Concrete advice

[Ian Stirling]

Cement is the fine powdery dust that is added to water, and makes a binder which holds aggregate (sand and rocks) together to make concrete.

Is the water coming in through the brick wall?

Basically, cement/water is the glue that holds the concrete together. The more water you have in this, the worse its mechanical properties are. You need enough cement paste to fill all the voids between the aggregate (sand/gravel/rocks)

The less void space the better, as it'll be stronger. This is why you mix rocks of different sizes, so that the little ones fit in between the gaps of the big ones, and the sand fills in the tiny gaps, with the cement only filling in what's left over.

[sneezy]

Firstly, is there a difference between concrete and cement?? I'm not sure.

I want to put a line of cement/concrete at a 45 degree angle along the edge of where a concrete floor meets a brick wall. I also want to do a bit of pointing - doesn't have to look pretty because it's not on show. I'm trying to stop water ingress.

I'm not sure about the whole sand and cement ratio thing. I've also noticed that you can buy it ready mixed but I guess the mixture isn't always suitable otherwise you wouldn't be able to buy sand and cement seperate. So what mixture do I need? Are there are any common mistakes I might make, or things not to do?

I had a look for an idiots guide on the net but I just ended up more confused :)

TIA

[N. Thornton]

you dont want concrete in either case. COncrete is sand, cement and stones, typically.

right, its often not.

depends on your wall. For soft Victorian bricks the usual recommendation is 1:1:6, thats cement: hydrated lime: sand, by volume. For the 45 degree bit people often use something weaker, so if movement occurs the new mortar breaks rather than break the fabric its stuck to. More like 1:6 to 1:8 - 1:8 is very weak. But, as ever, it depends.

Dont strip out all the mortar all the way across before replacing it :) That could be a big mistake!

why not tell us more details. Are these interior or exterior, what kind of wall, etc etc

Regards, NT

[stuart noble]

Ready mixed gen purpose mortar should be fine for this. If you mix it with

1PVA:4 water instead of just water you'll get better adhesion and more water resistance. For serious water ingress you might need to look at a better additive though.