Method to make an arm in a composite material for articulated or telescopic modular structures for the distribution of concrete, which comprises a first step in which one or more layers of fiber (16, 25) are distributed on the external surface (17) of a male-type mold (11) having a cross section defined by at least a substantially flat wall. The section of the male mold (11) reduces, in at least one of its sizes, passing from one of its ends to the other. The fibers (16) may or may not be pre-impregnated with a resinous matrix. The method also comprises a second step in which the male mold (11) is ejected from the product thus obtained, a possible heat treatment being provided, for example in autoclave, before the extraction of the product.