3m long concrete viaduct - "XPS foam" mould

ThomasDadDurham

ThomasDadDurham

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I havent made a thread for my layout build yet - but if you want a nosy and are still on the dumpster-fire that is Facebook, have a look Langley Sodor Railway Facebook.
Heres an overview of what I built in summer 2024, and where the viaduct will be going:
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Soldering iron and foam
So this method all came about because of the astronomical price of Urethane Rubber. I wanted to make a completely bespoke viaduct to fit in an existing spot, as I'd already built the raised areas and inclines. I had seen a sectional mould that an American had made via Google, which I knew I could replicate, but the costs when I started estimating climbed rapidly, 3k plus for each mould section, potentially double or even triple that, given the cost increases since that fella had posted his work on another forum years ago.So I figured as concrete formwork is traditionally EPS/XPS, why not carve the brickwork into XPS with a soldering iron. Heres my proof of concept test:
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I liked the 'stone' effect the soldering iron gave, so I started mocking up what I needed with a wood template, then given the dimensions and that of the space I had a 3D model done up (had a little help here as Im rubbish at Blender) - with a metal section for maximum clearance for the tracks running under, and a gentle curve.
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Next was finding someone willing to cut the forms for me - No way could I do this with a benchtop hot wire cutter, and the angles needed to be exact so it would fit in with the existing raised areas. Luckily I found one fairly easily: foamcutting.co.uk/. Heres the negative (the bits removed from the form)
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After a 5hr round trip, where I numbered all the bits on collection for re-assembly (they hadnt got the arches perfectly identical, so this became essential) the parts were brought back to my workshop, and re-assembled. I started reinforcing the sides by layering up thin ply, and clamping it while it dried. Finding an effective glue here was a nightmare. Resin-containing wood glues were recommended online, but they just didnt cut it for the wood-to-styrene. I did use wood glue on the layering of ply.
Contact cements melt styrene, so no go there. In the end, gorilla glue was the winner. But you have to clamp it well. LOTS of clamps. Because it wants to push the parts apart as it expands as it dries - what makes it so good for sealing two bits of porous foam together.
You can see as the ply is drying, the layer of paper I inserted to make a template for cutting the 'detail layer' of XPS.

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Marking up the XPS and carving. Oh, the fumes are very toxic so I had a professional filter mask on for this, plus my workshop has fitted extraction.
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Starting to fit it all in.
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An early test fit of the long section
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This is where Im at now!
  • I need to finish the 'short section'.
  • I need to drill through all the arches, run some threaded rod through it and use some big repair washers to clamp it all together. (Ever tried to find an 800mm drill bit? That was fun!) The F clamps alone arent enough for concrete, and if they failed the whole thing would just pop open! So its belt and braces. Threaded rod and big washers (as you would for concrete formwork normally) plus I'll leave the F clamps on the top as its poured in situ.
  • Rebar to be cut and bent to reinforce.
  • Im going to give the whole inside a brushed on coat of wax too, before pouring.
  • Ive got some cement colours to experiment with - going for a sandstone shade to match the stonework/remind me Durham viaduct. It looks like I need to use a 5:1 ratio of buff to brown, and go easy on it, as the colours are STRONG when you add a lot, so a small amount will dye the concrete from its natural silver-grey to more of a stone-y shade. I'll have to carefully measure my mixes to ensure even colour throughout! Doing various experiment batches is my next job.
Stay tuned!
 
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that looks like a lot of work.

i went the easy way. i made not a mold, but the bridge (4metre/12foot) with foam and soldering iron.

Stonebridge

SOUTHERN & GULF RAILROAD an indoors largescale modelrailway layout - S & G RR eine Grossbahn Modellbau Innenanlage. In English and German - Auf Deutsch und Englisch.
kormsen.info
 
korm kormsen said:
that looks like a lot of work.[...]
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It has been, BUT, its no less work than the rubber mould route and its cost me a few hundred pounds so far, vs a few thousand. Blender tells me I need 800kg or so of concrete (0.3 cubic metres volume approx) so thats another £150 in cement and ballast. For permanent and big a structure its going to be it seems worth it.
I had originally wanted to do a simpler route of cladding with bloxxs.de products, but Brexit induced VAT costs ruined that route for me :(
 
Paul M said:
I assume you mean 800mm long! Some masonry drill bits are pretty long, if it helps
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Not getting one of those through plywood :P
In the end theres such a thing as a 'formwork' drill bit, for exactly this purpose - getting threaded rod through large forms before you pour the concrete. Who knew! Took a lot of Googling before I discovered them. Theres wood auger bits up to 600mm, but no more, and its a 720mm mould (700 form, 100 styrene, 20 ply)
 
Hi , there are extension rods you can add to spade bits, 1/2" size upwards.( to clear the Ferrell on the extension)
They come in 300 and 600mm lengths. Just add as many as you need.
We drill down cavity walls when adding extra cables to a house.
You can go up to a couple of meters before they get a bit too wobbly to control well.
I have used them horizontally plenty of times too, just a bit harder to control. Maybe practise on some scrap, mock/up first.
Hope that helps, awesome looking mould hope the viaduct works out for you.
 
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The moment of truth is almost here. I have to move the trackbed of the down-ramp a touch but thats easily done once I have the mixer on site.

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Colour tests, waiting on these to cure. Im using very small amounts because I just want to *tint* the grey, not dye it a bold mustard or chocolate colour as these dyes are intended for:
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That looks excellent! When I cast a couple of over bridges in concrete quite a few years ago, I used hot glue to create the stonework courses but that didn't give the coarse stone finish which your method gives.

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More info here - How I cast two concrete overbridges

.... and ......


Rik
 
HobbitFertang said:
Very impressive! This should look really good. Great colour too. . . . . I love the smell of cement in the morning:):cool:
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It dries a lot lighter, it'll be quite a light sandstone-y brown. You can already see how much lighter the smaller section is and theres only 12hrs or so between them.
 
Picture taken with flash as Im in the shade.
20250520_131016[1].jpgSo I started removing in the point thats going to be least seen, the back of the short section (as there'll be a rockery there, where my rubble and waste pile is).
I poured it 24hrs before the long section, on Sat, and its now Tuesday, but its still probably too soon! There was a lot of crumbling you can see at the bottom right - also Ive a nagging feeling I didnt mix the concrete right for the second of three pours here, thats probably why that corner crumbled. I'll leave the front another day or two then try to do it.

It is a nightmare to get off. Individual bits of the pink do pop off easily with a screwdriver, but it doesnt just 'peel' off like I'd hoped slathering it in wax would do. I probably should have bit the bullet and painted it in a layer of urethane rubber 1-2mm thick - although for all I know, that would have torn and stuck in the details too and been just a different type of bother.

My method has become the following - tear and smash off the EPS white polystyrene in big chunks. Get as much off the XPS Pink foam off as possible with progressively finer tools, scraping following the pattern: Chisel, Screwdriver, Wire Brush. Then Blowtorch the awkward flecks (mask on) and wire brush again.

If the whole thing has big crumbly gaps, Im going to mix up a render and slather it on, and re-carve the stonework, but Im hoping its just this short section and a mix of my needing to give it another 48hrs/having messed up a mix on this section! *fingers crossed*
 
ThomasDadDurham said:
Picture taken with flash as Im in the shade.
View attachment 342881So I started removing in the point thats going to be least seen, the back of the short section (as there'll be a rockery there, where my rubble and waste pile is).
I poured it 24hrs before the long section, on Sat, and its now Tuesday, but its still probably too soon! There was a lot of crumbling you can see at the bottom right - also Ive a nagging feeling I didnt mix the concrete right for the second of three pours here, thats probably why that corner crumbled. I'll leave the front another day or two then try to do it.

It is a nightmare to get off. Individual bits of the pink do pop off easily with a screwdriver, but it doesnt just 'peel' off like I'd hoped slathering it in wax would do. I probably should have bit the bullet and painted it in a layer of urethane rubber 1-2mm thick - although for all I know, that would have torn and stuck in the details too and been just a different type of bother.

My method has become the following - tear and smash off the EPS white polystyrene in big chunks. Get as much off the XPS Pink foam off as possible with progressively finer tools, scraping following the pattern: Chisel, Screwdriver, Wire Brush. Then Blowtorch the awkward flecks (mask on) and wire brush again.

If the whole thing has big crumbly gaps, Im going to mix up a render and slather it on, and re-carve the stonework, but Im hoping its just this short section and a mix of my needing to give it another 48hrs/having messed up a mix on this section! *fingers crossed*
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Oh should have mentioned it, moulds normally oiled to assist removal but that woukd have possibly damaged the master in your case. Leave all for a week or so once the mould has been removed to fully harden off then as said cleaning with wire brush should not do much damage.

If you ever do another of these, possibly a slightly higher mix of 2 cement to 4 sand with a touch of washing up liquid in the water, SBR or Water Proof PVA in the mix would not hurt.

Your suggestion to flame out ant stubborn poly may work, but use a mask to protect from the fumes.
 
It might be that your stonework has been carved a bit deep, making it brittle. It does look good though, so hopefully after another drying session it'll be fine
 
Its definitely just a section where the concrete wasnt mixed properly, as I peel off the front its a lot tougher and no crumbling sections. I got someone to help me with the pour for the second section and he told me to let the dry ingredients mix for a bit before adding water. I was just bunging everything in at once and letting it mix, so its likely the short section I poured alone has lumps that arent properly mixed in.
Because I used a very fine 6mm gravel (the same one I use for ballast) the ratios were:
1-and-a-half bucket cement
2 bucket 6mm gravel
2 bucket sharp sand
2 cups 'buff' and 1 cup 'brown' cement dye powder (this produces a light brown sandstone-y shade)

The front - much nicer see:
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