| dc.description.abstract | In recent years, there has been a growing demand in the construction industry for slender, durable, and high performance structural elements. This has led to the development of an advanced cementitious composite material known as ultra high performance concrete (UHPC), which has gained significant attention primarily due to its enhanced mechanical and durability properties. However, the high cement content and associated costs have limited its widespread application in the construction industry. The studies reported in this thesis address these limitations by formulating a sustainable fibre-reinforced UHPC incorporating coarse aggregate and low-carbon supplementary cementitious materials (SCMs). Firstly, the aggregates were optimized experimentally based on their packing density and validated using the modified Toufar model (MTM), with reasonable agreement observed between experimental results and model predictions. Secondly, a Taguchi-grey relational analysis was used to determine the optimal SCM blend, contributing to reduced cement usage and enhanced performance. An optimum binder content was identified based on the target paste volume. The resulting UHPC, reinforced with hybrid fibres, exhibited improved mechanical properties and reduced porosity due to the synergistic effect between coarse aggregate and steel fibres.
Beyond UHPC mixture design, the thesis focuses on the compressive fracture process in UHPC under the framework of non-extensive statistical mechanics (NESM). Acoustic emission (AE) waveform parameters were simultaneously recorded to capture the evolution of internal damage. The AE inter-event time was analyzed using the q-exponential function, and the Tsallis entropic index (q-index) was determined to characterize the nature of microcrack interactions at various loading stages. A q-index greater than unity (q-index > 1) was associated with long-range interactions and stress redistribution, indicative of enhanced fracture resistance through microstructural toughening mechanisms such as fibre bridging and aggregate interlock. As the material approached failure, the q-index tends to unity, reflecting a dominance of short-range interactions, localized damage, and a transition to macroscopic failure, aligning with Boltzmann-Gibbs (BG) statistical mechanics.
Furthermore, the thesis investigated the energy accumulation and dissipation mechanisms in UHPC under unconfined uniaxial compression using q-index. A theoretical model connecting the power-law probability distribution of AE energy to q-index was applied, with temporal q-index variations reflecting damage evolution across different loading phases. The variation in q-index was associated with microcrack interaction forces, influencing stress redistribution and energy evolution. The q-index served as an indicator of energy accumulation and dissipation in cementitious materials and effectively captures the nonlinear microcrack interactions governing the fracture behaviour.
In addition, natural time (NT) analysis was applied to the AE time series to enhance the accuracy of failure prediction in UHPC subjected to compressive fracture. The key NT parameters, including the variance (k1), entropy change (DS), and complexity measure (Li), were used as precursors to identify the critical transition stage preceding macroscopic fracture. A critical value of k1 ≈ 0.07, a global minimum in DS, and an abrupt rise in Li were observed before the mainshock event. This behaviour was accompanied by a sudden drop in q-index, further confirming the onset of localization and instability. The fracture behaviour of UHPC exhibited strong resemblance to the Olami–Feder–Christensen earthquake model, evidenced by a sharp increase in cumulative AE energy after the critical region.
Eventually, this thesis presented a comprehensive fracture analysis of UHPC under Mode I crack tip deformation by integrating digital image correlation (DIC) with AE testing. The spatial evolution of the fracture process zone (FPZ) and the temporal variation of q-index index were simultaneously analyzed to characterize microcrack interaction mechanisms. The results indicated that hybrid fibre reinforcement significantly enhanced energy dissipation, stabilized crack propagation, and promoted a more ductile failure through increased crack path tortuosity and sustained microcrack interactions. The dual parameter approach using length of FPZ (lFPZ) and q-index provided an effective framework to link spatial damage progression with the statistical nature of fracture processes in UHPC.
The thesis integrates material optimization with statistical mechanics based characterization to understand the fracture processes in UHPC and to establish robust precursors for failure prediction. The findings facilitate the development of sustainable, fracture resistant UHPC and support advanced strategies for structural health monitoring and failure mitigation in fibrous cementitious composites. | en_US |
