Every coconut product leaves something behind. The water and flesh get used, and the shell often becomes a disposal problem. Researchers in road engineering see an opportunity in that leftover.

The case for coconut shell asphalt starts with abundance. A review of coconut shell properties describes it as an abundant agricultural waste, and found that the shell has lower water absorption and better strength than traditional aggregates. Water is one of asphalt's main enemies, so lower absorption is a useful trait.

A broader literature review examined 31 academic articles published between January 2011 and September 2022. Its authors wanted to find a second use for coconut shell and fibre as partial replacements for aggregate in pavement mixes, rather than letting them become a source of contamination.

The results are not uniform, which is worth stating plainly. According to one review, coconut fibre can raise stability, skid resistance and resilient modulus, while coconut shell can improve indirect tensile strength and static creep behaviour. A mix with 0.75% coconut fibre improved skid resistance but did not improve resistance to higher road surface temperatures. For heat, the shell looks like the more promising ingredient.

Within the shell, the charred form attracts the most attention. A review comparing materials found that coconut shell ash showed better strength than rice husk ash and palm kernel shell ash, which the authors attribute to its higher alumina and silica content.

Another study used coconut shell ash to replace granite dust as a filler in hot mix asphalt containing reclaimed asphalt pavement. Microscope analysis of the optimum mix showed a rough surface texture and better interlocking between aggregates, producing a denser mixture.

So the environmental argument and the engineering argument point in the same direction. Still, "promising" is the honest word. Supply chains, processing costs and consistency of the shell material would all need to be worked out before a road agency commits. Those questions are largely unanswered in the studies reviewed.