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Concrete / shuttering gurus - a question for you

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 Fraser 23 Dec 2017
If I wanted to cast a shuttered/board-marked concrete (interior) wall, but just as a decorative'facing', rather than a structural element in its own right, what's the minimum thickness I'd be looking at? Presumably it needs some sort of mesh or key to prevent it spalling from the substrate - in this case a common brick party wall - but besides that, does 50mm o/a sound too thin? It would be about 4-5m wide and would span about 5m vertically, but this could have a mid-height joint if necessary. For the sake of argument, ignore any support required at the base - that's easy enough to resolve.

I'm looking for something that would give the same effect as some of the images in the link below:

https://www.dezeen.com/2017/12/22/home-interiors-board-marked-concrete-arge...



 Wainers44 23 Dec 2017
In reply to Fraser:

The problem you have is that "concrete" includes a coarse aggregate, normally no smaller than 10mm, so unlike a mortar or screed or render material it generally has to be thicker.

I wouldn't try anything less than 75mm in ordinary concrete, but you might be able to find an additive which will allow you to go thinner...maybe?? Even then I would mechanically fix a lath (metal mesh) back to the brickwork as you also have a problem with potential sheer, as the two materials could be radically different in both strength and movement properties.

You are probably better going for an old fashioned "render and set" which is cement and fine aggregate only, so not really concrete, but someone with a bit of skill may be able to create the look you want by hand?
 Mick B 23 Dec 2017
In reply to Fraser:

Have you thought about 'pre-casting' the concrete on a horizontal surface and attaching it to the wall,
You would achieve the effect by nailing rough sawn 4x1 boards to a sheet of ply or straight onto supporting timbers.
The key is to vibrate the concrete well in the mould.

I would definitely get some reinforcing mesh in there though.
 DD72 23 Dec 2017
In reply to Fraser:

I couldn't help wondering why you are going for the effect when that finish is normally a product of the construction process and the reason behind showing it is that you are being honest about that process and embracing the use of concrete as a construction material.
 MG 23 Dec 2017
In reply to Fraser:

Take a look at ferrocement. Might be easier to apply.
OP Fraser 23 Dec 2017
In reply to DD72:
Simply because I really like the aesthetic, but I agree with your comment in principle.


In reply to Wainers44 & Mick B:

Thanks for the comments and suggestions. I'd definitely assumed a s/s mesh would be a prerequisite and getting decent vibration in a potentially narrow slot was one of my main concerns. I'd thought it could maybe be done in 3 or 4 separate pours to try and guarantee getting the mix to full depth and coverage. The idea of forming a horizontal mould first is interesting but I feel that might generate is own issues in terms of lifting it and getting it into place without breakage. So that means thicker than required once in situ, which increases weight etc.

I like the batten-effect where you express the separate joints between adjacent boards and the finished face isn't a flush plane. Comment re the aggregate size is noted, thanks.

Re the shear issue, I'd been imagining either a cantilevering angle bolted to the party wall below floor level, or simply a dwarf wall tied into the party wall in some way and take the new leaf down onto that.
Post edited at 19:56
OP Fraser 23 Dec 2017
In reply to MG:

Not heard of that, thanks. A quick Google though suggests it's a hand-applied finish, presumably trowelled, so that wouldn't give the shuttered/boarded finish I'm after.
 daWalt 23 Dec 2017
In reply to Fraser:

what Wainers said.
I'm assuming this is retrofit to an existing wall - otherwise it would be much easier to just pour a conc wall the forget the brick.

theoretically it would be fine to fix a mesh (A193 or A 252 reinforcement mesh? - no design services rendered here) and pour a thin (say 75mm) single-side shutter with a small aggregate size conc (20mm stone size is normal - this is far too big, it won't move though the remaining gap between the rebar and the shutter / wall properly).

but, be aware - pouring concrete isn't as easy as it looks:
Wet conc is effectively liquid, it needs to be poured. that is to say if you're planning for it to be the full height of the internal wall access might be tricky to let you pour it in from the top.
Because it's liquid the shutters need to be quite robust; a 75mm thick wall is quite wee but single side shutters are notorious for kicking out under the weight of wet conc.
Conc needs to be vibrated into place otherwise you get honeycomb and blow holes (air bubbles / pockets). Can't get a poker down a 75mm gap where there's re-bar in the way - you'll have to vibrate the shutter.
Beware of cold joints - where the conc starts to cure before more is poured on top.

if you're pouring in stages you should scabble the surface to expose the aggregate, much easier to do early while it;s still curing. if it's a DIY job I'd say just jetwash the surface after 12hrs while it's still green and this'll remove the top cement surface.

many things are possible, but the effort is sometimes disproportionate to the end result.
it's the detail of where the wall and ceiling meet and how this is finished might need a bit of thought.
 jon 23 Dec 2017
In reply to Fraser:

Can't help with the technical side, of course... Our neighbour is in the throes of building a very contemporary, completely concrete house, using exactly this feature for the external walls (not sure how the interior will be, but he's a fan of concrete... ). He's already completed phase 1 which was an infinity swimming pool, and the bits that are visible look fabulous. I thought the whole of the house with the same finishing effect might be a little overpowering but having looked at the drawings it seems that it will be broken up with a large amount of glazing and, because it's facing due south, sun 'visors' (not sure of the correct term) running full length of the south elevation. Throw in a flat roof planted with lavender - which we will look down on - and I think it'll be a very interesting and attractive building. No help to you, of course, but just a thumbs up for the effect!
 daWalt 23 Dec 2017
In reply to Fraser:

I would be tempted to say it would be easier to install a thin layer, more like molded plasterwork, rather than proper poured concrete.
wire mesh fixed as backing,
pourable coarse sand cement grout,
build up in layers of a few battens at a time,

if you're DIYing this:
do a test mix and pour - be careful to keep the conc / grout mix consistent
and keep it very well hydrated (I'd say proper wet) so that you don't get shrinkage cracks.
if you've ever plastered onto brick; the brick will suck the water out, and the more rich the conc/grout (more cement) the more it'll heat up and dry out while curing.

sounds like a proper project
 ian caton 23 Dec 2017
In reply to Fraser:
No no.

1 cubic meter of concrete weighs 2.5 tonnes. Do your maths. How will it stay put, lots of problems.

What you do is render it (10mm ish thick) then pattern it with a plank or planks. You can rent polyurethane moulds. SBR in render aids bond and plasticity. This is proper work.

You will need to research renders for colour ( you can through colour ) and bond. Play with old bricks.

You will need spray on liquid release agent so plank or mould doesn't stick. You will need to seal the finish.

Look through suppliers of materials for pattern imprinted concrete, some do a system for walls, but I can't remember which ones.

You can also get precast panels, or get somebody to do it for you.

https://www.i-faux.com/concrete-1
Post edited at 21:09
 DD72 23 Dec 2017
In reply to Fraser:

Interesting thanks, I hadn't realised this was a thing.
 JLS 23 Dec 2017
In reply to Fraser:
As someone has said earlier, I think realistically you are looking at a render solution, where the "shutter board effect" is pressed into the render, as being your best option. Actually pouring and compacting a 50-75mm skin of concrete would be a nightmare. Beyond that there would be temporary and permanent structural considerations... You wouldn't want that much concrete falling over on you while you watch the telly.
Post edited at 21:49
OP Fraser 23 Dec 2017
In reply to ian caton, daWalt et al:

Interesting link thanks for that. Pity it's American but there might be a UK equivalent.

I've also been thinking just now more towards the rendered solution for numerous reasons. I'm pretty familiar with standard shuttered concrete and have probably watched a few thousand cube being poured at work. Our practice even did some Cathedral-grade, shuttered concrete on a job a few years ago, but the main contractor had nightmares achieving the required finish! The problem for me is really a lack of space to form the necessary thickness you'd normally be looking at, hence the original query.

I'll have a contractor in on the house for all the rest of the structural works, so will speak to those tendering for the job about alternative options to achieve the required effect. I think a very wet, rendered skim coat of <insert final material here> and quickly whack up some sort of pattern-embossing shutter/press is likely to be the most successful option (or least unsuccessful!)

In reply to jon:

Sounds very nice, I'm now able to better picture the landscape you're looking out onto! Brise-soleil is the term you're searching for I think.


In reply to JLS:

It's in the middle of the plan where the new stair is going in ... I'll just run past it. Risk mitigated! ;P


Or just go for the safe, easy option and get that funky wallpaper!
 jon 23 Dec 2017
In reply to Fraser:

Brise-soleil, that's it. Funnily enough it's the same in French
 aln 24 Dec 2017
In reply to Fraser:

If your house is anything like the examples in your link, why are you asking on here?
 Wainers44 24 Dec 2017
In reply to aln:

And if money is really no object, you can get exactly the effect you need in GRC,...glass fibre reinforced concrete panels.

They can produce pretty much any shape or finish. It's much stronger and lighter than concrete. It's the same stuff that they make Aco drainage channels out of.

I have bought it a few times...hundreds of copings for walls at HMS Raleigh in Plymouth, and also as cladding panels for a shopping centre. Looks great, but it is pricy.

Quite a few places make the stuff, send out a few emails? If you stumble across someone who has that mold finish in stock it might only cost a fortune!!
 AJM79 24 Dec 2017
In reply to Fraser:

Hi Fraser,
DO NOT, I repeat do not make any structural changes to your party wall (including adding weight) without speaking to your neighbours as this is illegal under the party wall act and could leave you liable for any damages that they incur. I would strongly advise that after speaking to them and agreeing the work that you employ the services of a structural engineer and have it all signed of properly. I'm a builder and I wouldn't touch it without a structural engineer and building control covering me. If you just want the effect then why not just fix shuttering boards up and paint them grey.
 JLS 24 Dec 2017
In reply to Fraser:

Looks like a few experiments imprinting this stuff would give you the look you are after...

youtube.com/watch?v=G7ZbJUZXNgw&

OP Fraser 24 Dec 2017
In reply to jon:
Sadly not enough 'soleil' up here to warrant many!


In reply to aln:

> If your house is anything like the examples in your link, why are you asking on here?

Unfortunately, it's not. The style of house is irrelevant really, it's the effect I'm asking about and I'm asking on here as there is often a good depth of knowledge in areas I know nothing or little about. There used to be a poster who was a concrete casting expert an I'd hoped he might be able to shed some light.

In reply to Wainers44:

Money is an object, quite a major one in fact! I should maybe have stated in the OP that I'm an architect so am familiar with most standard construction techniques and materials, including GRC. You're right, that would be an expensive option, but GRG may be possible (certainly thinner) and I think would be a bit cheaper. But anything requiring special moulds to made up off-site for casting is going to add cost, so I had hoped that whatever the solution it could be done in-situ by your average builder rather than a specialist.

In reply to AJM79 :

Fair points - don't worry, it'll all be by the book. Pretty sure the 'Party Wall Act' isn't actually applicable in Scotland but thanks for raising it as a potential issue. I have already spoken to the neighbour and have an SE in place. I need to apply for a warrant for the other works being carried out so, if ultimately appropriate, this element of the works will be included in the application. Re your final suggestion - I think that even with all the best will in the world, painted timber will always look just that, not like shuttered concrete.

Once the project quotes come in, the wall will probably end up being painted plaster anyway (or I could take it back to bare brick) so this may all be academic!
Post edited at 09:29
OP Fraser 24 Dec 2017
In reply to JLS:

> Looks like a few experiments imprinting this stuff would give you the look you are after...


Interesting idea cheers, but imo the finished article looks more like a bad plastering job than concrete.
 ian caton 24 Dec 2017
In reply to Fraser:
"Very wet skim coat, quick shuttering"

Hmmm.

The devil will be in the detail.

If the coat is too thin, say less than 5mm, the pattern will go into that part of the render that has dried out from bonding to brickwork, and by so doing, in parts, break the bond.

If it is too wet it will slump and Slough off the wall.

Quick with the shuttering will mean minutes with your idea imho.

You need to be patterning the render when it is going firm due to set rather than from drying out. Thus giving you loads more time to get it flat etc. (Though adding time pressure at a different stage) But then you will need to apply reasonable pressure to get a good print.

P.s. I have done a lot of decorative overlay of concrete.
Post edited at 11:55
OP Fraser 24 Dec 2017
In reply to ian caton:

Cheers for the info. I suppose till I know how even the brickwork behind is, I won't know the theoretical minimum thickness of coating I would require. For the sake of argument, say the brick is very smooth, do you think 20-25mm of whatever coating would be reasonable?
 ian caton 24 Dec 2017
In reply to Fraser:
I would do rough and ready experiments. But would start with 10 mm. 20 way too much.

It would help if the pattern you use doesn't press too deep. Say 2mm. If the render is clay-like at printing, you will get a good crisp imprint.

But you will need to learn stuff like how the render behaves on the wall. The top will dry before the bottom as the water falls by gravity. So do you render the bottom first? Then where do you start printing to take account of how the set progresses?

Maybe you render it very steadily, so you know which bit is going to set first, but then it's always warmer at the ceiling than the floor. So would be tempted to start at the top so you know that if you can keep in front of the set you are on a winner. And if the last row is crap it's not so visible at the bottom..

Also how long a time window do you have to print? So how many people with how many bits of wood. And how heavy a club do you need to hit it with, so you have the right tools.. You will need a saw and off cuts to do little fiddly bits.

If you do it and you start printing at the bottom say, as you approach the top I would texture the top few inches before you print it, because the last row is bound not to fit and that would save you having to cut planks to fit when it is going hard. You can free hand plank end marks in with a bolster.

Remember with render, if a patch is bad or you didn't get it all stamped before it went hard, you can scutch it off and redo it.

You will need to research getting the bond to work. Part of me says you want to needle gun the lot so you are dealing with clean brick and a good profile.

Do it when it's cold, you will have more time.

I can't overemphasise how physically demanding this work can be. Plastering is a walk in the park by comparison. If you decide SBR is the way to go, it is really claggy stuff that doesn't trowel flat easily, but moves with the trowel. If the crispness of planking is important to you this could be a stumbling block.

Experiment, experiment, experiment.
Post edited at 14:53
 ian caton 24 Dec 2017
In reply to Fraser:

The wallpaper option looks good to me
OP Fraser 24 Dec 2017
In reply to ian caton:

Thanks very much for all that detailed information - it's very useful and, if I'm honest, making the wallpaper solution more attractive by the minute! Whatever finish I go for, it'll need to be done before a new stair gets installed against the same wall, so time will be at a premium when the contractor is on site. Based on your comments above, this is definitely an additional factor to be considered. The work won't be starting for several months yet at the earliest, so I've time to rethink and come up with something I can afford, that a contractor can do, and will give an appearance I'm going to be happy with.

Cheers all for the various suggestions and advice - there's plenty of food for thought.
 ian caton 24 Dec 2017
In reply to Fraser:

Render takes weeks to dry out.

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