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2022 | Book

Fibre Reinforced Concrete: Improvements and Innovations II

X RILEM-fib International Symposium on Fibre Reinforced Concrete (BEFIB) 2021

Editors: Prof. Pedro Serna, Prof. Aitor Llano-Torre, Prof. José R. Martí-Vargas, Prof. Juan Navarro-Gregori

Publisher: Springer International Publishing

Book Series : RILEM Bookseries


About this book

This volume highlights the latest advances, innovations, and applications in the field of fibre-reinforced concrete (FRC), as presented by scientists and engineers at the RILEM-fib X International Symposium on Fibre Reinforced Concrete (BEFIB), held in Valencia, Spain, on September 20-22, 2021. It discusses a diverse range of topics concerning FRC: technological aspects, nanotechnologies related with FRC, mechanical properties, long-term properties, analytical and numerical models, structural design, codes and standards, quality control, case studies, Textile-Reinforced Concrete, Geopolymers and UHPFRC.

After the symposium postponement in 2020, this new volume concludes the publication of the research works and knowledge of FRC in the frame of BEFIB from 2020 to 2021 with the successful celebration of the hybrid symposium BEFIB 2021. The contributions present traditional and new ideas that will open novel research directions and foster multidisciplinary collaboration between different specialists.

Table of Contents

Correction to: Wind Tower FRC Foundations: Research and Design

In the original version of the book, the following belated correction has been incorporated: The author’s name changed from “Giulio Zanis” to “Giulio Zanito” in Chapter 71 and in the index. The book and the chapter have been updated with the changes.

Technological Aspects

Effects of Fly Ash Content and Curing Age on High Temperature Residual Compressive Strength of Strain-Hardening Cementitious Composites

Class F fly ash is an integral component of most Strain Hardening Cementitious Composites (SHCCs). It reacts with the calcium hydroxide produced in the primary hydration of cement. As calcium hydroxide disintegrates at lower temperatures than other cement hydration products, reducing it through the pozzolanic reaction with fly ash has been shown to improve the thermal stability of SHCC. However, the degree (or extent) of the pozzolanic reaction increases with curing age. Therefore, the effects of fly ash content on the residual compressive strength of SHCC after exposure to elevated temperatures must be investigated at different curing ages, which is the motivation behind this study. Three different SHCCs with fly/ash to cement (FA/c) weight ratios of 1.2, 2.4, and 3.6 were tested at 28 days and 150 days under residual conditions after being subjected to high temperatures of up to 600 °C. The results show that the beneficial effects of high FA/c ratios on the residual compressive strength, associated with the accelerated pozzolanic reaction of fly ash at high temperatures, are lost at the long curing age.

Optimization of Functionally Graded Concretes Incorporating Steel Fibres and Recycled Aggregates

Functionally graded material (FGM) refers to a class of material produced with grading composition and structure to achieve enhanced performance compared to homogeneous materials. Several studies have explored the application of the concept of FGM to enhance the flexural behaviour of concrete, producing functionally graded concrete (FGC). Previous results indicated that FGC produced with fibre reinforced recycled aggregate concrete (FRRAC) exhibited higher residual flexural strength than homogeneous FRRAC for ratios of reinforced height to total beam height (h/H) equal or higher than 0.75, demonstrating the benefits of FGC with FRRAC. Hence, this study aims to verify the optimum value of h/H to obtain the highest residual flexural performance of the FGC. To achieve this goal, an experimental program was carried out, in which, FGC fabricated with FRRAC was assessed under bending considering a content of fibre of 0.50% in volume, and values of h/H ranging from 0.70 to 1.00. The effect of h/H in fibre orientation was also evaluated using the inductive method. The results indicated that the highest residual flexural strength is obtained with h/H = 0.90. However, a balance between pre-cracking and post-cracking behaviour should be defined for each application, resulting in optimized values of h/H in FGC with FRRAC. Furthermore, since the fibre orientation was not affected by h/H, the same orientation factor used in the design of the fibre reinforced concrete elements can be adopted for FGC elements, increasing the potential application of FGC.

Mix Proportioning of Fiber Reinforced Self-compacting Concrete Adopting the Compressible Packaging Method: Comparison of Two Methods

Fiber reinforced self-compacting concrete (FRSCC) is a material that combines the advantages of self-compacting concrete and fiber-reinforced concrete, which can act on two problems of conventional concrete, improving concrete in the fresh state eliminating the need for vibrations with its high workability, in addition to increasing the ductility and toughness of the concrete due to the inclusion of the fiber. This material can be used in structures with high reinforcement rates, allowing a more efficient concreting and, at the same time, reducing the reinforcement rate. There are several ways to dosage self-compacting concrete, one of the most accurate being is the Compressible Packaging Method (CPM). This method is based on the solution of packaging dry mixtures in all components used in the concrete dosing. However, FRSCC dosage studies using CPM are still incipient. There are some ways to consider the effect of fiber on concrete, one approach assesses the effect of fibers considering a perturbed volume that they can generate in the mixture, another simulates the effect of fibers through the concept of equivalent diameter. This work examined these two methods, comparing the compactness results obtained experimentally for 3 types of steel fiber and one synthetic fiber, seeking to evaluate what is the most efficient way to consider the effect of the fibers on the CPM. The results showed that the two approaches can be used for dosage of the FRSCC, however, for larger fiber volumes (0.09% for synthetic fiber and 2% for steel fiber), the second mentioned approach presented the best results.

Influence of Steel Fibers on Damage Induced by Alkali-Silica Reaction of Concrete with Reactive Sudbury Aggregates

The main consequence of Alkali-Silica Reaction is the formation of cracks due to expansion, which can lead to a reduction in resistance capacity of the section of the structural element. The addition of steel fibers (micro fibers and Macro fibers) in the concrete, leads to considerable changes in the material properties in terms of its mechanical and durability performance, even under operating loads. This involves reducing or eliminating the formation of cracks, thus limiting the penetration of corrosive agents. In order to understand the behavior of steel fibers on concrete subjected to ASR due to the presence of highly reactive coarse aggregate (Sudbury aggregate), a 4-month accelerated laboratory testing campaign was conducted with four mix designs (plain concrete, 0.5% Macro fiber, 1.0% Macro fiber, 1.0% Hybrid: 0.5% Macro and 0.5% micro fiber). To speed up the ASR reaction process the samples were exposed to 50 °C temperature and 100% R.H. in a moisture chamber for the entire maturation period. Prisms, cubes, cylinders and beams were cast and tested for: longitudinal expansion, dynamic elastic modulus, damage rating index, compressive strength, static modulus of elasticity and flexural tensile strength. The main result was that the 1.0% Macro mix design is the one that most mitigates the formation of cracks.

Mechanical Properties

Effect of Alkali Treatment to Improve Fiber-Matrix Bonding and Mechanical Behavior of Sisal Fiber Reinforced Cementitious Composites

The current use of dispersed fibers in cementitious matrices is focused on the enhancement in structural performance and mitigation of the effects of shrinkage, which results in microcracks in the cementitious matrix. Natural fibers appear as a low cost and eco-friendly alternative. However, these fibers are degradable in alkaline environments, resulting in changes in the mechanical performance of the composite. These fibers are also susceptible to volume variation with moisture presence, which results in interface degradation. Therefore, the main goal of this work is to evaluate the effect of alkali treatment in order to overcome these limitations and successfully utilize these materials in several applications. For this purpose, sisal fibers with 50 mm length were subjected to 1, 5 and 10 wt.% alkali solutions for chemical modification. The effect of the treatment was evaluated by pullout tests on untreated and treated fibers on a free calcium hydroxide matrix. Additionally, the mechanical performance of fiber reinforced concrete was analyzed through three-point bending tests. Treated fibers presented a brittle behavior in the pullout test. The alkali treatment did not contribute to an increase in the flexural performance of the composite. Similar values of residual strength in the post-cracking region were reached for untreated and treated fiber reinforced cementitious composites.

Drop-Weight Impact Test for Fibre Reinforced Concrete: Analysis of Test Configuration

Impact resistance represents a key property of Fibre Reinforced Concrete (FRC). Recently, the authors proposed a repeated drop-weight test for FRC impact characterization that, among other advantages, is able to distinguish the contribution of different types and dosages of fibres both at cracking and in cracked state. The impacts are applied on a simply supported notched prism of 150 × 150 × 300 mm, being the adopted span length 240 mm, the notch depth 25 mm and the projectile mass 5 kg. An experimental and numerical parametrical study was carried out in order to analyse the effect of the specimen geometry on impact test results. The span length (240, 350 and 500 mm), the notch depth (10, 25 and 50 mm), the specimen width (70, 100 and 150 mm), and the projectile mass (5, 10 and 20 kg) were considered as variables. The numerical model was developed to analyse the stress distribution before the first crack appears (elastic behaviour). In order to corroborate the numerical model, the impact load was measured in some tests with a dynamic load cell. Numerical results concerning the effects of the above-mentioned variables on impact response are presented.

Influence of Specimen Size for Impact Testing of Fiber Reinforced Cementitious Composite by Charpy Test

To evaluate the behaviour of fiber reinforced cementitious composites under impact loads, different impact tests can be used, such as drop-weight, Charpy test, gas gun and others. The Charpy impact test was initially developed for metallic materials, however, unlike what is observed for metallic materials, there are no standards or technical procedures for cementitious materials or fiber reinforced cementitious composites. Thus, an experimental program was carried out to determine the influence of the specimen size on the energy absorption capacity measured by Charpy impact test. Composites specimens were produced with different dimensions and mixtures. The minimum dimension of the specimen was equal to twice the fibre length. The mixtures were fabricated with cement, fly ash, sand and polyvinyl alcohol (PVA) or polypropylene fibers (PP). The PP fibers used in the composite mixtures were 6 mm and 12 mm length and PVA fiber was 8 mm and 12 mm length. For comparative purposes, a reference plain mortar mixture was also produced. Mechanical behaviour of the specimens was measure by using bending and compression tests and impact behaviour by Charpy impact test. The results indicate that the bending behaviour under static loads, including the ultimate bending strength, deflection capacity and toughness, were improved by increasing the fiber length, in all the PVA and PP composites and by increasing the PP fiber content. The impact energy absorption of fiber reinforced cementitious composite depends on the fibre content and length. The result of impact energy absorption obtained by Charpy test using different specimen sizes cannot be comparable, because the impact resistance depends on the size specimen used in the Charpy test. In the specific case, when the specimen size increases, there is a decline in the energy absorption capacity for PVA fiber composite; conversely, the trend is opposite for mortar or when PP fiber is used.

The Effect of Organic Fiber Hybridization on Fresh and Hardened Concrete Properties

The present study aims to evaluate the effect of two different fiber lengths (19 and 30 mm) and morphology (twist and wave) hybridization on both fresh and hardened properties of ordinary concrete. The fiber hybridization consists on the use of fiber with different form, types or morphology in the same concrete mixture. In this study, the total dosage of fiber used was fixed at 0.12% and five fiber proportion has been tested as 0/100%; 30/70%; 50/50%; 70/30% and 100/0% of fiber with 19 mm length and 30 mm respectively. The obtained results indicate that this combination of fibers leads to enhance both the plastic viscosity and the constant viscous (decrease) for all mixtures comparing to that of concrete mixtures with one fiber type. In regards to hardened properties, it has been found that the fiber hybridization with 50/50% proportion increases the mechanical strength and the elastic modulus of concrete.

The Antifragility of FRC in the Crack Pattern of Reinforced Concrete Ties

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.

Fatigue of Plastic Fibre Reinforced Concrete in Bending: Assessment and Prediction

The present paper deals with an experimental study on the flexural fatigue behaviour of pre-cracked polypropylene fibre reinforced concrete with two different volume of fibre. Mechanical response was evaluated through compressive strength, elastic modulus and static bending test. Fatigue test considered an initial crack width accepted in the service limit state and the evolution of the crack opening displacement of the beams subjected to a prescribed number of cycles (1,000,000 or 2,000,000). After the cyclic load, the post-fatigue residual strength was evaluated and compared to the static response. Results suggest that the mechanism of crack development is independent of the adopted fibre content. The post-fatigue strength seems to be unaffected by accumulated damage due to cyclic load and the static load-crack opening displacement curve might be used as a criterion to predict the residual strength. Furthermore, a conceptual model is proposed to predict the crack opening as a function of number of cycles in view of accumulated fatigue damage. The equation was validated for different fibre content and polypropylene fibres.

Fatigue of Cracked Steel Fibre Reinforced Concrete Subjected to Bending

This paper presents an experimental investigation on the post-crack flexural fatigue behaviour of a steel macrofiber reinforced concrete (SFRC) and a high performance steel microfiber reinforced concrete (HPFRC), on notched beams considering the crack opening for serviceability condition. Different load levels were applied by means of three-point bending tests in order to verify the fatigue life. Performance of SFRC and HPFRC was compared under cyclic dynamic tests. Higher load levels seem to lead to failure through a continuous pull-out of the fibres, generating a more ductile response. Smaller load levels can be responsible for the progressive weakening of the fibre-matrix interface through micro-cracks. The conducted probabilistic approach has demonstrated to be suitable to predict the flexural fatigue life of pre-cracked SFRC and HPFRC for a desired probability of failure. From the experimental intrinsic scatter of the fatigue phenomenon, in particular for high levels of applied fatigue load, the amount of fibres in the cracked cross section seems to play an important role in withstanding the fatigue load.

Direct Tensile Tests of Supercritical Steel Fibre Reinforced Concrete

Steel fibre reinforced concrete (SFRC) becomes increasingly interesting for structural design and application. However, to reinforce structures just with steel fibres – not including any rebar – supercritical fibre contents are essential to ensure hardening behaviour in the post-cracking domain. Material properties are usually determined from experiments conducting three- or four-point bending tests. Specific conversion factors capture the softening behaviour and enable to transform flexural into tensile strengths. Own experiments prove that fibre contents of 1.8 Vol.-% yield flexural strengths of about 8 MPa. To get definite and reliable tensile strengths, direct tensile tests on optimised bone-shaped specimens made of supercritical SFRC are proposed here. As a specimen a slab (w × h × l = 200 × 100 × 720 mm3) is casted horizontally. That way, fibre orientation and distribution representative for practically relevant slabs with 10 cm thickness are simulated. To eliminate the so-called wall-effect that occurs during casting, the edges are cut off by water jet cutting before testing. Two pairs of displacement transducers on each face of the slab record the crack opening over a measuring length of 100 mm on the top and 400 mm on the bottom face. A new test set-up is introduced. Loading is applied to the specimen by friction using pre-tensioned threaded steel rods. Coating with an epoxy resin and corundum guarantees the required coefficient of friction. Displacement transducers on the top and bottom of the specimen record the relative displacement between the specimen and the test station. Axial loading is induced by a triangular steel structure (framework). Strain gauges on the outer faces of the diagonal struts control inevitable eccentricities of the load transfer. Consequently, highly accurate measurements are recorded. During testing, the crack flanks are slowly pulled apart from another (up to 4 mm) but without complete separation. On average a maximum tensile strength of 3 MPa and a coefficient of variation of 11% for maximum force is recorded what indicates a small scatter and highly accurate strengths.

Uniaxial Tension Tests of Steel Fibre Reinforced Concrete with AE Monitoring

The uniaxial tensile behaviour of steel fibre reinforced concrete (SFRC) is an important material characterisation. However, uniaxial tensile tests (UTT) are difficult to perform and thus less reported in literature compared to bending tests. This paper investigates the uniaxial tensile behaviour of four SFRC mixtures. An advanced UTT setup was developed with acoustic emission (AE) monitoring. Besides monotonic loading, also progressive cyclic loading has been applied. Two hooked end steel fibre types are both used in a content of 20 and 40 kg/m3. These fibre characteristics and their distribution and orientation have a large impact on the load-displacement behaviour. Micro-cracking is detected by means of AE activity. Failure mode analysis shows the onset of macro-cracking by the downshift of average frequency and the increase of rise angle. A larger amount of frictional damage results in more AE events during unloading and an increased AE energy amount. Localisation of AE events validated the position of the fibres in the fracture plane. During the different loading stages, the crack’s initiation and development are accurately localised. Lastly, predictions of the tensile strength and behaviour according to Model Code 2010 are in good agreement with the experimental results.

An Experimental Study on the Flexural Fatigue Behaviour of Pre-cracked Steel Fibre Reinforced Concrete

Fatigue behaviour of concrete has become an increasingly popular topic in the last century, especially with the development of railway bridges. Fatigue loading may appear in various forms, from physically applied loads, to indirect loads, including corrosion and thermal fatigue among others. These fatigue loadings may occur independently or in conjunction with the applied fatigue loadings, which could exacerbate the fatigue process and likely decrease the lifespan of the structure. The mechanisms of fatigue failure in concrete may be divided into three phases: (1) crack initiation, (2) progressive growth of micro-cracks, and (3) convergence of micro-cracks to form macro-cracks. In fibre reinforced concrete (FRC), energy is dissipated in the wake of the crack tip, which increases the load carrying capacity, thereby providing post-cracking ductility. Unlike ferrous materials, concrete was found to exhibit no fatigue limit after 2 million load cycles. However, its performance may be influenced by stress levels, load frequency, boundary conditions, matrix composition, and number of applied cycles. In this paper, the flexural fatigue behaviour of pre-cracked steel fibre reinforced concrete was investigated. Various pre-cracks and load levels were considered. X-ray Computed Tomography (CT) scans were implemented to determine the extent of damage to the fibres within the concrete matrix after fatigue loading.

Experimental Study on Punching Shear Strength of Fiber Reinforced Concrete Slabs

This paper is part of an ongoing research project on punching shear strength of reinforced concrete flat slabs incorporating macro synthetic fibers. Results obtained from testing six slabs (1500 × 1500 × 150 mm) are reported and discussed in this paper. The slabs were reinforced with flexural reinforcement ratio of 0.9% and macro synthetic fibers were added at two volume fractions of 0.5% and 1.0%. Slabs were tested in simply-supported configuration system with center point load increasing monotonically in displacement control mode. Loads, slab deflection, strain in the concrete and steel were measured in the experiment and crack patterns and failure mode were observed. The results revealed noticeable enhancement in the punching shear capacity, cracking behavior, energy absorption and ductility of the reinforced concrete slab. The addition of macro synthetic fibers increased the punching shear strength by up to 17% and the energy absorption by up to 73% for the tested FRC slabs relative to the control slab. Furthermore, the slab deflection profile showed that the addition of fibers modify the shape of slab deflection which indicates that fibers help engaging the whole slab in carrying the applied load.

Mechanical and Fracture Behaviour of an HPFRC

The current paper analyses the mechanical and fracture behaviour of a High-Performance Fibre Reinforced Concrete (HPFRC). An HPFRC was developed in a previous stage aiming to simultaneously, maximise aggregates content, achieve a compressive strength of 90–120 MPa and maintaining self-compactability (SF1+VS2). The benefits of fibres hybridisation (using fibres with lengths of 13, 35 and 60 mm) on flexural strength are investigated using the wedge-splitting test, in order to achieve the highest performance while keeping a relatively low fibre content. The final selected mixture was characterised in terms of workability, compressive strength and modulus of elasticity. Six notched prismatic specimens were subjected to three-point bending tests, according to EN 14651, for classification according to the MC2010. Based on the bending tests data, the simplified linear characteristic tensile stress vs. crack opening displacement relationship of the HPFRC was evaluated according to MC2010 and two other analytical approaches available in the literature.

Property Assessment of Self-compacting Basalt Fiber Reinforced Concrete

The aim of the research was to determine the suitability of chopped basalt fibers to reduce the brittleness of high performance concrete (HPC) while maintaining the requirements for self-compacting concrete (SCC). The study investigated the influence of the basalt fiber volume content on the fresh properties of self-compacting concrete and the mechanical properties of basalt fiber reinforced high performance concrete. Basalt fibers were added at the ratios of 0, 0.025, 0.05, 0.075, 0.125, and 0.25% by volume to the concrete mixtures in which 46% of the cement was replaced by ground granulated blast furnace slag (GGBS). The water to binder ratio (w/b) in the SCC mixtures was kept constant at 0.32. The influence of fiber volume content on the fresh properties of SCC, including filling ability (slump diameter, flow time) and passing ability (L-box) of the mixtures as well as mechanical properties such as the compressive strength, tensile splitting strength, flexural strength, and toughness indexes were analyzed after the moist curing of concrete at 7 and 28 days. The toughness, as the one of an important parameter for fiber reinforced concrete, was determined in accordance with ASTM C 1609. None of the basalt fiber reinforced concrete beam specimens reached the intended deflection of span/150. The load-displacement curves showed a rapid decrease in load after peak load, and the beam specimens reinforced with chopped basalt fibers showed their little potential for producing ductile HPC. Nevertheless, the higher content of basalt fibers improved the strength, and toughness of the SCC up to 0.25 vol.% fibers, and therefore the use of these fibers improved the overall SCC performance. The mechanical properties of the sustainable SCC mixtures containing the high content of GGBS as a cement substitute and a low content of basalt fibers falls within ranges suitable for structural engineering applications.

Influence of Steel Fibers and Casting Direction on the Bond Between Concrete and Reinforcement Bars

This research aimed to analyze the stress transfer mechanism in specimens under direct tensile stress, through the cracking process on two types of self-compacting concrete mixtures, without fibers and using steel fibers (hybrid reinforcement), which were cast horizontally and vertically. To achieve the purpose, prismatic specimens were produced and their mechanical properties evaluated through of the stiffness, load and displacement of the cracking process characterization. The specimens used dimensions of 150 × 150 × 750 mm and were cast in the vertical and horizontal position with 20 mm steel bars longitudinally, centralized in the cross section. The composition of self-compacting concrete using fibers has a volumetric fraction of 1.5%. The results of experimental tests indicated different crack patterns and fracture mechanism. To mixture without fibers, the vertical direction casting group shown a cracking process synchronously on each specimen faces, demonstrating homogeneity of the mixture and less presence of voids in steel-concrete interface. Moreover, both groups without fibers, vertical and horizontal, performed a brief cracking level, quickly reaching the steel bar stiffness. On the other hand, the steel fibers presence stiffened the prismatic rods, which resulted in gradual levels of stress transfers between steel and concrete. The first crack opening in specimens of horizontal direction casting group showed increase in load and stiffening modulus, when compared to vertical group, and a steady stiffening modulus after. The vertical group performed a gradual decrease stiffening modulus in post-first crack stage. The specimens face of vertical group in the post-cracking stage showed regular and simultaneous spacing between the cracks, while the horizontal group performed an asymmetric crack pattern. Thus, the direction of horizontal casting influenced the inside distribution of fibers, resulting in a favorable mechanical orientation caused during the casting of the fresh mixture.

Evaluation of Behavior of the Joint Between Two Concrete Layers During Splitting

The evaluation of behavior of the joint between two concrete layers during splitting was given on the basis of the results of wedge splitting tests (WST). The wedge splitting test was performed on a cube with an edge of 20 cm, which was made of two layers. There is a joint between the layers that is loaded by the splitting force. The upper (second) layer of concrete was placed after 2, 3, 4, 7 and 10 days. The WST was performed at the age of the second layer of 28 and 56 days. An important conclusion of these investigations is that the joint must be treated, with a roughness that ensures the resistance of the joint against crack propagation. The fibres have a big impact on this.

Short-Term Effects of Moderate Temperatures on the Mechanical Properties of Steel and Macrosynthetic Fiber Reinforced Concretes

Fiber reinforced concrete (FRC) is a special type of concrete with improved mechanical properties due to the introduction of fibers. Macrosynthetic fibers have been recently proposed as structural reinforcement and further investigations are required to evaluate their performance in aggressive environments wherein their properties may change due to factors such as temperature.This research examines the effect of moderate temperatures on the short-term behavior of pre-cracked and non pre-cracked FRC specimens with steel and polypropylene-based macrosynthetic fibers. A concrete type C30/37 was chosen to be as close as possible to common practice, as well as the fiber type and their content (2.1% and 0.7% respectively). The experimental campaign consists of 72 beams which were tested according to a modified procedure based on the standard EN 14651:2007 + A1:2007 in order to assess their residual flexural strengths at target temperatures (−15, 20 and 60 ℃). Additionally, 18 cubes were produced to perform compression strength tests at these environmental conditions. Slump, density, air content and compression strength at standard conditions were also tested to characterize the mixes. To guarantee that the interior of the specimen has reached the target temperature during the tests, all specimens were exposed 72 h at these target temperatures before testing. During the test, the variation of internal temperatures of the specimens was greatly reduced by means of a custom insulation system. Part of the analysed beams were pre-cracked at room temperature at the age of 28 days up to a crack opening equal to 0.5 mm. These pre-cracked beams were re-loaded again at the selected moderate temperatures to investigate the effect of the temperatures on large crack mouth opening displacements in already cracked elements when compared to uncracked elements.The results obtained show differences in the behavior depending on the type of fiber. The steel and polypropylene fiber reinforced concretes investigated in this study maintained overall good residual strength values at the temperatures selected after 3 days of exposition to moderate temperatures.

Residual Flexural Strength of SFRC: A Multivariate Perspective

The main contribution of steel fibres to the hardened state performance of steel fibre-reinforced concrete (SFRC) is the residual flexural strength the material exhibits, which is commonly characterised by the residual flexural strength parameters (fR1, fR2, fR3, and fR4) as defined by EN 14651. A database of values of residual strength parameters corresponding to hundreds of prismatic specimens from different SFRC mix designs has been put together from previously published papers. Multiple linear regression has been applied to derive a model which relates these parameters to the steel fibres aspect ratio, length and volume fraction as well as the relative amounts of the SFRC mix constituents. The model obtained presents a very good fit to the data collected, and its relatively simple specification makes it a promising tool to optimise SFRC mix designs from the point of view of residual flexural strength. The effect of fibre dosage and dimensions and that of their interactions with other mix design parameters such as water, cement, or aggregate contents are analysed by means of response surface plots representing the average trends reproduced by the model. These modelling and analysis efforts are part of an ongoing study, and this paper focuses on the residual flexural strength parameters fR1 and fR3. In relation to the dimensions of the fibres, the effect of fibre length on residual flexural strength has been found to be comparable to that of fibre volume fraction. This, together with the sensitivity of residual flexural strength to the fibre aspect ratio, leads to the conclusion that it is not necessary to use steel fibres in high dosages to proportion SFRC mixes with better-than-average levels of residual flexural strength. The key points emerging from the interpretation of the proposed model are presented and discussed in the context of the wide range of SFRC mixes represented by the database it is based upon.

Dynamic Behaviour of Steel Fiber Reinforced Concrete Plates Under Gun Fire and Free Fall Tests

Due to an increasing threat of attacks with small arms or explosions on (government) buildings or structural facilities requiring special protection, protection against such impacts is becoming more and more important. For this reason, the Chair for Concrete Construction at the Institute of Structural Engineering at the Bundeswehr University Munich is conducting research into the improvement and development of effective structural protection systems. For this purpose, a protective layer of metal is used, which is either concreted into a steel fiber reinforced concrete slab with concrete compressive strength class of C40/50 (e.g. ring mesh) or subsequently applied to one side of the hardened slab surface (e.g. metal foam). The protective function thus achieved is validated by means of gunshot tests. Thus, in a first step, the crater ejection on the protective side, i.e. the side facing away from the bombardment, is documented. This is followed by an evaluation of the crater volume by weighing the tested specimens. The mass determined by the difference (before-after) is verified by means of a geometric control calculation.Based on the collected results, it was shown that calibers with a dimension of 7.62 × 51 mm are absorbed by the concrete slab and thus do not pose any danger to people and material on the protective side. Despite the positive results of the protective coatings used, great research efforts are still required to optimize the protective wall panels in order to reduce the mass, size and flight distance of the concrete debris to a level that meets the safety requirements, in addition to preventing the passage of the projectile, in order to ensure the greatest possible protection.The aim of the optimization is the best possible coordination of the material compositions, layer types, layer thicknesses and geometry of the protective layers. Based on this, the optimized protective wall panels are integrated into a completely designed and functional wall system, i.e. from the foundation to the technical control equipment for an effective and practical solution for the protection of critical infrastructures. In addition to the effectiveness of the protective wall panels, economic efficiency is a decisive criterion. Therefore, the use of materials is also to be optimized to produce the thinnest and lightest panels possible, which nevertheless fulfill all safety requirements as a result of impact loads. To investigate the behavior of the panels under impact loads, a free fall setup was developed at the institute. This allows a quantification of the panel damage due to impact loads because of differently shaped free-falling metal bodies as well as a comparison of the damage pattern between the gunshot and free fall tests.

Spinability and Characteristics of Particle-Shell PP-bicomponent Fibers for Crack Bridging in Mineral-Bonded Composites

Polypropylene (PP)-fibers are one of the most widely used polymer fibers for several different applications in fiber reinforced concrete due to their availability, low price, chemical inertness and stability in high alkaline environment. In order to improve the fracture energy and toughness of fiber-reinforced mineral-based composites under impact loads, the energy absorption provided by the fiber material itself but also the failure mechanisms in the fiber-matrix interphase play a crucial role. A desirable pull-out behavior for high energy absorption is achieved for polymer fibers with high tensile strength in combination with high surface roughness. Based on this knowledge, new PP-bicomponent fibers have been developed containing different particles (e.g. Al2O3, CaCO3) in the outer shell in order to generate a rough fiber surface. In this work the results of first spinning trials of PP-bicomponent fibers produced by a lab-scale spinning equipment are presented. The fibers` tensile strength and particle distribution along the surface was determined depending on the drawing ratio. In single-fiber pull-out tests the fibers enabled high energy absorption compared to state-of-the-art PP-fibers. Furthermore, the structure of the fibers surface before and after pull-out was analyzed by scanning electron microscopy and revealed enhanced mechanical interlocking.

Damage of Fracture Properties of Polyolefin Fibre Reinforced Concrete Under High Temperature

Concrete has become the most common construction material, showing, among other advantages, good behaviour when subjected to high temperatures. Nevertheless, concrete is usually reinforced with elements of other materials such as steel in the form of rebars or fibres. Thus, the behaviour under high temperatures of these other materials can be critical for structural elements. In addition, concrete spalling occurs when concrete is subjected to high temperature due to internal pressures. Micro polypropylene fibres (PP) have shown to be effective for reducing such spalling, although this type of fibres barely improves any of the mechanical properties of the element. Hence, a combination of PP with steel rebars or fibres can be effective for the structural design of elements exposed to high temperatures. New polyolefin fibres (PF) have become an alternative to steel fibres. PF meet the requirements of the standards to consider the contributions of the fibres in the structural design. However, there is a lack of evidence about the behaviour of PF and elements made of polyolefin fibre reinforced concrete (PFRC) subjected to high temperatures. Given that these polymer fibres would be melt above 250 °C, the behaviour in the intermediate temperatures was assessed in this study. Uni-axial tests on individual fibres and three-point bending tests of PFRC specimens were performed. The results have shown that the residual load-bearing capacity of the material is gradually lost up to 200 °C, though the PFRC showed structural performance up to 185 °C.

Evaluation of the Inertia Force in Compressive Impact Loading on Steel Fiber-Reinforced Concrete

Steel-fibre-reinforced concrete (SFRC) is a strain rate sensitive material and, therefore, its dynamic and static compressive behaviour can be significantly different. In the present study, the effect of loading rate on the compressive behaviour of SFRC with 1% hooked end steel fibres is experimentally investigated. During impact loading, an inertia force is created due to acceleration along the specimen, whose effect in the range of impact is studied for a comprehensive assessment of the dynamic analysis of SFRC structures. For the evaluation of the inertia force, an instrumented drop-weight test setup is used, which includes two fast response loadcells with capacities of 1000 and 2000 kN on top (impact force) and bottom (reaction force) of specimen. The drop-weight impact tests were performed with three different drop heights, corresponding to maximum strain rates that ranged from 1 to 50 s−1. Two high-capacity accelerometers (5000 g) were mounted in the middle of the cylindrical specimens to obtain the cylinder acceleration response. The results show that, by increasing the strain rates, compressive strength, maximum acceleration at the middle of cylinder, and inertia force are increased. The results in terms of the ratio between inertia and impact load of specimens are presented and discussed.

Long-Term Properties

International Round-Robin Test on Creep Behaviour of FRC - Part 2: An Overview of Results and Preliminary Conclusions

The International Round Robin Test (RRT) on the creep behaviour of Fibre Reinforce Concrete (FRC) cracked specimens organised by the RILEM Technical Committee 261-CCF was outlined in the BEFIB2016 Proceedings. The objective of this paper is to present an overview of the results and preliminary conclusions derived from the RRT four years later. A total of 124 specimens with either steel or macro-synthetic fibres were tested in 16 different laboratories spanning across 5 continents and following four methodologies: flexural creep of small-scale prisms, direct tension creep and flexural creep of both square and round panels. Shrinkage and creep in compression were also assessed. Specimens were subjected to sustained load for 360 days. Then, they were unloaded and left to rest for 30 days to assess the creep recovery. Finally, specimens were tested to failure to assess the residual behaviour after one-year creep test. Although a general guideline was defined for the testing procedure, each laboratory had slightly different equipment and methodology. RRT results supported the identification of the main parameters affecting the creep results. Different variables, delayed deformations, methodologies and procedures, equipment and some parameters calculated from RRT results were analysed. The preliminary conclusions from the RRT are summarised hereafter.

Evaluation of Creep and Shrinkage of Patented Mixture of UHPC with Applied Heat-Treatment

In this paper the description and evaluation of results of experimental verification of rheological properties of patented mixture of ultra-high-performance concrete (UHPC) is presented. Specimens were cured with various curing regimes including curing by an increased temperature and in a water saturated environment. For the evaluation of the results an adapted model B4 is used, which is considered the most advanced rheological material model based on great consistency with large set of experimental results. It seems to be viable for use for prediction of creep and shrinkage of UHPC as it predicts long-term strains by incorporating effect of volume of additives and admixtures used in the fresh concrete. Model B4 also takes into effect thermal treatment of fresh concrete which accelerates cement hydration in early age. Current model B4 has several limitations that are often exceeded by characteristics of UHPC. In this paper, these limitations are identified and viable adaptation of model B4 is presented.

Cracking Behaviour of FRC Members Reinforced with GFRP Bars under Sustained Loads

GFRP bars are regarded as an alternative to steel reinforcement in marine and aggressive environments. However, there are some shortfalls to the use of GFRP reinforced members in flexure, which the addition of fibres can redress. This paper is concerned with the effect of synthetic fibres on the cracking behaviour of GFRP reinforced members. A number of FRC beams reinforced with GFRP bars were tested in flexure, considering different synthetic fibre contents and GFRP bar diameters. The flexural loads applied were representative of service conditions and were sustained for 90 days. The short- and long-term cracking behaviour was analysed in terms of crack spacing, distribution and development in pure bending sections. It was concluded that synthetic fibres increased the cracking moment capacity by up to 20% and reduced the crack width and crack spacing by up to 63% and 31%, respectively.The accuracy of the models available in current codes to predict crack width and crack spacing was assessed by comparing the experimental results to theoretical predicted values. The accuracy of crack spacing, and crack width predictions was found to vary with fibre content, and higher discrepancies were associated with higher fibre contents. This study shows that current prediction models for crack width and spacing need updating to make them better suited to elements reinforced with GFRP bars adequately considering the contribution of synthetic fibres.

Experimental Investigation on the Influence of Temperature Variations on Macro-synthetic Fibre Reinforced Concrete Short and Long Term Behaviour

In the last decades, fibre reinforced concretes became widely adopted in structural applications. Nevertheless, some elements of their mechanical behaviour are not fully understood. For instance, the effect of environmental conditions on the short and long-term behaviour of these materials has been studied to a limited extent only.In this perspective, the present paper presents the results of a large experimental campaign involving flexural tests on Macro Synthetic Fibre Reinforced Concrete (MSFRC) specimens under short- and long-term loads. Two different polypropylene Fibres were used, with dosages of 8 kg/m3 and 10 kg/m3. The effect of temperature on the short-term behaviour of these materials was investigated by performing three-point bending tests at 20 °C and 40 °C in cracked and uncracked conditions. The effect of temperature variations on long-term deformations was studied by means of four-point bending tests on pre-cracked notched beams at increasing temperatures, from 20 °C to 40 °C. The paper presents the test results as well as analyses the effective number of fibres crossing the cracks.

Short and Long Term Behaviour of Polypropylene Fibre Reinforced Concrete Beams with Minimum Steel Reinforcement

Fibre reinforced concrete (FRC) has quickly become an attractive solution for increasing both the mechanical performance and sustainability of concrete structures. In particular, polymeric fibres are increasingly recognized as offering significant benefits for FRC structural applications, especially in areas such as durability. Nonetheless, the behaviour of such FRC remains to be fully understood, especially from the perspective of long-term effects such as shrinkage and creep. Therefore, in this study, a comprehensive experimental programme is carried out for short- and long-term characterization of polypropylene FRC (PPFRC). The experimental program consisted of producing concretes C40/50 with 0, 3 and 9 kg/m3 of polypropylene fibres. Besides specimens for testing mechanical properties, shrinkage and creep in compression and under bending were tested. Finally, full-scale 3–m span beams with minimum steel reinforcement (0.18%) were tested until failure and under sustained loads. The results are analysed and the contribution of polypropylene fibres to reducing deflections and crack widths is assessed, with no evidence of local flexural failures due to tertiary creep phenomena. The results of this study can provide a contribution towards a fuller understanding of PPFRC structural behaviour and its future incorporation into design codes.

Analytical and Numerical Models

Computational Mesoscale Modeling for the Mechanical Behavior of Fiber Reinforced Concrete

In the last decades, fiber-reinforced concrete (FRC) has emerged in the civil engineering industry. Due to its excellent mechanical properties and functionality as crack propagation control, several numerical investigations have been carried out to study these composites. However, only few technical standards are established for these materials. However, the increase in computational cost in order to explicitly represent the fibrous reinforcement in FE models and the strategies to simulate the interfacial relations between each phase are among the greatest challenges of mesoscale modeling. This paper proposes a numerical methodology to simulate the mechanical behavior of fiber-reinforced concrete in a mesoscale approach with the Finite Element Method (FEM). The mesoscale level is the fiber scale; in this sense, the cementitious matrix, the discrete and random fiber reinforcement, and the interfacial transition zone (ITZ) are explicitly represented in the computational models. Finally, the formulation is validated against experimental data available in the literature. Moreover, the simulations considering this new mesoscale element formulation present reliable results close to the experimental responses.

Numerical Multi-level Model for Fibre Reinforced Concrete: Validation and Comparison with Fib Model Code

In this contribution, a Finite Element modelling scheme for steel-fibre reinforced concrete (SFRC) is proposed with which the post-cracking response of fibre reinforced structural members can be predicted. In contrast to the common guidelines, the post-cracking response of SFRC is derived from the actual fibre properties instead of indirectly from bending tests. The numerical model is designed to directly track the influence of design parameters such as fibre type, fibre orientation, fibre content and concrete strength on the structural response. For this purpose, sub models on the single fibre level are combined into a crack bridging model, considering the fibre orientation and the fibre content, and are integrated into a finite element model for the purpose of numerical structural analysis. The predictive capability of the proposed numerical multi-level model for SFRC is systematically validated by means of test series performed on the fibre, crack and the structural level. The experimental study comprises pull-out tests of Dramix 3D fibres, uniaxial tension tests involving different fibre contents and fibre types as well as three-point bending tests on notched beams with 23 and 57 kg/m3 Dramix 3D fibres. Furthermore, the results are compared to the modelling approach presented in the fib model code 2010 and an inverse analysis approach.

Isotropy-Based Analytical Model to Estimate the Residual Strength of FRC

This study describes an analytical model to estimate the post-cracking strength of FRC under flexural stresses in a three-point bending configuration. Considering the content and the number of fibres within concrete, statistical distributions are implemented to assign randomly the position and orientation of each fibre. Different degrees of isotropy can be defined depending on the properties of concrete, this leading to different distributions and orientations of the fibres according to the type of concrete. Pull-out laws are also implemented in the model to define the pull-out characteristics of the fibres at the crack section and determine the residual strength of FRC based on the contribution of each fibre to resisting the flexural load. Therefore, and assuming that only fibres contribute to the tensile strength after cracking, the pull-out load of all the fibres combined with the sectional equilibrium can be used to determine the post-cracking strength of FRC. Assembling the contribution of all the fibres provides a representative curve of the post-cracking strength of the analysed element. The estimations of the post-cracking strength curves are able to reflect the influence of the content and the orientation of the fibres, as well as the effect of the specimen dimension and the type of concrete, defined by the degree of isotropy.

Different Approaches for FEM Modelling of Strain-Hardening Cementitious Composites

Strain-hardening cementitious composites (SHCCs) are a special class of fibre-reinforced concretes which develop multiple, fine cracks when subjected to an increasing tensile load. This ensures remarkably high strain capacities of up to several percent, making SHCC an advantageous material both for new structures subjected to high mechanical loading or in severe environmental conditions and for the retrofitting/strengthening of existing structures by application of thin SHCC layers.Finite element modelling (FEM) can be an efficient tool to predict the behaviour of SHCC under different loading types. This paper lists some recent advances in finite element modelling of strain hardening fibre reinforced cementitious composites. Based on a comprehensive literature review two main approaches are highlighted, the continuum model approach and the advanced lattice modelling approach. This paper provides an overview of both approaches and their functionalities with respect to each other, based on results from literature. The work reported in this paper is part of a larger study into the use of SHCC for shear strengthening of existing concrete structures.

Exploring the Performance of a Single Panel SFRC Slab Under a Point Load with Fe Analysis

This study explores the performances of a single round panel without conventional longitudinal reinforcement using a nonlinear finite element analysis (FEA) software, ABAQUS. Experimental data from an established work on a full-scale steel fibre-reinforced concrete (SFRC) panel with 1.0% fibre volume were validated using FEA. A constitutive model was used to interpret the tensile behaviour of SFRC panel, and concrete damaged plasticity (CDP) of ABAQUS deployed for both tensile and compressive behaviour of the panel. The model was relatively stiffer than the experiment. The experiment result has a failure load of 38.0 kN while the FEA failure load was 37.9kN, thou stiffer than that of the experiment. This shows the FEA simulate the behaviour of the round panel under central loading. The parametric studies were carried to investigate the impact of steel fibre on the tensile behaviour of SFRC round panel by varying the fibre volume (1%, 1.25%, 1.50%, 1.75%, 2.00% and 2.5%) and characteristic strength (30MPa, 40MPa, 50MPa and 60MPa). The results of the central displacements, yield and peak loads, strengths, ductility, strain, and crack patterns. The FEA results show that the more the fibre volume, the more the strength. The FEA was able to provide an essential performance profile of SFRC round-panel.

An Analytical Study of Shear Transfer Mechanisms in Macro-synthetic Fibre Reinforced Concrete

Several researches have demonstrated the effectiveness of fibres to provide post-cracking strength to concrete elements. This is due to the potential of fibres to bridge the crack faces and continue transferring stresses along the shear crack by different shear transfer mechanisms. Several models have been stablished in order to explain the mechanical action of fibres in a shear crack, most of them explained as a function of parameters such as the fibre type, dispersion, inclination, aspect ratio or pull-out stress of the fibre. However, these models have been stablished only for steel fibres, which limits their use to different fibre’s materials. Within this framework, the present research first investigates the different shear transfer mechanisms acting in a shear crack, and evaluate the possibility of characterize the fibre shear transfer mechanism by means of the residual flexural tensile stresses (RFTS). For this, an analytical model that involves the transfer mechanism acting in a shear crack is developed. Analytical results are compared against experimental results obtained after testing 20 pre-cracked push-off specimens of plain concrete and macro-synthetic fibre reinforced concrete. Results evidence the great differences among aggregate interlock models available in the literature against the existing differences among the fibre ones. Finally, the feasibility of using RFTS to characterize the contribution of fibres to transfer shear stresses is observed.

Creep of Macro Synthetic Fibre Reinforced Concrete: Experimental Results and Numerical Model Calibration

The mechanical performance of fibre reinforced concrete presents aspects still under investigation, mostly those regarding the long-term behaviour. Even if creep and shrinkage are two well-known phenomena that characterize concrete, in case of FRCs, and in particular of macro-synthetic Fiber reinforced concretes (MSFRCs), there are no reliable models for predicting their long term-behaviour, because of the interaction between concrete, fibre creep and bond. In addition, temperature is a further variable to control since it affects the material performance.In this perspective, the present paper shows the experimental results of a large campaign of creep tests performed on macro synthetic fibre reinforced concrete specimens. The material tested had a compressive strength of about 55 MPa and it is reinforced with 8 kg/m3 of polypropylene crimped fibres. The experimental investigation is carried out by performing creep compression tests on cylinders and direct tensile test on notched cylinders. In addition, the tensile behaviour of the single fibre under sustained load is analysed. The tests were conducted in a humidity and temperature controlled chamber. Furthermore, the temperature was increased from 20 °C to 30 °C after a time of 50 days of testing in order to understand how this condition modifies the creep deformations evolution of the material.The paper shows also the initial calibration of a numerical model based on the Lattice Discrete Particle Model (LDPM) theory. The LDPM is one of the most validated theories able to reflect the actual coarse aggregate distribution of a quasi-brittle material, i.e. concrete. Currently this theory has been extended to include the fibres reinforcement. The aim of the big study presented would be to elaborate a predictive model for the MSFRCs accounting also for concrete and polymers long term behaviour.

Statistical Modelling of Flexural Fatigue Response of Steel Fibre Reinforced Concrete

Steel fibre reinforced concrete (SFRC) is widely recommended where fatigue is a predominant mode of failure, such as concrete bridges, offshore structures, and concrete pavements. From a detailed literature review, it was observed that the different fibre parameters like fibre type, fibre volume, aspect ratio, etc., influence the crack resistance performance and thus, the fatigue endurance limit. This paper attempts to understand the effect of various test parameters (stress ratio, frequency) and fibre parameters (length, diameter, volume of fibres, aspect ratio, and reinforcing index) on the fatigue life of SFRC based on available literature data. Studies involving SFRC with volume fractions of 0.13–2.0% are included in the analysis. The objective of the current study is to propose a generalized fatigue model, based on statistical analysis of data available from literature, for pre-cracked SFRC with the consideration of the above variables. A multiple linear regression (MLR) analysis using SPSS software was used to perform analysis of variance (ANOVA). The most significant parameters, obtained from the analysis, for the prediction of fatigue life of pre-cracked SFRC are stress ratio, length of fibre and reinforcing index. The generalized expression can be used for the prediction of the post cracking fatigue life of SFRC for volume fractions within the range of 0.13–2.0%. It is envisaged that these generalized models could be integrated into any SFRC design methodology requiring fatigue response prediction, leading to better-optimized designs. Overall, this study expands the state-of-the-art on fatigue behaviour of SFRC and its analysis.

Numerical Modeling of the Steel Fiber Reinforced Concrete Behavior Under Combined Tensile and Shear Loading by a Micromechanical Model Taking into Account Fiber Orientation

The mechanical behavior of steel fiber reinforced concrete (SFRC) is strongly dependent on the cracks bridging brought by fibers. Thus, the fibers orientation in SFRC is one key factor. This research extends a micromechanics-based model to describe the shear transfer in addition to tensile mechanisms at the crack surface and to establish a base at micro-scale for the further homogenization at the macro-scale. The shear effect is described as a function of the fiber pullout process, the stress across the cracks is then derived from the integration of the product of the fiber pullout function by the probability of the fiber location and orientation. The simulation results provide the SFRC behavior under mixed-mode displacement of crack (slip and opening). The influence of material parameters is investigated.

Predicting the Residual Flexural Strength of Concrete Reinforced with Hooked-End Steel Fibers: New Empirical Equations

To characterize the tensile behavior of Steel Fiber Reinforced Concrete (SFRC), international codes generally adopt performance-based approaches that require to perform either indirect or direct tensile tests on concrete samples. The fib Model Code 2010 recommends to assess the tensile performance of SFRC by a Three Point Bending test on a notched beam able to provide a series of residual strengths corresponding to different crack mouth openings detected at midspan. Therefore, when designing SFRC structures, the tensile properties considered in the safety verifications must be checked by laboratory tests involving a suitable number of beam samples. On the contrary, especially in case of preliminary structure sizing, designers need simple tools for estimating the potential tensile performance of concrete by starting from its basic properties. The present paper proposes two equations for predicting the residual strengths fR1 and fR3 included in most of the equations reported by the fib Model Code 2010 for safety verification of SFRC members. The concrete compressive strength, the fiber aspect ratio and volume fraction are the only three parameters included in the formulations. The assessment of the model effectiveness has been based on a statistical analysis including the adoption of a modified Demerit Point classification method. The good predicted performance of the proposed equations has been also proved by comparison with other similar models reported by literature.

Structural Design

Design and Performance of a Precast Bridge Barrier with Ultra-high Performance Fibre Reinforced Concrete (UHPFRC)

The use of precast concrete barriers instead of cast-in-place barriers avoids early-age cracking due to restrained shrinkage and thermal effects and accelerates the construction process. For these reasons, Polytechnique Montreal has developed since 2009 several durable precast concrete barriers with fiber reinforced concretes and various connection types to the bridge slab. Due to the excellent mechanical and durability properties of ultra-high performance fibre reinforced concrete (UHPFRC), it was integrated recently in a precast barrier concept developed for the Quebec Ministry of Transportation (QMT). This paper presents the concept and the mechanical behaviour under eccentric loading of the TL-5 precast hybrid barrier composed of a NSC core and a UHPFRC shell and connected to the bridge slab through a reinforced UHPFRC connection. An experimental test was performed on a full scale 2 m precast hybrid barrier and was reproduced numerically. The validated model was then used to reproduce the Canadian Highway Bridge Design Code (CSA-S6) loading conditions and validate the performance of the hybrid barrier. The model showed that the hybrid barrier capacity is 69 % and 43 % higher than the ultimate design load required in CSA-S6 when submitted to centered and eccentric loadings, respectively.

Shear Behaviour of V-shape Webbed Steel Fibre Reinforced Concrete Beams

Asymmetrical three-point bending tests were performed on beams with four different types of V-shaped/tapered webs (V-beams) containing various percentages of steel fibres by volume (0, 0.6 and 1%) as well as longitudinal reinforcement in the form of two 16 mm diameter steel bars. The test results revealed that the ultimate shear capacity increases significantly with increasing fibre content and larger web cross-sectional areas. The normalised shear stresses at ultimate resistance are relatively constant for the various types of V-beams indicating that shear capacity is highly dependent on the entire cross-sectional area of the web. An average increase in shear strength of 49% for beams containing 0.6% fibres and 74% for beams containing 1% steel fibres was observed for V-beams when compared to their reference beams containing no fibres. Furthermore, the results show that the widely used assumption where shear is resisted entirely by an effective rectangular cross-sectional area calculated as the product of the narrowest part of the beam (bbww) and the effective depth (dd) can result in over-conservative shear predictions for V-beams.

Punching Shear Resistance of SFRC Flat Slabs with and Without Punching Shear Reinforcement

The punching shear capacity of steel fibre reinforced concrete (SFRC) flat slabs without shear reinforcement has been the subject of extensive investigations over the past 40 years. Past investigations have shown an increase in the failure load of around 30 to 60 % and a change to a very ductile failure mode. However, most of these tests were carried out on slabs with a thickness of less than 150 mm, and are thus of only limited practical relevance. In addition, only twelve tests are available with a combination of SFRC and conventional shear reinforcement.To partly make up for this lack of experimental data, a total of ten punching tests were carried out at the Technical University of Munich over the last three years. The experimental program included three reference tests without steel fibres and seven SFRC tests. All specimens were manufactured with conventional flexural reinforcement, a slab thickness of 250 mm and four specimens had stirrups or shear studs as punching shear reinforcement. The parameters varied were the fibre type and content as well as the flexural reinforcement ratio. All of the SFRC specimens showed a significant increase in the bearing and deformation capacity compared to reference specimens.

Elevated Steel Fibre Reinforced Concrete Slabs and the Hybrid Alternative: Design Approach and Parametric Study at Ultimate Limit State

Application of Fibre Reinforced Concrete (FRC) in pile supported flat slabs is definitely challenge in term of a structural application of this material. Possible substitution of traditional reinforcement by steel fibres in the concrete mix drew attention of researchers due to clear benefits that could be provided by this technological alternative. This statement has been already proven by existing examples, in which during the construction, the optimization of resources, reduction of execution time and required labor force was highlighted. Nevertheless, considering the knowledge base associated with structural behaviour of FRC, it has been confirmed that hybrid solutions consisting in FRC with a moderate residual flexural tensile strength combined with conventional reinforcement placed in those zones where the higher bending moments are expected can be even more attractive from the technical point of view.Taking into account the abovementioned, the potential application of Hybrid Reinforced Concrete (HRC) for elevated slabs was studied. By modifying the ratio fibre/rebar content for different structural geometries, a parametric study was carried out in order to evaluate various solutions in terms of structural capacity in accordance with the requirements of Ultimate Limit State.

Shear Behavior of Hollow-Core Slabs Reinforced by Macro-synthetic Fibers

Several researches demonstrated that the addition of fibers in correct proportions can significantly increase the shear behavior of reinforced concrete elements, allowing to partially or totally replace the traditional web reinforcement. However, despite of this increase of knowledge about fiber reinforced concrete, there is still a gap in the applicability of fibers as shear reinforcement in certain precast and prestressed structural elements where the use of conventional transverse reinforcement is difficult due to their manufacturing process. In this context, the present paper presents the experimental results of full-scale tests on Hollow-Core Slabs (HCS) made with Polypropylene Fiber Reinforced Concrete (PFRC). In order to analyze the HCS end zones (critical zones in shear), several tests were performed on slabs adopting a shear-span-to-effective depth ratio equal to 3.5. Results show that macro-synthetic fibers are able to improve the shear strength of hollow-core slabs. The experimental results obtained were compared against the predictions of four international codes standards (Eurocode 2, ACI 318-11, Model Code 2010 and EN1168) as well.

How Can We Verify Structural Members Made of FRC Only?

The fib Model Code for Concrete Structures 2010 (MC2010) recognizes fiber reinforced concrete (FRC) as a new structural material, favoring its usage in innovative structural applications. According to MC2010, FRC can be adopted as the only reinforcement in structures provided that both strength and ductility requirements are fulfilled. This paper presents a discussion on the MC2010 requirements for FRC structures applied to both experimental and numerically simulated structural elements, proposing a refined set of equations for the ULS and SLS verification of FRC structures (not containing any classical reinforcement).

Codes and Standards

Eurocode 2 – Annex L – European Harmonized Standard for Steel Fibre Reinforced Concrete

The current Eurocodes are under revision and estimated to be available in 2023. For the first time in the European history, Eurocode 2: “Design of concrete structures” will be extended with a European-wide harmonized annex covering steel fibre reinforced concrete. The work on Annex L – Steel Fibre Reinforced Concrete has already started in 2012 and significantly benefitted from the work carried out for the fib Model Code for Concrete Structures 2010. The use of performance classes of Model Code 2010 as well as parts of the design approach were the basis for the new steel fibre reinforced concrete annex. In addition, the latest state of science has been used to prepare a powerful but, in the same way, easy-to-use design document for structural engineers, covering both ultimate and serviceability limit states for steel fibre reinforced structures, with or without reinforcement.

Reliability of Shear Strength Models for Fibre Reinforced Concrete Members Without Shear Reinforcement

The scope and amount of fibre reinforced concrete (FRC) structural applications have seen significant increases. This means that safe and reliable ultimate limit state (ULS) models are necessary for FRC structural members. Among these, shear strength of FRC members without shear reinforcement is highly important due to the brittleness of shear failure. Because of this, the fib Model Code 2010 introduced two shear strength models: an empirical model based on Eurocode 2 and a physical model based on the Modified Compression Field Theory. However, a comprehensive reliability assessment of these models has been lacking. Therefore, in this study, the safety format of these models is assessed and the partial safety factors for FRC in shear, γc and γF are updated. As a first step, a large database of experimental results on FRC beams is used to determine model uncertainties. Following this, a comprehensive parametric probabilistic analysis is performed using the First Order Reliability Method to determine the adequate values of γc for different target reliability indices β. The results of this study show that in order to reach typical reliability indices used in ULS design, γc and γF values need to be increased for FRC members without shear reinforcement for both models proposed by the fib Model Code 2010.

Yield Line Design for SFRC Elevated Slabs

The paper aims at discussing the application of Yield Line methods for the design of elevated slabs made of Steel Fibre Reinforced concrete. The paper considers three different case studies of Fibre reinforced concrete slabs ranging from laboratory small case test to real scale application. In particular, three different slabs are considered: circular simply supported slab (diameter 60 cm and thickness 2 cm), two different configurations of centrally loaded slabs with four point supports with 2 m span and a real scale elevated slab on columns with clear span of 6 m and nominal thickness of 20 cm. The paper discusses the approach that is going to be proposed by the Annex L of Eurocode 2 for the design of FRC structures and particularly compare the results of such a design approach and the experimental results.


Code Provisions for Shear Strength in Prestressed FRC Members: A Critical Review

Since its introduction, FRC has gained quite high attention from precast industry for the potential advantages resulting from partial or total removal of conventional reinforcement, especially related to shear. As compared to non-prestressed elements, the use of precompression may allow to significantly reduce the required amount of transversal reinforcement, taking advantage of the shear force carried by prestressing tendons/cable. As proved by several research studies, the adoption of fibers may further reduce shear reinforcement, which could be only limited to support areas or to those regions where the prestressing action is not fully developed.The current structural codes are generally very conservative in predicting the shear strength of these elements. Therefore, new research studies are needed to improve the accuracy of models for calculating the shear strength. Special focus could be also done to existing prestressed structures, which might benefit from more refined models in terms of reduction of structural repairing costs.This paper presents a database of shear tests of prestressed FRC members, emphasizing the rather conservative predictions of the prEN 1992-1-1 (2020) and fib Model Code 2020 drafts. A critical discussion will follow.

A Short Review on the Utilization of Basalt Fibres in Concrete

In recent days, fibre reinforced concrete already gained significant momentum to be used for the structural applications due to its better mechanical properties than the normal concrete. Steel fibres, synthetics fibres, glass fibres, carbon fibres, natural fibres etc. are few commonly used fibres in concrete mixes to improve the mechanical properties, post cracking behaviours, durability properties etc. for using in structural applications. In recent days, basalt fibres are also gaining interest to be used in concrete mixes due to its better physio-mechanical properties and economic production processes. This paper intends to provide a systematic short review of the recent developments, experimental test results of the basalt fibre reinforced concrete desired to utilize in the structural applications. Promising advantages of the utilization of basalt fibres are highlighted based on the published works, and problems are also highlighted.

Quality Control

Using Energy Absorption Capacity to Determine Residual Resistances of FRC

In this paper, the residual strengths and flexural toughness of fibre reinforced concretes (FRC), determined from the load - CMOD response obtained by mean of flexural test conducted according to EN 14651, are related experimentally through non-linear fits, which depend on the fibres content and crack opening. On the other hand, it is experimentally verified that there is a linear relationship between the flexural toughness and the energy absorption capacity of FRCs determined by testing square panels following EN 14488 – 5.Using the proposed relationships, the residual strengths of FRCs have been estimated from the energy absorption capacity, with differences between experimental and estimated values less than 3%.

Case Studies: Structural and Industrial Applications

Use of Steel-Fibre Reinforced Concrete to Extend Service Life of Temporary Safety Concrete Barriers

Temporary concrete safety barriers used by the Quebec Ministry of Transportation (QMT) have very short service life, mainly due to repetitive damages resulting from low velocity impacts occurring during their handling procedure. Utilization of steel fibre reinforced concrete (SFRC) in barrier represents an economical alternative to delay crack initiation and propagation. The quasi-static and pseudo-dynamic flexural and shear behaviours of concretes containing 0, 0.5 and 1% in volume of steel macrofibres were evaluated to select an appropriate SFRC to enhance the barriers service life. The maximal flexural and shear quasi-static strengths of SFRC increased by more than 40% in comparison to normal strength concrete (NSC) and the flexural and shear failure energies exploited in service condition also increased by more than 170%. Increasing the loading rate enhanced the maximal strengths by more than 12% for all the tested concrete, while keeping similar strength improvement between SFRC and NSC. Satisfactory results obtained at materials scale convinced QMT to complete a pilot application. 50 SFRC temporary safety barriers containing 0.5% of steel fibres were produced in a precast plant and installed on construction sites to establish their performance (cracking, spalling) during 1.5 years in comparison to 50 NSC barriers. Toughness of SFRC led to significant decrease of the damages observed in-field in the SFRC barriers in comparison to the NSC barriers.

A Fiber Reinforced Concrete for a Nuclear Waste Container

For the production of caissons, which are cubic containers for nuclear waste conditioning, the use of a fiber reinforced concrete in alternative to a classic reinforced concrete is studied. A numerical analysis approximating the behaviour of a caisson under load is presented in this paper. This numerical analysis allows for identifying the concrete properties that lead to satisfying structural performance of the caisson under load. This information is on his turn used in a second numerical analysis of laboratory bending experiments on notched beams according to the EN 14651 and round panel testing according to the ASTM C1550. The result of this second numerical analysis is a limit curve which identifies the mechanical performance that should be attained in these tests with 95% confidence. It allows thus for easy discriminating between an adequate concrete composition and a non-adequate one. The experimental work focused first on identifying a suitable fiber among three possible commercial candidates. Next a fiber dosage was defined and the concrete composition was submitted to small variations while verifying his performance according to EN 14651. It could be demonstrated however small changes in the concrete in terms of e.g. a variation in the water-to-cement ratio for 0,45 to 0,50 do not influence the performance according to EN 14651 in a significant way. Also it is demonstrated in this paper a concrete with Bekaert’s 3D 80/30 hooked end steel fibers at a dosage of 55 kg/m3 does not perform sufficiently well for the production of these caissons. The development of an adequate concrete would probably demand for higher fiber loads beyond 55 kg/m3 or a significant change in other concrete constituents beyond the current investigated ranges.

Design and Execution of Floors on Ground and Industrial Pavements with Fibre Reinforced Concrete

Steel and synthetic macrofibres are widely used as a replacement for nominal reinforcement in industrial floors and pavements. Some of the properties of the wet and hardened concrete enhanced by the fibres provide superior behaviour to the slabs containing conventional steel bars reinforcement. Fibres control the extent and size of cracks formed due to drying shrinkage and allow the extension of the slabs joint spacing. Fibres improve the post-cracking properties of concrete increasing flexural and ultimate load carrying capacity of the slabs. Finally, the combination of fibres with an expansive agent to obtain a “Shrinkage-Compensating-Fibre-Reinforced Concrete” allows the design of large slabs with almost no cracks and with tight joints widths. Although the same considerations for a good concrete mix design without fibres are also good for a FRC, care should be taken to avoid increasing potential shrinkage and curling using inappropriate materials and proportions. Examples of large floor and pavement works in Argentina using these technologies are presented, mentioning the different fibre types incorporated and explaining the FRC properties considered in the design. Comparison between laboratory and on field results is shown. FRC characterization included the evaluation of workability, free and restrained shrinkage, compressive and flexural residual strengths. In addition, comparisons between polymer and steel FRC behaviour are made.

Design Optimization of Fibres Reinforced Concrete Railway Tracks by Using Non-linear Finite Elements Analysis

Between 2007 and 2014, IFSTTAR, Alstom and other industrials partners have developed a new concept of railways track called New Ballastless Track (NBT). The concept was validated under 10 million fatigue cycles on a real-size mockup at IFSTTARA first numerical study, using a non-linear model was performed to evaluate the possibility of replacement of the original reinforced concrete layer of the track slab by a steel fiber reinforced concrete, to simplify the construction of the NBT track and to take advantage of the redistribution of mechanical stresses on a hyper-static structure. This study led to the conclusion that this replacement was very relevant.This paper is on the optimization of this Fibres Reinforced Concrete Railway Tracks solution by using the same non-linear numerical model. It is shown that this optimization procedure leads to a significant reduction of CO2 emissions compared with the initial one.

Elevated Flat Slab of Fibre Reinforced Concrete Non-linear Simulation up to Failure

The growing use of fibre reinforced concrete (FRC) on structural concrete has made that several codes and guidelines have included models for design in which the traditional reinforcement has been substituted partially or totally. Among the industrial applications, fibres as reinforcement of elevated flat slabs is gaining interest due the post-break bearing capacity of the material. This technology has already been used for real scale structures. This research contribution is focused on a parametrical analysis of FRC elevated flat slab by means of non-linear finite element simulation. The model is calibrated and compared with the real scale experimental test and with experimental slabs tested up to failure that can be found in the literature. The main goal of this paper is to carry out a parametrical analysis of the slab combining different types of reinforcement: FRC and hybrid reinforcement (fibres + conventional reinforcement) under design loads eventually choosing an amount of fibres for an actual test The results demonstrated that the combination of fibres and rebar improves the structure against failure, reduces deformation and presents a crack pattern better for cracking control.

TBM Thrust on Fibre Reinforced Concrete Precast Segment Simulation

Fibre reinforced concrete (FRC) is gaining acceptance as a structural material for casting precast segments as this has proven to lead to various advantages respect to the traditional reinforced concrete, especially for improving the crack control during transient loading situations. Concentrated loads induced during the excavation stage by Tunnel Boring Machines (TBMs) is still a matter of discussion into the tunnelling construction field, this having a strong impact from both technical and economic perspectives. In this regard, although specific codes and guidelines have been published and intense research has been carried out, this pointing out the benefits of using FRC as partial or total substitution of conventional reinforcement; however, there is still not a thorough research on the effectiveness of the FRC strength class (as defined into the fib Model Code 2010) to control crack widths in tunnel linings during the TBM thrust phase. To this end, the objective of this research contribution consists in carrying out a parametric analysis, considering different FRC strength classes (including hybrid reinforcements), related with the crack patterns due to the TBM-thrust on FRC segments. This was performed by means of a comprehensive non-linear finite element (FE) numerical simulation. The results derived from this research are meant to be useful for those stakeholders involved in the design of precast tunnel linings.

Design, Specification and Failure Investigation of Fibre Reinforced Concrete Ground Bearing Industrial Floors and Hardstandings and Pile Suspended Industrial Ground Floors

This paper discusses the significant observations of the Author’s involvement in the design and failure investigation of many fibre concrete floors and pavements over the last two decades. Serviceability failures of ground bearing industrial floors are more common than overloading. Nonetheless the extension of the related thickness design methodologies to hardstandings often fails to adequately recognise that the dominant failure mechanism is fatigue; such designs remain empirical but lack comprehensive substantiating trial data. Some methods of suspended fibre concrete floor design misrepresent the true global factor of safety, although full-scale load testing demonstrates that structural performance is not well represented by standard beam tests, and the true global factor of safety is close to that required by typical industry standard guidance.

Self-compacting Steel Fibre Reinforced Concrete: Material Characterization and Real Scale Test up to Failure of a Pile Supported Flat Slab

Steel Fibre Reinforced Concrete (SFRC) is increasingly being used in the construction industry providing structural, technological and economic benefits. However, this relatively new material has not demonstrated its full potential due to presence of certain aspects related to both concrete mix and structural design that should be further investigated. Specifically, pile-supported flat slabs is an interesting field of application of the SFRC; however, there are still aspects needing attention and solutions in order to make this structural application more attractive from both economic and technical points of view, these being (among others): the effect of new types of fibres on material properties and more insight regarding the structural capacity at both serviceability and ultimate limit state. Complementing the existing knowledge in aforementioned areas might lead to considerable expansion of SFRC application in elements with high structural responsibility.To this end, an extensive experimental program was carried out within the industrial-oriented project eFIB that contained characterization of 15 concrete mixes with different fibre types and its content (up to 120 kg/m3) and construction of 10 × 12 m SFRC flat slab. This prototype was loaded in different stages with permanent and life-loads in order to study both cracking and time deformation responses; the structure was eventually led to failure. The results of the described research are presented and discussed herein.

Structural Behavior of Precast Tunnel Segments Reinforced by Macro-synthetic Fibers During Temporary Loading Phases

Fibre Reinforced Concrete (FRC) has been increasingly used in precast tunnel linings. FRC, with or without conventional reinforcement could be an adequate solution for fulfilling the structural requirements. The design process of segmental concrete linings in ground conditions generally refers to the final permanent embedded in ground condition as well as to temporary phases. Among these temporary phases, the TBM thrust jack phase is a crucial loading condition during construction, which may noticeably govern the segment design. Tunnel elements having an internal diameter of 3.50 m and a thickness of 0.20 m were studied through full-scale tests to shed new lights on the possible use of macro-synthetic fibres in segmental lining. Precast segments were tested under bending in order to reproduce typical flexural conditions occurring during de-moulding, storage and transportation. Moreover, in order to study lining behaviour during TBM operations, segments were also tested under high concentrated loads for reproducing the excavation phase. Three main reinforcement solutions have been analysed: macro-synthetic fibres only (MSFRC segments), combination of an optimized amount of rebars and macro-synthetic fibres (Hybrid segments) and the reference solution made by conventional bars reinforcement (RC segments).

Fibre Reinforced Cement Sheaths for Zonal Isolation in Oil Wells – Quantification and Mitigation of Shrinkage-Induced Cracking

The formation of cracks in the cement sheath and delamination at the cement to steel casing interface due to shrinkage compromises the overall structural stability, imperviousness and durability of oil wells. Particularly, the flow of downhole fluids (e.g. methane or oil) through the cement sheath has become an environmental issue both in offshore and onshore oil wells. This study investigates the impact of fibre reinforcement on the initiation and propagation of cracks in a simulated oil-well section. The study combines a modified setup based on the ASTM-C1581 ring-test with Digital Image Correlation (DIC) in order capture, quantify and measure the initiation and propagation of cracks in the hardening cement and characterize the cracking pattern observed due to autogenous and drying shrinkage deformations and resulting self-induced stresses in the section. The experimental results obtained show the beneficial effect of fibre reinforcement on reducing the extent of cracking by increasing the post-crack ductility of the hardening cement. However, fibre reinforcement had a negligible role in preventing cracking initiation, which is governed by the cement matrix and degree of restraint. This study highlights the benefits of fibre reinforcement as a mitigation measure to reduce shrinkage-induced damage in oil-well cements.

Recent FRC Developments in Uruguay: Quality Control, Durability and Three Structural Applications

FRC in Uruguay is usually used in pavements and slabs-on-ground; design based on empirical experience of suppliers and post-cracking strength is not controlled. This paper presents the results of five research projects developed in Uruguay to address FRC use and to show its potential. A) Montevideo test (MVD), a stable test, which requires simple equipment and uses small samples was developed. Correlation between MVD and EN14651 show a qualitative and quantitative equivalence between them. B) The extent to which the cracking improvement given by fibres collaborates with concrete durability, was experimentally studied in beams exposed to a chloride environment. C) A direct experimental assessment of the SFRC orientation factor was performed for an elevated slab. D) A cross-section of Eladio Dieste’s famous vaults system is composed by a layer of masonry with steel in between them and a layer of mortar with steel mesh reinforcement. Through an experimental research, an equivalent response was obtained by sprayed FRC as a replacement of the mortar-mesh layer. E) Precast concrete sandwich panels, composed of two concrete layers separated by a layer of insulation, were experimental and numerically studied. Traditional reinforcement in the concrete layers was replaced by fibres, optimising the panel geometry and reinforcement.

Large-Scale Pressure-Swelling Tests on Panels Made of Strain-Hardening Cement-Based Composites with Different Bedding

Strain-Hardening Cement-Based Composites (SHCC) are a material class of short-fiber-reinforced concretes which has a strain-hardening behavior due to micromechanical adaptation of the concrete matrix to the short fibers under tensile stress after occurrence of the initial concrete failure. Since its first scientific description in the early 90s, this material has been a constant topic in the research area. In the meantime, extensive knowledge of mechanical properties, cyclic properties, fiber-matrix interactions, modeling and durability aspects is available.Nevertheless, the number of implemented practical application is relatively small, which is justified on one hand to the barriers from standardization and approval procedures and on the other hand by missing knowledge on the transferability of the results from the laboratory scale on test specimens in the scale 1:1.In addition to possible applications in building construction, the material seems to have the potential in road construction. It could combine the advantages of the two proven construction methods, asphalt and concrete construction, with the exclusion of some disadvantages, like joints, rods and others. In particular, a joint less and robust concrete construction could be realized by SHCC.This article presents results from the central loaded square panel tests with different bedding materials. The size of the panel was 2.5 m by 2.5 m and was loaded with up to 4 million cycles. The height of the loading was started on the single wheel load according to German road-building standards and during the experiment increased to 4 times.The relevant deformation and internal strains were measured and evaluated. After the loading the crack patterns were analyzed and specific cracks were investigated in detail, i.e. small samples from a specific crack were gathered out of the main block and the crack surface were analyzed in the electron microscope. The found failure mechanism were compared with cyclic tests on lab scale specimens to create a missing link between the two scales.

SFRC Underwater Slab for the Potsdamer Platz (PP) Berlin

Daimler-Benz (DB) the well-known car-producer decided to make the largest immobile investment in Europe, in the heart of Berlin. DB bought a large area in 1988, just near the wall. Nobody knew at that time that the wall would be taken down by the East Berlin people in November 1989. Since our office have made an engineering task in Stuttgart for DB, at a building in 8-m-deep groundwater, for DB it was clear that the same engineer should make the job as well in Berlin. Nobody knew at that time, that the water conditions of those two projects were completely different. In the project we handled before, the groundwater could be lowered by pumps. DB wanted the same concept for the PP. But in Berlin the water conditions are completely different. The depth of the water for the building pit was between 20 to 22 m and the water could not be lowered. The level of the groundwater was not allowed to be touched because the Berlin people get all their drinking water from the perfect filtered groundwater. The idea of a steel fibre concrete slab could be a solution for this project. However, we had no guideline, no code and no experience. Therefore, our procedure to design a safe and stable SFRC underwater slab was based on tests in scale but also on tests in the field with real dimensions. Our main task was to convince the authorities from the Senate of Berlin, DB and the German Railway Authorities. With this concept we got the agreement to build the deepest pit ever constructed before in a depth of 22 m.

Polypropylene Fibre Reinforced Concrete for the Structural Panels of the Pavillions of the Motril Port (Spain)

Two new pavilions for the Motril’s Port, finished in October 2020, were designed, and constructed considering polypropylene macro-fibre reinforced concrete (PPFRC) as structural material. The main purpose of this decision was to avoid the use of any type of steel reinforcements in the elements exposed directly to the marine environment (ex., façades) so that the durability and service life were positively benefited altogether with the maintenance costs and the aesthetic requirements. The object of analysis of this scientific contribution are the 2250 × 3480 × 150 mm PPFRC precast panels designed with a C50/60 with PP macrofibres as unique reinforcement. These panels are part of the external envelope and also provide structural support to the roof (and the loads to which this is subjected to). The PPFRC panels resulted from an optimized-oriented design integrating aspects related with material properties, structural behaviour, construction process and production. The solution proposed is, ultimately, oriented to maximize the sustainability of these type of structural elements.

Use of Macro Synthetic Fibre in Segmental Tunnels

Fibre reinforced concrete is becoming widely utilized in segmental linings due to the improved mechanical performance, robustness and durability of the segments. Further, significant time and cost savings can be achieved in segment production and by reduced reject or repair rates of segments during temporary loading conditions. The replacement of traditional rebar cages with fibres further allows changing a crack control governed design to a purely structural design with more freedom in detailing. A further benefit of replacing steel rebar cages by fibres is the significantly reduced carbon footprint.Recently completed research at the University of Brescia on full scale tunnel segments reinforced with macro synthetic fibres (MSF) confirmed the feasibility as well as the fulfilment of the structural requirements of the Model Code 2010. Parallel, field experience has shown that MSF reinforced concrete segments perform robust and dependable even under very difficult conditions.This paper presents and discusses the key findings from the segment research as well as three successfully completed reference projects using MSF as primary structural reinforcement.

Design and Verification of Elevated Slabs Made with Hybrid Reinforced Concrete: Case Studies

The literature has recently reported many real applications with the use of Fiber Reinforced Concrete (FRC) for the construction of elevated slabs. Most of these case studies suggests the use of steel fibers as the only reinforcement in addition to the Anti-Progressive Collapse (APC) used to prevent brittle failure mechanisms. However, this reinforcement arrangement does not usually represent an optimized design solution as high amounts of fibers (Vf > 70 kg/m3) are often required for reaching the minimum capacity to resist design loads as well as to the internal actions due to restrained shrinkage.This paper focuses on the design of FRC elevated slabs by using the most recent design provisions reported in the fib Model Code 2010. A simplified design procedure based on a consolidated design practice is described. The use of a properly designed combination of fibers and conventional reinforcement, referred to as Hybrid Reinforcement, is proposed. Different case studies taken from the literature are considered to prove the effectiveness of the proposed design approach.

A New Sustainability Assessment Method for Façade Cladding Panels: A Case Study of Fiber/Textile Reinforced Cement Sheets

As the building sector is one of the leading responsible for energy consumption and CO2 emissions, criteria of sustainability, availability, and recyclability should be considered for developing materials even in the envelopes. Façade, as the first element against the undesirable external impact, may contribute to building sustainability by reducing the amount of energy consumption and providing indoor environment quality for the inhabitants. The envelope excluding its aesthetic function should fulfill certain requirements such as strength, flexibility, ductility, lightness, thermal and acoustical insulation, durability, and sustainability. Fiber/Textile cement sheets as an interesting architectural material attract great interest during the last decade, especially those reinforced with more sustainable fibers like vegetables or textile wastes. In this sense, this paper presents a novel model to evaluate the sustainability index of façade cladding panel, especially the fiber/textile cement board. To this end, a new model for assessing objectively the façade cladding sustainability was designed and developed based on MIVES according to the value function concept and seminars of experts.

Validation Testing of Precast Tunnel Lining Segments Using Polymeric Fibers

Fiber reinforcement in precast tunnel segments is quite attractive since it reduces the cost and labor associated with the fabrication process while it improves the post-cracking behavior considerably. Among the benefits, one can address improvements in handling, fatigue, impact resistance and durability while reducing the crack widths significantly. Flexural tests of full-scale precast tunnel segments are conducted using a newly developed testing laboratory for large structural testing. Using the closed-form material properties obtained from the flexural beam specimens, the response of full sections is predicted and the test results are compared with the full-scale test results obtained on the testing facility. Using the proposed methodologies, one can develop proper material and structural models for the accurate design of tunnel segments due to their unique design characteristics.

Wind Tower FRC Foundations: Research and Design

The design of massive structures, like cast-on-site foundations for wind towers, is not an easy task for design. In fact, this kind of structure is not specifically taken into account by Eurocode 2 standard. The huge concrete volume requires a huge amount of reinforcement, even if only the minimum amount is selected, and it is not clear if the lateral confinement exerted by the truncated cone geometry, mainly loaded along only one radial direction, corresponding to that characterized by the fastest wind direction, really requires the huge transversal reinforcement conventionally introduced in current wind towers. Within the research programme developed in 2019 between Enel Green Power, the Enel Group company dedicated to the development, construction and operation of renewables across the world, and Politecnico di Milano, an experimental investigation, carried out on prototypes characterized by reduced scales (1:15 for the whole structure and 1:4 for the investigation of the only core foundation, where the cylindrical stem is anchored), was conceived to calibrate a reliable modelling with the aim of being extended to full-scale structure, optimizing the required reinforcement. In this perspective, the use of fibre reinforced concrete can significantly improve the construction time by giving a specific toughness to the whole foundation, limiting also the crack opening and the correlated durability problems in the serviceability limit states.

Textile-Reinforced Concrete

Development of Polymeric Textile Reinforced Concrete Structural Members

The life cycle costs of structural systems are a function of raw materials, labor, energy, environmental impact, serviceability, and durability. Textile Reinforced Concrete (TRC) using alternative reinforcing systems are cost competitive and have gained popularity among new construction materials. TRC composites are uniquely lightweight with a very high specific strength, stiffness, and ductility that compete and outperform light gage steel and wood products. TRC’s high potential durability and high strength is amenable to continuous production and formability, thus making it highly sustainable. An effective manufacturing technique was developed using automated pultrusion process for efficient production of TRC structural sections. The objective of this study is to develop and characterize the properties of PP based mesh reinforcement. Mechanical properties of textile reinforcement systems are used to justify the use of textiles for use in the production of TRC composite structural sections. Test results of flexural and tension specimens are discussed in terms of closed loop test results as well as Digital Imgae Correlation (DIC) technique. The study indicates that PP-TRC composites with 4% (representing both warp and weft directions) textile volume fraction can reach maximum tensile strength of 11 MPa with strain capacity of 23%. Flexural samples show an apparent flexural strength of 40 MPa and deflection capacity of up to 40 mm for a flexural sample on a span of 254 mm.


Mechanical Evaluation of Na-Based Strain-Hardening Geopolymer Composites (SHGC) Reinforced with PVA, UHMWPE, and PBO Fibers

Strain-hardening geopolymer composites (SHGC) show increased deformation capacity due to a multiple cracking tolerance under tensile loading. To evaluate their mechanical performance, a common metakaolin-based mixture was produced. Three types of short fibers were evaluated as disperse reinforcement: polyvinyl alcohol (PVA), ultra-high molecular weight polyethylene (UHMWPE), and poly(p-phenylen-2,6-benzobisoxazole) (PBO). The composites’ mechanical features were analyzed in compression, three-point-bending, and tension tests with subsequent Environmental Scanning Electron Microscopy (ESEM) analysis of the fracture surfaces. Digital Image Correlation (DIC) was used to evaluate the extent of multiple cracking and crack widths under uniaxial tension. Additionally, single-fiber pullout tests were performed. PBO-based composites yielded the highest mechanical properties, reaching a 4.8 MPa peak stress in tension at a strain level of 2.3%, with a larger number of cracks. PVA and UHMWPE-based materials, however, demonstrated a lower mechanical performance, because of their larger diameter, lower mechanical properties and fiber-matrix adhesion.

Nano-technologies Related to FRC

Influence of Dispersion Methods of Microcrystalline Cellulose on the Mechanical Behavior of Cement Pastes

The idea of using cellulose-based reinforcement is to explore all the benefits of the cellulose, the main component responsible for the strength of plants. With notable mechanical and physical properties, microcrystalline cellulose (MCC) inclusions are able to reinforce different types of matrices. In cementitious materials, the investigations about microcrystalline cellulose inclusions are quite recent. Moreover, the challenge involving dispersion is one of the limiting factors with regard to the full effectiveness of their properties. In order to assess the effect of the inclusion of MCC on the mechanical properties of cement pastes, compression and four-point bending tests were carried out with several contents of MCC, from 0.05% to 1% in mass of cement. The present work also aims to compare different methods of dispersion of microcrystalline cellulose into water by means of mechanical and chemical approaches, for further addition in cement pastes. The methods include mechanical stirring, superplasticizer addition, surfactant addition, and ultrasonication. Results show an impressive enhancement of the flexural properties of more than 5 and 8 times for strength and modulus, respectively with 0.75% of MCC. The use of ultrasonication as a dispersion method showed to be the most effective to disperse 0.05% of MCC.

Effect of Nano-SiO2 Coating on the Mechanical Recovery of Debonded Fiber-Cement Interface Under Water Curing

Recent studies have revealed that interfacial shear strength at debonded fiber/matrix interface can recover under water/dry conditioning cycles which is mainly attributed to the formation of CaCO3 that fills the cracks and enhances the interfacial friction. However, the CO32− ions (i.e., dissolved CO2) is often scarce at the deep part of a FRCC, significantly limiting the interfacial healing. In this study, SiO2 nanoparticles were successfully coated onto PVA fiber surface via sol-gel method, which is confirmed by SEM-EDS characterizations. The SiO2 nanoparticles coating layer can react with Ca(OH)2 to form C-S-H gel that refines the interfacial zone and potentially promote the healing degree after water conditioning. Single-fiber pull-out and preload-reload results indicate that the SiO2 coating not only enhanced the original interfacial bond, but also greatly helped the recovery at the debonded-and-healed interface. A healing degree estimation way has been proposed in this study and the rationality also has been elucidated.

Piezoresistivity of Carbon Black/Cement-Based Sensor Enhanced with Polypropylene Fibre

In this study, polypropylene (PP) was added to develop carbon black (CB)/cementitious composites as cement-based sensors. The mechanical properties and piezoresistivity were been experimentally investigated. The compressive strength slightly decreased, while the flexural strength was significantly increased with the increased amount of PP fibres. The improvement is mainly achieved by the reduced CB concentration in cement matrix and the excellent tensile strength of PP fibres. Under the cyclic compression, the piezoresistivity increased by three times for 0.4 wt% PP fibres filled CB/cementitious composite, regardless of the loading rates. The flexural stress sensing efficiency was considerably lower than that of compressive stress sensing, but it increased with the amount of PP fibres. Electrical conductivity increased with the amount of PP fibres, due to the enclosed CB nanoparticles and more conductive passages. Moreover, fitting formulas were proposed and used to evaluate the self-sensing capacity, with the attempts to apply cement-based sensors for structural health monitoring.


Development of Eco-Efficient UHPC and UHPFRC by Recycling Granite Waste Powder (GWP)

Ultra-high performance concrete (UHPC) is a class of cement-based composites with enormous potentials for sustainable construction thanks to its outstanding compressive strength and cracking resistance. While the former allows saving material volumes and reducing the structural weight, the latter confers unprecedent durability. However, one of the major limiting factor of the industrial use of UHPC drawbacks it is high cost of raw materials. Moreover, UHPC are often characterized by high cement content which not only remains unreacted for a considerable percentage, but it also contributes to the anthropic carbon emission. To reduce the environmental impact of UHPC and initial cost, granite waste powder (GWP) which is locally available as an industrial by-product of the granite quarry in Quebec was used to partially replace cement. UHPC mix-designs with low cement content (400 kg/m3) with 200 kg/m3 of recycled granite powder were produced by assuring satisfactory workability and the compressive strength. The present results compare the developed UHPC and UHPFRC using granite powder with a commercially available UHPC using various indicators.

Flexural Behaviour of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) Under Monotonic Loads and Loading-Unloading Cycles

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is an innovative material with great mechanical and durability performances, high ductility and toughness. Although the mechanical behaviour of UHPFRC has been extensively studied in the last years, the damage mechanisms and permanent strains of this material when subjected to flexural loads need to be further investigated, in order to quantify and to better predict the performance of UHPFRC structural elements.This work presents the results of an experimental study on the UHPFRC bending behavior. Both static and cyclic loading-unloading bending tests were performed. The effects of brass-coated steel fibers (diameter of 0.20 mm and length of 13 mm) on the flexural behavior of UHPFRC was investigated, varying the amount of fibers up to 2,5% by volume. Four-point bending tests were performed on prisms with different geometries (30 × 70 × 280 mm3 and 70 × 70 × 280 mm3). Particular attention was paid in the UHPFRC post-cracking behaviour, in order to evaluate the strain-softening and/or strain-hardening phases. Damage progress, number and width of cracks were monitored by means of a Digital Image Correlation (DIC) system on both the frontal and bottom surfaces of the specimens.Finally, a phase-field model has been implemented in a FE code and numerical simulations have been performed to better understand the effects of different fiber dosages on the mechanical behavior of UHPFRC specimens under cyclic loads. Concrete matrix and fiber reinforcement have been modeled as brittle and elasto-plastic phases of a mixture, whose internal energies are enriched by non-local damage and plasticity contributions.

Application of 3D Digital Image Correlation to Capture the Impact Beahviour of UHPFRC Plate

In this study, a repeated drop-weight test on a 50 mm thick four-side fixed UHPFRC plate was carried out. A high-speed 3D Digital Image Correlation technique (3D DIC) was employed to capture the full-field response of the plate during impact loading. The DIC results were validated against the measurements of physical accelerometer and strain sensors attached on the top surface of the plate. The DIC results in combination with a polynomial surface fitting method was used to obtain the mid-point deflection. Replicability of the plate’s dynamic response indicated that only negligible damages occurred in the plate due to repeated drops from 0.5 m. During the subsequent impact from 2.0 m, the overall performance was governed by flexure. Lastly, the plate failed with local failure and flexure after impacted from 3.0 m.

Experimental Characterization of the Tensile Constitutive Behaviour of Ultra-High Performance Concretes: Effect of Cement and Fibre Type

The Research Project ReSHEALience has been launched in 2018 in the framework of the European Programme Horizon 2020. The project aims at developing a new approach for the design of structures exposed to extremely aggressive environments, based on durability and life cycle analysis. In this regards, the starting point is represented by new advanced Ultra-High Performance Fibre Reinforced Cementitious Composites (UHPFRCCs), called Ultra-High Durability Concretes (UHDC) because of their enhanced durability obtained by means of engineered composition, which should be characterized by strain hardening behaviour under tension in both ordinary and very aggressive conditions. In this context, the first step is to develop an effective approach for identifying the main parameters describing the overall behaviour in tension. In the present study, indirect tension tests have been performed via 4-Point Bending Tests (4PBT). Starting from the test results, a combined experimental-numerical identification procedure has been implemented in order to evaluate the effective material behaviour in direct tension in terms of stress-strain law. Finally, the comparison among three mixes differing for fibre and cement type is reported in terms of tensile response and post-crack localization behaviour.

Mechanical and Durability Assessment of Concretes Obtained from Recycled Ultra-High Performance Concretes

The aim of this work is to analyse the mechanical and durability properties of Recycled Ultra High Performance Concretes (RUHPC) containing different amounts of recycled fine aggregate obtained from crushing Ultra High Performance Concretes (UHPC). This paper summarizes and compares the results from different experimental campaigns carried out in the framework of the ReSHEALience project (Rethinking coastal defence and Green-energy service infrastructures through enhanced-durAbiLity high-performance cement-based materials) which has received funding from the European Union’s Horizon 2020 programme (GA 760824). Mechanical performance was evaluated by means of compressive and flexural tests, whereas durability was evaluated by means of chloride penetration, chloride migration and water absorption capillary tests. The results indicated that replacing 50% or 100% of natural aggregates with recycled aggregates did not significantly affect neither compressive strength nor flexural strength. In the case of high replacement rates, a slight decrease in workability was detected, but the mix retained its self-compacting properties. RUHPC had similar durability performance as UHPC. In conclusion, the results have shown that it is feasible to produce RUHPC; the recycled fine aggregate has shown great potential to be used in the production of new UHPC. Scalability of the recycling procedure to industrial level was also addressed in order to pave the way towards the uptake from the different value chain actors of the construction industry of the innovation potential demonstrated by the research.

An Experimental Evaluation of Direct Tensile Strength for Ultra-high Performance Concrete

Ultra-high performance concrete (UHPC) has emerged as an advanced material due to its superior mechanical properties and excellent durability. In addition to very high compressive strength exceeding 150 MPa, the tensile strength of UHPC is much larger than that of normal concrete and considered as an important parameter in structural design. To determine the tensile strength, test methods usually include the splitting test, double punch test, flexural test, and direct tension test. Therefore, it is needed to evaluate these methods to get accurate values of tensile strength of UHPC and UHPFRC. The direct tension tests were adopted by many researchers and also considered as a reliable method. In this study, direct tension tests were conducted to evaluate the tensile strength of UHPC. The UHPC was designed to obtain a nominal compressive strength varying between 150 and 200 MPa at the age of 28 days. In addition to plain UHPC, micro steel fibers were added into UHPC mixture with volumes of 1% and 2%. There were two types of prisms for direct tensile tests: (1) Prisms of 40 × 40 × 160 mm without notches; (2) Notched prisms of 40 × 40 × 80 mm. The difference between the direct tension tests on prisms with and without notches was clarified and discussed. Finally, the tensile strengths of UHPC and UHPFRC obtained from this study were compared with those suggested by several guidelines.

Finite Element Modelling of UHPFRC Tensile Bars

In the present paper, a numerical modelling study on the uniaxial tensile behaviour of reinforced UHPFRC ties by means of a non-linear finite element model (NLFEM) is carried out. The results obtained from the simulation done by the NLFEM are compared to the results from an experimental programme adopted. These tensile bars (ties) modelled in this work are cast using UHPFRC with 160 kg/m3 of steel fibres and tested in a direct tensile test. The NLFEM developed for UHPFRC reinforced flexural beams used in previous research is improved and applied in order to validate the mechanical tensile characterisation of UHPFRC when direct tensile reinforced elements are considered. As it happens with the flexural elements, in this case the shrinkage and tension stiffening effects are essential in the model to simulate the reality of the tensile test. After the NLFEM simulation, very accurate results are obtained that lead to consider the reliability of the NLFEM model developed not only for flexural reinforced elements, but also for direct tensile ones.

Fibre Reinforced Concrete: Improvements and Innovations II
Prof. Pedro Serna
Prof. Aitor Llano-Torre
Prof. José R. Martí-Vargas
Prof. Juan Navarro-Gregori
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