Purposeful setting of the bond properties and durability of ductile, strain-hardening composites based on recyclable polymer fibers and sustainable matrices

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Strain-hardening cement-based composites (SHCC) with high durability can be achieved by reinforcement with short polymer fibers. Prerequisites for this are well-balanced fiber-matrix interactions and stable interphase properties over the entire service life. In the previous project it was proven that the interaction with concrete matrices of high cement content and, hence, the composite ductility can be specifically influenced by surface modification of the fibers. In the project presently applied for, the understanding of these interactions is to be deepened by the systematic variation of the surface properties of fibers made of recyclable polymers and transferred to novel, sustainable concrete matrices. In addition, investigations on the long-term behavior of the sustainable SHCC will be included.The project is subdivided into two subprojects. In the first subproject, high-modulus polymer fibers (PE, PP, PET) are to be modified by chemical bonding and cross-linking of different substances in order to achieve a wide range of different surface properties (non-polar, acidic, basic) and to improve the chemical resistance of the fibers in the alkaline matrices. One point of emphasis is to be sectional modification, which should enable strong anchoring of the fiber ends in the matrix and allow debonding of the fiber in the middle portion during the crack-bridging process.In the second subproject, the modified fibers will be embedded in sustainable concrete matrices. In accordance with the LC³-concept (Limestone Calcined Clay Cement), the cement clinker content will be halved and such industrial byproducts as fly ash and silica fume will be excluded by using calcined clay and limestone. The fiber-matrix interactions will be quantified by single-fiber pullout tests and the fiber-matrix interphases formed will be morphologically, then physically and chemically characterized to relate the properties of fiber and matrix. Furthermore, failure mechanisms of the fiber are to be analyzed. The stress-strain behavior of the highly ductile SHCC is to be investigated by means of uniaxial tension experiments. Thereby, the influences of the mixing regimes in respect of fiber and particle dispersions as well as fiber-matrix-interaction will be characterized. In long-term exposure tests subject to different environments, the influence of the fiber modifications on the durability of the fiber-reinforced composites will be clarified.Based on the results of both projects, an empirical material model and a concept for a targeted design of the fiber-matrix interaction of sustainable SHCC is to be developed.