A few years ago on my 15 year old concrete block house, one of my exterior window sills started cracking severely and eventually started falling off in large chunks. We had new windows installed, and that work basically did the old broken window sill in for good. I decided to try to form a new concrete window sill on my own. You can see in the photo below that the missing window sill and the broken pieces on the ground after I had ripped off all the loose pieces.
In the photo below, you can clearly see the rebar showing throughout the length of the fractured concrete surface. I guess that rebar didn’t help the sill stay in one piece. It actually helped break it apart once the crack had formed in the area of the rebar.
I got the surface really clean and free of dust and conrete pieces. Then I painted on concrete adhesive that I bought at my local home improvement store. I’ve never used this stuff before, so I don’t know how well it works. As you can see from the photo below, I then mounted some wood strips that were about a half an inch thick to the wall with tapcon screws. These wood strips would become the bottom and side surfaces for my concrete sill mold.
Then I screwed a 1×6 plank that I had laying around to the wood strips. I mounted the top of the plank at the same height that I wanted the top of the new window sill to end up being. I then mixed up some concrete and poured it into the mold. I made sure that the top surface of the new sill slanted down and away from the house. Otherwise, water would pool instead of running off. The photo below shows the mold filled with concrete and curing with plastic around it.