1 An experimental and numerical investigation on the viscous behavior of FRP materials Luigi Ascione, Valentino Paolo Berardi, Anna D’Aponte Department of Civil Engineering, University of Salerno, Italy E-mail: l.ascione@unisa.it , berardi@unisa.it , adaponte@unisa.it Keywords: FRP viscosity , structural retrofitting, constitutive laws. SUMMARY. The present paper shows the results of the creep test program on several GFRP laminates and constituent phases, subject to different stress values, under constant environmental conditions. The tests are currently being carried out at the Testing Laboratory of Material and Structures of the Department of Civil Engineering of the University of Salerno. 1 INTRODUCTION The rheological behavior of materials has an important role in the field of Civil Engineering. One aspect that is particularly relevant is represented by creep phenomenon, in that the continually increasing strain can compromise the durability of structural elements. The viscous effects result particularly marked in the case of fibre reinforced composite materials (FRP – Fibre Reinforced Polymer), due to the presence of the polymeric matrix, with it being sensitive to viscous phenomena. The problem has been studied by many authors both from theoretical and experimental points of view [1-9]. Current literature does not provide final conclusions on this important topic. Studies developed in the fields of aeronautic and naval engineering on the creep properties of FRPs cannot be directly applied to Civil Engineering applications, primarily due to the different processing techniques of these materials. From a practical point of view, the international guide lines on structural retrofitting with FRP, including the recent CNR-DT 200/2004 [10-12], indirectly take into account the viscous phenomena by introducing suitable coefficients, which further reduce FRP design stress. Within the context, the authors have recently formulated, in the field of linear viscoelasticity, a mechanical model capable of predicting the viscous properties of a FRP laminate starting from those of the single phase (matrix and fibre). This model assumes basically the perfect adhesion between the components, fibres and matrix and the linear viscoelastic behavior of the components. Its validation has been developed by comparing the theoretical data with the experimental ones available in literature, related to creep tests on both the composite as well as the single phases. A good agreement has been observed [13-19]. The limited amount of both theoretical and experimental data on the subject has motivated the interest to study in further detail the phenomena, with reference to the experimental characterization of the rheological properties of several different types of GRFP laminates and constituent phases, widely used in the field of Civil Engineering. In addition, the data obtained from the tests will be used to further validate the proposed mechanical model. The present paper shows the first results of the creep test program on several GFRP laminates and constituent phases, subject to different stress values, under constant environmental conditions. The tests are currently being carried out at the Testing Laboratory of Material and Structures of the Department of Civil Engineering of the University of Salerno.