# Forum Home Renovation Concreting  Post Hole Concrete Mix

## jjcp

Hi All,  
I decided i'll make my own concrete mix (1:2:3 ratio) to fill in the post for the retaining wall/deck I plan to build. I have a couple of questions with making the mix:  
1) What type of sand should I use? From my search I should be looking at Sydney or Newcastle washed sand. Am I right and which one?
2) Size and type of aggregate. Is 10mm blue metal ok? Could I get away with using recycled ag instead since its cheaper?
3) I need around 1.3m cube of concrete. What is the quantity of sand and aggregate should I order? A tonne of each enough?  
thanks  :Smilie:

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## METRIX

I decided i'll make my own concrete mix (1:2:3 ratio) to fill in the post for the retaining wall/deck I plan to build. I have a couple of questions with making the mix:  
1) What type of sand should I use? From my search I should be looking at Sydney or Newcastle washed sand. Am I right and which one? *Washed sand, don't use brickies sand as it contains clay which is not the best for a good concrete mix* 
2) Size and type of aggregate. Is 10mm blue metal ok? Could I get away with using recycled ag instead since its cheaper? *20mm max, whatever you want that is cheapest, such as bluemetal, some gravel contains clay or fine dirt particles etc so stay away from that* 
3) I need around 1.3m cube of concrete. What is the quantity of sand and aggregate should I order? A tonne of each enough? *For a 1:2:3 @ 1.3cube you will need  21 bags of concrete, approx 860kg of sand, and approx 1.3ton of aggregate*

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## Marc

1.3m of concrete? 
Mix it by youself?
In a wheelbarrow?
A lot of mixing. 
Anyway, I suppose you can do it bit by bit.
1 cement, 2 sand, 3 stone is a very bony mix, way more than you need. Tyr 4 stone, 3 sand, 1 cement. Or buy sand and stone mix and do 5/1 easy.  
Sand ... if you really want to mix from scratch ... I suppose you are going to buy sand from a landscape supplier right? Not picking it up at the beach yourself right? Well in that case just ask for sand for concreting. As long as you don't buy filling sand or bricky sand or rendering mix or mm thats it I think ... you'll be OK. 
Forget it mate and buy ready mix concrete ..  :Smilie:

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## jjcp

cheers guys. Appreciate the responses  :Smilie:   
@marc. What do you mean by bony mix?  
Yeah I would be buying the sand from a landscape supplier or building supplier which ever is the cheapest. 
Ready mix concrete as in bags or by truck? I did look at concrete taxi but then realised I don't have the time to do it in one go. Plus doing it this way is cheaper (I think)

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## Marc

Don't know about cheaper but hey, It can be fun. 
Bony, too strong, way too strong. Buy ready mixed sand and gravel from the same place that sells sand. 5 bucket of sand and gravel mix to one of cement and one of water. Don't overdo the water or your concrete will be too weak. Buying sand and gravel mix you deal with one heap of stuff only and you will need a lot of stuff. If you need very strong concrete use 4/1 sand and gravel mix.
I have mixed many meters of concrete for strip footings or post footings done bit by bit on my own and I have always used a mixer. Just don't underestimate how much concrete 1.3 cubic meters is.

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## METRIX

It will be slightly cheaper for you to make your own compared to delivered, but not by that much that I would consider mixing that much, if you cannot take the concrete all in one hit, then you have no choice.
I would recommend buying a concrete mixer, then selling it when your finished, you will be there for ever and a day hand mixing 1.3 cubic. 
For example if you are going to go for your 1:2:3 mix 
Bluemetal ~ $76 / ton  Approx $114 for your need
Sand ~ $68 / ton for your needs $68
Cement ~ $6 / bag for your needs $126 
Add $60 for delivery your up for $368 
Delivered will be around $450 / cubic 
If you went the pre-bagged stuff you would need approximately 129 bags ~ $7 / bag total $ 903 
Your mix of 1:2:3 is usually used for driveways or floors, a mix of 1:3:3 is recommended for footings, and only enough water to combine the mix, if you over water it you weaken the concrete.

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## Marc

Interesting in a simple matter of concrete mix ratios there is such discrepancies. Because you have written 1/3/3 I looked it up only to find a smorgasbord of mix ratios all different not by much but different. 
The only consensus seems to be in the 3/2/1 ratio for the rest it's all over the place. By the way I have always said the aggregate first then the sand and then the cement. It does not really matter as long as we understand each other. 
What matters when making concrete is the sand and cement ratio. The aggregate is there for bulk and crack prevention and is proportional to the sand. So concrete with 2:1 sand cement ratio is bomb proof, next is 2.5:1 then 3:1 3.5:1 and 4:1 not sure if there is use for weaker mixes. 
3:1 is middle of the road stuff, yet since it has 50% more sand than the 2:1 mix, it must have more coarse aggregate so to me 3:1 mix should have 4 parts of stone, yet some websites do say 3/3/1 or rather 1/3/3, go figure.

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## METRIX

Yes, everyone has a slightly different ratio, I usually go by Cement Australia's recommendations, shown below but looking up Boral they also recommend different ratios.
Concrete Australia are probably more conservative, where Boral might go for bulk product such as more stone as it's a cheaper mix to make ?? 
At the end of the day, if you stick close to all the recommendations you cant go too wrong for what the OP needs. 
When ordering from a supplier, they will supply the correct mix as per your requirements, for small jobs by the DIY the ratios can be slightly off and it won't matter.
Most important thing for a DIY is not to add too much water, or it doesn't matter what ratio mix you do you will weaken the final product.

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## Marc

Wow, 3/1.5/1 now that is my kind of mix, add to it 19mm reo and you make a bomb bunker, ha ha

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## Moondog55

When ordering bulk around here you can always specify what you want delivered.
Choose by strength; 24 MPa which is standard; 32 which is strong and they just started with 56MPa which is pretty much bullet proof if you need to build bomb shelters
Also aggregate size from 6mm to 20mm. 6mm grit needed if it is going to be pumpered apparently, I was told by the concrete contractor who delivered when we had the stumps done that a lot of restumpers spec the 32 as it sets quicker due to the higher cement ratio

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## jjcp

thanks guys for the tips. 
I got my ratio from Boral's website. I figured it would fall under 'High strength & watertight' application. Maybe I got a little carried away :P hahaha 
So after ringing a few places I managed to get these prices: 
-Sydney Sand $54/tonne
-10mm blue metal $65/tonne
-Cement $6.30/bag
+$20 delivery 
Might not be a significant savings but I'm doing my landscaping from scratch with a limited budget so every dollar counts  :Smilie:  
and as for the cement mixer. Ill start looking at ebay. 
cheers.

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## Random Username

As an extra point, you don't necessarily need to add water for in-ground use... 
 ASABE Technical Library :: Abstract  *In-Situ Hydration of a Dry Concrete Mix*  Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan ASABE    Citation:  Paper number   034003,  2003 ASAE Annual Meeting                                                                              .  @2003      
Authors:   David R Bohnhoff, Zachary D. Hartjes, David W. Kammel, Nathan P. Ryan     
Keywords:   Concrete, Cement,  Cement hydration, In-situ hydration,  Hydration, Concrete placement, Dry  concrete mix, Concrete moisture  content, Concrete testing   Hydration of a dry  concrete mix after the mix has been covered with soil is herein  referred to as in-situ hydration. In this study, a series of dry  concrete mix footings were hydrated in-situ by burying them in sand and  subjecting them to different water treatments. Footings were removed and  cored at 4, 12 and 24 weeks. Compression tests on these cores showed  that in-situ hydration could produce concrete with strength comparable  to a normally hydrated mix. Additional research is needed to determine  how in-situ hydrated concrete strength is affected by aggregate  properties, initial compaction, confinement pressure, dry mix uniformity  after placement, as well as conditions related to water movement into  the confined mix.

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## Marc

> As an extra point, you don't necessarily need to add water for in-ground use...

  That is not necessarily true. House footings ar in ground and I have yet to see footings done with dry mix. 
You can get away with using dry premixed concrete for fence post providing there is enough moisture in the ground and you don't go troppo with the hose and wash the cement down the hole, however in general that is poor advice. Post that must hold vertical loads need to have a substantial pad under it.

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## shauck

> That is not necessarily true. House footings ar in ground and I have yet to see footings done with dry mix. 
> You can get away with using dry premixed concrete for fence post providing there is enough moisture in the ground and you don't go troppo with the hose and wash the cement down the hole, however in general that is poor advice. Post that must hold vertical loads need to have a substantial pad under it.

  I'd have to agree. Fine for fence posts ( I do it that way when it suits), not fine for retaining walls. How many leaning retaining walls have you seen because of poor construction? I think this would fit in that category.

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## r3nov8or

Can someone check my maths for me please? We needed to fill 4 holes with concrete, each 350mm (diametre) x 900mm (depth). We went and ordered 0.4m3 of sand and aggregate (plus bags of cement). They dumped it in the trailer and away we went.  
(My maths ended up as 0.086m3 per hole, times 4 = 0.344m3) 
We got home and were able to fill only 2 of the holes, with a little left over! Was my maths terrible or were we ripped off?

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## METRIX

> Can someone check my maths for me please? We needed to fill 4 holes with concrete, each 350mm (diametre) x 900mm (depth). We went and ordered 0.4m3 of sand and aggregate (plus bags of cement). They dumped it in the trailer and away we went.  
> (My maths ended up as 0.086m3 per hole, times 4 = 0.344m3) 
> We got home and were able to fill only 2 of the holes, with a little left over! Was my maths terrible or were we ripped off?

  You might have been ripped off. 
Each hole with no post in them will take 0.087 cubic with zero allowance for wastage, 4 holes should take 0.346 cubic.

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## r3nov8or

Thanks Metrix. Yep we ordered a little more than the calculation, but basically received half what we needed (and paid for). Might have to have a chat with them.

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## jjcp

just a couple more questions. 
1) Any specific brand of cement I should buy? Hardwicks, Bastion any good?
2) Do I let the footings dry first before I put the post in the hole? 
I think that will be all for my concrete questions. Just need to find the time to start my project  :Smilie:

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## Marc

Now you got me confused.1) No idea, others may know to me, cement is cement, don't know if there is a difference between brands.2) I thought you were cementing the post in. So you are doing a concrete pad and then ... ?Wooden post? Gravel around it?

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## jjcp

ooppss sorry I don't think I said the correct term. 
What I ment for my second question is do I put cement first on the hole and let it dry then put the post in and fill the hole with cement? Here is a diagram of what I was trying to ask. Hopefully that made sense

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## Marc

If the post is steel, cement it in all in one go, making sure you keep it off the ground a minimum of 100mm, 200 better. If it is a wooden post, make a decent size pad, let it set and then sit the post on it and fill with gravel all around, no cement...providing the dimensions of the post, the depth of the hole and it's diameter all are within specifications.

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## johnc

Actually plenty would just use concrete all round for the wooden post as well, however following on from Marc's post if you opted for concrete pad rather than using gravel which can create a sump I would use the soil pulled from the hole when you dug it and ram back in hard.

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## Marc

Yes, many would do it that way. 
I think that wooden post in concrete sit in a puddle all the time and end up rotting quicker if not under cover that is. I have demolished my fair share of wooden post set in concrete and all were well and truly rotten worst than straight in the ground. 
Gravel in my way of thinking keeps the post dry, or dryer than soil anyway.
I suppose it all depends on location, soil, type of wood, cover or not ... not enough information.

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## shauck

Is it a fence, a footing or a retaining wall post?

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## jjcp

It's for both retaining wall post using 200*75 sleepers and deck posts using galv stirrups.

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## Marc

Mm ... retaining wall ... lateral forces ...  I'll abstain from commenting on the retaining wall without more info, for the deck posts using stirrups, obviously you need to concrete them in all in one go. My recommendation is to use stirrups that have a solid metal rod and not a hollow tube.

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## johnc

> Yes, many would do it that way. 
> I think that wooden post in concrete sit in a puddle all the time and end up rotting quicker if not under cover that is. I have demolished my fair share of wooden post set in concrete and all were well and truly rotten worst than straight in the ground. 
> Gravel in my way of thinking keeps the post dry, or dryer than soil anyway.
> I suppose it all depends on location, soil, type of wood, cover or not ... not enough information.

  I'd agree that just using concrete on wooden posts creates a bucket effect and encourages rot. I actually prefer to use the soil that came out of the hole as I reckon it is less likely to hold water once tamped in. From my experience gravel can become a bit of a sump. Does of course depend on soil type, also how in love you are with a crowbar, if you have never used one or done next to no manual work then a bucket of premix looks very attractive regardless of what you an I might think.

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## Marc

I wonder if it would be possible to concrete the wooden post in the following way. Dig a 150mm deeper hole than required, fill the first 150mm with 20mm gravel, place the post on top of the gravel and concrete all around it with thick concrete not too watery. This would consolidate the sides of the post and still leave a drain under it. (?)

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## johnc

Possibly, another is to just put the post in the hole, concrete around it leaving water to pass through. Again depends on soil, would make little difference in sandy soil but a different matter in clay. Mind you that is fence posts not stumps which must have concrete under. I just prefer original soil, sandy gravel will shed water in most soil types anyway. Old style is timber stump with timber baseplate and brace, rammed earth, but that's pre commercial bagged cement.

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## shauck

With deck, being stirrups, no problem. Concrete under stirrups is the way. With retaining wall, although it's correct practice to concrete under posts, I'd just concrete around posts. There's no downward force. Make sure you're holes are as deep as is required for the height of the wall. Also, ensure the bottom of your hole is solid, no loose material. Should be in clay layer. No less than 600mm and any wall over 600mm, will be as deep as the wall is high. For example an 800mm high wall will have 800mm deep post holes. If you put concrete under the post, add 100mm for that. If you have walls over 1m, I can give you a PDF that details correct procedure and has tables for working out correct spacings and timber sizes.  
Best to use a concrete truck. You say you don't think you can do it. If you set up all your posts and brace them properly and set up your deck bearers, also braced and hang the stirrups from bearers, over the holes, the concrete pour will be much quicker. I'd still get a second person to either barrow for you, while you come after them and clean up each hole, checking for plumb/level as you go (mostly with the posts, but also the bearers maintain their line), making sure each hole is sufficiently filled and shaped to let water run off. It won't take long at all. Much easier than mixing concrete

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## Marc

> Much easier than mixing concrete

  I second that

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## Dimi

> I wonder if it would be possible to concrete the wooden post in the following way. Dig a 150mm deeper hole than required, fill the first 150mm with 20mm gravel, place the post on top of the gravel and concrete all around it with thick concrete not too watery. This would consolidate the sides of the post and still leave a drain under it. (?)

  Good one Marc. I agree this is the best way by far if using timber post in cement. Also there's timber and there's timber. Real Ironbark or real Messmate is what they used to use allot on docks and lasts a long time. It's bacteria that start the rotting process you want dry or drenched not warm and damp/moist. Piers will usually start rotting around the water line. You can go treated but remember it leaches CCA and its no good for a lot of plants. I realize it's a retaining wall just using dock scenario as example

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## jjcp

Sorry for the lateish response and thanks again for the tips. :2thumbsup:  
@ marc.
The retaining wall height ranges from 800-400mm. The soil I have I would think is heavy clay. 
With the deck post, I’m struggling to find a post with a solid metal rod and 600mm in length, in fact the only manufacturers I found so far are Pryda and Dunnings both of which has a hollow tube. Any ideas where to get them? 
I read from other member’s deck project that they used high wind post stirrups instead. What do you think of these? It might be a little over engineering in my case since I don’t live in a high wind area. 
@shauck
Yeah Ideally I would like to pour the concrete in one go but time is a major factor which I don’t have. Maybe that would change as I get closer with starting this project. Btw, how long does a concrete truck/taxi usually stay before they start charging extra? 
@dimi 
There will be vegetables/fruits on top of the retaining wall so CCA is out of the equation (from what I read it only leaches out minor levels of the bad stuff- but I don't want to risk it). Never knew sourcing 200mm*75mm*3m arsenic free sleepers would be so hard.

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## Marc

Ok, haha, this is a long thread.
Retaining wall as per Shauck notes, post go 800 deep for 800 high wall, 600 for 600 high wall. Easy no mistakes. Concreting of posts as per your choice. I would set the post on gravel and concrete around it, others may have a different view. Depends also on the wood used, I suppose you would use treated pine H4. Cyprus pine is also a good choice.
Stirrups. Why 600 long? Stirrups have different length to be able to level a deck over uneven terrain, you don't need to have the stirrup rod 600 deep in the concrete. I suppose you could but past 250 or so it makes no difference. 
The high wind stirrups are very good and give you better lateral resistance because they are essentially double with those two flat bars on each side however I suspect you only need a normal stirrup some 300 long with a nice solid rod not tube. Pryda is the common brand at Bunnings. 
You need to find out the depth and size of your footings according to soil condition. A guesstimate would be 300x300 400 deep as a minimum

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## jjcp

hahaha yeah it turned out to be a long thread but this should answer all the basic question for newbies like me (well it did for me anyway)  :Smilie:

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## shauck

> @shauck
> Yeah Ideally I would like to pour the concrete in one go but time is a major factor which I don’t have. Maybe that would change as I get closer with starting this project. Btw, how long does a concrete truck/taxi usually stay before they start charging extra?

  Not sure how long before they charge. Here, the concrete supplier even helps if it looks like it's needed. Bought him a bottle of whiskey once to thank him. Country towns rock.

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## magicpossum

Re timber in concrete sump effect, Has anyone tried the paint-on bitumen membrane products (eg Ormonoid) designed for heavy duty waterproofing? You'd think with the in- ground timber coated in bitumen, rotting would at least be delayed. 
Re concrete ratios, Most landscaping suppliers do a concrete blend of sand and stone to which you can simply add your cement poweder and water to. This would simplify the mixing process a bit. stone:sand ratios shouldnt be too critical for mass 'crete footings.

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## r3nov8or

> Re timber in concrete sump effect, Has anyone tried the paint-on bitumen membrane products (eg Ormonoid) designed for heavy duty waterproofing? You'd think with the in- ground timber coated in bitumen, rotting would at least be delayed.
>  ...

   Our local council does this, it extends to a couple of inches above ground level, as they are now using Cypress pine for all public timber (e.g. seats,. playgrounds etc) rather than treated pine.

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