Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection

Permanent URI for this collectionhttps://hdl.handle.net/11147/7148

Browse

Filters

20252

Settings

Author: search.filters.author.Demir, UgurPublisher: search.filters.publisher.Elsevier Science inc

Search Results

Now showing 1 - 2 of 2
  • Article
    Repair and Strengthening of Fire Damaged Concrete Cylinders Using FRP Confinement: Tests and Analytical Modelling
    (Elsevier Science inc, 2025) Demir, Ugur; Ilki, Alper
    This study examines the effects of fiber-reinforced polymer (FRP) repair and strengthening on the axial stress-strain behavior of concrete columns after exposure to realistic fires. A total of 30 plain concrete cylinders, each measuring 150 x 300 mm, were cast for this investigation. Of these, three specimens were kept as reference at ambient temperature, while the remaining were exposed to ISO-834 standard fire for durations of 30, 60 or 90 min, with nine specimens in each duration group. After natural cooling, the heated specimens were categorized into three groups: i) three were left unconfined, ii) three were repaired and strengthened using two layers of carbon FRP sheets, and iii) three were repaired and strengthened with four layers of carbon FRP sheets. This study employs realistic ISO 834 fire scenarios and investigates CFRP confinement with up to four layers, addressing high confinement demands beyond current literature. The results showed that transverse confinement provided by carbon FRP sheets significantly improved axial strength and deformability for all specimens, while it did not fully restore the axial stiffness achieved before fire exposure. The effectiveness of FRP confinement increased with longer fire exposure durations. Additionally, two analytical models proposed previously for predicting the axial strength and ultimate strain of FRP confined fire-damaged concrete were evaluated in terms of their accuracy. The accuracy of the predictions was reduced with an increase in exposure temperatures for both models. Therefore, a new model is proposed within the scope of study, which shows good agreement with the novel test results.
  • Article
    Impact of High Axial Stress on Seismic Behavior of Substandard Reinforced Concrete Columns
    (Elsevier Science inc, 2025) Gundogan, Safiye; Demir, Ugur; Turan, O. Tugrul; Ilki, Alper
    The seismic performance of reinforced concrete (RC) buildings, particularly those constructed without adequate seismic detailing, remains a critical concern in earthquake-prone regions worldwide. Many of these buildings, often referred to as substandard RC structures, were built before modern seismic codes were established and are characterized by poor material quality and inadequate construction practices. The Southern T & uuml;rkiye earthquakes on 6 February 2023 underscored the urgent need to better understand the seismic behavior of these substandard structures, which frequently fail to meet modern design standards and are prone to damage or collapse. Substandard RC columns, characterized by low concrete strength and inadequate transverse reinforcement, are susceptible to severe seismic damage, increasing the risk of collapse and life loss. While numerous studies have experimentally examined the seismic behavior of RC columns under low to moderate axial load to capacity ratios (typically below 0.30), these conditions do not accurately reflect the reality of many existing substandard columns that are frequently subjected to higher axial compression stresses. This study addresses this critical gap by presenting the first experimental data on the seismic behavior of full-scale, substandard RC columns under high axial load ratios (0.30-0.80). The analysis focused on lateral load-displacement relationships, ductility, plastic hinge length, stiffness, energy dissipation capacity, and residual displacements. Increases in axial load led to more brittle failure modes, reduced displacement ductility and an extended plastic hinging zone. High axial loads also caused accelerated stiffness degradation, reduced cumulative energy dissipation, and progressive residual deformations. Analytical models overestimated deformation capacity, making them unreliable for substandard RC columns under high axial stress. Additionally, predictions using plastic hinge length formulas underestimated the values at high axial loads. The study also evaluated the performance of widely used concrete confinement models in predicting the moment-curvature responses and corresponding ductility for substandard RC columns with low compressive strength and subjected to high axial stress. These findings underscore the critical need for refined modelling approaches and assessment methodologies to improve the seismic evaluation of substandard existing buildings.