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Zararlı bir ortamda betonun uğradığı hasarın hasar mekaniği ile incelenmesi

Application of damage mechanics to the corrosion of concrete

  1. Tez No: 39646
  2. Yazar: YILMAZ AKKAYA
  3. Danışmanlar: PROF.DR. SAİM AKYÜZ
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1994
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 80

Özet

ÖZET: Güvenilir yapılar tasarlandırılırken betonun ilerde uğrayabileceği hasarları ve bu hasarların derecesini önceden tahmin etmek çok önemlidir. Bu şekilde önceden alınacak önlemlerle yapı daha güvenilir hale getirilebilir. Çeşitli çevre koşullan ve mekanik şartlar altındaki betonun sürekliliği bileşenlerinin fiziksel ve kimyasal yapısına bağlıdır. İstenmeyen çevre koşullarına ve mekanik şartlara maruz kalan beton gittikçe artan mikroyapısal değişiklikler sebebiyle mukavemet kayıplarına uğrar. Bu değişiklikler mekanik özellikleri zayıflatarak hasara sebep olur. Bu hasarın derecesinin önceden tahmin edilebilmesi için hasar olayı mekanik terimler cinsinden formüle edilmelidir. Böylece analitik ve hesaplamalı teknikler kullanılarak çeşitli mühendislik problemleri analiz edilebilir. Bu çalışmada piyasada yaygın olarak kullanılan katkılı portland çimentolarıyla üretilmiş betonların, zararlı kimyasal eriyiklerle teması sonucu uğradığı hasar incelenmiştir. Bu hasar üzerinde betonun fiziksel özelliklerini büyük ölçüde belirleyici olan su/çimento ve çimento dozajının etkisi araştırılmıştır. Zararlı kimyasal ortam olarak gübre fabrikaları ve depolarında şiddetli bir hasara sebep olan amonyum sülfat ve amonyum nitrat ortamı kullanılmıştır. Deneysel çalışmalarda 3 ayrı su/çimento oranı (0.51, 0.62 ve 0.71 ) ve 3 ayrı çimento dozajı (250, 300 ve 350 ) kullanılmıştır. Deneysel çalışmaların sonucunda betonda hasarla beraber fiziksel özelliklerde ve mekanik dayanımlarda zayıflama görülmüştür. Zamanla bu zayıflamanın hızlandığı da tesbit edilmiştir. Amonyum nitratın ve amonyum sülfatın sebep oldukları hasarlar arasında da bir fark görülmüştür. Amonyum nitrat ortamında görülen çözme etkisi sebebiyle hasar hızlı bir şekilde meydana gelirken, amonyum sülfat ortamında görülen şişme etkisi hasan geciktirmiştir. Mekanik deneylerden bulunan sonuçlardan hareketle hasar olayı“Hasar Mekaniği”terimleri cinsinden ifade edilmiştir.

Özet (Çeviri)

SUMMARY APPLICATION OF DAMAGE MECHANICS TO THE CORROSION OF CONCRETE Modern engineering materials are subjected to unfavorable mechanical and environmental conditions. These materials loose their strength because of agressive attacks and corrosion due to the accumulation of microstructural changes. Concrete is a widely used engineering material, so research is needed to illuminate the subject of“ Corrosion of Concrete ”. There are different types of conrete corrosion, 1. Chemical Attacks : a. Internally Induced : Example; alcali-silica reaction or high sulphate content of materials in concrete mixture. b. External Agents : Example; ground water, sea water, agressive carbondioxide, deicing salts, sulphates or acids. 2. Physico-mechanical Attacks : Example; freezing and thawing, wetting and drying 3. Indirect Attacks : Example; corrosion of reinforcing steel bars. In this thesis agressive solutions ( external agents ) are studied.Some important agressive chemicals in the solutions are, 1. Acids 2. Sulphates 3. Magnezium Salts 4. Clors 5. Sea Water 6. Bases 7. Ammonium Salts There are some causes for low chemical resistance of concrete, these are, 1.Compositional Properties : a. High alkalinity of the portland cement paste. b. Presence of calsium hydrates and monosulphates. c. High surface area of calsium hydrates and monosulphates. vi2.Microstructural Properties : a. The porous nature of concrete. b. The microstucture of the near interfacial zone. There are also other important factors effecting the chemical agressive attack on concrete. Some of them are, 1. Temperature and humidity of environment. 2. Time of exposion. 3. Concentration of agressive ions in water. 4. Stability of products of chemical reactions. 5. Flowing of aqueous media. 6. Mechanical properties of the material subjected to agressive ions. 7. Physical properties of the material subjected to agressive ions. 8. Chemical properties of the material subjected to agressive ions. The physical properties of the concrete is determined by the amount and type of cement, water, aggregate and additives that are used. Physical nature of the concrete plays a big role in the durability problem because of the pores it contains. This porozity causes transportation of agressive liquids and gases into the concrete. The water-cement ratio is one of the dominant physical factors on the durability due to its effect on permeability, unit weight, workability and porozity. The aim of this study is to evaluate the effect of water-cement ratio on the durability of concrete in terms of“Damage Mechanics”. This study is presented in 6 chapters. In the first chapter basic knowledge about the physical and chemical properties of concrete is given. The effect of water-cement ratio on the properties of concrete is also stressed in this chapter. In the second chapter general knowledge about the effect of agressive chemical waters on the durability of concrete is mentioned. In the third chapter basic approach of damage mechanics and the influence of diffusion of some elements on fracture is given. The methods of experiments and the materials that are used in the concrete mixture are handled in the fourth chapter. The characteristics of the concrete compositions, the properties of agressive mediums are given in this chapter. The test results obtained for hardened concrete are listed and tabulated in the fifth chapter. And finally in the sixth chapter, appraisals of test results and discussion on the effect of agressive media is handled. The application of damage mechanic equations and conclusions are also given in this chapter. In tiie experiments, grading curve of aggregate mixture was kept constant and regulated to be near the B-16 curve. The maximum aggregate size was also kept constant and chosen to be 16 mm, The cement content of the mixes were 250, 300 and 350 Kg/m3. The water-cement ratios were 0.51, 0.62 and 0.71. In fertilizer factories and stores, members made with pozzolanic portland cement concrete are damaged by agressive chemical attack. Ammonium salts cause viiextensive corrosion of concrete. Production and storage of fertilizers are important problems, therefore ammonium salts were chosen to be agressive medium. Ammonium nitrate and ammonium sulphate are very harmful to unstable cement paste. They change into nitric acid and sulphuric acid so serious damage occurs. Ammonium also causes ion exchange damage. By bacterial nitrification damage proceeds. The autotrophic bacterias multiply in a medium having a pH value between 7 and 10 and in the presence of calcium carbonate which is a source of carbondioxide. The ammonium nitrate and sulphate in the aqueous solution and the alkalinity of the solution provide a suitable medium. By nitrification the ammonium ions are converted to nitrate. Thus, the medium is automatically enriched in terms of nitrate ions, therefore becoming more corrosive. The chemical reactions of ammonium nitrate are Ca(OH)2 + 2NH4N03 + 2H20 -* Ca(N03)2.4H20 + 2NH3 C3A.6H20 + Ca(N03)2.4H20 -? C3A.Ca(N03)2. 1 Ori.0 and ammonium sulphate, (NH4)2S04 + CaO.H20 -? CaS04.2H20 + 2NH3 3CaO.Al2O3.6H2O + 3(CaSO4.2H20) + 20H2O - *. 3CaO.Al203.3CaS04.32H20 After the experiments it is seen that all the physical and mechanical properties of concretes decreased with time of immersion in the agressive solutions. It is also observed that increasing water-cement ratio increases the damage also. It is essential to formulate this damage phenomenon in terms of mechanics if we want to estimate the value of damage when designing reliable structures. Then, it will be possible to analyse various engineering problems using analytical and computational techniques. In general a theoretical description of damage can be rather complicated. Because the experiments in this field are diffucult. Therefore as a rule, experimental data are scarce. Determination of functions and constants, which play a role in the complex variants of the theory, from available experimental data is often practically impossible. vuiFor the phenomenological description of microstructural changes it is necessary to introduce, according to the principles of irreversible thermodynamics, some internal variables ( hidden parameters ), supplementing the set of thermodynamic ( observable ) parameters of the basic undamaged state ( elastic, plastic, creep, etc. ) The set of parameters describing the damage is characterized by some mathematical entity which we denote by“-?& 0) Where A0 is the initial area of the undamaged section and A is the fractured, lost area as a result of damage. Here the damage variable is considered as scalar and it is a positive monotically increasing function. Sometimes it is convenient to use the function V = I”» = TT (2) Where A0-A can be interpreted as the actual area of the section. This function is called“continuity”. It is a positive monotonically decreasing function. IXIn the case of uniaxial tension, the actual stress is, ?“- ? (3> Where aa is the stress related to the undamaged area of the section, a is the nominal stress. In the case of uniaxial tension a simple form of the kinetic equation of damage is, ^ = -A(f)”(4) Where A and n are coefficients. From experimental results the following equation is obtained, where a and b are coefficients, t is time (day), V=l-atb (4a) Diffusion of some substances can significantly reduce the long term resistance of materials. It depends on the gradient of their concentration and the level of stress. The process of diffusion can be considered as independent of the stress field. But the fracture process depends essentially on the stress state and on the concentration of the element in the material. In this case we can write the coefficient A, A = A,q (5) Where q is the concentration of agressive substance. We can define q, as a function of time as follows, 2 Jl2 q = q.(i-^) (6) If we substitude (6) in (5) and (4), we can get dtp

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