Ron May's comment about the mandated inclusion of dark aggregate in
sidewalks gives rise to a question I have:

In several buildings in St. Louis, built as far as I can tell abround
100-1910, the concrete has mixed into it small particles of coal.  It
becomes particularly evident over time, as moisture gets into the coal,
causing it to expand, and ultimately force small spalls off.

I've asked several local current construction firms and architects:  why?

Does anyone out there have some sources to point me to, regarding why  in
a building dark inclusions were involved (particularly as this concrete
would, for the most part, not have been visible like the situation ron May
mentions).

dave browman