concrete.tectonics.architecture.ideas.inspiration.experiments

A flexible Jumping formwork

EXPERIMENT V            JUMPING FORMWORK
In this experiment I sought to incorporate the semi-flexible surface to the jumping formwork system

OBJECTIVE
– Investigate the possibility of incorporating a semi-flexible surface to a jumping formwork system
– Casting in a larger scale
– Exploring different formwork materials
– Achieving a higher quality of formwork that allows the mould to be reused
– Investigate the detailing of the formwork

EXECUTION AND PRELIMINARY CONSIDERATIONS
SIZE OF FORMWORK
Not being set on a fixed geometry yet, I choose to work with what in the previous experiment is referred to as pattern II. Mainly because the formwork of this design is easy to produce and the geometry fitted the materials at hand. Each triangle had a height and width of 210 mm. This translates into a scale of roughly 1:5 of the actual final formwork. I envisage that each triangle of a full-scale formwork would have dimensions from 1000 – 1500 mm. It’s a balance of choosing a scale where the design produce panels that is able to generate a spacious quality (and not merely is perceived as a pattern) and having a formwork that is physically manageably by a human.

I decided to produce a mould where the base-geometry was repeated 6 times (12 triangles on each panel). As it is a jumping formwork system I need to always use 1 row of geometry to clamp the formwork to the structure. This requires that the base-geometry always is repeated minimum 2 times in each panel. By using an extended version of the essential formwork, it is possible to build larger structure in fewer castings.

I conctructed my formwork as a double layered surface: A flexible surface to incorporate the bends and a reinforced backside to hold the rigid geometry. The rigid geometry was made up by 0,5 mm MDF. The edges of the triangles where all cut in a 45 degree angle, allowing the geometry to bend both ways. Each triangle had 0,7 mm hole drilled in the middle, which were to be treaded with a 0,6 mm cable. The cable where to act as a clamp, replacing what in the previous experiments where screws. I used cable glands to fix the length of the cable. The glands were tightened to maintain the desired distance between the formwork panels.

Finding a material suited to the flexible surface proved to be quite the challenge. Unsure of how much plasticity to allow I decided to test a few different types before applying it to my rigid formwork. I found a ribbed rubber that seemed to have the desired amount of flexibility without the elasticity. Despite having conducted a glue test on the rubber, where everything seemed fine. The material once applied to the formwork, had some sort of chemical reaction with the glue. The conclusion being that I decided to change to a more firm plastic material. The new material was a polyethylene, estimated 0,2 mm.

The jumping formwork structure is casted in sections. Once the first casting has cured, the formwork can be removed and reattached to cast the next section. In addition to the original cast I was able only able to produce a recast in the longitudinal direction before the submission deadline of this rapport. Nevertheless I intend to cast my next extension of the structure in a vertical direction.

EVALUATION
MATERIALS
The 0,5 MDF was easy to work with (it was possible to cut it without using a saw), but had the disadvantage that it doesn’t respond well to moisture. The MDF would be acceptable as a disposable formwork. However I would like to use a more durable material, possibly plywood or a steel sheet, in my further experiments.
The flexible plastic membrane did the job. The structure had a beautiful surface and sharp edges. The only problem being that the plastic had fractured in a few of the cuts and allowed the glue to react with the concrete. When concrete cures alongside contract glue, the chemical reaction leaves sandy trails in the cast. In my further investigations I have been looking into using a 0,75 -1 mm PET plastic. It seems once it has been pre-cut, it has the ability to bend without shoving any signs of fracture.
The cables and cable glands held up perfectly. They were able to withstand the pressure of the concrete and where easily removed. I think even in a larger scale it will be necessary to use some sort of flexible cable or wire, as it has to be able to incorporate the slight difference of the entry/exit holes. I imagine that it is possible to obtain cables that are suitable and well proportioned in a 1:1 scale.

CLAMP HOLES
The formwork only had one hole positioned in each triangle. Surprisingly the single hole was enough to clamp the formwork with minimal leakage during the recast. Ideally I should have placed a series of clamping holes along each triangle edge. This would allow the formwork to have a tighter grasp with a smaller chance of the concrete leaking on the already cured cast.

REINFORCEMENT AND JOINTS OF THE CASTINGS
I had no problem joining the second cast to the original. Despite second cast had to join a smooth concrete surface, the bond between the two seems quite strong. In a larger scale one could imagine that reinforcement would be added to take the tension in the joint. I would like to do an experiment where the reinforcement is casted into either side of the cast to strengthen the joints.

AN ADHESIVE-FREE DOUBLESKIN FORMWORK
I imagine that my final formwork will be a glue-free mould. Instead of permanently attaching the two layers of the formwork I want to keep them separable. The rigid and heavy outer-formwork will be elements that can be assembled and dismantled on site. The flexible inner-formwork will be a lightweight membrane that likewise can be de/attached to the mould. I envisage that the two layers of formwork could be held together by the same bolts/cable glands that control the deformation.

SEMI-FLEXIBLE SURFACE AS A JUMPING FORMWORK
I succeeded to use the semi-flexible formwork as a jumping formwork. Casting additional sections in either direction is straightforward. There are however some limitations of the formwork: Same issue as noticed in experiment III. Forcing the formwork to obtain a too steep curvature will generate difficulties when having to fill the mould. Also when the formwork is curved in a vertical direction, the structure is not able to hold a straight top and bottom edge. This issue arises because of the difference between a straight and curved surface area. As this is a rule of geometry, I see no way of ‘solving’ this problem without taking a change of direction of my investigation. Instead I would like treat this issue as a designing factor: For instance the base and plandesign of the structure  determine how much the structure are allowed to curvate in section. This would create a sort of essential cross-reference between the horizontal plan design and the vertical sections of the structure.

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