Alper Ilki

COMPRESSIVE BEHAVIOUR OF CARBON FIBRE COMPOSITE JACKETED CONCRETE WITH CIRCULAR AND NON-CIRCULAR CROSS-SECTIONS

This paper describes the concentric compression test results of carbon fibre reinforced polymer (CFRP) composite sheet jacketed concrete specimens with circular, square and rectangular cross-sections. In the experimental program, pre-damaged specimens and repeated compressive loads were considered as well as undamaged specimens and monotonic compressive loads. The contribution of CFRP composite jackets to the compressive behaviour of the specimens is evaluated quantitatively, in terms of strength, longitudinal and lateral deformability and energy dissipation. Simple analytical expressions are proposed for compressive strength and ultimate axial strain that are valid for CFRP composite jacketed concrete with circular, square and rectangular cross-sections. The analytical results obtained by the proposed expressions are in good agreement with the experimental data obtained in this study, as well as the experimental data available in literature.

Behavior of FRP-Retrofitted Joints Built with Plain Bars and Low-Strength Concrete

Two series of tests on eight full-scale exterior beam-column joint subassemblages built with plain bars and low-strength concrete were conducted. No transverse reinforcement was present in the joint cores. In the first series of tests, which included three specimens, the behavior of joints before fiber-reinforced polymer (FRP) retrofitting was investigated. In the second series, which included five specimens, the behavior of the FRP-retrofitted joints was investigated. The six specimens consisted of a column, an in-plane beam, a transverse beam, and a slab part, and two specimens were plane members without transverse beams and slabs. The utilized retrofitting scheme is easily applicable for actual exterior beam-column joints, even in the presence of a transverse beam and a slab. Two types of strength limitation were observed for specimens in the first series. The strength of the specimen with beam longitudinal bars sufficiently anchored to the joint core was limited by the shear strength of the joint. The strengths of the other two specimens were limited by the slip of the beams’ longitudinal bars at their anchorages. In the second series of tests, significantly better performance was obtained both in terms of shear strength and ductility, provided that the slip of the beam bars was prevented. Furthermore, by using a simple theoretical algorithm based on truss analogy, the strength and deformability characteristics of the tested reference and FRP-retrofitted joints are predicted with reasonable accuracy. The same algorithm is used for predicting the joint shear strength of specimens tested by other researchers, and satisfactory agreement is obtained between the predictions and test results.

Seismic Retrofit of Brittle and Low Strength RC Columns using Fiber Reinforced Polymer and Cementitious Composites

The objective of this study is to investigate the seismic performance of reinforced concrete columns constructed with low quality of concrete and insufficient transverse reinforcement before and after retrofitting. Totally twenty nearly full size specimens with rectangular cross-section were tested under constant axial load and reversed cyclic lateral loads. Both longitudinal and transverse reinforcements were plain bars. Ten of the specimens had insufficient lap splice length of longitudinal reinforcement between stories as well. Both pre-damaged and undamaged columns with these deficiencies were retrofitted with FRP (fiber reinforced polymer) composite jackets or prefabricated HPFRCC (high performance fiber reinforced cementitious composite) panels. The test results showed that all reference specimens, which were not retrofitted, failed with a premature loss of performance either due to buckling of longitudinal reinforcement or loss of bond, while retrofitted ones exhibited a significantly superior performance, particularly in terms of ductility. It should be noted that the enhancement in performance was less remarkable for the specimens with inadequate lap splice lengths. An analytical work is also presented for prediction of the behavior of the specimens both for continuous and lap spliced longitudinal reinforcement cases.

Full-Scale Shaking Table Tests on a Substandard RC Building Repaired and Strengthened with Post-Tensioned Metal Straps

The effectiveness of a novel Post-Tensioned Metal Strapping (PTMS) technique at enhancing the seismic behavior of a substandard RC building was investigated through full-scale, shake-table tests during the EU-funded project BANDIT. The building had inadequate reinforcement detailing in columns and joints to replicate old construction practices. After the bare building was initially damaged significantly, it was repaired and strengthened with PTMS to perform additional seismic tests. The PTMS technique improved considerably the seismic performance of the tested building. While the bare building experienced critical damage at an earthquake of PGA = 0.15 g, the PTMS-strengthened building sustained a PGA = 0.35 g earthquake without compromising stability.

SEISMIC RETROFIT OF INFILLED REINFORCED CONCRETE FRAMES WITH CFRP COMPOSITES

Retrofit of existing poor reinforced concrete frame structures is a major concern for Turkey. Considering the large stock of existing poor buildings and economical situation of the country, research on economical and easily applicable retrofitting techniques is important. Although the infill walls are not considered as structural members during design, the observations after earthquakes have shown that they are important sources of strength, stiffness and damping during earthquakes. Consequently, keeping infill walls in their locations during earthquake by preventing premature damage either due to in-plane or out-of-plane loads can help the structure maintain a significant amount of its unaccounted strength, stiffness and damping characteristics. In this study, 6 reinforced concrete frames, two bare, two infilled and two with CFRP retrofitted infill walls were tested under constant axial load and reversed cyclic lateral loads. At the end of the tests, it was seen that retrofitting of infill walls with CFRP composites in diagonal direction provided significant enhancement in lateral strength and stiffness.

An exploratory study on perceptions of seismic risk and mitigation in two districts of Istanbul.

Istanbul is one of the worlds cities most vulnerable to seismic events. According to seismologists, the probability of a severe earthquake in the next 30 years is approximately 40 per cent. Following an outline of the seismicity of this vital Turkish city and a summary of current seismic risks and mitigation studies, this paper presents the results of a survey conducted in two districts of Istanbul, Avcilar and Bakirkoy. The survey comprised some 60 questions on the seismic risk perceptions of individuals and requested basic personal data, such as on age, education level, employment type, financial income, and gender. Despite various differences among the survey population, such as academic background and level of financial income, responses were surprisingly similar, especially in terms of having no plan for a safer house. The data may help those planning mitigation programmes and public awareness campaigns on preparedness and particularly mitigation in highly vulnerable regions.

Effects of Reinforcement Corrosion on the Performance of RC Frame Buildings Subjected to Seismic Actions

AbstractCorrosion of reinforcing bars is a common deficiency for existing RC buildings, which may have significant adverse effects on seismic performance of those buildings. The main effects of corrosion of reinforcing bars are strength loss as a result of reduction of the effective cross-sectional area of the bars, reduced bond strength between concrete and bars as a result of the increased volume of the bars because of rust products, and reduced elongation capacity of the bars as a result of the concentration of plastic deformations at sections of corrosion pits. The last item (concentration of plastic deformations of reinforcing bars only at the most heavily corroded sections rather than occurring all through the potential plastic hinge length) has a very remarkable adverse effect on the displacement capacity of existing buildings. It is known that the displacement capacity is as important as strength, particularly for the buildings that are designed to exhibit ductile behavior during earthquakes. In t...

Pullout Performance of Fully and Partially Bonded Retrofit Anchors in Low-Strength Concrete

Although steel reinforcing bars can be used as post-installed anchors to integrate new structural members to the existing structural system in the seismic retrofit of reinforced concrete structures, premature cone-type concrete failure can reduce the bars’ efficiency when they are anchored into low-strength concrete. This study investigates pullout behaviors of fully and partially bonded chemical anchors post-installed in low-strength concrete. The test parameters are the embedment length, the bonded length of the anchors, and the humidity and dust surface conditions of the anchor holes. The pullout behavior is examined through the load-displacement curves, modes of failure, and anchorage strengths of specimens. The results show that the pullout performance of partially bonded, post-installed chemical anchors embedded in low-strength concrete is significantly better than conventional fully bonded post-installed chemical anchors, which suggests that partially bonded anchoring of deformed bars could be useful for anchorages in seismic retrofit applications. A model is proposed for predicting the pullout strength of anchors that fail due to a combination of concrete cone and bond failures. A good correlation is found between the model predictions and test results from this study compared with previously-published experimental findings.

Effects of Loading Rate and Duration on Axial Behavior of Concrete Confined by Fiber-Reinforced Polymer Sheets

In this study, 18 concrete cylinder specimens were tested either under uniaxial compression at different loading rates or exposed to sustained axial stresses after being jacketed externally with carbon-fiber-reinforced polymer (CFRP) sheets. The specimens were cast using medium strength concrete. All the specimens had identical dimensions and level of confinement. Loading rate and applied sustained stress level were the main test parameters. Applied loading rate varied between 0.0002 and 0.04 strain/min. Four stress levels between 0.52 and 0.85 f cc ′ (0.90 and 1.46 f co ′ ) were used in short-term creep tests. Test results showed that the stress-strain behavior of CFRP confined concrete was influenced by the change in loading rate, and CFRP confinement provided considerable increase in the creep performance of concrete. The strength enhancement was more pronounced for specimens loaded at higher strain rates, while specimens loaded at slower strain rates exhibited better deformability. Results obtained fr...

AXIAL BEHAVIOR OF RC COLUMNS RETROFITTED WITH FRP COMPOSITES

Fifteen RC columns and one plain concrete column with inadequate transverse reinforcement were tested under uniaxial compression after being jacketed externally with carbon fiber reinforced polymer (CFRP) sheets. CFRP layer thickness, cross-section shapes and concrete strength were the test parameters. External confinement of columns with CFRP sheets resulted in an increase in the strength and ductility. The behavior of the CFRP jacketed columns was also predicted by the proposed stress-strain model. There was reasonable agreement between analytical behavior and experimental data.