# Forum Home Renovation Brickwork  Can I use H-blocks in a curved wall?

## GOM

I am in the process of building a bushfire bunker (see my separate thread "Help with design for fire bunker"). I have received some terrific advice, but now I need some more help (my wife would say in more ways than you can imagine). 
The two walls are curved, following the edge of a concrete water tank with a 6m diameter as shown in the plan below:   
and in section:   
I had planned to use Besser blocks type 20.48 ("H-blocks") because the walls will be core-filled. From this site I gather that H-blocks are easier to core-fill and also make a stronger wall. However, I have realised that it might be difficult to build the curving walls using H-blocks as it will leave a large gap to fill with mortar on the outer side, as shown in the following sketch:   
These represent two different ways I could arrange the blocks for the inner wall. I have calculated that there would be a 37mm gap to fill if I leave the standard 10 mm gap for mortar on the inside bend (i.e. the left-hand diagram). If I butt the blocks straight up against each other on the inside, there will still be a 27mm gap to fill, and virtually no room for mortar on the inside bend (i.e. the right-hand diagram). 
Of course if I got really keen I could chop off some of each block, but with over 200 blocks involved, I don't think that will happen. 
So, if I stick with H-blocks and try to patch over the 27 (or 37) mm gap with mortar, is it likely to blow out when I do my core filling? Or, should I go with 20.42 Channel Blocks? The latter would serve to contain the grout when poured, and I could patch over the gap for cosmetic purposes. They are said to be easier to lay for an amateur, but are harder to core fill and would make for a weaker wall. 
All suggestions gratefully received. 
Cheers, GOM

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## Maverick2000

I'd be inclined to choose the no gap on the inside and 27mm gap on the outside, I built a few retaining walls using H blocks and from what I could find out mortar in between the blocks doesn't really do anything as there is such a small surface area on the ends. The core filling will fill a fair bit of the 27mm gap (you could always put some tape over the gaps when you corefill or you could just put a render on the outside). 
I assume you'll be running reo horizontally as well as vertically?

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## GOM

Maverick2000 wrote:   

> The core filling will fill a fair bit of the 27mm gap (you could always put some tape over the gaps when you corefill or you could just put a render on the outside).

  My worry is not so much filling the gap for cosmetic reasons, (hopefully this is a project that I will never use!) As you say, I could render it later if I'm fussy. I'm more worried about stopping the grout spilling out when I fill the cores (especially at the bottom where the pressure would be higher). I don't think my almost non-existent skills as a brickie are enough to patch over a 27mm gap well enough to stop a fairly runny grout mix from spilling out. Is there such a thing as a tape that will stick to Besser blocks and be wide enough to cover a 27mm gap?   

> I assume you'll be running reo horizontally as well as vertically?

  Yes, there's so much steel going into this thing I think it's helped end the Global Financial Crisis. That's one reason I'd prefer to be able to use H-blocks, as I understand they make it easier to core-fill, whereas the reo will tend to obstruct it. 
Cheers, GOM

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## cherub65

With all the effort you are  going too, cutting every second block is the preferred option, when core filling a gap of 37mm could blow out as the concrete mix is very viscose using smaller aggregate and more cement. Really they cut like butter with a diamond blade on a grinder. Plus make sure you fully secure formwork to the clean out blocks, lay blocks facing each other cut strips  of plywood and brace in between both walls. If unable to brace you drill two small holes in line with the vertical reo bars then tie off with tie wire doubled up ( if you dont get what i mean ill draw diagram latter) This is very important process as the weight of wet concrete is underestimated often and the last thing you wont is to be shoveling concrete from inside your bunker.

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## GOM

Cherub65 wrote:   

> cutting every second block is the preferred option

  Hmmm. That seems to be more viable than trying to patch a 37mm gap. It's important because I'm in the process of setting out my starter bars before I pour the slab. If I shave 27mm off each block, that will be a difference of over half a block's length after ten blocks. The problem I can see then though, is that it would be hard to line my blocks up with the cleanout blocks (which I don't think I can cut). It would be tricky to set out the starter bars so that they get through both the cleanout blocks and the cut-down H-blocks.   

> make sure you fully secure formwork to the clean out blocks, lay blocks facing each other cut strips of plywood and brace in between both walls

  I had planned to use the "Cleanout Biscuits" (number 20.45A) to close over the Cleanout Blocks (number 20.45), but the plywood would be cheaper and I assume you would have to brace the "Biscuits" anyway.   

> If unable to brace you drill two small holes in line with the vertical reo bars then tie off with tie wire doubled up

  Yep, I understand that. You're tying the plywood back to the reo. 
Thanks for the useful comments. 
Cheers, GOM

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## Blocklayer

It will be difficult to lay H blocks on that radius and fill the outer perps. H blocks, when core filled are stronger and much more water resistant (because there is no through mortar joint), but you might end up with lots of mortar down the cores trying to fill the outside perps, especially if youre new to blocklaying. I wouldnt think tape is an option, the pressure of the core fill is amazing and is even higher (more volume) with H blocks. Use the tightest inner perp you can (4 or 5 mill). You will need to cut the end blocks (length) anyway, so keep the bond tight and cut last block to fit. Cutting the blocks is obviously the neatest way to do it and easiest to lay, but the cutting takes a while and is dangerous. Before you decide on any option (or cut too many blocks), lay a dry course to get a look at it. Youll soon see if you think it will work with or without H blocks and /or cutting and you can more easily work out the starter placings.  <O :Tongue:  Look here http://www.blocklayer.com/ArkWall.aspx for a calculator to work out inner and outer perp sizes, bond and diagonals to lay to (if you have access to the centre point).   :<O :Tongue: </O :Tongue:

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## GOM

Blocklayer wrote:   

> you might end up with lots of mortar down the cores trying to fill the outside perps

  Wow, you've introduced me to a whole new term "perps". A trifle disturbing when I tried an on-line dictionary and found it can mean "REALLY good marijuana. short for purple, referring to one of the many strains of purple weed". Then I found it means the perpendicular mortar joint (as opposed to the bed joint).   

> Cutting the blocks is obviously the neatest way to do it and easiest to lay, but the cutting takes a while and is dangerous

  I think that, given all these problems, it will be better to use 20.42 Channel Blocks rather than H-blocks. The wall will not be as strong, but it seems to me that the structure is over-specified anyway. It won't be quite as waterproof, but I will be using plastic sheeting and/or bitumen paint against the soil cover. They may also be easier for an amateur to lay (though harder to fill). 
Thanks for the help. 
Cheers, GOM

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## cherub65

Have you considered brickwork if not confident with h blocks? 
Boral do a fire rated brick

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## GOM

Cherub65 wrote:   

> Have you considered brickwork if not confident with h blocks?

  Yes I did consider bricks. The problem is though that the walls need to take lateral strain, mainly to allow for the sideways force of several tonnes of water-saturated soil in the middle of winter. Sure, I'll have agi pipe at the base, covered by aggregate, but the soil will push on the aggregate, and the aggregate will then push on the wall. So, I need reinforcing steel in the wall. I could make a double-wall from bricks and then have reinforced concrete in that, but I think that would be slower and more expensive than besser blocks. 
The design I'm following is basically the one for a fully-propped wall in the Hanson brochure:   
The way I understand it, part of the sideways forces at the top of the retaining wall are taken up through the roof. This reduces the requirement for a deep "keel" on the retaining wall side (on the left of my bunker), but these forces are then transmitted through to the opposite wall. That means that both my walls need to be reinforced. I am pretty confident I can make a strong enough wall out of Channel-blocks rather than H-blocks. 
Thanks for the advice. 
Cheers, GOM

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## Dan574

Maybe too late but have you considered using a crib wall system for the retaining side of things then a steel frame with hebel panels on the outside.  Your going to a lot of expense (not that this a bad thing) for a very solid structure when there are alternatives that are just as good for a light weight structure that is more diy friendly.

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## GOM

Dan574 wrote:  

> have you considered using a crib wall system for the retaining side of things then a steel frame with hebel panels on the outside

  I didn't look into a crib wall, mainly because of space limitations. I can only go out about 1.6m from the water tank without interfering with a 4WD access track nearby.  
I did look at the Hebel panels though (but probably not thoroughly enough). In fact, when I first started looking into fire bunkers I thoughts that AAC products in some form would be the bees knees for a bunker (high fire resistance, light weight, low heat conductivity). Concrete, by contrast, has a fairly high heat conductivity, so I will be relying on the earth cover for most of my heat protection. However, I thought that for the strength I would need in the panels (particularly in the roof) I would need the Hebel panels with built-in reinforcing. This would be necessary to withstand, say, a 30 metre gum tree falling on it. Then I thought that the reinforced panels would be difficult to cut to the curved shape I need. It seems to me that the Hebel products would be great for a rectangular design, but difficult with curves. 
As you say, it is probably too late at this stage, but I could perhaps have come up with a rectangular design that would have achieved the same result, but which would have allowed me to use the Hebel panels. Next time. 
Cheers, GOM

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## cherub65

I built this in my sleep last night, formed the whole thing up then just pumped the concrete, worked out well.

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