Shear strengthening of reinforced concrete beams with SHCC-FRP panels

Tese de Doutoramento em Engenharia CivilThe main purpose of this thesis is to assess the potentialities of Hybrid Composite Plates (HCPs) for the shear strengthening of reinforced concrete (RC) beams. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates applied according to the Near Surface Mounted technique (NSM). Due to the excellent bond conditions between SHCC and CFRP laminates, these reinforcements provide the necessary tensile strength capacity to the HCP, while the high post-cracking tensile deformability and resistance of SHCC avoids the occurrence of premature fracture failure of this cement composite in the stress transfer process between these two materials when the HCP is crossed by a shear crack. Different HCPs with different CFRP laminates percentage are adopted for the shear strengthening of the beams. Two different processes are investigated to apply the HCPs to the beam’s concrete substrate: by using epoxy adhesive; in addition to the epoxy adhesive it is also used mechanical anchors. From the results it was verified that when epoxy adhesive is only used, the shear strengthening contribution of the HCPs is limited by the tensile strength of the concrete substrate of the strengthened beams. Mechanical anchors were therefore used in order to prevent a premature debonding of the HCPs and apply a certain confinement to the concrete in the zone of the beam to be strengthened, resulting favorable effects in terms of shear strengthening. Experimental programs with intact and damage RC beams are carried out to assess the shear strengthening contribution of HCPs. The results obtained from these tests are also used for determining the predictive performance of the analytical model developed to evaluate the shear resistance of RC beams strengthened with HCPs. To further explore the potentialities of HCPs for the shear strengthening of RC beams, advanced numerical simulations are performed by using a FEM-based computer program, whose adequate predictive performance is demonstrated by simulating the experimental tests carried out.O principal objetivo desta tese é avaliar as potencialidades de um novo tipo de painel compósito hibrido no reforço ao corte de vigas de betão armado (BA). Este fino painel, designado na língua inglesa por Hybrid Composite Plate e com a abreviatura HCP, é composto por argamassa reforçada com fibras, a qual após endurecida é reforçada por finas lâminas de polímero reforçado com fibras de carbono (CFRP) aplicadas segundo a técnica “Instalação próxima da superfície” que na bibliografia em Inglês é conhecida por NearSurfaceMounted (NSM). Estes laminados de CFRP são instalados em finos entalhes executados no painel e colados com resina epóxi. Esta argamassa apresenta comportamento de endurecimento em tração, designada na língua inglesa por StrainHardeningCementitious Composite (SHCC), pelo que após o início da fendilhação da matriz desenvolve capacidade crescente de resistência à tração até nível de extensão relativamente elevado.O elevado desempenho em tração do SHCC permite mobilizar eficazmente a elevada resistência em tração dos laminados de CFRP, pois as microfibras que reforçam o SHCC evitam a ocorrência de rotura precoce por fratura durante o processo de transferência de tensões destes para o meio circundante, situação esta que ocorre quando o HCP é atravessado por fenda de corte que se desenvolve na viga a reforçar. Diferentes configurações de HCP foram investigadas para o reforço ao corte de vigas de BA, em especial a percentagem e inclinação de laminados de CFRP e o processo de fixar os HCPs às faces laterais da viga a reforçar. No que respeita a este último aspeto, dois métodos foram estudados, um em que o HCP é aplicado recorrendo apenas a adesivo epóxi, e um outro em que para além do adesivo epóxi o painel é fixo recorrendo a ancoragens mecânicas. Os programas de ensaios experimentais evidenciaram a elevada eficácia desta técnica de reforço, principalmente quando os HCP são aplicados com adesivo e ancoragens, pois o nível de reforço garantido pelo HCP aplicado exclusivamente por adesivo está limitado pela resistência à tração do betão do substrato da viga a reforçado, dado ocorrer destacamento precoce do HCP. Este destacamento é evitado recorrendo a ancoragens mecânicas, as quais permitem ainda introduzir algum confinamento no betão da viga a reforçar dado poderem ser aplicadascom uma certa pós-tensão, resultando maior eficácia de reforço ao corte de vigas de BA conseguida com a presente técnica. Recorrendo a conceitos e formulações existentes no quadro do comportamento e reforço ao corte de vigas de BA, foram desenvolvidas duas novas formulações, cuja capacidade preditiva foi avaliada não somente com os resultados obtidos nos programas experimentais executados, mas também recorrendo a resultados experimentais existentes na bibliografia da especialidade. No intuito de melhor compreender as potencialidades da técnica de reforço desenvolvida neste trabalho, recorrendo a estudos paramétricos, foi inicialmente avaliada a capacidade preditiva de um modelo constitutivo existente no programa de elementos finitos FEMIX, fazendo a simulação dos ensaios executados experimentalmente. Constatou-se que o modelo constitutivo que modela o amolecimento em tração e em corte do processo de abertura e deslizamento de uma fenda em betão permite simular com elevado rigor quer a resposta deformacional, como o padrão de fendilhação registado nos ensaios experimentais. Utilizando este modelo, foram efetuados estudos paramétricos para evidenciar a influência de parâmetros relevantes na eficácia da técnica de reforço proposta

Shear strengthening of reinforced concrete T beams with Hybrid Composite Plate (HCP)

This paper aims to evaluate the effectiveness of hybrid composite plates (HCPs) technique for the shear strengthening of the reinforced concrete (RC) T cross-section beams. HCP consists of a thin plate of strain hardening cementitious composite (SHCC) reinforced with carbon fiber reinforced polymer (CFRP) laminates. Two HCPs with different CFRP laminates percentage (ρfw=0.08% and ρfw=0.14%) were adopted for the shear strengthening of the beams. The HCPs were bonded to substrate in two different ways. In the first case, the HCPs were bonded using epoxy adhesive, whereas in the second case they were bonded using epoxy adhesive and fixed by mechanical anchors. The effectiveness of this technique was limited by the tensile strength of the concrete cover of the strengthened beams. Therefore, in the second case, mechanical anchors prevented a premature debonding of the HCPs and a certain concrete confinement was applied in the zone of the beam to be strengthened, resulting in favorable effects in terms of shear strengthening. Advanced finite element method (FEM) based numerical simulation was performed by using a constitutive model, whose predictive performance was demonstrated by simulating the experimental tests carried out. After demonstration of the good predictive performance of the numerical model, a parametric study was carried out to study the influence of the shear reinforcement ratio, and the influence of thickness of the HCPs on the beam’s load carrying capacity.Fundação para a Ciência e a Tecnologia (FCT) - PTDC/ECM/114511/200

Shear properties of the strain hardening cementitious composite material

Recently, Strain Hardening Cementitious Composite (SHCC) material has been used for the shear strengthening and the structural rehabilitation of reinforced concrete structures. However, the shear behavior of this material has not been yet fully understood due to lack of an appropriate and accurate direct shear test method. This paper aims to investigate the shear properties of the SHCC material. For this purpose, Iosipescu shear test was selected, where loads are applied in antisymmetric four points bending, assuring a pure shear section at the center of the specimen. A special geometry for the specimen was adopted in order to assure a uniform shear stress distribution in the pure shear section. This experimental test can characterize the shear behavior of SHCC material. The experimental test was simulated by the FEM-based computer program, FEMIX. To predict the average shear stress-sliding response, the shear crack softening diagram, available in the multi-directional fixed smeared crack model, was used. After demonstration the good predictive performance of the numerical model, a parametric study was carried out to evaluate the influence of shear retention factor, fracture energy of mode II, and crack shear strength on the average shear stress-sliding response of the SHCC. The advantage of SHCC instead of conventional mortar was also studied.Fundação para a Ciência e a Tecnologia (FCT) - PTDC/ECM/114511/200

A design approach to determine the shear capacity of reinforced concrete beams shear strengthened with NSM systems

This paper present a design approach to predict the shear capacity of reinforced concrete (RC) beams strengthened with fiber reinforced polymer (FRP) laminates/rods applied according to the near surface mounted (NSM) technique. The new approach is based on the simplified modified compression field theory (SMCFT) and considers the relevant features of the interaction between NSM FRP systems and surrounding concrete, like debond and concrete fracture. In the SMCFT model, the shear strength of a RC element is a function of two parameters: the tensile stress factor in the cracked concrete ( β ), and the inclination of the diagonal compressive stress in the web of the section ( θ ). However, this approach is not a straightforward design methodology due to its iterative nature. A sensitivity analysis is carried out to assess the relative importance of each input parameter that mostly affect the shear capacity of RC beams shear strengthened according to the NSM technique. Taking into account the obtained results, equations to determine β and θ without recurring to an iterative procedure are derived. The experimental results of 112 beams shear strengthened with NSM FRP are used to appraise the predictive performance of the developed approach. By evaluating the ratio between the experimental results and the analytical predictions, an average value of 1.14 is obtained, with a coefficient of variation of 13.1%, being safe estimations 87% of the predictions.The author acknowledges the support provided by FP7-PEOPLE-2013-ITN in the project “endure - European Network for Durable Reinforcement and Rehabilitation”, proposal MC-ITN-2013-607851.info:eu-repo/semantics/publishedVersio

Hybrid Composite Plates (HCP) for shear strengthening of RC beams

The potential of a hybrid composite plate (HCP) for the strengthening of reinforced concrete (RC) deep beams is evaluated. HCP are composed of a CFRP sheet that is glued to the external surface of a thin plate made of strain hardening cementitious composite (SHCC). These panels were glued to the lateral faces of RC deep beams. Three groups of shear strengthened RC beams were tested under three-point bending load configuration. CFRP sheet, SHCC plate or HCP were individually applied to the lateral faces of shear deficiently reinforced beams to compare the effectiveness of these different strengthening schemes. The load-mid span deflections of these beams are compared to the response of the control beam. The maximum load carrying capacity and its corresponding mid-span deflection, crack pattern and the initial flexural stiffness are the studied parameters.Fundação para a Ciência e a Tecnologia (FCT

Materiais de matriz cimentícia de elevado desempenho para o reforço estrutural

No presente trabalho avaliam-se as potencialidades de materiais com endurecimento em tração (designados na bibliografia inglesa por “Strain Hardening Cement Composites-SHCC”) no reforço à flexão de estruturas com comportamento frágil, e no reforço ao corte de vigas de betão armado (BA). No reforço à flexão foi aplicada uma camada de SHCC, de 15 e 20 mm de espessura, na face de tração de vigas de alvenaria formadas por tijolos maciços de argila ligados por argamassa de baixa resistência, tendo-se constatado que esta técnica de reforço, de fácil e rápida execução, permite aumentar significativamente, quer a capacidade de carga como a deformabilidade deste tipo de elementos estruturais. Para o reforço ao corte de vigas de BA foram pré-fabricados painéis de SHCC de 18 mm de espessura, os quais foram aplicados nas faces laterais das vigas a reforçar por intermédio de adesivo epóxi. Um dos grupos de vigas foi reforçado aplicando nas suas faces laterais manta de fibras de carbono (CFRP) segundo a técnica “Externally Bonded Reinforcement-EBR”, sobre a qual foi aplicado o tipo de painel de SHCC anteriormente referido, recorrendo a adesivo epóxi. Os resultados dos ensaios demonstraram que ambas as técnicas, somente com painel de SHCC, e com SHCC mais manta de CFRP, permitem aumentar significativamente a rigidez e a capacidade de carga de vigas de BA, em especial neste último caso, em resultado da adequada sinergia de efeitos dos materiais envolvidos.In the present work the potentialities of strain hardening cement composites (SHCC) are assessed for the flexural strengthening of brittle structural elements, as well as for the shear strengthening of reinforced concrete (RC) beams. For the flexural strengthening, masonry beams, formed by massive clay bricks bonded by low strength mortar, were strengthened with a layer of SHCC of 15 or 20 mm thickness applied in the tensile surface of these beams. The results have evidenced the possibility of increasing significantly the load carrying capacity and the deformability of this quite brittle type of structural elements. For the shear strengthening of RC beams it was explored the potentialities of applying prefabricated panels of 18 mm thickness on the lateral faces of the beams. In one of the groups of beams, a sheet of carbon fibre reinforced polymer (CFRP) was combined with the SHCC. The results demonstrated the capacity of these techniques to increase the load carrying capacity and the stiffness of RC beams failing in shear, mainly when SHCC is combined with CFRP, due to the favorable synergy effect of the ductile character of SHCC and the high tensile strength and elasticity modulus of CFRP.Fundação para a Ciência e a Tecnologia (FCT

Shear strengthening of damaged reinforced concrete beams with hybrid composite plates

This paper aims to evaluate experimentally the potentialities of Hybrid Composite Plates (HCPs) technique for the shear strengthening and repairing of reinforced concrete (RC) beams. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates. For this purpose, an experimental program composed of four Rectangular and five T-cross section beams was executed to assess the strengthening efficiency of HCPs technique. There were two control beams without any type of shear reinforcement, and seven beams strengthened with HCPs. The HCPs were applied to the lateral faces of the beams by using a combination of epoxy adhesive and mechanical anchors. The bolts were applied with a certain torque in order to increase the concrete confinement. To have a better understanding of the shear behavior of SHCC material, Iosipescu shear tests were carried out, and the results were used to derive their fracture mode II parameters. To further explore the potentialities of the HCPs technique for the shear strengthening, the experimental tests were simulated numerically by a FEM-based computer program considering the material properties obtained experimentally. After demonstration of the good predictive performance of the numerical model, a parametric study was executed to investigate the influence of some parameters on the load carrying capacity of the strengthened beams, namely: i) Use a mortar instead of the SHCC; ii) application of different applied torque level to the mechanical anchors; iii) different bond condition between HCPs and concrete substrate.Clever Reinforcement Iberica for providing the CFRP laminates and epoxy, Sika for the sand and adhesive, Grace for the superplasticizers, Dow Chemical Co. for viscous modifying agents, ENDESA Compostilla power station for the fly ash, and Casais for assisting in the execution of the beams. FCTinfo:eu-repo/semantics/publishedVersio

Shear strengthening of reinforced concrete beams with hybrid composite plates

The effectiveness of Hybrid Composite Plates (HCPs) for the shear strengthening of the Reinforced Concrete (RC) beams was assessed by an experimental program. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates. Due to the excellent bond conditions between SHCC and CFRP laminates, these reinforcements provide the necessary tensile strength capacity to the HCP. Two HCPs with different inclination of CFRP laminates (45º and 90º) were adopted for the shear strengthening of RC beams by bonding these HCPs to the lateral faces of the beam with an epoxy adhesive. The results showed that these HCPs have assured a significant increase in terms of load carrying capacity, mainly those with inclined laminates. The SHCC surrounding the CFRP laminates in the HCP has offered effective resistance to the degeneration of micro-cracks on macro-cracks, which has avoided the occurrence of premature mixed shallow semi-pyramid-plus-debonding failure modes registered currently when using the NSM-CFRP technique. Advanced numerical simulations were performed by using a FEM-based computer program, whose predictive performance was demonstrated by simulating the experimental tests carried out. In this context a parametric study was executed to evaluate the shear strengthening efficiency of the arrangement and percentage of CFRP laminates in HCPs, as well as the influence of using mechanical anchors to avoid premature detachment of the HCPs.“PrePam –Pre-fabricated thin panels by using 17 advanced materials for structural rehabilitation” with reference number of PTDC/ECM/114511/200

Assessing the applicability of a smeared crack approach for simulating the behaviour of concrete beams flexurally reinforced with GFRP bars and failing in shear

Numerical simulation of beams failing in shear is still a challenge. With the scope of verifying the applicability of smeared crack approaches to simulate the behavior of reinforced concrete (RC) beams failing in shear, a set of concrete beams reinforced with longitudinal glass fiber reinforced polymer (GFRP) bars, experimentally tested up to their failure, and comprehensibly monitored, are numerically simulated. The simulations are carried out with a multi-directional fixed smeared crack model available in the FEMIX computer program that has several options for modeling the crack shear stress transfer, which is a critical aspect when simulating RC elements failing in shear. The predictive performance of the numerical simulations is assessed in term of load vs deflection, crack pattern at failure, concrete strains in critical shear regions, and moment–curvature relationship. The influence on the predictive performance of the following modeling aspects is also investigated: finite element mesh refinement; simulation of the crack shear stress transfer by using the classical shear retention factor and a crack shear-softening diagram; bond conditions between flexural reinforcement and surrounding concrete. The simulations carried out demonstrate that small dependence of the results on the finite element mesh refinement and adequate crack patterns can be obtained with refinement levels suitable for design purposes and taking into account the actual computer performances, as long as a crack shear-softening diagram is used. However, the predictive performance of the simulations depends significantly on the values adopted for the parameters that define this diagram, as demonstrated by the performed parametric studies.The first author aims to acknowledge the support provided by FCT through the research project ICoSyTec -Innovative construction system for a new generation of high performance buildings, with reference: POCI-01-0145-FEDER-027990

Development of a high-performance concrete deck for Louisiana’s movable bridges: numerical study

Louisiana has approximately 160 movable bridges, mostly in the southern part of the state. This places Louisiana among the states with the highest inventory of movable bridges in the nation. The typical deck systems in these movable bridges are steel grids. Records show that steel grids have had maintenance issues. An alternative ultra-high/high performance concrete (UHPC/HPC) bridge deck system is proposed for Louisiana’s movable bridges. This system consists of precast waffle slab deck panels, which are reinforced with glass fiber reinforced polymer (GFRP) bars as positive moment reinforcement, and a two-way carbon fiber reinforced polymer (CFRP) mesh as top reinforcement. Several validated nonlinear finite element analyses were performed to simulate the behavior of the precast panels from the onset of loading to failure. It is concluded that the precast concrete waffle slabs provide a viable alternative to steel grids by supplying load capacities that surpass service level and ultimate level load demands and deflection capacities that are within code specified limits.This study was sponsored by the Louisiana Transportation Research Center (LTRC). The authors thank the members of the project review committee for their comments and their thorough review of the work performed to date. The authors also thank Lafarge North America for donating Ductal to conduct the material characterization study. The opinions expressed herein are those of the authors and do not necessarily reflect the views of the sponsor.info:eu-repo/semantics/publishedVersio