Browsing by Author "Aslanbay, Yuksel Gul"

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Citation - WoS: 7
Citation - Scopus: 8
Stress and Damage Distribution Analysis of Steel Reinforced Geopolymer Concrete Beams: Finite Element Method and Experimental Comparison Under Varying Design Parameters
(Elsevier, 2025) Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Aslanbay, Huseyin Hilmi; Aslanbay, Yuksel Gul
Geopolymer concrete (GPC) is a sustainable and eco-friendly alternative to ordinary Portland cement-based concrete (OPC). However, its application in reinforced concrete structures remains limited due to insufficient research on structural performance. This study examines the effects of tensile reinforcement ratio, sodium silicate/sodium hydroxide ratio, and curing method on GPCreinforced concrete (GPC-RC) beams. Experimental and numerical bending tests were performed on GPC and OPC beams with similar tensile reinforcement and strength properties. Load- displacement and moment-curvature relationships were obtained and compared, while stress and stiffness behaviors were analyzed numerically. The results show that curing methods and reinforcement ratios significantly influence GPC beam behavior. In GPC samples, numerical and experimental displacement and load values differed by approximately 10 % at both yield and ultimate points. For OPC, these differences were 35 % and 14 % at the yield point and 17 % and 25 % at the ultimate point. GPC exhibited distinct stress and damage distribution characteristics compared to OPC. The finite element models were statistically validated, confirming their consistency with experimental results. These findings contribute to the understanding of GPC's structural behavior and provide guidance for its design and optimization in reinforced concrete applications.
Citation - WoS: 27
Citation - Scopus: 29
Determination of Stress-Strain Relationship Based on Alkali Activator Ratios in Geopolymer Concretes and Development of Empirical Formulations
(Elsevier Science Inc, 2023) Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Atas, Oguzhan; Aslanbay, Huseyin Hilmi; Aslanbay, Yuksel Gul; Altun, Fatih
Fly ash-based geopolymer has recently gained attention of researchers due to its potential application, as well as being an alternative binder with low emissions compared to ordinary Portland cement (OPC) in concrete production. Studies which are conducted on the design and mechanical properties of structural members produced from fly ash geopolymer concrete (GPC) are very important in terms of increasing the use of this concrete. The aim of this study is to obtain experimental data on the effect of sodium silicate/sodium hydroxide (SS/SH) and alkali activators/fly ash (AA/FA) ratios on the mechanical properties of a low calcium heat-cured fly ash geopolymer. In addition, it is to reveal the similarities and differences of OPC and GPC by comparing the mathematical formulations in existing regulations and concrete models with experimental data. Thus, geopolymer cylinder concrete samples were produced using 15 different mixtures with SS/SH ratios of 1.5, 2.5 and 3.5, while AA/FA ratios of 0.4, 0.5, 0.6, 0.7 and 0.8. At the end of the study, the optimum SS/SH ratio was obtained as 2.5. A decrease in the AA/FA ratio increases the compressive and splitting tensile strength, while an increment increases the ductility and consuming energy. In addition, the relationship between the experimental data and the splitting tensile strength and modulus of elasticity formulations depending on the compressive strength given in other studies and regulations as a part of literature was investigated, and then, two alternative empirical formulations considering the ratios of alkali activators were proposed at the end of the regression analysis. When the stress-strain relationship of OPC concrete models and GPC mixtures were compared, the closest unconfined concrete model for GPC concrete was the Hognestad model.
Citation - WoS: 1
Citation - Scopus: 1
Experimental and Numerical Analysis of Damage and Crack Behavior in Geopolymer and Ordinary Portland Cement Reinforced Concrete Columns
(Elsevier, 2025) Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Aslanbay, Yuksel Gul; Aslanbay, Huseyin Hilmi
This 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.
Citation - WoS: 50
Citation - Scopus: 53
Comprehensive 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, Oguzhan
By 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.
Numerical Analysis and Experimental Comparison of Stress and Stiffness Parameters of Steel Reinforced Geopolymer Concrete Columns
(Elsevier Sci Ltd, 2026) Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Aslanbay, Huseyin Hilmi; Aslanbay, Yuksel Gul; Altun, Fatih
Despite extensive research, Geopolymer concrete (GPC) lacks reinforced concrete construction and design specifications. Developing such specifications requires comprehensive studies to promote the use of GPC, which is known for its superior performance and environmental benefits compared to ordinary Portland cement concrete (OPC). This study numerically investigated and compared the behavior and strength of fly ash-based geopolymer-reinforced concrete columns with the experimental results. Comparisons with OPC were made based on existing specifications. Herein, FEM analyses were conducted on 16 GPC and 4 OPC columns under eccentric axial compressive loads. Parameters such as eccentricity, reinforcement ratio, curing method, and activation solution ratios were varied. According to average numerical results, the GPC columns have 7% more moment capacity and 30% more curvature values than OPC. Moreover, GPC columns absorbed more energy than OPC columns. Also, GPC columns have higher axial load and bending moment carrying capacities than OPC for numerical results. Error analysis between FEM and experimental data revealed a strong correlation, with MAPE values of 8.88% (axial load) and 7.20% (moment) for GPC columns, confirming the reliability of the numerical model. ACI 318 and Eurocode 2 specifications were deemed applicable for GPC columns, provided axial loads are limited per TEC 2018.
Citation - WoS: 20
Citation - Scopus: 22
Comprehensive Analysis of Experimental and Numerical Results of Bond Strength and Mechanical Properties of Fly Ash Based GPC and OPC Concrete
(Elsevier Sci Ltd, 2024) Aslanbay, Yuksel Gul; Aslanbay, Huseyin Hilmi; Ozbayrak, Ahmet; Kucukgoncu, Hurmet; Atas, Oguzhan
Nowadays, 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.