Proceedings of the End of Action Conference
Budapest, Hungary - 3-5 April 2017

Local Organising Committee
György L. Balázs (Chair) - Prof. Dr.-Ing., Head of Department
Sándor Sólyom - PhD student, MC Fellow

Department of Construction Materials and Technologies
Faculty of Civil Engineering,
Budapest University of Technology and Economics
H-1111 Budapest, Műegyetem rkp.3
HUNGARY

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Plenary Session I

 

An Innovative Structural and Energy Retrofitting System for Masonry Walls using Textile Reinforced Mortars Combined with Thermal Insulation: Mechanical and Fire Behaviour

T. Triantafillou, K. Karlos, K. Kefalou, E. Argyropoulou

The University of Patras, Greece

Masonry walls have been proven to be prone to failure during high or moderate intensity earthquakes or high wind pressure, hence they represent a significant hazard to life safety. Moreover, structural decay due to ageing or cumulative seismic‐induced damage poses a direct threat to the preservation and safeguarding of masonry structures that comprise an important part of many countries’ cultural heritage. Thereby, there is an urgent need for upgrading existing masonry structures, both in seismic areas and in non‐seismic areas.
In addition to structural (including seismic) retrofitting, given the high energy consumption associated to old buildings and their significant environmental impact, there is a strong need for effective solutions for the building envelope energy retrofitting. Towards this goal, a wide range of solutions has been proposed, with external and internal insulations becoming increasingly popular, due to high energy savings, the quick and easy application and the low cost.
The present study presents an innovative system for both structural (including seismic) and energy retrofitting of masonry walls, involving the combination of textile reinforced mortar (TRM) and thermal insulating materials. The system was tested on brick masonry wallettes subjected to out‐of‐plane cyclic bending. Some of the wallettes were subjected to fire testing prior to mechanical testing, to assess the effectiveness of the new system under realistic fire conditions. From the results obtained in this study the authors believe that TRM jacketing may be combined effectively with thermal insulation, which can be fire‐resistant too.

 

 

The unsolved issues in the design process of FRP strengthened RC members

Christian Carloni & Mattia Santandrea

University of Bologna, Italy

Externally bonded fiber reinforced polymer (FRP) composites have been extensively used to strengthen reinforced concrete (RC) members. FRP composite systems have a high strength-to-weight ratio, require relatively limited time to cure, and have mechanical properties that can be engineered to meet the desired structural performance. Code and guidelines have been issued in several countries and practitioners are familiar with this technology. However, there are some key factors of the design process of strengthened members that have not been fully investigated or understood. For example, the relationship between full-scale beam tests and direct shear tests. This presentation will point out the unsolved issues and weaknesses of the current approach used to design RC members strengthened with FRP.

 

 

Seismic and extreme loadings

 

Collapse testing of full-scale RC buildings with or without seismic retrofit of columns with FRP jackets

Erkan Tore, Mustafa Comert, Cem Demir, Alper Ilki

Istanbul Technical University, Turkey

Two full-scale three story reinforced concrete frame buildings representing existing sub-standard constructions were built and tested simultaneously under reversed cyclic lateral loads. The columns of one of the buildings were retrofitted with external FRP jackets for enhanced ductility and shear capacity. The vertical loads on the first story columns were approximately 25 to 45% of the axial capacities of the columns determined without consideration of longitudinal bars. Some columns of the buildings were flexural-critical and others were shear-critical. Therefore, contribution of external FRP jackets for both flexural and shear strength enhancement could be observed through the experimental work. The reference building without retrofitting totally collapsed very suddenly in a brittle manner at about 1% lateral drift of the first story, whereas the identical building with FRP retrofitted columns could sustain lateral loads up to lateral drift of 15% of the critical first story (about 400 mm lateral drift at only the first story). The test results very clearly and convincingly demonstrated the potential of seismic performance enhancement achievable through use of FRP sheets in the form of external jacketing of the columns.

 

Theoretical and Experimental Study on the Shear Resistance of Precast RC Wall Panels

Carla Todut, Valeriu Stoian, Daniel Dan, Tamas Nagy-Gyorgy, Mihai Fofiu

Politehnica University Timisoara, Romania

The paper presents a part of the results from an experimental program developed to study the seismic performance of precast reinforced concrete wall panels with openings. The precast reinforced concrete walls investigated in this study meet the requirements of Eurocode 8 for walls designed to DCM (medium ductility) as large, lightly reinforced walls. The specimen characteristics and reinforcement configuration were taken from a typical Romanian project used widely since 1981 and scaled 1:1.2 due to the constraints imposed by the laboratory facilities. The experiental tests revealed a shear type of failure that is influenced by the opening type. These panels were damaged under cyclic lateral loads and thereafter rehabilitated and retested. A theoretical study was also performed, considering the recommendations given in EC 2 and CNR, and concluding remarks were taken within the limitations of the current research.

 

Effect of FRP confinement on the axial stress-strain behaviour of rubberised concrete

Samar Raffoul, David Escolano Margarit, Reyes Garcia, Maurizio Guadagnini, Kypros Pilakoutas

University of Sheffield, United Kingdom

During the past few years there has been significant research on the use of waste tyre rubber as partial replacement of concrete mineral aggregates. Despite having numerous potential environmental and structural benefits (e.g. higher deformability and damping capacity), the use of rubberised concrete (RuC) is often limited to non-structural applications due to its low compressive strength and stiffness. This is mainly attributed to the low stiffness of the rubber but also its high Poisson’s ratio, which leads to tensile stress concentrations, excessive micro-cracking and premature failure.
This paper investigates the use of Aramid and Carbon FRP sheets as external confinement to RuC with high rubber content to limit crack propagation and mitigate losses in compressive strength. It is shown that the unique premature volumetric expansion and large lateral deformation capacity in RuC can be exploited to activate confinement pressure at earlier stages, leading to higher effectiveness, compared to confined conventional concrete. The confined-RuC cylinders reached compressive strengths of 90 MPa (about 100% increase in strength) and unprecedented axial strains of around 5.5% for four layers of AFRP confinement. Such confined-RuC can be used for structural applications with high strength and high-deformability requirements.

 

Experimental Study on Highly Deformable Coupling Beams

David Escolano-Margarit, Samar Raffoul, Matteo Di Benedetti, Maurizio Guadagnini, Kypros Pilakoutas

The University of Sheffield, United Kingdom

Reinforced Concrete (RC) structural coupled walls have long been used in medium to high rise buildings as a primary lateral force resisting system for wind and seismic loads. When a wall system is subjected to large lateral actions, the coupling beams are required to transfer high shear forces and undergo large deformations. Therefore, the performance of the entire lateral resisting system depends on the deformation capacity of the coupling beams.
This paper presents a novel solution for highly deformable coupling beams using confined Highly Deformable Concrete and presents the results of a preliminary experimental investigation. Highly Deformable Concrete utilises recycled rubber particles as replacement for both fines and coarse aggregates. Although the inclusion of rubber in concrete can lead to a significant reduction in compressive strength and stiffness, rubberised concrete exhibits higher deformability, ductility and energy dissipation capacity than traditional concrete when confined with steel jackets or FRP.
The results of an experimental program on 1:2 scale coupling beams with regular and confined Highly Deformable Concrete are presented and discussed to assess the performance of the proposed novel solution. Two different reinforcement details (flexural and diagonal) are considered. All specimens are subjected to the same loading history comprising a series of quasi-static load cycles at increasing levels of maximum displacement. The results of the tests indicate a better seismic performance of the confined Highly Deformable Concrete coupling beams, in terms of ductility, deformability and energy dissipation capacity compared to conventional RC coupling beams.

 

Literature Review on Blast Protection by Externally Bonded FRP Reinforcement

A.Maazoun , S.Matthys , J.Vantomme

Magnel Laboratory for Concrete Research Structural Department, Faculty of Engineering & Architecture, Ghent University, Belgium

One of today’s state-of-the-art techniques is the use of Fibre Reinforced Polymer (FRP) composites as Externally Bonded Reinforcement (EBR). This method consists in gluing strips or flexible sheet in the tension zone to increase the resistance capacity or service behaviour of structures. The use of FRP as externally bonded reinforcement has been demonstrated as a very efficient technique mainly for static load conditions. More recently, a number of studies have also been conducted regarding the use of FRP EBR for strengthening critical infrastructure (concrete and masonry) against blast loading. This paper presents a brief literature review of research on FRP EBR and blast loading and describes the efficiency of FRP composites for blast protection.

 

Bond in strengthening applications

 

Open Issues on the experimental setup in TRM-to-substrate shear bond tests

Francesca Roscini, Stefano De Santis, Gianmarco de Felice

Roma Tre University, Italy

Textile Reinforced Mortar (TRM) strengthening systems, comprising high strength fabrics bonded with inorganic (cement or lime mortar) matrices, are currently receiving great attention as an effective, sustainable and compatible solution to retrofit existing structures. The advantages provided by the use of a mortar matrix in place of the epoxy resin employed in Fibre Reinforced Polymers (FRP), in terms of behaviour at high temperatures, possibility of being applied to wet or irregular surfaces, durability, and cost-efficiency, make TRM externally bonded reinforcement particularly competitive, especially for applications to masonry structures. Nevertheless, the bond strength of TRM systems may be lower than that of FRPs, especially with lime-based mortar matrices, and even more complicated, since a relatively broad range of failure mode may take place. Different experimental methods have been developed to investigate of the TRM-to-substrate bond behaviour, including single-lap/double-lap setups, single prism/double prism setups, push-pull/pull-pull setups, and three/four point bending setups. Each of them has advantages and disadvantages related to the reliability of load and slip measurements, complexity of specimen manufacturing and handling, number of sensors to install to monitor the specimen during test execution and to process test data. No standards exist that provide recommendations or guidance.
This work presents the TRM-to-masonry bond tests carried out in Roma Tre University with different experimental setups, including single-lap direct-shear scheme with the unbonded textile either left bare or embedded into the mortar matrix, double-lap double-prism push-pull and four-point bending setups of straight or curved substrates. The incidence on test results (bond strength, failure mode, load-slip response) of both the properties of the TRM system (textile material, fabric architecture, strength and stiffness of the matrix) and the setup is discussed. Digital Image Correlation was used in all the tests to integrate traditional measurement methods, validate displacement data, and derive additional information on crack pattern (occurrence, spacing, width and evolution of cracks).
The work aims at contributing to the existing knowledge on the bond behaviour of mortar-based composites and to the development of standardized guidelines for shear bond tests, which are needed to improve the reliability of the mechanical parameters of the TRM systems and the safety level of reinforced structures.

 

Use of image correlation systems to study FRCM-concrete joints

Cristian Sabau, Jaime Gonzalez, Lesley Sneed, Carlo Pellegrion, Gabriel Sas, Björn Täljsten

Lulea University of Technology, Sweden

This paper presents a non-contact measurement approach, based on digital photogrammetry, applied to the experimental study of the bond behavior of fiber reinforced cementitious matrix composite (FRCM) - concrete joints tested in single-lap direct shear tests. The use of digital photogrammetry techniques and traditional contact measurement approaches for determining displacement and strain are investigated and compared. The results show that measurements of strain in the fiber bundles determined using the image correlation system (ICS) correlate well with those obtained from electrical strain gauges. However, differences of 38% to 52% were observed between the maximum strain measured with either ICS or electrical strain gages attached to the fiber bundles and the maximum strain in the fiber bundles computed from the maximum applied load. ICS is also used to measure slip and strain of bare fiber bundles, and results show that the load distribution among fiber bundles is nonuniform. The proposed measurement approach shows higher spatial measurement resolution and increased accuracy compared to traditional contact approaches by enabling measurements in each fiber bundle and overcoming the need to attach additional elements to the tested specimen.

 

Bond behaviour of a mechanical anchorage system for Externally Bonded Reinforcement

Cristina Barris, Luis Correia and Jose Sena-Cruz

Universitat de Girona, Spain

Carbon Fibre-Reinforced Polymer (CFRP) laminates have been proven to be adequate materials to strengthen concrete structures in flexure. However, in some applications, special end-anchorage systems are required at the end of the laminate in order to transfer the high shear stresses typically developed between the FRP reinforcement and the concrete substrate, especially when the Externally Bonded Reinforcement (EBR) technique is used. In spite of mechanical anchorage systems been a promising solution to avoid premature FRP peeling-off failure, the influence of several parameters, such as the laminate width or the level of confinement, still requires better understanding.
This contribution presents an experimental programme on 6 prismatic concrete specimens of 200×500×800 mm3 strengthened with CFRP laminates that have been mechanically anchored to the concrete substrate through a metallic plate. The programme aims at investigating the influence of the laminate width and the level of confinement of the anchorage on the overall bond response. For this purpose, three different laminate widths (50, 80, and 100 mm) and two torque levels provided by the screws that fasten the mechanical anchorage (30 and 100 N·mm) have been tested. Results show that the metallic mechanical anchorage gives adequate bond performance even at low levels of confinement for laminate widths of 50 and 80 mm, however, for a laminate width of 100 mm, higher levels of confinement are required so as to avoid slippage of the CFRP.

 

Effect of sustained load on the residual capacity of concrete strengthened with NSM FRP strips

Mohamed Emara, Cristina Barris, Marta Baena, Mohamed Moawad and Lluís Torres

Universitat de Girona, Spain

As a part of an experimental program concerning the long term bond behavior of Near-Surface Mounted (NSM) FRP and concrete, the aim of this contribution is to study the effect of sustained load and environmental conditions on the load carrying capacity and failure modes of single shear pull-out specimens strengthened with NSM CFRP laminates. Pull-out specimens were prepared, using different bonded lengths, and divided into three groups, the first group was tested after curing, the second was left for three months then tested, while the last group was subjected to sustained load for 1000 hours then tested. Bond slip curves, failure load, maximum slip and failure mode were obtained. Besides, Digital Image Correlation (DIC) was used during tests to register the change in strain distribution, to better understand the results, and displacement field of tested specimens to be locally compared with results obtained from LVDTs. Results showed that, the studied parameters affects both failure mode and load carrying capacity of tested specimens. Besides, good correlation between DIC results and LVDTs was found.

 

Global stiffness of RC elements strengthened with CFRP laminates bonded on flexible adhesives

Derkowski Wit, Kisała Dawid, Zając Bogusław, Kwiecień Arkadiusz

Cracow University of Technology, Poland, Poland

The Flexible Joint Method uses polymers as adhesives for FRP strengthening of building structures. These polymers have a much lower stiffness than the conventionally used epoxy resins. The effectiveness of such solution for masonry structures has been confirmed in many studies. The impact of this strengthening method on the global stiffness of the RC beams will be presented in the paper.
The results of research carried out at the accredited Laboratory for Building Materials and Structures, where five RC beams strengthened with CFRP strips on various polymer adhesives were tested under monotonic load will be given and discussed.
Flexible adhesives were used also in a new, innovative solution for the damaged civil engineering structures, for example those in seismic areas. This protection system improves mostly the ductility of the strengthened element, which significantly increases the safety of the damaged structure, especially for the time required for evacuation of people and the introduction of necessary safeguards. The tested RC beam, which was previously destroyed in fatigue test, was repaired and strengthened by means of the CFRP wherein a part of them was bonded on a very soft polyurethane, and the others on the commonly used adhesive resin.
The article will discuss also the nonlinear analysis of tested beams using Ansys environment. The Willam-Warnke model was used for concrete and the standard bilinear models were used for steel and CFRP elements. The adhesion layer was modelled as spring elements to take into account the slippage at the interface.


Strengthening of masonry structures

 

Research on NFRCM Masonry reinforcements

Antonella Cecchi, Claudia Brito de Carvalho Bello, Daniele Baraldi, Emanuele Reccia

Università IUAV di Venezia, Italy

Here a research program on Natural Fibers Reinforced Cementitious matrix (NFRCM) is proposed. In the last years, the interest about Fiber Reinforced Cementitious Matrix (FRCM) systems has consistently increased. Different typologies of fibers may be adopted in FRCM. Here attention is paid to the use of eco-compatible material, in particular, to the possibility of replacing the most commonly used synthetic fibers with natural ones.
The purpose is to develop a mortar-based composite material which is totally eco-friendly, in order to provide a sustainable solution for masonry retrofitting.
The research program relies on a combined approach based on experimental and theoretical analysis. Experimental tests are ongoing in collaboration with company partners to define mechanical characteristics of materials. The tests results are combined with FEM numerical approach, in order to investigate the behavior of composite systems made by different typologies of natural fibers embedded in mortar matrix.
The principal aims of research are to characterize NFRCM mechanical behavior, to assess structural performance and durability and to evaluate the improvement of masonry response in terms of resistance and ductility. The issue is topical also in relation with the new technical regulation development about fiber composites.

 

Study of the crack propagation in FRCM-masonry joints using a meso-scale three-dimensional finite element approach

D'Antino T, Mazzucco G, Salomoni V, Pellegrino C

Politecnico di Milano, Italy

Fiber reinforced cementitious matrix (FRCM) composites represent a promising alternative to the use of fiber reinforced polymer (FRP) composites for strengthening existing structures. FRCM composites are comprised of a high-strength fiber net (textiles) embedded within an inorganic matrix, which is responsible of the stress-transfer between the composite and the substrate. Failure of FRCM composites comprising one layer of fiber is generally reported to be deboning from the substrate. Depending on the FRCM material employed, failure may occur at different interfaces, such as the matrix-fiber interface or matrix-substrate interface. When stitch-bonded textiles, i.e. textile where longitudinal and transversal bundles are firmly connected, are employed, failure generally occurs due to cracking of the matrix.
In this paper, the bond behavior of FRCM-masonry joints comprising one layer of a basalt stitch-bonded textile embedded within a lime-based matrix is studied by means of single-lap direct-shear tests. A meso-scale three-dimensional finite element approach is then employed to study the formation and propagation of the matrix cracks, which eventually led to failure. The finite element analysis was carried out employing a dynamic explicit approach, which allowed for overcoming convergence difficulties associated with severe nonlinearities of the model.

 

Prediction of shear and compression strength of masonry structural elements reinforced by FRM (Fiber Reinforced Mortar)

Alessio Cascardi, Francesco Micelli, Maria Antonietta Aiello

Università del Salento, Italy

The use of Steel Reinforced Mortar (SRM) and Fiber Reinforced Polymer (FRP) have been experienced as strengthening techniques for masonry structures in the past. SRM has widely evidenced its vulnerability to electrochemical corrosion while FRP have been proved to be hardly compatible with historical masonry not only from a mechanical point of view but also because of the application of that material often does not accomplish the stringent rules of conservation and limits the breathability of the existing masonry. A new generation of composites, named Fiber Reinforced Mortar (FRM), consisting in a fiber grid and a non-polymeric matrix, seems able to overcome the drawbacks related to the application of SRM and FRP. In fact, the utilized grids (glass, carbon, basalt) do not easily corrode and an inorganic matrix better adapts to the masonry substrate. Hence the necessity of assessing theoretical predictions of the improved load-capacity provided by the FRM-application.
The present paper aims to illustrate two analytical formulations for computing the shear strength of the FRM strengthening masonry panels subjected to in-plane shear force and the compressive strength of the FRM-confined masonry columns. Experimental databases were collected from the literature in order to perform advanced calibration with modern techniques such as the linear multiple regression and the artificial neural networks. Simple models are presented and discussed while the precision of the predictions is tested on the basis of experimental outcomes. The comparison with other available analytical models is also made in order to evaluate the effectiveness of the proposed formulations.

 

Full scale tests on timbrel masonry vaults strengthened with Steel Reinforced Grout

Stefano De Santis, Francesca Roscini, Gianmarco de Felice

Roma Tre University, Department of Engineering, Italy

The vaults of numerous existing structures need retrofitting to ensure an adequate safety level according to current standard codes. An experimental investigation will be presented, carried out in the laboratory on three full scale vaults, having 2.9m span, 5.5cm thickness and 50cm width. One of them was tested unreinforced, while the other two vaults was strengthened with a steel textile applied with lime based mortar (SRG) either at the extrados or at the intrados. Aiming at reproducing the actual condition of brick vaults in historic constructions, the specimens were provided with buttresses and filling. The load was applied under displacement control over the filling at 1/3 of the span and increased cyclically up to failure. In addition to traditional equipment, Digital Image Correlation was also used to measure the displacements of the brick units, detect crack development, and record the displacement field of the filling material. To this aim, the experimental setup was provided with a Plexiglas spandrel wall that allowed the movements of the fill soil to be monitored. The tests showed the increase in load-carrying capacity provided by SRG and the modification of the failure mode, contributing to improving the knowledge on the structural behaviour of reinforced masonry vaults and to the development of suitable retrofitting techniques.

 

 

Internal FRP reinforcement

 

Numerical modeling of the GFRP rebar and concrete bond

Mohammadali Rezazadeh, Valter Carvelli, Ana Veljkovic

Politecnico di Milano, Italy, Italy

The mechanical behavior of the bond between concrete and fiber reinforced polymer (FRP) bars often controls the structural performance of concrete elements reinforced with these type of composite materials mainly when FRP debonding failure occurs at the ultimate stage. Considering the available scientific literature in this area, the FRP-to-concrete bond behavior has been extensively investigated experimentally. Comparatively, much less attention has been directed toward to the numerical analyses of this bond behavior due to the difficulties to develop an accurate numerical model to simulate the relevant interface properties. However, performing the numerical analyses parallel to the experimental tests are an important aspect to better understand the influence of critical parameters on the FRP-to-concrete bond behavior. Hence, the main purpose of the current paper is to develop a 3D finite element (FE) model capable of considering the bond behavior of GFRP bar-concrete interface under monotonic loading condition. The bond behavior in the FE model includes a damage model based on the following governed laws: 1st linear elastic stress-separation and shear stress-slip relationships; 2nd damage initiation criteria; and 3rd damage evolution laws. In this regard, an exponential function is used as damage evolution law for the softening branch of FRP-to-concrete bond behavior. At the end, the predictive performance of the developed FE model is demonstrated by comparing with the relevant experimental results, and the critical parameters of this FE model were discussed.

  

Prediction of the Mechanical Response of Concrete Beams Strengthened with Internal Reinforcement in Composite Material

Todor Zhelyazov, Ted Donchev

Technical University of Sofia, Bulgaria

In this article the mechanical response of concrete beams, strengthened with internal reinforcement in composite material is discussed. Specifically, concrete beams with pre-tensioned flexural reinforcement in basalt fiber, shear links and top longitudinal reinforcement in steel are modeled. Constitutive laws are defined for all materials. Elastic- plastic response of steel is modeled by assuming bi-linear kinematic hardening. Response of basalt fiber is assumed to be linear and elastic until rupture. Two alternative material models are considered for concrete. The first one is a ‘standard’ crush-crack concrete material model implemented in the existing commercials finite element codes (e.g. ANSYS). The second one is a hybrid constitutive law: mechanical response of concrete in compression domain is modeled by assuming a multi- linear isotropic hardening whereas response in tension domain is simulated by introducing a damage variable in the material stress-strain relationship. Results obtained by finite element simulation are compared to experimental data.

 

A novel concept of crack analysis: prediction of mean crack spacing in FRP reinforced concrete beams

Gintaris Kaklauskas, Mattia Francesco Bado

Vilnius Gediminas Technical University, Lithuania

The current study proposes a simple, mechanically sound and transparent analytical approach for crack analysis of concrete members reinforced with steel or FRP bars. The philosophy behind the proposed methodology is to establish mean spacing between the primary cracks through the compatibility of the stress transfer and mean strain approaches. The governing parameters of crack spacing are obtained by equating mean strains of the tension reinforcement defined by these approaches. The model assumes that a single RC block of a length of mean crack spacing represents the averaged deformation behavior of the cracked member. Based on the experimental evidence, reinforcement strain within the block is characterized by a strain profile consisting of straight lines representing zones with different bond characteristics. A limited comparative analysis has demonstrated that the predictions of mean crack spacing by the proposed model agree well with the tests.

 

Effect of DIC setup and analysis parameters on the measurement of deformation in load tested FRP RC beams

Matteo Di Benedetti, Cristina Barris, Maurizio Guadagnini, Lluis Torres

The University of Sheffield, United Kingdom

In the past decade, the use of digital image correlation (DIC), a contactless measuring technique for full-field displacements and strains, has been increasingly spreading. However, despite its handiness, flexibility and density of measurable points, DIC cannot still compete in terms of resulting measurement accuracy with standard state of the art strain measurements. While this represents a non-negligible issue in the case of strain evaluation in brittle material testing like reinforced concrete (RC), optimizing setup and data analysis can yield sufficiently reliable results.
This paper aims to assess the reliability of DIC for measuring displacement and strain in FRP RC elements. For this purpose, three large-scale GFRP RC specimens, having different combinations of shear and flexural reinforcement ratios, were load tested using an asymmetric three‑point bending arrangement. Two- and three-dimensional DIC were used in parallel, with the latter having the advantage of not being affected by out of plane motions. The parameters examined affected both the experimental setup and the analysis of the acquired images and included angle between the optical axes of the two cameras; surface texturization methods (airbrush and spray painted speckles); image analysis with different combinations of subset and step size.
DIC measurements are compared with well-established measurements techniques and with the expected theoretical values. In addition, the effect of the examined parameters on the displacement and strain measurements is discussed and commented upon.

 

Column Strengthening

 

Resilience of Noncircular Reinforced Concrete Columns Strengthened With Hybrid Elastic Confinement

Theodoros Rousakis

Democritus University of Thrace, Greece

The paper investigates the resilience characteristics of reinforced concrete (RC) columns externally strengthened with hybrid elastic confinement, subjected to axial compression tests. The columns had square section with 150 mm side, corner radius of 17 mm and 750 mm height. They contained 4 longitudinal ribbed bars of 8 mm diameter and B500C quality. Further, smooth transverse steel stirrups, with 5.5 mm diameter and S220 quality were spaced at 100 mm centers all over the columns’ height. Thus, slender bars (s/ØL = 12.5) were designed to simulate old-type deficient columns. External wrapping included bonded unidirectional glass Fiber Reinforced Polymer (FRP) sheets or unbonded, non-impregnated polypropylene fiber ropes (PPFR) or braided basalt fiber ropes (BFR). The present study focuses on the resilience features of columns with hybrid GFRP composite polypropylene rope or composite basalt rope confinement. No column presented FR fracture up to early termination of the tests while they exhibited unique load regaining capacity after GFRP fracture. All tests with hybrid GFRP sheet and FR wrapping were stopped for safety reasons at strain levels higher than 5% because of global instability related issues (associated with disintegration of the concrete core and bars’ buckling). This paper reveals the unique damage build-up and control at member level that may prevent collapse of critical infrastructure. A new generalized design concept is concluded towards enhanced inherent resilience of similar members. It suggests that adequate suitable confinement with continuous elastic flexible rope may preserve damage-sensitive-restriction in a way that globalizes damage and maximizes energy dissipation.

 

An Overview of Steel-Reinforced Grout (SRG) Jacketing Applications

Georgia E. Thermou, Iman Hajirasouliha

The University of Sheffield, United Kingdom

This paper aims to present an overview of the different applications of a relatively new local intervention method, the Steel-Reinforced Grout (SRG) jacketing. The SRG jacketing system comprises composite fabrics that consist of high strength steel cords embedded in an inorganic matrix. The new composite system is simple and fast in its application and has been successfully applied for confinement of plain and reinforced concrete, shear strengthening of beams and repair of pre-damaged old-type columns. This paper summarises the information related to the confinement effectiveness of the SRG jackets, the strength and deformation capacity increase observed in the experimental studies and the capability of the proposed method in preventing premature failure of RC structural members with substandard detailing.

  

A concrete damage plasticity model for the modeling of FRP confined columns

Chia Farahmandpour, Sophie Dartois, Marc Quiertant, Yves Berthaud, Helene Dumontet

Université Pierre et Marie CURIE, France

Wrapping of reinforced concrete columns with high-strength Fibre-Reinforced Polymer (FRP) material is an effective method to enhance their axial load-carrying and deforma-tion capacities. However, proper design of such retrofitted columns requires an appropriate concrete model that must describes confinement sensitivity (i.e increase of the strength and ductility) of concrete under the triaxial stress state that results from dilation cur-tailing of concrete core by the FRP jacket. Consequently, dilation behaviour of concrete under multiaxial stress conditions should be captured by the model. In this paper, a damage plasticity model for concrete material is then presented. The dilation behaviour is predicted using a non associated Cam-Clay type potential function. This confinement-sensitive constitutive model closely reproduces strain-softening and stiffness degradation of concrete but also displays its transition from brittle to ductile behaviour as confinement increases. The application of the model is demonstrated in the case of active confinement and of FRP (passive) confinement. It is found that essential mechanical features of the confined concrete can be reproduced for both active and passive confined concretes in practical engineering uses.

 

Bond and Long term behaviour of FRP reinforcement in concrete

 

Bond behaviour of GRFP bars to concrete in beam test

Renata Kotynia, Damian Szczech, Monika Kaszubska

Lodz University of Technology, Poland

Bond behavior between reinforcing bars and concrete is a key problem to understand behavior of reinforced concrete members. Good performance of FRP reinforced concrete requires adequate interfacial bond between bars and concrete, mainly due to surface preparation. The authors' own experimental program includes twelve beam bond tests carried out on rectangular beams consisted of two concrete blocks connected by a continuous GFRP bar in tension and by a steel hinge in compression. Two main parameters were investigated in the tests: bar diameter and thickness of a bottom concrete cover.
The aim of the tests was to determine the bond behavior of GFRP bars to concrete on the beam bond setup. A test program contained twelve rectangular concrete beams with a cross section of 150 x 200mm specimens with the length of 800mm. The beams used in the tests consisted of two concrete blocks. Two main parameters were investigated in the tests: the diameter of GFRP bars and the thickness of the bottom concrete cover. Each type of the beam was duplicated in order to collect double test results. The thickness of the concrete cover and the bar diameter varied in order to investigate their influence on the GFRP to concrete bond behavior.
The GFRP bars indicated good bond behavior to concrete, mainly due to the ribs on the bar surface. Failure mode was most often caused by splitting of the ribs along the GFRP bars. The results of the test indicated the decrease in the ultimate shear bond stress with the increase in the bar diameter regardless of the thickness of the concrete cover. The decrease in the concrete cover caused the decrease in the shear bond stress for all bar diameters.

 

Durability of bond between FRP rebars and concrete

Sandor Solyom; Gyorgy L. Balazs

Budapest University of Technology and Economics, Hungary

Interest on Fibre Reinforced Polymer (FRP) is increasing in recent years. While results on short term bond behavior are available in a relatively large number for FRP rebars, lack of data can be observed for long term behavior. Nevertheless, it is important to study if the FRP rebars are able to adequately transfer the interactional forces into the concrete and vice versa over the full design lifetime of the structure.
An experimental program to investigate the time dependent behavior of bond between internal FRP rebars and concrete when subjected to different environmental conditions is presented in this paper along with the results and discussion on the effect of different parameters. Environmental conditions are: freeze/thaw cycles in air or immersed in saline solution and wet/dry cycles. The test parameters are: type and duration of conditioning (50 and 100 days) and FRP bar type and diameter.Conclusions include interpretation of the possible bond strength reduction ratio for different conditioning and duration (highest in case of freeze/thaw cycles in saline solution).

 

GFRP bar and concrete bond under cyclic loading

Ana Veljkovic, Marcin Michal Haffke, Valter Carvelli, Matthias Pahn

Politecnico di Milano, Italy

This experimental research reveals some aspects of fatigue behaviour of the bond between glass fibre reinforced plastic (GFRP) rebars and concrete. Fatigue of this type of bond is still insufficiently investigated and understood, although it represents an important aspect in FRP reinforced concrete structural design. Standard pull-out test setup is modified by positioning the bar eccentrically to account for concrete cover effect on the bond behaviour. Beside concrete cover, the concrete strength is also set as variable, while bar type and diameter (GFRP ComBAR f8) maintained constant throughout whole experiment. Quasi static pull-out tests are supposed to estimate the bond strength that is used afterwards as reference for fatigue tests. 60% and 70% of static bond strength is adopted as the maximum load during cyclic loading, with the aim to estimate the fatigue life of bond under different load levels. Tests were running until reaching bond failure or one million cycles. In the latter case, bar was pulled out quasi-statically to estimate the residual bond strength. None of specimens exposed to fatigue at lower load level did not fail after one million cycles, whereas specimens under higher load level revealed that higher concrete quality attained longer fatigue life. In case of specimens treated with lower load level, post-fatigue bond strength compared to the pre-fatigue quasi-static one remained unchanged. Damage evolution and residual bond strength and stiffness are discussed in detail considering all mentioned parameters.

 

Long term behaviours of GFRP rebar in concrete elements under sustained loads and aggressive environment

Hamed Fergani, Matteo Di Benedetti, Maurizio Guadagnini, Cyril Lynsdale, Cristina Mias

The University of Sheffield, United Kingdom

Glass fibre reinforced polymer (GFRP) bars have been increasingly used as an alternative material for reinforcing concrete (RC) structure where corrosion of steel rebar is an issue. However, GFRP bars in concrete structures can be exposed to a combination of high alkaline environment, humidity, sustained stress and high temperatures, which could affect their mechanical properties and consequently the long term structural performance of the reinforced members. Although a large amount of research on GFRP RC beams has been carried out over the years, very limited studies focused on their durability. The main aim of this study is to investigate the combined effects of severe environments and sustained loads on the long term performance of GFRP bars in concrete members. The experimental programme included tests on GFRP RC concrete beams and prisms. Initial conditioning consisted of applied sustained stress levels corresponding to tensile strains in the reinforcement equal to 3000 me and 5000 me as well as exposure to tap water at 60°C for up to 12 months. Tensile and flexural tests were performed to examine changes in tension stiffening and flexural behaviour due to the effect of hydrothermal conditions and sustained stress. The findings of this study will be useful to civil engineering community interested to use GFRP bars in hot and wet weather conditions as main reinforcement for concrete structures.

 

Plenary Session II

 

Innovative solutions for seismic retrofit of existing RC buildings with poor quality concrete

Del Vecchio C., Di Ludovico M., Balsamo A., Prota A., Manfredi G.

University of Napoli “Federico II”, Italy

Recent devastating earthquake outlined the high vulnerability of existing reinforced concrete (RC) buildings to seismic actions. The lack of seismic detailing often led to the shear failure of unconfined beam-column joints limiting the entire structural performances. Significant research effort promoted the use of composite materials as an efficient, cost-effective and easy to install retrofit solution. This led to a massive use of Fiber Reinforced Polymer (FRP) systems in the reconstruction processes followed to the recent seismic events (L’Aquila, 2009; Emilia, 2012). Nowadays, these strengthening solutions are common in the design practice and simple and reliable design formulations are available.
Although the FRP strengthening and other retrofit techniques allowed to significantly increase the seismic capacity of most of the damaged RC buildings by L’Aquila earthquake, a significant portion of buildings were demolished due to economical inconvenience of the retrofit solution, poor quality concrete or high residual drift.
This paper focus on the seismic retrofit of beam-column joints using innovative retrofit solutions. The recently developed design formulation for the FRP strengthening is illustrated and the limits of this technique when applied to a poor concrete substrate are discussed. An alternative retrofit solution consisting on an innovative application of High Performance Fiber Reinforced Cementitious Composites (HPFRCC) is proposed. Two beam column joints with very poor quality concrete (i.e. fcm<12 MPa) were extracted from a real building damaged and demolished after the L’Aquila earthquake (2009). The joints were tested in the as-built or HPFRCC strengthened configuration with a cyclic load protocol to demonstrate the effectiveness of the proposed solution.

 

CFRP Strengthening of Metallic Members; state of research at Empa

Elyas Ghafoori, Masoud Motavalli

Empa, Switzerland

Although strengthening of concrete members using carbon-fibre reinforced polymer has become a common practice in civil engineering, the situation is not the same for strengthening of metallic structures. This paper aims to review the main differences between strengthening of concrete and metallic structures, and also to identify obstacles in front of CFRP strengthening of metallic members. Furthermore, a review on the most recent prestressing systems that have been developed at the Swiss Federal Laboratories of Materials Science and Technology (Empa).


Strengthening of timber and steel structures

 

Research projects on the use of nano-composites for rehabilitation, protection and durability of wood and timber structures: an overview.

Clara Bertolini Cestari, Stefano Invernizzi, Tanja Marzi

Politecnico di Torino - DAD, Italy

The presentation illustrates the results obtained during recent research projects, coordinated by the Department of Architecture and Design of Politecnico di Torino, aimed at increasing the performance of timber structures with the use of innovative protection treatments, eco-friendly and based on the use of nanotechnology.
Nano-coatings (in particular based on the use of nano-silica, nano titanium oxide, silver ions, tourmaline, fluorinated silica nanoparticles and CNTs) for wood rehabilitation and protection have been tested and assessed, with the aim of achieving a lower environmental impact than existing products, allowing to reduce maintenance costs and at the same time improving the performance and functionality of wood (i.e. water resistance, fire resistance, antibacterial properties, self-cleaning, reinforcement).
Special attention was paid on the characterization of these innovative nano-coatings and mechanical tests have been carried out on timber structural elements, with comparative analysis between traditional and nano-structured materials.
These research projects were carried out in the frame of short-term projects, granted by the European Fund for Regional Development for industrial research or experimental development activities that involve the active participation of regional small and medium enterprises.
The multi-disciplinary research group has seen the participation of experts of different sectors involved, in particular: experts in mechanical resistance of materials and diagnostic methods for its evaluation, chemical engineers for the physical-chemical characterization, experts in nano materials, technology of architecture and geomatics.

 

Strengthening of timber structures with FRP's - Design guidelines for FRP damage and delamination

K.U. Schober

Mainz University of Applied Sciences, Institute of Innovative Structures (iS-mainz), Germany

Fiber-reinforced plastics can be effectively used to improve strength and stiffness especially for timber beams. In the bonding region, a distinction must be drawn between the fundamentally different kinds of fracture. In addition, there is the delamination of the individual lamina with a negative effect on the overall stiffness. Lamina by lamina analyses make it possible to dimension FRPs which experience a predominantly plane stress state. An appraisal of the delamination risk cannot be made on that basis. In regions subject to a three-dimensional stress state, such as in the end-anchorage zone of FRP strengthened timber structures, the laminate will also be subject to a three-dimensional stress state.
Failure criteria implemented in many finite element programs do not take into account the heterogeneous nature of the lamina, treat it as an isotropic material and deliver results which probably fall on the unsafe side. Therefore, criteria that distinguish between the different kinds of fracture have be used and implemented in the FEM program via an interface and by modifying the implemented criteria. The successive failure of laminates in FRP strengthened timber structures is treated by a layer-by-layer failure analysis accounting micro damage, matrix cracking and debonding. This paper gives a state-of-the-art summary of design approaches for FRP damage and delamination to introduce on-site bonding of FRP to timber as a new way in design for structural repair and rehabilitation.

 

The influence of the accelerated aging of reinforced beech wood on its selected physical and mechanical properties

Izabela Burawska, Magda Lopatowska

Warsaw University of Life Sciences - SGGW, Poland

The aim of work was to assess the impact of accelerated aging on physical and mechanical properties of reinforced with glass fiber beech wood. The reinforcement method consisted in wrapping with the use of epoxy adhesive. The accelerated aging was performed on the basis of Matejak et. al method, involving cyclic soaking and drying of wood. One cycle lasted 24 hours. Six cycles of accelerated aging were performed, which corresponds to 1-year natural aging. The test program assumed to determine changes in density, as well as bending and tension strength along the fibers caused by aging. Already six cycles of artificial aging caused a significant drop in the density of timber samples. Also compressive strength along the fiber in case of non-reinforced samples decreased significantly. Samples reinforced with glass fiber did not show a decrease in compressive strength. However, all samples that were tested in bending (non-reinforced and reinforced) showed a significant reduction in strength in comparison to control samples.

 

Application of CFRP for fatigue strengthening of old steel structures

Pavel Ryjáček, Jan Vůjtěch, Martin Vovesný

CTU in Prague, Czech Republic

The purpose of this work is to analyse the behaviour of deteriorated steel reinforced with CFRP under fatigue loading. A series of experiments was performed in the last year, together with the numerical study. The strengthening was done on the mild steel with different level of surface deterioration (corrosion pits or corrosion holes). The specimens are strengthened by using of hand laid wet layup composites. They are subjected to fatigue loading and the difference between the fatigue life reinforced and unreinforced specimens is observed. The 1,5-3,0 extension of the fatigue life was observed. The results are discussed, together with the impact of the freezing cycles.

 

Adhesively bonded composite reinforcement against fatigue crack propagation in the case of old steel structures

E. Lepretre, S. Chataigner, L. Dieng, L. Gaillet

IFSTTAR, France

After corrosion, fatigue phenomenon is the main cause of damage in old metallic structures. Fatigue cracks appear in stress concentration area subject to high stresses, and can lead to the ruin of the bridge. The main objective of the present work is to study the effectiveness of adhesively bonded carbon fibre reinforced polymer (CFRP) laminates in reinforcing fatigue crack emanating from rivet hole. Thus, investigations on small-scale specimens were done. These specimens consist of metallic plates with center hole from which one single crack emanates. Two metallic materials, puddled iron and mild steel, and two reinforcement processes were used. These reinforcement processes consist of Normal Modulus (NM) and Ultra High Modulus (HHM) CFRP laminates. Furthermore, symmetrical and un-symmetrical reinforcement configurations are considered as well as pre-stressing NM laminates before application. The experimental results showed firstly the efficiency of the different studied reinforcement configurations in slowing down crack propagation. For puddled iron plates, a maximum increase in fatigue life of 327% compared to un-reinforced specimens was observed. The results allowed obtaining modified stress intensity factors for the studied geometries and reinforcement. A finite element model is currently being settled to be able to determine it for other geometries or reinforcing techniques.

 

 

Shear and torsion

 

Shear tests of GFRP reinforced concrete beams without stirrups

Renata Kotynia, Monika Kaszubska, Joaquim A. O. Barros

Lodz University of Technology, Poland

Fiber reinforced polymer (FRP) bars due to their excellent corrosion resistance, high tensile strength, good non-magnetic properties have been proposed for reinforcing concrete structures instead of traditional steel. Many research use glass fiber reinforcement as it is much cheaper than carbon one. The aim of this paper is to analyze the influence of longitudinal GFRP reinforcement on shear capacity of concrete beams without stirrups and to investigate a dowel effect of this reinforcement on the shear strength.
The paper presents experimental test results of T- shaped, single span, simply supported beams without stirrups reinforced with longitudinal GFRP bars. The following parameters were investigated in the research: flexural reinforcement ratio, number of reinforcement layers, number and diameter of bars in one layer.
Test results indicated no significant influence of the longitudinal reinforcement on the shear strength of the beams. A beneficial effect of two layers of the tensile reinforcement on the shear capacity was observed only in the beams with the reinforcement ratio of 1.80%. The beams reinforced with two layers of GFRP reinforcement showed more extensive crack pattern than the beams with one reinforcement layer. It confirms that application of two reinforcement layers more effectively enhance the shear strength mainly due to higher tension stiffening effect provided by the former flexural configuration in the beams with two reinforcement layers.
Digital image correlation system (DIC) shows increasing popularity especially in the complex stress state like shear. DIC is an innovative non-contact optical technique for the study of crack propagation and material deformation. The paper presents comparison of test results registered by LVDT and DIC system. The main aim of the research was to investigate the shear strength and crack propagation until failure. The paper shows an interesting analysis of crack pattern and concrete strain in the shear region measured with LVDTs and DIC.

 

Shear strength and size effect in RC beams with internal FRP reinforcement

Szymon Cholostiakow, Matteo Di Benedetti, Emanuele Zappa, Maurizio Guadagnini

The University of Sheffield, United Kingdom

The shear performance of RC beams is greatly affected by their size. Although various studies have examined this phenomenon in steel RC beams, research on size effect in FRP RC beams is still limited. FRP RC beams can exhibit large strains and significant cracking even at load levels below their ultimate limit state, and this can result in a more pronounced size effect. This paper discusses an experimental study examining size effect in GFRP RC beams with overall height varying from 260mm to 460mm, but otherwise similar in geometry and material properties. All beams were unreinforced in shear and designed to ensure shear failure. 3D DIC was employed alongside strain and displacement transducers to gain additional insight into the relative development and degradation of shear resisting mechanisms. The results of this study help to better understand the influence of size on shear performance of FRP RC beams and can assist in improving existing FRP shear design recommendations.

 

Deep Embedment FRP bar technique for shear strengthening of continuous RC T-beams

Vesna Raicic, Prof Tim Ibell, Dr Antony Darby, Dr Mark Evernden, Dr John Orr

University of Bath, United Kingdom

Extending the service life of a structure is a fundamental aspect of sustainability. The use of a civil engineering structure may change throughout the course of its life, as for example its whole function can change, loads can increase and/or higher building standards are required and the structure must be made in accordance with these demands. Fibre-Reinforced Polymer (FRP) systems are high-strength, lightweight reinforcement in the form of fabric sheets, thin laminates, or bars that are bonded to concrete members with epoxy adhesive to increase their load carrying capacity.
Improvement of shear resistance represents a major challenge since Externally Bonded Reinforcement (EBR) or Near Surface Mounted (NSM) techniques are not able to mobilize truss action as FRP materials cannot be anchored into compression zones of continuous reinforced concrete T-beams; similarly, debonding of FRP laminates within low strains raises additional problems. An alternative strategy for shear strengthening of reinforced concrete beams, even in cases where the webs of the beams are inaccessible is the Deep Embedment (DE) or Embedded Through Section (ETS) technique.
In line with this technique, vertical or inclined holes are drilled through the beam section, and steel or FRP bars are inserted into these holes and epoxy bonded to the concrete. In this way, the top and the bottom chord are connected through the additional DE bars. Therefore, the primary focus of this research is to deepen the knowledge on the behaviour of such strengthened continuous structures where large shear forces are combined with large negative bending moments.

 

Effect of material and amount of external reinforcement on shear strengthening of RC beams with U-shaped TRM jackets

Tetta Zoi, Koutas Lampros and Bournas Dionysios

University of Patras, Greece

This paper presents an experimental study on shear strengthening of reinforced concrete (RC) beams with U-shaped TRM jackets focusing on the following investigated parameters: (a) the amount of external TRM reinforcement ratio, rf, by means of using different number of textile layers and different types of textile fibre materials (carbon, glass, basalt) and (b) the textile geometry. In total, 16 RC beams with rectangular section were constructed and tested as simply supported under (three-point bending) monotonic load. The results showed that: (a) TRM is very effective when the failure is attributed to debonding of the jacket from the concrete substrate; (b) the trend of effective strains for carbon, glass and basalt TRM jackets is descending for increasing values of the TRM reinforcement ratio, rf, when failure is associated to debonding of the jacket and (c) the effect of textile geometry is significant only for low values of rf, resulting in variances in the capacity enhancement and the failure modes, whereas this effect is drastically mitigated by increasing the external reinforcement ratio.

 

Torsional strengthening of thin walled tubular reinforced concrete structures using NSM FRP

Chandan C Gowda, Joaquim A O Barros & Maurizio Guadagnini

University of Minho, Portugal

Strengthening of bridges in torsion is an area of civil engineering where limited research is available, specially for techniques using fiber reinforced polymer. Every year more and more problems related to the topic is emerging in many countries such as Australia, Portugal, USA etc. They are supposed to be caused by poor quality of construction, upgrades in codes, changes in building usage etc. As a result, there is a need to perform more innovative and detailed study in this area. Few researches are available using conventional strengthening methods like span reduction, composite strengthening, adding an additional concrete layer etc.
The research presented in this paper is concentrated on strengthening of reinforced concrete beams in torsion using innovative strengthening method viz., using fiber reinforced material by near surface mounting method (application of FRP material with adhesive on thin grooves which are extracted on the surface of the concrete member). A new test setup is proposed to perform the experimental tests by fixing one end of the beam and loading through the other end.
The work consists of performing numerical simulations on RC beams to evaluate different parameters like the concrete strength class according to Eurocode and reinforcement ratios giving a better idea for the proposal of strengthening techniques. The whole test setup is analyzed numerically to prepare for the experimental work, since only half the beam will be tested. The paper presents these numerical results on strengthening techniques and experimental test setup.

 

 

Long term response of strengthened structures

 

Analytical and numerical time-dependent behaviour of RC slabs strengthened with NSM CFRP strips

Patricia Silva; Lluis Torres; Miguel Azenha; Jose Sena-Cruz

University of Minho, Portugal

The present work includes, firstly, a résumé of an experimental program with RC slabs strengthened in flexure with CFRP strips according to the NSM technique and, secondly, analytical and numerical simulations of these slabs. The experimental program consisted on submitting the slabs to, simultaneously, sustained loading and one of the following environmental conditions, during a specific period of time: (i) laboratory environment (control specimens); (ii) immersion in water; (iii) immersion in water with chlorides; (iv) wet dry cycles with water with chlorides. After two years, these slabs were unloaded and, then, were monotonically tested up to the failure in a quasi-static manner. Based on the results obtained in experimental tests, analytical predictions of the time-dependent behaviour of slabs were carried out based on the age-adjusted effective modulus and bi-linear method, revealing good predictive results when applied to this type of structures. With the purpose of better understanding the results observed in time-dependent and failure tests of slabs, several numerical models based on element finite method were also developed. In general, the used existing tools allowed to simulate with enough accuracy the experimental responses.

 

Corroded RC Beams at Service Load Patch Repaired and Strengthened with NSM CFRP Strips

Garyfalia Triantafyllou, Theodoros C. Rousakis, Athanasios I. Karabinis

Laboratory of Reinforced Concrete, Democritus University of Thrace (D.U.Th.), Greece

The present paper investigates experimentally the efficiency of patch repair and near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) strips for corroded reinforced concrete beams (150x300 mm, l=1920 mm) under service loads. One RC beam was non-corroded, while two RC beams were corroded under the same conditions using accelerated electrochemical technique. Then, they were subjected to vertical service loads that corresponded to 60% (BC1) or 75% (BC2) of the load at steel yielding of the non-corroded beam. After the first loading, the beams were subjected to further corrosion (medium level). Then, they were tested under the same serviceability load and then corroded again (heavy level). Finally, all beams were tested again up to the same service load. Longitudinal cracks due to corrosion and flexural cracks due to loading were recorded at the end of each corrosion-loading cycle. Test results show an increase of the crack width and deflection of both beams as corrosion progressed. The deflection of beam BC1 was measured at each corrosion-loading cycle equal to 5.53 mm, 6.16 mm and 7.5 mm respectively, where for beam BC2 increased with a higher rate equal to 6.79 mm, 9.04 mm and 11.47 mm respectively. Then, two NSM CFRP strips were implemented after replacing damaged concrete cover and treating the corroded steel reinforcement for each beam. All beams were loaded up to failure. Despite the heavy corrosion, the patch repair and NSM strengthening of the beam resulted in enhanced load-bearing capacity when compared with the non-corroded control beam.

 

Fatigue behaviour at elevated temperature of RC slabs strengthened with EB CFRP strips

Juan Manuel Gallego, Christoph Czaderski and Julien Michels

Empa, Switzerland

Lateral cantilevers of highway concrete box-girder bridges are able to suffer fatigue damage due to cyclic loads produced by heavy traffic. In the last years the concern about the effects of cyclic loads on these elements has increased as a result of, among other reasons, the increase of loads with respect to the initially expected load for existing structures where fatigue requirements were not accounted for in the design stage, and the limited warning alert of typically brittle fatigue failures. In order to increase their load-bearing capacity in flexure, these cantilever slabs can be strengthened on the upper side in transverse direction with Externally Bonded (EB) Carbon Fiber Reinforced Polymer (CFRP) strips.
Previous tests have shown that the typical fatigue failure mode at room temperature of reinforced concrete beams strengthened with EB CFRP strips is due to fatigue failure of the longitudinal steel reinforcement. Besides, temperature measurements on existing reinforced concrete bridges strengthened with EB CFRP strips indicated that these strips, and hence the epoxy adhesive, can reach elevated temperatures during the service life due to the sun heating in warm periods. This fact could affect the fatigue life of such strengthened slabs due to the modification of the epoxy properties at high temperatures. At elevated temperatures, fatigue failure of strip/epoxy/concrete joints could happen before fatigue failure of the longitudinal steel reinforcement due to the fatigue degradation of such joints.

 

Fire behaviour of FRP material for EB strengthening

Phi Long Nguyen, Xuan Hong Vu, Emmanuel Ferrier

Université Claude Bernard Lyon 1, France

This paper investigates the tensile properties of CFRP through tensile test of coupons with or without thermal insulation. The experiments combine in the same time mechanical testing and thermal exposure thanks to an original testing machine. The authors aim to experimentally investigate the performance of CFRP, subjected to two different regimes of elevated temperature loading and mechanical loading. In the first regime (test at constant temperature level), the mechanical performance of CFRP, with and without being protected by a thermal insulation, is identified at different temperature levels (varying from 20°C to 600°C), while in the second regime (test at constant working ratio of CFRP), the varieties of its thermal performance at different mechanical loading condition is investigated. The results from the first regime shows that the ultimate strength and the Young modulus of CFRP (without thermal insulation) generally reduce when the applied temperature level increases. The ultimate strength of CFRP (without thermal insulation) reduces about 40% when the temperature increases from 20°C to 200°C. This value reduces 55% and 85% when temperature levels are respectively 400°C and 600°C. The Young modulus of CFRP (without thermal insulation), in a different tendency, firstly increases 15% when the temperature level increases from 20°C to 200°C then reduces about 21% and 26% at the temperature levels of 400°C and of 600°C respectively. The results from the second regime show that the rupture temperature of the CFRP material (without thermal insulation) reduces when the level of imposed mechanical loading on the tested material increases. Correlations and comparisons between two testing regimes are proposed and discussed.
The thermo-mechanical performance of CFRP, with being protected by a thermal insulation, has also been identified by different tests carried out at the second regime (test at constant working ratio of CFRP). The tests carried out on CFRP, with thermal insulation, allow characterizing the real effectiveness of thermal insulation material for the protection of CFRP subjected to thermal and mechanical loading.

 

Fatigue behavior of structural adhesives for bridges

Anastasios P. Vassilopoulos, Maria Savvilotidou, Thomas Keller

EPFL, Switzerland

This paper presents the results of an experimental program designed to study the effects of ageing and wet environment on the fatigue behavior of epoxy resin used in bridge applications. Specimens were cured and kept under a variety of environmental conditions in room conditions and in water environments in order to simulate the ageing of adhesives in bridges up to 30 years. Experimental results have indicated that a typical power law S-N equation can describe the fatigue stress vs. life behavior of the examined material under different gravimetric conditions. The slope of the curve was found to be in the range of other polymers and polymeric composite materials. The cyclic strain behavior and the hysteresis loops obtained at different gravimetric conditions and stress levels assisted by examination of the fatigue failure surfaces using a digital handheld microscope reveal the damage mechanisms. The results of this work showed that the examined epoxy resin can sufficiently sustain reasonable amount of loads, above the serviceability levels seen by adhesives in structural joints in bridges for extended periods of life.

 

 

Material development and characterization

 

Natural fiber based composites for structural solutions: proposals

Clara Bertolini Cestari, Claudia Brito de Carvalho Bello, Tanja Marzi, Alberto Viskovic

Università IUAV di Venezia, Italy

Ongoing research activities for innovation in the fields of structural engineering and construction techniques, considering bio-composites materials, are presented. Natural fiber composites used in structures solutions improve sustainability, mechanical resistance, ductility; furthermore can reduce material weight and costs. There is a wide range of possible applications of natural fiber based composites as structural solutions: Natural Fiber Reinforced Polymer (NFRP) and pultruded elements; natural fibers combined with cementitious mortar to obtain structural reinforcements by composite systems, bio-concrete and bio-reinforced mortars; exploiting nano-technologies to obtain bio-nanocomposites with increased mechanical resistance and surface protection functions (waterproofing, UV, fire-resistance).
Natural fibres (i.e. bamboo, hemp, cardoon, sisal, jute, ramie) represent a sustainable alternative to replace synthetic and petroleum-based materials into composites and can be combined with bio-resins for a totally bio-composite. The research effort is to characterize their mechanical performance and to extend their use, assessing their long-term durability and LCA. This brief presentation can constitute a starting base for a wider research network, in which it is fundamental a multidisciplinary dialogue between different sectors as structural engineering, chemistry, materials technology and enterprises in the fields of structural engineering, construction, innovation technology, nano-materials and composite materials.

 

Biomixture with vegetable fibre fabrics: testing in progress

Francesca Giglio, Giulia Savoja

Mediterranea University of Reggio Calabria, Department of Architecture and Territory, dArTe, Italy

The Ph.D. research is conducted in the Mediterranea University of Reggio Calabria.
The aim of this research is to study and to test bio-mixtures for laminas to use in construction field components. The research is carried out within the experiment of new mixtures for composites. In particular, these mixtures are based on vegetable fibre fabrics and bio-based epoxy resin. In fact, one of the main purpose is to make a contribution to the environmental impacts of traditional composites, both as regards their production phase and with respect to the end of life cycle. The experimentation focuses on vegetable fibres that could have a good development in South Italy, to promote local production chains about renewable and sustainable sources.
The paper describes part of the conducted laboratory tests on different kinds of laminas, for material and mechanical characterization. Furthermore, the paper identifies some possible applications in construction sector and it begins to analyse possible future prospects.

 

Numerical examples for calculation of laminated composite plates with delamination

Marina Rakocevic

University of Montenegro, Faculty of civil Engineering, Montenegro

Inter-laminar cracking can occur due to a defect in the production, transport, storage or during exploitation load. Instability in the production process, imperfections of various nature and thermal or chemical components of the laminate components, may also be the cause of the initial delaminations.
This paper presents a model for calculating the realistic stress-strain state of simply supported laminated composite plates with delamination by using a layerwise theory. Internal delamination which is stable under exploitation transverse load, with the assumption of no further growth or propagation, is considered. It is possible, by using layerwise theory, to obtain stresses and displacements in the cross-section of the plate with delamination. Presented model is based on the mathematical model of Reddy's layerwise theory for laminated composite plates. Stresses and displacements are calculated using the double trigonometric series with discretization through the thickness of the plate.

 

Implementation of the factorial experimental design for improving the production of composite pipes for construction

Vineta Srebrenkoska, Renata Kotynia, Silvana Zezova, Sara Srebrenkoska

Faculty of Technology, University Goce Delcev, Stip, R.Macedonia, Macedonia

In the present work, the attempt was made to assess the applicability of the full factorial experimental design in predicting the hoop tensile and compressive strengths of glass fiber/epoxy resin composite pipes. The preparation of the composite pipes was conducted in accordance with the full factorial experimental design by using of three parameters and two levels of variation. The winding speed of the composites was taken to be the first factor, the second was the fiber tension and the third winding angle. The first factor low and high levels were set at 5,21 m/min and 21 m/min, respectively, for the second factor at 634N and 60N, respectively, and for the third factor at 100 and 900. To approximate the response i.e. the mechanical properties of the composite pipes within the study domain (5, 25 - 21) m/min x (34 - 60) N x (10 - 90) 0, the first order linear model with the interaction was used. The influence of each individual factor to the response function was established, as well as the influence of the interaction of the two and three factors. The hoop tensile properties of selected specimens were determined based on split-disk tests while for the compression tests, the tubular specimens were tested and reliable results were obtained with low standard deviations. It was found that the estimated first-degree regression equation with the interaction gave a very good approximation of the experimental results of the hoop tensile and compressive strengths of composite pipes within the study domain.

 

Summary of activities at partner institutions

 

Basalt fibers as composite material for structural elements

Eythor Thorhallsson & Jonas Thor Snaebjornsson

Reykjavik University, Iceland

During recent years, the structural and composite laboratory at Reykjavik University (SEL) has perform novel experimental and analytical research using continuous basalt fibers as structural material for timber, composite and concrete structures. The paper will give an overview of these experiments and draw some general conclusions based on the experience accumulated.
The experimental programs involves the following specimens: Bending test of concrete beams with slack basalt bar and prestressed reinforcement (BFRP) situated in tension zone. Monitoring of pre-tensioned BFRP bars to estimate loss of prestress. Production of 20 mm concrete plates with basalt fibermats as tension reinforcement. Testing and comparison of circular and rectangular concrete columns with ordinary steel reinforcement with wrapped BFRP strengthened columns of same geometry and material. Concrete beams with BFRP sheets glued with mortar on the tension side of the beams. Gluing BFRP mats on the sides of concrete beams to improve shear strength behaviour. Glulam beams were strengthened gluing one-directional basalt mats to the bottom lamella. Composite plate specimens were designed using basalt fibers and polyester resin and tested for tension, compression and shear forces using a standard ASTM test.
In near all cases analytical and/or numerical models were compared with the experimental results. An environmental study was carried out as well to compare BFRP beams with RC beams. The main conclusion is that BFRP are promising structural material for various types of civil engineering structures and could complement and even replace conventional design in near future.

 

FRP composites for structural strengthening - Review of research performed at Politehnica University Timisoara

Tamás Nagy-György, Codruț Floruț, Valeriu Stoian

Politehnica University Timisoara, Romania, Romania

The paper presents briefly all the important research performed at Politehnica University Timisoara in the field of structural strengthening using FRP composites, from the beginning until today. The presentation will start with studies on strengthening timber beams with GFRP bars using NSMR technique (from 1983), followed by a program on retrofitting of masonry and RC walls for shear using EBR methods. The paper discusses results on dapped-end beam retrofitted using different qualities and arrangements of FRPs and recommendations for future applications. The effect of confinement on beams strengthened for cyclic reversal loads is also discussed, together with special anchorage details developed by the research team. In the research performed on slabs with cut-out openings the use of a mixed NSMR-EBR technique is presented. The effectiveness of FRPs on seismic retrofit of RC shear walls with cut-out openings is also discussed through a series of specific detail with the most important conclusions. In the final part of the paper ongoing research on RC and masonry walls are shown, as well as actual trends and perspectives of the use of FRP, from the authors’ perspectives.

 

Testing of Composites for Strengthening of RC Structures

Dr Miroslav Cerny

Czech Technical University in Prague, Klokner Institute, Czech Republic

The methods and results of material and structural testing of composites for strengthening of reinforced concrete structures (conclusion of CTU KI COST work) will be given. Some applications of strengthening RC structures by composites will be shown as well.

 

Prefabricated and prestressed elements

 

Thin-walled precast concrete sandwich panels with GFRP reinforcement and connectors. Experimental and theoretical study

Marcin M. Haffke, Matthias Pahn

Technische Universitaet Kaiserslautern, Germany

Prefabricated modular sandwich panels consist normally of two steel-reinforced concrete wythes and core layer including insulation material and connectors joining mechanically all the layers. Their use in the construction grew increasingly over the past decades due to accuracy and simplicity of assembly at a construction site and their good thermal characteristics. The market of energy-efficient, sustainable housing systems is predicted to grow in the following years.
Replacement of reinforcement and connectors by glass fibre reinforced polymers (GFRP) in prefabricated sandwich panels is one of the application where the unique chemical and mechanical properties of GFRP can be fully utilized. Possible, due to corrosion-resistance of GFRP, reduction of the concrete cover improves thermal properties of panels and reduces their weight, transport costs and overall concrete consumption.
Experimental campaign was conducted to investigate the flexural behaviour of the panels and their components. Specimens with various types of GFRP and steel reinforcement of the wythes under different conditions of composite action were tested and the results were compared. The mechanical behaviour of sandwich panels was analysed with the focus on the flexural behaviour of the GFRP reinforced thin-walled concrete wythes performing as a part of sandwich panel.
The investigation programme and the test results are presented and discussed. Sandwich panels reinforced with GFRP rebars show good mechanical behaviour in the range of service load and high degree of composite action. The results of the conducted tests show high potential for use of GFRP reinforcement in prefabricated thin, energy-efficient sandwich panels.

 

GFRP reinforced concrete sandwich panels: thermo-mechanical behaviour

Valter Carvelli, Andreas Schmitt, Marcin Haffke, Matthias Pahn

Politecnico di Milano, Italy

The thermo-mechanical response of sandwich panels was experimentally investigated. The panels had two external concrete layers reinforced with glass fibre reinforced polymer (GFRP) rebars and an internal expanded polystyrene insulation layer. These are typical for low load bearing panels in façade claddings. The heating condition was such that internal GFRP rebars in one concrete layer were exposed, for different times, to temperature higher than the glass transition temperature of the resins. Such condition allowed assessing the variation of the mechanical behaviour, in terms of deformability and load carrying capacity, of the panels with pre- and post-heating three points bending tests. As main outcome, the elevated temperature produced significant modification of the insulation layer and considerable reduction of the initial global stiffness and of the load carrying capacity, while the GFRP rebars were not apparently modified.

 

Assessment of ductility and deformability approaches for concrete members prestressed with CFRP tendons

Katarzyna Zdanowicz, Steffen Marx

Leibniz Universitat Hannover, Germany

Fiber Reinforced Polymer (FRP) tendons are nowadays becoming an alternative to steel prestressing reinforcement. However, a linear-elastic behaviour of FRP composite reinforcement up to failure and their brittle nature lead to inefficient applications of such reinforcement so far. In contrary to members with steel reinforcement, FRP-reinforced sections are over-reinforced, which does not contribute neither to lower brittleness of such structures nor to cost efficiency, but indeed provides sufficient margin of safety.
To avoid high reinforcement ratios and over-reinforcing there is a necessity for a reasonable design approach for FRP reinforced and prestressed concrete elements, which will ensure sufficient ductility of a structure or appropriate signalisation of forthcoming failure. Ductility of structures with FRP reinforcement can be currently evaluated (in form of ductility or deformability indices) with several approaches, which are available in the literature and adopted by some codes. The aim of this study was to compare a broad spectrum of experimental results with calculated indices in order to assess how relevant these ductility approaches are for members prestressed with carbon FRP tendons. For this purpose a database with flexural tests results of CFRP prestressed concrete members was created. The ductility approaches were then verified to judge which of them are the most suitable for various types of CFRP tendons, cross-section shapes and concretes.

 

Prestressing of RC beams with internal BFRP reinforcement: tendencies and latest developments

Ted Donchev

Kingston University - London, United Kingdom

The reinforced concrete (RC) elements internally reinforced with Fibre Reinforced Polymer (FRP) bars are getting increasingly popular due to lack of corrosion, lack of electro-magnetic effects on different type of equipment and the high strength of this type of reinforcement. The main obstacle for their wider application, especially in areas such as marine structures, coastal protection and chemical plants is the higher deformability of such elements when loaded on bending. One promising way for solving of this problem is via prestressing of FRP reinforcement.
The paper presents summary of the results obtained from research conducted in Kingston University - London, UK. The focus of the research is the estimation of the deformability of pretensioned RC beams with BFRP reinforcement with additional considerations for factors influencing their behaviour, such as anchoring, losses of prestress and creep rupture effects. The obtained results are analysed and compared with results from similar investigations conducted by other authors.