Konstantinos Katakalos

Shear Behavior of Rectangular Beams Strengthened with either Carbon or Steel Fiber Reinforced Polymers

The aim of this study is to upgrade the shear capacity of reinforced concrete (R/C) beams strengthened with either Carbon (CFRP) or Steel (SRP) fiber Reinforced Polymers with strips having the form of either closed or open hoop external transverse reinforcement. This investigation also includes the use an anchoring device. Seven prototype specimens in need of shear strength upgrade were tested having a span of 3000mm. The strengthened R/C beams along with a non-strengthened control R/C beams were tested monotonically under four point bending loading conditions. The experimental results indicate that the shear failure of the closed hoop strengthened beams is accompanied with the tensile fracture of the CFRP/SRP strips together with an increase in the shear capacity. A considerable increase in the shear strength was observed when open hoop CFRP/SRP strips with no anchoring were used; however, the debonding of these strips posed a limitation to this upgrade. When these open hoop CFRP/SRP strips were provided with anchoring devices the shear strength of the R/C beams was further increased and the shear failure was accompanied this time by either the fracture of the CFRP/SRP strips or the failure of the anchoring system. In conclusion, a shear strengthening system for R/C beams utilizing open hoop CFRP/SRP strips with properly designed anchoring it is easily applicable, it increases significantly the shear capacity of such R/C beams and it exploits at the same time the high tensile strength of these fiber polymer materials in a cost effective way. An analytical procedure used to predict the shear strength for of such R/C beams yielded good comparison with the measured values.

Numerical simulation of the shear behaviour of reinforced concrete rectangular beam specimens with or without FRP-strip shear reinforcement

The successful validation of a numerical model is presented that can realistically approximate the shear behaviour of reinforced concrete (R/C) rectangular beams strengthened against shear with externally applied open hoop fibre reinforcing polymer (FRP) strips. For this purpose, the measured load-deformation response of ten (10) full-scale R/C beam specimens is utilised. These specimens were loaded monotonically in a four-point bending arrangement up to failure. Open hoop FRP strip shear reinforcement was applied externally to upgrade the shear capacity of eight (8) R/C beam specimens. Four of these specimens had these FRP strips without anchorage, whereas for the other four the FRP strips were attached together with novel anchoring devices. This successful numerical simulation predicts with a very good degree of approximation the observed load-deformation behaviour and the ultimate shear capacity of all these specimens as well as the observed modes of failure including diagonal concrete cracking, debonding of the FRP strips in the case of no anchoring, or the plastification of parts of the anchoring devices plus the adjacent crushing of the concrete.

The Influence of Concrete Surface Preparation when Fiber Reinforced Polymers with Different Anchoring Devices are Being Applied for Strengthening R/C Structural Members

The aim of this study is to investigate the influence of concrete surface preparation when either steel or carbon fiber reinforced polymers (FRP) are applied for strengthening applications of R/C members. The present study also investigates the anchorage or not of the applied FRP strips on the volume of concrete. For this purpose special concrete specimens were fabricated and were used to attach CFRP or SRP strips with or without anchoring and with or without contact surface treatment. The experimental results indicate that the concrete surface preparation is important and results in an increase of the load bearing capacity when the FRP strip is not anchored. When an anchoring device is employed, the concrete surface preparation is of no significance. With a properly designed anchoring device, a significant increase in the bearing capacity was observed and the failure was that of the fracture of the FRP strips for all such specimens. The highest FRP material exploitation was achieved in the specimen that utilises the patented anchoring device together with two layers of SRP strips. Debonding of the FRP strips, or failure of the anchoring device, results as was to be expected, in relatively unsatisfactory FRP material exploitation.

Mechanical Behavior of High Temperature Hybrid Carbon Fiber/Titanium Laminates

The aim of this paper was to investigate the tensile and flexural properties of hybrid laminates made with titanium sheets and high modulus carbon fiber composites. Grade II titanium was used, which exhibits great high-temperature performance and creep resistance, low weight, and high strength. An inorganic fireproof matrix, known as geopolymer, was used to fabricate the high modulus carbon fiber composites. Previous studies have shown that these composites are strong, durable, lightweight, and can exhibit excellent performance up to 400°C. In the present study, a number of specimens were tested in uniaxial tension and four-point bending after exposure at elevated temperatures. The results indicate that the addition of carbon fibers can reduce the weight and increase the stiffness of the pure titanium. Moreover, the hybrid laminates are stronger and stiffer than the sum of the individual strengths and stiffnesses of the parent materials. An important finding is that the interlaminar bond is strong, and as a result no delamination failures were observed.

EXPERIMENTAL INVESTIGATION AND NUMERICAL SIMULATION OF SHEAR BEHAVIOUR OF RC T-BEAM SPECIMENS WITH OR WITHOUT SFRP-STRIPS UNDER CYCLIC LOADING

The successful validation of a numerical model is presented that can realistically approximate the shear behavior of reinforced concrete (RC) T-section beams strengthened against shear with externally applied open hoop steel fiber reinforcing polymer (SFRP) strips. For this purpose, the measured load-deformation response of three (3) full-scale R/C beam specimens is utilized. These specimens were imposed in a cyclic loading sequence up to failure. Open hoop steel FRP strip shear reinforcement was applied externally to upgrade the shear capacity of two (2) RC beam specimens. For the first specimen the SFRP strips were applied without anchorage, whereas for the other strengthened specimen the SFRP strips were attached together with a novel anchoring device. The successful numerical simulation predicts with a very good degree of approximation the observed load-deformation behavior and the ultimate shear capacity of all these specimens as well as the observed modes of failure including diagonal concrete cracking, debonding of the CFRP strips in the case of no anchoring, or the plastification of parts of the anchoring devices plus the adjacent crushing of

INVESTIGATION OF NOVEL ANCHORING DEVICES FOR OPEN HOOP CFRP STRIPS BONDED ON R/C ELEMENTS UNDER MONOTONIC AND CYCLIC LOADING-UNLOADING CONDITIONS

Results obtained from research conducted in the framework of an initiative introduced by the Hellenic Earthquake Planning and Protection Organization aiming to support relevant provisions of the Greek Code for the repair and strengthening of R/C structures are presented and discussed. When trying to utilize fiber polymer sheets, as external reinforcement attached on R/C elements, their potential of high tensile strength is prevented by the FRP sheets’ debonding mode of failure thus limiting the exploitation of the material. In order to confront this limitation the effectiveness of two novel types of anchoring devices that can be incorporated together with such FRP strips was studied. For this purpose special unit beam concrete specimens were fabricated and were used to attach the open hoop carbon strips (CFRP). The novel loading arrangement developed at Laboratory of Strength of Materials and Structures was utilized to apply the necessary forces to these unit beam specimens together with instrumentation capable of capturing the behavior of these specimens up to failure. Studying in this way the transfer of forces from the open hoop FRP strips it could be demonstrated that when this type of retrofitting was accompanied with a properly designed anchoring device, a significant increase in the bearing capacity of the tested specimens was observed. The present investigation incorporates also the influence of number of cycles, in a loading-unloading way, on the overall behavior of the strengthening scheme, in terms of ultimate strength and observed mode of failure. Finally, this paper includes the most important findings of a supplementary numerical investigation, using the FEA software ABAQUS, aimed to study the behaviour of the novel anchoring device which is patented in Europe under the patent number WO2011073696.

THE DYNAMIC RESPONSE OF A MOCK-UP OF THE RESTORED ANCIENT NORTH WALL SCHEME FOR THE MACEDONIAN PALACE AT VERGINA - GREECE.

Abstract. This paper presents summary results from a numerical study that examined the dynamic behaviour of a mock-up of the proposed retaining wall scheme aimed at supporting the North ancient wall of the Macedonian Palace at Vergina, Greece. Part of this retaining wall scheme represents the ruined ancient wall that will be restored. This wall will have at its base those monoliths of the ancient that survive in good condition so that can be used again. For the rest of the wall new monoliths will be used which will be constructed in-situ for this purpose. The restored wall will be partly supported by an additional wall to be built at its back which will also support the ruins of the Macedonian palace. The restored ancient defense wall will be connected at its back with special ties to the rest of the retaining wall scheme. The stability of the restored ancient wall is examined through a mock-up that simulates the geometric non-linearities from the sliding and rocking of the monoliths as well as those from the special ties. The friction characteristics between the new prototype monoliths were investigated experimentally at the laboratory.

INFLUENCE OF THE LOADING RATE ON THE AXIAL COMPRESSIVE BEHAVIOR OF CONCRETE SPECIMENS CONFINED WITH SRG JACKETS

In the present paper, an experimental study was carried out where the main objective was to investigate the favorable confinement characteristics of steel reinforced grout (SRG) jackets on the compressive behavior of unreinforced concrete prisms under monotonic and cyclic axial loading. SRG jackets were made by applying steel fiber reinforced fabrics of reduced density combined with cementitious grout that serves as the connecting matrix. For this purpose, one layer of the novel jacketing system was applied to a number of unreinforced concrete prismatic specimens constructed for a moderate concrete cylindrical compressive strength of 25 MPa. The parameters of this investigation were: a) the density of the fabric used in the steel reinforced concrete jackets and b) the rate of axial loading. The density of the fabric was either medium (2 cords/cm) or relatively low (1 cord/cm). The specimens were subjected to monotonic concentric uni-axial compression load applied either in a slow rate reaching the maximum load in 400 secs or at a axial load rate ten times faster. Both the slow rate load and the fast rate load were applied in the following manner. Initially, approximately 50% of the maximum load was reached in 3 load-unload cycles that were followed by 3 loadunload cycles at approximately 75% of the maximum load before the specimen reached the maximum load and failure at a final loading cycle. The experimental sequence was carried out at the Laboratory of Strength of Materials and Structures of Aristotle University of Thessaloniki. The magnitude of the maximum load reached, as well as the mode of failure attained at the end of each loading sequence was monitored. From the obtained results, the influence of the loading rate on the ultimate compressive load and on the mode of failure in relation to the SRG jacket characteristics was studied. The whole effort intended to examine the effect on the response behavior of SRG jacketed specimens when axial load is dynamic in nature.

A Hybrid Device for Anchoring FRP Sheets Aimed to Enhance the Flexural Capacity of RC Elements

Abstract: The potential of externally applied FRP (fiber-reinforced plastic) sheets, being employed in retrofitting schemes aimed to repair and strengthen RC (reinforced concrete) structural elements damaged by prototype strong earthquakes, is presented and discussed in this study. The limitation of the debonding mode of failure of these FRP sheets is highlighted and the necessity to develop efficient anchoring devices for these FRP sheets is underlined. The behavior of such a novel HAD (hybrid anchoring device), capable of anchoring CFRP (carbon fiber reinforcing plastic) sheets to RC structural elements, is presented and discussed. The behavior of the device itself was studied through a 3D non-linear numerical simulation at the preliminary design stage in order to establish certain desired features such as the ductile behavior of the device itself as well as the satisfactory performance of the FRP sheets wrapped around this device. This HAD was next applied as part of a strengthening scheme aimed to upgrade the flexural capacity of an RC bridge-type pier specimen subjected to a cyclic seismic-type loading sequence. The obtained results demonstrated an increase in the specimen’s flexural capacity by 100% as well as a similar increase in its capability of dissipating energy in a ductile manner during the cyclic load sequence. Moreover, the employed 3D non-linear numerical simulation yielded reasonably good agreement between the measured and the predicted cyclic response of this specimen strengthened by CFRP layers, which were anchored by the novel HAD. The successful behavior of this novel HAD, which has been patented with No. WO2011073696, is currently being tried with a number of other retrofitting schemes employing FRP sheets externally attached on RC structural elements.