably. What does a sheet of polythene cost? Is it worth the risk? The slab will be damp and your bitumen will peel. It's a no brainer.

 

[DIYedboy]

I would have to blind all the hardcore, secondly the slab is deeper around the perimeter which all adds up to a pain in the bum. If I can get away with giving myself an easy time without any detrimental results, that would be the way to go.

Peeling bitumen...............good point, hadn't thought of that

 

[jeds]

I am building a summerhouse and will sit it on a 4 inch concrete slab that I will put metal mesh in.

In the past I have put a DPM underneath. This time I don't want to as I will paint bitumen on top of the slab.

As there isn't a DPM. will the metal mesh rust and split the slab over time?

No it won't. Concrete is alkali and metal doesn't rust in an alkali environment. It may start to rust if exposed to high levels of acid - as in so-called concrete cancer. But with 50mm of cover that will take about 75 years.

 

[Nigel_Cro]

A bit late to this, but...

As long as there is sufficient cover, the rebar will not rust.

So called concrete cancer is caused where the cover is insufficient and water can get to the rebar, rust sets in and causes expansion which blows the cover off, this obviously allows more water to come into direct contact with steel making the rust worse... and so on.

The problem is sometimes exacerbated by the non-use of Sulphate Resistant Cement in areas where it should be used - i.e. in coastal environments, or where regular contact with water is anticipated.

If you think about it, concrete structures are exposed to water every day and aren't falling down all over the place, Well, the good ones aren't anyway.

 

[DIYedboy]

Out of interest, precast concrete garages are notorious for being damp, is this purely because the floor doesn't have a DPM or some other reason?

 

[Jackrae]

Cold uninsulated wall + wet car or cooling air temperature = condensation = "damp" garage ?

 

[DIYedboy]

The garage I refer to hasn't had a car in It for 20 years, a brick walled garage is also uninsulated but they don't seem to suffer the same fate