title: Shear behaviour of ultra-high performance concrete beams at elevated temperatures
Creators: name: Yang, Jiacheng
Publisher: name: Griffith University
Year: 2024
Subjects: subject: UHPC beam
subject: shear-dominant
subject: high temperature
subject: fire

description: Ultra-high performance concrete (UHPC) is known for its remarkable strengths (over 120 MPa in compressive strength and 10 MPa in tensile strength) and excellent durability. These advantages promote its wide range of applications in construction, such as large-scale buildings, large-span bridges, coastal buildings, and even retrofitting projects and exterior facades, demonstrating its development potential in reinforced concrete structures. The different material components and mechanical properties lead to a distinct shear resistance mechanism for UHPC beams. For example, the higher tensile strength provided by steel fibre on the web and bottom areas, the lack of aggregate interlocking along shear cracking, and the smaller cross-sectional dimensions. The shear performance of UHPC beams under fire exposure has not been explored yet. The UHPC beams may be highly susceptible to shear failure under fire conditions for several reasons. The shear resistance contributors to UHPC beams primarily consist of the concrete, steel fibres, and stirrups. The ultra-high mechanical properties of UHPC lead to a smaller cross-sectional size for UHPC beams. Considering the special thermal parameters of UHPC, the temperature of UHPC beams may increase more rapidly. This may lead to faster strength deterioration of the concrete in the compression zone, stirrups, and acceleration in steel fibre softening. Therefore, with the diminution in material strength and effective cross-sectional area, the ultimate shear capacity of UHPC beams reduces consistently under fire exposure, elevating the risk of shear failure. Due to the multitude of factors influencing shear performance of beams at high temperatures, the shear failure is unpredictable and uncontrollable. Additionally, shear failure can lead to beam fractures, which are not conducive to overall structural stability and post-fire repair. Therefore, it is necessary to explore the shear performance of UHPC beams. However, the existing calculation methods for the shear capacity of UHPC beams are semi-empirical and lack evaluation by massive experimental data. Furthermore, there have been no experimental or numerical studies on the shear performance of UHPC beams under fire exposure. Consequently, fire safety design recommendations for shear-dominant UHPC beams have not been generated. In this research, extensive experimental and numerical studies were carried out to fill these gaps. First, shear tests were carried out at ambient temperature. Eight full-scale UHPC beams were cast and tested to explore the influence of the longitudinal reinforcement ratio and shear span to depth ratio on the shear performance of UHPC beams. Through a comparative study with the collected shear test results, the reliability of the current shear bearing capacity design formulas for UHPC beams was evaluated. On the basis of that, a modified formula was proposed and verified. Second, ten full-scale UHPC beams dominated by shear were tested under ISO 834 fire curves with the aid of a large fire furnace. These experimental beams were exposed to elevated temperatures while being subjected to constant loads. The applied constant loads were calculated according to the derived formula of shear capacity from the shear tests at ambient temperature. The influence of several parameters on the fire performance of shear-dominant UHPC beams was explored. These parameters included load ratio, shear span to depth ratio, stirrup ratio, and longitudinal reinforcement ratio. Fire resistance, displacement changes, temperature distributions, and crack patterns were collected during the fire tests. The crack patterns and failure modes of UHPC beams were observed after the fire exposure and were compared with the shear test results at ambient temperature. Furthermore, through a comparative study, the applicability of the current fire resistance design specifications was determined. Third, the numerical studies were conducted using the commercial software ABAQUS. A three-dimensional finite element model was created to predict the shear performance of UHPC beams under fire exposure by using a sequential thermomechanical coupling method. Additionally, the user-defined material subroutine was used to compute the effects of transient strains and creep strains. Next, the accuracy of the proposed finite element model was then validated by the results obtained from conducted fire tests. Moreover, parametric studies were conducted with an emphasis on the effect of stirrup cover thickness, cross-sectional width, load ratio, and shear span to depth ratio on fire resistance. Finally, based on the results of the parametric analysis, design suggestions for fire resistance for UHPC beams were proposed by constraining the minimum cross-sectional width and minimum stirrup cover thickness, and the corresponding construction measures were provided. This offers theoretical and technical support for the fire safety of UHPC beams.