Browsing by Author "Akca, Abdullah Huzeyfe."

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    Mechanical behavior and recovery of fiber reinforced concrete after high temperature exposure
    (Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2017.) Akca, Abdullah Huzeyfe.; Zihnioğlu, Nilüfer Özyurt.; Gökçe Ahmet.
    In spite of the importance of strength regain of re damaged concrete in terms of repair and rehabilitation, very few studies had been conducted. Most of the available data are in qualitative form and restricted to strength recovery only. In this study, rehydration and deterioration processes of concrete after high temperature exposure were investigated in details. Therefore, air and water re-curing methods were applied to air entrained ber reinforced concrete incorporating di erent type of cementitious materials. Before and after re-curing processes, compression tests were conducted to determine mechanical properties and SEM, XRD and TG analyses were carried out to determine the morphological changes in concrete. Moreover, heated surfaces of concrete were monitored during air-recuring period and the relation between crack growth rate and compressive strength reduction were investigated. Deterioration in mechanical properties and changes in microstructure of concrete were observed after heating and these deterioration and changes continued during air re-curing period. On the other hand, recovery in mechanical properties and morphology of concrete were observed after water re-curing period. And nally, similar tests were conducted on reinforced concrete beams and prominent e ects of air and water re-curing were also observed in these specimens.
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    Microstructural development and resulting mechanical properties of seawater mixed concrete
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Aydoğan, Olcay Gürabi.; Zihnioğlu, Nilüfer Özyurt.; Akca, Abdullah Huzeyfe.
    According to the “Water Report” published by United Nations in 2018 (UN World Water Report, 2018), two thirds of the world’s population live in areas that experience water shortage for some time in a year. Ongoing studies and predictions represent that water shortage problem will increase in the near future. Based on that information, many researchers worldwide focus on the subjects such as waste water management, use of seawater for several applications and etc., under the scope of sustainability. Construction industry is represented as one of the moderately water dependent sectors in the “2016 Water Report” (UN World Water Report, 2016) of United Nations. Concrete sector is also one of the big water consumer sectors, since concrete is the most used construction material worldwide. In the scope of this study, the main objective was to understand microstructural development and resulting mechanical properties of seawater concrete. Most of the studies found in literature focused on changes in mechanical behavior when seawater was used as the mix water for concrete. However, the number of studies that examine and explain the reason for changes in mechanical behavior is yet very limited. 8 different concrete mixtures were produced in the scope of this study. Type of mix water (seawater and tap water) and type of cement (PC and Sulfate resisting cement) were varied. Macro synthetic structural fibers were also used to see the effectiveness of these fibers when combined with seawater mixed concrete. Detailed microstructural analyses were carried out as well as mechanical strength tests to understand and evaluate the reasons for changes in the material behavior when seawater was used as the mix water for concrete.
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    The effects of polyproylene fiber and air entraining admixture addition on high performance concrete exposed to elevated temperatures
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2011., 2011.) Akca, Abdullah Huzeyfe.; Zihnioğlu, Nilüfer Özyurt.
    As the use of HPC increases in structures, the risk of being exposed to fires also increases. Unlike NSC, strength and durability of HPC are more influenced by fire conditions and also HPC is more prone to spalling at elevated temperatures due to its dense microstructure which limits the ability of water vapor to escape from concrete. Recent studies showed that addition of PP fibers into HPC is an efficient way to avoid spalling of concrete because PP fibers melt in concrete above 170 o C and form microchannels which permit the evacuation of gases in pores. On the other hand, using various additives such as mineral and chemical admixtures in HPC was found to be efficient to limit the reduction in strength and durability of HPC. Therefore in this study, PP fiber and AEA with various ratios were used in HPC incorporating blast furnace slag so as to create interconnected reservoirs and to improve fire resistance of HPC. Nine mixes of HPC with 0.24 water-to-binder ratio and various PP and AEA contents were produced and 18 series were obtained. These series subjected to 300 o C, 600 o C and 900 o C with a heating rate of 10 o C/min and after air cooling, residual compressive strength tests were conducted. The heated specimens were observed both at macro and micro scales to investigate the color changes, cracking and spalling of HPC at various temperatures. Also, TGAs were performed on powder samples from each nine mixes. Results showed that addition of AEA diminished the decrease in residual strength but this result was found to be irregular after 300 o C, PP fibers reduced the residual strength. The collaboration of AEA and PP fibers decreased the risk of spalling of HPC. Also, size of specimen was found to be important in deterioration of HPC.
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    The effects of post fire re-curing on mechanical properties and recovery of fiber reinforced concrete
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Noori, Mohamed Adil Noori.; Zihnioğlu, Nilüfer Özyurt.; Akca, Abdullah Huzeyfe.
    High temperatures or fire may cause significant damage in concrete. Moreover, further deterioration continues at the post heating stage. Although, there are few studies on the post fire rehabilitation of concrete in the literature, most of them focused only on the re-strengthening of concrete. On the other hand, the post fire activities in concrete have a significant impact on both mechanical and microstructural properties of concrete. Therefore, in this study new solution based re-curing techniques were utilized on the concrete specimens at the post heating stage. 7 days of post fire re-curing was applied in order to be more effective, practical and cost effective compared to the longer re-curing periods. At every stage of heating, cooling and re-curing; destructive and non-destructive tests were performed on cubic and prismatic specimens in order to determine the residual mechanical properties and to monitor the extent of recovery. Some of these specimens were also subjected to TGA analyses to better understand the microstructural changes of concrete following heating and the effect of re-curing methods. The results showed further deterioration in air re-cured concrete but recovery in water and solution based re-cured concrete.