Thread: Concrete type for footings
13th Jun 2007, 10:08 AM #1
Concrete type for footings
I have a question regarding the type of conrete to use for footings for a deck/pergola. I just tried to get a price for readymix concrete delivered but they told me I would need to specify the exact type and strength of concrete I want. They did however give me a quote for 20/20/80 (???) at around $220 cubic metre.
What does everyone use for concrete footings? I have seen discussions saying to avoid using the quick set stuff due to an apparent non-structural rating.
Do you mix it yourself?
I have ten posts to set (using galv stirrups) and was going to use a hole 450mm dia and 600mm deep. That's approx 0.1m3 per hole.
Also are there any suggestions for how to keep the top neat. Do you form up some type of box at ground level? Or some type of cylindrical formwork? I want to keep the top slightly above ground level so I can ensure a proper water run off.
Many thanks in advance!
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13th Jun 2007, 11:01 AM #2
20/20/80 means 20mpa (cured concrete strenght) / 20mm aggregate (gravel) / 80mm slump, this is a form of testing the water content to which wet concrete is place in a cylindrical cone and will slump down 80mm when the cone is removed.
Subject to what your local building code requirements are you may be able to use 15/20/80.
For finishing around the top/ground you could go to all the trouble of setting up some formwork or just do it freehand. As its under a deck and will probably never be seen freehand will be the cheapest way to go but that your call.
13th Jun 2007, 11:11 AM #3
Concrete suppliers like many specialist building suppliers like to intimidate people that are new to the game or dont speak in their own code.
You get used to it after awhile. Best way to deal with suppliers when you are not too sure of what you want is to give the first one you speak to the third degree and make notes on a pad.
The next one will think you half know what you want and the third will think your a tradie!
The 20/20/80 is a refference to strenght -20 mpa which is fine unless you need s super strong batch 32mpa (unlikely) the 80 (From Memory) refers to the slump - dont worry about this too much as the driver can wet the batch down a bit if its not runny enough.
Can't recall what the other 20 refers to.... not important so just ask for 20 mpa concrete they give you standard 80 slump anyway unless you ask otherwise.
I would have thought $220 a cube is a bit steep but I haven't bought any for a while.
13th Jun 2007, 04:28 PM #4
14th Jun 2007, 10:03 AM #5
Thanks for that, it makes sense now! And good idea Bleedin Thumb about the questions - I've used that tactic before to good effect.
Now should I order premix or mix it myself onsite? I would have to barrow premix from the street round the side of the house to the back (luckily slightly downhill...) Do you have enough time with premix to set 10 stirrups before it hardens? Is 1 cube of premix too small an order for delivery?
If I mix it onsite it will all have to be mixed in a barrow - sounds like hard work!
What are the advantages/disadvantages of premix vs mix yourself?
Are they the same strength?
Effort or time required?
14th Jun 2007, 10:37 AM #6
For cost effectiveness, conveniece, consitant strenght and miminal effort readymix would be the way to go. Most readymix mobs will supply a mimimum of 0.2 cubic metre
If you want to mix yourself it means getting in sand, gravel and cement which means over-ordering/leftovers, in-consistant strenght (which I wouldn't worry to greatly about). You could hire/borrow a cement mixer.
Re the concrete going hard, you need to remember that as soon as the wet concrete hits the surrounding soil, the soil is going to start sucking the water out of the wet concrete thus going hard, unless you have well and truly soaked the holes beforehand.
I would be setting up all of the stirrups in place before I placed/poured any concrete. Readymix mobs charge waiting time after an initial period.
14th Jun 2007, 07:17 PM #7
Mix it yourself or buy ready mix?
A bit like "how long is a piece of string ?"
Mix yourself- should be cheaper, you are in control, do two holes, fix you brackets. Do two more, same, have a rest, have a beer.etc. No rush, but slower. Spread it over a weekend. You can still get good concrete if you use a mixer.
Ready Mix- we have these little 1.6 cu.M trucks specially for small awkward jobs. Pump up the tyre on your barrow, clean the crap out of your way. Don't let the driver overload the barrow and get most of the weight over the wheel. Run it in, 1.2cu.M (don't order short) say 12 or 15 barrows. P.O.P. All over in no time. Better if there are two of you, one on the barrow and one helping then setting up the brackets.
14th Jun 2007, 11:23 PM #8
Downhill is good for moving anything heavy. If the ground is lumpy or somewhat soft, lay some planks for a runway for the barrow. If not enough planks, have an assistant leapfrog the ones you have as you move the barrow. If you hire a mixer for DIY, remember you'll have to move the mixer back uphill to return it to the rental yard.
JoeOf course truth is stranger than fiction.
Fiction has to make sense. - Mark Twain
15th Jun 2007, 02:34 PM #9
Here in Brisbane I'm paying $143/cu.m + GST for 20/20. A builder I know who does about 30 houses/yr pays about $115+.
18th Jun 2007, 09:46 PM #10
Remember if getting a mini mix that they only allow a certain amount of time per cubic to unload the truck and then you pay waiting time..So if you positioning shoes or posts and wheelin the concrete expect to pay a waiting fee...
Ring up and ask for "AAAAAh cubic of 20Mpa concrete mate and not real runny mate cause i gotta put some footins in that i want to be able tah position straight away and how long ya give me to unload it mate.. What sort a notice ya need to get it to site to Aiih!"
If done right you'l sound like a tradie if you get it wrong you'll sound like a knob
We mix onsite usually cause it gives the time needed to position the stirrups and posts without rushing
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