WoS İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/394
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Article Structural Behavior of Geopolymer Reinforced Concrete Beams: Experimental, Numerical, and Code-Based Assessment(Springer, 2025-08-11) Ozbayrak, Ahmet; Kucukgoncu, HurmetThis study experimentally investigates the flexural performance of heat-cured low-calcium fly ash-based geopolymer concrete (GPC) beams reinforced with ribbed steel bars, focusing on the effects of reinforcement ratio, alkaline activator concentration (SS/SH), and curing regime. Fifteen full-scale beams, including twelve GPC and three OPC specimens, were tested under four-point loading to evaluate load-deflection and moment-curvature behavior. Despite a lower compressive elastic modulus, the results showed that GPC beams exhibited comparable or superior cracking and ultimate moment capacities relative to OPC beams. Increasing the reinforcement ratio enhanced load capacity but reduced ductility in both systems, with GPC beams showing more brittle post-yield behavior. Numerical models based on OPC parameters were developed in SAP2000 to compare with experimental GPC moment-curvature data, revealing good agreement in the linear range but notable differences in post-yield response. The study also examined the microstructure of failed GPC beams via SEM, XRD, and EDX analyses to correlate matrix morphology with mechanical behavior. Finally, moment capacities calculated according to ACI 318 and TS 500 provided conservative estimates, supporting the safe applicability of current design codes to heat-cured GPC beams. These findings demonstrate that GPC, when properly proportioned and cured, is a viable structural alternative to OPC for reinforced concrete members.Article Citation - WoS: 35Citation - Scopus: 29Microstructural Analysis of Low-Calcium Fly Ash-Based Geopolymer Concrete With Different Ratios of Activator and Binder Under High Temperatures(Springer Heidelberg, 2024-06-25) Kucukgoncu, Hurmet; Ozbayrak, AhmetGeopolymer concretes have emerged as an alternative to traditional Portland cement concretes with high strength, good durability, well corrosion performance and high-temperature resistance, and being a sustainable and environmentally friendly material. In this study, a comprehensive microstructural analysis of low-calcium fly ash-based geopolymer concrete samples with different alkali activator to binder ratios was conducted after exposure to temperatures ranging from 400 to 800 degrees C. The experimental results of the geopolymer concrete specimens found out significant findings, including a notable loss of mass and an approximate 80% decrease in compressive strength after exposure to 800 degrees C. The microstructural analysis underlined crack formation, voids and porosities in the geopolymer matrix at elevated temperatures, affecting the physical and mechanical properties of the material. The study presents significant insights into the behaviour of low-calcium fly ash-based geopolymer concrete with different binder and alkali activator ratios under high temperatures, revealing the performance of geopolymer concretes in extreme environments and the effect of incompatibility between geopolymer concrete and aggregate due to thermal temperature effects on this performance.Article Citation - WoS: 2Citation - Scopus: 2Experimental and Numerical Analysis of Damage and Crack Behavior in Geopolymer and Ordinary Portland Cement Reinforced Concrete Columns(Elsevier, 2025-10) Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Aslanbay, Yuksel Gul; Aslanbay, Huseyin HilmiThis study evaluates and compares the structural behavior of reinforced concrete columns produced with geopolymer concrete (GPC) and ordinary Portland cement (OPC) under eccentric axial loading, aiming to investigate GPC's potential as a sustainable alternative. A total of twenty columns with varying longitudinal reinforcement ratios, curing methods, eccentricities, and geopolymer formulations were experimentally tested. Displacement measurements at each load increment were obtained using a total station, allowing crack pattern tracking at key stages such as first cracking, yielding, and ultimate failure. These observations were schematically documented using AutoCAD. Additionally, finite element models were developed in ABAQUS using the Concrete Damage Plasticity (CDP) model, with material parameters calibrated based on experimental compressive and tensile strengths, elastic modulus, and fracture energy. Results indicated that increasing eccentricity reduces axial load capacity while increasing lateral deformation. While the reinforcement ratio did not significantly affect axial strength, it increased displacement demand. Due to their distinct microstructural characteristics, GPC columns exhibited greater deformation capacity and narrower, more localized crack patterns than OPC columns. Furthermore, the sodium silicate/sodium hydroxide (SS/SH) ratio and curing duration significantly influenced the structural response of GPC. Numerical simulations showed strong agreement with experimental results regarding load-displacement behavior and damage distribution. These findings demonstrate that GPC can serve as a reliable and sustainable alternative to OPC in structural column applications, provided its specific material properties are considered in design and analysis.Article Citation - WoS: 3Citation - Scopus: 3Equivalent Stress Block Parameters for Fly Ash-Based Geopolymer Concrete Structural Elements(Ernst & Sohn, 2025-03-06) Ozbayrak, Ahmet; Kucukgoncu, HurmetResearch on the design of structural members made from geopolymer concrete (GPC) remains limited. This study investigates the applicability of equivalent rectangular stress block parameters, traditionally used for reinforced concrete design, in GPC structural elements. We conducted experimental tests on 20 columns (16 GPC, 4 Ordinary Portland Cement [OPC]) and 15 beams (12 GPC, 3 OPC) produced using fly ash-based GPC and standard OPC. These tests involved subjecting the specimens to various loading conditions to measure their ultimate compressive strength and strain. The findings demonstrate that these factors significantly influence the stress block parameters in GPC samples. Notably, parameters k(1) and k(3) were compatible with ACI 318 and Eurocode 2 standards, with deviations within acceptable limits, supporting GPC's potential for use in conventional reinforced concrete frameworks. The study also reveals that GPC columns and beams have higher balanced reinforcement ratios than OPC, due to GPC's increased deformation capacity and strain values. According to the results, the average balanced reinforcement ratio of GPC column specimens is 30% higher than that of OPC, while that of GPC beam specimens is 6% higher. Variations in alkaline activation and curing methods did not significantly impact the equivalent stress block parameters. The change between the average equivalent stress block parameters obtained from GPC and OPC beam samples varies between 1% and 5%, while the change in column samples is around 1%. Although the longitudinal reinforcement ratios in the tests are variable, the averages of the calculated equivalent stress block parameters are close. The experimental results align with numerical analysis, emphasizing GPC's suitability as an alternative material in structural applications. These findings provide a basis for incorporating GPC into existing design standards, with adjustments for its distinct mechanical behavior.Article Citation - WoS: 23Citation - Scopus: 25Comprehensive Analysis of Experimental and Numerical Results of Bond Strength and Mechanical Properties of Fly Ash Based GPC and OPC Concrete(Elsevier Sci Ltd, 2024-02) Aslanbay, Yuksel Gul; Aslanbay, Huseyin Hilmi; Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Atas, OguzhanNowadays, materials in the more environmentally friendly waste product class, which can be an alternative to standard Portland cement (OPC), are frequently used by researchers in concrete production. One of these, namely fly ash-based geopolymer concrete (GPC), should demonstrate its superiority over OPC in terms of chemical and mechanical properties to enhance its utilization. One of the mechanical properties of GPC is the bond strength between reinforcement and concrete. In this study, it was aimed to obtain bond strengths by performing tensile tests on GPC samples with varying sodium silicate/sodium hydroxide (SS/SH) and alkaline activator/fly ash (AA/FA) ratios. A pull-out experimental setup was prepared in accordance with RILEM Standard. Experimental results were compared with numerical results obtained from finite element models designed in ABAQUS software and were found to be compatible. When evaluated in terms of peak load and max bond stress values, GPC is superior to OPC. Compared to OPC an increase in the SS/SH ratio enhances mechanical properties such as compressive strength and bond load, whereas an increase in the AA/FA ratio with a value of 0.7 in the series has the opposite effect. In the finite element models, stress values are higher in samples with an AA/FA ratio of 0.5 compared to other ratios. An increase in the AA/FA ratio leads to a decrease in stress values. The analytical results are demonstrated that the proposed model can be utilized to assess the bond strength performance between traditional reinforced concrete and fly ash-based geopolymer concrete. Additionally, as a result of experimental studies, a formula that can be used to estimate bond strength based on GPC compressive strength and shows the superiority of GPC compared to studies in the literature has been proposed.Article Citation - WoS: 53Citation - Scopus: 55Comprehensive Experimental Analysis of the Effects of Elevated Temperatures in Geopolymer Concretes With Variable Alkali Activator Ratios(Elsevier, 2023) Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Aslanbay, Huseyin Hilmi; Aslanbay, Yuksel Gul; Atas, OguzhanBy growing population and rapid urbanization, demand for concrete increases exponentially. Researches on use of fly ash material in waste product class for concrete production are important to produce concrete more environmentally friendly. However, there is a need for more research to use geopolymer concrete (GPC) in every field where ordinary Portland cement concrete (OPC) is used. Therefore, it is crucial to experimentally investigate thermal properties as well as me-chanical properties of geopolymer concrete. As investigated thermal properties, the main factor affecting strength development of GPC is alkali activator ratios. In this study, GPC prism samples with nine different compositions, produced by various alkali ratios. After flexural strength tests, they were cut into cubes and exposed to 400 degrees C, 600 degrees C and 800 degrees C, then they were subjected to compressive strength tests. Results obtained from different AA/FA and SS/SH ratios were eval-uated as mechanical properties at ambient temperature and physical, mechanical and micro-structural properties at elevated temperature. An empirical formula, which considers the effect of activator ratios, was proposed to calculate flexural strength depending on compressive strength of samples at ambient temperature. As an increase of SS/SH and AA/FA ratios, compressive strength increased, while flexural strength decreased. The increase in AA/FA ratio decreased compressive strength of samples exposed to high temperatures, while increase in SS/SH ratio did not deter-mine at elevated temperatures. There is an inverse change with AA/FA ratio and parallel change with SS/SH ratio between compressive strengths of samples at ambient temperature and exposed to high temperature.