# Forum Home Renovation Brickwork  Block wall core fill problem.

## Jake.

Hi all,  
Im building a 2.6m block retaining wall soon & will have a builder mate do the concreting but i have to 
organise the digging, steel etc ready for the concrete.
Im a sparky so im fairly new to this.
I have engineering drawings done although hand drawn so i have a couple of questions. 
1800mm high is the max i can go with 200mm blocks so i have to use 300mm blocks 4 blocks high (courses?)
The problem i face is when i core fill it the concrete will spill out the rear 100mm of the uncovered 
300mm block.
Someone suggested i core fill the 300mm blocks first before the brickie starts laying the 200mm blocks 
on top but to me this sounds like the wall won't be as strong as there is a join between the upper & 
lower core fills. If this is correct the 200mm blocks will only be strengthened by the vertical reo 
@400. Is this ok ?
Or i thought i could have the brickie cover the open section behind the wall with flat blocks & core 
fill the whole wall at the same time.....to me this will be stronger. 
Also what does B/U & B/O mean when written after say N16@400 ?     
Cheers.

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## Dirty Doogie

I'll take a guess and say Bent Up and Bent Over.  
The concrete core fill wont overflow through the open section of the 300 mm block as much as you might think. Maybe a little will come out but it is not going to keep pouring out. The blocks absorb the water in the core fill and a stiff plug of concrete forms fast. If you fill the wall lengthways (like it is supposed to be done) you pause when you've gone one course up higher than the 300 blocks.

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## Jake.

Hi DD, 
I thought the mix would be quite liquidy & with 1.8m of head above it i recon it would spill out. Can only guess as i don't really know. 
Anyway, for some reason my initial post has disappeared so i will re post the question again. 
Cheers.

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## brickman

Have been doing a few of these lately seem to be the latest in engineering design ,  I usually lay the 300s then place cement sheet (3.5mm) ripped down to 120 mm wide with a grinder on the back third of the block then lay the 200s on top of the 300s in line with the face so that the back of the block sits on the cement sheet with a thin layer of mortar and it caps off the rest of the 300 block . Most builders tend to think u need to concrete fill the 300s then lay the 200s till u explain this method which eliminates a cold joint , and all of a sudden they take ownership of the idea. cheers

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## brickman

One other thing it also pays to cement a splay on top of the cement sheet with left over concrete or mortar prior to tanking to eliminate the edge where water may penetrate.

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## Jake.

"Cold joint", thats what its called hey. 
Personally i don't like the idea of two separate core fills so i will talk to the bricky about your cement sheet idea. I can see that working well......cheers.  :2thumbsup:  
As for water proofing im going to seal the back of it with bitumastic paint. 
If thats the case of only one pour then i wont need to do the two separate vertical reo bars with the n16 over lapping the n12 like in the drawing.
I should be able to drop down a 2.6m n16 bar after the block layer has gone. Might be cheaper, will find out.

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## DaveD-75

Without having a great deal of knowledge on the subject.... is there a reason why you can't use 300 blocks the whole way up?  Maybe I'm just mis reading your drawing...

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## Jake.

300mm blocks cost about a $1.50 more than the 200mm ones so considering i will probably need 700-800 blocks thats i big difference in price. 
Also the core fill will be a lot more & that isn't cheap either.

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## autogenous

> I usually lay the 300s then place cement sheet (3.5mm) ripped down to 120 mm wide with a grinder on the back third of the block then lay the 200s on top of the 300s in line with the face so that the back of the block sits on the cement sheet with a thin layer of mortar and it caps off the rest of the 300 block

  If you lay the sheet down, place a straight edge at 120, scribe the sheet a couple of times with the tip of a trowel, then place straight edge under scribe that the sheet will snap perfect down the scribe. 
Great idea, Ive only ever laid any one thickness to the top. Ill mention it to the Site manager next time I do one.   :2thumbsup:

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## Jake.

Yeah its great the info you come across on forums, exactly what i was after.  :Cool:

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## Stringy

Had the same design retaining wall built, brickies ran a row of normal bricks along the back edge where the gap is, this worked fine!

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## Jake.

Normal bricks ? 
Where i live 200x400mm besser bricks are the norm.  :Smilie:

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## Dirty Doogie

HMM - I thought you had to leave the 300mm blocks open at the top for the pour. 1) so the cores can be rodded (Mortar pushed around and compacted) and 2) a nice head of grout indicates the lower blocks are filled.

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## brickman

The top of the wall is still open above the 200's and when u drop the full length y-12 bar down after the core fill you rod it up and down as u place it in the core and with an 1800 head height of core fill above the 300,s I will personally guarantee there will not be a void in the 300's , cheers

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## Brickie

> Normal bricks ? 
> Where i live 200x400mm besser bricks are the norm.

  Then they are blocks and not bricks.  :Tongue:

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## Jake.

> u drop the full length y-12 bar down after the core fill

  I wish, the drawing shows the lower half with N16 & then N12 at the top half with a 800mm over lap. 
Basically the same cost to just use one length of N16 (2.5m) & will save the mucking around of tying the two together.

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## Jake.

...

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## Jake.

> Then they are blocks and not bricks.

  Can't argue with that, does it show i don't know what im talking about ?  :Biggrin:

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