Stress- and temperature-dependent creep behaviour of polymer fibre reinforced concrete

• Applicants: Professor Dr.-Ing. Steffen Anders; Dr.-Ing. Dominik Brands; Professor Dr.-Ing. Jörg Schröder
• Subject Area: Construction Material Sciences, Chemistry, Building Physics ; Applied Mechanics, Statics and Dynamics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 533826000

Concrete is known for its susceptibility to cracking. Therefore, reinforcing steel is usually used. To limit crack-widths in structures or for slabs of logistic-areas concrete is often reinforced solely using steel-fibres. Besides possible corrosion, energy input and CO2-emissions of steel production are critical aspects regarding current social challenges. Polymer-fibres are a sustainable alternative to partly replace steel. They have already been used successfully in several projects, nationally and internationally. However, in design for load-bearing structures polymer fibre reinforced concrete (PFRC) needs specific approval in Germany and in other countries. The main obstacle for further application is the lack of knowledge on creep behaviour and effects of temperature on the behaviour of polymer-fibres and PFRC. Consequently, a scientific analysis of the potentials of PFRC is required, based on a comprehensive description of the temperature- and stress-dependent creep behaviour as well as a thermodynamically consistent modelling. Initially, creep behaviour of single polymer fibres is investigated depending on stress and temperature and will be modelled with "generalised rheological models". Afterwards, short- and long-term tests in direct tension and flexure are needed to calibrate the numerical models. Furthermore, these tests are important to connect the results to performance tests in existing regulations. In addition to temperature and stress-dependent description of the material behaviour, it shall be shown that the "Master Curve Approach", which is known for polymers and that is standardised e.g. for asphalt is suitable to reduce currently demanded test durations of 12 to 18 months to 4 to 6 months. Due to the visco-elastic properties of polymer fibres with elongations up to 30%, the concept of “generalised stress-strain measures“ is consistently implemented in the models. Based on preliminary considerations, we expect realistic rate-dependent stress-strain curves for the polymer-fibres when using the non-linear rheological model by Schapery. Throughout the project, experimental results are used for calibration. Furthermore, the "time-temperature superposition principle" to consider the temperature dependencies. For the description of the concrete, a phase-field model is used to cure the pathological mesh dependency of the FEM due to softening material behaviour. Modelling of fibre-concrete interaction is initially carried out for selected boundary value problems within the framework of the rebar element technique. Subsequently, fibre orientation- and distribution functions are used to model practically relevant applications together with a phenomenological model. Since the applicants have achieved convincing results for steel fibre reinforced concretes in previous research projects with comparable concepts, a realistic characterisation of PFRC seems possible with the planned comprehensive extensions of experiments and models.

• DFG Programme: Research Grants