Betonarme Kolonların Depreme Karşı Güçlendirilmesinde Aramid Lifli Polimer Güçlendirme Donatılarının Kolon Temeline Ankrajı

Abstract

Türkiye, dünyadaki en aktif fay hatlarından birinin üzerinde bulunmaktadır. Geçtiğimiz yüzyılda yıkıcı büyük depremler meydana gelmiştir ve bu depremlerde çok sayıda can kaybı yaşanmıştır. Mevcut yapıların, düşük beton basınç dayanımına, yetersiz etriye aralığına, düz donatıya ve bindirme boyuna sahip olması bu kayıpların asıl nedeni olarak tespit edilmiştir. Betonarme kolonların yeterli eğilme dayanımına sahip olmaması nedeniyle binalarda ciddi hasarlar yaşanabildiği için depreme karşı betonarme kolonların eğilme dayanımlarının arttırılması, mevcut binaların deprem performanslarının iyileştirilmesi için önem kazanmaktadır. Bu amaçla, tez çalışması kapsamında, betonarme kolonların deprem etkilerini temsil eden eksenel yük ve tersinir tekrarlanan yatay yükler altında eğilme dayanımlarının arttırılması amaçlanmaktadır. Eksenel yük betonarme kolonların eksenel yük kapasitesinin yaklaşık % 20’si mertebesinde uygulanmıştır. Tersinir tekrarlanan yatay yükleme durumu için betonarme kolonların aramid lifli polimer şeritler kullanarak eğilme dayanımlarının arttırılması bugün itibariyle dünya üzerinde hiç bir ulusal/uluslararası yönetmelikte yer almamaktadır. Bu tez kapsamında yapılan deneysel ve analitik çalışmalar sonucunda ilgili yönetmeliklerin güncellenmesinde kullanılabilecek mühendislik parametrelerinin ortaya konulması hedeflenmiştir. Yapılan çalışmada, dikdörtgen enkesitli (200 mm×300 mm) dört adet betonarme kolon numunesi imal edilmiştir. Numuneler ülkemizdeki güçlendirme gereksinimi duyulan yapıların çoğunluğunu yansıtması açısından düşük beton dayanımına sahip, düz donatılı olacak şekilde ve sargılanma bölgesinde etriye sıklaştırması yapılmadan üretilmiştir. Güçlendirme aşamasına kadar aynı tipte imal edilen dört adet betonarme kolon numunesinden bir tanesi referans numunesi olarak kullanılmak amacıyla güçlendirilmemiştir. Diğer üç adet betonarme kolon numunesine (LAM, LAM-LAM, LAM-PB) güçlendirme uygulaması yapılmıştır. LAM ve LAM-PB numunelerinde aramid lifli polimer şeritler kolon kısa yüzeyine yerleştirilmiş ve doğrudan temele ankre edilmiştir. Kolon ve temelin birleştiği yüzeyde aramid lifli polimer şeritlerin potansiyel kusurlarından kaçınarak, daha iyi bir performans elde etmek için LAM-LAM numunesi için ilave aramid lifli polimer şerit kullanılmıştır. LAM ve LAM-LAM numunelerinde boyuna aramid lifli polimer şeritler yüzeye tamamen yapışmış olmasına karşılık, LAM-PB numunesinde kritik bölgede (kolon-temel birleşim bölgesi) şekildeğiştirme birikimini azaltmak ve aramid lifli polimer şeridinin lokalize hasarını önlemek için ankraj bölgesinde aramid lifli polimer şeridinin bir kısmı koli bandı ile sarılarak yapışma doğrultusunda kısmi süreksizlik sağlanmıştır. Güçlendirilen numunelerin üçü de paspayı sıyrılarak ilk kez İstanbul Teknik Üniversitesi Yapı ve Deprem Mühendisliği Laboratuvarı’nda uygulanmış olan bir metod kullanılarak güçlendirilmiştir. Bu anlamda, beton örtüsü kaldırılan numunelerin üzerine, boyuna doğrultuda aramid lifli polimer şerit yapıştırılmış, daha sonra boyuna doğrultuda güçlendirilen üç adet betonarme kolon numunesinin üzerine iki kat karbon lifli polimer kumaş sargı yapılmıştır. Deney değişkeninin aramid lifli polimer şeritlerin temele ankrajı olduğu bu çalışmada sonuçlar dayanım, süneklik, enerji yutma kapasitesi, rijitlik, kalıcı deformasyonlar ve göçme modları bakımından değerlendirilmiştir.

Turkey has been located on one of the most active earthquake zones in the world and has experienced several major destructive earthquakes in the last century. There were many of casualties left behind these earthquakes, resulting from substandard detailing and material characteristics of existing buildings. Insufficient flexural capacity of reinforced concrete columns may cause severe damages during earthquakes. Therefore, enhancing the flexural behaviour of the columns against seismic actions is critical to prevent potential heavy damages or collapses. Many existing reinforced concrete structures were constructed with substandard characteristics. Low quality concrete, insufficient transverse reinforcement and insufficient flexural strength are among the most common deficiencies. As it is well known, concrete cover is generally deteriorated due to corrosion of internal steel reinforcing bars of substandard structures constructed with low strength concrete. While substandard structures are in need of retrofitting, particularly in seismic areas, problems such as financial constraints, disturbance to occupants and functions of the structures are among major obstacles for retrofit interventions. Fiber reinforced polymers can provide feasible retrofit solutions with minimum disturbance to occupants. In recent years, use of fiber reinforced polymers in construction industry have become quite common. They offer feasible and innovative solutions for seismic retrofitting due to their lightweight, high tensile strength and noncorrosive character. In this study, the main aim is to investigate the flexural seismic performance of substandard reinforced concrete columns retrofitted with embedded longitudinal fiber reinforced polymer reinforcement. In this study, four cantilever reinforced concrete columns were constructed using low strength concrete and plain reinforcing bars for representing relatively old substandard structures, and then were tested under reversed cyclic lateral and constant axial loads in a quasi-static displacement-controlled manner before and after retrofitting. While lateral loads were applied through displacement cycles in pushing and pulling directions with increasing amplitudes, axial load was kept constant at approximately 20% of the axial load capacity of the columns. Three different anchorage details were designed for embedding aramid fiber reinforced polymer laminates to the existing footing, which constitutes main test variable. The reference specimen, denoted as REF, was tested without any retrofit, and the other columns (LAM, LAM-LAM and LAM-PB) were tested after they were retrofitted with aramid fiber reinforced polymer reinforcement in longitudinal and carbon fiber reinforced polymer reinforcement in transverse directions. The longitudinal aramid fiber reinforced polymer reinforcement embedded in the columns was anchored directly to the footing in case of LAM and LAM-PB specimens, while additional aramid fiber reinforced polymer anchorage reinforcements were used for the specimen LAM-LAM to achieve a better performance by avoiding potential failures of fiber reinforced polymer reinforcement at the interface of the column and the footing. Contrary to the fully bonded anchorages of longitudinal fiber reinforced polymer reinforcement in case of the specimens LAM and LAM-LAM, the anchorage of the fiber reinforced polymer reinforcement in case of specimen LAM-PB was intentionally partly debonded from concrete by using insulating tape. Partial debonding of aramid fiber reinforced polymer reinforcement in the anchorage zone was for preventing the localized damage of the aramid fiber reinforced polymer reinforcement at the interface of column and footing by avoiding localization of stresses at this critical zone. The applied retrofitting technique was introduced by Goksu et al. (2012). The aim of the applied retrofitting technique is the enhancement of column flexural capacity using aramid fiber reinforced polymer reinforcement under cyclic lateral loading in the presence of constant axial load without a significant sacrifice from deformation capacity. On the other hand, like many existing substandard RC columns, the spacing of the transverse reinforcement of the specimens was also insufficient, which might cause deficiencies in terms of ductility and shear strength after fiber reinforced polymer retrofitting. Therefore, after the intervention made for flexural strength enhancement through longitudinal aramid fiber reinforced polymer reinforcement, the specimens were also jacketed externally with carbon fiber reinforced polymer sheets in transverse direction. As known, while external fiber reinforced polymer confinement has a remarkable contribution to deformability and shear strength, its contribution to flexural strength (which is the main issue investigated in this study) is marginal with respect to the contribution of longitudinal aramid fiber reinforced polymer reinforcement. Nevertheless, both contributions were taken into account in the analytical calculations in this paper. Test results, supported by the analytical calculations, revealed that the cyclic flexural capacity of the substandard reinforced concrete members can be enhanced significantly by the use of aramid fiber reinforced polymer pultruded laminates through the applied retrofitting method. Current technical guidelines/documents do not allow use of longitudinal fiber reinforced polymer reinforcement for flexural retrofit under cyclic loading in potential plastic hinge regions of reinforced concrete members. This is basically due to concerns related with ductility. A significant enhancement was obtained in lateral flexural strength through the proposed retrofitting method. Furthermore, it was seen that the cyclic lateral drift capacities of the retrofitted columns was as high as 3%, which can be deemed as quite satisfactory against seismic actions. The comparison of the experimental data with analytical calculations revealed that a conventional design approach assuming composite action between concrete and fiber reinforced polymer reinforcement can be used for flexural retrofit design. As a summary, the higher strength of the specimen LAM-LAM is due to the contribution of additional AFRP anchorage reinforcement to the flexural capacity. The specimen LAM-PB behaved similar to the specimen LAM in terms of maximum lateral load and fracture of AFRP reinforcement at 3% drift. The sudden remarkable loss of strength upon exceeding the drift ratio of 3% was due to the fracture of one of the AFRP reinforcement within the isolated section. It was observed that the other AFRP reinforcement was decomposed locally just above the isolated height, while most of the individual fibers did not fracture at the decomposed section. This behavior allowed the column to remain in the elastic range with increasing drift ratios. When compared with the specimen LAM, it was observed that the partially bonded AFRP reinforcement (in case of specimen LAM-PB) did not affect the lateral load capacity, however, affected the seismic behavior after reaching the ultimate lateral load by presenting more pinched hysteresis curves with limited plastic deformations even up to large drift ratios (8% drift ratio). Therefore, this type of anchorage system can be utilized to limit the residual displacement (residual damage). It is important to note that all significant damage was accumulated at the base of the columns, since the columns were wrapped with CFRP sheets in transverse direction along the full height. Consequently, the crack width reached several centimeters at the intersection of the column and the footing. This type of damage may be quite disadvantageous in case of earthquakes since the distribution of plastic deformations through the potential plastic hinge length is prevented due to presence of a rigid transverse CFRP jacket. The accumulation of a remarkable portion of plastic deformations only at the interface of the column and the footing may significantly reduce the overall drift capacity of the column. Since drift capacity is essentially important for a satisfactory seismic performance, this kind of reduction in drift capacity should be avoided. Strip type external CFRP confinement may allow better distribution of plastic deformations along the height of the column.