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.
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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.
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