International Concrete Abstracts Portal

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.

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 the interface shear between ordinary concrete and ultra-high-performance concrete (UHPC) connected with glass fiber-reinforced polymer (GFRP) reinforcing bars. Following ancillary tests on reinforcing bar fracture under in-plane shear loading, concrete-reinforcing bar assemblies are loaded to examine capacities and failure modes as influenced by the size, spacing, and number of the reinforcing bars. While the shear behavior of bare reinforcing bars is primarily governed by the orientation of the load-resisting axes in the glass fibers and their volume, the size and spacing of the reinforcement largely control the interface capacity by affecting the load-transfer mechanism from the reinforcing bar to the concrete. The degree of stress distribution affects the load-displacement response of the interface, which is characterized in terms of quasi-steady, kinetic, and failure regions. The primary failure modes of the interface comprise rebar rupture and concrete splitting. The formation of cracks between the ordinary concrete and UHPC results from interfacial deformations, leading to spalling damage when applied loads exceed service levels. An analytical model is formulated alongside an optimization technique. The capacities of the interface in relation to the reinforcing bar rupture and concrete splitting failure modes are predicted. Furthermore, a machine learning algorithm is used to define a failure envelope and propose practice guidelines through parametric investigations.

Recent tests showed that anchorage failure could be the primary mechanism that limits the strength and deformation capacity of column-footing connections. An experimental program consisting of the reversed cyclic load testing of 16 approximately full-scale column-footing subassemblages was thus conducted to investigate the effect of various reinforcement details on connection strength, drift capacity, and failure mode. The main parameters evaluated were type of anchorage for the column longitudinal bars (either hooks or heads), extension of column transverse reinforcement into the footing, and longitudinal and transverse reinforcement ratios in the footing. Test results indicate that even when column longitudinal reinforcement extends into the joint with a development length in accordance with ACI 318-19, a cone-shaped concrete breakout failure may occur, limiting connection strength and deformation capacity. The use of transverse reinforcement in the connection over a region extending up to one footing effective depth away from each column face proved effective in preventing a concrete breakout failure. However, for the specimens with column headed bars, extensive concrete crushing adjacent to the bearing side of the heads and spalling beyond the back side of the heads led to significant bar slip and “pinching” in the load versus drift hysteresis loops at drift ratios greater than 3%. The use of U-shaped bars in the joint between the column and the footing or slab, as recommended in ACI 352R-02, led to improved behavior in terms of strength and deformation capacity, although it did not prevent the propagation of a cone-shaped failure surface outside the joint region. Based on the test results, the basic concrete breakout strength, Nb, corresponding to a 50% fractile, in combination with a cracking factor ψc,N = 1.25, is recommended when using Section 17.6.2. of ACI 318-19 for calculation of concrete breakout strength in connections similar to those tested in this investigation.

Through uniaxial tensile tests, the mechanical behavior of bone shaped concrete reinforced with glass textile and carbon textile impregnated with epoxy resin was verified using a stress-strain response curve. It was observed that elements reinforced with glass fabric presented different mechanical responses depending on the textile reinforcement rate. In samples with two layers of glass fabric, three stages were formed, as predicted in the literature. In the specimens reinforced with only one layer, the structural incapacity of the element was observed. For samples reinforced with carbon textile, there were problems with slipping and spalling caused by the concentration of stress at the ends of the piece. Even so, it was possible to clearly determine the three stages in the curve response of the material. The stresses experimentally obtained in the elements reinforced with carbon textile obtained results approximately five times greater than those of the glass fabric.