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To assess the durability of Reinforced Concrete (RC) structures, a model capable of predicting the crack pattern of RC ties is herein introduced. Based on the classical tension-stiffening equations, such model provides the transfer length, which in turn depends on the bond-slip mechanism between steel and concrete. The aim is to compute the length of a tie which shows a single crack in the serviceability stage. In this particular situation, if the geometry does not change, transfer length only depends on the strength of plain or fiber-reinforced concrete (FRC). Nevertheless, the experimental investigation, performed on RC and R/FRC ties with the same geometrical and mechanical properties, reveals two different crack patterns. Specifically, RC ties show multiple cracking, whereas only one crack tends to appear in presence of FRC. This dichotomy can be ascribed to the so-called antifragility, which can be considered as the capacity of FRC to gain strength from its intrinsic disorder.

The environmental impact of the Portland cement production and the large use of cement-based building materials is a growing problem [...]

Natural wool is a good insulating material, both thermally and acoustically. With the increase in demand for the usage of waste materials, other applications have been found, such as the use of wool as a fibre reinforcement in mortars and concretes. Unfortunately, wool, like other natural organic materials, dissolves in alkaline environments and, consequently, the performance of wool reinforcement cannot be guaranteed for a long time. To address this issue, three series of wool-reinforced mortar beams, with various contents of alkalis in cement, were investigated. The chemical compatibility and the effects of alkalinity on mechanical performance were investigated by testing the beams under three-point bending and, subsequently, by analysing the microstructure of the mortars using energy-dispersive X-ray spectroscopy. The results showed that the lower the alkalinity of the cement paste, the better the resistance of wool fibres in the cementitious matrix, thus guaranteeing larger post-cracking residual stresses in the wool-reinforced mortars.

Among the techniques used to retrofit existing reinforced concrete structures, methods involving Ultra High Performance Fiber Reinforced Cementitious Composites (UHP-FRCC) are widely regarded. However, current practices make the use of this material for in-situ application expensive and complicated to perform. Accordingly, a new method to strengthen existing concrete beams by applying a precast UHP-FRCC layer on the bottom side are introduced and described herein. Two test campaigns are performed with the aim of defining the best conditions at the interface between the reinforcing layer and the existing beam and to reducing the environmental impact of UHP-FRCC mixtures. As a result, the eco-mechanical analysis reveals that the best performances are attained when the adhesion at interface is enhanced by means of steel nails on the upper surface of the UHP-FRCC layer, in which 20% of the cement is replaced by fly ash.

To study the brittle/ductile behavior of concrete beams reinforced with low amounts of rebar and fibers, a new multi-scale model is presented. It is used to predict the flexural response of an ideal Hybrid Reinforced Concrete (HRC) beam in bending, and it is validated with the results of a specific experimental campaign, and some tests available in the technical literature. Both the numerical and the experimental measurements define a linear relationship between the amount of reinforcement and the Ductility Index (DI). The latter is a non-dimensional function depending on the difference between the ultimate load and the effective cracking load of a concrete beam. As a result, a new design-by-testing procedure can be established to determine the minimum reinforcement of HRC elements. It corresponds to DI = 0, and can be considered as a linear combination of the minimum area of rebar (of the same reinforced concrete beam) and the minimum fiber volume fraction (of the same fiber-reinforced concrete beam), respectively.

In this paper, an overview of the latest research activities in the field of cement-based composites incorporating sheep wool reinforcement is presented. First, the characteristics of this type of natural fibre are described. Then, the current use of sheep wool fibres in cement-based composites is discussed. The research problems regarding the properties of cement matrix composites reinforced with sheep wool are divided into four groups: thermal and acoustic properties, mechanical behavior, durability issues, and microstructure aspects. The latter two groups are analysed separately, because both durability and microstructure are of particular importance for future applications of wool reinforcement. Finally, the main directions of future researches are presented.

Ultra-High Performance Fiber-Reinforced Concretes (UHP-FRC) subjected to uniaxial tensile loads are investigated in the present paper. The study comprises a new procedure to assess the effectiveness of the hybridization, herein obtained by reinforcing UHP-FRC with micro and macro steel fibers. A comprehensive experimental campaign is also performed on monofiber and hybrid UHP-FRC. In all the concretes, the distance between the cracks and the minimum fiber volume fraction, which produces strain hardening response and multiple cracking, are theoretically and experimentally evaluated. If the bond parameter of the macro-fibers is properly calculated, the results of the analytical model, in terms of crack-spacing vs. fiber volume fraction, are in good agreement with the test data. Moreover, to increase the number of the cracks, and to reduce crack spacing, the hybridization is suitable only when the amount of macro-fibers is within a well-defined range.

Three series of tests performed on fibre-reinforced gypsum composites are described herein. Sheep wool fibres and hemp fibres were used as reinforcement. The aim was to evaluate the capability of these biomaterials to enhance the fracture toughness of the gypsum matrix. The mechanical properties were measured by means of flexural tests on small specimens, whereas scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction were used to analyse the microstructure and composition of the fibres and of the gypsum composites. As a result, wool fibres were shown to improve the mechanical performance of the gypsum matrix, better than hemp fibres. This is due to the high adhesion at the interface of the fibre and gypsum matrix, because the latter tends to roughen the surface of the wool and, consequently to increase the bond strength. This preliminary research carried out shows that this type of biofiber—a waste material—can be considered a promising building material in sustainable and environmentally friendly engineering.

Ultra-High Performance Fibre-Reinforced Cementitious Composites (UHP-FRCC) show excellent mechanical performances in terms of strength, ductility, and durability. Therefore, these cementitious materials have been successfully used for repairing, strengthening, and seismic retrofitting of old structures. However, UHP-FRCCs are not always environmental friendly products, especially in terms of the initial cost, due to the large quantity of cement that is contained in the mixture. Different rates of fly ash substitute herein part of the cement, and the new UHP-FRCCs are used to retrofit concrete columns to overcome this problem. To simulate the mechanical response of these columns, cylindrical specimens, which are made of normal concrete and reinforced with different UHP-FRCC jackets, are tested in uniaxial compression. Relationships between the size of the jacket, the percentage of cement replaced by fly ash, and the strength of the columns are measured and analyzed by means of the eco-mechanical approach. As a result, a replacement of approximately 50% of cement with fly ash, and a suitable thickness of the UHP-FRCC jacket, might ensure the lowest environmental impact without compromising the mechanical performances.

To reduce the mass of CO2 released into atmosphere by the construction industry, the performance strategy can be adopted. It is based on the use of High-Strength Concrete (HSC) in alternative to Normal-Strength Concrete (NSC). Such concretes are herein considered to design the reinforced concrete structures of three buildings, having 14, 30 and 60 floors, respectively. For each building, the structural analyses, carried out for four classes of concrete (i.e., C25, C40, C60 and C80) in accordance with Eurocode 2, provides different dimensions of the structural elements. In other words, the amount of CO2, released in the atmosphere due to the production of the structural materials, is a function of both concrete strength and height of the building. As a result, the minimum impact of low-rise buildings occurs when the structural elements are made with NSC. Conversely, only when HSC is used to cast the structural elements of tall buildings, can the carbon footprint be effectively reduced. Keywords: Performance strategy, Carbon footprint, FEM analyses, Reinforced concrete structures

In the last decades, a large number of studies have been carried on the use of rubber particles derived from end-of-life tyres as partial replacement of stone aggregates. Despite a significant decrease in concrete mechanical properties, such as compressive strength, has been observed, rubberized concrete (RuC) shows an increment of ductility, energy dissipation and damping capacity. Nevertheless, experimental researches on the behaviour of RuC in structural members is quite lacking, especially referring to the interaction between rubberized concrete and steel rebar. For this reason, the bond-slip behaviour between RuC and deformed steel bars is investigated in this paper with the aim of evaluating the influence of volumetric aggregate substitution. A reference ordinary Portland concrete (OPC) mix and four different RuC mixtures, with increasing percentages (i.e., 6%, 12%, 18% and 24%) of rubber particles as partial replacement of fine natural aggregates, have been tailored and tested. The results showed a decay, up to 20% in respect to the reference mix, in bond strength when RuC contains more than 12% in volume if rubber aggregates. Bond strength decrease has been correlated with the simultaneous decrement, up to 37%, in compressive strength observed for the same percentages of aggregates substitution. On the contrary, residual bond stress slightly increases of about 10%, similarly to concrete toughness, with the content of rubber.

To reduce the environmental impact of cement-based composites used by the construction industry, stone aggregates can be substituted by unconventional aggregates, such as the rubber from end-of-life tyres. This substitution strategy can be applied to both mortars and structural concretes. Nevertheless, each application requires a specific particle size distribution, to the point that the maximum aggregate size of concrete can be 10 times larger than that of mortar. Consequently, the mechanical response under bending actions, ruled by the fracture energy, can drastically change with the structural dimension. This size-effect is particularly evident in the experimental analyses performed herein, which include three point bending tests on mortar prisms and concrete slabs. As a result, the fracture toughness always increases in presence of rubber, and in the case of large particle size the increment of ductility can lead to the increment of the flexural strength.

It is well known that the concrete in-dustry has a strong environmental impact on our planet. First, there are the huge volumes of material needed to produce the billions of tons of concrete worldwide each year. Then there are the CO2 emis-sions caused during the production of Portland ce-ment. Together with the energy requirements, water consumption and generation of construction and dem-olition waste, appears evidente as the concrete is not particularly environmentally friendly or compatible with the demands of sustainable development. In this context large number of papers focused on new concrete materials that would be more suitable with environmental point of view. Among these ma-terials, one of the most discussed is rubberized con-crete. Rubberized concrete is obtained by incorporat-ing rubber particles, obtained from recycled end-of-life tyres, as a replacement for mineral aggregates.

To design the retrofit and/or conservation of existing reinforced concrete structures, an assessment of concrete strength is generally required. The current methodologies consist of destructive tests, based on uniaxial compression performed on concrete cores extracted from a structure, and/or non-destructive tests, in which the strength of the concrete is indirectly estimated by measuring other physical properties (like ultrasonic pulse velocity). Nevertheless, in several situations (e.g. the seismic vulnerability assessment of long-term service structures), the traditional tests cannot be performed. Hence, a new simplified approach is introduced and described in this paper. This can be applied to structures located in a precise geographical area, where the average strength of concrete (and the relative variance) is a function of the year of construction. Such a relationship is summarised by the so-called strength curves statistically computed from a huge database stored in the Department of Structural and Geotechnical Engineering of Politecnico di Torino (Italy). Relating to a concrete dam and a stadium built during the 1950s and in 1967, respectively, the strength predicted by these curves is in good agreement with the measurements of destructive tests.

Ultra High Performance – Fiber Reinforced Cementitious Composites (UHP-FRCC) has successfully developed in the last decades. They show excellent mechanical properties and durability, due to the highly dense microstructure produced by extremely low water/binder ratio and large amounts of cement. Thus, replacing part of the cement content with High Volume Fly Ash (HVFA) could be an effective way to improve also the ecological properties of UHP-FRCC. Indeed, most of the carbon footprint of cement-based composites is caused by the CO2 production of cement manufacturing. On the other hand, HVFA tends to reduce the mechanical properties of UHP-FRCC, especially at the early age. To evaluate the effect of some cement replacement (20, 50 and 70% by weight), a series of uniaxial compression and tensile tests have been performed and described in this paper. At the same time, through the Eco-Mechanical Index (EMI), ecological performances are evaluated on the basis of the measured mechanical properties. As a result, the application of UHP-FRCC in full-scale structures (e.g., composite concrete columns) is really convenient in the presence of HVFA, although the material properties (the compressive strength, in particular) show an opposite trend.

Durability of reinforced concrete (RC) structure is strictly connected to the capacity of concrete mass to protect the embedded reinforcement from corrosion. As cracks are almost inevitable, crack width is a fundamental parameter that needs to be controlled during the serviceability stage of RC members. In fact, not only the direct ingress of aggressive agents, such as oxygen and water, is a function of crack width, but also concrete carbonation and the chloride ion penetration are accelerated by the presence of wide cracks. Accordingly, the aim of present research project is to assess the width by using a new system, based on the optical conoscopic holography. It provides the non-contact measure of crack profile, taken at the end of each loading cycle, in ties subjected to sets of repeated loads. Both in plain and fiber-reinforced concrete (FRC), more than one crack width can be measured for the same crack.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; Dams are critical infrastructures whose failure would entail serious consequences for community safety. Although large dams represent the most dangerous items, small size dams may be critical, as a large number of these constructions are built in the proximity to inhabited zones. Earthquake is one of the hazards that may affect an existing dam. To assess safety and plan investments, dam owners need to create prioritisation sequences of interventions through a protocol considering technical, political and societal aspects. It is presented herein a specific procedure, based on a multi-criteria analysis with a rapid screening survey of the infrastructures, which addresses to classify the seismic criticality of dams considering the effects (in terms of loss of life and property) of a failure scenario subsequent to an earthquake. A protocol is used to define a prioritisation of interventions that mitigate the seismic risk. It has been applied to a group of 9 small concrete dams and 17 fixed weirs, built from 1920 to 1990 in Aosta Valley (Italy). As a result, about 30% of the existing dams, which are more vulnerable than fixed weirs, shows the highest level of seismic criticality, and needs to be retrofitted in accordance with the current standards.

Ultra-high-performance fiber-reinforced cementitious composite (UHP-FRCC) has been successfully developed over the last decade. UHP-FRCC shows very high strength and high durability because of its very low water binder ratio and dense microstructures. Moreover, by incorporating a combination of steel and mineral fibers with different sizes in the mixture design, UHP-FRCC also exhibits high ductility and high energy absorption capacity, even under uniaxial tensile stress. The high ductility of UHP-FRCC materials shows strain hardening behavior with multiple cracking after the occurrence of the first crack. While extensive research on the tensile and bending behavior of this composite under short-term loading is reported, only limited results can be found on time-dependent behavior of UHP-FRCC under sustained long-term loading, which is strongly related to the situation of real structures. This paper presents experimental results concerning the time-dependent behavior of cracked UHP-FRCC specimens under sustained bending load. In bending creep test, sustained load with 50 % of the pre-cracking load was applied to a pair of prismatic specimens of 50 × 100 × 400 mm under a 4-point bending setup. The performance of UHP-FRCC is compared with those of conventional fiber reinforced cementitious composites (FRCC) using steel and synthetic fibers, which are investigated within the Round Robin Test (RRT) organized by RILEM TC 261-CCF. The results showed that UHP-FRCC exhibited smaller creep deformation at the sustained loading condition than the conventional FRCC, even when the initial deformation of the UHP-FRCC is much larger than the conventional FRCC. Cracking pattern, in terms of crack width and number of cracks, was also studied in both composites under sustained loads. As a result, the superior resistance of UHP-FRCC was obtained with respect to conventional FRCC.

In recent years the term design has been overexploited in popular magazines, newspapers and on television. Schools and universities have increased the courses on offer which now include very different design disciplines, from social design to systemic design. The latter, in particular, is the ability to outline and plan the flow of matter and energy from one system to another, within metabolization processes which reduce the ecological footprint and generate a remarkable economic flow. Today designers are asked to strategically design a scenario that doesn’t focus only on product innovation as an end in itself, but involves developing broader issues which require the input and expertise of other fields of learning. These issues necessarily include non-traditional economic and industrial models (i.e. Circular Economy, Blue Economy and Bioeconomy). This paper aims to create bridges between the world of construction engineering and design practices based on non-linear industrial models. It stimulates a discussion about systemic design challenges in relation with the concrete structures, evidencing some challenges, open questions and possible new directions.

Man-made composites, like reinforced and fiber-reinforced concrete elements, behave similarly to natural composite structures (e.g., brittle rocks lying on a ductile substratum). In both cases, the cracking phenomenon of brittle layers depends on the scale of observation and is ruled by the Golden Ratio. Thus, a unique size-effect relationship, herein called Golden Scaling Law (GSL), is introduced and used to predict the crack pattern of concrete and rock structures. In accordance with several experimental data, GSL permits to calculate the values of crack width and crack spacing when the geometry of crack patter is known at a lower scale. Moreover, GSL can be applied to large composite structures without knowing the material performances, but by testing prototypes of lower dimensions.

Both in architecture and arts, the golden ratio has been taken into consideration most exclusively for its geometrical properties. Specifically, among all the proportions, the golden ratio can inspire beauty and aesthetic pleasure. Indeed, it has driven, in an implicit or explicit manner, the construction of buildings for centuries. Nevertheless, as discussed in the present paper, also fracture mechanisms in brittle and quasi-brittle materials call the golden ratio into play. This is the case of fracture energy and fracture toughness, in which the irrational number 1.61803 recurs when the geometrical dimensions vary. This aspect is confirmed by the results of different experimental campaigns performed on concrete and rock beams and ice sheets. In other words, it can be argued that the centrality of the golden ratio for quasi-brittle structures has profound physical meanings, as it can bring together the aesthetic of nature and architecture, and the equilibrium of stress flow in solid bodies.

the elastic approach of eco-mechanical performances of concrete is presented. In particular the fracture energy aspects are investigated, taking into account aggregates and fibres

The environmental performances of concrete structures have been largely investigated in the last years. However, mechanical behavior and ecological aspects are not always combined in these analyses. Thus, two low-carbon footprint self-consolidating concretes have been subjected to uniaxial compression loads, in order to measure the strength and the dissipated energy during failure. Starting from these tests, new eco-mechanical ratios are herein introduced with the aim of tailoring eco-friendly concrete with acceptable mechanical performances. As a result, the best structural and environmental performances can be achieved not only by replacing cement with mineral admixtures, but also adding steel fibers.

To reduce the environmental impact of traditional lightweight concrete (TLC), porous aggregates can be substituted by rubber granulates. The mechanical properties of such rubber lightweight concrete (RLC) are investigated and compared with those of TLC made with expanded clay aggregates. Uniaxial compression and three point bending tests were performed for assessing the mechanical and ecological performances of the two mixtures, containing or not plastic fibers. As a result, when compressive strength is the functional unit of the analyses, TLC performs better than RLC. Conversely, fiber-reinforced RLC is the best solution when flexural strength and structural ductility are the required performances.

The fiber contribution to the ultimate limit state capacity of precast and cast-in situ tunnel linings is analytically investigated. By means of a numerical model, capable of computing the interaction curves of reinforced concrete cross sections subjected to combined compressive and bending actions, the mechanical performances of plain and fiber-reinforced concrete are compared. As a result, a new index is introduced to quantify the effectiveness of fiber addition. The higher the efficiency index, the higher the amount of steel reinforcing bar that can be removed from a plain concrete cross section. The application to real concrete linings, where shear resistance is ensured without shear reinforcement, shows that a large volume of rebar can be saved by the presence of steel fibers. This gives significant advantages in terms of durability and rapidity of tunnel construction.

The mechanical response of fiber-reinforced concrete (FRC) beams depends on the amount of fibers, and the transition from brittle to ductile behavior in bending is related to a critical value of fiber volume fraction. Such quantity, which is mechanically equivalent to the minimum amount of steel rebars in reinforced concrete beams, can be defined according to the new approach proposed herein. It derives from the application of a general model and from the introduction of the so-called ductility index (DI). When FRC beams show a ductile behavior DI is positive, whereas DI is negative in the case of brittle response. Both the theoretical and experimental results prove the existence of a general linear relationship between DI and the fiber volume fraction. Accordingly, a new design-by-testing procedure can be used to determine the critical value of fiber volume fraction, which corresponds to a ductility index equal to zero.

The structural behavior of lightly reinforced concrete (LRC) and fiber-reinforced concrete (FRC) beams in bending is similar and depends on the amount of reinforcing bars and fibers, respectively. For this reason, a unified approach is introduced herein for the evaluation of the minimum reinforcement. Such approach is based on the definition of the ductility index (DI), which is a function of the difference between the ultimate bending moment and the effective cracking moment. DI is higher than zero when LRC and FRC beams show a ductile response, whereas it is negative in the case of a brittle behavior. In accordance with several experimental data, the linear dependence of DI on the amount of reinforcement can be demonstrated. Thus, a new design-by-testing procedure can be established to determine the minimum reinforcement (that is, reinforcing bars or fibers) of a concrete beam, which corresponds to DI equal to zero

A size-effect theory is herein introduced to investigate the average expansion of concrete containing free lime (CaO) and impurities. By drawing an analogy with the structural size-effect, the phenomenon of CaO hydration shows a characteristic length, whose dimension remains constant at all scales. If a grain of free lime is assumed to be a sphere, only a thin layer on the external surface hydrates and doubles the volume, as confirmed by the results of 2D finite element analyses and by some experimental observations. For practical applications, a simplified chemo-mechanical model of CaO hydration is eventually proposed, to predict the consequences of uncombined lime in concrete structures.

Several engineering problems, such as the collapse of borehole in rocks, can be investigated by means of multiaxial compression tests. However, the brittle failure that occurs during the excavation of circular tunnels in rock masses cannot be completely reproduced by laboratory tests on hollow cylinders having a diameter of few centimetres. Thus, referring to these specimens subjected to hydrostatic pressure, and made of concrete or other quasi-brittle materials, a numerical model capable of predicting their structural response at different scales is here presented. Due to the absence of confinement, strain localization and crushing failure appear around the inner surface of the cylinders, which behave nonlinearly even in the case of low pressures. Similarly to cement-based beams in bending, this phenomenon can be modelled by adopting a suitable stress-inelastic displacement relationship for the post-peak response under compression. With regard to hollow cylinders subjected to multiaxial compression, it is possible to predict the size effect on fracture strength, which needs to be taken into account when laboratory tests are used to analyze the borehole failures. The effectiveness of the proposed approach is checked by comparing the computed results with the test data measured by Elkadi and van Mier [Elkadi AS, van Mier JGM. Experimental investigation of size effect in concrete fracture under multiaxial compression. Int J Fract 2006;140(1-4):55-71] on concrete specimens.

The ductility of High Performance Concrete (HPC) can develop both in tension and compression. This aspect is evidenced in the present paper by measuring the mechanical response of normal vibrated concrete (NC), self-compacting concrete (SC) and some HPCs cylindrical specimens under uniaxial and triaxial compression. The post-peak behaviour of these specimens is defined by a non-dimensional function that relates the inelastic displacement and the relative stress during softening. As a result, in normal and self-consolidating concrete, fracture toughness in compression increases in the presence of active confinement. Moreover, HPC specimens, which can also show strain hardening in tension, provide a very ductile behaviour even in absence of confinement. In particular, during the post-peak stage, the ductility of HPC is comparable with that of NC or SC at 1MPa of confining pressure. Moreover, the performance of fiber-reinforced composites can be quantified by the distributed confining pressure generated by the fibers. The presence of HPC in compressed columns is therefore sufficient to create a sort of active distributed confinement, and improve both the mechanical behaviour of concrete and its durability.

Fiber-reinforced concrete (FRC) showing strain hardening after cracking is commonly defined as High Performance Fiber-Reinforced Cementitious Composite (HPFRCC). In the post-cracking stage, several cracks develop before complete failure, which occurs when tensile strains localize in one of the formed cracks. As is well known, multiple cracking and strain hardening can be achieved in cement-based specimens subjected to uniaxial tension by increasing the volume fraction of steel fibers with hooked ends, or by using plastic fibers with and without steel fibers, or by means of high bond steel fibers (e.g., twisted fibers or cords). To better understand why, in such situations, high mechanical performances are obtained, an analytical model is herein proposed. It is based on a cohesive interface analysis, which has been largely adopted to investigate the mechanical response of FRC or the snubbing effects produced by inclined fibers, but not the condition of multiple cracking and strain hardening of HPFRCC. Through this approach, all the phenomena that affect the post-cracking response of FRC are evidenced, such as the nonlinear fracture mechanics of the matrix, the bond–slip behaviour between fibers and matrix, and the elastic response of both materials. The model, capable of predicting the average distance between cracks as measured in some experimental campaigns, leads to a new design criterion for HPFRCC and can eventually be used to enhance the performances of cement-based composites.

The high ductility of Fiber Reinforced Self-consolidating concrete (called Sismabeton) can be developed not only in tension but also in compression. This aspect is evidenced in the present paper by measuring the mechanical response of normal concrete (NC), plain self-compacting concrete (SC) and Sismabeton cylindrical specimens under uniaxial and triaxial compression. The post-peak behaviour of these specimens is defined by a non-dimensional function that relates the inelastic displacement and the relative stress during softening. Both for NC and SC, the increase of the fracture toughness with the confinement stress is observed. Conversely, Sismabeton shows, even in absence of confinement, practically the same ductility measured in normal and self-compacting concretes with a confining pressure. Thus, the presence of Sismabeton in compressed columns is itself sufficient to create a sort of active distributed confinement.

New procedures to design cast-in-situ steel fiber reinforced concrete (SFRC) tunnel linings are briefly presented in this paper. The ductile failure of such cement-based structures is ensured by adding a suitable amount of steel fibers to ordinary steel bars. The capability of SFRC to carry tensile stresses, also in the presence of cracks, allows designers to reduce the minimum area of ordinary steel reinforcement, generally computed in compliance with American or European code requirements. In the serviceability stage, to evaluate crack widths more accurately, a suitable block model is introduced. This model is able to take into account the bridging effect of fibers, as well as the bond slip between steel bars and concrete in tension. The proposed approaches have been successfully applied to the design of tunnel linings in Italy.

The cracking phenomenon of members made of traditional reinforced concrete (RC) or by combining steel fibers and traditional reinforcing bars (R/FRC) is analyzed in this paper. Referring to the tensile zone of a beam subjected to bending moment and axial load, a unique block model is introduced to predict crack widths, crack lengths and crack distances. The proposed approach, based on the nonlinear behaviour of the cracked cement-based material in tension, and on the bond-slip interaction between rebars and concrete, provides crack patterns similar to those observed in different experimental campaigns. Cracks in R/FRC beams are generally narrower, and originate at small distances, than those observed in RC beams having the same geometry and reinforcement ratio. This is entirely due to the presence of fibers, here considered through the Reinforcing Index, which is the product of volume percentage and fiber aspect ratio. However, the beneficial effect of fiber-reinforcement vanishes with the increase of structural dimension. Thus, in the case of massive structures, it appears necessary a direct calculation of the crack width, even in the presence of fibers.

To increase the crack bridging capacity of concretes and mortars, fibers are often added to cement-based matrices. As a result, the fracture energy of a fiber-reinforced concrete (FRC) is the sum of the cohesive forces of the matrix and of the pullout resistance given by the fibers crossing the crack. In the case of fibers randomly inclined with respect to crack surfaces, the so-called frictional snubbing effect must be added to the previous contributions, if the energy released during crack growth has to be computed. Of course, snubbing forces are not present when fibers are perfectly orthogonal to crack surfaces. As the classical formulas are not always effective in the evaluation of snubbing phenomenon, a cohesive interface model is here proposed for a more precise definition of the frictional snubbing forces. The model is able to predict adequately the experimental results obtained by pulling out steel fibers at different inclinations angles from cement-based matrices. By analyzing fibers with different bond properties, the proposed model can be also used to define the crack bridging capacity of both bonded and unbonded steel fibers. Moreover, it seems to suggest a more simplified approach for the definition of the postcracking response of FRC.

The nonlinear behavior of fractured quasi-brittle materials is conventionally modeled with a fictitious crack model, which relates stresses on the crack surfaces to the corresponding crack widths. Its definition for fiber reinforced concrete is only possible by introducing a cohesive model for the matrix, and by modeling the pullout of randomly oriented fibers. To this aim, a new cohesive interface model, able to predict effectively the pullout response of inclined fiber, is presented in this paper. Based on the nonlinear behavior of steel fibers and cementitious matrixes, the proposed approach also takes into account the bond-slip relationship between the materials. By means of an iterative procedure, numerical results similar to experimental data can be obtained. In particular, maximum pullout forces at given inclination angles, as well as the complete pullout load vs. displacement diagrams, can be correctly predicted. Moreover, according to test results, the proposed approach shows, from the first pullout stage, the dependence of the response both on crushing of cementitious matrix and on yield strength of steel fibers.

To model the behavior of precast segmental tunnel linings under thrust jack loading, cylindrical concrete columns, subjected to concentrated loads, are investigated. Specifically, uniaxial compressive tests have been performed on specimens made with self-compacting concrete and reinforced with two different amounts of steel fibers (0, 30 kg/m3, respectively). In addition, to measure concrete properties, three point bending tests have been carried out in accordance with code rules. As a result, not only the fracture toughness of concrete in compression is improved, but also the ultimate and the maximum splitting loads are remarkably increased by the presence of long fibers. Accordingly, a new formulation of the classical Leonhardt’s approach is eventually proposed. Finally, based on the results of the present research, it seems reasonable to replace reinforced concrete with SFRSCC in structures subjected to high concentrated loads.

As the mechanical response is better than in plain cement-based mortars, fiber-reinforced mortars are widely used in the construction industry. Specifically, the fracture toughness in tension increases with the volume and the aspect ratio (i.e., the ratio between length and diameter) of the fibers, which are generally made with polymeric (e.g., polyethylene, polyvinylchloride, etc.) or inorganic (e.g., glass, carbon, etc.) materials, or with steel. However, some vegetal fibers, such as bamboo and hemp, have been also introduced in the last decades. To produce new mortars with animal fibers, the use of wool as fiber-reinforcement is investigated for the first time in the present paper. According to UNI EN 196-1-2006, three point bending tests have been performed on three series of beams: plain mortar, mortar reinforced with 1% in volume of wool, and mortar reinforced with 1% in volume of treated wool. In the latter case, wool is previously treated with atmospheric plasma in order to modify the nano-metric properties of the fiber surface. As a result, both the flexural strength and the ductility increase when wool, plain or treated, is added to cementitious mortars. In other words, wool does improve the mechanical and ecological performances of cementitious mortars and creates a link between textile and construction markets., Academic Journal of Civil Engineering, Vol 33 No 2 (2015): Special Issue - ICBBM 2015

The Soleri Viaduct, one of the most important infrastructures in Northern Italy, has been used as an access road to the town of Cuneo for more than 80 years. It crosses the River Stura di Demonte and is about 750 m long. In a special maintenance project of the Viaduct, the current reinforced concrete slabs of the sidewalk zones need to be substituted by similar structures, but made with lightweight concrete and without the traditional steel reinforcing bars. A full-scale experimental program was performed on the new slabs to evaluate the effectiveness of the unconventional reinforcement, which consists of polymer fibers randomly dispersed within a cementitious matrix. As a result, an analogy can be drawn between the evaluation of the minimum amount of fibers and the computation of the minimum reinforcement ratio in concrete beams. A general approach, aimed at designing slightly reinforced concrete structures, is therefore introduced and applied for the fiber-reinforced lightweight concrete slabs investigated herein.

In lightly reinforced concrete (RC) structures, the area of steel cannot be lower than a minimum value, so that the ultimate limit state can be reached under a yielding moment higher than the cracking moment. Also in the serviceability stage, a minimum amount of reinforcement should be provided in tensile zones, in order to reduce crack widths. In fiber-reinforced concrete (FRC) members, due to the presence of structural fibers in the cementitious matrix, the minimum amount of steel area can be significantly reduced. Fiber can guarantee tensile stresses in a cement-based matrix even in the presence of wide cracks. Therefore, for the same cross-section of steel, a reinforced FRC member in bending can show higher bending moments, and reduced crack widths, than those measured in classical RC beams. This is particularly true in case of massive members, like the structures of tunnel linings. For such elements, and starting from the constitutive relationships recommended by Rilem TC 162-TDF, a new approach for the evaluation of minimum reinforcement area is proposed in this paper. By means of this nonlinear model, it is possible to calculate a reinforcement area lower than that calculated according to Eurocode 2 and Rilem TC 162-TDF prescriptions.

The transition from the uncracked to the cracked phase in lightly reinforced concrete beams was experimentally investigated by subjecting five full-scale beams to three-point bending. All beams had the same dimensions and the same percentage of flexural reinforcement, but varying diameters and number of reinforcing bars. By means of transducers, strain gauges, and geometric moire, the crack profiles and the concrete strain field around the cracks were measured. In this way, it was possible to observe and to reproduce numerically the crack onset and its stable propagation up to the unstable behavior that follows the effective cracking moment. During the growth of the crack, the moment rotation diagrams, the values of tensile strains in concrete, and the shape of crack profiles demonstrate the dependence of the mechanical response on bar diameter. Consequently, to avoid brittle failure, the minimum reinforcement ratio for lightly reinforced concrete beams should likewise be a function of bar diameter.

To evaluate the strains in a steel bar of a reinforced concrete structure under repeated loads, a mechanical model is proposed. More precisely, a reinforcing bar embedded in a concrete cylinder subjected to uniaxial actions is modelled. In the proposed one-dimensional approach, both the nonlinear fracture mechanics of concrete and the bond-slip between the two materials are taken into account, since they affect the structural response of several reinforced concrete structures in the serviceability conditions and in the ultimate limit state. When cyclic actions are applied and previous cracks close up again, the interaction between the fictitious crack model of concrete in tension and the bond-slip near the crack is investigated. During the unloading phase of each cycle, only through refinement of the damage effects, with regard to the classical bond and cohesive models, can the hysteretic behaviour of the reinforced concrete structures be modelled. The strains in the bar, computed with the proposed model, seem to be in good agreement with those measured in the tests made by Bresler and Bertero.