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Spawling Concrete

Started by marc_reusser, May 23, 2009, 02:11:53 PM

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marc_reusser

A while back I posted this comment in a thread over on the MIG forum...and I thought it might be of some interest here.


"The plaster wall post for Jacqs stucco technique reminded me of a sim technique that can be used to represent chipping and spawling concrete surfaces.

Using plaster/Hydrocal, the initial pour/layer has some small grains of silica/sand added it (to represent the aggregate in the concrete)...this is then allowed to dry, and can be stained as needed (or the entire piece can be stained/finished later). A second very thin layer of just plaster is then applied over it, and finished or sanded smooth (but not enough to show/wear through to, the first layer)...this top layer can then be chipped and flaked off in areas to expose the coarser (\"inner core\") part of the concrete (the inner core/first layer can also be chipped deeper at the same time if desired, to vary the effect). I will try and do an SBS of this as part of an upcoming project. "



Ken (Marken) from over on RRL inquired weather I had done an SBS yet; I told hom I would promptlydo so......and so ....last night was a real comedy of errors.....and so far the experiment has been fraught by missteps on my part.....though the results so far have been "wanting" to put it kindly......I do see that there is potential in this approach, and could yield some very nice results if done right...especially in the larger scales like 1/35 to 1/16.

So despite my reticence to post this.....I will try and put my money where my mouth is...[in my defense I tried to work fast and took a lot of shortcuts :roll: :D ].... here is how it went so far:

I made a random sized mold by sticking some balsa wood to a piece of scrap Masonite, using the 3M double sided mounting film/tape.



I did not have anything on hand that felt like the right size for aggregate, so I crushed some Arizona Rock Co scenic material in a mortar & pestle.....it was going to have to do for this (an HO or N-scale ballast would have worked better).



I wanted to try doing some exposed rebar, so I took some .020" brass rod and made a small piece of reinforcing web. In the photo the top & right intersections have been soldered, while all the others have been tied, like the prototype, using .006 brass wire. Once done the whole piece was dipped in a blackening solution (like "blacken-it" only in bulk from a hardware & restoration supply store)



Next was how to create a textured finished sub surface for the future spawling......once again I improvised, and used some too coarse patio paver sand that I had in my back yard...this was mixed with some of the crushed rock, and spread in a thin layer on the bottom of the mold. Next, following Marcel Ackle's method, the rebar web was set in a small pile of sand (the sand acts as a barrier for the plaster...so wherever the web is covered with sand the plaster will not encase it.)



This is where I then had some headaches...and definitely an area that need refining and further experimentation. Crushed stone was then mixed with Hydrocal roughly a 40 stone/ 60 hydrocal, mix...to this I added water, tinted with some Woodland Scenics "Concrete" coloring. Either because I played with the mixing too long, or something with the coloring or added rock, but the plaster seemed to "kick" very oddly and quickly as I was pouring it onto the mold, and I ended up having to spread it around using a trowel (not really what you want to do over a loose sand base, as it forces the base into the plaster mix a bit too much.) This image shows what will be the back, non-visible side of the pour. After the image was taken and the plaster had set a bit bore I scored this so I could more easily break it into four pieces.



Being the impatient guy that I am, I broke apart the mold while the casting was still very damp and the mix was still not really set....but i was somewhat concerned that if I waited till it was to dry, the sand layer would embed itself too much in the pour, and become difficult to remove/clean. In the image, on the left you can see how a piece looked when it was removed from the mold, on the right is another piece that has had the sand brushed away/off. [One serious drawback working with the pieces still this damp is that they are extremely brittle/flakey, and the surface is very soft...so the brushing tended to soften some of the coarser surface texture I wanted from the sand.]



Here are the other two pieces after being completely cleaned. I bent the rebar web a bit using tweezers, holding it so as to make sure not to put any stress on where it was embedded in the casting. Note on the rebar piece how softened some of the surface and broken edges are....this is the issue that I was referring to re, the brushing.



These pics were taken this morning after allowing the pieces to dry/cure. Despite adding what I thought was a lot of tint to the plaster mix, this photo shows the color comparison between my pour and the concrete step in my backyard. Even taking into account "scale effect", it is still too light and will need to be corrected with later staining.



This is a close-up of the exposed rebar piece. Hopefully I can make the edges a bit crisper in the final piece by careful additional chipping. [For scale reference, in 1/35 scale, the rebar grid is about 18 scale inches (45 scale cm) on center.]



I have a number of things that I would tweak/address on another go around,...but most obvious is dealing with the proper sized aggregate, adding some rib texture to the rebar (probably by rolling the rod between a hard surface and a coarse file) and being more patient with the drying and removal process.

Next will be adding the "surface finish" over these pieces, and seeing how it will work with the spawling idea. If I am not too many sheets to the wind this evening, I will give it a go.


Cheers,


Marc
I am an unreliable witness to my own existence.

In the corners of my mind there is a circus....

M-Works

marc_reusser

#1
Just a tiny bit of quick progress (can't do more because my three-legged cat has decided my workbench is a nice place to nap this afternoon.)

I did some quick initial stain/washes to the pieces using a very diluted random mix of Panzer Aces #315 "Light Mud" and Model-Color #986 "Deck Tan".  The original color can be seen on the piece in the upper right (this is the un shaded/washed back of the piece). This will later be augmented with some greys, once the finished plaster layer is applied.

The rebar pieces also received some color using a semi-dry brushing of LifeColor #UA-701 "Rust Dark Shadow" (from their diorama series) and Bragdons "Dark Rust" pigment.





Marc
I am an unreliable witness to my own existence.

In the corners of my mind there is a circus....

M-Works

shropshire lad

This is an excellent experiment and it looks like , with a few more attempts , you are going to come up with a very realistic concrete . I shall now look at any sand that I use in a different light. Not so much the results I might get in 1:1 scale but more the results I could get in 1/35th scale .
  I have had similar funny results when I've added WS pigments to plaster . I guess if you know that setting times are altered you can work around it .
  I wouldn't get too hung up about the final colour of the concrete because , as you know , there is no such thing as , the right or wrong colour, as it will vary from region to region and the differing colours of the sand used . I would be happy with any one of the colours you used .

  This looks like another tutorial that will get filed in the " new technique" folder .

   Nick

marc_reusser

Thanks Nick...don't save it yet...could still all go to hell. ;D


Hi Dave:

I had considered painting the rebar before pouring.....and were it a more complex piece with small areas of rebar showing at divots or opengs in the slab, i would probably do so....at least the initial base drybrushed color. Applying the pigments prior to casting it, could potentially create an interesting effect if the pigment bleeds a bit into the surounding plaster...sort of like real rust...but it could potentially be tough to control. Even with pre-painting though there would probably be a fair bit of touch-up and weathering needed on the rebar after the casting, cleaning, staining, etc.

Most of the changes I would do are those mentioned in the SBS so far, along with much tighter "quality control" , things like pour consistency, making the molds the correct height for the final slab thickness, trimming it clean and flush to that height. Taking more time to make the sand layer in the mold more even. All these things would make the coming steps of "surfacing" the slab pieces much easier, consistent and controllable......as  it is I will have to continue "winging it"

Marc
I am an unreliable witness to my own existence.

In the corners of my mind there is a circus....

M-Works

Krusty

That's some superb concrete and the technique shouldn't need too much more development to sort out the few minor issues.

Seems to me that it would be particularly useful to look into some sort of threaded rod for the rebar in order to reproduce the way traces of concrete stick to it when exposed. I recall a 1930s concrete bridge being demolished near here last millenium. It looked like a WWII war scene for a couple of weeks, with twisted reinforcing sticking out of the remains of the abutments, but the colour was grey, with no trace of rust for about a week. The attached photo shows a wall that had been broken for well over over a year when photographed (and there's plenty of rain and saltspray hereabouts to promote rust growth), yet there's still plenty of evidence of concrete on the rebar.
Kevin Crosado

"Caroline Wheeler's birthday present was made from the skins of dead Jim Morrisons
That's why it smelt so bad"

shropshire lad

Kevin ,

   Those bars look rather painful if you landed on them . The photo may be deceiving but the concrete looks pretty close to the road ,so  I'm surprised that they are allowed to stay there by the "powers to be" .

   Nick

marc_reusser

Nick:
That's probably in that country down under filled with hearty former British criminal stock....a few rebar punctures to them is no big deal....besides by their standards you would be considered a fool for impaling yourself in the first place.  ;) ;D

Kevin:
Thanks for the photo. Good reference. I wish more of the plaster had remained/accumulated on the bar....it is one of the things that I would change next time around. I think though that the directional aspect of treaded rod would be too noticeable if used for the rebar, that's why I was thinking of the annealed wire rolled both directions under a coarse cross-cut file, so that the pattern is not directional, yet gives the indentations for the concrete remains and rust to accumulate.
Would you mind if I used, and credited, the photo over on MIG?


Marc
I am an unreliable witness to my own existence.

In the corners of my mind there is a circus....

M-Works

jacq01


    Marc,

     good SBS.  Usefull as I am preparing the concrete footings for the mill and the casing for the dam part.

    why use plaster ?
    I have done in H0 a concrete road ( plates with cracks ) with grouting cement some 12-15 years ago.
    For the mixture, very fine (extra sieved) river sand was added as the mixing instructions.
    The liquid used was water with approx 15% white glue and a few of drops of washing liquid.

   
   
     The terrace and bike path are made as described. Carved while still wet.


   

    The road approx 6" wide.  Thickness  1/8"

    The module is still in use with the club  ;) ;)

     Layering can be done with gesso and cement/sand powder similar as the plastered wall on the mill.
     Experimenting will be necessary as I don't know how wll gesso adheres to concrete.

     


      Jacq

 
put brain in gear before putting mouth in action.
never underestimate the stupidity of idiots
I am what I remember.

Krusty

#8
 
QuoteThose bars look rather painful if you landed on them . The photo may be deceiving but the concrete looks pretty close to the road ,so  I'm surprised that they are allowed to stay there by the "powers to be" .

Umm, actually, it is right at the edge of the road, but it was the powers that be that put it there. It's at the start of a steepish section of winding road. The locals complained about corner-cutting trucks being a threat to pedestrians, so the council bureaucrat solution was to build a wall along the edge of the footpath to keep the trucks back. I guess the fact it only took a couple of weeks before the first bits were knocked out kinda proved the residents' point. Strangely, the wall was patched a few months ago and so far has held up fine. Perhaps it bit back sufficiently hard the first time to make the truckies take a bit more care now.


QuoteThat's probably in that country down under filled with hearty former British criminal stock.

Nah. That's the West Island. We're much nicer over here.


QuoteWould you mind if I used, and credited, the photo over on MIG?

Fine by me.
Kevin Crosado

"Caroline Wheeler's birthday present was made from the skins of dead Jim Morrisons
That's why it smelt so bad"

lab-dad

great ideas Marc.
Between you and Chuck some real concrete ideas! ha ;D ha ;D ha  ;D
I have been thinking of ideas to achieve the texture for the rebar but have come up flat
May be the rum will be helpful?.....
good stuff! (the sbs)
-Marty

marklayton

Marty -

Faithfully duplicating the texture of rebar would be tough at scale, but from many angles, it looks a bit like coarse thread.  It might be possible to use a lathe with a cut-off tool of the correct width for the spacing between the ribs, and pick the correct threading rate to cut a sort of coarse thread.  The only problem would be supporting such thin rod, so I suspect that only short lengths would be possible.  Maybe it would be best to hand-carve a 2" long master from brass and have it reproduced by an investment casting shop.

I enjoy working with rebar in the forge.  Twisting can produce some interesting patterns.

Mark
He who dies with the most tools wins.

marc_reusser

#11
Thanks for all the comments & input guys..

After a run to Home Despot yesterday, I was able to move forward with this experiment.

First all the the pieces were sanded smooth at the edges. At this point I was not yet sure if I wanted to try making a larger surface consisting of 4 pieces for this experiment, or just doing small individual ones. Just in case, I made sure they would all line-up with each other.



A  frame was made from Balsa that was just slightly higher/deeper than the pieces. Tamiya masking tape was used and shaped where I would later want the spawled effect at the broken edge.




Into this frame was troweled a mixture of Spackle, WS "Concrete" coloring, and finely sieved (through nylons) crushed stone (same one as used for initial pieces).



Once dry the frame was removed, and I started experimenting on one of the pieces.

Here is the overall view of the piece, after using an Xacto to spawl the edge, and some surface areas. The piece was then given very thinned random washes of the Vallejo acrylic, followed by random washes of grey & black water colors. When dry, it was given a random dabbing of some Bragdons powders, which were then disoolved/dispersed onto the piece using Odorless Thinner/Turpentine. Plant material is a combination of seed pods from my yard, Woodland Scenics ground foam, Silfor grass, and the leaves are pieces of dried spices (parsley). The dirt is dirt. The barrel is 1/35 scale.









I am not really satisfied with the results. There are a lot of issues that bother me:

1.) Due to the extremely uneven initial surface (because of poor fore-thought and quality control), the spawling layer ended up being too uneven in depth...and just too thick overall, thus not creating the sharp shallow spawls I wanted. I had to go too deep in many places to reach the initial surface, and thus not achieving that thin hard edged spawl.

2.) I should have given the plaster a full day to really cure. When applying and working the washes, the brush too easily smoothed out some of the edges and surfaces of the spawls (this is especially noticeable on the one at the edge).

3.) The two problems above combine to give the piece more of an appearance of old stone/limestone rather than concrete.

4.) The dirt is too coarse and too loose, for what one would most typically find in these kind of divots.  I also feel the color is off. This would likely be represented better by using primarily pigments (held in place with the pigment fixer) for the dirt, with just a few small real dirt pieces added in to represent a couple of pebbles.

There are other smaller issues as well, but they are mainly due to my need to practice a bit more of the coloring and finish.



Marc


I am an unreliable witness to my own existence.

In the corners of my mind there is a circus....

M-Works

marklayton

Found a couple of images of spalling concrete...





Mark

He who dies with the most tools wins.

Ken Hamilton

Lots of interesting problems & solutions in this thread. 
Concrete is tough to duplicate and all these ideas and techniques are greatly appreciated!
Ken Hamilton
www.wildharemodels.com
http://public.fotki.com/khamilton/models/

marc_reusser

#14
Thanks for the comments and input. Thanks also for the pics Mark.

The type of spawl appearance (sans the rebar) where the smooth outer surface gives way to the coarse inner is what I was basically after.  I need to keep practicing/working on making the more even & thinner top surface.

Much appreciated.


Here are some more pics of examples:


This one shows the start of a spall/spawl (can be spelled both ways):



Here on some older concrete you can see the surface gone. Some small areas of surface still show.


Some more surface spawling:




This shows a complete failure:



More exposed rebar:






Cheers,

Marc
I am an unreliable witness to my own existence.

In the corners of my mind there is a circus....

M-Works