Concrete Garden Bridge

Background

My parents were having some landscaping done and they wanted a little bridge over a dry stream bed. All the options they could find were either too expensive, or not what they wanted. Always eager to do more ferrocement work, I offered my services. Against me were professionally crafted stone, wood, and cast cement; I had creativity and hard work on my side.
A special thanks to my wife Anna for taking the photographs and encouraging me during the long hours it took to complete this project.

Planning

I knew the volunteered labor an cheap materials would keep the price extremely reasonable. The "not what they wanted" part is a trick though. Initial discussions were fruitless (people don't know what they want). Next I took some location photos and modeled a bunch of options in Blender. My favorite is the bottom right design.

The concept renders allowed them to say "I like this, but not that"; Much more helpful. After a few iterations we agreed on the shape and I did a detailed model with internal structure. This is a bit narrower than the final design, but you get the idea. The materials ended up being about $35.

Re-bar Armature

Once the design was approved I bought the re-bar and got to work cutting, bending, and welding it into shape. This is the largest, most precise, most complicated armature I've done to date, so it took a while. The method to cut and bend the rebar is the same here as I used in the Ferrocement Mushroom project, so I'm going to skip all that and just show the finished product.

The frame is not quite square, and that's intentional. Can't see it? Compare (above) the leftmost and rightmost stringers. They are farther apart on the far side, and closer together on the near side. A perfectly square bridge wouldn't look quite right on a curving path, so I've designed the bridge to curve ever so slightly. The effect is nearly invisible, but when finished, it will be square to the path at both ends.

There was one more change between design and construction, the step was removed. You'll notice that the render has an extra stringer, and some triangular supports on both ends. We decided a step would be more of a tripping hazard than a smooth curve (plus the curve was easier to make! Double win!).

Wire Mesh

The wire mesh went on fairly easily. I used some one inch chicken wire on the walkway, as well as half inch all over. This part takes a while too, so here are some pics of the finished mesh.




The black wire is mild steel that we used to sew the skin onto the skeleton. I would say more about the method, but what? Wrap that stuff on tight!

Applying the Concrete

Here's where the concrete goes on. The first step is to get a platform to work on. This was made with some leftover lumber from another project. Omnidirectional access is helpful when you're laying the mud on. Here is the progress after putting a first layer on. I used my gloved hand to push the thick mixture down through the chicken wire, and a trowel underneath to keep it from falling all the way through. Pretty ugly, but this will all be covered up later.

As I added this first batch of concrete, the center began to sag. Somehow, I had failed to account for this in my design. If I had been thinking ahead, I would have made the center ridge higher, and had it all sag to a level surface. Oh well.

As with all of my previous ferrocement creations, most of concrete is applied by hand. Gloves are important, of course, as the lime will destroy your skin; Even if it didn't the abrasive sand would.

After the rails were partially done I went back to the walking surface. This is a critical part of the bridge; visible and functional. Unevenness here will make it hard to walk confidently. Basically, you want this surface flat and good looking. Because the layer underneath has set up a bit, I can use a much thinner mixture (more water) which is easier to work and smooth.

After another hour or so of working on the walking surface, I'm ready to call it a day. It turns out that I didn't get it quite flat, and there's a shallow dip right in front of me, but it's close enough. This is the final coat for the walking surface.

There's still a long ways to go, but I didn't take any pictures, so here's the summary. After a few hours, use a brush to uniformly texture the existing surfaces. After a day, do a rough sanding of the walking surface. Let the concrete set up for a week. Finish sanding the walkway (not totally smooth, but the gritty texture isn't comfortable on bare feet). Flip the bridge over and put a thin layer on the underside, to fill in all the ugly holes. Let the concrete set up for another week. Turn it back right side up and put on the rails and trim. Let it set for a few hours and then use a file to shape the rails. Let it set for another week. Between the above picture and the finished product is about a month of periodic work and waiting.

Finished Bridge

The work is done and here it is in situ.

A hand-made 300 pound solid steel and concrete bridge. It took two men, two boys, and a teenage girl to move it into place. Soon afterwords, it passed the stress test (everyone in the family jumping up and down on it) with flying colors. Hopefully it will serve well (as an elegant and functional bridge over a decorative river) for years to come.

If you want a ferrocement sculpture or construction of your own, feel free to contact me through my commission page.
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