After testing out bracing, I've now moved back to the main set of physical testing, focused on fabric formwork. The latest aspect of the process that I've decided to test is possible reinforcement strategies. The three that I have looked at so far are fibre reinforcement, textile reinforcement and metal mesh reinforcement. These have all yielded differing results, some more successful than others.
I am aware of the limitations of this physical process at such a small scale and in imperfect testing conditions (as under lockdown I limited by what can be achieved within my flat, rather than having the full capabilities of the workshop and building materials laboratories), so I have also looked to the principles behind them and precedents of their use.
As when I was doing the tests of the different types of fabric, it was important to control and keep constant all of the other variables as much as possible, so that it was only the reinforcement strategy that was changing. I picked the form of the first tensioned model and used this for each test.
Principles and precedents
With fibre reinforcement, fibres are added into the casting material mixture whilst it is in its wet state, prior to application, making it an easy strategy to apply to anything freeform, such as the fabric formwork forms being proposed. The fibres provide tensile reinforcement which 'knit' the cast material together when it cracks.
Fibre-reinforced concrete particle diagram
Fibre-reinforced concrete force diagram – fibres in the lower half of the beam 'knit' it together under tension
Textile reinforcement is laid within the casting material, so requires cutting and shaping to some regard, unlike fibre reinforcement. As textiles are flexible though, this is easier than working with metal rebar or mesh. An example of textile reinforcement being used with concrete to achieve a long structural span with a very thin concrete section is the bridge in Albstadt-Ebingen by the Institute of Structural Concrete at RWTCH Aachen University, Germany. This uses carbon fibre as the textile.
Albstadt-Ebingen bridge – carbon fibre mesh (top) and completed (bottom)
More conventional rebar mesh, is much heavier and more difficult to work with than textile reinforcement. Another drawback is the need for adequate concrete cover over the reinforcement, to prevent corrosion of the reinforcement, which would compromise the whole structure. This is not the case with textile, as shown in the diagram below, and explains how the bridge example above was possible.
Steel reinforced concrete cover and thickness compared with textile reinforced concrete
Metal mesh being bent into shape (left) and bespoke hyperbolic paraboloid reinforcement being carefully constructed in a Felix Candela example
Fibre reinforcement physical test
For the fibre test, I used nylon line, like fishing wire, cut into approximately 20mm lengths. I chose this as it is flexible, but will hold its form with a good degree of stiffness at this sort of length. To use the material efficiently and speed up the process, I only applied it to the peaks, where I thought that the tensile forces would be the greatest. A first thin layer of plaster was applied to the fabric, then the fibres added onto and into it whilst it was still wet, and then more plaster was added on top.
Process of adding fibres to the peaks, pressed into an initial layer of plaster, before covering with more plaster
The resulting model broke into five pieces – naturally not the result that I had been hoping for! However, I don't completely discount this strategy for a number of reasons.
It didn't break where the fibres had been placed.
Where it broke, the plaster was very thin – in focusing on adding the fibres before the plaster set, I had neglected to build up the areas sufficiently.
It was only under the duress of pulling the modelling off the base and peeling that fabric that it broke. If the fabric had been left as a permanent formwork, this would not be an issue.
Broken pieces, showing the thinness of the plaster and a fibre holding two pieces together
Textile reinforcement physical test
I used Modroc for the textile test, using it on its own rather than with fabric underneath it. I stitched two pieces together, then stretched it over the bamboo, and then applied water to it. Then I added a thin layer of plaster on top of this.
Modroc stretched into place (left) and then with water and a thin layer of plaster applied, showing naturally-formed ribs
This was a successful test. The model stands of its own accord, despite being very thin. It is interesting to see how ribs have naturally formed running between the anchoring points in the base and over the tops of the bamboo. I believe that this gives it greater strength and rigidity, much more than if it had stayed as a flat section.
Final result, showing naturally formed ribs and folds, very thin section, and spaces created
Metal mesh reinforcement physical test
The final test was metal mesh. This would be mesh made of steel rebar at full size, but the analogue that I used at this scale was aluminium contour mesh. Like with the Modroc test, I elected to leave the fabric out and see how the process would fare just on its own.
Aluminium contour mesh tensioned into place (left) and a first layer of plaster added, showing holes where plaster had run off or dripped through
The final result of this one can also be considered a success – free-standing and with a fully covered surface. There are drawbacks to this method, however:
It uses a lot of reinforcement material.
As a solid and inflexible material (at full-size) it is hard to work with, and the shape has to be manually created by labourers, meaning that the interest of the fabric formwork forms are lost.
A lot of casting material was wasted, coming through the holes and falling onto the base – of course, using fabric underneath this would have prevented that.
Final result, showing how plaster has keyed into the mesh to leave an interesting 'messy' underside
Reflection
I feel that the physical tests on their own, whilst interesting process to carry out, were limited in what they could inform me about the real-world, full-size process. For example, I know that fibre reinforcement can be a very effective strategy, but working at this scale with quite crude resources and equipment, my test wasn't a fair representation of that. However, by researching principles and precedents for each, I feel that I've gained a good understanding of the merits and drawbacks of each.
With this in mind, I believe that the best strategy for my project will be fibre reinforcement. However, more importantly, I think the fact that I have been exploring compressive vaulted forms is going to be key. This is because if I can have the structure working fully in compression, the need for reinforcement is largely negated. Fibre reinforcement can then become the secondary reinforcement strategy.
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