Why would precast or prestressed concrete change youngs modulus? It is an inherent property of the mix design, not dependant on how the concrete is used or placed.
Just use your normal code relationships for calculating youngs modulus irrespective of the final construction.
It does depend on placement and curing to a degree. Precasting might use vibrating forms and stream curing which have effects. That said, I've never seen it all collected in one document and it's quite variable, so I'd simply use the code if not any more critical than usual.
