International Concrete Abstracts Portal

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.

This study introduces a new anchorage strategy using ultra-high-performance concrete (UHPC) to attach unbonded post- tensioning (PT) strands to existing foundations. This solution complements a seismic retrofit scheme investigated by the authors, which transforms nonductile cast-in-place reinforced concrete (RC) shear walls into unbonded PT rocking shear walls following concepts of selective weakening and self-centering. In the proposed PT anchorage scheme, mild steel reinforcements are inserted through the shear wall thickness and into the foundation. Subsequently, UHPC is cast around the wall base, forming a vertical extension connected to the foundation, which is used to anchor the unbonded PT strands. The feasibility and performance of the anchorage scheme was investigated through a combination of laboratory testing and numerical simulations. Pullout testing on four scaled-down anchorage specimens was conducted in the laboratory. Hairline cracks were observed in the UHPC during testing. Additionally, three-dimensional (3-D) finite element (FE) models were created, validated, and used to study the performance of the proposed anchorage scheme under lateral loading. The simulation results support the effectiveness of the proposed anchorage strategy.

Corrosion of prestressing steel can threaten the durability of prestressed concrete. To ensure the durability of unbonded post-tensioning (PT) systems, it is crucial to investigate the effects of construction defects such as grease leakage and high-density polyethylene (HDPE) sheath damage. This study quantified the thickness of grease coating (PT coating) and HDPE sheath damage as experimental variables. An accelerated corrosion test was conducted in two environments: 1) chloride ions only (Cl–); and 2) both chloride ions and dissolved oxygen (Cl– + DO). The corrosion current density and weight loss of prestressing strands and the suspended concentration density of corrosion cell solution were measured to quantify the corrosion performance. Increasing the grease coating thickness over 0.3 mm (0.012 in.) did not significantly enhance corrosion resistance. Realistic levels of HDPE sheath damage had no significant detrimental effects on durability; however, excessive HDPE sheath area loss must be avoided for long-term durability. It was examined to quantify the interrelationship between three data—electrochemical measurement, weight-loss, and suspended concentration density—as quantitative corrosion data. The findings of this study can serve as a basis for developing durability-related provisions, as well as controlling the construction defects of unbonded PT systems in field applications.

An approach to improve the progressive collapse resistance of conventional reinforced concrete (RC) frame structures was put forth by using unbonded post-tensioning strand (UPS). Two UPSs with straight profiles were mounted at the bottom of the beam section. A static loading test was conducted on an unbonded prestressed RC (UPRC) beam-column subassemblage under a column removal scenario. The structural behaviors of the test specimen such as load-carrying capacity, failure mode, post-tensioning force of the UPSs, and reinforcing bar strain were captured. By analyzing the results of the tested substructure, it was found that the compressive arch action (CAA) and catenary action (CTA) were sequentially mobilized in the UPRC subassemblage to avert its progressive collapse. The presence of UPSs could significantly improve the load-carrying capacity of conventional RC structures to defend against progressive collapse. Moreover, a high-fidelity finite element (FE) model of the test specimen was built using the software ABAQUS. The FE model was validated by experimental results in terms of the variation of vertical load, horizontal reaction force, and post-tensioning force of the UPSs against middle joint displacement (MJD). Finally, a theoretical model was proposed to evaluate the anti-progressive collapse capacities of UPRC subassemblages. It was validated by the test results as well as the FE models of the UPRC subassemblages, which were calibrated using the available experimental data.

This study used finite element analysis to examine how tendon configuration affects the temperature behavior of post-tensioned concrete structures during fire exposure. The thermal behavior of various tendon configurations was modeled, showing good agreement with experimental data. Parametric studies found that unbonded single-strand tendons (S) and prestressing (pretensioned) strands (R) had lower thermal resistance than bonded post-tensioned tendons (B), unbonded post-tensioned tendons (U), and grouted extruded-strand tendons (G). The S and R specimens stayed at or below the critical temperature for one-way slabs, validating current safety codes. The B, U, and G specimens remained well below critical temperatures, indicating that a thinner concrete cover might suffice. These findings highlight the need to consider tendon configuration in structural fire-resistance evaluation and incorporate heat resistance assessment to ensure the safety and efficiency of prestressed concrete structures during fires.