Structural behavior of hybrid GFRP and steel reinforced FRC prestressed beams

The present thesis intended to contribute for the development of a new generation of high durable and sustainable reinforced concrete (RC) beam structures submitted to flexural loading, by combining the benefits that Glass Fiber Reinforced Polymers (GFRP) and steel bars can provide: the former due to their corrosion immunity, and the latter derived from their high ductility. Furthermore, High Performance Fiber Reinforced Concrete (HPFRC) was developed to improve the ductility of such innovative structures. To avoid corrosion, steel bar was placed with a HPFRC cover thickness, higher than 100 mm, while GFRP bars were applied in the near tensile surface of the HPFRC beams. In addition, the GFRP and steel bars were applied with a certain pre-stress level. The prestressing optimized their reinforcing capabilities, and increased the service load carrying capacity of the beam. On the other hand, conventional shear reinforcements were not used, and they were totally replaced by HPFRC material. Due to the quite high post-cracking tensile strength and energy absorption capacity that HPFRC attained, the composite system showed adequate shear resisting, and also enhancement in the structural performance at both Serviceability and Ultimate Limit States (SLS and ULS). The work started with the assessment to bond behavior between GFRP and HPFRC through experimental tests and analytical investigation. The structural performance of this hybrid prestressed GFRP-steel reinforced HPFRC was investigated by performing four-point bending tests on beams with I-shaped cross section under both monotonic and fatigue loading conditions. Moreover, an extensive analytical formulation was developed in order to theoretically address to the main structural aspect of the tested beams. The obtained experimental results were captured well using the respective results from the analytical study. Finally, finite element (FE) simulations were carried out using two well-known modelling approaches available in the literature for concrete elements in form of both 2D and 3D models. The results obtained from these models were promising, aA presente tese pretende contribuir para o desenvolvimento de uma nova geração de estruturas de betão armado, submetidas a esforços de flexão de elevada durabilidade e sustentabilidade, combinando os benefícios do uso de varões de polímeros reforçados com fibras de vidro (GFRP - Glass Fiber Reinforced Polymers) com os de varões de aço convencional: os primeiros devidos à sua imunidade à corrosão, enquanto que os segundos devido à sua ductilidade. Para além disso, foi desenvolvido um betão reforçado com fibras (HPFRC - High Performance Fiber Reinforced Concrete) de alto desempenho de modo a melhorar a ductilidade destas estruturas inovadoras. Para evitar a corrosão, o varão de aço foi colocado com um recobrimento superior a 100 mm, enquanto que os varões de GFRP foram aplicados junto à superfície mais tracionada das vigas de HPFRC. Adicionalmente, os varões de aço e de GFRP e foram aplicados com um determinado nível de pré-esforço. O pré-esforço potenciou os reforços usados para o comportamento em serviço da viga. Por outro lado, haverá que referir que não foram usadas armaduras convencionais de reforço aos esforços transversos (estribos), tendo sido totalmente substituídos pelo HPFRC. Devido à elevada resistência à tração e à elevada capacidade de absorção energia na fase de pós-pico que o HPFRC apresenta, o sistema estrutural mostrou adequada resistência aos esforços transversos, e também melhoria no desempenho estrutural, tanto para os Estados Limite de Serviço, bem como Últimos. O trabalho iniciou-se com o estudo da aderência entre o GFRP e o HPFRC através de ensaios experimentais e investigação analítica. O desempenho estrutural deste sistema híbrido foi investigado através da realização de ensaios experimentais em vigas com secção transversal em forma de I, sob quatro pontos de carga, em condições de carga monotónicas e de fadiga. Além disso, foi desenvolvida uma formulação analítica extensa com o objetivo de contemplar do posto de vista teórico, os principais aspetos estruturais das vigas ensaiadas. Os resultados experimentais obtidos foram simulados com rigor suficiente por intermédio destes estudos analíticos. Finalmente, foram realizadas simulações numéricas com recurso ao método dos elementos finitos utilizando, para tal, duas conhecidas abordagens disponíveis na literatura na simulação do HPFRC, recorrendo a modelos 2D e 3D. Os resultados obtidos a partir destes modelos numéricos foram promissores, podendo ser usados em futuras análises e desenvolvidos no âmbito do estudo desta área.The financial support provided by the Portuguese Foundation for the Science and Technology (FCT), with grant number SFRH/BD/77409/2011, and the research project DURCOST with the reference number PTDC/ECM/105700/2008

Tension-stiffening model for FRC reinforced by hybrid FRP and steel bars

This paper presents a tension stiffening model for Fiber Reinforced Concrete (FRC) tensile member reinforced by hybrid glass fiber reinforced polymer (GFRP) and steel bars. The model is developed through an explicit analytical bond formulation by considering a four-linear bond shear stress-slip relationship to simulate the bond behavior between reinforcing bars and surrounding FRC. The model is also capable of simulating both the fiber reinforcement contribution and the yielding stage of steel bar at cracked section. Additionally, a FE Model is carried out using a multi-directional smeared crack approach for modeling cracking process in FRC, and adopting interface finite elements to simulate the bond behavior between reinforcements and FRC, whose constitutive model was defined from the aforementioned bond law. Both the analytical and numerical approaches showed a good agreement with some recent experimental results on tension-stiffening in the literature. Finally, an extensive parametric study is performed by using the analytical model, and the influence of the involved parameters on the tension-stiffening and cracking behavior of hybrid GFRP/steel FRC tensile member is investigated.QREN project n. 30367 – UrbanCretePortuguese Foundation for Science and Technology (FCT) - SFRH/BD/77409/201

Deflection and cracking behavior of SFRSCC beams reinforced with hybrid prestressed GFRP and steel reinforcements

In the present work, the deflection and cracking behavior of I-shaped cross-sectional beams of Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC) reinforced in flexure with hybrid prestressed steel strand and glass fiber reinforced polymer (GFRP) bars was investigated. Combining prestressed GFRP bars of relatively low elasticity modulus, but immune to corrosion (located with a small concrete cover), with prestressed steel strand (with higher concrete cover to avoid corrosion), a good balance in terms of reinforcement effectiveness, ductility, durability and cost competitiveness can be obtained. The steel strand aims also to assure the necessary flexural strengthening of the beams if GFRP bars become ineffective in case of fire occurrence. This work presents and discusses the results obtained from the experimental study of the beams tested in flexure under monotonic loading conditions. Additionally, the predictive performance of the available formulation in the design codes for the case of Fiber Reinforced Concrete (FRC) and FRP reinforced Concrete (FRP-RC) was assessed to be used for the proposed hybrid system.The first acknowledges the PhD grant SFRH/BD/77409/2011 provided by FCT (Fundação para a Ciência e a Tecnologia), while the third author acknowledges the research grant in the ambit of the project DURCOST - Innovation in reinforcing systems for sustainable pre-fabricated structures of higher durability and enhanced structural performance, PTDC/ECM/105700/2008. The research carried out in this paper was supported by DURCOST. The authors also thank the collaboration of the following companies: Sireg and Schoeck for providing the GFRP rebars, Casais to manufacture the moulds, Ibermetais for supplying the steel fibres, Secil/Unibetão for providing the Cement, and BASF for supplying the superplasticizer

Analytical bond model for GFRP bars to steel fiber reinforced self-compacting concrete

The objective of this study is to present a computational algorithm to analytically evaluate the bond behavior between GFRP bar and steel fiber reinforced self-compacting concrete (SFRSCC). The type of information to be derived is appropriate to study the flexural behavior of SFRSCC beams reinforced with GFRP bars in terms of serviceability limit states requirements; in fact the bond between bars and surrounding concrete influences significantly the crack width and crack spacing. The proposed bond model was established by calibrating the parameters of a multilinear bond-slip constitutive law using the experimental results of pullout bending tests carried out by the authors, taking into account the experimental pullout force versus slip at loaded and free ends. According to the comparison between theoretical and experimental pullout force-slip, an acceptable accuracy of the model was observed. Additionally, by considering the proposed bond-slip relationship, a parametric study was carried out to evaluate the influence of the involved bond-slip law’s parameters on the maximum force transferred to the surrounding concrete. Finally, the development length of two GFRP bars utilized in the experiments (deformed and smooth bars) was determined by means of the proposed model, and it was compared with the values recommended by codes.Fundação para a Ciência e a Tecnologia (FCT

Shear capacity of HPFRC beams flexurally reinforced with steel and prestressed GFRP bars

This paper presents the relevant results from an experimental program to assess the shear capacity of high performance fiber reinforced concrete (HPFRC) beams flexurally reinforced with a hybrid system of passive steel and prestressed GFRP longitudinal bars. Three series of two beams with different level of prestressing were tested. The effect of prestressing level on the shear capacity of the beams was the main investigated parameter. The results showed an enhancement of the load carrying capacity, ductility and energy absorption with the increase of the prestress level. Based on the obtained results, the predictive performance of the analytical formulations of CEB-FIP Model Code 2010 and RILEM TC 162-TDF for the shear capacity of FRC beams was assessed. Both formulations seem appropriate for design purposes, but the CEB-FIP formulation predicts more conservative shear capacity. The experimental results demonstrated that the prestressing level has an effect on the shear capacity much higher than the one recommended by the codes

Interfacial bond behaviour of GFRP bar in self-compacting fiber reinforced concrete

In an ongoing research project, discrete steel fibers are being used in a self-compacting concrete (SFRSCC) to replace completely steel stirrups for pre-fabricated beams reinforced longitudinally with pre-stressed glass fiber reinforced polymer (GFRP) and steel bars. To take the advantages of the non-corrodible character and high tensile strength of GFRP bars, the minimum SFRSCC cover needs to be determined in order to assure the adequate bond performance between these bars and the surrounding SFRSCC. Since bond of the longitudinal bars has a relevant impact on the cracking behavior of RC elements (crack opening and crack spacing), an extensive experimental program composed of pullout bending tests was carried out where the influence of the following parameters was assessed in terms of bond behavior: GFRP bar diameter, surface characteristics of the GFRP bars, bond length, SFRSCC cover thickness. The local bond law was derived from inverse analysis and it was used to define the slip mode of the constitutive law adopted for interface finite elements. These interface finite elements were used to assess the crack opening and crack spacing on SFRSCC beams flexurally reinforced with GFRP bars. This paper resumes the experimental program, describes the strategy to derive the local bond law and presents and discusses the numerical simulations

Experimental and theoretical study on bond behavior of GFRP bars in steel fiber reinforced self compacting concrete

To estimate the cracking and the deformational behavior of steel fiber reinforced selfcompacting concrete (SFRSCC) beams reinforced with glass fiber reinforced polymer (GFRP) bars, it is fundamental to understand the interfacial bond behavior of embedded bars. Hence, the evaluation of the bond behavior between GFRP and (SFRSCC) was investigated in this study. A closed-form formulation was derived, adopting a new local bond stress-slip relationship. Furthermore, an experimental program composed of pullout bending tests was carried out in order to assess the influence of the following parameters on the bond behavior: bar diameter, bar surface treatment, embedment length and SFRSCC cover thickness. Finally, a numerical simulation was performed with a FEM-based computer program in order to simulate the bond behavior between GFRP bar and SFRSCC by means of a non-linear bond-slip relationship assigned to the interface finite element. The predictive performance of the theoretical models was appraised by comparing experimental and numerical results

Shear resistance of SFRSCC short-span beams without transversal reinforcements

Corrosion of steel reinforcements, especially stirrups, is considered as one of the most common reasons that shorten the service life of the reinforced concrete structures. This study aims to replace the stirrups of the beams by means of a tailor made steel fiber reinforced self-compacting concrete (SFRSCC). A hybrid flexural reinforcement system was used for all these beams, composed of glass fiber reinforced polymer (GFRP) rebars placed near to the outer surface of the tensile zone and steel reinforcements positioned with higher SFRSCC cover to be protected against the corrosion, which is considered another strategy for enhancing the durability and attending fire issues in terms of safety at ultimate limit states. The effectiveness of varying the prestressing force applied to GFRP bars to improve the shear capacity and failure mode of the designed elements is evaluated. By considering the obtained experimental results, the predictive performance of some analytical formulations for the shear resistance of fiber reinforced concrete beams was assessed. All formulations demonstrate acceptable accuracy for design purposes, but the one proposed by CEB-FIP Model Code 2010 predicts more conservative shear resistance.European Regional Development Fund (FEDER) - “Inotec”, with reference number 23024Portuguese Foundation for Science and Technology (FCT) - “SlabSys-HFRC”, with reference PTDC/ECM/120394/201

Effect of fiber dosage and prestress level on shear behavior of hybrid GFRP-steel reinforced concrete I-shape beams without stirrups

Corrosion of steel reinforcements embedded in concrete elements is generally known as one of the most common reasons that shorten the service life of the structures. The present study aims to contribute in overcoming this problem by replacing steel stirrups as shear reinforcement of concrete beams using a steel fiber reinforced self-compacting concrete (SFRSCC). In the present research the potential of SFRSCC for improving the shear resistance of the beams without stirrups is explored. In order to further reduce the risk of corrosion in this type of beams, a hybrid system of flexural reinforcement composed of a steel strand and GFRP rebars is applied and properly arranged in order to assure a relatively thick concrete cover for the steel reinforcement. The GFRP bars are placed with the minimum cover thickness for providing the maximum internal arm and, consequently, mobilizing efficiently their relatively high tensile strength. The effectiveness of applying different dosages of steel fibers and varying the prestress force to improve the shear behavior of the designed beam are evaluated. By considering the obtained experimental results, the predictive performance of a constitutive model (plastic-damage multidirectional fixed smeared crack model) implemented in a FEM-based computer program, as well as the one from three analytical formulations for estimating shear resistance of the developed beams were assessed. The FEM-based simulations have provided a good prediction of the deformational response and cracking behavior of the tested beams. All the analytical formulations demonstrated acceptable accuracy for design purposes, but the one proposed by CEB-FIP Modal Code 2010 predicts more conservative shear resistance.The first and second authors, respectively, acknowledge the research grant in the ambit of the project “UrbanCrete”, with reference number of 30367, supported by the European Regional Development Fund (FEDER), and “SlabSys-HFRC”, with reference PTCD/ECM/120394/2010, supported by the Portuguese Foundation for Science and Technology (FCT). The authors also thank the collaboration of the following companies: Tensacciaci in the name of Eng. F. Pimenta for the assistance on the application of prestress reinforcements, Sireg and Schoeck for providing the GFRP rebars, Casais to manufacture the moulds, Exporplas for supplying the polypropylene fibers, Secil/Unibetão for providing the Cement, BASF for supplying the superplasticizer and CiviTest for collaborating in producing the specimens

Mechanical behavior of concrete prisms reinforced with steel and GFRP bar systems

Being immune to corrosion, and having a tensile strength up to three times higher than structural steel, glass fiber reinforced polymer (GFRP) bars are suitable for reinforcing concrete structures exposed to aggressive environmental conditions. However, a relatively low elasticity modulus of GFRP bars (in respect to the steel) favors the occurrence of relatively large deformability of cracked reinforced concrete. Lack of ductility and degradation of properties under high temperature can be also identified as debilities of GFRP bars over steel ones. Combining GFRP and steel bars can be a suitable solution to overcoming these concerns. Nevertheless, the application of such hybrid reinforcement systems requires reliable material models. The influence of the relative area of GFRP and steel bars on the tensile capacity of cracked concrete (generally known as tension-stiffening effect), was never investigated from the experimental point of view, mainly crossing results from different tools on the assessment of the cracking process. This paper experimentally investigates deformations and cracking behavior of concrete prisms reinforced with steel bars and GFRP bars in different combinations. The test results of 11 elements are reported. A tensile stress-strain diagram is conceptually proposed for modelling the tension-stiffening effect in elements with such hybrid combination of the reinforcement. The cracking process in terms of crack width and crack spacing is analyzed considering the hybrid reinforcement particularities and a preliminary approach is proposed for the prediction of the crack width for this type of reinforced concrete elementsResearch Council of Lithuania (Research Project S-MIP-17-62). The second author also 590 wish to acknowledge the support provided by FCT through the PTDC/ECM591 EST/1882/2014 projec