We recently replaced two window sills on a residential home in Leaside, Toronto which was built in 1939. These pictures show the sill and it’s construction. At a glance it looked like limestone. The corners look like they were tooled by a mason. However, there was aggregate on the face of the window sill which threw me off as to what the sill was made of.
When we started demo we realized it was an early man made product. It appears there is an inner core with rebar and an outer visible layer. The outer layer appears to possibly be a lime based concrete. The inner core has larger aggregate and must have been intended for strength. The two layers are bonded well together.
The sill cracked along the position of the rebar possibly due to the rusting process. It makes me wonder if the sill would have lasted longer had no rebar been used. This sill is common around the Leaside area and further a field in Toronto. The design of the product with the margin tooled corners imitates the common finish on natural stone from buildings constructed in the 19th century and early 20th century. It has lasted a decent amount of time but when it comes to making building materials nothing beats god.