An analytical model to simulate tension stiffening mechanism in moment-rotation response of FRC flexural members with tensile longitudinal reinforcement

Report 12-DEC/E-2

A model to simulate the moment-rotation and crack width of FRC members reinforced with longitudinal bars

The present work describes a model for the determination of the moment–rotation relationship of a cross section of fiber reinforced concrete (FRC) elements that also include longitudinal bars for the flexural reinforcement (R/FRC). Since a stress–crack width relationship (σ–w)(σ–w) is used to model the post-cracking behavior of a FRC, the σ–w directly obtained from tensile tests, or derived from inverse analysis applied to the results obtained in three-point notched beam bending tests, can be adopted in this approach. For a more realistic assessment of the crack opening, a bond stress versus slip relationship is assumed to simulate the bond between longitudinal bars and surrounding FRC. To simulate the compression behavior of the FRC, a shear friction model is adopted based on the physical interpretation of the post-peak compression softening behavior registered in experimental tests. By allowing the formation of a compressive FRC wedge delimited by shear band zones, the concept of concrete crushing failure mode in beams failing in bending is reinterpreted. By using the moment–rotation relationship, an algorithm was developed to determine the force–deflection response of statically determinate R/FRC elements. The model is described in detail and its good predictive performance is demonstrated by using available experimental data. Parametric studies were executed to evidence the influence of relevant parameters of the model on the serviceability and ultimate design conditions of R/FRC elements failing in bending.This work is supported by FEDER funds through the Operational Programme for Competitiveness Factors – COMPETE and National Funds through FCT – Portuguese Foundation for Science and Technology under the project PTDC/ECM/105700/2008 – “DURCOST - Innovation in reinforcing systems for sustainable pre-fabricated structures of higher durability and enhanced structural performance”. The second and third author wish to acknowledge the grant provided by this project and FCT (SFRH/BD/71934/2010), respectively

Design-oriented approach for strain-softening and strain-hardening fibre hybrid reinforced concrete elements failing in bending

In the present paper a design oriented model is proposed to evaluate the flexural resistance of elements of fibre reinforced concrete (FRC) of tensile strain-softening or tensile strain-hardening behaviour and strengthened longitudinally by steel bars in the tensile zone of a rectangular cross section. The cross sectional moment-curvature response predicted by this model is used in a numerical approach to predict the load-deflection relationship of beams failing in bending. To appraise the predictive performance of this approach, the results obtained in an experimental program with shallow beams of steel fibre reinforced self-compacting concrete and strengthened with distinct reinforcement ratios are compared to those estimated by the developed model. The predictive performance of the model is quite satisfactory, taking into account the simplified approaches adopted in order to have a closed form solution that can be used in the scope of design FRC elements failing in bending.Quadro de Referência Estratégico Nacional (QREN) - 345

A design-based approach to estimate the moment-curvature relationship of fiber reinforced elements failing in bending

Relatório A0.T0.UM.

A design model for fibre reinforced concrete bending elements with longitudinal pre-stressed steel and FRP bars

A close form solution to calculate the moment-curvature and load-deflection response of strain-softening and strain-hardening fibre reinforced concrete (FRC) elements failing in bending and reinforced longitudinally with pre-stressed steel and fibre reinforce polymer (FRP) bars is presented. This hybrid reinforcement is used for the development of high durable pre-fabricated and cost competitive beams. Pre-stressed FRP bars are applied with the minimum concrete cover, in order to take into account the benefits derived from the relatively high tensile strength of these bars and their immunity to corrosion. Pre-stressed steel bars, with a larger concrete cover have the purpose of providing the necessary ductility and assure the resistance of the beam in case of a fire occurrence. To replace completely the steel stirrups, a high performance fibre reinforced concrete is used. The predictive performance of the model is assessed by taking advantage of FEMIX, a FEM-based computer program. The model is finally utilized in a parametric study in order to evaluate the impact of post-cracking performance of FRC and applied pre-stress percentage on structural performance of FRC beams

Evaluation of the influence of post-cracking response of steel fibre reinforced concrete (SFRC) on load carrying capacity of SFRC panels

To develop a reliable methodology for the design of steel fibre reinforced concrete (SFRC) slabs, an extensive experimental program was carried out with SFRC square panels simply supported in their contour. By adopting a moment-rotation approach, a numerical model was developed capable of taking into account the constitutive laws of the SFRC for the prediction of the force-deflection response of variety of panel tests recommended in the international standards. The predictive performance of the model was assessed by considering results available in the bibliography and those obtained on the experimental program. The proposed model was utilized in a parametric study to assess the influence of toughness classes of SFRC on the behaviour at serviceability limit conditions, on the load carrying capacity, and on the deformational response of SFRC round panels.This work is supported by the FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project SlabSys-HFRC-PTDC/ECM/120394/2010. The first author acknowledges the FCT PhD Grant SFRH/BD/71934/2010. The authors would like to acknowledge the materials supplied by Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), OmyaComital (limestone filler), and Pegop (Fly ash). Special thanks for CiviTest Company that developed the SFRCs and executed the specimens of the experimental program.Fundação para a Ciência e a Tecnologia (FCT

A design model for strain-softening and strain-hardening fiber reinforced elements reinforced longitudinally with steel and FRP bars

A close form solution is developed for the prediction of the moment-curvature relationship of cross sections of fiber reinforced concrete (FRC) elements failing in bending, and reinforced longitudinally with steel and fiber reinforced polymer (FRP) bars. The FRP bars are installed with the largest possible internal arm, e.g. with the minimum concrete cover that assures the bond conditions for a sound stress transfer from FRC to the FRP bars. The model is also able of simulating the flexural strengthening contribution provided by FRP bars installed according to the Near Surface Mounted (NSM) technique. To have good protection conditions against corrosion, the steel bars are applied with a relatively thick FRC cover. Since steel stirrups are the reinforcement with the smaller concrete cover thickness, they are the most susceptible to corrosion. In the reinforcement concept to be developed in the present research program, steel stirrups are replaced with discrete fibers. This hybrid reinforcement aims to develop high durable pre-fabricated elements that fail in bending. The proposed analytical formulation can simulate FRC with strain softening or strain hardening behavior. In the present work, the formulation is described and its predictive performance is appraised.The study reported in this paper is part of the research programs "DURCOST", PTDC/ECM/105700/2008, supported by FCT, and "PONTALUMIS", QREN, Project No. 3456. The first and third authors wish to acknowledge the support provided by project PONTALUMIS, while the second author acknowledges the support of DURCOST

Integrated approach for the prediction of crack width and spacing in flexural FRC members with hybrid reinforcement

In this paper, a new model is developed based on the moment-rotation approach to predict average crack width and average crack spacing of flexural elements made of fibre reinforced concrete (FRC) that also include longitudinal steel and/or fibre reinforced polymer (FRP) bars. The post-cracking behaviour of FRC is simulated by a stress-crack width relationship, while the interaction between concrete and longitudinal reinforcement is modelled by a multilinear shear stress-sliding diagram based on experimental evidence. For assessing the predictive performance of the developed model, an experimental program was executed with this type of structural elements, where the moment versus average crack width and crack spacing were recorded. The good predictive performance of the model was also demonstrated by using experimental results available in the literature. The predictive performance was, in general, better than the predictions from RILEM TC 162-TDF and fib Model Code 2010project ICOSytec, project number 027990, Announcement 02/SAICT/2017, financed by FCT (Protuguese Foundation for Science and Technology) and co-funded by FEDER through Operational Competitiveness and Internationalization Programme (POCI

Comparison of the value of multidetector-row computed tomography in diagnosis of biliary obstruction reasons with the results obtained from invasive procedures

زمینه و هدف: تصویر برداری تشخیصی نقش اساسی در تشخیص غیر تهاجمی علل انسداد صفراوی و غربالگری بیماران با ریسک بالا را دارا می باشد. این مطالعه با هدف مقایسه ارزش سی تی اسکن مولتی اسلایس در تشخیص علل انسداد صفراوی با یافته های حاصل از روش های تهاجمی انجام شده است. روش بررسی: در این مطالعه توصیفی ـ تحلیلی به روش سرشماری، کلیه بیمارانی که از بهمن ماه 1388 تا خرداد ماه 1390، (50 بیمار)، دارای تشخیص انسداد صفراوی، بر مبنای یافته های کلینیکی و سونوگرافیک بودند، با روش سی تی اسکن مولتی اسلایس (MDCT) بررسی شدند و گزارش رادیولوژی آن ها با یافته های حاصل از سایر روش های تهاجمی شامل تکنیک کلانژیوگرافی معکوس از طریق آندوسکوپ (ERCP)،PTC (Percutaneous transhepatic cholangiography) و یا جراحی مورد مقایسه قرار گرفت. یافته ها: یافته ها نشان داد که حساسیت، ویژگی و دقت سی تی اسکن مولتی اسلایس در تشخیص علل بدخیم انسداد صفراوی به ترتیب 100، 4/88 و 94، در تشخیص علل خوش خیم انسداد صفراوی به ترتیب 75، 100 و 96، در تشخیص سنگ صفراوی به ترتیب 4/94، 100 و 98 بود و رابطه معنی داری بین شدت اتساع مجاری صفراوی و خوش خیم بودن یا بدخیم بودن عامل انسدادی وجود داشت (001/0>P). نتیجه گیری: بر مبنای یافته های این مطالعه، سی تی اسکن مولتی اسلایس روشی سریع ،غیر تهاجمی و با دقت بالا در تشخیص علل انسداد صفراوی و افتراق عوامل انسدادی خوش خیم از عوامل بدخیم بوده و برنامه ریزی قبل از عمل جراحی مفید می باشد