The Bureau of Land Reclaimation claims that there were 96 fatalities during the building of HOOVER Dam but no entombments. Boulder Dam is what FDR’s Secretary of the Interior Harold Ickes (his excretable son by the same name worked for President Clinton) called it, but Congress never changed its name.
A body falling into concrete would create a void, and compromise the structural integrity.
https://www.usbr.gov/lc/hooverdam/history/essays/fatal.html
More likely the body would be compacted by the wet concrete pressing from every direction,ending with a skeleton clothed in the solids after the 70 percent of the human that is liquid has been squeezed out.
I would guess death would be horrifying ,painful,and take less than a mknute...assuming continuos pour.