Concrete strength - not so good....

[men8ifr]

I'm rendering a wall with a concrete and hydralime mix plus concreting in a lintel above a garage door - the lintel sits on top of a concrete block wall.

The problems are the lintel got knocked (upwards) by the door - didn't think I hit it that hard and it seperated the lintel from the wall. I'm thinking the concrete was not so strong...

Also I rendered a wall and when drilling a hole quite a bit of conncrete/render flaked off. Again I'm thinking it's not very strong.

Some causes may be:

Lintel and the top of the wall were not PVA'd before concreting - should they be, will it make a stronger join?

The wall was done on Saturday and due to the weather dried quick and on Sunday I drilled it, is that too soon? Also maybe the weather dried it too quick?

The concrete seems to have partially set in the bag i.e. it's not a fine powder but I have to scrape it with a trowell to get it into a powder - has the concrete expired?

I'm using hydralime (and less concrete) in the mixes - on the bag it says 'improved performance' but maybe it's not as strong. I'm also using the 'strong' recipe on the bag so am expectign it to be as hard as ummm concrete.

 

[noseall]

Are you saying you used lime when constructing a support lintel from scratch?

Lime will entrain air into a mix thus making it weaker if you like, but it will be easier to use. The use of lime is usually associated with colour matching, or used in mortars or renders that need to be less 'rigid' than cement based mortars.

A lime/cement render for instance will allow the passage of water more readily than say a cement only counterpart. Also, lime /cement mortars are more flexible than those that are cement only based.

However, if it is strength you want, particularly regards structurally relevant items such as lintels, then lime should not be used.

Are you confusing the words concrete, cement and mortar?

 

[men8ifr]

I'm probably confusing all the words!

I used the same mix to render an external wall and fix the lintel. Confusingly the cement and lime bags both say for improved performance use lime including for a strong mix.

I will try again using only sand + cement.

One question though do I need to PVA the surfaces before concreting?

 

[noseall]

One question though do I need to PVA the surfaces before concreting?

No. Just wet them with cement's favourite friend - water.