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Self-compacting fibre reinforced concrete (SCFRC) is a useful material in precast industry, leading to a significant reduction in construction time and costs, and possibly even labour shortages. The addition of fibres enhances the behaviour of structures at both serviceability limit state (durability, cracking, deformation) and ultimate limit state (bearing capacity and ductility), allowing in many cases to replace, totally or partially, the web reinforcement. In this context, the present paper focuses on the use of macro-synthetic fibres as shear reinforcement in self-compacting concrete prestressed double tees. Experimental results of full-scale tests on six prestressed double tees made in self-compacting concrete and self-compacting polypropylene fibre reinforced concrete are presented. Firstly, shear tests were conducted in areas where the prestressing is uniformly distributed, in order to evaluate the possibility of replacing the minimum amount of shear reinforcement by macro-synthetic fibres only. Secondly, several tests were also carried out at the end zones, which are more critical in shear since the beneficial effects of prestressing is not totally active. Experimental results show that macro-synthetic fibres can be used as shear reinforcement both in the zone with uniform prestressing (minimum shear reinforcement) and in the end zones (shear reinforcement required by equilibrium). Finally, the experimental results obtained were compared against the predictions of three international codes: Eurocode 2, ACI318-11 and Model Code 2010. [ABSTRACT FROM AUTHOR]

The use of fiber reinforced concrete in tunnel linings, with or without conventional rebars, has increased in the two last decades, especially with regard to precast tunnel segmental linings. In addition, there is a general growing interest in the scientific community on macro-synthetic fibers for use in underground structures. Within this framework, the present study investigates the possibility of using polypropylene (PP) fiber reinforcement in hydraulic precast tunnel segments by means of an experimental program on six full-scale segments of Monte Lirio hydraulic tunnel (Panama). Three different reinforcement solutions were studied both under flexure and point load test: typical conventional reinforcement generally adopted in practice (reference samples, RC); PP fibers only (PFRC specimens); combination of PP fibers and conventional reinforcement (hybrid solution, RC + PFRC segments). Based on the loading configurations considered, experimental results showed that PP fibers (with or without reinforcing bars, depending on loading conditions) represent an efficient reinforcement for hydraulic precast tunnel segments. PP fibers can be used as flexural, splitting and minimum shear reinforcement, while concerning the spalling reinforcement, only PP fiber reinforcement could be used even if the combination with conventional reinforcement (hybrid solution) guarantees a better control of spalling cracks for high load levels. [ABSTRACT FROM AUTHOR]

The local splitting behaviour in Fibre Reinforced Concrete (FRC) elements under a high concentrated load is gaining a growing interest, especially with reference to precast tunnel segments. In fact, during the lining construction process, high concentrated thrust jack forces are introduced in the segments leading to both high compressive stresses under the thrust shoes that can provoke a concrete crushing and tensile transversal stresses that can cause cracks and, eventually, a splitting failure. In order to investigate the main involved mechanisms during either splitting or crushing failure, a simplified analytical study is firstly presented on the two load configurations generally adopted in practice, namely the Line Load (LL) and the Point Load (PL). Based on this analytical study, experimental tests on Steel Fibre Reinforced Concrete (SFRC) specimens subjected to a LL configuration have been carried out. Two different steel fibre types have been studied. Experimental results indicate that the use of a low amount of fibres significantly enhances both the splitting bearing capacity and the ductility. Fibres allow a stable propagation of the splitting crack both in terms of crack opening and development, as a result of a transverse stress redistribution that continuously provide equilibrium with the external applied load. [ABSTRACT FROM AUTHOR]

The design process of segmental concrete linings in ground conditions generally refers to standard load cases of de-molding, storage, embedded ground condition and grouting process. Nevertheless, the application of the Tunnel Boring Machine (TBM) thrust jack forces is a crucial temporary loading condition during construction, which may govern the design procedure as well as the other stages. Tunnel segments were traditionally reinforced with conventional rebars in order to resist the tensile stresses at both Serviceability (SLS) and Ultimate Limit States (ULS). In the two last decades, Fiber Reinforced Concrete (FRC) has been also used in several precast tunnel segments with or without conventional rebars. For structural purposes, steel fibers are generally used, even though some types of structural macro-synthetic fibers, which are able to impart significant toughness and ductility to concrete, have been recently introduced in the market. For these reasons, an experimental program aimed to investigate the local splitting behavior in the segment regions under the TBM hydraulic jacks was carried out at the University of Brescia. Tests on concrete prisms (with or without fibrous reinforcement) under Line Load (LL) or Point Load (PL) configurations were carried out in order to evaluate the beneficial effects of polypropylene (PP) fibers in controlling typical splitting cracks occurring under the jack loads, which represent one of the most severe loading conditions for tunnel segments. Experimental results show the feasibility of using polypropylene fibers in tunnel segments, since they significantly enhance both the splitting bearing capacity and the ductility. Fibers lead to a stable development of the splitting crack, which allows a redistribution of stresses after cracking. However, the effectiveness of fibers is influenced by the casting direction, which leads to a different fiber orientation. When the latter is favorable (fibers expected to be mainly transversal to the splitting crack) the bearing capacity is higher than in case of an unfavorable casting direction (fibers expected to be mainly parallel to the splitting crack). [ABSTRACT FROM AUTHOR]

Many experiments available in the literature show that fibres, when provided in sufficient amount to guarantee an adequate toughness to Fibre Reinforced Concrete (FRC), are significantly effective as shear reinforcement. However, most of the experiments relate to elements reinforced by steel fibres and very few present research on structures produced with other kinds of fibres, i.e. polypropylene (PP) fibres, which can present a series of advantages (durability and corrosion avoidance above all). Today PP fibres might be able to impart significant toughness and ductility to concrete. This paper reports the results of a recent experimental programme on reinforced concrete (without any shear reinforcement, PC) and Polypropylene Fibre Reinforced Concrete (PFRC) Wide-Shallow Beams (WSBs) subjected to shear, focusing on the role of the width-to-effective depth ratio ( b / d ) and on the beneficial influence of newly developed PP fibres. Fourteen WSBs with different widths and depths, fibre content and, also, minimum amount of classical shear reinforcement, were tested. Results show that PC WSBs having width-to-effective depth ratio from 2 to 3 reach a higher ultimate shear stress (compared to the corresponding not-wide PC beams), generally disregarded by most of standards. Moreover, the ability of PP fibres in promoting a substantially more stable shear behaviour was experimentally observed, as well as the possibility of using PP fibres as minimum shear reinforcement. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports an experimental campaign on Reinforced Concrete (RC) Wide-Shallow Beams (WSBs) with or without fibres, tested under shear and flexure. A wide-shallow beam is a rather frequent structure in residential buildings in Southern Europe (as in Italy and in Spain). In order to study both the shear and flexural behaviour of WSBs and evaluate the possibility of substituting the minimum conventional transverse reinforcement required by Eurocode 2 with steel fibres, full-scale beams have been tested. Specimens, all 250mm deep, had two different widths, fibre contents and also, minimum amount of classical shear reinforcement. Results evidenced that a relatively low volume fraction of fibres can significantly increase shear bearing capacity and beam ductility. Moreover, WSBs did not show the typical brittle failure in shear, even without any shear reinforcement, as the effect of fibres was more prominent than in deep beams. Peculiarities of WSBs were evidenced in terms of enhancements both in shear and in flexure. Experimental results have been evaluated in terms of strength, ductility, post-cracking stiffness, shear and flexural cracking, collapse mechanism and fibre effect. [Copyright &y& Elsevier]

Corrosion of rebars is one of the main issues affecting the service life of Reinforced Concrete (RC) structures. It is well-known that the use of fibers enhances the mechanical behavior of RC structures at both Serviceability Limit States (SLS) and Ultimate Limit States (ULS). In the former, fibers mainly influence the cracking pattern, leading to narrower and more closely spaced cracks. The crack width is a key parameter for controlling the durability of RC structures along with concrete quality and the thickness of the concrete cover. Even though many research studies have been carried out on this topic, the behavior of RC elements, with and without fibers, in both cracked stage and aggressive environments is still not well understood. This article describes an innovative test procedure specifically developed to evaluate the chloride-induced corrosion in RC elements with and without fibers in service condition (under tensile stress and in the cracked stage). Prismatic specimens 90 × 90 × 830 mm with Ø12 rebar (called hereafter tension ties) were subjected for 280 days both to a tensile load and to wet-dry cycles in a water solution containing 5% of NaCl. Experimental results indicate that this test procedure was suitable to assess the chloride-induced corrosion in tensile elements in the cracked stage. Furthermore, a positive influence of fibers was observed, while no relationship between crack width and pitting depth was found. An innovative test procedure to evaluate the chloride-induced corrosion in RC and FRC elements in service condition Evaluation of chloride-induced corrosion in tensile elements Positive influence of fibres on the reduction of the corrosion rate due to a better crack control [ABSTRACT FROM AUTHOR]

• A novel research on the use of polypropylene fibres as shear reinforcement of hollow-core slab end zones. • Successful use of polypropylene fibres for enhancing the shear strength of hollow-core slabs. • Shear tests on full-scale hollow-core slabs varying the shear span-to-effective depth ratio. Experimental and numerical research studies demonstrated that the addition of fibres in correct proportions, enhances the shear behaviour of Reinforced Concrete (RC) elements, allowing to totally or partially replace the conventional web reinforcement. However, despite of this increase of knowledge about Fibre Reinforced Concrete (FRC), there is still a gap in the applicability of fibres as a shear reinforcement in certain prestressed structural elements where the use of conventional transverse reinforcement is difficult due to their manufacturing process, e.g. extruded elements in dry concrete. In this context, the present paper evaluates the possibility of enhancing the shear strength of Hollow-Core Slabs (HCS) by using Polypropylene Fibre Reinforced Concrete (PFRC). HCS can be critical in shear at their end zones, since these zones are disturbed regions (already stressed in tension by splitting forces) in which the beneficial effects of the prestressing actions on the shear strength are not active yet. Five full-scale HCS (42 cm high, 120 cm wide and 600 cm long) were tested under shear loading (1 in RC and 4 in PFRC). Two tests were performed on each slab varying the shear-span-to-effective depth ratio (a/d = 2.8 according to EN1168 and a/d = 3.5). Results show that macro-synthetic fibres are able to improve the shear strength of hollow-core slabs of about 25%; furthermore tests according to EN1168 are more affected by arch actions as compared to a/d = 3.5. Finally, the comparison between experimental results and predictions of four international codes (Eurocode 2, ACI 318-14, Model Code 2010 and EN1168), highlighted the need of improving the actual shear formulations. [ABSTRACT FROM AUTHOR]

• A novel research on the use of hybrid reinforcement in precast tunnel segments for metro tunnel lining. • Successful combination of macro-synthetic fibres and bars for reinforcing precast tunnel segments. • Flexural and point load tests on full-scale precast tunnel segments for metro tunnel lining. The addition of fibers has been proven successful to simplify reinforcement in precast tunnel segments, allowing as a function of both segment typology and fiber reinforced concrete (FRC) toughness a total or partial replacement of the conventional reinforcement. Results from an experimental research aimed at comparing the structural behavior of segments made with conventional (only rebars, RC) or hybrid (rebars + fibers) reinforcement are presented. The experimental program consisted of flexural and point load tests (which reproduces the jack actions during TBM operations) carried out on four large-scale precast tunnel segments representative of a metro tunnel lining characterized by an internal diameter of 5.80 m and a thickness of 0.30 m. The main goal of the experimental program was to evaluate the possibility of using macro-synthetic polypropylene fibers (Polypropylene Fiber Reinforced Concrete, PFRC) in combination with a lower amount of conventional rebars (optimized reinforcement, RCO) for guaranteeing the required segment performance. Experimental results indicate that macro-synthetic fibers may be very effective in combination with conventional rebars to withstand the main stresses that arise in a segment both at initial and final phases, proving that the adoption of hybrid reinforcement solution using macro-synthetic fiber is possible for metro tunnel lining. [ABSTRACT FROM AUTHOR]

• Experimental and numerical study on the flexural behavior of elevated slabs in fibre reinforced concrete. • Impact of fiber orientation on the flexural behavior of elevated slabs. • Positive influence of 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • From splitting to crushing collapse, by adopting SFRCs with adequate toughness. • SFRC samples with limited toughness manage equilibrium with longer splitting crack. • High FRC performance is useless when concrete crushing is governing. • Fiber orientation & distribution strongly affect bearing capacity & failure mechanism. • SFRC with shorter fibers can achieve a better crack control at crack-formation stage. Concrete elements subjected to high compressive loads applied over a small contact area is a typical engineering problem. The partially loaded area leads to high compressive stresses underneath the loading zone, that can cause crushing failure, and tensile transverse stresses, which could provoke splitting failure. Rebars are typically adopted by designers in Reinforced Concrete (RC) solutions to cope with these tensile stresses. However, there is a growing interest in reinforcement solutions based on Fiber Reinforced Concrete (FRC). Within this framework, a broad experimental program was carried out to shed new lights on the behavior of Steel Fiber Reinforced Concrete (SFRC) specimens subjected to partially loaded area and to compare their structural performance against RC samples. Prismatic elements reinforced either by reinforcing bars or different steel fiber types and amount were subjected to a load applied over a partial area. Different casting directions, favoring different fiber orientations, were considered as well. The goal is to evaluate the crack control promoted by fibers and the different failure mechanisms of SFRC samples. Results confirmed both the ability of steel fibers to increase the splitting bearing capacity and the importance of fiber orientation. SFRC characterized by a proper post-cracking performance can move the collapse from splitting to crushing and provide a cracking control similar to RC specimens. [ABSTRACT FROM AUTHOR]

• A morphological and mechanical characterization of corroded rebars. • A chloride content analysis in cracked specimens exposed to chloride-rich environment. • Perpendicular to crack wall, positive influence of fibres on the reduction of chloride content. 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. [ABSTRACT FROM AUTHOR]