Tuesday, March 11, 2014

ASCE Concrete Frisbee

ASCE Concrete Frisbee was part of a multi-university Civil Engineering Student Conference.  My project’s vision was to try different mold and construction techniques.  I decided on liquid Polyurethane casting to accomplish detailed form work.  I also experimented with color pigments, stains, dyes, and a variety of lightweight aggregates and property-modifying admixtures.

The final product, dubbed "1738" in honor of Bernoulli's Hydrodynamica
I used Inventor to CAD the formwork before starting construction.  This was an important step in justifying the procurement of pricey Polyurethane rubber.  It helped everyone understand the process.

Expanded view of the male mold, plastic form, female mold, and foam pre-form
The following images illustrate the process of making a Polyurethane mold from liquid urethane rubber.

Carved a void in a block of Styrofoam
Lined the pre-form with an impervious layer
Mounted the plastic cast into the Styrofoam case
Batched the rubber and hardening agent
Poured in to the form work then let cure for 72 hours
The structural concrete mix consisted of Type I Portland Cement with expanded slate as the primary aggregate.  Primary reinforcement was a net of alkali resistant glass (ARG).  Secondary ARG chopped strands were dispersed into the mix to reduce shrinkage cracking.  Metal oxide pigments were blended in for aesthetic purposes.  Water reducing and superplasticizing admixtures were batched with the wet components.  The final mix was laid by hand between the male and female molds.

Durability tests were heuristic.  I flexed the disc by hand and applied lateral pressure to visually detect flexure.  A coating of orange marking chalk amplified any imperfections in the discs.  Before competition, I attempted to FEM a point load (to simulate a lateral collision).

Finite Element Analysis of 1738
At competition, aesthetics judging went exceedingly well.  The judges seemed impressed by the product’s form, color, and quality.  The accuracy competition involved throwing the disc as close as possible to a fixed cone.  My team’s best attempt was still 8 feet out.  The best distance throw achieved stable flight and went 153 feet before striking the side of a building.
After the building collision
Primary and secondary reinforcement maintained the integrity of the disc.  With the broken pieces still attached, the durability score was not horribly affected.

Although a rubber mold yielded impressive results, the technique was expensive and sensitive to environmental factors while curing.  Nevertheless, this project proved that a Concrete Frisbee could be a real Frisbee.  In a future iteration, I would introduce more lightweight concrete aggregates and put more care in to the coloration of the final product.

As part of a larger conglomerate of student engineering projects (including Concrete Canoe), Frisbee served as a testing ground for new methods.  The smaller scale of the project allowed for more liberal application of unproven ideas and methods.

2nd place for Concrete Frisbee at the 2008 PSWRC