The basis for ductility evaluation in SFRC structures in MC2020: An investigation on slabs and shallow beams

The paper presents a synthesis of an extensive experimental campaign on linear and two-dimensional steel fiber reinforced concrete (SFRC) structural elements carried out to check the ductility requirements aimed at guaranteeing limit analysis approaches for the computation of ultimate load-bearing capacity of SFRC structures; special attention is devoted to the role of the degree of redundancy of the structure. In particular, full-scale shallow beams and slabs reinforced with steel fibers (with or without conventional longitudinal reinforcement) were tested in two different laboratories: the Politecnico di Milano (PoliMI) and the University of Brescia (UniBS). In this experimental campaign, two different fiber contents and fiber types were considered. The experimental investigation, carried out within the activities to support Annex L of Eurocode 2, was fundamental also for developing the design rules included in the fib Model Code 2020 and allowed to formulate conclusions regarding optimization of the mix design, ductility, and design prediction at the ultimate capacity

Effects of pitting corrosion on the mechanical behavior of Tempcore steel bars

La corrosione delle barre di armatura è uno dei principali meccanismi di degrado delle strutture in calcestruzzo armato (CA) e la valutazione del comportamento meccanico di strutture già soggette alla corrosione è diventato un compito di primaria importanza. Tali strutture vengono generalmente costruite con barre Tempcore, caratterizzate da una microstruttura composta da tre costituenti strutturali: un anello esterno di martensite, la zona di transizione in bainite e il nocciolo centrale in Perlite/Ferrite. Questo articolo studia il comportamento meccanico di questa tipologia di barre soggette a corrosione. Sono infatti state studiate delle barre estratte da dei provini fessurati in calcestruzzo armato che sono stati sottoposti a cicli di asciutto-bagnato (con cloruro di sodio al 5%) per 280 giorni. Questo processo di corrosione ha portato ad avere una profondità di pit che ha interessato sia lo strato di martensite che la zona di transizione di bainite

Morphological and mechanical characterization of reinforcement in cracked elements exposed to chloride-induced corrosion

In Reinforced Concrete (RC) structures exposed to chloride-rich environments, reinforcement corrosion might be already initiated (especially at crack location). A deep knowledge on the correlation between the mechanical properties of corroded bars and corrosive attack morphology, useful for the evaluation of the residual bearing capacity of structure, is still lacking. In this paper, morphological and mechanical characterization of steel reinforcement embedded in RC specimens (with and without fibers) both in cracked stage and subjected to cyclic exposure to a 5 % NaCl solution for 280 days was carried out. Although a chloride content higher than the critical chloride threshold was also measured in correspondence of uncracked concrete, corrosive attacks were observed only in correspondence of cracks. Pits were mostly characterized by a shallow morphology, which led to a rebar diameter loss ranging between 1 and 10 %. Furthermore, the results showed that both yielding and ultimate load of corroded rebars can be predicted by combining circular residual cross-section and mechanical properties of uncorroded rebars

Influence of fiber orientation and structural-integrity reinforcement on the flexural behavior of elevated slabs

In the last decades, research works regarding the behavior of Fiber-Reinforced Concrete (FRC) showed that the use of fibers enhances the mechanical behavior of Reinforced Concrete (RC) structures both at Serviceability Limit State (SLS) and Ultimate Limit State (ULS). It is also well recognized that the design of these elements should take into consideration fiber orientation since the residual properties of the material can be different depending on the fiber inclination with respect to the cracking plane. This article presents the results of an experimental program aimed at evaluating the influence of fiber orientation on the flexural response of elevated slabs. Both Vibrated Fiber-Reinforced Concrete (VFRC) and Self-Compacting Fiber-Reinforced Concrete (SCFRC) were analyzed. Seven elevated slabs (250 × 250 × 15 cm) were cast following the most common casting process adopted in practice by using VFRC (2 specimens), SCFRC (4 specimens) and plain concrete (1 specimen). The post-cracking mechanical properties evaluated by three-point bending tests on 192 notched beams sawn from three FRC slabs showed that, for a given fiber type and amount, the fiber orientation in slabs was comparable varying both concrete workability and casting procedure. The structural behavior of the remaining four slabs was studied both by experimental tests and numerical analyses. The slabs were simply supported at the corners and subjected to center-point loading. Results showed that the response in flexure was related to the average residual properties obtained from small beams sawn from slabs. Finally, the importance of having a structural-integrity reinforcement in FRC elevated slabs under flexure was also underlined by the numerical study

A practical approach for monitoring reinforcement corrosion in steel fiber reinforced concrete elements exposed to chloride rich environments

The use of fibers in Reinforced Concrete (RC) elements changes their cracking pattern, leading to narrower and more closely spaced cracks. In addition, the presence of fibers can improve the steel-to-concrete bond behavior reducing the steel-to-concrete interface damage after cracking. Cracks and steel-to-concrete interface damage work like paths, for aggressive agents, to reach the rebar in cracking elements, reducing the initiation period of the corrosion process, and favoring the corrosion at the intersection between cracks and reinforcement. In this context, this article discusses an experimental program on tension ties mechanically cracked and exposed to a chloride-rich environment with the purpose to check and eventually adequate the typical electrochemical measurements, used in the case of RC elements, for Steel Fiber Reinforced Concrete (SFRC). Adjustments related to degradation morphology and interference of steel fibers are proposed and used on Fiber Reinforced Concrete (FRC) elements for monitoring the influence of cracks and fibers on corrosion propagation. Finally, these proposed modifications were checked and validated against the results obtained by monitoring SFRC beams in the cracked stage

Effects of fibre orientation on the behaviour of RC beams in vibrated and self-compacting concrete

Structural design of fibre-reinforced members should take into consideration fibre orientation since the residual properties of the material can be different as a function of the cracking plane position and inclination. Designers must be aware of the possible negative influence of fibre orientation and they may take advantages from its positive effect. Some international standards (i.e., fib Model Code 2010 and German standard DafStb) define a factor that should take into account the orientation effects. In this context, the present paper presents the results of an experimental program aimed at evaluating steel fibre orientation in RC beams. Three beams (0.15 m x 0.70 m x 4.10 m) were cast at Magnetti Building (Carvico, Italy) factory following the most common casting process adopted in practice by using vibrated and self-compacting concrete

The basis for ductility evaluation in SFRC structures in MC2020: An investigation on slabs and shallow beams

The paper presents a synthesis of an extensive experimental campaign on linear and two-dimensional steel fiber reinforced concrete (SFRC) structural elements carried out to check the ductility requirements aimed at guaranteeing limit analysis approaches for the computation of ultimate load-bearing capacity of SFRC structures; special attention is devoted to the role of the degree of redundancy of the structure. In particular, full-scale shallow beams and slabs reinforced with steel fibers (with or without conventional longitudinal reinforcement) were tested in two different laboratories: the Politecnico di Milano (PoliMI) and the University of Brescia (UniBS). In this experimental campaign, two different fiber contents and fiber types were considered. The experimental investigation, carried out within the activities to support Annex L of Eurocode 2, was fundamental also for developing the design rules included in the fib Model Code 2020 and allowed to formulate conclusions regarding optimization of the mix design, ductility, and design prediction at the ultimate capacity

Influence of fibre orientation on the structural response of FRC slabs

The structural design of elements reinforced by fibres requires to take into consideration fibre orientation, since the material residu-al properties can vary as a function of element zone and cracking plane inclination. Therefore, designers have to consider the influ-ence of fibre orientation, both for its negative and positive effects. In this context, the present paper presents the results of an ex-perimental program aimed at evaluating fibre distribution and orientation in FRC slabs, as well as design implications at both mate-rial and structural level. Four elevated slabs (2.50 m x 2.50 m x 0.15m) were casted at Magnetti Building factory following the most common casting process adopted in practice (in order to be as close as possible to real situations). Two slabs were cut either orthog-onal or parallel to casting direction in order to obtain small beams 15x15x55 cm, while the remaining two slabs were first subjected to flexural tests and then numerically studied. Both vibrated and self-compacting concrete reinforced by 50 kg/m3 of steel fibres were studied