What concrete mix to use?

r011ingthunder

I want to put a concrete barge on the gable end of the house. I've already got someone to put up the shuttering, but now I need to fill it. I've been advised to use a "strong, dry mix". Would you agree? If so, what ratio mix would you use? 5:1? 4:1? Maybe even 3:1? What are the pitfalls with a strong mix? (I'm thinking cracking).
Oh, and how dry is a "dry" mix? I'll be using standard cement, and plasterer's sand to give a smooth finish as I want to paint it afterwards.
Any advice appreciated as it's gonna happen Saturday / Sunday.

ircoha

the dry relates to the fact that if it is 2 wet it wont stay in the shuttering.

if too strong [4:1 or so ] there is a risk of cracking unless u keep it fairly damp for a few day, cover with wet canvas sacking or old towels etc.

Re the smooth finish if u use a bit of proper plasticiser, then u can get a smooth finish, even with normal concrete mix of gravel and cement.
{ do not use washing up liquid as the plasticiser, it weaken the concrete}

I think you need some gravel in the mix, I think all plastering sand will be much more prone to cracking due to the thickness.

However I hope someone else will answer that

r011ingthunder

Thanks for that, I would never have thought of using washing up liquid for it anyway!
I reckon I can figure a way to keep it damp for the few days although I'd doubt there is that many old towels knocking around.
If I need gravel in the mix, then what do you recommend?
4:1:1 (Plasterer's sand : Gravel : Cement)?
What's the story with plasticiser? How much of that do you use? Will it work with shuttering? How does it work?
Sorry for all the questions.

ircoha

r011ingthunder wrote: »

Thanks for that, I would never have thought of using washing up liquid for it anyway!
I reckon I can figure a way to keep it damp for the few days although I'd doubt there is that many old towels knocking around.
If I need gravel in the mix, then what do you recommend?
4:1:1 (Plasterer's sand : Gravel : Cement)?
What's the story with plasticiser? How much of that do you use? Will it work with shuttering? How does it work?
Sorry for all the questions.

The questions are never the problem ,its the answers:)

Given that the gravel will contain sand maybe 2:2:1 but I would like a second opinion.
re what it does etc see http://www.antel-uk.co.uk/tds/aa01.html
http://www.uk.graceconstruction.com/product.cfm?mode=a&did=1&id=28

u will need very little.
I use it in any concrete floors as it vastly improves workability and u need less water: be v careful when adding the water if u have the plasticiser as it will go from very workable to goo in a flash.
The other point I like is that it is much easier to get a trowel finish, which requires the cement to come to the surface.

start at the bottom of your shuttering and work up the slope

re the washing up liquid: I was getting my retaliation in first as there was an discussion early this year on this forum where I got some abuse for stating that using it is wrong:)

r011ingthunder

Thanks for all that, so at the moment I need:
2 : 2 : 1
Gravel : Sand : Cement
with a small amount of plasticiser (I read 250 mls per 50 kg of cement of one of your links ircoha)
Add the plasticiser to the water, and mix until "gooey"?
Start from bottom of shuttering and work upwards.
Any other tips?
Thanks!
r011in
PS Will the stones in the gravel not drop to the bottom of the mix and show themselves all along the bottom of the barge when I take the shuttering down?

ircoha

r011ingthunder wrote: »

Thanks for all that, so at the moment I need:
2 : 2 : 1
Gravel : Sand : Cement
with a small amount of plasticiser (I read 250 mls per 50 kg of cement of one of your links ircoha)
Add the plasticiser to the water, and mix until "gooey"?
Start from bottom of shuttering and work upwards.
Any other tips?
Thanks!
r011in
PS Will the stones in the gravel not drop to the bottom of the mix and show themselves all along the bottom of the barge when I take the shuttering down?

NOT GOOEY, until it is workable and no more

if it is gooey it will run down the shuttering and flow out like....out of a nappy

read up on slump test: you need minimum slupm
http://images.google.co.uk/images?sourceid=navclient&hl=en-GB&ie=UTF-8&rls=GGLJ,GGLJ:2006-43,GGLJ:en-GB&q=slump%20test&oe=UTF-8&um=1&sa=N&tab=wi

re the gravel, if you have not worked with concrete before then perhaps best to leave out gravel and just keep it damp for about 3 days.
You need to keep it wet beacuse all concrete needs water to cure proiperly and if you dont keep it wet it heats up in proportion to the amt of cement: more cement more heat: covering it with plastic is not enough.
also if frosty you need to protect it

r011ingthunder

Thanks for all your help, I'll let you know how I get on. One things for sure, given the weather, I'd doubt I'll have a problem keeping it wet!

sesswhat

I'd be interested to hear how you got on as I will be doing something similar.

r011ingthunder

As soon as the weather permits I'll start it. I will of course let you know how I get on. If I can I'll take pics