<https://www.americanroadmagazine.com/forum/topic/1933-building-the-illinois-national-road/#comment-17957>
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"Jim, \n  \nAnother reason occurs to me to support a date earlier than 1925.  I read the Illinois planned road bids for the 1920 season and they didn't yet know what materials would be used because bids for both concrete and brick were coming in.  So they might well have used brick in 1920.  But the clear move was to concrete, and Illinois was already testing center joints, etc. in concrete.  It would take a little more effort, but I bet I would find that contracts let after 1920, or perhaps 1921 were concrete, not brick.  It would be fairly easy to confirm my hunch as the reports are on the web. \n  \nOf course this is all conjecture on my part, so take it with a grain of salt. \n  \nDave \n  \nKeep the Show on the Road \n"
<https://www.americanroadmagazine.com/forum/topic/1933-building-the-illinois-national-road/?tab=comments#comment-17947>