With that in mind, I could now design the length of each bridge. One was to be a single track bridge of 36" long, and two were to be a double track bridge of 24" long.

The hardest part of building a concrete bridge is the forms. It takes a little time to first sketch out what kind of bridge that is needed, and second figure out how to construct the form to make the bridge. The bridges that I poured are pretty straight forward with very little ornamentation.

To get started, you need to decide what the forms are to be made of. I used 3/8" BC plywood and 1/8" luen for two of the bridges, and 1" foam insulation for another. The one I will discuss here is the plywood and luen.

Once the size of the bridge has been chosen, build the form around those dimensions. In other words, the form has the bridge size as its inside dimensions and everything else is added to that. The bridge that I designed is 7" wide, 24" long and 2" thick.

The next step after deciding what size of bridge that will be built is drawing out the plans of the bridge. The bridge I built consisted of 4 concrete parts: the bridge deck, the bridge abutment, the retaining wall and the wings.

The form for these three parts is basically an open top box. The bridge deck is 7" wide X 24" long X 2" thick and the form was made out of 3/8" plywood. There are two of these bridges. The bridge abutment is 7" tall X 16" long X 3" thick and the form was made out of 3/8" plywood. There are two of these abutments. The bridge retaining wall is 7" tall X 16" long X 1" thick and has 3/8" plywood sides and 1/8" luen ends and bottom. There are two of these abutments.

Once I had cut out all my parts, the next step was assembling the individual forms. The forms were held together with #4 X 3/4" wood screws. These are small enough to not to split the plywood but strong enough to keep the wood from bowing and warping from the water in the cement. When it comes to the number of screws in the forms, the more the better. If the form is to reused, the extra screws will help the form last longer.

I predrilled all my holes before assembling.

Reinforcement is important in small concrete structures just as it is in large structures. I used 1/2" square wire cloth that can be bought at any lumber company. This is inexpensive and can be shaped to fit the form which is being constructed. 1/2" square wire cloth also allows the concrete to flow easily and evenly in the form. I allowed 1/2" space between the wire cloth and the inside of the form. To hold this 1/2" space, I randomly nipped one end of a cross wire here and there and bent it outward as a spacer. On the 1" thick retaining wall, I bent several wires both ways to center the wire cloth. The reinforcement in the bridge deck was bent down 1" on both sides to position the body of the wire cloth in the middle of the 2" thick concrete. The reinforcement in the abutment is in the shape of a rectangle 1/2" from the inside of the form. The reinforcement in the abutment and retaining wall stop 1/2" short of the top of the form.

Also I used the same 3/8" water pipe in the retaining wall. I countersank (2) #10 X 3" long machine screws in the bridge abutment to hold it and the retaining wall together. This prevents the two from separating as the ground shifts with the seasons. The #10 machine will slip inside the water pipe with ease. Drill the holes in the abutment form 1/8" larger than the screw so it will slip out after the concrete is set. This is because wood will slightly increase in size when it absorbs water and thus the hole will shrink in size.

Now that the forms are screwed together and before the concrete is mixed, coat the inside of the form with motor oil and insert the reinforcement wire. The motor oil prevent the concrete mix from sticking to the wood.

When assembling the 1" retaining wall form, assemble all pieces except for one form side wall. Coat the inside with motor oil, slip 2 extra screws through the back of the form and through the water pipe. Also insert the reinforcement wire at this point. Place the remaining coated form side in place with the 2 extra screws sticking through the holes and assemble it to the other form pieces.

Before I mixed the concrete, I used a 1/4" square wire cloth to screen out the larger gravel in the Quickcrete concrete. To make sure the concrete flows evenly in the forms only small gravel works well.

I used a small storage container, a child's plastic sand shovel and plastic hoe to mix the concrete. The hoe just fits inside the container. The container allows you to mix just what you need as you need it.

Several batches of concrete will need to be mixed to fill up all the forms. I purchase Quickcrete in 80 lbs. bags for about $5.00 a bag from the lumberyard. You will need to purchase (2) bags to pour one set of forms. Mix the first batch soupy so that it will not set up while you mix the second batch of concrete. Agitate the concrete as it is poured in the mold for it to evenly flow in the mold and to work out all the air bubbles. The second batch can be made a little stiffer since there is ample water in the first batch to rake off and smooth the concrete at the form top.

The parapet walls are held in place after the bridge is dried by coat hanger wire inserted into the bridge deck right after the concrete is poured. I cut the coat hanger wire in 1 1/4" lengths. They are held in place with a 1" strip of luen cut the length of the bridge. I spaced 5 wire pieces evenly starting 1" from each end and 3/16" from the edge.

The wings are constructed and poured separately. I made the wings mold by purchasing a sheet of 1" foam insulation. This is extruded polystyrene foam insulation. It comes in 4' X 8' sheets. It runs about $10 to $12 dollars a sheet. Dow Chemical and Owens Corning are two companies that manufacture the foam. The wing is 6" X 6" X 1/2". They are in the shape of a triangle with ends blunted 1". I used my Dremel and routed out a 1/2" deep mold in the foam the shape of the wing. I coated the mold with motor oil similar to the other forms and poured the concrete. 4 wings will need to be poured.

After 7 days, remove all the forms except the bottom. You will find that after you remove the forms that the concrete is still damp. Allow at least another day for the concrete to dry.

After the concrete is completely dry, I coated the outside of the concrete with a soupy mixture of cement. This seals the concrete from the elements and gives the bridge parts a nice appearance. The strip luen that holds the coat hanger studs can be seen in the background.

The cement coating dries in a short time. After it is completely dry, you can install the bridge on your railroad. To construct the parapet wall , I cut a piece of cypress 3/8" X 3/8" X 24". A parapet wall is the 6" X 6" retaining wall that one may see on top sides of a railroad bridge. Wooded railroad trestles with a ballasted deck use wooden parapets held in place by screws. Concrete railroad bridges either use cast in place parapets or ones brought in and held in place by rebar. It's purpose is simply to hold the subgrade under a railroad track in place. I layed the wood stock pieces beside the coat hanger wire studs in the top of the bridge and mark the wire locations. Next I drilled 1/8" diameter holes at these locations and in center of the wood stock, sanded the wood and painted it a color close to the cement. After 2 coats of paint, the wood stock was slipped over the wire studs and I had parapet walls on my bridge. The ballast could then be spread on the track.

It took me about 4 weeks to construct the double bridge. Most of the time is spent waiting for the concrete to cure but in the end, the results will be pleasing.

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