CRACK-BRIDGING BEHAVIOUR OF POLYMERFIBRES IN STRAIN-HARDENING CEMENT-BASED COMPOSITES (SHCC) SUBJECT TO ALTERNATING TENSION-COMPRESSION CYCLIC LOADING

The thesis at hand presents the results of experimental investigations into the crack-bridging behaviour of polymer fibres in SHCC subject to alternating tension-compression loading regime. The investigations covered monotonic loading as well. The experimental programme included fibre tension tests, single-sided, single fibre pull-out tests, double-sided, single and multiple fibre pull-out tests as well as microscopic analysis of the specimens after testing. The bridging and pull-out behaviour of single PVA fibres embedded in cement-based matrix were comprehensively characterised and described by a new model. The Locking Front Model explains different phenomena of interaction between fibre and matrix after full debonding. Furthermore, the interaction and damage mechanisms under cyclic loading were understood. The damage types depend on various parameters such as the fibre inclination angle to the crack plane. Above all, however, the deterioration of the bridging capacity results from the damage of the fibres between crack faces in alternating tension-compression regime. The severity of damage is mostly determined by the number of cycles, compressive stress level and crack width. The results of experimental investigations at the micro- and meso-level were analysed further to establish a multi-scale approach for describing the behaviour of a single crack in the composite. The Non-Simultaneity Hypothesis is proposed, which suggests that the crucial events of fibre bridging action may occur non-simultaneously with increasing crack opening displacement and the bridging parameters may be reliably determined based on the overall behaviour of a group of specimens. Additionally, the Three Stage Micromechanics-based Model is developed to describe the bridging behaviour of the fibres with different embedded lengths. The parameters of the model were obtained according to the overall bridging behaviour and Non-Simultaneity Hypothesis. The parameters were validated by comparing the prediction with experiments, observed bridging behaviour in the tests with other embedded lengths or multiple fibres. In the framework of the novel concept, Criterion-Dependent Reference Volume (CDRV), the effective volume fractions of the fibres assuming non-uniform distribution of the fibres were determined over the length of a hypothetical specimen. The behaviour of a single crack was then predicted at the composite level and compared to the equivalent experimental results. The whole multi-scale approach manifests a considerable capability for analysing the behaviour of Fibre Reinforced Cement-based Composite (FRCC). Eventually, the concept of Representative Continuum with Predetermined Cracking Sequence (RCPCS) is briefly explained for describing the stress-strain behaviour of SHCC for further development of the multi-scale approach.