Development of hybrid composite plate (HCP) for strengthening and repair of RC structures

Tese de Doutoramento em Engenharia CivilThis research work deals with development of a novel retrofitting element for RC structures designated as “Hybrid Composite Plate (HCP)”. This prefabricated element is composed of a thin strain hardening cementitious composite (SHCC) plate reinforced with either near surface mounted CFRP (NSM-CFRP) laminates, designated as HCP(L), or externally bonded CFRP (EB-CFRP) sheets, designated as HCP(S). From the materialstructural point of view, this system benefits from the high ductility of SHCC and the high tensile strength of CFRP in retrofitting of RC structures. HCP is essentially tailored to be significantly free of the shortcomings identified in the most advanced available retrofitting techniques, such as textile reinforced mortar (TRM) and conventional FRP systems. Furthermore, it is possible to attach this proposed system to the RC members by means of either adhesive, chemical anchors or a combination thereof. The investigation carried out is mainly dedicated to the development of HCP and assessment of its structural efficiency for upgrading/repairing RC members with a variety of retrofitting demands. In this framework, series of experimental tests are executed to assess HCP retrofitting efficiency for upgrading shear, flexural and energy dissipation capacity of RC members. Results of these experimental tests confirmed HCP’s high potential for retrofitting RC structures. An analytical approach is presented to estimate the ultimate flexural capacity of the beams with an HCP attached to their soffit, which is further complemented with a numerical strategy to predict the load-deflection response of such retrofitted beams. The proposed analytical and numerical approaches accurately predict the flexural capacity and load-deflection response, of flexurally strengthened beams using HCP. Finally, adopting a combination of experimental tests and finite element modelling, recommendations for an optimized HCP(L) and its connection with concrete are provided. The local bond stress-slip models at the interface of CFRP-SHCC and interface of HCP(L)- concrete are determined. Based on results obtained, equations correlating the pull force capacity of the HCP(L) to the CFRP-SHCC bond length for CFRP laminates with two different axial stiffness are derived.Esta tese apresenta a investigação realizada para o desenvolvimento de um novo elemento visando o reforço de estruturas de betão armado (BA) designado por Hybrid Composite Plate (HCP). Este elemento consiste num painel pré-fabricado composto por uma fina camada de material de matriz cimentícia apresentando comportamento de endurecimento em tração ( SHCC) reforçada laminados de matriz polimérica reforçada com fibras de carbono (CFRP) inseridos à superfície (Near Surface Mounted), designado por HCP(L), ou com manta de CFRP aplicada segundo a técnica de colagem exterior (externally bonded, EB-CFRP), designadas por HCP(S). Do ponto de vista estrutural, este sistema beneficia da alta ductilidade do SHCC e da elevada resistência à tração do CFRP no reforço de estruturas de BA. O HCP afigura-se como uma solução apropriada essencialmente por não apresentar as deficiências identificadas nas técnicas mais avançadas de reforço estrutural atualmente disponíveis, tais como TRM (Textile Reinforced Mortar) e sistemas FRP (Fiber Reinforced Polymer). O sistema proposto permite uma ligação aos elementos de BA através de resina epóxi, ancoragem química ou uma combinação entre estes. A investigação realizada foi dedicada ao desenvolvimento do HCP e avaliação da sua eficiência estrutural para melhorar ou reparar elementos de BA para uma variedade de exigências de reforço. Com este objetivo, uma série de ensaios foram realizados para avaliar a eficiência do reforço do HCP ao corte, flexão e capacidade de dissipação de energia de elementos de BA. Uma formulação analítica foi desenvolvida para estimar a resistência à flexão de vigas de BA reforçadas com HCP ligado à sua face inferior. Esta abordagem foi completada com um modelo numérico para prever a resposta carga-deformação destas vigas. A formulação analítica e o modelo numérica propostos previram com precisão, respetivamente, a capacidade de flexão e a resposta carga-deformação de vigas reforçadas com HCP. Finalmente, com base na combinação de resultados de ensaios experimentais e modelos de elementos finitos, são fornecidas recomendações para a otimização do HCP(L) e a sua ligação ao betão. Para a caracterização das interfaces CFRP-SHCC betão-HCP(L) foram determinadas leis tensão versus deslizamento. Com base nos resultados obtidos, apresentamse as equações que relacionam a capacidade de carga à tração do HCP(L) com o comprimento de ligação do CFRP-SHCC composto por camadas de CFRP dotadas de rigidez distinta.Fundação para a Ciência e a Tecnologia (FCT) SFRH/BD/65663/2009.Fundação para a Ciência e a Tecnologia (FCT) PTDC/ECM/114511/200

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

Effects of curing conditions on crack bridging response of PVA reinforced cementitious matrix

The effect of the different curing conditions on the response of fiber crack bridging of PVAECC is studied. The self compacting PVA-ECC was cast into the moulds to produce four similar rectangular panels. These panels were kept under different curing conditions for 28 days. The tensile specimens were cut from these panels, and after executing a notch they were tested under tensile loading. The stress versus crack opening relationship for the specimens extracted from panels cured at different conditions is obtained, and the derived results are compared. The dispersion of results, the stress at crack initiation, the maximum fibers bridging stress and the corresponding crack opening, the absorbed energy and the fibers bridging stiffness are the investigated parameters. Also macroscopic images of the fibers in fracture surface are used to interpret the obtained data. As a result of this study a significant influence of different curing conditions on the fiber crack bridging response was observed

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

Strain hardening fiber reinforced cement composites for the flexural strengthening of masonry elements of ancient structures

To assess the strengthening ability of a strain hardening cementitious composite (SHCC), a layer of SHCC was applied to masonry beams subjected to bending. When compared to the strengthening performance of steel fibre reinforced self-compacting concrete (SFRSCC) layer for this type of brittle beams, the SHCC presented better workability in fresh state, and provided a higher load carrying capacity and deflection ductility even with a smaller layer thickness. By using the data derived from the experimental tests with the constituent materials of the strengthened masonry beams, the behaviour of the tested strengthened masonry beams was numerically simulated with good accuracy.The study presented in this paper is a part of the research Project titled "PrePam - Pre-fabricated thin panels using advanced materials for structural rehabilitation" with reference number of PTDC/ECM/114511/2009 supported by FCT. It is also conducted as part of the MSC-SAHC Master's program supported by the European Commission. The authors also thank the collaboration of the following companies: Sika for providing the sand, Grace for the superplasticizers, Dow for the viscosity modification agents, ENDE-SA Compostilla power station for the fly ash, and SECIL for supplying the cement. The first author acknowledge the PhD grant SFRH/BD/65663/2009 provided by FCT

Flexural strengthening of masonry members using advanced cementitious materials

Two different cement based fiber reinforced composites for the flexural strengthening of masonry beams under monotonic loading are studied. Steel Fiber Reinforced Self- Compacting Concrete (SFRSCC) with tensile strain-softening behavior, and PVA fiber reinforced cement based mortar (SHCC) with tensile Strain-Hardening were the developed composites. Both composites were applied on the tensile surface of masonry beams and the effectiveness of this technique for the flexural strengthening of these quasi-brittle structural elements was assessed by performing four point beam bending tests. Both materials contributed effectively to increase the load carrying capacity and ultimate deflection ductility of the tested masonry beams, but, higher average values were obtained for these two indicators of the strengthening effectiveness when using a layer thickness of SHCC that is 2/3 of the thickness of SFRSCC. Furthermore, much more homogenous results, in terms of forcedeflection relationship, were obtained with masonry beams strengthened with SHCC than with SFRSCC

Development of hybrid composite plate (HCP) for the repair and strengthening of RC elements

Hybrid Composite Plates (HCPs) made of a Strain Hardening Cementitious Composite (SHCC) and reinforced with Carbon Fiber Reinforced Polymer (CFRP) materials are developed by taking the synergetic advantages of SHCC and CFRP for the retrofitting of reinforced concrete (RC) structures. Thanks to the high ductile character of SHCC, this prefabricated plate can be attached to the substrate using a combination of adhesive and chemical anchors to assure an effective transference of forces between these elements, leading to a high mobilization of the tensile capacity of the CFRP. This paper reports the most relevant results of a series of experimental tests performed to assess the effectiveness of this innovative technique for the repair/strengthening of RC elements. Enhancements obtained in both shear and flexural capacity of strengthened RC beams, in shear capacity of a repaired RC beam, as well as in the repair of a severely damaged interior RC beam-column joint, have demonstrated the high effectiveness of this technique.Fundação para a Ciência e a Tecnologia (FCT

Novel prefabricated panels for the repair of damaged interior RC beam-column joints

A novel prefabricated panel is introduced for the repair and strengthening of damaged interior RC beam-column joints. This prefabricated hybrid composite plate (HCP) is made of a Strain Hardening Cementitious Composite (SHCC) reinforced with CFRP materials. Besides the higher durability of this system when compared to EB-FRP, thanks to the high ductility of the SHCC, anchors can be used in combination with adhesive to attach the HCP to the concrete substrate. The first section of this paper reports the methodologies to apply HCP for the repair of two severely damaged interior RC beam-column systems. To evaluate the performance of this rehabilitation technique, the repaired specimens were subjected to the same loading pattern as it was imposed to their virgin states. Since both repaired specimens showed a higher lateral load and energy dissipation capacities than the corresponding values in their undamaged state, the effectiveness of the HCP for the repair of damaged interior beam-column joints is proved.Fundação para a Ciência e a Tecnologia (FCT

Preliminary assessment on the development of cost-competitive PVA-fiber reinforcement mortar

Relatório 10-DEC/E-0