We Keep Putting Stuff in Asphalt Concrete

There are two parts to the “holy grail” of asphalt concrete pavement performance: slow pavement cracking and reduce rutting caused by high truck traffic volumes. To this end, after modifying the aggregate blend, state highway departments and research universities seek additives to the simple mixtures of asphalt cement and well-graded aggregate.

Most recently, we have seen the introduction of plant-based additives as recycling admixtures. Before that, ground up tires, finely ground glass, and ground roofing shingles have been added to asphalt mixtures with the normal successes of trial and error. Short fibers, polypropylene or Kevlar, may hold asphalt concrete together during the asphalt cement oxidation and micro-cracking under truck loading. There is a fine balance between a stiff asphalt cement to resist rutting but having more cracks versus a softer asphalt cement to minimize cracking but allow more rutting.  

The additive approach has a long history. Back in the early 1960s, the Johns Manville company launched an effort to make asbestos fiber an anti-crack additive to asphalt concrete.

The hazards of asbestos fibers may have been unknown at that time. But our point is there always seems to be organizations looking to put waste products in pavements for disposal. Right now, we are seeing an increase in vegetable oils in recycling and seal coating. It is wise to be skeptical of additives until there is reliable performance history available. Remember, you are buying an asset with an expected 16-to-20-year life expectancy.

Modern Paving Techniques with Asbestos Fiber  https://youtu.be/OERZBoUfHY8?t=52

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