International Concrete Abstracts Portal

Fiber-reinforced polymer (FRP) reinforcements have been utilized in versatile forms to enhance durability performance and consequently to attain longevity of concrete structures in recent construction practices. The shear strength of FRP-reinforced concrete (FRP-RC) beams holds significant importance in structural design. However, there exists inherent analytical uncertainty concerning shear in concrete members due to the distinctive material characteristics of FRP bars compared to conventional steel reinforcements, such as their low axial stiffness and bond properties. This study aims to identify the shear resistance mechanisms developed under combined actions between concrete and FRP reinforcements. To this end, the dual potential capacity model (DPCM) is extended to FRP-RC beam members subjected to shear and flexure, and an attempt was also made to derive a simplified method. To validate the proposed approaches, a total of 437 shear test results from RC members incorporating FRP bars were utilized. Findings indicate that the proposed methods can provide an acceptable level of analytical accuracy. In addition, a significant shift in the shear failure mode of FRP-RC members with no stirrup was observed from the compression zone to the cracked tension zone as the FRP reinforcement ratios increased. Conversely, when FRP stirrups were added, the shear failure mode was mostly dominated by the compression zone.

Ultra-high-performance concrete (UHPC) enables thinner, longer-span elements with fewer or no reinforcing bars. This study investigates the shear behavior of rebar-free UHPC panels with a thickness of 4 in. (101.6 mm) and 2.0% volumetric content of straight steel fibers. The panels were tested under combined shear and axial loads using the Universal Panel Tester (UPT) facility. The UPT experiments were complemented with small-scale direct tension tests (DTT) and large-scale tension strip tests (TST) to investigate the effect of UHPC tensile characteristics on shear. The panels exhibited ductile responses with post-peak residual shear capacities higher than 20% of the maximum shear stress, with the TSTs providing an improved correlation to UHPC shear than the DTTs. Test results showed that the relative effect of axial loads on UHPC shear can be greater than the relative effect on conventional concrete per ACI 318. It was also found that a correlation exists between fiber alignment and UHPC’s tensile behavior, which can alter the localization stress by as much as 39%.

This paper presents the torsional behavior of hollow reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) U-wraps. Test parameters involve a variable wall thickness in the section and the width and spacing of the externally bonded CFRP sheets. An experimental program is conducted with 27 beams (3 unstrengthened and 24 strengthened) to examine their capacities, shear flows, and force distributions when incorporating a ratio of 0.27 to 0.46 between the areas of the hollow and gross cross-sections. The stiffness and capacity of the test beams are dominated by the wall thickness and the effectiveness of CFRP-strengthening becomes pronounced as the void of the beams decreases. The presence of CFRP redistributes internal shear forces in the cross section, which is facilitated by narrowing the spacing of the U-wraps. The effective zone of CFRP retrofit is positioned near the outer boundary of the strengthened section. Regarding crack control, multiple discrete U-wraps with narrow spacings outperform wide U-wraps with enlarged spacings. While the location of a shear-flow path is dependent upon the wall thickness, the width of the U-wraps controls the effective shear-flow area of the beams. The size of the void is related to the stress levels of internal reinforcing components, including yield characteristics. Transverse stirrups are the principal load-bearing element for the unstrengthened beams; however, the reliance on the stirrups is reduced for the strengthened beams because the U-wraps take over portions of the torsional resistance. Through a machine learning approach combined with stochastic simulations, design recommendations are proposed.

This paper presents the implications of creep-fatigue interactions for the long-term behavior of bulb-tee bridge girders prestressed with either steel strands or carbon fiber-reinforced polymer (CFRP) tendons. A large amount of weigh-in-motion data incorporating 194 million vehicles are classified to realistically represent live loads. Computational simulations are conducted as per the engagement of discrete autonomous entities in line with time-dependent material models. In general, the properties of the CFRP tendons insignificantly vary over 100 years; however, the stress range of CFRP responds to fatigue cycles. Regarding prestress losses, the conventional method with initial material properties renders conservative predictions relative to refined approaches considering time-varying properties. The creep and fatigue effects alter the post-yield and post-cracking responses of the steel- and CFRP-prestressed girders, respectively. From deformational capability standpoints, the steel-prestressed girders are more vulnerable to fatigue in comparison with the CFRP-prestressed ones. It is recommended that the fatigue truck and the compression limit of published specifications be updated to accommodate the ramifications of contemporary traffic loadings. Although the operational reliability of both girder types is satisfactory, the CFRP-prestressed girders outperform their steel counterparts in terms of fatigue safety. Technical findings are integrated to propose design recommendations.

Either subjected to tensile or compressive loads, concrete is susceptible to the effect of sustained loading. To address this, common practice in building guidelines typically involves applying a sustained loading factor ranging from 0.6 to 0.85. Given that the shear capacity of structural members without shear reinforcement is linked to the concrete strength, one might question whether there is a comparable sustained loading impact on shear. To address this inquiry, a total of 18 reinforced concrete beams without shear reinforcement were subjected to prolonged sustained loading, with a load intensity factor (ratio of ‘applied sustained shear load’ to ‘short-term shear resistance’) ranging from 0.88 to 0.98. Several beams endured the sustained loading test for an extended period, close to a decade before the test was terminated. Interestingly, in contrast to concrete subjected to direct compression or tension, it was observed that sustained loading did not affect the shear capacity. Some early results of this experimental study, where concrete beams were subjected to up to 4 years of sustained loading, have been previously published. This paper concludes the results of the testing campaign up to a decade of sustained loading with additional results and findings.