How The Rubber Met The Road

Something great was being cooked up in Mrs. McDonald’s kitchen in the 1960’s. Her husband, Charles McDonald, an Engineering Supervisor for the City of Phoenix at the time — melted, mixed, (in Mrs. McDonald’s kitchen) and spread out his tire taffy on the roads of Phoenix and became known as “The Father of Asphalt Rubber”.

Mr. McDonald was an old school engineer whose materials laboratory consisted  of a small windowless room where a couple of guys broke concrete cylinders with  a hand operated hydraulic press and made soil proctors behind file cabinets. He was looking for an inexpensive way to patch pavement to prolong its life and to protect the subgrade. Mr. McDonald envisioned a pavement salve that was water-tight,  could flex without rupturing and could be inexpensively made and successfully applied to deteriorated pavement. Mr. McDonald was a creative and resourceful  man with a steely sense of resolve that led him to melt, mix, and apply several iterations of his rubberized asphalt mixtures. He had technicians document and  test his countless experiments of paving repair compounds and they had a few humorous incidents along the way.

It was a hot day in Phoenix and Mr. McDonald wanted to test his asphalt-rubber patch on a pothole in extreme weather conditions. This experiment was typically done in milder weather and had favorable results. His skeptical yet diligent technicians went out and applied the patch as directed, pulled off the release  paper and waited to see what happened when the patch was driven over. After the second or third car rolled over the patch, the patch broke loose and flailed and broke off as the bewildered driver continued on down the road. The two technicians looked at each other and said “Let’s get out of here”.

Another example of Mr. McDonald’s tenacity and resourcefulness was found in his brute force yet effective mixing technique. Mixing of the hot asphalt cement and ground recycled tire rubber was accomplished in a boot truck that would drive forward, break hard, then reverse and break hard again. The back and forth  motion created a sloshing effect that mixed the rubberized asphalt.

Continued use and experimentation with rubberized asphalt has generated new technologies and equipment throughout the world. Today in Phoenix, ADOT and MCDOT extensively uses rubberized asphalt mixes as finishing courses on major roads and freeways.

Thanks to Mr. McDonald for his curious mind and to his wife for letting him experiment in her kitchen!

Park, O.D., P.E., October 2011, Charles McDonald, Father of Asphalt Rubber.

Feliz, Phillip D., S.E.T, September 2012, Recommendations for Updating the City of Phoenix AsphaltRubber Concrete Specifications, Section 321 (Letter to the City of Phoenix – Street Transportation).

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