CFRP shear strengthening of reinforced concrete T-beams without stirrups

Ülkemizin büyük bölümünün deprem riskini taşıması nedeni ile yapı stoğumuz güçlendirmeye ihtiyaç duymaktadır. Kesme kırılmasının ani ve gevrek özelliği nedeni ile kesme güçlendirmesi daha da önem kazanmaktadır. Günümüzde, fazla sayıda olumlu özellikleri, Lifli Polimer (LP) malzemelerin kesmeye karşı güçlendirmede etkin olarak kullanılmasına neden olmaktadır. Bu makale Carbon Fiber Reinforced Polymer (CFRP) levhalarla güçlendirilmiş T-kesitli betonarme kirişlerin kesme dayanımlarını artırmaya yönelik deneysel bir çalışma içermektedir. Deneysel çalışma bir adet referans elemanı ve güçlendirme uygulanmış dört adet elemandan oluşmaktadır. Depremi benzeştirebilmek adına kirişlere tersinir-tekrarlanır yükleme uygulanmıştır. Güçlendirilmiş kirişlerde farklı tipte ve uygulama biçiminde LP kullanılmıştır. LP malzemesi tek ve çift doğrultulu olmak üzere iki çeşittir. Tek doğrultulu LP uygulamasında 45°, 60° ve 0–90°’lik uygulama biçimleri seçilmiştir. Son eleman ise çift doğrultulu ve çift katlı LP levhalar ile güçlendirilmiştir. LP’in yüzeyden ayrılmasını engellemek için yeni bir ankraj detayı geliştirilmiş ve kiriş gövdesinin yan yüzeylerinin alt ve üst kısımları ile tablaya uygulanmıştır. Deneysel sonuçlar kirişlerin dayanım ve davranışlarında referans elemana göre oldukça büyük iyileşmelerin olduğunu göstermiştir. En fazla dayanım artışı çift kat tek doğrultulu levhaların birbirine dik yapıştırıldığı elemanda kaydedilmiştir. Geliştirilen ankraj detayı başarılı olmuş; hem tabla ve gövdenin birbirinden ayrılmasını engellemiş hem de LP levhanın beton yüzeyinden ayrılmasına engel olmuştur. Ayrıca deneysel sonuçlar ACI–440 komisyon raporu kullanılarak analitik sonuçlar ile karşılaştırılmıştır.  Anahtar Kelimeler: Lifli polimer, kesme güçlendirilmesi, tersinir-tekrarlanır yükleme, ankraj.A great number of reinforced concrete (RC) structures constructed in the past are structurally unsafe according to today's design codes in our country. To overcome their poor behaviour and meet the expected requirements, deficient RC structures are in need of strengthening. Shear failure is brittle in nature and it comes without advance warning. Therefore, extra care must be conducted to overcome that unwanted failure. Carbon Fiber Reinforced Polymer (CFRP) is widely preferred to strengthen and upgrade existing structures because of its superior mechanical properties. This paper presents results of an experimental study conducted on shear strengthening of RC T-section beams with CFRP sheets under cyclic loads. None of the five beams had internal shear reinforcements. Unidirectional and bidirectional CFRP sheets were used in this work. All strengthening scheme was bonded to the sides of the beam. Strengthening was not applied to control specimen K1. K2 was strengthened with two ply unidirectional CFRP sheets. The first ply was applied 45° inclination to the beam axis (45°) and second ply was bonded perpendicular to the first ply (135°). K3 was similar to the K2. K3 was also strengthened with two ply unidirectional CFRP sheets but K3's strengthening scheme was applied 60° and 120° angle to the longitudinal axis of the beam, respectively. K4 was strengthened with two ply unidirectional CFRP. The first ply was bonded parallel to beam axis (0°) and the second ply was bonded perpendicular to the first ply (90°). B4's strengthening procedure was constituted whether it acted like a bidirectional CFRP application or not. K5 was strengthened with two ply bidirectional CFRP. Shear strengthening of beams may to be problematic since they are cast monolithically with slabs. This phenomenon increases the difficulty of anchoring the FRP at the beam-slab junction and intensifies the risk of deboning failure. However, bonding FRP on both sides, or the sides and the soffit provide some shear strengthening of that kind members. Hence, the different mechanical anchorage detail was developed in this study. Anchorages were included 50x50x5 mm square steel plates, L shaped 50x50x5 mm steel plates and 10 mm diameter threaded rods. For all tests, cantilever beams were tested under cyclic load. Loading was increased up to the yielding load level or till the failure of the beam. Four linear variable displacement transducer (LVDT) were used to gauge the displacements. Also strain gauges were attached to CFRP surface to measure strains. To obtain strengthening effect, all strengthened beams were compared with the reference specimen with the help of response envelopes. Response envelopes were drawn by connecting peak points of the loading cycles. A minimum increase of 112% and a maximum of 131% in shear capacity was obtained from the strengthened specimens. K2 and K3 were strengthened two-ply unidirectional CFRP but implementation angle's were different. The results show that 45° and 60° application types had nearly the same strength and behaviour. K4 were strengthened two-ply unidirectional CFRP (0°-90°). To imitate bidirectional CFRP, two perpendicular unidirectional plies were performed to strengthen K4. On the other hand, K5 was strengthened by two-ply bidirectional CFRP. Consequently, K4 and K5 showed similar strength and failure mode. Strain values can be used to indicate the contribution of CFRP to the shear resistance. It is therefore essential to evaluate the strain activity of the strengthened specimens to determine to contribution. The maximum strain of K4 was 0.0027 mm/mm, which corresponds to 68% of the expected value outlined in the ACI-440 committee report. A low strain level of the specimens can be explained by fewer shear crack propagations, deficient bond or success of the strengthening scheme. Besides, maximum strain value of K5 and K3 exceeded the 0,004 mm/mm, ACI-440 committee report expected value. The shear resistance due to CFRP was compared to the nominal shear resistance predicted by the ACI-440 committee report. Analytical equations that were suggested by the ACI-440 committee report was denoted the closest results with the specimen that was strengthened with two-ply unidirectional CFRP. The predicted shear capacity for K4 was 9% less than that of the experimental result for ACI-440. Keywords: CFRP, shear strengthening, cyclic load, anchorage