Sunday, April 08, 2007

How did I make the platforms?

The platforms were cast from concrete to precisely fit their locations. After considering a range of options (wood, plastic, tiles), I decided concrete would be the most appropriate - it is durable, cheap and easy to construct.

Two methods were used:
  • for the terminus, the platforms were cast in situ using shuttering. The platforms here are primarily straight so simple shuttering was feasible.
  • for the through station, the platforms were prefabricated using moulds. The platform here is curved throughout its length and hence shuttering would have been tricky.

Method 1 - shuttering
The platform height was set at 3 cm. This was decided by measuring from the trackbed to the lowest step on one of the coaches.

Stage 1
Strips of 3/4" (2 cm) pine were cut to 3 cm sufficient to form the shuttering for one of the platforms. The width of 3/4" provided the right amount of clearance between the coach steps and the platform edge.

Stage 2
The pine shuttering was positioned around the edges of the area to be made into the platform. To accommodate the curves, the shuttering was cut into shorted lengths (approx 15 cm). The shuttering butted up to the sleepers (ties).

Stage 3
A strong mix of concrete was prepared (see mixing the concrete below) - using 2 parts sand, 2 parts fine gravel (Alpine grit) and 1 part cement.

Stage 4
The concrete was carefully troweled into the shuttering and gently tamped down to ensure there were no air bubbles. The surface was smoothed over with a trowel.

Stage 5
The edges of the platform were masked to a width of around 1.5 cm with offcuts of plywood. Sand was then sprinkled on to the centre part of the platform using a sieve ('borrowed' from the kitchen) to add texture similar to tarmac.

Stage 6
When the concrete had hardened to its 'green' state (ie not fully set) the ply masks and shuttering were removed and the concrete was scribed with the blade of a screwdriver to represent stone slabs. The platform was left to dry (covered at night as there was a risk of a frost).

Method 2 - Prefabrication

I initially considered setting up some sort of shuttering in situ, but rejected this as the platforms were curved along their whole length on both sides and I could see no way of reliably fixing it in place - so I opted for prefabrication.

Stage 1A newpaper template was cut to fit between the tracks.

Stage 2
A felt tip pen was attached to a coach (requiring the widest clearance) which was then run along the tracks to mark the optimum position for the platform. As an extra check, other stock was run along the track to see if there was likely to be any snags.

Stage 3
The paper template was transferred to sheets of 1/2" laminated chipboard and pine (recycled from some redundant shelving). This was then cut to shape with a jigsaw. The sections were placed between the tracks to check clearances (by running various bits of rolling stock alongside).

The height of the platform was determined by measuring from the trackbed to the lowest step on one of the coaches (3 cm). Strips of 1/8" ply were cut to 4.5 cm to take account of the platform height and the thickness of the chipboard. These strips were then screwed to the sides of the chipboard templates to form the moulds.

Rather than creating a single mould, the platform was divided into sections - to make the casting process easier, to allow for some adjustment when fitting, and also to allow for expansion and contraction when the platform is completed.

Stage 4
The insides of the moulds were greased (with standard motor grease) to prevent the cement from sticking. Before the cement was poured into the moulds, miniature stone slabs (obtainable from Miniature Brick Bargains - were pressed into the grease along the sides of the mould to act as facings.

A strong mix of concrete was made (2 parts sand + 2 parts fine gravel + 1 part cement - see below) and poured into the moulds. When the concrete was partially set, stone slabs were pressed into it along each platform edge and sand was sprinked on using a fine sieve to represent the gravelly surface of the platform.

Note: The block in the left hand section is to give clearance for a point motor.

Stage 5
When the concrete had set (after about three days) each section was removed from its mould. Owing to the fragility of the concrete, the sides of the moulds were unscrewed rather than attempting to prise or shake them out. The sections were then loosely positioned to check for clearances by running stock along the tracks. They were scrubbed to remove all traces of grease and then the sections were cemented into place and left to set.

How I mixed the concrete
I do not profess to be an expert, but the technique I use for mixing concrete was taught to me by my dad - who seemed to know how to do most practical things. Anyway, it works ..............

Stage 1
Two shovels of sand and two shovels of gravel (Alpine grit) were put on to an old sheet of plywood I've used for many years for mixing concrete.
Stage 2
A shovel full of cement was added.
Stage 3
The ingredients were mixed thoroughly with the shovel.
Stage 4
A depression was made in the middle of the mix and water added. It's important not to add too much water initially as it can wash the cement powder away.
Stage 5
The ingredients were mixed again, adding water until it reached the consistency of thick cream or custard.

[Update April 2009] After two years, the platforms have fared well.

This photo (above) shows the main terminus platform which was cast in situ. No cracks, but a bit slab-like.

By contrast, the bay platform looks quite good. While the cement was wet, I covered the edges with some strips of wood and sprinkled dry sand over the centre of the platform. When the cement had gone 'green' (ie had not fully set) I removed the shuttering and the strips of wood and scribed blocks along the edges with a screwdriver. Once the cement was fully set I brushed off the loose sand and, as you can see, the result is quite realistic - particularly as little pieces of moss are beginning to populate the cracks.

The pre-cast platforms have also survived well. I had wondered if the small tone slabs would become dislodged, but as you can see they are still very much in evidence. A few did become dislodged immediately after the casting process, but none since.

No comments: