PSI - Issue 66

Bineet Kumar et al. / Procedia Structural Integrity 66 (2024) 337–343 Author name / Structural Integrity Procedia 00 (2025) 000–000

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binder content and notably low water-to-binder ratio (w/b), usually falling within the range of 0.14 to 0.20. The incorporation of various pozzolanic and filler materials such as silica fume, quartz powder, quartz sand, superplasticizers, and steel fibers contributes to UHPFRC’s superior mechanical properties. These include high levels of compressive and tensile strength, outstanding ductility, and increased fracture energy. Such characteristics make UHPFRC a highly effective material for a diverse array of construction applications, including military and civil infrastructure, such as fortifications, nuclear waste containment structures, highway bridges, and high-rise buildings. A comprehensive understanding of UHPFRC’s mechanical behavior is crucial for the accurate design of structural components and for the calibration and validation of constitutive models. The mechanical and fracture characteristics of cementitious materials, which are inherently quasi-brittle, have been shown to be dependent on factors such as size and geometry, as widely documented in existing scientific studies (Kumar and Ray (2022), Accornero et al. (2022), Kumar and Ray. (2023). However, there remains a lack of detailed understanding regarding how these size and geometric factors specifically influence the performance of ultra-high-performance fibre reinforced composites. Bridging this knowledge gap is critical to optimize the design of structural elements utilizing UHPFRC, necessitating focused research into the effects of geometry-dependent mechanical properties on their overall behavior. The scientific community has also highlighted the importance of "wall effect" caused by fibre orientation, which becomes a significant factor contributing to the size effect, particularly in fibre-reinforced composites (FRCs) (Carpinteri et al. (2023), Accornero et al. (2023), Mobasher et al. (2021)). As specimen size increases, the randomness of fibre orientation tends to rise. In contrast, in smaller specimens, long fibres often align horizontally along the boundaries, enhancing their effective length for bridging across developing cracks (Yao et al. (2021), Monterio et al. (2023)). This improved fibre-bridging capability in smaller specimens, compared to larger ones, suggests the necessity of studying fibre orientation alongside specimen size to understand the wall effect mechanism comprehensively across different fibre contents. In the present study, the fracture behavior of center-point bend beams under flexural loading has been examined, considering variables such as specimen size, fibre percentage, and orientation. Geometrically similar beam specimens of three different sizes have been prepared, each with two different fibre contents: 1.5% and 2.5%. Load CMOD (Crack Mouth Opening Displacement) behavior has been recorded during center-point loading tests. Subsequently, inverse analysis has been performed by combining tensile stress profiles derived from both stress strain and stress-crack width relationships, leading to the development of an analytical formulation. The proposed material model takes into account the effects of specimen size, fibre orientation, and fibre yielding. Calibration of the model parameters has been achieved by aligning the proposed model results with experimental findings, followed by validation. This formulation offers insights into predicting various fracture stages, bridging mechanisms, fibre pull-out behavior, and crack propagation trends, while also considering the influences of The UHPFRC composites in this research has been produced using Type-1 Portland cement, which has a specific gravity of 3.15. Sand served as the fine aggregate, with a specific gravity of 2.65 and a fineness modulus of 2.5. To lower the porosity, quartz powder (particle size of 20 μ m) have been added alongside a polycarboxylic ether-based high-range water reducer, maintaining a water-to-binder (w/b) ratio of 0.18. To evaluate the influence of fiber reinforcement, hooked-end steel fibers measuring 30 mm in length and 0.6 mm in diameter have been mixed into the composite at proportions of 1.5% and 2.5% by volume. After curing for 28 days, the concrete cubes achieved compressive strengths of 128 MPa and 142 MPa for 1.5% and 2.5% fiber contents, respectively, with elastic moduli of 41,300 MPa and 44,000 MPa. The study examined the flexural performance of the composite, particularly in relation to specimen size, by using geometrically similar beam specimens in three sizes: small, medium, and large. The dimensions of these specimens, designed according to the RILEM guidelines, have been specified in Table 1, with span-to-depth and notch-to- specimen size and fibre content. 2. Experimental plan and results