If the original text was in English, the word used was most probably "concretized". There is a tendency for translators to render "concretar" to "concrete" in verb form, instead of the usual correct form "concretize".

The Oxford Spanish-English Business Dictionary translates concretar as "to fix, to set"

English definitions of the verb form concrete and concretize:

A. Concrete –verb (used with object) (concretes, concreting, concreted)

10 to treat or lay with concrete: to concrete a sidewalk.
11. to form into a mass by coalescence of particles; render solid.
12. to make real, tangible, or particular.
–verb (used without object)
13. to coalesce into a mass; become solid; harden.
14. to use or apply concrete.

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B. con·cre·tize (concretizes, concretizing, concretized)

to make concrete, real, or particular; give tangible or definite form to: to concretize abstractions.

http://dictionary.reference.com/browse/concretize

"This theory is concretized in a downward-sloping demand function for capital." is the appropriate rendering of this sentence. It is valid use of the term to express the view that a theory is "set" or "fixed" in the downward-sloping demand curve for capital. It means that economists can agree on the basis of the relationships shown by the demand curve. Unlike philosophers who can argue endlessly until the cows come home, economists convince one another and reconcile their views through the logic of graphs and formulas.