Making moulds...
...for casting your own bridge piers and abutments.

I thought that I had previously brought this over from the old place, but, since I can't find it anywhere, I'll start afresh.

Here's a simple method for making your own bridge piers and abutments, and it will work for tunnel portals and retaining walls, too.  With a little more work, you should also be able to create cast wall sections, which can be assembled into structures.

All of the bridge piers and abutments shown below were cast in Durabond 90 patching plaster, the same stuff as I used over screen for my scenery. It contains a catalyst, and will set in approximately 90 minutes, regardless of how thin or thick you mix it. When it first sets you can file or carve it fairly easily, but it becomes very hard when fully cured.
I made some simple 2- or 3-piece moulds from .060" sheet styrene, wiped the interior surfaces with some vegetable oil on a paper towel, assembled the moulds, then poured the plaster. For the piers shown in the first picture, I made only one mould, taller than the tallest pier, and designed to sit upside down for filling. Then, each pier was poured in succession to a height just a bit taller than what was needed in its final location. When removed from the mould, it was easy, using a mill file on the bottom surface, to bring it down to the proper height. On the piece that formed the bottom of the mould (which is the top of the finished pier), I cemented two small squares of .060" styrene, which left indentations in the top of the cast pier. I then cemented similar-size pieces on the bottom of the "steel" bridge supports, which fit into the indentations and keep the bridge in alignment. Like all of my bridges, they're removeable as a unit, simply by sliding the rail joiners clear at either end of the span. The abutments at either end of the bridge were cast in similar type moulds, but each one built to suit the site. When casting, by the way, the moulds were held together with elastic bands.

This is the mould for all of the piers of the bridge immediately behind the mould. As you can see, it's quite a bit taller than needed here, or indeed, for any of the piers on this bridge - you never can tell when it might come in handy for another, taller, bridge, though.  I simply made a pencil mark inside of the mould to denote how high the plaster needed to be poured, allowing a little extra for trimming.


It's built of .060" sheet styrene, with more of the same material used as stiffeners along the edges. The mould walls and the stiffeners are stepped at the edges, so one face interlocks and overlaps with the adjoining pieces.


This is the bottom of the mould, which will be the top of the cast pier. Note how there's a larger piece upon which the mould sits while being filled with plaster, with a smaller piece which fits inside of the vertical parts of the mould. This is the form for the top surface of the pier. You can also see the two small squares of styrene cemented to this which will form the two bridge-locating indentations in the top of the pier.


Here's the mould completely disassembled. I used an X-Acto knife to score and scrape lines to represent the form boards which often show on the prototype, although they're not very noticeable on the pier castings, especially under fluorescent lights. If you use Durabond for your castings and want to add carved-in stonework detail, do so immediately upon removing the part from the mould - as it cures, it becomes extremely hard and more and more difficult to work.


And a different view:


The large styrene blocks on the exterior of the mould are to keep the elastic from slipping out of place during the pouring process - in the first two pictures, the centre elastic should be above the blocks, not below.

The abutments for this bridge were somewhat complicated by the fact that the basic scenic landforms were in place, but a few judicious smacks with a hammer "adjusted" any protruding elements. I cast the abutment in the mould shown, then, after it was installed, used the flat pieces, propped and braced as required, to cast the wing walls "in situ".
Here's the mould for the support abutment:



Of course, the mould would sit like this to be filled, propped up and supported by scraps of wood. The bottom of the freshly-cast abutment was trimmed with a knife to roughly match the contours of the "ground", then set in a thin bed of wet plaster to affix it in place.


Because of the "step" required for the bridge support members, I had to make the bottom of the mould more complicated than that for the piers. Here it is partially separated (the "tab" protruding from below the lifted section is designed to slip over the outer face of the part below, sliding between the raised stiffeners on either side):


And from the opposite side - the narrow vertical piece on the lifted part fits over the outside face of the rear of the mould, and between the stiffeners on either side, effectively interlocking the parts of the mould together.


Here's the same mould completely disassembled. Note how the edges of the adjoining sections are stepped to lap one another, and the tabs that help to hold everything in alignment. Even with the tabs, use ample elastic bands to ensure that the mould stays together. The plaster is fairly heavy, and it's also a good idea to vibrate the freshly-filled mould to help remove any air bubbles, although any surface defects can be filled with a bit of fresh plaster applied after the mould is removed.


Here's a mould used for a different bridge, one where the spans are of different depths. Again, the mould is designed to be filled while upside down, as shown, and is quite a bit taller than required for this particular location.


And a view of the three pieces which comprise the mould. The sides sit atop the largest piece of the base, with the next smallest area forming the bottom of the mould (and what will be the top of the pier). The rectangular block will form the seat for the deeper of the two girders shown in the previous picture.


The sides of the mould were made perpendicular, as the finished pier is fairly short, but you could also do a taller one with tapered sides and a similar top. The "form" detail is visible on the inside face of the mould. I used a double thickness of material for the wide face, and narrow stiffeners on the end pieces to attain the required overlap at the corners.

After the plaster pieces were thoroughly hardened, I coloured them with a wash of thinned PollyScale paint, then glued them in place using yellow carpenter's glue. Even though they've been in place for a while, they've yet to be weathered.

I hope that some of this information will be of use when constructing your bridges.

Thanks for bringing that thread over Wayne. I was looking for this one a couple of weeks ago to show a fellow modeler, but couldn't find it. I never thought about the old place. I just don't go there any more.
Just now replaced the photos in this thread. If you happen to come across one of my similar "how I did it" type of threads where the photos are missing, please send me a PM and I'll try to restore them as soon as I can (and if I can find the photos Crazy Misngth ).

Great article which I am happy to have seen the link on MR.
What was the paint color mix formula you used on hese cement pieces?
Thank you--jd
Hi jd, and Welcome to Big Blue - it's good to see you here.

As for the paint, I honestly don't recall. I may have started with PollyScale's "Aged Concrete", but I usually just dump in a little of this and a little of that, until it looks acceptable to my eye. The only time I make a record of a colour mix is when it's to match a specific prototype, such as a colour for a diesel or unusual freight car. For example,I've painted hundreds of freight cars, and few, if any, used a "boxcar red" right out of the bottle. Even for those painted in the same session, I will alter the colour as I paint, so that only a few will be the same. And of course, once they've been weathered, it would be difficult to find an exact match.
I'm not sure what you have in the way of paint on-hand or what's currently available, but I'd suggest that you check out prototype examples in your area (take photos, if possible) and then select a paint which is close. I don't really understand too deeply the machinations of colour mixing, but I seem to have a pretty-good natural affinity for it , so it's difficult for me to suggest specific colours you might use to alter the basic colour with which you choose to work. While you can lighten with white and darken with black, other light or dark colours can also accomplish changes - for example, yellow will shift red to orange, depending on how much you use.
When you look at prototype concrete, try to forget any preconceived ideas about what colour it should be, and instead look at the colour it actually is. Some new concrete looks pretty grey, but older stuff is seldom grey at all - try starting with a light grey (easier to darken a paint than lighten it), then add some yellow - do this on a scrap of plastic or other impermeable surface, using "brushloads": dip a brush in your basic colour 2 or 3 times, deposit it on the plastic, then wipe the brush (no need to fully clean it), and quickly dip it once into the yellow, and stir that, using the brush, into the grey. How does it look? What do you think it needs? Since you're not using great amounts of paint, it's easy to simply play at this until you start to see how the colours work with one another.
Another option is to use paint which has been labelled by the manufacturer as "Concrete" (aged or not) and either use it as-is or alter it, then, once your concrete work is dry and fully cured, alter it further with weathering. For weathering, you can use a wash of thinned paint (if water-based, adding a drop of dish detergent to the thinned paint will allow it to flow more readily) or weathering powders, or you can use an airbrush.
If your first attempts at colouring the concrete or weathering it are not to your liking, don't be afraid to go back and redo it. A concrete pier has little in the way of details which will be obliterated by more paint. Wink

Oh, and one more thing: if you're in the U.S., your paint will have no "u" in its colour. As far as I know, that won't affect your results. Crazy Misngth

Hi jd and welcome to Big Blue, its a good place to be, lots of interesting stuff, friendly people with a whole range of knowledge on different topics.

Having worked in building and construction, I can verify doctorwayne's method of casting and his observations on the colour of concrete.
Would you believe that when freshly poured at the initial set and for the first 24 hours or so, concrete is actually green both in colour and how it is described.
This is the time to strip formwork and also clean threads on holding down bolts, as the concrete is easy to clean off or chip away and the curing slab provides
warmth to any residual curing water so your hands dont get cold. Leave it a day or two longer and it becomes a different story and the water is now cold. No fun. Nope
The slab then goes very white, blindingly so in the harsh Australian light, and then after another couple of days it finally starts to go concrete grey and then dirty
from all the building activity going on. This describes natural, run of the mill, non coloured concrete, concrete with mix plant added colour plays differently.
Then over the years concrete ages and gets all sorts of stuff spilled on it or ground into it and takes on a very unique character.
An example of this would be where the roof overhangs a concrete path/driveway it remains remarkably clean and smooth, but where exposed to the rain it becomes
dirty looking with a rougher finish.
Start looking, I mean really looking at the concrete things around you and you will notice just how varied it is.
Fake It till you Make It, then Fake It some More

Thanks for that very complete explanation and good series of photos, particularly the replacement of photos.

I'm going to have to take some of that data into account when I cast my pieces for my Thomas viaduct

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