The role of the structural characteristic length in FRC structures (original) (raw)
Related papers
Analysis of Flexural Behaviour of Reinforced FRC Members
In the present paper, the flexural behaviour and crack propagation in reinforced FRC members are analysed and discussed, this is also compared with conventional reinforced concrete members. Special attention is given to how the combined effect of fibre bridging and reinforcement bars influence the structural behaviour in the serviceability-and ultimate limit state. The effects of the fibre bridging are investigated by means of analytical models and finite element analyses, both based on non-linear fracture mechanics and uni-axial material properties.
Use of fiber reinforced concrete in bridges – Metrorrey Line 2 case study
Engineering Structures
An assessment concerning the structural applicability and performance of fiber reinforced concrete (FRC) is presented for different bridge elements and within a design framework. FRC as the main bearing material in structural members has evolved from low-demand applications to increasingly ones, where bending and shear are the main internal forces. In actual applications, this was reflected with initial slab-on-grade cases, through tunnels, and later moving towards elevated slabs. Past experiences show that FRC has notable features regarding ultimate capacity and serviceability performance (i.e., enhanced crack control). These capabilities allowed for optimizations such as material savings, reduction of intensive labor during construction, or extended durability. Considering FRC's enhancements from previous applications, a case study based on the Metrorrey Line 2 lighttrain viaduct (Mexico) is developed. The case study aims to assess the structural performance that FRC can deliver within bridge geometries, loads, and specific conditions. Two numerical models considering different transversal post-tensioning configurations are developed based on the reference structure. The use of these two numerical models aims to broaden the applicability of this study to most U-shaped light-train viaducts. The design is based on current and future standards and recommendations, being prEN1992-1-1:2021, EN1992-1-1:2004, and fib Model Code 2010. After the numerical models and structural analysis, different sectional analyses at ultimate and serviceability levels are carried out, considering both conventional and fiber reinforced concretes. From the sectional results, FRC can provide reductions to reinforcement quantities at ultimate load levels, which are tied to the initially required reinforcement ratio (in other words, linked to the internal forces existing in the element). When higher reinforcement ratios are necessary, FRC optimizations point toward serviceability limit states, especially on the crack width reduction and the potential to reduce or suppress any additional reinforcement due to crack limitations.
Structural Concrete, 2016
The paper focuses on the reliability of the design approach proposed in Model Code 2010 for the estimation of the ultimate capacity of fibre-reinforced concrete (FRC) elevated slabs on the basis of different tests for material characterisation. The fracture properties of the material are determined through three-point bending tests on notched beams, and through double edge wedge splitting (DEWS) tests carried out on cylinders cored in the full-size test structure. As a case study, an FRC elevated flat slab is considered, consisting of 9 bays (panels) of a 6×6 m² dimension and supported by sixteen circular concrete columns having a thickness of 0.2 m and a full-size of 18.3×18.3 m². The ultimate bearing capacity of the slab experimentally determined is compared with the design value predicted by means of a procedure based on limit
Analytical Prediction of Crack Width of FRC/RC Beams Under Short and Long Term Bending Condition
It is well known that fibers are effective in modifying the cracking pattern development of concrete structural element, causing an higher number of cracks and, consequently, lower crack spacing values and narrower crack widths compared to the matrix alone. This effect could be exploited to improve durability of Reinforced Concrete (RC) structures, especially of those exposed to aggressive environments. The analytical prediction of crack width and spacing in Fiber Reinforced Concrete (FRC) structural elements in bending is still an open issue. A crack width relationship for RC elements with fibers similar to those developed for classical RC structural members would be desirable for designers. The recent development of important technical design codes, such as RILEM TC 162 TDF and the new MC2010, embrace this idea. However further validation of these models by experimental results are still needed. On the other hand, the study of the influence of a sustained load on crack width in presence of the fiber reinforcement is a topic almost unexplored and important at the same time. In the present work, the cracking behaviour of full-scale concrete beams reinforced with both traditional steel bars and short fibers has been analyzed under short and long term flexural loading. A theoretical prediction of crack width and crack spacing was carried out according to different international design provisions. The analytical results are discussed and compared in order to highlight the differences between the models and to check the reliability of the theoretical predictions on the basis of the experimental data.
Engineering Structures, 2015
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 ( w ) is used to model the post-cracking behavior of a FRC, the -w directly obtained from tensile tests, or derived from inverse analysis with threepoint notched beam bending tests, can be adopted in this approach. For a more realistic assessment of the crack opening, a bond stress vs. slip relationship is assumed for the simulation of 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 ambiguous 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.
Fibre-reinforced concrete infibModel Code 2010: principles, models and test validation
Structural Concrete, 2013
In the fib Model Code for Concrete Structures 2010, fibre-reinforced concrete (FRC) is recognized as a new material for structures. This introduction will favour forthcoming structural applications because the need of adopting new design concepts and the lack of international building codes have significantly limited its use up to now. In the code, considerable effort has been devoted to introducing a material classification to standardize performance-based production and stimulate an open market for every kind of fibre, favouring the rise of a new technological player: the composite producer. Starting from standard classification, the simple constitutive models introduced allow the designer to identify effective constitutive laws for design, trying to take into account the major contribution in terms of performance and providing good orientation for structural uses. Basic new concepts such as structural characteristic length and new factors related to fibre distribution and structural redistribution benefits are taken into account. A few examples of structural design starting from the constitutive laws identified are briefly shown. FRC can be regarded as a special concrete characterized by a certain toughness after cracking. For this reason, the most important constitutive law introduced is the stress-crack opening response in uniaxial tension. A wide discussion of the constitutive models introduced to describe this behaviour, which controls all the main contributions of fibres for a prevailing mode I crack propagation, is proposed. The validity of the models is discussed with reference to ordinary cross-sections as well as thin-walled elements by adopting plane section or finite element models.
Study of Dimensioning Aspects of FRC Based on the Beam Flexion Theory
RILEM Bookseries, 2020
The fiber reinforced concrete (FRC) have a higher load capacity in the post-cracking stage, however, this increase only occurs if the concrete is dosed and applied properly and, for this, there are standards that establish design aspects for use of the FRC. However, in these documents, the portion of the resulting resistant strength of the fibers is not fully described in their equations and is determined for specific situations. Therefore, and with the objective of obtaining the positioning and the resultant of the fibers of a model of fluid concrete reinforced with fibers, this work presents a study related to the determination of the resultant of the fibers in the stretched part of the concrete. This determination was obtained based on the bending theory in beams, through the three-point bending test standardized by EN 14651 (2007) [8], where both beams with steel fibers and polymeric fibers were used. From this, the equation obtained analytically was compared with those determined by international codes and it was realized that, when comparing the three methods used, the intermediate value corresponded to the determined analytically, where the results showed variations due to the different forms of arrangement that each method uses.
On the evaluation of the structural redistribution factor in FRC design: a yield line approach
Materials and Structures, 2016
Fibre-reinforced concrete (FRC) is a material that can be characterized by a high standard deviation in the post-peak tensile region. As consequence, structures made of FRC show a too safe prediction of the maximum bearing capacity when derived from characteristic values identified by means of small standard specimens. The Model Code 2010 has introduced a coefficient, named structural redistribution factor, that is able to take into account a reduced variability of the structural response when compared to that of material. A simplified procedure to provide an upper bound estimation of the structural redistribution factor based on the yield line method and able to take into account the material heterogeneity is presented. As case studies, a FRC full-scale elevated flat slab, a slab on ground and a full-scale beam are considered. Keywords Fibre-reinforced concrete (FRC) Á Structural redundancy Á Material heterogeneity Á Redistribution coefficient Á Yield line Abbreviations List of symbols CMOD Crack mouth opening displacement COD Crack opening displacement COV Coefficient of variation CTOD Crack tip opening displacement FRC Fibre reinforced concrete d f Fibre diameter E Young's modulus f c Concrete compressive strength f eq1 Average flexural nominal strength in the CTOD range between 0 and 0.6 mm f eq2 Average flexural nominal strength in the CTOD range between 0.6 and 3 mm f R1 Residual flexural nominal strength of FRC corresponding to CMOD = 0.5 mm f R3 Residual flexural nominal strength of FRC corresponding to CMOD = 2.5 mm f Fts FRC serviceability uniaxial tensile residual strength f Ftu FRC ultimate uniaxial tensile residual strength f If Flexural nominal stress corresponding to a CTOD equal to 25 lm f L Limit of proportionality k Subgrade modulus K Fibre orientation factor K Rd Redistribution factor K MC Rd
PERFORMANCE OF FRC AT DIFFERENT LEVELS OF CONCRETE STRENGTH
IAEME, 2019
The high performance, arresting crack propagation, improvement ductility, toughness and residual strength after first crack formation and avoiding the growth of micro cracks has been gained fiber reinforced concrete (FRC) its popularity in the last decades. These improvement properties may be affected by concrete strength level. This paper investigated experimentally the effect of concrete strength and steel fiber volume fraction (V f %) on the mechanical properties, strength relations and flexural energy of FRC. Three concrete strength levels representing normal strength concrete (NSC), moderate strength concrete (MSC) and high strength concrete (HSC) were designed. Hook-ends steel fibers (SFs) of length 35 mm and fiber aspect ratio of 43.75 were used by volume fractions of 0, 0.5%, 1.0% and 1.5%. The experimental results showed that the addition of SFs to the three types of concretes caused significant improvement on their mechanical properties involving compressive, indirect tensile and flexural strengths by different percentages. The maximum enhancement was recorded for NSC and the minimum was for HSC. The strengths relations were estimated and correlated. The addition of SFs to different types of concretes showed also a noticeable improvement in their flexural energy.
Flexural Modeling of Steel Fiber-Reinforced Concrete Members: Analytical Investigations
This paper presents an analytical model for the determination of the ultimate flexural capacity of steel fiber-reinforced (SFR) concrete rectangular sections and their associated crack width using the principles of strain compatibility and force equilibrium. The proposed model considers an elastic perfect-plastic model for compression and an elastic constant postpeak response of SFR concrete in tension. Unlike other flexural models available in the literature, the proposed model considers in the analysis the random distribution and orientation of steel fibers at a cracked section. The model was verified using existing experimental results, and it fairly predicted the flexural capacity of the SFR concrete beams. A parametric study using the proposed model was conducted to assess the effect of the fiber parameters and the strength of the concrete on the ultimate flexural capacity of a section and its associated crack widths. A normalized design chart is also presented to simplify the analysis procedure, and this can be used in the selection of fiber parameters (volume fraction and aspect ratio) during flexural design of SFR concrete members.