Re: A stream for the Elderbury & District Light Railway
Posted: Fri Nov 09, 2018 3:04 pm
Well, I ve been a busy bunny for the last couple of weeks. Having got the basics of the stream sorted I decided it was time to get serious about the railway bridge over it. My initial thoughts were to make a mould and pour it in solid concrete, in situ. As I've posted before, I have previously made a bridge support using a plywood mould with the mortar courses applied to the inside with hot melt glue, but that was a simple slightly tapered box. This bridge is much more complicated, being on a skew for starters, plus needing the arch of course, plus I really wanted to cast the parapets as an integral part, plus I would need to cast it with the stream liner already in place. I started by making a kind of 3D template out of some foamboard, on the basis that its easy to cut and I could replace bits if the geometry came out wrong. It wasn't as bad as I feared so started to apply the mortar courses as well, thinking that perhaps I could reinforce it and actually use it as the mould itself rather than using ply.
At this point Greg was posting his Bridges of The Sandstone & Termite, thread, with a link to his web page on using Hebel blocks. I had seen this before but forgotten about it but the picture of his huge viaduct got me thinking. Why couldn't I carve a bridge from Thermalite blocks ( the UK version of Hebel)? It also jogged my memory that Rik used similar methods for Beeston Castle. So last weekend I made a trip to Wickes and bought a couple of blocks for a whole £1-60 each! They aren't actually Thermalite, Wickes sell them as "Aerated blocks" but they seem to be much the same. Although in theory 200mm thick Thermalite/Celcon blocks are available, as far as I can see you have to order a pallet load, individual blocks are only easily available in the 100mm thick version so I needed two and would have to 'glue' them together. They have rather rough faces which are bit difficult to smooth but I got rid of most of the roughness by laying the two blocks together and simply rotating one on the other like a pair of millstones. The one on the left is smoothed and the RH one is as they come.
I made a template for the shape of the arch which was copied onto one face. Then the two blocks were clamped rough sides togther and angled parallel lines drawn across the bottom to match the angle cut through the track base. The template was then used again to mark the opposite arch. Finally the two blocks were separated and the template used again to draw arches on the inner faces. I then used an old panel saw to cut the vertical faces of the arches and a masonry bit in an electric drill to chain drill the curves. The curves were then approximately smoothed to the required shape, although final shaping would have to wait until the two halves were fixed together.
Next I used the saw again to cut out the track bed from the top of each block, leaving a side piece sticking up to become the parapet wall in due course.
To fix the two pieces together I used Stixall, as recommended by Phil ( Lonsdaler) and it seems to have worked well. To hide the join on the inside of the arch, I made up a slurry of waterproof PVA with some of the sawn block dust, and used it as a grout. It dries slightly darker but is approximately the right colour - a grey line can just be made out in the picture below.
Next I cut the spare out of the blocks at each end by sawing down parallet to the arch sides and then sawing horizontally to meet them. This allows the bridge to drop into the hole in the trackbed with ( hopefully) enough play to allow for the stream liner and underlay.
I then cut the parapets down to the right height and had a minor disaster, one of the overhanging parapet ends broke off. It turned out that there was an internal airhole right at the junction with the main block which made it very weak. I tried using Stixall to glue it back together, and although it did stick, it left the joint rubbery and flexible, so I made another slurry, this time using SBR and block dust, and grouted the gaps all round. Again there is a darker shade but I think it will be ok once everything is finished.
At this point I started scribing the blockwork onto the faces.
The off-cuts from the top of the parapet walls were using to form coping stones which were glued on by liberally damping both faces and then painting with SBR. Once rigid I again grouted the gaps with SBR and dust.
The join in the broken parapet can be seen as a pale grey diagonal line towards the LH end of the rear parapet
In this picture I had started colouring one face before I remembered to take a photo!
This is the bridge with a first thin coat of Everbuild mortar colour (Buff plus a touch of Brown). It actually looks better, less blotchy, in reality than the photo, but I'm wary of putting on too much colour since I'm trying to get close to a pale Cotswold colour. I can always go over it with more, but I can't take it off!
Finally I took it outside and placed it loosely into position to get a feel for the eventual look - clearly it does need more colour, but I'm very wary of it going too yellow, like the top shows, and adding a little more brown tends to make it go a reddish shade.
At this point Greg was posting his Bridges of The Sandstone & Termite, thread, with a link to his web page on using Hebel blocks. I had seen this before but forgotten about it but the picture of his huge viaduct got me thinking. Why couldn't I carve a bridge from Thermalite blocks ( the UK version of Hebel)? It also jogged my memory that Rik used similar methods for Beeston Castle. So last weekend I made a trip to Wickes and bought a couple of blocks for a whole £1-60 each! They aren't actually Thermalite, Wickes sell them as "Aerated blocks" but they seem to be much the same. Although in theory 200mm thick Thermalite/Celcon blocks are available, as far as I can see you have to order a pallet load, individual blocks are only easily available in the 100mm thick version so I needed two and would have to 'glue' them together. They have rather rough faces which are bit difficult to smooth but I got rid of most of the roughness by laying the two blocks together and simply rotating one on the other like a pair of millstones. The one on the left is smoothed and the RH one is as they come.
I made a template for the shape of the arch which was copied onto one face. Then the two blocks were clamped rough sides togther and angled parallel lines drawn across the bottom to match the angle cut through the track base. The template was then used again to mark the opposite arch. Finally the two blocks were separated and the template used again to draw arches on the inner faces. I then used an old panel saw to cut the vertical faces of the arches and a masonry bit in an electric drill to chain drill the curves. The curves were then approximately smoothed to the required shape, although final shaping would have to wait until the two halves were fixed together.
Next I used the saw again to cut out the track bed from the top of each block, leaving a side piece sticking up to become the parapet wall in due course.
To fix the two pieces together I used Stixall, as recommended by Phil ( Lonsdaler) and it seems to have worked well. To hide the join on the inside of the arch, I made up a slurry of waterproof PVA with some of the sawn block dust, and used it as a grout. It dries slightly darker but is approximately the right colour - a grey line can just be made out in the picture below.
Next I cut the spare out of the blocks at each end by sawing down parallet to the arch sides and then sawing horizontally to meet them. This allows the bridge to drop into the hole in the trackbed with ( hopefully) enough play to allow for the stream liner and underlay.
I then cut the parapets down to the right height and had a minor disaster, one of the overhanging parapet ends broke off. It turned out that there was an internal airhole right at the junction with the main block which made it very weak. I tried using Stixall to glue it back together, and although it did stick, it left the joint rubbery and flexible, so I made another slurry, this time using SBR and block dust, and grouted the gaps all round. Again there is a darker shade but I think it will be ok once everything is finished.
At this point I started scribing the blockwork onto the faces.
The off-cuts from the top of the parapet walls were using to form coping stones which were glued on by liberally damping both faces and then painting with SBR. Once rigid I again grouted the gaps with SBR and dust.
The join in the broken parapet can be seen as a pale grey diagonal line towards the LH end of the rear parapet
In this picture I had started colouring one face before I remembered to take a photo!
This is the bridge with a first thin coat of Everbuild mortar colour (Buff plus a touch of Brown). It actually looks better, less blotchy, in reality than the photo, but I'm wary of putting on too much colour since I'm trying to get close to a pale Cotswold colour. I can always go over it with more, but I can't take it off!
Finally I took it outside and placed it loosely into position to get a feel for the eventual look - clearly it does need more colour, but I'm very wary of it going too yellow, like the top shows, and adding a little more brown tends to make it go a reddish shade.