Thread: Structural strength of builders cement

Pete ,
what is the concrete for , how thick and wide is it , and are there any steel reinforcing bars in it ?

Hi Manuka

The concrete is for shade sail footings. The reason Im asking is that the company that we are getting to make our shade sail is fine with me putting in the posts (as I need to put in a post already to do with another part of the house). They will specify footing sizes (which off the top of my head are diam. 450mm and 1000mm deep), but they also specify a concrete strength to go with these footings, and I really dont want to %$#@ it up!:P Ive heard bad things can happen with the tension in shade sails...

Cheers,
Pete

PS - no reinforcement other than the 100x100 mm RHS for the post.

Pete ,
What concrete strength do they specify ?
It seems to me that the mix you have is ok for that .
After all , the concrete is only serving as bulk and weight to the butt of the post .

Manuka,
They specify 20MPa concrete. Im just concrened on the massive torque that the concrete has to resist (Posts are exposed 3m) I know that rapid set is weak with only about 5MPa, but just wanted to check.

Cheers.

Rapid set ? Is that what you are using ?
The stuff that you shovel in dry and then pour in the water ?

Hi Pete,
I normally work on 4:1 (by volume) for a "normal" strength concrete and 3:1 for high strength. 5:1 is a little weak and I would normally only use it for fencing type work.

Originally Posted by Manuka Jock

Rapid set ? Is that what you are using ?
The stuff that you shovel in dry and then pour in the water ?

Manuka,

Nup wouldnt touch rapid set with a 20 foot pole for structural work - this has been hammered into me by every builder I talk to...

Vernon - 5:1 is what the back of the concrete bag says, and what the landscaping place said, but I think I'll add a bit more cement just make sure.

Thank you both for helping me out!

Cheers,
Pete

no worries Pete

Originally Posted by MrPotatohead

PS - no reinforcement other than the 100x100 mm RHS for the post.

I would also weld some reo crossbars to the post so that the concrete can key it self to the post better

I would use min 4:1, 5:1 is too weak as said above. You should either have a flange welded to the foot of the post or drill right through and insert some reo, or weld some reo on as above. Have fun, when I did my shade sails those holes looked big enough to bury someone!

Cheers
Michael

Concrete Mix
Light domestic footings
1 cement
2.5 clean sharp sand
5 aggregate ( 50/50 mix ie size 50 of 20mm/ 50 of 10mm)

Reinforced beams,floor slabs , drive ways and paths
1 cement
2 sand clean sharp
4 aggregate ( 50/50)

High Strength, thin reinforced walls and columns
1 cement
1 sand clean sharp
2 aggregate ( 50/50)

In your situation I would have gone for a mix as follows
1 cement
3 sand
4 aggregate

With the steel posts if you drill a couple of 10mm holes in opposite directions in the area set in concrete and then run a couple of 250mm long x 10mm diam lengths of reinforcing rod through the holes this will tie the steel column to the concrete.
Note- Do not measure by the shovel full as it is not accurate. Use a bucket
How many holes involved as it may pay to get a
" Mini Mix" delivered.

Cheers.
Mac

My son and I put up a shade sail at my villa on the south coast put in ten bags of rapid set, pole has not moved one inch hole was fairly deep about 2'6' square from memory and about 4 foot deep.has not moved through all that wind we had on the weekend.
Still trying to sell it so I can move back home,I want to get my dough back I put into it and dont want to sell it to some smarty who is going to resell it to make a quick buck.
Spring time they come out of the woodwork over here(buyers that is)

How deep is the hole for this post?

I could have sworn when we did our sail post we dug ours about 600mm deep?

I was on the ground reaching down into the hole...as I have the longest arms in the family :P

I'd agree with Malcolm on proportions if you can afford it, otherwise . Having some 20 ml ag in with the 10 ml as he said is good advice- two different sizes of stone give a better structure... but no smaller than 10 ml. How are you mixing it? Mixing the cement for a good while by itself in water with a good paint mixer gives a stronger result.