International Concrete Abstracts Portal

This paper presents an experimental study on the residual bond of glass fiber-reinforced polymer (GFRP) rebars 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°C (77°F) to 300o°C (572o°F), material and push-out tests are conducted to examine the temperature-dependent properties of the constituents and the behavior of the interface. Also performed are 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 300o°C (572o°F). Thermal damage to GFRP, in the context of microcracking, is observed after exposure to 150°C (302°F). Fourier transform infrared spectroscopy reveals prominent wavenumbers at 668 cm^-1 (263 in.^-1) and 2,360 cm^-1 (929 in.^-1), related to the bond between the fibers and resin in the rebars, while spectroradiometry characterizes the thermal degradation of GFRP through diminished reflectivity in conjunction with the peak wavelength positions of 584 nm (2,299×10^-8 in.) and 1,871 nm (7,366×10^-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 the 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 150o°C (302o°F). Design recommendations are provided for estimating reductions in the residual bond of the GFRP system exposed to elevated temperatures.

Previous research studies have been conducted to study the seismic response of low-aspect-ratio RC shear walls when designed using normal-strength reinforcement (NSR) versus high-strength reinforcement (HSR). Such studies demonstrated that the use of HSR has the potential to address several constructability issues in nuclear construction practice by reducing the required steel areas and subsequently rebar congestion. However, the response of nuclear RC shear walls (i.e., aspect ratios of less than one) with both HSR and axial loads has not yet been evaluated under ground motion sequences. As such, most nuclear design standards restrict the use of HSR in nuclear RC shear wall systems. Such design standards do not consider the influence of axial loads when the shear strength capacity of such walls is calculated. To address this gap, the current study investigates the influence of axial load on the performance of nuclear RC shear walls with HSR when subjected to ground motion sequences using hybrid simulation testing and modelling assessment techniques. In this respect, two RC shear walls (i.e., W1-HSR and W2-HSR-AL), with an aspect ratio of 0.83, are investigated. Wall W2-HSR-AL had an axial load of 3.5% of its axial compressive strength, while wall W1-HSR had no axial load. The test walls were subjected to a wide range of ground motion records, from operational basis earthquake (OBE) to beyond design basis earthquake (BDBE) levels. The experimental results of the walls are discussed in terms of their damage sequences, cracking patterns, ductility capacities, effective periods, and rebar strains. The test results are then used to develop and validate a numerical OpenSees model that simulates the seismic response of nuclear RC shear walls with different axial load levels. Finally, the experimental and numerical results are compared to the current ASCE 41-23 backbone model for RC shear walls. The experimental results demonstrate that walls W1-HSR and W2-HSR-AL showed similar crack patterns and subsequent shear-flexure failures; however, the former had wider cracks relative to the former during the different ground motion records. In addition, the axial load reduced the displacement ductility of wall W2-HSR-AL by 18% compared to wall W1-HSR. Moreover, the ASCE 41-23 backbone model was not able to adequately capture the seismic response of the two test walls. The current study enlarges the experimental and numerical/analytical database pertaining to the seismic performance of low-aspect-ratio RC shear walls with HSR to facilitate their adoption in nuclear construction practice.

The reduction in shear capacity when using recycled coarse aggregate (RCA) made from crushed concrete is evaluated in terms of tensile cracking and fracture-surface characteristics. An experimental investigation into the fracture and flexure-shear behaviors of recycled aggregate concrete (RAC) is presented. Replacing natural aggregate in concrete proportioned for 30 MPa (4350 psi) compressive strength with RCA results in lower compressive and tensile strengths. The tensile fracture-surface characteristics vary between RAC and natural aggregate concrete (NAC). While the surface area created in the tensile fracture of RAC is larger than that of NAC, the fracture surface profile in RAC has a smaller roughness than NAC. In the flexure-shear response of reinforced concrete beams, the dilatancy determined from the slip and crack opening displacements measured across the shear crack is smaller in RAC than in NAC. The failure in the reinforced beam is due to the frictional stress transfer loss across the primary shear crack. There is a larger decrease in the shear capacity with the use of RAC than indicated by the reduction in compressive strength. The reduced shear capacity of reinforced RAC is due to the combined influences of reduced tensile strength and crack surface roughness. The design provisions require calibration for crack surface roughness when using RAC in structural applications.

Linear structural analysis is the method of choice commonly used by practicing engineers to support the seismic design of a structure. The structural models are developed in commercial software and incorporate stiffness modifiers, which lower the stiffness of the members, in recognition of all the sources of flexibility that occur upon cracking of the concrete. This paper describes a mechanics-based model to compute the stiffness modifiers for columns with a circular cross section. The mechanics-based model accounts for five modes of deformation observed. Calibration of this model was performed with a database of tests reported in the literature on 22 circular-section columns that exhibited ductile response. The paper ends by describing a simplified method for use in design. The mechanics-based model and the design method yield an effective column lateral stiffness that closely aligns with the values obtained from the column database.

The term “mass concrete” characterizes a specific concrete condition that typically requires unique considerations to mitigate extreme temperature effects on a structure. Mass concrete has historically been defined by the physical dimensions of a massive concrete element with the intent of identifying when differential temperatures may induce early-onset cracking, leading to reduced service life. More recently, in addition to differential temperature considerations, extreme upper temperature limits have been imposed by the American Concrete Institute to prevent long-term concrete degradation. Studies dating back to 2007 show shafts as small as 48 in. (1.2 m) in diameter can exceed both differential and peak temperature limits; in 2020, augered cast-in-place piles as small as 30 in. (0.76 m) in diameter exceeded one or both limits. This suggests the term “mass concrete” is misleading when considering today’s high-early-strength or high-performance mix designs. This study applies numerical modeling coupled with field measurements to investigate the effects of concrete mix design, drilled shaft diameter, and environmental conditions on heat energy production and temperature. Further, the outcome of this study focuses on developing criteria that combine the effects of both size and cementitious material content to determine whether unsafe temperature conditions may arise for a given drilled shaft design.