Shown here is a test casting, in Hydrocal, of a section of the concrete arch bridge. I wanted to test how the plaster would exit the mold. It was difficult, even in this small section, and even though I had built the mold with a 3 degree taper for the pieces forming the small enclosed arches. The outside pieces of the mold all are removable and that was fine, so I decided to make each of the individual small arch pieces separately removable, so one by one they can be eased out of the whole piece. Hopefully I will have a photo of a completed full casting sometime in the near future. I was pleased by the look of the piece and anxious to see a whole side intact!
Designing my layout, I wanted to incorporate different types of bridges (I suppose that’s a thought of many model railroaders), but I wanted to design some of my own bridges in a freelanced but believable way. There are a number of concrete arch bridges in my area of NW Philadelphia including the Walnut Lane Bridge and the Henry Avenue Bridge, both Highway spans but seemingly constructed very similar to many railroad bridges of the same type. I also referred to the terrific book “Bridge and Trestle Handbook” by Paul Mallery to keep me building a basically sound engineered structure.
The Drawing shows my design for a concrete arch bridge to span a gorge that will be right in the front of my layout.
The photo shows the form I built to cast the bridge (actually half the bridge) as two castings back to back will form the bridge.
The form is made from Wood and sintra.