MECHANICAL AND FRACTURE STUDIES ON ULTRA-HIGH PERFORMANCE FIBRE REINFORCED CONCRETE

Abstract

Ultra high-performance fibre reinforced concrete (UHPFRC) is an advanced cementitious composite material with high compressive strength, and enhanced durability properties. A unique blend of pozzolanic materials, filler, and high range water reducer fibres with a low water-binder ratio produces a denser mix with enhanced durability. The Incorporation of steel fibres improve the tensile characteristics and crack resistance of the composite thereby, making it suitable for diverse applications such as bridges, buildings, off-shore structures, and rehabilitation projects. However, higher binder content, environmental impact, high cost, and lack of standard guidelines are some of the major aspects that hinder the application of UHPFRC in the construction industry. In this study, an attempt has been made to develop an environmentally sustainable UHPFRC composite by incorporating industrial waste materials such as fly ash, silica fume, and quartz powder. Apart from mix design and evaluation of mechanical properties, another important aspect that requires attention is the understanding of the fracture behavior of UHPFRC composite. The unique micro-structure and enhanced tensile characteristics demand an in-depth investigation into fracture and damage mechanisms. The fracture behaviour of UHPFRC is complex due to inherent material heterogeneity. Such a material, when loaded, develops a large size inelastic zone, named the fracture process zone (FPZ), ahead of the crack tip. This process zone, governed by various toughening mechanisms namely, matrix cracking, fibre-debonding, fibrebridging, fibre pull-out etc., and contributes towards quasi-brittle nature and overall fracture behavior of UHPFRC. One of the key benefits of UHPFRC is its ability to enable the design of structures with smaller cross-sections, resulting in lighter, more efficient constructions. The reduced dead weight of structural members makes it more susceptible to fatigue failure. Therefore, the investigation of repetitive loading to evaluate the fatigue performance of UHPFRC is important and has been attempted in the present study. In this thesis, extensive experimental investigations have been performed on geometrically similar notched beam specimens under monotonic and fatigue loadiii ing conditions. Three different beam sizes were tested, and the fracture analysis was performed using non-destructive techniques such as digital image correlation (DIC) and acoustic emission (AE). These techniques allowed for monitoring crack growth and damage evolution throughout the loading process. The fatigue behavior of UHPFRC has been explored through repetitive loading, revealing a three-stage fatigue fracture process: crack initiation, stable crack growth, and catastrophic failure, similar to conventional concrete. AE techniques revealed that tensile microcracks were the primary failure mechanism, with fibre reinforcement effectively mitigating local damage. Characterization of various fracture parameters, including fracture process zone was made by utilizing AE and DIC techniques. Further, to differentiate damage mechanisms under monotonic and fatigue loading, K-means clustering was employed. Fibre pull-out was observed to be the dominant mode of damage for failure under fatigue. The proposed work contributes towards improved understanding of fracture and damage mechanisms in UHPFRC composite.