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Admixtures are defined as those products that are incorporated in the moment of the process of mixture of the concrete in a quantity not bigger than 5 by mass of the cement %, with relationship to the cement content in the concrete, with object of modifying the properties of the mixture in .state fresh and/or hardened. The behaviour of the admixtures depends on its chemical and ionic composition, the organic functional groups present, and the structure of the polymer and the distribution of molecular weight of the different polymers. In the present work the techniques and methods of characterization physical-chemistry, chemistry and ionic, structural, as well as of the polymers that constitute this admixtures, are described. A lot of techniques have been employed like: ionic chromatography, ultraviolet-visible spectroscopy (UV-VIS), Fourier transform infrared spectroscopy (FTIR), Fourier transform Raman spectroscopy (FT-Raman), nuclear magnetic resonance spectroscopy (1H-RMN and 13C-RMN), gel permeation chromatography (GPC). Two commercial admixtures have been selected to carry out this characterization, a superplastificant based on policarboxilates, and a reducer of the shrinkage based on polipropilenglycol.