BSC2021 - Daeothar's collapsed overpass

[Daeothar]

A little bit of progress, after sitting idle for about a week, while I pondered my options in regards to the building of the concrete parts.

There'll be no avoiding it; I'm going to have to build moulds and start casting my own 'concrete'!

Here's the current state of affairs; XPS foam ramp has been glued into place, and the same corrugated cardboard templates are still being used to mock it all up.





I've spent a pretty hour by now studying concrete bridge constructions and gathering pictures of both complete and collapsed bridges, so I've got a pretty clear idea of how this is going to look.

Here's part of the mood board I created, just to get an impression:



The plan now is to create the overpass (well a small part of it anyway) in 'concrete', using real life construction methods (in miniature), and then destroy it, as much according to my plan as possible, and work from there in finishing it.

The intact ramp would be looking something like this:



And when the first road section collapses, it will be canted and bent in such a way, it'll be a (dangerous) ramp down, back onto the table:



There'll be an interesting dilemma after this stage though, as I will be using pre-pigmented plaster, and the ground cover will also be natural, to match the cover of my table (sand, gravel, tilegrout and a bit of pigment). In fact there'll be very little painting to be done, bar the details! So will I stop putting up pics once I start the ground cover, or will I continue till I start the details?

Meh; that's for later. Right now, I'm going to build me some moulds...

[OSHIROmodels]

Looks promising and I’m looking forward to seeing the mould work 

Still think you should have a red shirt poking out the rubble 

[Daeothar]

Looks promising and I’m looking forward to seeing the mould work 

Cheers James This whole plaster 'concrete' thing is new to me, so I'll be just as interested to see the outcome!

Still think you should have a red shirt poking out the rubble 

I like the idea, so I'll see what I can do about that

So, I just tried to twist some rebar, but that stuff's more difficult than it looks. Even though this wire is extrmely flexible and soft, it's still quite the task to get it nice and evenly patterned, and this first try just won't do.



I might simply pop over to a DIY store tomorrow to get half a meter or so of chicken wire-mesh. You know; the 1cm square type. That'll be so much easier than trying to get this stuff to properly line up. I'll be willing to sacrifice some realism for that convenience. After all, most of it will be hidden, cast into the concrete anyway...

[Daeothar]

And putting my shopping slot reservation where my keyboard was, I popped over to the hardware store this morning, and bought me a 50x500cm roll of birdmesh. This is enough to last me 9 lifetimes (when I stick to miniatures, obviously).

I cut off a piece to fit the cardboard template I cut, and immediately it was clear that this looks ten times better.



So I then got to work creating a mould for the most important piece; the collapsed section. I will need two of these, as the part of the ramp that's up on supprts on the far edge of the base obiously will have to continue the same curvature.

I built it out of styrene sheet. There were several considerations for this: I have all the materials readily available, and I'm well versed in polystyrene construction. It's  got a smooth surface, and it's also quite flexible, which comes in handy when de-moulding.

Also; the only other obvious method would be to create a master, and then pour a mould around that. This is something I haven't done before, and getting the supplies is both time consuming (in research and lead-time) and expensive.

So I came up with this:





And it came together really quickly too.

The next step was fitting the rebar into the mould in such a way that de-moulding would not be an issue. I started out with styrene supports glued to the bottom, half the height of the mould. The rationale was that the small glue points would break off the bottom when getting the cast out of the mould, but I changed my mind after I glued them in.

Instead, I went for a hanging construction, with two beams with thin metal hooks supporting the frame from above, and this was a winning idea.



However, before I poured the plaster, I took out the frame one last time and painted it a chocolate brown colour, with some brighter rust brown stipling on top. After all, once the rebar was covered with plaster, it would be impossible to paint*.



Then it was on to the most exciting part of this session; the plaster!

I tinted the dental plaster I always use with some Vallejo weathering powders; Light Siena and Carbon Black. The result was a stunningly accurate concrete colour, but I will be sourcing some cheaper pigments for this, considering that quite a bit is required and the Vallejo powders are rather dear...

It was then that I discovered two flaws in my process. One: I should have made the mix a lot thinner. As it was, it dried very, very quickly, and pouring quickly became plastering instead. Not good; I'll remember that one for next time.

Two: The supporting beams holding up the rebar were flush with the top of the mould, meaning I could not smooth out the surface as much as I'd have liked, and together with the thickness of the plaster, I could not fully reach under the beams.

Only time will tell if the cast will be usable in the project, or if I will need to cast up a few new ones. This was pretty much a proof of concept trial, and all things considered, I'm pretty chuffed with it so far.



The de-moulding went really smoothly too.

I had feared that I would loose the mould after each casting, but the shape came out perfectly without even a hitch, and it is fully re-usable.



The surface of the cast is ridiculously smooth too. In fact it's so smooth that I will probably rough it up before painting on the stripes and weathering it. It's really perfect. On the top surface anyway; the bottom (which obviously was the top when still in the mould) is not smooth at all, due to the thickness of the plaster and the beams that were in the way.

Next up will be another one of these (possibly two), before I cut up the mould and re-use the material to create moulds for the other components. I'll need quite a few beams, tiles and at least one column. So I'll be casting for some time to come I think.

Which is not a problem; it's fun (as long as everything goes as planned of course ). But the funnest part will be breaking the concrete up, revealing the rebar underneath!




* I hope I'm staying within the rules here, but I had to paint them before starting the casting process. It's only a very minor detail of the entire build...

[OSHIROmodels]

Cracking stuff 

[Will Bailie]

I love it!!  One of the amazing aspects of this contest is seeing detail like this.  I have been on job sites where my responsibility was to ensure that the contractor installed the rebar correctly before he poured the concrete and hid it from view.  So I'm thrilled to see your miniature rebar.  And yes, I'll also be excited to see you smash it up, too!

[AKULA]

That’s dedication to your craft

 

[Billchuck]

Those look great, I love watching projects like this.

To support the rebar, you could loop a piece of fine wire around one of the junctions and twist it together into a tail.  Do 4 or 6 of these spaced out and cut them to length, and they'll serve as legs to hold the rebar at the correct height in the mold.

[Andym]

Fantastic! I love the whole process you've created!

Could you not scribe cracks into the flat plaster? There was a tutorial I ended up using a while ago for scribing into MDF. Seems to me that would work for this. Along with break bits and scribing, its going to end up soooo realistic! I'm really looking forward to this!

[carlos marighela]

Most impressive, that concrete looks very good.

[FreakyFenton]

Amazing work and top effort! 

[pistolpete]

Nice work. I suppose another option would be to pour plaster halfway, then add rebar then top off with more plaster. Kind of like the 1:1 scale models.

[gamer Mac]

Is your table going to be strng enough to support all this weight.
I take it you are going to build the rod supports out of plaster as well?
Going to look great when finished
you have got me thinking of projects where I could use this?

[Daeothar]

Is your table going to be strng enough to support all this weight.
I take it you are going to build the rod supports out of plaster as well?

It'll most likely survive this stress-test; I just re-enforced it with a steel frame, but a good thing I didn't decide to build a nuclear powerplant gone off, because that would require a lot of concrete.

Then again; I've always wanted to build a S.TA.L.K.E.R. table...

But yes; I will be doing the entire bridge/overpass construction in 'concrete', as this entry has turned out to be a bit of an experimental one for me, so why not go the whole length, right?

Some progress again though:

I cast a second curved piece, with rebar in it, but stupidly forgot to paint the rebar before pouring, so that one will remain a test piece I'm afraid.

A new method of holding the rebar in place worked very well; I simply bent four loose ends down and cut them at approximately 5mm, so the rebar stood on its own during pouring. It did not float up (much?) or move, and the 'legs' are not even visible on the eventual road surface (so there must have been some floatage). I will be using this simple method on all further pieces from now on.

After drying though, I discovered that the pigments were not in the exact proportions as they were in the first one, since I had simply eyeballed that one. So both of the first pieces would not be going into the final terrain, since I do want all the concrete parts to have the same(ish) colour.

I already mentioned how pigment-hungry this concrete mix is, so I went and got me a small range of artist pastels, that almost perfectly match the colours of the Vallejo weathering powders I used previously. A whole heck of a lot cheaper per gram too btw...

So three times' the charm, and I weighed every component of the mix this time (and wrote them down of course ), cut and painted the rebar, and poured the thing.

Turns out I had mixed exactly the right amount too, so double yay!

This mix is a tad darker than the previous two, but that's because I specifically wanted that, as especially the first one was way too light still. The mix will dry significantly lighter in tone though, so we're going to have to see if I hit the spot this time.

In the meantime, I have started experimenting with breaking up the re-enforced concrete of the first piece.









This take significant force! I've broken this piece by hand, and it took several localized breaks to look like collapsed concrete, but I'm chalking this one up as a full win! The results are awesome already , and I can see a lot of promise for the end result...

[gamer Mac]

Looking great
Where did you source the artist colours?