How I cast two concrete overbridges

I decided it was time to start detailing parts of the line, so began with the area around Bulkeley station. This included adding a cottage, a road and two overbridges. These would also act as scenic divides where the mine branch and the mainline disappear behind the shed.

After altering the landscaping and adding the cottage and and road (see Progress Report 27 and How I made a road), I considered how I would construct the overbridges. I could have made them from exterior ply but decided to experiment with casting them from cement.

Firstly, I cut two templates from cardboard for the sides of the bridge.

To check clearances, I used a couple of the tallest, widest rolling stock and trimmed and taped on additional pieces until the templates more or less fitted the space. I then drew round the templates on some 8mm ply and cut them out with a jig-saw.

After a little extra tweaking, the sides were fitted into the space and clearances were checked once more.
A card template was then cut to act as the support for roof of the arch.

The template was used to cut the arch roof from corriflute (corrugated plastic sheet).

Sides for the arch were cut out in a similar way, and they were fixed to the sides with hot glue.

A strong mix of concrete was then prepared (2 parts sand, one part gravel, one part cement) to the consistency of custard. Red cement dye was added with the water to colour the concrete to resemble the local red sandstone.


The mix was was then poured into the mould and tamped down to try and disperse any air bubbles.
The hot glue proved insufficient to hold the corriflute in place so it needed to be shored up with suitable pieces of wood offcuts. Two lengths of surplus brass rail were embedded in the concrete over the arch for reinforcement.

Lengths of corriflute were fashioned for the inside faces of the parapets. These were filled with concrete, with wooden spacers to keep them apart.

While the concrete was wet, coarse builders' sand was sprinkled on the road surface and pressed down with a dry paintbrush.

Once the concrete had set to its 'green' state (ie partially dried out and still soft), the mould was carefully removed and courses of stonework were scribed into the sides with a 3" nail and then lightly brushed over with a soft paintbrush.

With the first bridge, I made three mistakes:

1. I relied on the hot glue to hold the corriflute in place
   
2. I made the shuttering for the inside of the sidewalls too deep and hence there was a gap between them and the roadway
3. I removed the shuttering too soon (around 12 hours after I'd poured in the cement mix)

As seen above I had to shore up the mould to contain the cement but worse still, the side walls were joined insufficiently well to the main structure - and hence fell off when I was trying to scribe them. They then had to be pieced back together with epoxy filler

A watery mix of concrete was then 'painted' into the cracks to improve their appearance.

To avoid the problems encountered with the first bridge the following tactics were used for the second bridge:

1. I reinforced the mould after hot gluing the corriflute by wrapping wire around the mould


2. I made the shuttering for the side walls less deep so it was resting on the roadway rather then being embedded in it (obvious really).
I also reinforced the concrete by pushing a couple of lengths of wire down into the side walls while the cement was wet:


3. I left the concrete for 24 hours before removing the shuttering and also moulded stonework on the inside of the mould with hot glue to represent cement courses:

I found that the shuttering for the first bridge more or less fitted into the location for the second bridge with some slight adjustment (ie attacking the the rockwork with a hammer).

It wasn't a perfect fit, but close enough for any gaps to be plugged with a few off-cuts and soem strategically placed rocks.

Once the second bridge is fully set, this area will be tidied up by removing the brickwork in the background and landscaping around the roadway.

I'm also going to have to realign the track under the second bridge.

But overall, I have achieved the effect I was after..........

Update - 23/12/12

The bridges have now been in situ for two and a half years and are beginning to weather nicely into the surrounding landscape.

You still can just make out the repairs to the first bridge carried out to remediate the damage caused by my impatience......

.... but the epoxy resin in the cracks has more or less blended with the original concrete mix.

The second bridge is also blending into the background as the moss and green mildew take a hold.

During the bridges' first winter I covered both bridges to protect them from frost damage but I forgot to do this during the second winter and so far I have not yet covered them against the elements this winter.

However, over the next couple of days I intend to drape them in some old sacking which might not look too pretty but will hopefully deter Jack Frost's prying fingers.

Update 10 Jan 2020
The bridges have now been in place for nearly ten years. I never did get around to sealing them or covering them during the winter as frost protection and yet they have survived. One was damaged when a branch fell on it while pruning the apple tree above it. It was easily repaired with some epoxy masonry adhesive - see Progress Report 76

The area around the bridges has now been landscaped

--o0o--