International Concrete Abstracts Portal

Flexural strength and ductility of exclusively bonded or unbonded steel prestressed concrete (PC) members are well covered and documented in the literature and codes of practice. However, current design methods are limiting the use of hybrid (i.e., a combination of unbonded and bonded steel and Fiber Reinforced Polymer (FRP)) tendons, particularly when using brittle material such as FRP tendons. In this paper, a general procedure for evaluating the nominal moment capacity and ductility of hybrid PC members was developed using the strain compatibility approach. The procedure is applicable for members with any combination of bonded or unbonded steel and FRP tendons. Using a capacity design approach based on strain compatibility, the ductility performance of several hybrid systems with different parameters was compared. The parameters included, among others, the level of “net tensile strain” in the tension reinforcement at nominal strength adopted in ACI 318-19 as a measure of ductility; concrete compressive strength; and the newly defined hybrid prestressing ratio (HPR). HPR represents the ratio of the moment contribution of the unbonded tendons to the total moment capacity of the member with hybrid tendons. Non-linear analysis was carried out to generate the entire load-deflection and moment-curvature responses of the different systems. The accuracy of the nonlinear analysis was verified by comparing with available experimental data and the analysis results were used to compare traditional curvature ductility measures of the various systems against the ductility measure specified in the ACI Building code. A design example is provided in Appendix A to illustrate the use of the strain compatibility approach.

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.

Numerous shear tests on high-strength high-performance fiber-reinforced cementitious composites (HS-HPFRCC) and ultra-high-performance Concrete (UHPC) over the last three decades have enriched the understanding of their shear strength. This study integrates these experiments, which focused on specific shear strength parameters, into a comprehensive analysis. The Initial Collection Database, containing 247 shear tests, was developed for this purpose. From this, the Evaluation Shear Database was derived using specific filtering criteria, resulting in 118 beams pertinent to HS-HPFRCC and UHPC materials. These databases are accessible to the engineering community for advancing the evaluation and development of shear strength formulations in structural design codes. This study concludes with an analysis of a subset of the Evaluation Shear Database, consisting of beams with reported uniaxial tensile strength. This analysis demonstrates the Evaluation Shear Database's applicability and highlights limitations in existing design equations. Notably, their reliance on a single predictor variable constrained predictive power.

This paper presents the implications of a hydrocarbon fire on the behavior of bridge girders prestressed with either steel strands or carbon fiber-reinforced polymer (CFRP) tendons. Stemming from a recent bridge fire that occurred in Lakewood, CO, numerical investigations are conducted employing a computational method called agent-based modeling to understand the intricate responses of these girders under thermomechanical loading. As the convection and radiation of surroundings are transformed to conductional thermal energy, the temperature of girder concrete rises, and the internal temperature differentials dwindle over time. Thermally induced damage in the prestressing elements is a function of distance from the surface heat. When loaded without thermal distress, the moment-carrying mechanism of the steel- and CFRP-prestressed girders is analogous; however, with the presence of heat, the development of lever arms and tensile strains of the girders demonstrates palpable differences. The maximum usable strains of the steel and CFRP vary with the degree of thermal exposure, thereby dominating the load-carrying capacity of the girders. From a design perspective, no evidence is noted to distinguish the performance of the steel- and CFRP-prestressed girders under service loadings, and contrary to commonplace notion, their fire ratings are found to be comparable because of sequential heat transfer.

The results of an experimental program conducted to evaluate the performance of shear-critical post-tensioned I-girders with grouted and ungrouted ducts are presented. The experimental program involved the design, construction, and testing to failure of six fullscale specimens with different duct layouts (straight, parabolic, or hybrid) and using both grouted or ungrouted ducts. All tests resulted in similar failure modes, such as localized web crushing in the vicinity of the duct, regardless of the duct condition or layout. Furthermore, the normalized shear stresses at ultimate were similar for the grouted and ungrouted specimens. The current shear design provisions in the AASHTO LRFD Bridge Design Specifications (AASHTO LRFD) were reviewed, and updated shear-strength reduction factors to account for the presence of the duct in the web and its condition (that is, grouted or ungrouted) were proposed. The data generated from these tests served as the foundation for updated shear-strength reduction factors proposed for implementation in AASHTO LRFD.