Standart altı betonarme kolonların yenilikçi malzemeler kullanılarak deprem performansının iyileştirilmesi
Improvement of earthquake performance of sub-standard reinforced concrete columns using innovative materials
Yazar: MUHSİN CAN LÜLECİ
Danışman: PROF. DR. ALPER İLKİ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Kamu Yönetimi = Public Administration ; İnşaat Mühendisliği = Civil Engineering
Dizin: Betonarme kolon = Reinforced concrete column ; Betonarme yapılar = Reinforced concrete structures ; Deprem davranışı = Earthquake behavior ; Deprem hazırlığı = Earthquake preparedness ; Deprem performansı = Earthquake performance ; Yapısal güçlendirme = Structural empowerment ; Zemin güçlendirme = Soil reinforcement

Özet:
Türkiye ve gelişmekte olan ülkelerin mevcut betonarme yapı stokunun birçoğu standart altı yapılardan oluşmaktadır. Standart altı olarak adlandırılan bu yapılarda gözlemlenen en yaygın eksiklikler düşük beton dayanımı, düz yüzeyli donatı kullanılması, yetersiz enine sargı donatısı ve yüksek eksenel yük oranı olarak öne çıkmaktadır. Bu yapılar deprem etkileri altında can kayıpları ve ekonomik kayıplara neden olabilir. Türkiye'deki mevcut betonarme yapı stokunun yaklaşık %80'ini 2000 yılı öncesinde inşa edilen yapıların oluşturduğu göz önüne alındığında, bu yapıların yıkılıp tekrar inşa edilmesi gerçekçi bir çözüm önerisi olmamaktadır. 2000 yılı öncesinde inşa edilen yapılardaki ortalama beton dayanımı yaklaşık 9-10 MPa'dır. Bu yapıların güçlendirilmesi ekonomik sebepler ve zaman açısından bir alternatif oluşturmaktadır. Farklı metotlar kullanılarak betonarme kolonlara yapılan sargılama işlemi, elemanın dayanım ve süneklik kapasitelerini geliştirmektedir. Son yıllarda popülerleşen lif takviyeli polimer ile yapılan güçlendirmenin hızlı ve kolay uygulanabilmesi, yapıya getireceği ağırlığın ihmal edilebilir düzeyde olması, yüksek çekme dayanımı ve yüksek rijitliğe sahip olması gibi avantajların yanında yüksek sıcaklık altında matrisi oluşturan polimer malzemenin dayanım ve rijitliğini kaybetmesi, uygulama sırasında çıkan zehirli gazlar nedeniyle insan sağlığına zarar vermesi ve ıslak zeminlerde uygulanamaması ise dezavantajlarıdır. Bu metoda alternatif olarak yapılan çalışmada yüksek en kesit en/boy oranına sahip betonarme kolonların deprem altındaki performanslarının yenilikçi bir yöntem olan püskürtme cam lif takviyeli harçlar ile güçlendirilmesi konusu araştırılmıştır. Cam lif takviyeli harç kırpılmış cam liflerin inorganik çimento bazlı matrise gömülmesi ile elde edilir. Cam lif takviyeli harç kolon yüzeyine direk püskürtülerek yapıldığından dolayı uygulama hızlı yapılmaktadır. Yapılan çalışma kapsamında 5 farklı en kesit en/boy oranına sahip (1, 2, 3, 4, 5) toplam 14 adet konsol kolon üretilmiştir. Üretilen numunelerin 7 adeti referans olarak kullanılırken kalan 7 adet güçlendirilerek sabit eksenel yük ve çevrimsel yatay yükler altında test edilmiştir. Numunelere donatı katkısı ihmal edilerek hesaplanan iki farklı eksenel yük oranı (0.35 f'cxAg ve 0.60 f'cxAg, f'c: beton basınç dayanımı, Ag: brüt enkesit alanı) uygulanmıştır. Numunelerde güçlendirme yapılırken en kesit en/boy oranı arttıkça azalan kesit etkinliği dikkate alınmış ve yanal sargı basıncını artırmak için uygulanan cam lif takviyeli harç kalınlığı kesit orta noktalarına doğru artan formda yapılmıştır. Yapılan deney sonuçlarına göre referans ve güçlendirilmiş numunelerin yatay yük ve süneklik kapasiteleri karşılaştırılmıştır. Elde edilen deney verilerine göre uygulanan güçlendirme metodu yatay yük taşıma kapasitesi ve sünekliği önemli oranda artırmıştır. Deneysel olarak elde edilen sonuçlar nümerik olarak da incelenmiş ve tutarlı sonuçlar elde edilmiştir. Yapılan araştırma sonucunda püskürtme cam lif takviyeli harç ile yapılan güçlendirmenin mevcut standart altı betonarme kolonların deprem performansını geliştirme konusunda alternatif bir metot olabileceği söylenebilir.

Abstract:
Most of the existing reinforced concrete building stock in Turkey and developing countries consists of substandard structures. The most common deficiencies observed in these substandard structures are low concrete strength, use of plain reinforcement, insufficient transverse  reinforcement and high axial load ratio. These structures can cause loss of life and economic losses under the effects of earthquakes. Considering that approximately 80% of the existing reinforced concrete building stock in Turkey consists of structures built before 2000, demolishing and reconstructing these structures is not a realistic solution. Strengthening these structures creates an alternative in terms of economic reasons and time. The wrapping process of reinforced concrete columns using different methods improves the strength and ductility capacities of the column. Reinforced concrete jacketing is more frequently used methods for repair or strengthening. This method consist of casting a concrete layer and adequate amount of longitudinal and transverse reinforcement. The efficacy of the technique on the structural behavior is related to the enlargement of the transverse cross section, which increases the load-carrying capacity and to the confinement pressure induced by the jacket in the inner column thereby ductility and strength increase and also prevent buckling of longitudinal reinforcement. Many studies and  field implementation has been accomplished in years and they have feedback about advantages and disadvantages about this strengthening technique. The main advantages of this method is lateral load capacity of buildings uniformly distrubited throughtout structure, thereby avoiding the concentrations of lateral load in some element. Hence majör strengthening of foundations can be avoided. However there are some disadvantages of associated with the column jacketing technique. Some cases the presence of beams may require most of the new longitudinal bars in the jacket to be bundled into the corners of the jacket. Some studies have proposed design rules and minumum 10 cm for cast-in-situ concrete so section dimensions significantly increased correspondingly this self weight of structure increased. Moreover, the jacketing technique disrupts the existing structures as drilling into the existing concrete structures to add dowel bars or connectors cannot be avoided. Steel jacketing method was reported to be one of the most efficient methods to increase the ductility of damaged concrete columns. The past studies prove that effectiveness of different steel jacketing methods about improving the shear strength and ductility of existing member. Stainless steel jacket can significantly increase the plastic deformations and exhibited good energy absorption. The strain hardening properties of stainless steel was observed to be beneficial in avoiding local buckling in steel members. According to the past research and field observations this retrofitting technique has some disadvantages. The effectiveness of this system is affected by shape and size especially section is square or rectangular the efficiency was found considerably low, require special attention when applying for satisfy design. This method suffers from corrosion and fire resistance and lastly moment transmission requires difficult and expensive arrangements because of this reason in most cases this system used for improving ductility, shear and axial capacity.  The steel plate jacketing method requires various works that demand specialization and skilled workers including anchor casting, field welding of steel plates and mortar grouting. Fiber reinforcement polymer become very popular in recent years because of several advantages these are tensile strength of continuous fibers approximately 2900-4600 MPa this value is higher than general steel yielding strength, approximately between 300-600 MPa, steel reinforcing bars supply constant lateral pressure after yielding but fiber reinforcement behave perfect elastic until failure so this material apply increasing lateral pressure when deformations increasing. Unlike steel, fiber reinforcement has advantage with respect to corrosion. Fiber reinforcement materials are flexible materials that allow free folding so adaptability to the shape of the structures is excellent. The fiber reinforcement polymer jacketing does not change stiffness of the members. The ease of application, high strength/weight ratio, minimal change in member dimensions are another advantages of this retrofitting technique. Several disadvantages associated with the use of FRP confinement are the high and the lack of pretension in the fibers which prevent the full utilization of FRP, its effectiveness significantly reduces when concrete materials are failed in compression. The rupture of fiber reinforcement often leads to brittle failure of structural members. Therefore, it is important to properly understand the safety allowance ratio against rupture of fibers. Effectiveness of FRP jacketing significantly affected by size and thermal resistance of FRP jacketing is low due to epoxy resins, some epoxy resins start softening at about 80°C. When resins soften the tensile strength of the FRP sheets decreases compared with the condition of sheets at a normal temperature. To generate fire resistance when applying the retrofitting method using fiber reinforcement, sufficient fire resistance covering is necessary, but it bring extra cost. The last disadvantage of this technique is that it does not apply for wet surfaces. In this study, as an alternative to this method, the issue of strengthening the performance of reinforced concrete columns with high cross-sectional aspect ratio under earthquakes with sprayed glass fiber reinforced mortars, which is an innovative method, was investigated. The difference from FRP jacketing methods is the used matrices changes from organic to inorganic thus enable some negative behavior of organic matrices. The glass fiber reinforced mortar is consist of glass fiber embedded in inorganic cementitious matrices. In this study fiber reinforced mortar sprayed directly onto the column surface and therefore, the application is fast. Advantages of this retrofitting technique can be list fire resistance, can apply on wet surfaces and also cementitious matrices is cheaper than epoxy resins. Disadvantages of this method can be list, provide effective confinement thickness of jacketing equal or greater than 20 mm so it causes increase dead weight of structure. Within the scope of the study, a total of 14 cantilever columns with 5 different cross-sectional aspect ratios (1, 2, 3, 4, 5) were produced. While 7 of the produced columns were used as reference, the remaining 7 columns were strengthened and tested under constant axial load and reversed cyclic loading. The cyclic loading step was determined according to ACI 374.2R-13. Two different axial load ratios (0.35 f'cxAg and 0.60 f'cxAg, f'c: concrete compressive strength, Ag: gross cross-sectional area) which are calculated by ignoring the reinforcement contribution applied to the columns. While strengthening the columns, the decreasing section efficiency factor as the cross-sectional aspect ratio increases was taken into account and the glass fiber reinforced mortar thickness applied in an increasing form towards the middle of the section. Before the application fiber reinforced mortar the columns corner points rounded and surface was cleaned. According to the obtained test data, the applied strengthening method significantly increased the lateral load carrying capacity and ductility. Experimentally obtained results were also analyzed numerically on open-source software known as opensees and consistent results were obtained. In opensees software for define to concrete material 'Concrete02' and for longitudinal reinforcement 'ReinforcingSteel' model were used. As a result of the research, it can be concluded that strengthening with sprayed glass fiber reinforced mortar can be an alternative method to improve the earthquake performance of existing substandard reinforced concrete columns.