Something to chew on

Exploring the legend of the asphalt chewing test

By Amma Wakefield, P.Eng.

If you have ever attended an asphalt mix design course, you may have heard about the chewing test as one of the first methods of characterizing asphalt cement. You were likely told that the test was conducted exactly as the name suggests.

And, if you sat in a mix design course that I taught, I may have been chewing gum… you know, for effect. So, imagine my surprise when I learned that there was a “chew machine.”

Specifically, it was called the Chew Ductility Machine, where a sample of asphalt about one centimeter thick was pulled apart utilizing a gear operated by a handwheel. A motor drive was preferred for better uniformity in the rate of pull, but in both cases, the sample was pulled until breaking, then the distance it stretched before breaking was recorded. The test and machine are referenced in a 1916 manual by Hubbard titled “Laboratory Manual of Bituminous Materials.”

Does that mean there was never a test where a person chewed asphalt to determine its quality? Where is the fun in that? Is chewing asphalt a myth?

Well, not so fast… in a 1905 journal “The Modern Asphalt Pavement” by Clifford Richardson, we find a reference to the chewing test that we are more familiar with. Richardson explains that the chewing test was used as a preliminary test for asphalt cement. It involved chewing a small piece of asphalt cement which had been cooled by pouring it into cold water.

Don’t try this at home kids! But a cooled asphalt cement was placed in the mouth and worked between the teeth.

Through chewing, the asphalt cement rapidly assumes the temperature of the mouth which, for normal body temperature is 98.4°F (36.9°C) and consistent. Richardson explained in the journal that the amount of effort needed to chew the asphalt showed whether it was harder or softer than what experience had taught about a proper consistency in asphalt. An experienced chewer could determine if the asphalt was within four or five points of the desired consistency. One reference stated the chewing test was conducted for 15 minutes!

Another reference in 1911 from the American Society of Municipal Improvement convention mentions the chewing test, where an asphalt consultant J.W. Howard added that the asphalt was of inferior quality “if it becomes like lard or slime.”

Who would have thought that there were actually two “chewing” tests, one by mouth and the other by machine?

I must confess, I would have been incredibly disappointed if I had learned that chewing asphalt to determine its quality was never really a thing… but 15 minutes though?

Wakefield is an Asphalt Institute Regional Engineer based in Ontario, Canada.