Scientists replaced 40 percent of cement with rice husk cinder, limestone crushing waste, and silica sand, giving concrete a rubber-like quality, six to nine times more crack-resistant than regular concrete. It self-seals, replaces cement with plentiful waste products, and should be cheaper to use.

[u/mvea]

https://newatlas.com/materials/rubbery-crack-resistant-cement/

[u/Opeewan]

Obviously they didn't and either came up with their recipe through trial and error or it was a lucky coincidence.

[u/Mattzocrazy → u/[deleted]]

To be fair, that does seem like a pretty reliable way to measure the tensile strength of a material over time, just get a burly fella to whack it with a pickaxe once a day and see how much each whack takes off and then measure based on the size of the fragments over time