Buy them, don't build them. Take advantage of the supplier's warranty as being fit for purpose, and the Building Inspector's accepance of that.

ya look at your area's building code it will tell you

Why do you want concrete? They will much heavier than either wood or light-weight steel framing, and be harder to install and support.

Concrete beams are usually constructed with precisely pre-tensioned(stretched) steel cables or rods embedded in the concrete. The concrete mix and pre-tensioning is usually dictated by the required beam loads. Trying to build them without careful design and execution will probably kill someone.

Suggest further research starting here...

http://www.spancrete.com/products.php