I wanted to pour the foundations for this bridge from plaster or anchor bolt cement. In the era this bridge was built the forms would have been made from dimensional lumber. I had used 1mm V grove styrene for bridge foundations on the Lake Cowichan bridge so I decided to do the same here. It creates lots of nice lines for form lines which can be sanded or otherwise altered to achieve lots of age and weathering affects. So I opted to use the same here.
For lake Cowichan I only needed two so I made two molds and destroyed them in the casting process. For this bridge I need 6 of one style and 4 of another. This meant making re-useable molds.
For the foundations at the north and south ends, they are square with an angled top. The nice thing about the V grove styrene is that it makes it really easy to make uniform cuts.
I started drawing it right on the styrene and cutting the shapes.
I then scribed and snapped where they were to angle up, being careful not to snap them right off.
I made angle braces to keep the two halves square and then once glued in place, I bent the angled parts in and secured them.
The last step was to build flanges so they could be clamped together.
For the foundations on the middle tower I had a lot more work to do. As with the first, I had no measurements and went based strictly on perspective and ratios in the photo comparing the foundation to the bridge structure.
I made a drawing of what I though the foot print would be and began cutting pieces based on that. I’m no engineer so this was very much a see how it fits as I go along type endeavour.
I started out making them quite tall:
But as I mocked it up I saw that the foot print at the top would be too small compared the the bridge leg so I shortened it which made that foot print larger… thank you again V groove siding.
Once I had the basic shape done I added the top layer and was left with two halves of a mold.
And then flipped it over and sat the tops down on two roof halves which would serve to strengthen the whole thing and act as a bottom of the mold. I also added reinforcements at corners and joints.
Finally I wanted to strengthen the outsides of the molds in general and had a tube of PL Premium construction adhesive near by…. And figured why not. Can’t get that crap off my cloths or tools as it’s very strong….
Also for the angled mold, I stumbled a bit on how to hold it together. Flanges would be awkward as it doesn’t sit flat faced down on a table, and so to make the two halves fit I think I would have had to glue the angled flanges on while paired together and feared a gluing then mold halves together. So I came up with an idea to drill holes at the corners and wire the halves together. Seemed to work of so it was time for a test pour!
Success! The molds aren’t exactly water tight but that’s ok. And there are some blemishes and flash… but I’m not making automobiles, so I’m happy with the result.
On the first I took it out after only a few hours. You can see that some of the “form lines” stayed in the mold making kind of an eroded or spalling effect.
The big question mark was would the more complicated multi piece mold survive?
And it did! I left it in over night so more of the detail survived. Future ones I’ll probably pull early so they look more beaten down.
Now for some prototype comparison.
And that’s that! Time to make a bunch more and let them cure for bit. I can make 1 or two a day so it’ll take at least a week.
Craig