Although the use of Fibre Reinforced Concrete (FRC) for structural applications is continuously increasing, it is still limited with respect to its potentials, mainly due to the lack of International Building Codes for FRC structural elements. Within fib (Federation Internationale du Beton), the Special Activity Group 5 is preparing a New fib Model Code that aims to update the previous CEB-FIP Model Code 90, published in 1993, that can be considered as the reference document for Eurocode 2. The New Model Code includes several innovations and addresses among other topics, new materials for structural design. In this respect, FRC will be introduced. The Technical Groups fib TG 8.3 “Fibre reinforced concrete” and fib TG 8.6 “Ultra high performance FRC” are preparing some sections of the New Model Code, including regular and high performance FRC. This paper aims to briefly explain the main concepts behind the structural rules for FRC structural design.

Fibre reinforced concrete: new design perspectives

Governing the dispersion and the orientation of fibres in concrete through a suitably balanced set of fresh state properties and a carefully designed casting procedure, is a feasible and cost-effective way to achieve a superior mechanical performance of fibre reinforced cementitious composites, which may be required by the intended application, even keeping the fibre content at relatively low values (e.g. around 1% by volume). In this paper the possibility of pursuing the above said “integrated” approach has been addressed in the framework of larger project focused on developing a deflection-hardening FRCC (DHFRCC), reinforced with 100 kg/m3 (1.27% by volume) of short steel fibres (13 mm long and 0.16 mm in diameter). The material has to be employed to manufacture thin (30 mm) roof elements, without any kind of conventional reinforcement, which have been anticipated to work, as simply supported beams, over a 2.5 m span. The study hence paves the way to the possibility of exploiting at an industrial level the correlation among fresh state performance, fibre dispersion and hardened state properties of self consolidating steel fibre reinforced concrete to achieve enhanced structural performance tailored to the specific application.

High mechanical performance of fibre reinforced cementitious composites: the role of “casting-flow induced” fibre orientation

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.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/suco.201300021

Fibre‐reinforced concrete in fib Model Code 2010: principles, models and test validation

Finite element modeling of steel-polypropylene hybrid fiber reinforced concrete using modified concrete damaged plasticity

With the number of 3D printed concrete structures rapidly increasing, the demand for concepts that allow for robust and ductile printed objects becomes increasingly pressing. An obvious solution st...

https://tandfonline.com/doi/pdf/10.1080/17452759.2018.1548069

Ductility of 3D printed concrete reinforced with short straight steel fibers

The paper presents an experimental and a numerical investigation on precast, prestressed reinforced concrete (RC) and steel fibre reinforced concrete (SFRC) roof elements. The element investigated has a complex geometry, because it is characterized by a thin-walled open cross-section and a long span. In order to reduce the total weight of the traditional RC element and favour an industrialized production process, the structure can be made of fibre reinforced concrete. This composite presents a significant toughness after cracking that can substitute the diffused reinforcement made of common steel-welded meshes, conserving the longitudinal prestressed reinforcement. The mechanical characterization of SFRC material has found recently a shared design approach that starts with the identification of the uniaxial tension constitutive law obtained from a standardized bent notched specimen. Nevertheless, for defined casting procedures of the structure, like in prefabrication, the identification of the uniaxial tension constitutive law can be performed by a four point bending tests on suitable unnotched specimens, able to take into account the effective fibre orientation in the structure and the real nominal thickness of the critical portion of the element. The latter two different experimental test procedures (on notched or unnotched specimens) lead to significant differences in the tension softening response. For this reason SFRC tension softening relations, coming from the previously mentioned experimental tests, are analyzed in this paper in order to evaluate their effects on the structural response of this large-scale roof element. The results of the experimental tests on the roof element presented in this paper show that second-order effects drastically anticipated the achievement of the longitudinal bending moment resistance calculated following the beam theory and neglecting transverse equilibrium and in-plane cross section deformation. Two numerical models are proposed in this paper to evaluate second-order effects in the resistance assessment of the precast structure. The first one is based on a plane section approach (PSA), while the second one is based on a non-linear finite element analysis (NLFEA). Both second-order effect and uniaxial tension constitutive relationship roles are examined in relation to the global response of the structure up to failure. The final remarks, coming from a careful comparison between experimental and numerical results, highlight that the failure is mainly led by a structural behaviour, because second-order effects prevail on non-linear response of SFRC materials adopted.

On the fracture behaviour of thin-walled SFRC roof elements

Structures made of a material with a very high standard deviation, such as fibre-reinforced concrete, exhibit an exceptionally safe prediction of the maximum bearing capacity when this is derived from characteristic values identified by means of small specimens. This is emphasized when the structures are characterized by high redundancy. In this regard, two reference tests representing two extreme situations are considered: a) simply supported unnotched full-scale beams characterized by a statically determinate loading scheme and b) full-scale slabs on the ground characterized by a statically indeterminate loading scheme. The Italian standard and, more recently, the fib Model Code for Concrete Structures 2010 have introduced a coefficient (structural redistribution factor) that is able to take into account the reduced variability of mechanical bearing capacity when associated with a large volume involved in the failure process and/or when the structure is able to redistribute stresses significantly, thus favouring the average rather than the minimum strength. A numerical procedure taking into account the expected heterogeneity of the mechanical characteristics in the structure is introduced for the first time to evaluate the redistribution factor.

The structural redistribution coefficient KRd: a numerical approach to its evaluation

On the connections between fresh state behavior, fiber dispersion and toughness properties of steel fiber reinforced concrete

With the purpose to develop a fast procedure of intervention for stabilising earth slopes, a new retaining structure system taking advantage of prefabrication, post-tensioning technique, high-perfo...

Daniele Dozio

Papers

Fibre‐reinforced concrete in fib Model Code 2010: principles, models and test validation

On the fracture behaviour of thin-walled SFRC roof elements

The structural redistribution coefficient KRd: a numerical approach to its evaluation

On the connections between fresh state behavior, fiber dispersion and toughness properties of steel fiber reinforced concrete

Assessment and control of a SFRC retaining structure: Mechanical issues