International Concrete Abstracts Portal

The calculation of the nominal flexural strength of concrete members prestressed with hybrid (i.e., a combination of bonded and unbonded (steel and/or carbon fiber reinforced polymer (CFRP)) tendons is dependent on determining the stress in the unbonded prestressed reinforcement. Current provisions in the ACI CODE-318-25 are only applicable to members with either unbonded or bonded steel tendons. Additionally, while ACI PRC-440.4R-04 is adopted for unbonded CFRP tendons, neither ACI provisions address the use of hybrid tendons. This paper presents a closed-form analytical solution for the stress at ultimate derived based on the Modified Deformation-Based Approach (MDBA) that is applicable to beams prestressed with unbonded, hybrid (steel or FRP), external with deviators or internal tendons, with and without non-prestressed reinforcement. An assessment of its accuracy and applicability in calculating the nominal flexural strength is examined using a large database of 330 beams and slabs (prestressed with steel and/or CFRP tendons) compiled from test results by the authors as well as those available in the literature. Results predicted by the proposed approach exhibited excellent accuracy when compared to those predicted using ACI CODE-318 or ACI PRC-440 stress equations. They also show that the approach is universally applicable to any combination of bonded and/or unbonded (steel and/or CFRP) tendons, span-to-depth ratio, as well as loading applications.

This study presents a comprehensive field investigation into the long-term behavior of unbonded post-tensioned (PT) concrete flat slabs using Smart Strands embedded with fiber Bragg grating (FBG) sensors. The monitoring program was conducted in a real-world building in Seoul, Korea, spanning over five and a half years and capturing continuous prestressing force and deflection measurements at multiple slab locations. Results revealed that approximately 5% of nominal strength of tendon prestress losses occurred within the first year, stabilizing thereafter, and that deflection patterns were significantly influenced by slab position and construction activities. Comparison with analytical models showed strong alignment, with ACI CODE-318-25 time-dependent coefficients accurately predicting long-term deflections after the early-age period. This study contributes valuable long-term data, validating design codes and guidelines and enhancing understanding of the time-dependent behavior of PT concrete structures.

Corrosion—one of the major threats to the integrity of concrete structures—can consequently affect structure serviceability and ultimate limit state, possibly resulting in failure. Glass fiber-reinforced polymer (GFRP) can be used as an innovative alternative for conventional steel reinforcement in concrete structures, effectively addressing corrosion issues. In addition to its corrosion resistance and high strength-to-weight ratio, GFRP is commonly selected for nonprestressed bars and stirrups due to its cost advantage over other fiber-reinforced polymer (FRP) materials. The study endeavored to provide a comprehensive overview of the shear resistance in GFRP-reinforced concrete (RC) beams with short shear spans. The manuscript aims to synthesize and analyze shear test data based on published studies on GFRP-RC beams with a short shear span (a/d = 1.5 to 2.5). A comprehensive literature review was conducted to compile a database comprising 64 short GFRP-RC beams to evaluate the efficiency of using the strut-and-tie model (STM) for predicting the shear resistance of GFRP-RC beams. The findings reveal that ACI 318-19 STM yielded the most accurate predictions of the shear resistance of GFRP-RC beams with a/d of 1.5 to 2.5, because the current ACI CODE-440.11-22 and ACI 440.1R-15 design codes and guidelines do not include shear equations using the STM for predicting the shear resistance of GFRP-RC beams. Based on the findings of this study, the results could contribute to establishing shear equations in the upcoming revision of the ACI CODE-440.11-22 and ACI 440.1R-15 design codes and guidelines, specifically tailored for designing short GFRP-RC beams using the STM. The study also provides sufficient data to apply the STM in the design of GFRP-RC beams.

This paper presents the effectiveness of various reinforcing schemes in the end zones of prestressed concrete bulb-tee girders. The default girder, provided by a local transportation agency, includes C-bars and spirals intended to control cracking, and is analyzed using three-dimensional finite element analysis. The formulated models are used to evaluate the breadth of end zones, strain responses, cracking patterns, damage amounts, and splitting forces, depending upon the configuration of the end-zone reinforcement. The number of C-bars is not influential in developing strand stress along the girder. The maximum principal stresses exceed the conventional limit within h/4 of the girder end, where h is the girder depth; however, the 3h/4 limit adequately encompasses the stress profiles, particularly in the web of the girder. The maximum tensile strain in the concrete varies with the elevation of the girder and the inclined strands cause local compression in the C-bars, while spiral strains are independent of the number of bars. By positioning the C-bars, the vertical strain of the concrete decreases by more than 15.9%, which can minimize crack formation. Whereas the short-term crack width of the girder may not be an immediate concern, its long-term width is found to surpass the established limit of 0.18 mm (0.007 in.). In this regard, multiple C-bars should be placed to address concerns about undesirable cracking. The splitting cracks in the girder, resulting from the strand angles and eccentricities, can be properly predicted by published specifications within the range of 0.2h to 0.7h, beyond which remarkable discrepancies are observed in comparison with a refined approach. From a practical perspective, two to three No. 6 or 7 C-bars spaced 150 mm (6 in.) apart are recommended in the end zones alongside welded wire fabric.

Current design assumptions for precast, prestressed concrete piles embedded in cast-in-place (CIP) pile caps or footings vary across states, leading to inconsistencies in engineering practices. Previous studies suggest that short embedment lengths (0.5 to 1.0 times the pile diameter) can develop approximately 60% of the bending capacity of the pile, with full fixity potentially achieved at shorter embedment lengths than current design specifications due to confinement stresses. This study experimentally evaluates 10 full-scale pile-to-cap connection specimens with varying embedment lengths, aiming to investigate the required development length for full bending capacity. The findings demonstrate that full bending capacity can be achieved at the pile-to-pile cap connection with shallower embedment than code provisions, challenging existing design standards and highlighting the need for more accurate guidelines for bridge foundation design.