International Concrete Abstracts Portal

Ultra-high performance concrete (UHPC) is a forward-looking material ideal for use in large-scale civil infrastructure systems. However, due to its unique mix, when UHPC is used in actual structures in conjunction with materials like steel reinforcement, it may lead to unexpected behavior. Therefore, this study analyzed the behavior of reinforced UHPC (R-UHPC) for use in actual structures, focusing specifically on beams among various structural components, with a particular emphasis on their flexural behavior. For this purpose, the study collected and comprehensively analyzed experimental data from flexural tests of R-UHPC beams conducted to date, identifying basic mechanics, peculiarities, and considerations in structural design. This study highlights that, besides the commonly known longitudinal reinforcement ratio, numerous factors such as beam length, height, number of tension reinforcement layers, strength, etc., can influence the flexural behavior of R-UHPC beams and demonstrate how these elements impact the performance.

Inverse analysis methods proposed by current standards for extracting the tensile properties of tension-hardening cementitious materials from indirect tension tests (for example, flexural prism tests) are considered either cumbersome and can only be performed by skilled professionals or apply to certain configuration and specimen geometries. Significant discrepancies are reported between the results of direct tension (DT) tests and inverse analysis methods. This has eroded confidence on flexural tests as a method of characterization of tension-hardening ultra-high-performance concrete (UHPC) and has motivated its abandonment in favor of DT testing. Additional concerns are size sensitivity, variability, and lack of robustness in the results of some methods. However, DT tests are even more difficult to conduct and results are marked by notable scatter. This is why some codes allow for bending tests at least for quality control of UHPC. To address the limitations of the bending tests in providing an easy and quick method for reliable estimation of the tensile characteristic properties of UHPC, a new practical method is developed in this paper based on a forward analysis (FA) of third-point bending tests. A unique aspect of the approach is that it considers the nonlinear unloading that occurs in the shear spans of the prism after strain localization in the critical region. The method was used to derive charts for direct estimation of the tensile properties from quality control bending tests, for the commonly used flexural specimen forms and material types. The goal of the study is to provide a practical alternative in characterization of tension-hardening UHPC materials. Results obtained using the proposed FA method are in good agreement with the tensile response from DT tests. However, it is noted that due to the presence of a strain gradient in bending tests and the larger strain gauge lengths employed in some DT tests, the strain values at localization from DT tests tend to be more conservative.

Ultra-high-performance concrete (UHPC) is increasingly gaining attention for structural applications, with structural fire safety being a key design factor. It is evident from recent research that UHPC structural members are prone to fire-induced spalling and have lower fire resistance than traditional concrete members. Currently, there are no specific guidelines for the fire design of UHPC members, and extending existing fire design provisions developed for conventional concrete members may not be appropriate considering the unique challenges posed by UHPC. This paper outlines the critical factors contributing to the lower fire performance of UHPC structural members, discussing these factors in detail, using data from both numerical and experimental studies. Based on the results from parametric studies, as well as observations from published data, a set of design guidelines for mitigating spalling and enhancing fire resistance of UHPC beams is proposed.

This paper presents a design model for the one-way shear of ultra-high-performance concrete (UHPC) beams without transverse reinforcement. The model unifies the shear design of UHPC with the ACI 318 shear design approach for conventional concrete. Hence, the proposed model accounts for the longitudinal reinforcement ratio and the axial load effects, while the tensile strength of UHPC replaces the concrete compressive strength term. The effects of fiber type, fiber alignment, beam shape, and beam size are incorporated through dimensionless parameters, with their values calibrated using UHPC beam and panel shear data sets. The proposed shear model was evaluated using a database of 137 UHPC nonprestressed and prestressed rectangular and I-shaped beam shear tests performed in the United States and elsewhere.

This paper presents an experimental study on the residual bond of glass fiber-reinforced polymer (GFRP) reinforcing bars embedded in ultra-high-performance concrete (UHPC) subjected to elevated temperatures, including a comparison with ordinary concrete. Based on the range of thermal loading from 25 to 300°C (77 to 572°F), material and pushout tests were conducted to examine the temperature-dependent properties of the constituents and behavior of the interface. Also performed were chemical and radiometric analyses. The average specific heat and thermal conductivity of UHPC are 12.1% and 6.1% higher than those of ordinary concrete, respectively. The temperature-induced reduction of density in these mixtures ranges between 5.4 and 6.2% at 300°C (572°F). Thermal damage to GFRP, in the context of microcracking, was observed after exposure to 150°C (302°F). Fourier transform infrared spectroscopy (FTIR) reveals prominent wavenumbers at 668 and 2360 cm–1 (263 and 929 in.–1), related to the bond between the fibers and resin in the reinforcing bars, while spectroradiometry characterizes the thermal degradation of GFRP through diminished reflectivity in conjunction with the peak wavelength positions of 584 nm (2299 × 10–8 in.) and 1871 nm (7366 × 10–8 in.). The linearly ascending bond-slip response of the interface alters after reaching the maximum shear stresses, leading to gradual and abrupt declines for ordinary concrete and UHPC, respectively. The failure mode of the ordinary concrete interface is temperature-sensitive; however, spalling in the bonded region is consistently noticed in the UHPC interface. The fracture energy of the interface with UHPC exceeds that of the interface with the ordinary concrete beyond 150°C (302°F). Design recommendations are provided for estimating reductions in the residual bond of the GFRP system exposed to elevated temperatures.