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SURI Project Explores Avenue for Recycling Plastic

Institute Report, August 2012    

When Cadet John McDonald '15 talks about his SURI project to evaluate how well concrete made partially from plastic performs according to engineering standards, two thoughts emerge: This project was a whole lot of work, with many opportunities for him to develop his problem-solving skills, engineering and otherwise.  And the project just might spawn a whole bunch more research.

The process was pretty straightforward: Collect and sort samples of each type of plastic from waste produced on post; shred it and replace some of the sand in the concrete mixture with the shredded plastic; mold the resulting concrete into cubes and test them for compressive strength.  Each step presented a separate challenge.

Maj. Jenny deHart, VMI physical plant sustainability coordinator and one of McDonald's SURI mentors, helped him refine his goal in using plastic and then offered him access to VMI's waste stream so he could get enough of each of the seven types.

Crozet Hall staff helped McDonald figure out how to complete the sometimes unappetizing task of washing food residues off the plastics without using too much energy or water.  Another challenge, to turn yogurt cups and Styrofoam packing materials and laundry detergent bottles into consistently shredded plastic, required more ingenuity.

"I sorted it by type, cut it with tin snips, and put it into an oven," said McDonald, who continued adding pieces till the plastic filled the cans he was using as molds.  "It made a solid plastic cylinder that I could use on a cheese grater."  McDonald spent an entire weekend hand-grating plastic on a cheese grater before he decided to borrow an idea from industry, where cheese graters spin.

"I put a bolt through the center of the chunk of plastic, put the bolt into a drill and ran it over a cheese grater attached to a vice," said McDonald.  "I didn't have a way to make the cheese grater spin safely so I made the cylinder spin instead."

Though McDonald was hoping the plastic might perform similarly to sand in concrete, he quickly discovered that measuring the volume of shredded plastic was more difficult than measuring the volume of sand.

"The plastics floated and all the tests for concrete design are done for things that sink," explained Lt. Col. Chuck Newhouse, associate professor of civil engineering and McDonald's other SURI mentor.  When his first thought, a French press, didn't work, McDonald created a suitable device using an Erlenmeyer flask.  He replace 25 percent of the volume of the sand in the concrete mixture with plastic and tested the resulting cubes for compressive strenth after 24 hours, three days and 28 days.

"The end product has been very useful," said Newhouse, who anticipates that the project will eventually produce an article in a peer-reviewed journal.  "We learned several new things that we could not have anticipated before the project started."

"On average, the plastic met 55 percent of the standard cube's strength," said McDonald.  Some did much better.  Best of all was expanded polystyrene, or Styrofoam, No. 6 plastic, which came in at 68 percent, which is pretty strong, he said.

"That can take roughly 3,500 pounds per square inch.  Not every use of concrete needs large compressive strength.  A bridge deck needs to be a lot stronger than a pedestrian sidewalk," said McDonald.  Other uses for such concrete range from landscaping pavers to base courses for roads to structures built with insulated poured concrete.

With the method for figuring densities sorted out, future projects could address specific issues.  Newhouse noted, for instance that the concrete mixtures were unexpectedly dry.  Another project might increase water content, which Newhouse suspects will increase the compressive strength of the concrete.

McDonald, who would like to pursue this research for his honors thesis, noted how the project benefited him.

"It was a lot of work, but its been really rewarding," said McDonald.  "This is the first time I've written a technical research paper.  ... Col. Newhouse and Maj. deHart gave me the opportunity to steer the project, ... and they were there to support me."