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Building a Concrete Viaduct


mick
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You'll most likely have seen from my layout thread that the Selby Garden Railway has a lengthy viaduct of wooden construction but due to the necessity to constantly protect it from the elements I am considering replacing it with a more permanent structure made from concrete. Does anyone have any tips or advice for making a concrete viaduct?

Firstly there's the required shuttering and the problem of how to produce the series of arched shapes. I've seen polystyrene used on one occasion but it looks messy. Plastic guttering would seem the best way to form the curve top of the arch with wood running down the insides of the arch from there but perhaps you can think of something more suitable?

Secondly there's the type of mixture to use for casting the actual viaduct. It seems to me that the smaller the ballast used the finer the resulting cast but what about the overall strength of the finished piece? Would internal reinforcement be advised and if so, what should I use? I've also seen some substances other than your normal sand & cement mixture which claim to give a much finer finish and a stronger product for outdoor but they are rather expensive.

I'm also unsure of the best way to secure the track to the top of the viaduct once it is constructed. I've thought about inserting short lengths of wood at 90 degrees to the tracks and setting them into the concrete a short distance apart so that the track can be pinned to them. Are there any other ways?

Mick

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  • 1 month later...
  • 2 months later...

G'day Mick, I have considered making a concrete viaduct myself and had a quote for one, price? $10,000 or 4,500 pounds eeeeeeeekkkk!

But for your almost ground level line get a copy of the Model Rail DVD "Garden Railway Expert." that may help you quite a bit. I had intended to put mine in the bottom left corner of my block. (where the blue King and 8F climb away in my you tube video.

Roy.

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$10,000 for a model viaduct? What size were they quoting for?

On reflection I just might discard my viaduct section in favour of a platform/station area. I'm not sure I'm up to the task of constructing a concrete one at the moment and the other 'to do' jobs are rapidly stacking up.

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Mick, I think it would be a great shame for you to remove your current viaduct, it's majestic.

Have you considered a better covering system to protect it. In the past I've used plastic cloches in my garden and they were a great way to protect plants in the spring. A google search for "plastic cloches" will give you all kinds of shapes, sizes and systems which may well be a lot easier to work with than your current way of working. You may be able to find something that allows you to run trains without having to remove the cloche.

Just a thought...

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chris said:

Mick, I think it would be a great shame for you to remove your current viaduct, it's majestic.

Have you considered a better covering system to protect it. In the past I've used plastic cloches in my garden and they were a great way to protect plants in the spring. A google search for "plastic cloches" will give you all kinds of shapes, sizes and systems which may well be a lot easier to work with than your current way of working. You may be able to find something that allows you to run trains without having to remove the cloche.

It's in a bad way Chris and I didn't take enough care with the construction hence the reason that the track on top is all over the place. It would also be better if positioned elsewhere because even though it's good to see, it's not easy to get any great shots of trains going over it without getting the house wall in the background. I'd almost made up my mind to replace it and while it will need replacing, perhaps I should look again at using concrete? The viaduct was my incentive to build - maybe now I know a little more I can make a better job the next time.

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  • 5 months later...

Hi Mick,

Jaybee and I are collaborating on his project - we've been friends for some years and naturally I jumped at the chance to help out, especially since my own project has been stalled due to 'austerity' measures and animals on the lines.....

He also wants to have a viaduct running up a good 4 metres of the garden, and we've spent a lot of time over the last few weeks researching the best way to do it. If you've got the money to throw at it, Garden Railway Specialists do some great pre-cast concrete modular segments, but for our purposes that worked out as a few hundred quid. What we've settled on, unless someone comes up with a better idea, is to use garden decking planks to form a basic structure, i.e. struts embedded in the ground and above to the correct height, and planking across for the trackbed. Once laid, the exact size for the arches and frontage will be known to the nearest millimetre.

Using polystyrene sheet, we then plan to carve to shape, then add the stonework detail, most likely using a heat prong tool (can't remember the exact name), then spray using Plasticote's polystyrene primer, and paint / dress appropriately, then affix to the wooden strut base. The under arches would then be built up and shaped using a fine mortar and painted to match.

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I've not entirely given up on the idea of building a proper cast viaduct, in fact I've been thinking it over again recently and looking at how others have tackled similar constructions.

If you can keep it watertight then a wooden framework should suffice and working with timber is a much easier task than messing around with concrete. Using the tarpaulin covers I can keep the rain out but I would really like to have the viaduct exposed all the time, open to the elements so that it weathers naturally. I have to say that the plywood on my viaduct has stood up fairly well over the past 2 years now but that's because I cover it whenever there's a chance of rain.

I rather like the modular approach which would mean making a mould from a master pattern to be cast in stone or concrete. There's some good ideas to be had by looking at the http://www.cainhowleystructures.co.uk/viaduct--bridges.html' rel="external nofollow">viaduct and bridges manufactured by Cain Howley Structures. If you can make the 2 master sections and produce the necessary moulds then you could make the viaduct whatever size you wished and at a reasonable cost - certainly less than some are quoting!

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  • 10 months later...

Hi Mick

As I said in my original post in the Planning forum I too have been thinking about concrete viaducts :D

Here is a poorly drawn idea for the basic pier former.

Pierformer.jpg

the loose block at the bottom of the drawing is screwed to the end of the former, in effect making a wedge shaped trough 6" wide at the bottom, 4" wide at the top & 4" deep. The length is up to you, I will need to make several of these formers, some as wide as 9" at the bottom, as my terrain slopes and I will need longer piers. I will be using fencepost concrete, as this sets up in less than 24 hours. The former will be well rubbed with candle wax to aid release of the pier and will be screwed together just in case it doesn't release ;).

I am working on the arch former, this will be constructed in a similar manner.

With this I think I will be able to make a kit of parts for a 6 foot long viaduct in one week!

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It seems to be that casting whole stretches would go faster.

If you come up with rounded top inserts for the gaps and make plywood sides you can just cast a whole section of viaduct in place. If you want it curved you just curve the ply sides.

You could make the curved inserts out of cut foam. Cut the foam with a hot wire.

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Would casting the whole lot in one go not make it brittle, Griff?

I was worried that I would pull off the formwork (we call it shuttering :) ) and the whole lot would collapse, but I suppose you could put some reinforcing bar into the ground where the piers are, and lay mesh along the deck above the arches?

You method is a good idea tho, have you used it yourself? (or seen it used?)

Hmm, foo for thought!

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mick said:

$10,000 for a model viaduct? What size were they quoting for?.

Originally Mick it was to be a three arch structure over 10m in length and 1.5m high. The builder had done a lot of work to my house but when I said no to the viaduct he refused to do anymore. I found that odd as no cheque that I'd given him previously had bounced and I just looked on it as sour grapes on his behalf. He was a local bloke and I recently saw him in town. I said G'day and he just ignored me. Odd.

If you do reconsider putting in a viaduct perhaps you could use rubbercrete on top of it to make the track bed and use large diameter PVC piping to form the archers with Thermalite blocks to form the piers. I know that sounds a little Heath Robinson but it could work and be stable and solid.

Roy.

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  • 3 years later...

Hi Mick. Have you thought of upvc soffat board to form your arches? It's flexible, cheap, and if you put some clingfilm over it before you pour in your concrete mix that should help release it when the time comes to remove the moulding. Hope this helps. Mark :D

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I gave up on the idea of casting a concrete viaduct Mark. Firstly we had an unexpected house move which brought an end to the Selby Garden Railway and then after starting my new layout, Worsley Dale, I came up with the idea of using those lightweight aerated blocks to form the viaduct piers and the rest is history. I certainly wouldn't ever contemplate casting a concrete viaduct now that I've used aerated blocks. The blocks are so much easier and in my opinion they give far better results.

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