# Forum Home Renovation Brickwork  Core Fill mix and advice please

## GAM

Hi All, 
I have been using this forum to answer all my DIY questions and never needed to post anything....great info everyone........but now I have one. 
I have a besser block wall 2.4mtr high x 9mtrs long with steel reinforcing all through. It has been core filled with a truck load of cement and I thought it was ready to go. BUT..........when drilling some holes in it I have found that there is an air space just below the top. i have drilled more small holes to see the extent and it is approx 1.5mtr x 1mtr that is still hollow. This would have been due to the fact that this section is near a corner and there was lots of steel at the top, so when they pumped it there must have been a blockage that stopped it all falling through. the top course however is filled with all the steel etc. 
Anyway, I think I really should fill it before the backfill goes in so I have a couple of questions: 
1) Any smart ideas on how I can fill it, My options seems to be to either chisel away the top course around the steel, remove about 3 blocks, refill then re-lay the top 3 blocks OR smash out some side blocks below the steel and make a ramp to slide the concrete in from the side. Any suggestions welcome. 
2) I have heaps of left over bags of cement, lime and a heap of loam (sand) from the laying. Can I use this to core fill without gravel, I am happy to use extra cement if needed? If so what ratios. 
Thanks
GAM

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## sports fan

did your blocklayer hose out the dags from inside the cores after he laid the wall?? to late now anyway! i would punch a hole at the top of the voids in the wall and pour in structural grout (m-bed)

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## Brickie

When core filling you have to order 1/4 fine, its a very fine concrete, if you don,t you get the problem you have described with blockages. 
It has nothing to do with the layer not hosing out, in fact Ive never heard of the practice?  :Confused:

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## journeyman Mick

Are you sure that every core is meant to be filled? Unless it's some type of super-duper retaining wall not every core is filled. I live in a region where besser block is the predominant building material and where buildings are designed with cyclones in mind. Generally there is a bond beam around the top of the building which has one or two Y12 bars in it (horizontal) and then there's vertical reinforcing and core fills at every corner, over and around every window and door opening and at 1200 centres.  
The top beam uses lintel or bond beam blocks which are "U" shaped in cross section rather than having the two cores in it. Where vertical reinforcement is required a "half and half" block is used (there's another name, but it's rude :Shock: ). This has a vertical hole or core in one half and a channel in the other. Other wise a standard block is used with a blanking plate over one hole. Vheck your plans/specifications to see what's required. 
Mick

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## Dirty Doogie

This is a retaing wall ??  As Mick says above - sometimes they dont fill the walls completely - a bond beam around the top is usually all that is filled unless it is a lower storey wall or a full height retaing wall.   If it is a retaining wall to the full height - then yes it should all be filled.

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## sports fan

> It has nothing to do with the layer not hosing out, in fact Ive never heard of the practice?

  in sydney for the first course in block retaining walls we lay 'clean out blocks' and then conventional blocks up from there.  we have horizontal and verical reo in the wall.. the blocklayer should be told to wash out the dags inside the core holes at the end of every days work before they go off.  the open face of the clean out block is formed up before concrete is poured.  if dags are left inside the core holes they create blockages and the concrete wont flow through as described..

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## Brickie

Ive worked on a few big core fill jobs over the years and not one of the has washed out the dags.. :Confused: 
They are still standing too BTW.  :Roll Eyes (Sarcastic):

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## Blocklayer

To get an idea what Micks talking about, take a look at this http://www.blocklayer.com/BlockWalls.aspx (Mouse over the image and titles above the blocks). 
Its easy to overdo the steel and block the cores. Like Brickie said, if the core fill is to course, it'll easily block. It also needs to be wet enough to run through the cores. Some even hose the walls before they core fill.
They used to try an make us wash out the cores around here, but that was because some blockies were so messy they were dropping mortar into the cores. They'd end up with a layer of mortar between the footing/slab and concrete core fill. If your blockie is clean, theres no need to wash out. And on a hot day, theres no way you'll wash out mortar layed in the morning, in the afternoon. 
If its a 2.4mtr high retaining wall, you'd better make sure its as its supposed to be, before you back fill it. A retaining wall that high here would most likely have the first metre or so (high) in 300 seies (300mm wide), have horizontal steels at least every second course, vertical steel every core and definately be solid filled. 
.

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## sports fan

> Ive worked on a few big core fill jobs over the years and not one of the has washed out the dags..
> They are still standing too BTW.

  i guess thats a reflection on the builders you work for

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## Brickie

> i guess thats a reflection on the builders you work for

  They are better in Vic? Is that what you mean?  :Confused:  :Confused:

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## GAM

Thanks all for your replies.
To answer a few questions: 
Yes it is a 2.4 mtr retaining wall.  I had engineering drawings done up for council and they required N16 bars at 200 centres vertically and N12 bars every 4th course horizontally.  However, they also requires 2 x N12 in Bond beam at the top of the wall.  The wall is made with "H Blocks" to allow it to be completely core filled from top to toe. 
Yes we used the extra fine core fill concrete, specially ordered in for the task and used about 6 metres of the stuff.  Even with the fine fill, it still got clogged at times due to the 2x N12's at the top.  We found we had to blast hose in to the holes as we were pouring to help it flow at times. 
So what i ended up with was a particular section where the main retaining wall has a corner in it.  This corner also has N12 corner bar in it to tie it to the main wall.  So at the top, we actually have 3 N12 bars in the section above the air bubble (2 x N12 Bond bars and one corner bar) which is why it musn't have flowed properly in this area. 
The bottom section is partly filled at this point as is has run through from the rest of the wall. 
Looks like my only option is to smash the top, refill the hole and re-lay the top course again. 
GAM

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## GAM

OK, so I have fixed it and found the problem. 
The core fill was effected not only by the 3 pieces of steel under the top course effecting flow, but also the fact that my bricky put some standard blocks in the top 3 courses around the effected area instead of H blocks. This stopped the sideways flow of the core fill and effectively compounded the problem hence leaving the air bubble. 
I decided not to smash the top course as it is full of steel and effectively a good bond beam as it is, so I opened up the side of 6 blocks and filled it by hand from the side. They were all standard blocks so each needed to be filled separately as there was virtually no sideways flow for the fill. Formed up the smashed brick on the top and "viola!" 
This baby isn't moving anytime soon now  :Biggrin:  
Gam

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## autogenous

> OK, so I have fixed it and found the problem.

  I know you've sorted the issue of the core void in the wall. Once the concrete sets and there is a void you are left with potentially a cold/dry joint in the concrete when its filled after. Its always good to wet the surrounding area when you are attempting to patch an area. It may pay to drill and chemset bars into the existing concrete you are trying to correct. 
Washing out blockwork is a great concept but not always practical. If the mortar is wet up the top and dry down the base unless someone is prepared to wait around after hours till the top dries a little washing the block cores out can wreck the work. 
If someones a little hubris with the hose it can affect the work, especially if the blocks are already holding moisture. 
If the mortar is slide spread properly on the blocks you shouldn't have that much down the cavity.  
Your objective is not to stick the core fill to the concrete footing, thats the job of the starter bars. 
It sounds like "possibly" bigger blocks should have been used as the structural steel is blocking access to the lower section of the wall. It always pays to put some water down the cores prior to core filling as well.

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## brickman

Most retaining walls have written on the engineering drawings cores to be well rodded and this will almost certainly have solved the problem in the first place but why not blame the brickie anyway . There is a always a problem with any core - filled reinforced wall when 6 " blocks are used as the size of the core is only 70 mm wide and for the life of me I cant work out why engineers specify 6 " blocks as opposed to 8 " blocks when the small variance in cost far outweighs the structural benefits of the bigger block. Glad to see u fixed the problem and the strustural strenght of the bond beam I am sure will last your lifetime , cheers.

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