Allow me to clarify. The problem with most polypropylene, nylon and most plastic fibers used in slabs is that the modulus is wrong. To use a high tech term they are "stretchy". Like trying to reinforce concrete with rubber bands. When loads are applied they stretch enough to allow the concrete...
Actually the GFRC panels can have the windows flush with the frame. You might take a look at the "GFRC Cladding Panels" booklet at Stromberg GFRC http://www.strombergarchitectural.com/materials/gfrc
it shows some examples and details.
http://www.strombergarchitectural.com/materials/gfrc
