Man, I did this when I was still living at my parents home. The threshold rotted away and I thought I would build a threshold that would never rot away.
I built the door frame jambs out of PVC and formed and poured the threshold...just can't remember what I used at the moment. Maybe the 5500 psi Maximizer before they went with that expanded shale aggregate they now use which us useless for finishing?
I used one of those ADA aluminum thresholds with stainless steel screws, and has looked great after 10 years. Probably good for another 150 years, lol.
Everytime, I step on these cheese aluminum (leak prone) theresholds on my house doors I tell myself I need to do this...