We're doing a thesis about adding plastic to cement. :)
as far as I know, we don't add plastic to cement. we do however, add plasticizers which aid in the concretes ability to flow.
not really, it may actually weaken the structure depending on the type of cement and what it's being used for. the best way to reinforce cement is to add steel rebar, this is one of the only known ways to actually make cement stronger and more durable for things like bridges and skyscrapers.
Plastic fibers have been used as an additive in concrete for years. There are two types....long strand and short strand. It is seldom used in commercial applications but frequently used in residential slabs. I have used this product and prefer the long strand. It's makes for a more difficult finish. The plastic fibers are not intended to eliminate rebar in footers or vertical support members. The product is primarily used in open slab areas. Google is full of info on slumps tests.
