By the middle of April 2008, we were into casting some of the bridge supports around the river. Our aim was to try getting the outside mainline operational so trains could once again begin to roll on the layout.
I cut through the chux/plaster shell and then made up some in-place balsa boxing for the bridge abutments. Next was to spray the balsa with water and make it nice and wet so it wouldn't suck the water out of the plaster once poured. I then mixed up a mix of about 50% plaster of paris and 50% cornice cement (about the consistancy of honey on a warm day) and poured it into the moulds. A lot of tapping of the balsa sides to release any bubbles and then time for it to dry and set. After about two hours, but whilst it was still moist, but set, I removed the balsa.
If you leave it to be totally dry, I've found that the balsa sticks too well and you can't get it off! If you take the balsa off too early, then you get "concrete cancer" like I did on one of the abutments. So it's a balancing act of letting it all set well, but not dried out too muxh before removing the balsa. The "cancer" actually added some nice aging to the concrete in the end.
Here are two abutments on the right hand side of the river. These will cater for the outside and middle mainlines. The lower of the two will get some patching done on it since there is a chunk missing.
And here is the abutment on the left side of the river for the middle mainline. Notice we'd already just done a quick wash of ochre on the rock moulds so they stood out a bit better for the time being.
The reason we used 50% plaster of paris and 50% cornice cement is that I've found that the cornice cement is a very fine and smooth one and adds a lot of strength to the whole thing when dry. I don't use all cornice cement as it does take a lot longer to dry than plain old plaster of paris.
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