Prestressing structural elements allows an increase in the spans of reinforced concrete structures. In the design and dimensioning of prestressed reinforced concrete structures it is necessary to know, that is to adopt a number of parameters which are empirically determined. Determining the level of the tension force of the cables is of primary importance for prestressed structures. The level of tensioning force, in addition to other parameters, greatly depends on the friction coefficient of cables and the imperfection coefficient of execution. These coefficients are defined by standards for the design or by the manufacturer of the prestressing system. The coefficients are defined in wide ranges, while for the design, the knowledge of the most accurate values is necessary in order to obtain a good design and cheaper structures. Through experimental work in-situ the range of the values of the coefficient of friction of cables is determined and compared with ranges defined by the standard.