Xylo

Sculpture Installation

Fall, 2017 - Fall, 2018

Xylo is a project that began early in the fall of 2017 during a class discussion to chart out the semester goals for a precast concrete class at Georgia Tech. We decided our goal would be to create a public art piece using only precast concrete elements. After observing the acoustical resonance of a piece of Ductal ultra-high performance concrete (UHPC) I proposed the idea that the piece could be a concrete musical instrument. After much discussion on what that could mean at an occupiable scale, my partner and I decided to use a repeating form to produce a percussion instrument similar to a xylophone; member length would dictate the tone of each piece.

Design

Using Grasshopper we were able to quickly manipulate both global geometry as well as fine tune member cross-section profile and depth for requisite structural performance.

Our final design consisted of 13 unique elements mirrored in both the X and Y axis to yield a final design of 50 elements; all achievable from a single rubber mold. The form is generated through a series of closed catenary curves that evolve throughout the piece to form a symmetrical oculus in the overall geometry. The defining curvature for each unique element is extracted from the master geometry.

Each member utilizes the same base geometry. Curvature and final length (removed from the top of the member) change to give each piece its unique form. A constant 4” depth along the strong axis is maintained for structural performance though the weak axis is manipulated through three primary stages. The bottom 30” of exposed member is a constant rectangle such that each member can bare against the members to either side. Beyond 120” of total length the profile becomes a constant ellipse such that when trimmed to length each member will have the same end profile. A circle is used to control the rate of transition from rectangle to ellipse.

Prototyping

Production feasibility arose as our first obstacle; the cost and time of producing an array of molds to satisfy each tone would be prohibitive. As a result, my partner and I decided to explore flexible rubber form liners a sustainable and cost-effective means for forming complex curvilinear geometries in architectural precast applications. Our design consisted of a single base shape manipulated to form multiple unique elements.

If the cost of custom one-off formwork is no longer a factor in the production of complex elements, precast concrete would become a much more viable option for systems containing multiple mold variations. The project gained traction and we were able to recruit a larger team to generate a full scale prototype of a sculptural installation to be exhibited at Fab-City Summit in Paris, France.

The initial proof of concept was a small 3” x 16” strip generated from a small hobbyist rubber kit. The accuracy of the results surprised everyone and was the catalyst for recruiting the team the following semester.

With a proof of concept we quickly jumped to a 1:1 scale and produced a 7’ section of the final form. High-density sign foam was CNC milled to produce the positive from which we were able to cast our rubber mold negatives. Luckily we encountered many issues along the way that provided valuable insight into the final production plan. Issues of mold deformation and “blowout” were the major hiccups that allowed us to make improvements that streamlined final production.

Full Scale Prototype

The cost and time required to produce a full 50 piece installation was unreasonable given the funding and length of the semester. We set our sights on creating a section of the final design as a full scale prototype that could be shipped overseas at a reasonable rate. Making a section of each end would allow us to create additional pieces in the future if needed.

The full scale rubber molds were produced in the same manor as the prototype but we used removable plywood sides as opposed to a single piece form to make the demolding process easier. Metal rods inserted into the foam during casting provided through-holes that were essential to the mold alignment during casting. They allowed the top and bottom pieces of the mold to stretch and compress uniformly such that during bending the ends remain aligned without one being longer than the other.

Dex Industries in Atlanta, GA was kind enough to lend us space in their casting facility including assess to their state of the art large batch mixer. We cast four batches of four unique elements to give us a total of sixteen elements to display in Paris.

After casting post processing was required, the pieces were trimmed to length, any excess material at the seams was removed, and the pieces were ground smooth with a wet polisher. In order to create a system able to produce multiple variations from a single mold we designed the last 3’ of the mold with a continuous cross section profile; all pieces were poured to the same length. This allowed each element to be trimmed to varying lengths without creating an array of shapes at the exposed end of each member. We attempted to create a concealed post-tensioning system running through the length of the base, however an 11th hour mistake rendered the system unuseable.

Paris Installation

As part of th Fab City Summit 2018 conference, a student exposition was displayed in Parc de la Villette for the month of July. This exposition was to highlight advancements in sustainability in design and production; Xylo was the only US based student project selected.

After post-processing, the pieces were crated and loaded into a shipping container bound for Paris. The assembly process went smoothly until we discovered a single eleventh-hour drilling mistake that left our post-tensioning system useless. Once the pieces were placed and fine tuned into position ratchet straps and friction solved our tensioning problem. Multiple straps were wrapped around the base of the sculpture on each side to clamp the pieces together; the friction between the elements rigidly held the middle elements in place. To hide and prevent tampering with the straps, the base was extended up the sculpture.

Production Modifications

The casting process was not perfect. The plywood used to create the top and bottom skin of the wooden mother molds was prone to bulging under the tremendous head pressure during the casting process resulting in a slightly “wavy” appearance on some of the members. We decided to beef up the exo-structure and create a thicker composite plywood top and bottom skin from multiple layers of thin plywood glued to create a single rigid sheet. The final results were perfect with the lone exception that they never fully cured due to freezing outdoor temperatures and were destroyed during removal after the final casting test.

Acknowledgments

I would like to give a special thank you to the PCI foundation for funding this project and Dex Industries for lending us the space and machinery needed to make it all happen.