PSI - Issue 64

André Weber et al. / Procedia Structural Integrity 64 (2024) 74–80 André Weber/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Most FRPs are known as durable and non corrosive materials. For the use in alkaline concrete standard test protocols have been established [ISO, ACI, CSA]. In special applications where further chemical attack cannot be excluded, additional tests have been performed. A special view on the behaviour of FRPs under sustained loads is needed for rebars and tendons containing glass and basalt fibres as these fibres themselves are not stable in alkaline environment without a good protection. In addition these fibres are susceptible to so called creep rupture. This susceptibility is managed in some guidelines for the dead loads only, while other guidelines take into account the whole combination of loads. From creep rupture resistance charts can be concluded, that the resistance against loads with shorter durations is higher than for loads with longer durations. This should be described with sufficient accuracy as well taken into account for the design against dead and live loads and their combinations. Nomenclature abZ allgemeine bauaufsichtliche Zulassung (national certification) EC Eurocode (unified European building code system EN 1990 ff) f fk characteristic value of (long term) tensile strength f fd design value of (long term) tensile strength  action combination factor t rep representative temperature t min minimum temperature t max maximum temperature R 10 property change for tenfold exposure time 2. Material resistance Most FRPs are known as durable and non corrosive materials. For the use in alkaline concrete standard test protocols have been established [ISO, ACI, CSA]. In special applications where further chemical attack cannot be excluded, additional tests have been performed. A special view on the behaviour of FRPs under sustained loads is needed for rebars and tendons containing glass and basalt fibres as these fibres themselves are not stable in alkaline environment without a good protection. In addition these fibres are susceptible to so called creep rupture. A special attention shall be paid to the maximum sustained stress in the reinforcement as generally a greater part of the load can be dead load. 2.1. Creep and Creep Rupture Creep is the steady increase of strain under a certain sustained load. For polymers this phenomenon is well known and described by different approaches. For solid materials like glass, basalt, carbon or aramid fibres the creep is much lower. (although glass as well as basalt is a subcooled liquid and aramid is a melt oriented highly crystalline polymer). The load is always redistributed from softer to stiffer and from faster creeping to slower creeping materials. For this reason in unidirectional pultruded bars with small imperfections under sustained load the following model of creep is proposed [Weber]. The failure probability of a random fibre is influenced by the external load, the orientation of the fibres, the individual strength of the fibres, the diameter of the fibres, through local load concentrations, temperature, external load, and various other smaller influences. If a single fibre is broken, the neighbour fibres can transmit this additional load within some length if the matrix is still intact and these fibres are not overloaded. So multiple of such small defects can occur all releasing a local small additional strain. Seen from external this can be detected as strain increase under constant load, what is the definition of creep.