Betonarme kirişlerde çelik halat kullanımının yapısal davranışa olan etkisinin irdelenmesi
Investigation of the effect of steel strands on structural behavior in reinforced concrete beams
Yazar: MUZAFFER YAZAR
Danışman: DOÇ. DR. BEYZA TAŞKIN AKGÜL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Betonarme = Reinforced concrete ; Betonarme kesitler = Reinforced concrete sections ; Betonarme çelik = Reinforced concrete steel ; Çekme donatısı = Tension reinforcement

Özet:
Bu çalışmada betonarme kirişlerde eğilme donatısı olarak çelik halat kullanılması deneysel ve kuramsal olarak incelenmiştir.
Çelik halatlar öngermeli veya artgermeli olarak önüretimli veya yerinde dökme betonarme yapı elemanlarında yaygın olarak kullanılmaktadır.
Bu çalışmada çelik halatlara herhangi bir kuvvet uygulanmadan, betonarme kirişlerde eğilme donatısı olarak kullanılması araştırılmıştır. Betonarme kirişlerde aynı kesitte aynı mekanik özelliklere sahip donatılar kullanılır. Burada ise aynı kesitte hem çelik halat hem de geleneksel betonarme çeliği kullanılmıştır. Çelik halatlar ile geleneksel betonarme çelikleri gerilme şekildeğiştirme özellikleri bakımından farklılıklar gösterir. Çelik halatların akma ve çekme dayanımları, kullanılan geleneksel betonarme çeliğinin akma ve çekme dayanımlarından yaklaşık 4 kat daha yüksektir.
Çelik halatlar ve geleneksel betonarme çeliklerinin aynı kiriş kesitinde kullanılmasının davranışa olan etkisini araştırmak amacıyla deneysel bir çalışma yürütülmüştür. 3 adet kirişi temsil eden, 1 adet kolonu temsil eden 4 numune hazırlanmıştır. İlk numunede sadece geleneksel betonarme çeliği kullanılmıştır. Diğer numunelerde hem çelik halat hem de betonarme çeliği kullanılmıştır. Çelik halatlı numunelerdeki çekme donatısı miktarı, geleneksel betonarme çeliği ile donatılandırılmış numunenin çekme kuvveti ile eşdeğer olacak şekilde belirlenmiştir. İlk üç numune 4 nokta eğilme testine, son numune 3 nokta eğilme testine tabi tutulmuştur. İlk 3 numune monotonik, son numune çevrimsel olarak yüklenmiştir. Numunelerin hiçbirine normal kuvvet etkitilmemiştir. Yapılan deneylerde kuvvet yerdeğiştirme, moment yerdeğiştirme eğrileri çıkarılmıştır. Deneyler süresince kirişlerde oluşan çatlaklar da gözlenerek, hasar oluşumları incelenmiştir. Deneyde elde edilen moment açıklık ortası göreli yerdeğiştirme sonuçları kullanılarak kirişlerin moment kapasiteleri karşılaştırılmıştır.
Deneysel çalışmanın doğrulanması için analitik bir çalışma yürütülmüştür. Kiriş numuneleri Vector5 programında modellenerek statik itme analizi yapılmıştır. İlk olarak sadece geleneksel betonarme çeliği içeren ilk numune için analitik çözümleme yapılarak, beton ve çelik malzeme modelleri doğrulanmıştır. Daha sonra aynı malzeme modellerine çelik halatın malzeme modeli eklenerek diğer numuneler için de modelleme yapılarak analitik sonuçlar elde edilmiştir. Malzeme modellerinde çelikler için çekme testi sonucu elde edilen malzeme özellikleri kullanılmıştır. Beton için ise Vector5 programında yer alan malzeme modeli kullanılmıştır.
Deney sonuçları hem kendi içinde hem de analitik çözümleme sonuçları ile karşılaştırılarak, çelik halatların geleneksel betonarme çelikleriyle beraber kullanıldığında davranışa olan etkileri ortaya konmuştur.

Abstract:
In this study, the use of steel strands as bending reinforcement in reinforced concrete beams is examined experimentally and theoretically. These strands are generally used in pre-cast or in situ cast reinforced concrete structural elements as prestressed or poststressed.
It is not common for steel strands to be used in reinforced concrete beams without applying any force. In the literature, there is a few of experimental or analytical studies on this subject. The studies in the literature research are generally the same type of reinforcement used in the same section. In this work, both the steel strands and the conventional reinforcements are used in the same section as tensile reinforcements. In this respect, this study differs from the studies in the literature. Since the aim of this study is to investigate the effect of high strength bending reinforcement on the behavior of reinforced concrete beams, the scope of the study is expanded and the bending reinforcements with different mechanical properties having high strength to affect the capacity of the reinforced concrete sections are added to the literature research.
It has been investigated to use steel strands as bending reinforcement in reinforced concrete beams without applying any force to the steel strands. Reinforced concrete beams are equipped reinforcements which have the same mechanical properties. Steel strands and conventional reinforcements differ in their stress strain properties. The yield and tensile strength of the steel strand are approximately 4 times higher than the conventional reinforcement's yield and tensile strength .
In this study, S420 reinforced concrete steel is used as bending and shear reinforcement as conventional reinforcement. The diameters of the reinforcements are 18 and 12 for longitudinal reinforcement and 8 mm for shear reinforcement respectively. Tensile testing is performed for 9 rebar sample, 3 for each diameter. Stress strain graphs are obtained and mechanical properties of rebars are determined. Strain values are read up to 7 percent in order to avoid physical damage to the extensometer. After 7 percent strain, the extensometer is removed and the tensile test is continued until the rebars reached their tensile strength. The yield and tensile stresses are determined 480 MPa and 560 MPa respectively. Concrete beam samples used in the experiment are casted with C30 concrete. Two concrete cube samples which have 150 mm of edge size are prepared to determine the compressive strength of the concrete. The compressive strengths obtained are 22-day concrete compressive strengths and converted from cubic compressive strength to cylindric compressive strength. The cubic compressive strengths are determined for two cube samples 32.2 MPa and 30.6 MPa respectively. Cylindrical compressive strength is calculated as 24 MPa. 0.5 inch diameter steel strands are used as a tensile reinforcement. Tensile tests are performed for a total of 3 steel strand samples. Strains are read up to 1 percent in order to avoid physical damage to the extensometer. Extensometer is removed after 1 percent. After 1 percent strain, the stress strain curve is approximately drawn. The tensile test is terminated when the steel strands lost their helix structure and reached tensile strength. Maximum strain is determined as 6.5 percent. The yield and tensile stresses are determined 1730 Mpa and 1960 MPa respectively.
An experimental study is conducted to investigate the effect of two different mechanical properties on the same reinforced concrete beam section. 4 reinforced concrete samples representing 3 beams and 1 column are prepared. Beams and column are numbered as 1, 2a, 2b and 3. Only conventional reinforcements are used in the first sample. In other samples both steel strands and conventional reinforcements are used. The amount of tensile reinforcement in the beams with steel strand is determined so that the tensile forces are the equal as the tensile force of the beam equipped with conventional reinforced concrete beam. According the equal tensile forces it is expected that beams must have close moment capacities. Moment capacity of the beams are compared by using the moment displacement results. The first three samples are subjected to the 4 point bending test and the final sample is subjected to 3 point bending test. The first 3 samples are monotonically, the final sample is cyclically loaded. None normal force is applied to any of the sample. In the experiments, force displacement and moment displacement curves are obtained.
During the experiments, cracks occurred in the beams are observed and damage occurrences are examined. The first bending cracks on 1, 2a and 2b beams occurred at loads of 18 kN, 22 kN and 17 kN respectively. It is observed that the cracks in the conventional sample are more frequent and narrow than those in the beam samples with steel strand. In the conventional sample, bending cracks are concentrated in the middle region. The cracks in the beams with steel strands have propagated faster. When the steel strands are used together with conventional reinforced concrete steels, the bond losses of the beams with steel strands prevents beams to reach the desired moment capacity and crack pattern.
An analytical study is conducted to verify the experimental study. Beam cross sections are modeled in Vector5 program, then pushover analysis is done. Vector5 is a computer program which performs nonlinear cross-sectional analysis in 2D systems including beam, column, shear wall and calculate for temperature, static and dynamic loading conditions. Initially, only the first sample of conventional reinforced concrete steel is used for analytical analysis and material models for concrete and conventional reinforcements are verified. Analytical results are obtained by modeling for other samples with the same material models verified by the first analysis. Experimental results are compared with analytical analysis results and the behavior of reinforced concrete beams are investigated. The effects of the steel strand on behavior of the reinforced concrete beams when used with conventional reinforcements have been demonstrated.
In the experiments, it is observed that steel strands could not reach desired strains and stresses due to slip and bond loss. For this reason, the moment capacity of the beams with steel strand is 40 percent less than that the beams with conventional reinforcements.
In the analytical study, static pushover analysis is performed for all beams. In the comparisons the pushover and experimental results are not close to each other. As it is seen in the comparison of the experiment results, a new tensile stress value is calculated for the steel strand due to the bond loss. This stress value is called as average stress. The average stress for yield and tensile stress of steel strands are defined to Vector5 program and static pushover analyzes are repeated. The results are closer to the experimental results. In other words, it is observed that the steel strands do not work as expected. This result is confirmed by the experiment and the analytical model.
Test samples are prepared outside and brought to the test area. Therefore, straingauges could not be attached to reinforcements. In order to calculate the rotation and curvature values, transducers are placed on the concrete side surfaces at the midpoints of the beams. Data obtained from transducers were not sufficient to calculate the rotation and curvature values. Therefore, moment curvature or moment rotation expressions could not be used in this study. The comparison of the experimental results and analytical results is performed by means of relative displacement in the middle of the beam and moment.