International Concrete Abstracts Portal

Low-cycle fatigue and monotonic tension tests were performed on steel reinforcing bars microalloyed using niobium and vanadium and processed by various hot-rolling and post-rolling cooling production strategies. The objective was to identify beneficial alloy designs and production techniques that deliver cross-diameter microstructures at different strength levels with improved fatigue properties. Bars were sourced from the United States and China to represent a range of alloy designs and production methods common in those countries. Parameters considered included the microalloying content of vanadium (V) and/or niobium (Nb), carbon content (C), overall alloy content (CE), hot-rolling/postrolling cooling strategies, microstructures/grain size, stress-strain tensile curve shape, hardness, and rib geometry. Ferrite fraction and grain size, average cross-section hardness, and bar deformations were found to be influential on fatigue life. Bar chemistries and processing techniques that result in increased ferrite fraction and reduced grain size are recommended to improve the low-cycle fatigue performance of reinforcing bars.

To reduce CO2 emissions of concrete, a slag-based zero-cement concrete (ZC) of high strength (60 MPa [8.70 ksi]) was developed. In the present study, cyclic loading tests were conducted to investigate the seismic performance of full-scale interior precast beamcolumn joints using the new ZC. One monolithic portland cementbased normal concrete (NC) beam-column joint and two precast ZC beam-column joints were tested. The test parameters included concrete type, fabrication method, and beam bottom bar anchorage detail. The structural performance was evaluated, including the strength, deformation capacity, damage mode, and energy dissipation. The test results showed that the structural performance of the precast ZC beam-column joints could be equivalent, or superior, to that of the monolithic NC beam-column joint. Although the reinforcement details of the ZC joints do not satisfy the seismic design requirements of special moment frames in ACI 318-19, the seismic performance of the ZC joints satisfied the requirements of ACI 374.1-05 and AIJ 2002 Guidelines.

To secure emulative seismic performances of precast concrete (PC) special moment frame buildings, two capacity-based connection design options (i.e., strong and ductile precast connections) are provided in the current ACI 318 code. However, the evolving performance-based seismic design and response evaluation requires a reasonable estimation of the energy dissipation and corresponding hysteresis damping characteristics so that their potential performance level can be properly predicted. Therefore, this study focuses on the seismic performances, especially the energy dissipation and damping performances of the code-compliant PC wide beam-column connections. Three PC-wide beam-column connection specimens under the ductile connection design principle with different joint details and an RC control specimen were fabricated and tested under reversed cyclic loadings. In addition, an energy-based macro modeling method was developed to characterize the cyclic responses including the damping response of PC wide beam-column connections. The test results revealed that code-required overstrength of shear friction strength between PC beam members and cast-in-place (CIP) concrete is crucial to achieving the ductile performance of precast connections. It also appeared that the energy-based macro modeling method can well capture the hysteresis features through the relationship between equivalent viscous damping (EVD) ratio and ductility capacity of PC-wide beam-column connections.

This paper presents cyclic test results of nine beam-column joints, fabricated with Grades 60, 80, and 100 (Grades 420, 550, and 690) No. 11 (D36) headed bars in joints with matching concrete strengths. With the exception of the joint shear determination, each specimen was designed in accordance with ACI 318-14 provisions for special moment frame joints. The headed bar embedment length was determined using 1.0fy in accordance with ACI 318-14, rather than the ACI 318-19 requirement to use 1.25fy to calculate the development length. Test results demonstrate that the ACI 318-14 development length can be used for closely spaced high-strength No. 11 (D36) headed bars in joints with Code-conforming transverse reinforcement. This paper also aggregates the test data with prior research and compares the data with descriptive equations and breakout methodology for assessing the development length and anchorage strength of headed bars. The results support several design recommendations for the ACI 318 provisions.

In this study, full-scale loading tests were conducted to investigate web-shear strengths of hollow-core slab (HCS) members strengthened in shear by using practically viable methods. All the HCS units used in the current test program were fabricated by using the individual mold method, not by the extrusion method, and the key experimental variables of the shear test were set as the presence of shear reinforcement, core-filling concrete, topping concrete, and also the magnitude of effective prestress. The shear force-displacement behaviors, crack patterns, and strain response of shear reinforcements were reported in detail. In addition, to identify the shear strength enhancement provided under various strengthening conditions in a quantitative manner, existing shear test results of series specimens, including a naked HCS member and corresponding composite HCS members with cast-in-place (CIP) concrete and/or shear reinforcements, were collected from literature. On this basis, a practical design expression capable of estimating shear strengths of HCSs strengthened with CIP concrete and stirrups was suggested based on the ACI 318 code equation. The proposed method evaluated the shear strengths of the collected specimens with a good level of accuracy, regardless of the presence of corefilling concrete, topping concrete, and shear reinforcements.