Bazalt, kalker ve kumtaşı agregaları içeren portland çimentosu ve kalisuyum alüminatlı çimento esaslı betonların yüksek sıcaklık performansı
High-temperature performance of CAC- and OPC-based concretes with basalt, limestone, and sandstone aggregates
- Tez No: 963153
- Danışmanlar: PROF. DR. HASAN YILDIRIM
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2025
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Yapı Mühendisliği Bilim Dalı
- Sayfa Sayısı: 131
Özet
Beton, sahip olduğu çeşitli üstün avantajları nedeniyle günümüzde en çok üretilen ve kullanılan malzemelerden birisidir. Bu avantajlarından en önemlileri yüksek dayanıma sahip olması, üretim maliyetlerinin düşüklüğü ve çeşitli çevresel koşullara karşı dirençli olmasıdır. Ancak, beton sahip olduğu bu özellikleri karşılaşabileceği çeşitli sert çevresel koşullar altında kaybetmeye başlayabilir ve hatta tamamen yitirebilir. Bu sert koşullardan en önde gelenlerden birisi de yangın veya başka bir olay sonucunda oluşabilecek olan sıcaklığın yüksek olduğu ortamlardır. Yüksek sıcaklık koşullarında beton elemanlar bulundurdukları bağlayıcı olan çimentonun kimyasal ve fiziksel içeriğine bağlı olarak farklı performanslar gösterebilmektedir. Günümüzde üretilen betonların ezici çoğunluğunda portland çimentosu (PÇ) kullanılmaktadır. Ancak sertleşmiş betondaki portland çimentosunun bileşenlerinden olan CH (Ca(OH)2) ürünlerinin yaklaşık 400-600 oC aralığında ayrışmaya uğramasıyla betonun basınç dayanımı dramatik bir şekilde azalmaktadır. Portland çimentosunun aksine hidrate CH ürünleri içermeyen kalsiyum alüminatlı çimentolar (KAÇ) yüksek sıcaklık seviyelerinde herhangi bir ayrışmaya uğramadığından bu sıcaklık değerlerinde dayanımlarını büyük oranda koruyabilmektedir [1][2]. Kalsiyum alüminatlı çimentoların üretimi çok az olduğundan dolayı literatürde kullanıldığı durumlarda gösterdiği performanslar hakkında sınırlı sayıda bilgi mevcuttur. Bu çalışmada da günümüzde beton üretimlerinde yaygın şekilde kullanılan kumtaşı, bazalt ve kalker agregalarının hem portland hem de kalsiyum aluminatlı çimentosuyla ayrı olarak kullanıldığı altı farklı beton karışımı tasarlanmış ve üretilmiştir. Bu karışımların tamamında 400 kg/m3 çimento kullanılmış ve 0.45 su/çimento oranıyla beton üretimleri gerçekleştirilmiştir. Her karışımda üretilen betonun bir kısmı 10x20 cm boyutundaki silindir numunelere, kalan beton ise 5x5 cm, 10x10 cm ve 15x15 cm boyutlarında ve herbirinin beton örtü kalınlıklarına 1200°C'ye kadar dayanıklı K tipindeki termokupl sabitlenecek şekilde özel olarak üretilmiş kalıplara yerleştirilmiştir. Üretimin ardından 28 gün boyunca suda bekletilerek kürü tamamlanan her karışımdaki tüm 10x20 cm'lik silindir numunlerin her biri sadece kendisi için belirlenen değere maruz kalacak şekilde 250 °C, 500°C, 750°C ve 1000 °C sıcaklık seviyelerinde en az 2 saat bekletilmiştir. Ardından bu numuneler yüksek sıcaklıklara maruz bırakıldıktan sonra soğutularak çevre sıcaklığına erişmesi beklenmiş ve nihai olarak bu numunelere basınç dayanımları ve gerilme-şekil değiştirme özellikleri belirlenmesi için ilgili deneyler gerçekleştirilmiştir. Yapılan bu deneylerin yanı sıra, her karışımda aynı şekilde ve adette bulunan, beton örtü kalınlıklarını temsil eden merkez noktalarına termokupl yerleştirilmiş olan numunelerde 0-1000°C arasında öngörülen sıcaklık-rejimine maruz bırakılarak beton örtü kalınlıklarında meydana gelen değişimler gözlemlenmiştir. Son olarak bu deneyler sonucunda iki farklı çimento türü ve bu çimentolarla ayrı ayrı kullanılan üç farklı agrega türünün yer aldığı altı farklı karışımının performansları karşılaştırılmıştır. Genel olarak, bu çalışma farklı çimento ve agrega türlerinin yüksek sıcaklık altında betonun performansını nasıl etkilediğini kapsamlı bir şekilde incelemektedir. Elde edilen bulgular, gelecekte yüksek sıcaklık koşullarına maruz kalan beton yapı elemanlarının tasarımında ve malzeme seçiminde önemli bir referans oluşturmayı hedeflemektedir. Ayrıca, 250 °C'den 1000 °C'ye kadar farklı sıcaklık seviyelerinde yapılan deneyler, betonun sıcaklığa bağlı davranışını ve dayanım değişimlerini daha kapsamlı bir şekilde anlamamıza yardımcı olmayı hedeflemektedir.
Özet (Çeviri)
Concrete consists of binder material, water, aggregate and admixtures, if needed, and is generally used in structures. The basic features required from concrete, in cases where it is used as a structural material, are maintaining its strength throughout its service life and protecting the other constituent materials from external effects, particularly when concrete is part of a composite material. One of the most important factors influencing the strength of hardened concrete is its resistance to environmental effects. The effects of durability problems on strength largely depend on the pore structure of the concrete and the properties of the materials present in it. There is a direct relationship between the strength of hardened concrete and its pore structure. According to this, the strength of concrete decreases as the number and size of pores in the concrete increase. This allows materials from the environment to penetrate the concrete and chemically react with its constituents, resulting in an increase in the volume of concrete. Thus, internal stresses develop within the concrete, leading to cracks that cause a significant loss of strength. Apart from this, by affecting the pore structure within concrete, ambient conditions can play a crucial role in decreasing its strength. In this context, ambient temperature may induce various physical and chemical reactions in the binder material and aggregates of concrete, which can ultimately lead to their decomposition. Therefore, the strength of concrete decreases with the progression of pore formation. Concrete can be used with many materials, particularly steel reinforcement. In such composite systems, concrete is required not only to maintain its mechanical properties but also to protect the accompanying materials from adverse environmental conditions. Thanks to its low-porous structure, concrete is able to meet this requirement by providing impermeability against harmful agents and exhibiting low thermal conductivity, thus protecting the accompanying materials from physical and chemical degradation. In reinforced concrete elements, under high temperatures caused by fire or other situations, concrete delays the transmission of heat to the rebars due to its low thermal conductivity. As a result, concrete helps mitigate high-temperature-related damage by providing more time for response and reducing harm to other materials, ultimately minimizing economic losses. In today's world, the performance of building material at high temperatures is crucial for both the safety of occupants and the structural integrity of buildings. To avoid the deterimental effects of this environment, many materials have been designed and produced. Concrete is one of them-a material that can be durable at elevated temperatures. However, concrete produced with Portland cement (PC) has some drawbacks in maintaining its mechanical properties at these levels. The compressive strength and other mechanical properties of concrete produced with Portland cement (PC) decline sharply above 400–600°C because the CH (Ca(OH)₂) in hydrated Portland cement begins to decompose at these temperature levels. Due to the shortcomings of concrete containing Portland cement (PC), calcium aluminate cement (CAC) was explored as a potential replacement. In contrast to Portland cement, CAC does not contain CH (Ca(OH)₂) in its hydrated phase. For this reason, it is considered a more favorable cement type for producing concrete with a service temperature above these levels and is often associated with the refractory concrete¬¬— a type of high-temperature-resistant concrete made with calcium aluminate cement (CAC) [1][2]. Despite its superior advantages, concrete containing CAC undergoes a conversion mechanism— a type of recrystallization of certain hydrated compounds — which leads to noticeable proportional decreases in strength. As a result, it is often considered a less preferable material for use in building construction [3]. In spite of the sheer amount of research regarding the performance of building materials, especially concrete, at high temperatures, their scopes and contents resemble one another and do not differ significantly, leading to a deficiency in literature. In the modern academic field and codes, most research discussing the performance of concrete elements at high temperatures involves the aggregates that can withold high temperatures and are conducted at temperatures below 800 oC. Moreover, although the amount of research related to refractory concrete has significantly increased in recent years, knowledge about calcium aluminate cement (CAC) and the comparison of its performance with Portland cement (PC) at high temperatures is still very limited and most of them are not prepared in detail [4]. In light of these factors, there should be new research in this field. In this research, the main goal is to contribute new and accurate knowledge and data about the high-temperature performance of concrete to the academic field by properly conducting relevant experiments and analyses. In the research, there are a total of six different types of concrete which will be separated into two main types: those produced with portland cement (PC) and those made with calcium aluminate cement (CAC), resulting in three concrete types for each group. The three types of concrete will be separately produced with three types of aggregates: sandstone, basalt and limestone. The water-to-binder ratio is 0.45 and cement dosage is 400 kg/m3, and both parameters remain same for all concrete. In the first phase, concrete was produced for each mixture. Part of the fresh concrete was cast into 10×20 cm cylindrical specimens, while the remaining portion was placed into specially fabricated cylindrical molds with dimensions of 5×5 cm, 10×10 cm, and 15×15 cm. K-type thermocouples—resistant up to 1200 °C—were embedded at the center of these molds, representing the concrete cover, to measure internal temperature changes. In the end of this phase, standard curing with water was applied for 28 days to the samples, which will be demolded the day after casting. Secondly, after 28 days curing of specimens in curing pools, the specimens will be exposed to high temperatures of 250 °C, 500°C, 750°C and 1000 °C for at least two hours, and each specimen will be ovendried at only one temperature level. When the temperature at the core of the specimens reaches the target degree, they will be removed and left to cool until their temperature equilibrates with the ambient temperature. This experiment type is the residual test which is one of the three main types of high-temperature experiments: 'stressed', 'unstressed' and 'residual'. For all methods, the specimen is taken out from the furnace when the temperature at its the center reaches the target temperature. However, there are other factors that vary from one another. In the 'stressed' test, the specimen is subjected to 0.4 fc stress in the furnace and is immediately subjected to relevant mechanical test upon removal from the furnace. In contrast, there is no such a thing that applying forces to specimen during heating in the 'unstressed' test. For 'residual' test, the specimen needs to be cooled after heating until its core temperature drops to ambient temperature: this process typically takes 24 hours [5]. Finally, the mechanical experiments will be conducted on the cooled samples to measure changes in compressive strength and stress-strain curves. Moreover, all newly fabricated 5×5 cm, 10×10 cm, and 15×15 cm cylindrical specimens, each containing a K-type thermocouple embedded at the core, were exposed to the same projected temperature–time curve. Based on the data obtained at different high-temperature values, the various statistical models will be developed to compare the behavior of these mixtures at ambient temperatures (approximately 23-25°C) with their behavior at high temperatures. At the high temperatures, the performance of concrete can vary depending on the type of cement that concrete contains. With the produced concrete mixtures, the aim is to determine the mechanical and elastic properties of these concretes in their hardened state when exposed to high-temperature. This study can be beneficial for dealing with certain problems and improving conditions in the construction industry. There are two main area where this research can come in handy. Firstly, with the infrastructure investments rapidly increasing, a large number of the tunnels are being constructed for railway and highway projects worldwide. When these structures are exposed to fire, the temperature of concrete elements forming the tunnels can swiftly increase. This is because extinguishing the fire could take long time, and the intensity of fire could quickly grow, often causing to structural damages and economic losses. Furthermore, except the tunnels, there is also need high-temperature resistant concrete for the industrial furnaces. As in the tunnels, formal regulations does not present suitable solutions for those furnaces. Since the service temperature is too high in those ovens, the structural elements making up the furnace need to endure these temperature levels. For such situations, modern codes and regulations may be insufficient to overcome them. Therefore, during the construction of these concrete elements, the findings and analyses from this research can help to construct durable and resilient structures. To sum up, this research comprehensively examines how high-temperature environments influence the performance of concrete containing different types of cement and aggregates. The obtained data are intended to serve as references for designing concrete exposed to high temperatures and for selecting appropriate materials for such conditions in the future. Additionally, this study aims to understand the behavior of concrete and the changes in its mechanical properties during rising ambient temperatures at different levels between 250 °C and 1000 °C.
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