Kypros Pilakoutas

STRUCTURAL REPAIR/STRENGTHENING OF RC COLUMNS

Abstract The susceptibility of the majority of the existing building stock to structural damage largely depends on the quality of design, detailing and construction. In many cases, the engineer can extend the life span of a building structure by utilizing simple repair/strengthening techniques. The choice of the repair/strengthening technique becomes, therefore, the decisive factor, since high cost would deter many building owners from executing essential repair works. In this paper, a cost effective and efficient technique for strengthening rc columns is described. The proposed technique involves post-tensioning metal strips around reinforced concrete columns, by using a strapping machine. The preliminary results of the experimental work carried out at Sheffield University indicate that such strengthening can increase member strength and ductility to higher levels than those possible by conventional reinforcement, at only a fraction of the time and cost required by alternative techniques.

Cyclic Behavior of RC Cantilever Walls, Part I: Experimental Results

The results of an experimental research program concerned with the response of reinforced concrete walls under cyclic loading are presented in this paper. Information pertinent to the experimental work, including description of the test rig, instrumentation of the specimens, control procedure of the tests, details of model manufacture, properties of materials used, and choice of the loading regime is first given. The description of the experiments is presented subsequently, together with details of the loading procedure and the crack development. Only brief comments on the results obtained are given in this paper further discussion of the experimental results and comparisons between experimental and analytical work are given in a companion paper

Concrete beams with externally bonded flexural FRP-reinforcement: analytical investigation of debonding failure

This paper studies the problem of early concrete cover delamination and plate-end failure of reinforced concrete beams strengthened with externally bonded FRP-reinforcement. The accuracy of analytical models and finite element (FE) methods for predicting this type of failure is assessed against published experimental data. Two design approaches based on the maximum concrete tensile strength and the shear capacity of concrete beams were examined first and it was found that linear elastic analysis cannot accurately predict the brittle plate-end concrete failure. It was also found that the extent of strengthening that can be achieved is limited by the shear capacity of concrete beams. The FE analysis is used to examine the effects of internal tensile reinforcement on the magnitude of principal tensile stresses in the critical region. The non-linear behaviour of FRP-strengthened beams is also examined in the FE analysis using the smeared crack model for concrete which is shown to adequately display the inelastic deformation of the beam. Finally, the mixed mode of failure due to the combined shear and concrete cover delamination is addressed through modelling plate-end and shear crack discontinuities using the discrete crack approach.

RC COLUMN STRENGTHENING BY LATERAL PRE-TENSIONING OF FRP

This paper presents a unique strengthening technique for existing concrete columns that use expansive materials to apply lateral pre-tensioning. The technique increases the capacity and ductility of a column as well as achieving better utilisation of the confining FRP (Fibre Reinforced Polymer) material. The confinement material properties and the confined cylinder performance are investigated experimentally. From the results, it is shown that it is possible to control the degree of applied pre-tension by controlling the amount of expansive material used. In addition, it is confirmed that jacketing columns by pre-tensioned FRP materials can increase the load bearing capacity up to 35% compared with no pre-tensioning and up to more than four times compared with unconfined concrete. The paper presents details of experimental work undertaken for the development of the confinement pressure with different confining materials (Carbon-CFRP, Glass-GFRP and Steel) and makes comparisons with predictive models.

Mechanical Characterization of Basalt FRP Rebars and Long-Term Strength Predictive Model

AbstractThe use of basalt fiber–reinforced polymers (BFRP) in construction applications is relatively new and, although its mechanical performance is expected to be similar to that of glass fiber–reinforced polymer, not many studies have addressed its performance in concrete and mortar environments. This paper examines the degradation of BFRP bars at their product development stage after exposure to accelerated environmental conditions and proposes a methodology to predict their long-term design strength. A total of 132 BFRP specimens comprising two types and seven different diameters were tested in tension after conditioning in pH9 and pH13 solutions at 20, 40, and 60°C for 100; 200; 1,000; and 5,000 h. Based on the results obtained and adopting the durability approach of industry standards for FRP reinforcement in concrete structures, a comprehensive long-term strength predictive model for fiber-reinforced polymer (FRP) bars in multiple environments is proposed and exemplified. The BFRP bars tested as p...

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.

Shear Performance of FRP Reinforced Concrete Beams

The paper deals with shear resistance of RC beams reinforced with fibre reinforced polymer reinforcement. The experimental programme presented here is part of an extensive research activity focused on comparing the development of shear carrying mechanisms in concrete beams reinforced with either steel or FRP reinforcement. Two sets of beams with equivalent geometrical characteristics were examined. Three beams were reinforced in flexure with conventional longitudinal steel bars and three beams with equivalent glass FRP bars. Shear reinforcement was provided in the form of transverse external links manufactured using glass fibres for the first set of beams and carbon fibres for the second. The experimental programme is described and findings of the project are reported and discussed in the context of the development of design guidelines urgently needed to support the wider use of FRP reinforcement for structural concrete applications.

Cyclic Behavior of Reinforced Concrete Cantilever Walls, Part II: Discussions and Theoretical Comparisons

The results of an experimental research program on the response of reinforced concrete (RC) walls under cyclic loading have been presented in Part I: Experimental Results. In this paper the general discussion is based on comparisons between experimental results and analytical solutions. The stiffness characteristics of reinforced concrete members determine the level of loading likely to be imposed and guidance is given as to suitable load-reduction factors. Limit states and deformational characteristics are investigated and observations are discussed.

Seismic Strengthening of Severely Damaged Beam-Column RC Joints Using CFRP

This article investigates the seismic behavior of three full-scale exterior reinforced concrete (RC) beam-column joints rehabilitated and strengthened with externally bonded carbon fiber–reinforced polymers (CFRP). The specimens had inadequate detailing in the core zone and replicated joints of a real substandard building tested as part of the EU-funded project BANDIT. Seven tests were performed in two successive phases. The bare joints were first subjected to reversed cyclic loading tests to assess their basic seismic performance. As these initial tests produced severe damage in the core, the damaged concrete was replaced with new high-strength concrete. The specimens were subsequently strengthened with CFRP sheets and the cyclic tests were repeated. The results indicate that the core replacement with new concrete enhanced the shear strength of the substandard joints by up to 44% over the bare counterparts. ASCE guidelines predict accurately the shear strength of the bare and rehabilitated joints. The CFRP strengthening enhanced further the joint strength by up to 69%, achieving a shear strength comparable to that of joints designed according to modern seismic provisions. Therefore, the rehabilitation/strengthening method is very effective for postearthquake strengthening of typical substandard structures of developing countries.

General Seismic Load Distribution for Optimum Performance-Based Design of Shear-Buildings

An optimization method based on uniform damage distribution is used to find optimum design load distribution for seismic design of regular and irregular shear-buildings to achieve minimum structural damage. By using 75 synthetic spectrum-compatible earthquakes, optimum design load distributions are obtained for different performance targets, dynamic characteristics, and site soil classifications. For the same structural weight, optimum designed buildings experience up to 40% less global damage compared to code-based designed buildings. A new general load distribution equation is presented for optimum performance-based seismic design of structures which leads to a more efficient use of structural materials and better seismic performance.