Concrete mix for footings for 1.2m high retaining wall

[richard7761]

I think I should use something like a 1 part cement 6 parts everything else mix. Specifically a 1:2:4 mix, that is 1 part regular cement, 2 parts sand, 4 parts aggregate (20mm).

I am wondering about using (washed) sandstone aggregate. Because I have lots of stones in a pile from the garden and it would save me money.

Comments appreciated. Thank you. Rich

 

[PrenticeBoyofDerry]

I would use a mix ratio of 4 parts 14-20mm gravel, with 2 parts grit sand and 1 part portland cement.
The washed stone should be fine to use as part of the 4 parts gravel, just mix them in evenly.

 

[richard7761]

I'm trying to figure out my sand and cement requirements.

Let's say that the block of concrete (footing) is 7m long, 23cm wide and 30.5cm deep. That comes to 0.49105 cubic meters.

Ratio is volume, not weight

One seventh part is 0.07015 cubic meters.

So, area-wise we get:

Cement (1 part): 0.07015 cubic metres

Sand (2 parts): 0.1403cubic metres

Aggregate (4 parts): 0.2806 cubic metres

Not sure what water does to these figures.

But, how do I know how many bags of concrete and sand to get? I'm not sure about that. Help appreciated. Thanks.

 

[theoldun]

Never heard of washed sand stone aggregate.
Course aggregate for concrete is normally crushed stone 37.5 screened down to 5mm angular shaped aggregate.
You can get different sized aggregate for different types of concrete.
Washed stone comes from quarries which normally have a large water content and are screened down to size required.
They are normally called stone rejects, pebbles or cobbles as they are more rounded than angular, the same as the stones you have harvested from your garden.
You can use these at a pinch for concrete depending on strength of concrete you require. Lets just say for the small amount of concrete that you require, we would not use them.
As you do not state what foundation is for, can not comment on foundation size, except to say that 230mm wide and 305mm deep are quaint measurements..
Builders are not rocket scientists and do not work quantities out to such detail as you have done.
State what foundation is for, and we feel sure some one will give you the quantities that you require.
oldun

 

[Norcon]

We use the round stone mix mostly as the finish is superior on a shuttered wall compared to the crushed stone mixes.
The down side is hair cracks can be more common although the correct use of reinforcement and expansion/contraction joints keeps them to a minimum.

 

[richard7761]

Okay, the thing to do is to establish quauntities required for 1 cubic meter.

I found a concrete calculator that gives materials quantities when using 1:2:4 ratio by volume:

http://www.source4me.co.uk/calculate_concrete_mix.php

For 1 cubic meter, you get the following figures:

--------------------------25Kg bags---40Kg bags
Cement: 320Kg -------13------------8
Sharp Sand: 600Kg----24------------15
Aggregate: 1200Kg--48------------30
Water:....176 litres

OR

Ballast (all in)-------1.67 tonnnes

Obviously this is scaled according to the cubic volume one has to make for the job.

 

[richard7761]

I think I now will be able to get the quantites required. By the way, the dimentions I gave is one half required. In the middle of footings for the full brick wall, is a line of 2" thick paving slabs that go down 600mm acting as a root barrier.

I really now just have a question as to the aggregate. I myself have crushed down larger sandstone chunks into about 20mm size chunks. Would I mix in with this some smaller chunks? Or just put in 20mm chunks?

 

[richard7761]

Just one more question. I want to have a straight edge to the footing. I aim to put in the ground, lengths of board. I might need to put in 3 or 4 over the 7m length of the footing. Now, when the footing is set, the idea is to remove sections, one by one, filling in the gap left between the set concrete and the earth after each board section is removed, until all boards are removed.

Would this present any problems? Thanks.

 

[richard7761]

Just one more question. I want to have a straight edge to the footing. I aim to put in the ground, lengths of board. I might need to put in 3 or 4 over the 7m length of the footing. Now, when the footing is set, the idea is to remove sections, one by one, filling in the gap left between the set concrete and the earth after each board section is removed, until all boards are removed.

Would this present any problems? Thanks.

I see were talking about wood formers, removed when concrete is set. The wall will actually be divided by a stretch of 2" thick paving stone that goes down 600mm. So, there will be two lengths of concrete to lay, one on either side of the paving.

It's just that I have this notion that when the wood former is removed, the concrete will want to turn over a bit and seperate from the paving stone, unless I remove the former in sections and fill in. I think unfounded, especially since the concrete will affix to the pavement which is solid in, but just checking.

 

[theoldun]

I think I now will be able to get the quantites required. By the way, the dimentions I gave is one half required. In the middle of footings for the full brick wall, is a line of 2" thick paving slabs that go down 600mm acting as a root barrier.

I really now just have a question as to the aggregate. I myself have crushed down larger sandstone chunks into about 20mm size chunks. Would I mix in with this some smaller chunks? Or just put in 20mm chunks?

Thought your dimensions were a bit quaint. What you are saying now is that you have a 450 wide found that you propose to drop a 600 slab down the middle of to act as root barrier.
Don’t know what book you dug that out from, but root barriers went out the window years ago, and you do not form a root barrier like that.
Have you got any trees adjacent to found?
Sandstone is one of the softest stones available, and we would never put it in a concrete mix. Your choice,

Just one more question. I want to have a straight edge to the footing. I aim to put in the ground, lengths of board. I might need to put in 3 or 4 over the 7m length of the footing. Now, when the footing is set, the idea is to remove sections, one by one, filling in the gap left between the set concrete and the earth after each board section is removed, until all boards are removed.
Would this present any problems? Thanks.

Your shuttering for concrete are the sides of your excavated trench. You do not need any shuttering as only the worms will see side of concrete.
Word of warning. Retaining walls even at 1200 high, if back filled full height require a design consideration in regard to materials being used, width of wall, width of founds for rotation, and excluding any live load above, the lateral thrust of the back fill and possible water content.
Angle of repose of existing ground is also a factor to determine the KN loading of back fill. In our opinion a 450 wide foundation is insufficient for a 1200 high retaining wall. If you are doing this all your self, you would be better to think along the lines of 2400x250x125 oak sleepers. Will last your life out if done correctly/.
oldun

 

[richard7761]

I do not worry about how the wall will be built, given the conditions. The wall will in fact only hold itself up, more-or-less, because the ground behind it is firm and has been for hundreds of years. I had a wall previously there for 20 + years with no movement and that had no foundations at all.

I do wonder though about using that soft sandstone as aggregate. Because it so very soft. Gravel I think is the proper aggregate.

Actually, I have just asked for a quote for concrete delivery. See what the quote is.