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    Engineering properties of hybrid polymer composites produced with different unsaturated polyesters and hybrid epoxy
    (Elsevier, 2024) Gokce, Neslihan; Eren, Sevki; Nodehi, Mehrab; Ramazanoglu, Dogu; Subasi, Serkan; Gencel, Osman; Ozbakkaloglu, Togay
    In this study, the mechanical properties of hybrid polymer composites produced with different unsaturated polyesters and hybrid epoxy resins are investigated. The composites were produced by blending unsaturated polyester resins (i.e., orthophthalic, isophthalic, and terephthalic) and bisphenol-A-based epoxy-vinyl ester resin to produce single, binary and ternary blends. In doing this, a total of 14 different combinations were produced. The results show that the binary and ternary polymer blends tend to improve almost all the tested properties of the polymer composites. Further, the fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) results confirmed that the reason for enahcned properties is due to better crosslinking and longer chains of polymers produced in binary and ternary mixtures. The absence of peaks determining the styrene polymerization character for all mixtures also demonstrates that the polymerization reaction takes place in all mixtures. It is also believed that the binary and ternary resin mixtures have developed higher energy absorption compared to single resin composites. All of the mentioned has been achieved while the gelation temperatures of the hybrid resin mixtures were not changed significantly and they began gelation at the expected temperature values. In addition to the gelation, peak exotherm temperatures, and barcol hardness values demonstrated that all mixtures achieved sufficient curing. The result of this study is significant and point to the great potential of producing high performance polymer composites through the use of binary or ternary resin mixtures.
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    Fiber-Reinforced Lightweight Calcium Aluminate Cement-Based Concrete: Effect of Exposure to Elevated Temperatures
    (Mdpi, 2023) Bideci, Ozlem Salli; Yilmaz, Hakan; Gencel, Osman; Bideci, Alper; Comak, Bekir; Nodehi, Mehrab; Ozbakkaloglu, Togay
    Calcium aluminate cements (CACs) are a group of rapid-hardening hydraulic binders with a higher aluminum composition and lower ecological footprint compared to their ordinary Portland cement (CEM) counterparts. CACs are commonly known to have higher thermo-durability properties but have previously been observed to experience a major strength loss over time when exposed to thermal and humidity conditions due to the chemical conversion of their natural hydrated products. To address this, in this study, silica fume is added to induce a different hydration phase path suggested by previous studies and utilized in conjunction with fiber-reinforced lightweight pumice to produce lightweight concrete. To closely evaluate the performance of the produced samples with CAC compared to CEM, two different types of cement (CEM and CAC) with different proportions of pumice and crushed stone aggregate at temperatures between 200 and 1000 degrees C were tested. In this context, sieve analysis, bulk density, flowability, compressive and flexural strength, ultrasonic pulse velocity and weight loss of the different mixes were determined. The results of this study point to the better mechanical properties of CAC samples produced with pumice aggregates (compared to crushed stone) when samples are exposed to high temperatures. As a result, it is found that CACs perform better than CEM samples with lightweight pumice at elevated temperatures, showing the suitability of producing lightweight thermal-resistant CAC-based concretes.
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    Investigation of electromagnetic interference shielding performance of ultra-high-performance mortar incorporating single-walled carbon nanotubes and steel fiber
    (Elsevier, 2024) Subasi, Serkan; Seis, Muhammet; Tekin, Ilker; Kazmi, Syed Minhaj Saleem; Munir, Muhammad Junaid; Gencel, Osman; Ozbakkaloglu, Togay
    For security amenities and key infrastructure, construction materials with extraordinary mechanical, durability, and electromagnetic interference (EMI) shielding performance are essential. This study investigates the EMI shielding performance of ultra-high performance mortar (UHPM) incorporating single-walled carbon nanotubes (SWCNT) and steel fibers. Currently, no such work is present in the existing literature. Eight mixtures were prepared with varying SWCNT dosages (0%-0.03 % by weight of cement) and steel fiber additions (1.2 % of the volume of the UHPM mixture). The performance of UHPM incorporating SWCNT and steel fibers was evaluated through flow diameter, compressive strength, flexural strength, Schmidt hardness, ultrasonic pulse velocity, EMI shielding performance, and scanning electron microscopy analysis tests. It is observed that increasing the SWCNT content enhances the compressive strength, flexural strength, ultrasonic pulse velocity, and Schmidt hardness of UHPM. The addition of steel fibers further enhances compressive (up to 22 %) and flexural (up to 92 %) strengths. In terms of the transmittance behavior, the improved EMI shielding performance of UHPM with the increasing SWCNT content is observed prominently at high electromagnetic frequencies (i.e., 2500 MHz-5100 MHz). However, the improved shielding performance is observed to be quite low, limited to 10 dB. Moreover, combining steel fibers and SWCNT enhances the EMI shielding performance of UHPM in terms of the transmittance behavior. As a result, UHPM incorporating SWCNT and steel fibers behaves as an absorbent material, shielding a significant amount of energy, approximately 45 dB, at a frequency of 5000 MHz. Based on the results, UHPM incorporating SWCNT and steel fibers can be used effectively as EMI shielding material. The findings of this study will enhance the practical applications of UHPM incorporating SWCNT and steel fibers.
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    The use of natural (coconut) and artificial (glass) fibers in cement - polymer composites: An experimental study
    (Elsevier Sci Ltd, 2024) Demirdag, Caner; Nodehi, Mehrab; Bideci, Alper; Bideci, Ozlem Salli; Tuncer, Metin; Gencel, Osman; Ozbakkaloglu, Togay
    Fiber reinforced concrete composites are a group of high-performance materials with considerably enhanced stress-strain properties. Similar effects can also be achieved using polymeric binders whereby the inclusion of the polymers as binder can significantly enhance the physical-mechanical properties of the resulting concretes. In this regard, the following study investigates the impact of utilizing natural (e.g., coconut) versus artificially manufactured fibers (e.g., glass fiber) in polymer-cementitious composites. In doing so, 14 mixes were produced using various ratios of the two fibers. To evaluate the properties of the produced samples a series of tests including flow diameters, unit weights, water absorption values, compressive strengths, flexural strengths and ultrasound transmission rates were determined. Also, to evaluate the microstructural cohesion of polymercement and polymer-coconut samples, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) has also been used. The results show that the surface texture of fibers can play a key role in major engineering properties of the fiber reinforced concretes and that the natural fibers have great potential to be used as high-performance materials in cementitious composites. Also, it is found that the use of polymer as the main binder can provide higher adhesion with fibers containing smoother surface (e.g., glass fiber) at the interfacial transition zone (ITZ). In the end, recommendation for future studies is also included.