An investigation on the application of hybrid glass fibre reinforced polymer (GFRP) and steel bars bars as longitudinal reinforcement for simple and continuous concrete beams is presented. Three simply and eleven multi-spans continuous reinforced concrete beams were constructed and tested to failure. Nine continuous and two simply supported beams were reinforced with a hybrid combination of both GFRP and steel re-bars at mid spans and internal support regions. In addition, two continuous concrete beams reinforced with either GFRP or steel bars and one simply supported beam reinforced with GFRP bars were tested as control beams. The beams were classified into two groups according to the reinforcement configurations. All specimens tested were 200 mm in width and 300 mm in depth. The continuous beams comprised of two equal spans, each of 2600 mm, while the simply supported beams had a span of 2600 mm. Unlike GFRP reinforced concrete beams, the hybrid and steel reinforced concrete beams failed in a favourable ductile manner and demonstrated narrow cracks and smaller deflections compared to the GFRP-reinforced control beam. The lower stiffness and higher deflection of GFRP reinforced concrete beams can be controlled and improved by the use of steel reinforcement in combination with GFRP re-bars. However, the ratio of GFRP to steel reinforcement is a key factor to ensure sufficient ductility and stiffness beyond the first cracking stage. The experimental results showed that the extent of moment redistribution in hybrid reinforced continuous beams depends mainly on the amount of hybrid reinforcement ratio in critical sections. Similar area of steel and GFRP bars in critical sections leads to limited moment redistribution whereas different amount of steel and FRP bars in critical sections leads to a remarkable moment redistribution. Design guidelines and formulas have been validated against experimental results of hybrid GFRP/steel reinforced concrete beams tested. The Yoon’s equation reasonably predicted the deflections of the hybrid beams tested whereas Qu’s model which is based on ACI 440.1R-15 underestimated the deflections of hybrid beams tested at all stage of loading after cracking. The ACI 440.2R-08 and Pang et al., (2015) equations reasonably predicted the sagging failure moment in most continuous hybrid reinforced concrete beams, whereas they underestimated the hogging flexural strength at failure of most hybrid continuous beams. On the other hand, the formulas proposed by Yinghao et al., (2013) was very conservative in predicting the failure moment at the critical sagging and hogging sections. On the analytical side, a numerical technique consisting of sectional analyses has been developed to predict the moment–curvature relationship and moment capacity of hybrid FRP/ steel reinforced concrete members. The numerical technique has been validated against the experimental test results obtained from the current research and those reported in the literature. In addition, a two-dimensional nonlinear finite element model was proposed using ABAQUS package. The proposed model was validated against the experimental results of the beams tested in the present research. / Higher Education Institute in the Libyan Government

No / This paper presents a numerical method for estimating the curvature, deflection and moment capacity of hybrid FRP/steel reinforced concrete beams. A sectional analysis is first carried out to predict the moment-curvature relationship from which beam deflection and moment capacity are then calculated. Based on the amount of FRP bars, different failure modes were identified, namely tensile rupture of FRP bars and concrete crushing before or after yielding of steel reinforcement. Comparisons between theoretical and experimental results of tests conducted elsewhere show that the proposed numerical technique can accurately predict moment capacity, curvature and deflection of hybrid FRP/steel reinforced concrete beams. The numerical results also indicated that beam ductility and stiffness are improved when steel reinforcement is added to FRP reinforced concrete beams. (C) 2015 Elsevier Ltd. All rights reserved,

The threat of terrorist bombings and accidental industrial explosions motivates the need for more economical and efficient blast-resistant construction techniques that offer enhanced levels of protection at reduced component damage levels. Despite having a high strength-to-weight ratio and being chemically inert, fiber reinforced polymer (FRP) reinforcing bars are not currently used in blast-resistant reinforced concrete due to their brittle nature and lack of ductility. However, the innovative use of blended mixtures of FRP and steel rebar as tensile reinforcement promises to address these limitations through self-centering behavior that provides reductions in residual damage and enhancements in flexural performance. This thesis presents the results of an experimental and analytical investigation on the effect of hybrid arrangements of glass fiber reinforced polymer (GFRP) and conventional mild steel reinforcement on the blast performance of reinforced concrete beams. Seven large-scale reinforced concrete beams with different combinations of tensile steel and GFRP rebar were designed, constructed, and tested under progressively increasing blast loading generated using the Virginia Tech Shock Tube Research Facility. The effect of hybrid reinforcing on the blast performance of the beams was evaluated based on the global response, failure mode, damage pattern, mid-span displacement, and support reactions of the tested beams. The results demonstrated several benefits in using hybrid arrangements of steel and GFRP reinforcement. Beams with hybrid reinforcing experienced reduced overall residual displacements compared with similar conventionally reinforced concrete members. This was attributed to the elastic nature of GFRP rebar which was found to produce a self-centering behavior that assisted in returning the hybrid members to their original undeformed position. This permitted the hybrid beams to safely experience larger maximum displacements at substantially less damage than all-steel construction. Furthermore, if the GFRP reinforcement did rupture, the presence of steel arrested hazardous component failure and provided additional energy dissipation and redundancy. Accompanying the experimental tests was an inelastic single-degree-of-freedom analysis to predict the displacement time-history response of the beams. Reasonably good predictions of response were obtained when the advanced material models and the effects of accumulated damage due to repeated blast testing were incorporated into the analytical predictions. Finally, a series of protective design recommendations and a new proposed response limit, that describes the level of damage achieved after a blast event, were established to encourage use of hybrid GFRP/steel reinforcement in blast-resistant construction. / Master of Science / The threat of terrorist bombings and accidental industrial explosions motivate the need for new blast resistant construction techniques. Despite having a high strength-to-weight ratio and being chemically inert, fiber reinforced polymer (FRP) reinforcing bars are not currently used in blast-resistant reinforced concrete due to their brittle nature and lack of ductility. However, the innovative use of blended mixtures of FRP and steel rebar as tensile reinforcement promises to address these limitations through self-centering behavior that provides reductions in residual damage and enhancements in flexural performance. Large-scale reinforced concrete beams with different combinations of steel and GFRP rebar were designed, constructed, and tested under progressively increasing blast loads, gen-erated by the Virginia Tech Shock Tube Research Facility. The results demonstrated that beams with hybrid reinforcing experienced reduced overall residual damage in comparison with similar conventionally reinforced concrete members. Additionally, if the GFRP rebar ruptured, the presence of steel prevented a brittle failure and provided additional energy dissipation and redundancy. The inelastic single degree of freedom model developed for this investigation resulted in an adequate prediction of the load-deflection characteristics record-ed from experimental testing. To encourage the use of hybrid FRP/steel reinforcement in blast-resistant construction, a series of protective design recommendations and a proposed response limit, that describes the level of damage achieved after a given blast event, were established.

Fibras vegetais de baixo módulo de elasticidade são conhecidas pela sua capacidade de aumentar a energia absorvida durante o carregamento dos materiais cimentícios, especialmente no estágio pós-fissurado. A utilização de nanofibras celulósicas pode contribuir para a tenacificação de matrizes frágeis, tanto por melhorar o empacotamento das partículas, com o refinamento de poros, quanto pela interceptação de fissuras na escala nanométrica, com a respectiva absorção de energia. A celulose nanofibrilada provém de um recurso natural, abundante e renovável, possui bom desempenho mecânico e superfície específica elevada, o que contribui para melhorar a adesão entre as partículas de cimento. Estes fatores justificam o uso da celulose nanofibrilada e a tornam uma boa alternativa como nanoreforço de materiais cimentícios. Com isso, o objetivo deste trabalho foi a produção de celulose nanofibrilada a partir de polpa organossolve de bambu, definindo a melhor condição para sua produção e posterior utilização como reforço em compósitos híbridos (reforçados na nano e micro escalas) em comparação a compósitos reforçados somente com microfibras (polpa) pelos processos de produção por sucção e prensagem, e extrusão. A celulose nanofibrilada foi produzida utilizando-se polpa não-branqueada e branqueada, por meio de 5, 10, 15 e 20 ciclos de nanofibrilação pelo processo grinding. Foram realizados testes químicos, físicos e mecânicos para definição da condição ótima de nanofibrilação. A celulose nanofibrilada não-branqueada produzida mediante 10 ciclos foi definida como a melhor opção para utilização nos compósitos híbridos, por possuírem maior módulo de elasticidade e, em razão da sua maior estabilidade estrutural, apresentam maior resistência à degradação em meio alcalino. Os compósitos foram submetidos à cura por carbonatação acelerada para mitigação da degradação da fibra pela diminuição do pH da matriz e também para refinamento dos poros. Os compósitos foram submetidos ao teste de envelhecimento acelerado por meio de 200 ciclos de imersão e secagem para análise da sua degradação. Os compósitos híbridos e reforçados somente com polpa aos 28 dias de cura e após o envelhecimento acelerado foram submetidos aos ensaios físicos, mecânicos e microestruturais para acompanhamento do efeito da celulose nanofibrilada nas suas propriedades. Nos compósitos produzidos pelos dois processos aos 28 dias não houve diferença estatística para as propriedades físicas testadas, comparando-se os compósitos híbridos e os reforçados somente com polpa. No processo de sucção e prensagem, embora útil para ajustes na formulação e na cura do compósito híbrido, não se percebeu contribuição estatisticamente significativa da celulose nanofibrilada na formação de pontes de transferência de tensões, e, portanto sem o correspondente aumento na resistência mecânica dos compósitos. Nos compósitos extrudados, a celulose nanofibrilada atuou de modo a melhorar o comportamento mecânico do compósito híbrido em comparação ao compósito sem nanofibras. Esta melhoria pode estar associada à maior adesão entre as nanofibrilas e a matriz cimentícia, o que foi atestado pela análise microestrutural (MEV) dos compósitos. Após o envelhecimento acelerado os compósitos com e sem nanofibras produzidos pelos dois processos não apresentaram redução do desempenho mecânico, o que se atribui à menor alcalinidade provida pela carbonatação acelerada. / Low elastic modulus vegetable fibers are known for their ability to increase the energy absorbed by cement based materials while they are loaded, especially during the post-crack stage. The use of cellulose nanofibers may contribute for toughening of brittle matrices and improving particle packing by both pore refining and crack intercepting at nanoscale, with the corresponding energy absorption. Nanofibrillated cellulose comes from a natural, abundant and renewable resources, it has good mechanical peformance and high specific surface, which contributes to improve the adhesion between the cement particles. These factors justify the use of nanofibrillated cellulose and give rise to an alternative nanoreinforcement for cement based materials. Thus, the aim of this work was the production of the nanofibrillated cellulose from bamboo organosolv pulp, establishing the best condition for its production and subsequent use as reinforcement in hybrid composites (both nano and micro-scale reinforcement) compared to composites reinforced with only microfibers (pulp), produced by the slurry vacuum dewatering followed by pressing and extrusion methods. The nanofibrillated cellulose was produced submitting unbleached and bleached pulps to 5, 10, 15 and 20 nanofibrillated cycles by the grinding method. Chemical, physical and mechanical tests were carried out to define the optimal condition to nanofibrillation. The unbleached nanofibrillated cellulose produced by 10 cycles was defined as the best option to be used in hybrid composites, since their greater modulus of elasticity and, because of their greater structural chemical stability, higher resistance to degradation in alkaline environments. The composites were subjected to accelerated carbonation curing process to mitigate thedegradation of fiber by reducing the matrix pH and also to refine the pores. The composites were subjected to accelerated aging process by means of 200 wet and dry cycles to assess their degradation. The hybrid composites and the composites reinforced only with pulp at 28 days and after accelerated aging were subjected to physico-mechanical and microstructural tests to study the effect of the nanofibrillated cellulose on their properties. There was no difference in the physical properties of the hybrid composites and composites reinforced with only pulp, produced by the two processes at 28 days. For the slurry vacuum dewatering followed by pressing process, although useful for adjustments in the formulation and cure hybrid composite, there was no statistically significant contribution of the nanofibrillated cellulose in the formation of stress transfer bridges, and therefore without a corresponding increase in the mechanical strength of the composites. For the extruded composites, the nanofibrillated cellulose improved the mechanical behavior of the hybrid composite compared to the composite without nanofiber. This improvement may be associated with greater adherence between the nanofibrils and the cement matrix, which was confirmed by microstructural analysis (SEM) of the composites. After accelerated aging, the composites with and without nanofibers produced by the two processes showed no reduction in mechanical performance, which is attributed to the lower alkalinity provided by the accelerated carbonation.

Fibras vegetais de baixo módulo de elasticidade são conhecidas pela sua capacidade de aumentar a energia absorvida durante o carregamento dos materiais cimentícios, especialmente no estágio pós-fissurado. A utilização de nanofibras celulósicas pode contribuir para a tenacificação de matrizes frágeis, tanto por melhorar o empacotamento das partículas, com o refinamento de poros, quanto pela interceptação de fissuras na escala nanométrica, com a respectiva absorção de energia. A celulose nanofibrilada provém de um recurso natural, abundante e renovável, possui bom desempenho mecânico e superfície específica elevada, o que contribui para melhorar a adesão entre as partículas de cimento. Estes fatores justificam o uso da celulose nanofibrilada e a tornam uma boa alternativa como nanoreforço de materiais cimentícios. Com isso, o objetivo deste trabalho foi a produção de celulose nanofibrilada a partir de polpa organossolve de bambu, definindo a melhor condição para sua produção e posterior utilização como reforço em compósitos híbridos (reforçados na nano e micro escalas) em comparação a compósitos reforçados somente com microfibras (polpa) pelos processos de produção por sucção e prensagem, e extrusão. A celulose nanofibrilada foi produzida utilizando-se polpa não-branqueada e branqueada, por meio de 5, 10, 15 e 20 ciclos de nanofibrilação pelo processo grinding. Foram realizados testes químicos, físicos e mecânicos para definição da condição ótima de nanofibrilação. A celulose nanofibrilada não-branqueada produzida mediante 10 ciclos foi definida como a melhor opção para utilização nos compósitos híbridos, por possuírem maior módulo de elasticidade e, em razão da sua maior estabilidade estrutural, apresentam maior resistência à degradação em meio alcalino. Os compósitos foram submetidos à cura por carbonatação acelerada para mitigação da degradação da fibra pela diminuição do pH da matriz e também para refinamento dos poros. Os compósitos foram submetidos ao teste de envelhecimento acelerado por meio de 200 ciclos de imersão e secagem para análise da sua degradação. Os compósitos híbridos e reforçados somente com polpa aos 28 dias de cura e após o envelhecimento acelerado foram submetidos aos ensaios físicos, mecânicos e microestruturais para acompanhamento do efeito da celulose nanofibrilada nas suas propriedades. Nos compósitos produzidos pelos dois processos aos 28 dias não houve diferença estatística para as propriedades físicas testadas, comparando-se os compósitos híbridos e os reforçados somente com polpa. No processo de sucção e prensagem, embora útil para ajustes na formulação e na cura do compósito híbrido, não se percebeu contribuição estatisticamente significativa da celulose nanofibrilada na formação de pontes de transferência de tensões, e, portanto sem o correspondente aumento na resistência mecânica dos compósitos. Nos compósitos extrudados, a celulose nanofibrilada atuou de modo a melhorar o comportamento mecânico do compósito híbrido em comparação ao compósito sem nanofibras. Esta melhoria pode estar associada à maior adesão entre as nanofibrilas e a matriz cimentícia, o que foi atestado pela análise microestrutural (MEV) dos compósitos. Após o envelhecimento acelerado os compósitos com e sem nanofibras produzidos pelos dois processos não apresentaram redução do desempenho mecânico, o que se atribui à menor alcalinidade provida pela carbonatação acelerada. / Low elastic modulus vegetable fibers are known for their ability to increase the energy absorbed by cement based materials while they are loaded, especially during the post-crack stage. The use of cellulose nanofibers may contribute for toughening of brittle matrices and improving particle packing by both pore refining and crack intercepting at nanoscale, with the corresponding energy absorption. Nanofibrillated cellulose comes from a natural, abundant and renewable resources, it has good mechanical peformance and high specific surface, which contributes to improve the adhesion between the cement particles. These factors justify the use of nanofibrillated cellulose and give rise to an alternative nanoreinforcement for cement based materials. Thus, the aim of this work was the production of the nanofibrillated cellulose from bamboo organosolv pulp, establishing the best condition for its production and subsequent use as reinforcement in hybrid composites (both nano and micro-scale reinforcement) compared to composites reinforced with only microfibers (pulp), produced by the slurry vacuum dewatering followed by pressing and extrusion methods. The nanofibrillated cellulose was produced submitting unbleached and bleached pulps to 5, 10, 15 and 20 nanofibrillated cycles by the grinding method. Chemical, physical and mechanical tests were carried out to define the optimal condition to nanofibrillation. The unbleached nanofibrillated cellulose produced by 10 cycles was defined as the best option to be used in hybrid composites, since their greater modulus of elasticity and, because of their greater structural chemical stability, higher resistance to degradation in alkaline environments. The composites were subjected to accelerated carbonation curing process to mitigate thedegradation of fiber by reducing the matrix pH and also to refine the pores. The composites were subjected to accelerated aging process by means of 200 wet and dry cycles to assess their degradation. The hybrid composites and the composites reinforced only with pulp at 28 days and after accelerated aging were subjected to physico-mechanical and microstructural tests to study the effect of the nanofibrillated cellulose on their properties. There was no difference in the physical properties of the hybrid composites and composites reinforced with only pulp, produced by the two processes at 28 days. For the slurry vacuum dewatering followed by pressing process, although useful for adjustments in the formulation and cure hybrid composite, there was no statistically significant contribution of the nanofibrillated cellulose in the formation of stress transfer bridges, and therefore without a corresponding increase in the mechanical strength of the composites. For the extruded composites, the nanofibrillated cellulose improved the mechanical behavior of the hybrid composite compared to the composite without nanofiber. This improvement may be associated with greater adherence between the nanofibrils and the cement matrix, which was confirmed by microstructural analysis (SEM) of the composites. After accelerated aging, the composites with and without nanofibers produced by the two processes showed no reduction in mechanical performance, which is attributed to the lower alkalinity provided by the accelerated carbonation.

Yes / This paper aims to investigate the flexural behavior of engineered cementitious composite (ECC)-concrete hybrid composite beams reinforced with fiber reinforced polymer (FRP) bars and steel bars. Thirty two hybrid reinforced composite beams having various ECC height replacement ratio and combinations of FRP and steel reinforcements were experimentally tested to failure in flexure. Test results showed that cracking, yield and ultimate moments as well as the stiffness of hybrid and ECC beams are improved compared with traditional concrete beams having the same reinforcement, owing to the excellent tensile properties of ECC materials. The average crack spacing and width decrease with the increase of ECC height replacement ratio. The ductility of hybrid reinforced composite beams is higher than that of traditional reinforced concrete beams while their practical reinforcement ratios are similar. Reinforced ECC beams show considerable energy dissipation capacity owing to ECC’s excellent deformation ability. Considering the constitutive models of materials, compatibility and equilibrium conditions, formulas for the prediction of cracking, yield and ultimate moments as well as deflections of hybrid reinforced ECC-concrete composite beams are developed. The proposed formulas are in good agreement with the experimental results. A comprehensive parametric analysis is, then, conducted to illustrate the effect of reinforcement, ECC and concrete properties on the moment capacity, curvature, ductility and energy dissipation of composite beams. / National Natural Science Foundation of China (51678514, 51308490), the Natural Science Foundation of Jiangsu Province, China (BK20130450), Six Talent Peaks Project of Jiangsu Province (JZ-038, 2016), Graduate Practice Innovation Project of Jiangsu Province (SJCX17-0625), the Jiangsu Government Scholarship for Overseas Studies and Top-level Talents Support Project of Yangzhou University