Transforming Construction: Advances in Fiber Reinforced Concrete

Strain-hardening cementitious composites (SHCC) are ductile concrete composites. Because of the incorporation of polymeric micro fibers, multiple cracking occurs under increasing tensile load in contrast to the brittle behavior of conventional concrete. This quality makes SHCC an interesting material for applications that require resistance to high-velocity impact. Mostly, polyvinyl alcohol fibers or high-density polyethylene fibers are used in SHCC, however, their high mechanical performance is accompanied by high costs resulting from the production by gel-spinning. In contrast, polypropylene (PP) fibers are produced by more efficient and less complex melt-spinning with comparably low energy consumption. They are widely used for other purposes in concrete already and can be a suitable alternative for the use in SHCC because of their good performance/ price ratio and alkali-resistance. On the other hand, their smooth surface and lack of active groups results in a poor wettability and insufficient interaction with the cement matrix, what usually results in lower SHCC mechanical performance. To overcome this issue, in this work a recycled PP (rPP) is used for the fiber melt-spinning. This combines the aspect of sustainability as well as the fact, that rPP shows potential to enhance fiber wettability because of a higher oxidation degree. The use of rPP could therefore be a way to improve the adhesion between fiber and matrix. However, the high shear stresses and temperatures during recycling by multiple processing may compromise the mechanical properties and limit the processability during spinning. Therefore, the aim of this work is to compare the surface properties of rPP and virgin PP (vPP) as well as their spinnability.

A major issue in the development of more sustainable fiber-reinforced cementitious composites is the control of the fiber-matrix interaction. In this study, ultra-high molecular weight polyethylene microfibers are surface-modified using two bio-based materials, dopamine and chitosan. Both coatings add polar groups to the surface of the initially nonpolar and non-reactive fibers; the combination of dopamine with chitosan may improve the alkaline stability of the films. Confocal and atomic force microscopy (AFM) depict changes of the surface morphology; zeta potential measurements show variations of the chemical surface properties of the fibers after modification. Unmodified and modified fibers are embedded in a novel LC3 matrix; the interaction between fibers and concrete matrix is studied by single fiber pullout tests. The thin polydopamine film improves both the wettability and the interfacial shear stress and pullout energy of the fibers in the cement matrix. The combined dopamine + chitosan coating exhibits a slightly weaker fiber-matrix adhesion.

This study was conducted to elucidate how fiber orientation in Ultra-High-Performance Concrete (UHPC) beams affects member capacity. Beam specimens were subjected to flexural loading tests and fiber orientation observation using X-ray micro-CT, and FEM analysis was employed to reproduce the flexural capacity. The experiment was designed with placement methods and center notch presence/absence as parameters. The results indicated that specimens with fixed placement exhibited 7% more flexural capacity than with moving placement, and that the tensile stress of UHPC contributed to this. Nonlinear FEM analysis was conducted with UHPC tension-softening curves as parameters, and curves obtained from the material testing and estimated from fiber orientation were used to deduce the flexural capacity difference from the experimental results.

The edge effect is important in the structural design of concrete elements as it may lead to an erroneous estimation of the mechanical response of concrete because of the difference in size between the test specimen and the full-size structure. This study analyses the influence of the pore and fiber distribution on the edge effect under compressive loads. For this purpose, three series of cylindrical specimens of steel fiber–reinforced concrete, with diameters of 75, 100, and 150 mm and a height/diameter ratio of 2 in all cases, were fabricated. Next, computerized tomography scanner was used to obtain the pore and fiber distribution inside the specimens. The images reveal that, near the cylinders’ walls, the pore and fiber content is lower than that in the cores. In addition, the thickness of the ring near the edge, where the microstructure is different, is independent of the specimen size. Moreover, it is determined that the mechanical capacity of the ring is lower than that of the rest of the specimen.

The study aims to enhance the ductility of Geopolymer Concrete (GPC) with the inclusion of hooked-end steel fibers. The influence of the fiber-matrix bond on the cohesive stress response in the fracture is evaluated for GPC and Normal Concrete (NC) matrixes. The fracture responses of GPC and NC with 25 kg/m3 hooked-ended steel fibers are evaluated experimentally. The pullout behavior of the steel fiber from the geopolymer and cementitious matrices is also determined experimentally. The fracture response of steel fiber-reinforced GPC is improved significantly when compared with steel fiber-reinforced NC, with a low dosage of fibers. The cohesive stress-crack opening relationship derived from the measured fracture responses indicates that a significantly higher crack closing stress is generated at smaller crack openings in GPC than in NC. The pullout resistance of the steel fiber embedded in the GPC matrix is significantly higher compared with NC. The improved bond between the GPC matrix and fiber contributes to higher debonding resistance in the early part of the fiber pullout. The higher peak pullout load resistance at low slip indicates better engagement of the fiber in providing crack closing stresses across a crack. The higher efficiency of the steel fiber in providing crack opening resistance contributes to significant improvement in the cohesive stress at small crack openings in GPC.

This study presents an advancement in the Brazilian test for assessing the tensile strength of concrete cubes through the introduction of a novel T-shaped loading mechanism equipped with a hinge arrangement. This innovation ensures the stability of the specimen and accurate centering of the load, leading to predictable crack propagation. Moreover, the research utilizes a cutting-edge Digital Image Correlation (DIC) system for the precise analysis of steel fiber-reinforced concrete specimens, enabling the identification of crack initiation points and their corresponding loads. The experimental findings confirm the efficacy of the DIC system as a superior alternative to conventional strain gauges, facilitated by the use of licensed GOM software for processing image series. This approach provides crucial qualitative DIC data, including detailed observations of major strains essential for understanding the local behavioral phenomena during indirect tensile tests. Particularly, the DIC system’s ability to pinpoint the exact moments of crack initiation significantly enhances the assessment of the indirect tensile strength and fracture behavior of concrete. Notably, matrix cracking in the tested steel fiber-reinforced concrete specimens was detected at roughly 60% of the tensile strength, highlighting the DIC system’s sensitivity and precision in early fracture detection. Overall, the integration of the T-shaped loading mechanism and the DIC system offers a comprehensive and robust framework for evaluating concrete’s tensile performance, markedly improving our understanding and assessment capabilities.

Definition, characterization and evolution of fracture process in concrete is still under discussion. This issue is particularly relevant for high- (HPC) and ultra-high performance concrete (UHPC), where steel fibers are added to ensure ductility and provide residual flexural strengths. As fibers add to the load bearing capacity by bridging cracks, more diffuse fracture develops, depending on the amount and type of fibers added. Understanding the development of this process contributes to a deeper insight into the mesoscale highly inhomogeneous fiber reinforced material.The primary fracture process at the crack tip, as well as fiber pull-out mechanisms, generate elastic waves that can be assessed using the acoustic emission (AE) analysis technique.Based on comprehensive low-cycle flexural tests using one HPC and one UHPC performed with AE detection, a post-processing approach for localization of AE events is outlined to define an “AE activity zone” representing the area of fracture process. In this paper, the standard deviation of the spatial dimensions of localized AE events is used to define such an activity zone. It is further shown, that the AE activity zone evolves differently depending on the compressive strength of concrete as well as the fiber content.

The feasibility of predicting the failure of fiber/matrix interface with electrical signals could be further developed into a unique self-sensing technology to monitor the post-cracking behavior (e.g., the very fine crack width) in conductive fiber-reinforced cementitious composite (c-FRCC), based on bottom-up scale-linking micromechanics. In the study, micromechanical behavior and steady electrical impedance were captured via a single carbon nanotube (CNT) fiber pullout testing and EIS measurement. It revealed non-linear cylindrical diffusion behavior at the low frequency, which shadowed the contribution of the fiber-to-matrix interfacial damage to the electrical response. For better quantitative analysis of reflecting mechanical and electrical correlation based on interfacial microstructure, chopped polyacrylonitrile (PAN) based carbon fibers were added to the percolation. After that, distinct physics-based impedance arcs were exhibited vis the EIS testing, corresponding to the matrix (high-frequency domain), fiber-to-matrix interface (intermediate frequency domain), and electrode-to-matrix interface (low-frequency domain), respectively. An exponential increase of resistance (r) with fiber displacement (u) was then observed in both the debonding and slippage stages. The contribution of each phase directly revealed that the change in the total electrical resistance (∆r) was mainly attributed to the matrix (high-frequency domain) via EIS analysis. During debonding, the weak disconnection of the CNT fiber and matrix compromised the conductive path in the matrix, while the dislocation of the CNT fiber relative to the matrix due to slippage altered the conductive path and both contributed to ∆r. The finding would be helpful in establishing electrical and mechanical modeling for the future parametric study and sensitivity analysis of the r-u relationship, further exploring the electrical and mechanical (like crack width) correlation of c-FRCC under the fiber bridging level.

This experimental study investigates the fibre recovery of fibre reinforced concrete. Specifically, a total of 24 fibre reinforced concrete specimens, each containing 30 kg/m3 of 4D Dramix® 55/50 BG fibres, were subjected to different recycling techniques. Initially, the fibre reinforced concrete specimens were crushed using a jaw crusher at the company AC Materials (Ghent, Belgium) until a concrete fraction smaller than 15 cm was achieved. In the second stage, the company Centre Terre et Pierre (CTP), located in Doornik (Belgium), used an impact crusher to maximize the fibre release. The current study indicates that the impact crusher, operating at 40 Hz with a crusher opening of 9 mm, recovered 85% of the fibres from the original fibre reinforced concrete specimens. In contrast, a lower frequency (30 Hz) showed a 5x higher amount of unreleased concrete-fibre components. Moreover, increasing the frequency to 50 Hz did not substantially enhance the fibre recovery, making 40 Hz with 9 mm opening the optimal fibre recovery setting in this study. Due to the recycling procedure, the recovered fibres are distorted, and consequently, the residual tensile strength capacity is slightly reduced compared to the virgin 4D Dramix® 55/50 fibres. The test data clearly show that the reduction is mainly observed at higher crack mouth opening displacements. The average reduction in the fR3-value is 0.7 MPa (relative decrease of 18%) with the incorporation of 30 kg/m3 recovered 4D Dramix® fibres. Besides a higher level of variability is observed in the test data. Nevertheless, these primary results clearly show the substantial fibre recovery potential and the feasibility of the existing constitutive tensile model in Eurocode 2 for the FRC mixture with recovered steel fibres.

The urgent need to reduce the environmental impact of building materials has led to the recent development of low-clinker binders, such as limestone calcined clay cement (LC3). The possibility of using LC3 to produce cement-based composites with high mechanical properties and reduced environmental impact is certainly of great interest. In this study, two LC3 mixtures with different mechanical performances (low and high strength) were investigated. Chopped carbon fibers coming from end-of-life prepreg carbon textiles (rCF), recovered through pyrolysis, and virgin carbon fibers (vCF) were used to reinforce the matrices. Different fiber dosages were investigated, up to 1.5% by volume. Fiber reinforced LC3 composites were characterized through compression and bending tests. The addition of both rCF and vCF results in a decrease in matrix workability, by increasing fiber volume. On the other hand, rCF were able to increase both the compression and bending performances of the cementitious composites, up to 66% and 53%, respectively. Remarkably, no significant differences were found in using rCF instead of vCF.

This study aims to investigate the effect of incorporating different quantities of polypropylene fibres recovered during concrete recycling or embedded in recycled aggregates on the mechanical properties of new polypropylene fibre-reinforced concrete. Both recovered fibre (at replacement ratios of 0%, 10%, 30% and 100%) and coarse recycled aggregate (0%, 100%) were used in new concrete. A polypropylene fibre content of 9 kg/m3 (1.0% by volume) was chosen for all concretes.. The compressive strength, elastic modulus and stress-strain behaviour of concrete in compression were tested. The residual tensile strength was also tested for concretes with recovered fibres and fibres embedded in recycled aggregate. The results show that mixing of recycled fibres recovered from concrete with virgin fibres is feasible without significant effect on the mechanical properties of concrete in compression.

Engineered Cementitious Composite (ECC) is a type of fiber-reinforced concrete (FRC) that exhibits tensile strain-hardening behavior. Magnesium-silicate-hydrate (MSH) is a class of magnesium oxide and reactive silica-based cementitious binders (MgO-SiO2) that develop strength via hydration similar to Portland cement. This study presents the mechanical performance of ECC developed using MSH binders to expand the range of binders for ECC and promote broader applications. The influence of a few material factors – fiber type (PVA and PE), fiber volume fraction (2%, 1%, and 0.5%), inclusion of aggregates (microsilica sand), and silica source (silica fume and metakaolin) – was investigated experimentally. The mechanical characterization was performed under unconfined uniaxial compression and uniaxial tensile tests. The tensile behavior was characterized using three parameters – first cracking strength, ultimate tensile strength, and tensile strain capacity. The tensile strain capacity was higher for PE fiber than for PVA fiber. Using PE fiber, the tensile strain-hardening can be achieved using a fiber volume fraction as low as 0.5% due to the high brittleness of the MSH matrix. The tensile strain-hardening behavior was retained even after adding microsilica silica sand with just a 1% volume fraction of PE fiber. The use of metakaolin increased the toughness of the cementitious matrix and thus reduced the tensile strain capacity compared to when silica fume was used as a silica source. Overall, all the composites showed tensile strain capacity above 3% and compressive strength greater than 40 MPa, successfully demonstrating the feasibility of using MSH binders to develop ECC.

The use of fibrillated biomass presents a novel opportunity for manufacturing concrete composites with low/zero carbon footprint. This methodology can be crucial for creating efficient decarbonization pathways and accommodating the ever-increasing performance demands in the construction industry. This research aims to develop sustainable, low carbon cementitious composites with high CO2 sequestration capacity, resiliency and ductility by reinforcing the cementitious matrix with carbon neutral waste cellulose fibers. Our results show that the use of small amounts of waste cellulose fibers (WCF) holds groundbreaking potential for the development of highly resilient, eco-efficient concrete composites, with up to 1.8x higher CO2 capture and mineralization ability and at least 50% increased pre-crack and post-crack tensile load bearing and energy absorption capacity over OPC concrete. Embracing such methodologies for mitigating environmental impacts is essential for the advancements in developing sustainable construction materials.

Column-supported solid reinforced concrete (RC) slabs are amongst the most popular forms of in-situ concrete frame construction for residential buildings. However, the partial or even total substitution of traditional reinforcement by fibres has emerged as an attractive alternative. The pioneer experiences provided encouraging results with respect to resource optimization and construction time reduction without jeopardizing the structural reliability. Nevertheless, the comparison of RC and fibre reinforced concrete (FRC) solutions is primarily cost-driven, i.e., the environmental and social factors are often disregarded, although those are of paramount importance for achieving long-term sustainable goals. Hence, this study focuses on the sustainability analysis of reinforced concrete flat slabs using a multi-criteria decision-making method. Primarily, a model based on criteria, indicators, weights, and value functions is developed and calibrated. Thereafter, the case study of a given geometry is designed considering three different alternatives: (1) RC solid slab, (2) hybrid fibre reinforced concrete slab (HFRC, reinforcing steel bars + fibres), and (3) FRC slab. Finally, the sustainability of these alternatives is assessed by means of the elaborated model. The obtained results evidence the potential of FRC and HFRC solutions in economic, environmental, and social terms. Both the model and results are expected to be useful for stakeholders in decision-making processes.

This paper gives an overview of parts of an industry-led project to evaluate the use of basalt fibre reinforced polymer macro fibre (BFRPmf) as dispersed reinforcement for energy-efficient production of precast concrete. For this purpose, the intrinsic properties of three BFRPmf designs were evaluated and their integrity and robustness during mixing were analysed during preliminary full-scale trials at FP McCann precast concrete factory in Knockloughrim. Furthermore, several fibre-reinforced concrete (FRC) mix designs were developed by the variation of the BFRPmf dosage. The fresh and hardened properties were measured to assist in optimising the mix design. The compressive and tensile strengths of the various FRC mixes were measured to characterise the effect of the variation in fibre dosage and the post-peak softening behaviour was investigated compared to a control mix with no fibres. These test results will be used to establish finite element models for verification of structural design and energy-efficient manufacturing of full-scale precast concrete products. In this paper, the laboratory results and contribution of BFRPmf to concrete strengthening and post-peak behaviour are outlined with a focus on the impact of the BFRPmf dosage at 0.5% of mix volume.

The use of steel fiber reinforced concrete (SFRC) for structural purposes has received a major enhancement since the publication of the fib Model Code 2010. The use of recycled concrete aggregates (RCA) in structural applications is also a growing practice. Therefore, it is expected that the combined use of SFRC and RCA will gain more space in structural applications. Assessment of this suitability is essential, and there is also the possibility of using alternative tests for this qualification, as allowed by the fib Model Code 2010. In this sense, the DEWS test (double-edge wedge splitting test) appears to be very promising since recent research has made its procedure and analysis of the simplest results, using only displacement control as a basis for material parameterization. Considering this, an experimental program was developed using SFRC with different levels of replacement of primary aggregate by RCA varying the steel fiber content to verify its influence on constitutive models. The simplified DEWS test proved to be an interesting alternative for the flexural test with some advantages such as providing good quality of results with a reduced volume of the material resulting in both economical and labor benefits. The results demonstrated that the content of RCA had minor effect on the post-cracking behavior of the material, indicating that it can be very interesting for more sustainable structural applications.

Infrastructure constructed around 60 years ago during Japan’s postwar period of high economic growth (1955–1973) is now facing the problem of aging. As a consequence, an era of maintenance and repair is approaching. There is expectation that sprayed ultra-high-strength fiber-reinforced concrete (UFC) will be a suitable repair material in many cases.UFC generally offers high strength, toughness, and durability. However, there is a problem with the workability of UFC because of its high viscosity, leading to experimental efforts aimed at achieving high strength, durability, pumpability, and adhesion when spraying UFC.This paper presents research on the hardening properties and physical properties of sprayed ultra-high-strength fiber-reinforced mortar containing a mix of copper and polypropylene fibers. The results confirm that, even with the addition of chemical fibers, the fresh properties remain similar to those of formulations containing only copper fibers. It is also found that spraying can achieve sufficient cover thickness and that on-site application is possible.

This paper presents a critical analysis of the main types of tensile test related to the determination of the tensile behavior of FRCs. This critical analysis clearly demonstrates that the bending test contributes to overestimate the FRCs mechanical performance when the cracks opening is related to the service limit state of a structure. The principal conclusion is that only uniaxial tensile tests are relevant for the intended objective. Two types of uniaxial tests are considered in relation with different connections between the specimen and the test machine: gripping and bonding connections. Some technical solutions related to each connection are proposed to perform relevant tests.

This paper presents a parametric study on the pullout performance of twisted steel fibre in normal concrete. The twisted fibre is 40 mm long with a rectangular cross-section with a diagonal length of 1.12 mm. The experiment parameters include the number of twists (4 and 6) and the fibre inclination angle, θ, (30°, 45°, 60°, 75°, and 90°). The results indicate that the inclination angle has a great effect on the pullout response contrary to the number of twists. Such a database can then be employed to develop analytical models of the behaviour of Fibre Reinforced Concrete (FRC) elements (compressive, tensile, and flexural), facilitating the optimisation process of such elements and leading to optimum designs.

Steel fibre reinforced concrete (SFRC) is a composite material whereby discontinuous steel fibres are randomly distributed throughout the concrete matrix. Since concrete is weak in tension, the fibres compensate for the lack of tensile capacity by improving the post-cracking ductility. It also enhances the concrete’s toughness, resistance to cracking, and fatigue life. The fibre volume dosage, fibre type, and fibre geometry, among others, are typically factors that influence the post-cracking behaviour of fibre reinforced concrete (FRC). While these factors may vary depending on the nature of the application, they not only influence the post-cracking behaviour, but the durability of the SFRC may also be compromised. Cracks at the surface of the concrete create ingress pathways for deleterious substances which may affect the durability of the composite. This research aims to investigate the mechanical behaviour and durability properties of uncracked hooked-end SFRC with fibre volume dosages varying from 0% to 1.0%. The results show an increase in mechanical properties with an increase in fibre volume dosage. In contrast, the durability properties showed improved oxygen permeability and minimal carbonation penetration at a 0.5% fibre dosage. However, an increase in the fibre dosage to 1.0% resulted in a decrease in oxygen permeability and an increase in carbonation, thereby illustrating the need to optimize the fibre dosage for favourable mechanical and durability properties.

In this study, a thorough evaluation of the toughening mechanism in cement-based nanocomposites reinforced with hybrid networks of carbon nanofibers (CNFs) and polypropylene microfibers (PPs) took place. The critical values of fracture toughness/stress intensity factor, $${K}_{IC}^{S}$$ K IC S , were experimentally determined on prismatic notched specimens of nano and micro scale fiber reinforced cementitious composites using the two-parameter fracture model (TPFM). The post-crack energy absorption capacity of the hybrid-composites was assessed by evaluating the dimensionless toughness index, I20, calculated through linear elastic fracture mechanics (LEFM) tests. The addition of CNF/PP networks at low volume fractions of about 0.1 vol% in cementitious matrix results in a significant improvement in the $${K}_{IC}^{S}$$ K IC S (85–240%) and 1.6 – 10x higher I20 compared to the CNF or PP reinforced materials. Relative to the single-scale fiber reinforcement, the synergy between the nano- and micro- scale fibers results in a multi-scale crack arresting distinctively increasing the toughening effect in the hybrid fiber-cementitious mortar nanocomposites.

Self-stressing steel fibre-reinforced concrete (SFRSSC) is increasingly utilised every day, with its areas of applicability expanding. One such application is ground-level elevated slabs on piles, along with elevated slabs on columns. Although regular steel fibre-reinforced concrete (SFRC) has been previously applied for such cases, albeit somewhat extensively, very few full-scale load-bearing tests have been conducted for these slabs under industrial-level loads up to failure. In Northern Europe, the use of SFRC in ground-level elevated slabs is somewhat common, either with or without additional traditional rebar reinforcement. On the other hand, SFRSSC is still an emerging technology with known benefits, and parts of the technology are yet to be discovered. No calculation method exists to validate the benefits of SFRSSC, specifically the self-stressing impact on load-bearing capacity at the ultimate limit state (ULS) and behaviour under serviceability limit state (SLS) loads. The aim of this presentation is to provide an overview of a comparative full-scale load-bearing capacity test up to failure, comparing two elevated slabs made of regular SFRC and SFRSSC loaded under similar conditions. Material properties were monitored by conducting an extensive battery of laboratory tests. The deformations, cracking, and shrinkage of the full-scale slabs were recorded using both standard sensors and innovative technologies, such as fibre optic sensors embedded in concrete and the digital image correlation method. The slabs were loaded with uniformly distributed sandbags up to the SLS load, and SLS criteria were measured. After unloading, the ULS load was applied; the slabs were evaluated and further loaded up to failure. The test results show that the slab made of SFRSSC had smaller deflections and crack widths, as well as a significantly higher load-bearing capacity.

Carbon Fibre Reinforced Ultra-High Performance Concrete (C-UHPFRC) is an innovative composite material, notable for its strength properties and the formation of fine multi-cracks. Non-metallic fibres in combination with UHPC offer advantages such as better surface properties and the absence of corrosion. Furthermore, they enable a more sustainable replacement for steel fibres.In cooperation with the industrial partners Implenia, KIBAG and newcycle, the University of Applied Sciences Munich (MUAS) conducted comprehensive laboratory tests regarding both rheological and mechanical properties. The study delves into a wide range of parameters, offering a thorough understanding of C-UHPFRC. The main emphasis was on the bending behaviour, with complementary investigations on compressive strength and Young’s modulus.The data analysis allows a comparison of various mix designs and highlights what the authors have identified as the limiting values for workability and critical fibre quantities for mechanical properties. These findings are based on interpretation of the data, framed within the context of this study’s objectives. Specimens that are over-critically reinforced with fibres show evidence of multi-cracking. An inspection for strain hardening behaviour was conducted based on the stress distribution approach of Swiss standard SIA2052. This property could initially not be proven, hence it is recommended to verify the applicability of the assumed stress distribution to carbon fibre-reinforced UHPC without further adjustments. Finally, the data obtained is classified according to SIA2052.The data from the systematic investigation is used to identify potential application scenarios, aiming to utilise the novel concrete technology to develop new and improved applications in civil engineering.

Current codes and recommendations are evolving towards an approach that proposes the orientation factor to consider differences between small-scale characterization steel fibre reinforced concrete (SFRC) specimens and the real-scale elements. Moreover, several research studies revealed that the use of constitutive models derived from three-point bending tests (3PBT) on notched beams tend to overestimate the bearing capacity of statically indeterminate two-way slabs. With this in mind, an experimental programme was carried out in order to evaluate the flexural behaviour of three SFRC slabs (3.0 × 3.0 × 0.1 m3) subjected to point load under a statically indeterminate test configuration. Thereafter, 88 cores were extracted from the tested slabs with the following analysis of fibre orientation by means of a non-destructive method. The results indicated that lower orientation numbers were observed in cylinders drilled from the slabs in comparison with those observed in the reference notched beams used for the material characterization (i.e., 3PBT). Based on these results, an orientation factor was analytically computed and considered to develop the constitutive model used to predict the ultimate load of the analysed SFRC slabs by means of yield line method (YLM). The prediction showed a good agreement with the experimental response; this evidencing that the orientation factor may permit to consider the differences caused by geometric and scale-induced factors between both prismatic notched beams used for SFRC characterization and real-scale slabs produced with the same material.

The paper describes full scale structural beam tests investigating the use of steel fiber reinforced concrete (SFRC) for steel-concrete composite structures. Steel profiles encased in reinforced concrete used for composite filler beam decks have been enhanced by high-strength steel fibres in the concrete matrix in addition to the conventional reinforcement bars.The obtained experimental evidence confirms that SFRC is advantageous for this specific application by improving the structural behavior both at Ultimate Limit State (ULS) as well as at Serviceability Limit State (SLS), therefore permitting a reduction of material consumption compared to the traditional solution to ensure a given design level. In fact, the enhanced compressive properties of SFRC, and namely the increased ductility compared to normal reinforced concrete, are relevant for encased steel-concrete sections as they allow to reach the complete yielding of the steel component also in compression. This ensures a higher bending moment resistance useful for ULS design. On the other side, the improved tensile properties of SFRC allow for a reduced cracking phenomena which permits to rely on an increased bending stiffness at SLS load levels compared to the use of normal concrete. This becomes relevant when deformation limits or vibration control becomes design decisive.

The remarkable advancements in concrete technology have yielded a plethora of specialized concrete types. Notably, steel fibre reinforcement (SFRC) boasts enhanced versatility, cost-effectiveness, and sustainability while drastically addressing traditional concrete's low tensile strength, cracking, and brittle behaviour. Despite extensive research in recent decades, crucial information is SFRC's resistance to localized tensile loads exerted by anchor bolts and the influence of potential non-homogeneous fibre orientations. This is particularly significant because anchors are often placed at component boundaries, where fibres tend to align parallel to the external surface due to the “formwork effect”. This study leverages an innovative experimental campaign utilizing 64 single bonded anchors embedded in concrete, with layered casting. This attempts to elucidate the influence of controlled unidimensional fibre orientation on the load-bearing capacity and behaviour of anchorages in SFRC, subjected to an axisymmetric stress field around the anchor axis.

Moment redistribution in structures is crucial for ensuring the safety, resilience, and efficiency of a building. There are few studies that address this phenomenon in fibre reinforced concrete (FRC), both with and without reinforcing bars. These studies used a two-span, one-way, continuous beam set-up, with point loads on each span. Although the system is, in theory, symmetric, the response is not symmetric due to differences in geometry, materials, and experimental errors. Therefore, a plastic hinge usually forms on only one of the two spans, in addition to the one over the central support. The asymmetric response makes the analysis cumbersome. In this paper, a different methodology for testing statically indeterminate FRC beams is presented. In it, a partially restrained support is materialized at both ends of a one-span beam. The configuration is completely described, which includes: load cells in the supports; LVDTs for midspan deflection, for the support rotation, and for crack opening displacements (COD); and Digital Image Correlation also for COD. Preliminary results of a beam tested in this configuration are shown and advantages and disadvantages are analyzed compared to the two-span, continuous beam set-up. Although the analysis of the proposed set-up is more straightforward, the rotational stiffness (κ) of the partially re-strained supports (defined as the relationship between the moment (M) and the angle of rotation (θ): κ = M/θ) should be carefully defined in the system to reflect the real behavior of FRC continuous members.

The implementation of jacketing stands out as a widely adopted approach to strengthen existing concrete structures. The performance of jacketed reinforced concrete columns is significantly influenced by the mechanical properties of the jacketing materials and the interfacial bond between the jacket and the core column. Strain-Hardening Cementitious Composite (SHCC), which is compatible with substrate concrete, can notably improve the performance of concrete structures due to its advantageous tensile characteristics. This study experimentally investigated the uniaxial compressive performance of concrete columns confined with a novel and sustainable jacket made of High-Tensile-Strength SHCC (HTS-SHCC). These innovative jackets were prepared by replacing 40% of Portland cement with limestone calcined clay (LC2). A total of 27 specimens with circular and prismatic geometry were prepared, including 9 control and 18 confined with two different mixes of HTS-SHCC. Each mix featured different fibre phases: 2 vol.% polyethylene (PE) fibres in the first mix, and a combination of 1.5 vol.% PE fibres and 0.5 vol.% steel fibres in the second mix. The peak strength, axial strain, and lateral strain increased by up to 55%, 129%, and 80%, respectively. However, these increases were higher in circular specimens and lower in prismatic specimens. The incorporation of rounded corners for prismatic specimens effectively altered the failure mode, subsequently improving both compressive strength and axial/lateral strain.

The revision of EN 1992-1-1 (Eurocode 2) published in November 2023 includes an Annex (L) for SFRC structures. Its provision for minimum flexural reinforcement is derived by solving the condition MR,min ≥ Mcr. This is similar to the plain RC provision, yet it includes the effects of the fibers, by considering their effective residual tensile strength fFtu,ef. However, for the common ultimate limit state plastic design of reinforced SFRC (R/SFRC) beams or slabs without any direct check of their rotation capacity, the code requires to increase As,min by a factor ‘αduct’. The default value of αduct is 2.0 and it can be different according to the National Annex. The reason for this provision is the effect of cracking localization on reducing the flexural ductility, as observed in R/SFRC beams with relatively low conventional reinforcement ratios. Experimental data shows that there exists a transition point (TP) in terms of the ‘effective reinforcement ratio’, below which a decrease of the reinforcement decreases the flexural ductility (opposite to plain RC beams). This paper presents examples of As,min calculated by the code’s provisions with the default factor and inspects their resulted values with experimental data. It is shown that if minimum ductility is wished to be maintained, as indicated by the TP, the default value should indeed be increased.

Prestressed concrete is the most extensively used material for railway sleepers (~75%), with the increased weight providing greater track stability suitable for high-speed traffic on modern tracks. Although the improved operational performance (i.e. higher bending strength & service life) also justified such adaptation of prestressed concrete over timber and steel sleepers, constantly increasing operational demands are now precluding them from fulfilling all railway industry requirements. These limitations typically include material deterioration, corrosion and premature cracking that may occur, thereby reducing the sleeper’s performance capacity, increasing maintenance frequency and associated costs. Therefore, macro synthetic fibre reinforced concrete (MSFRC) is suggested as a reinforcement technique owing to its improved post-peak flexural capacity, ductility and crack-arresting properties. In such scope, the residual behaviours of MSFRC are investigated by a series of material- and structural-scale experiments towards optimising (i.e. in terms of fibre dosage and fibre type) the MSFRC sleepers for future railway applications. Moreover, this paper will present the operational benefits of MSFRC over conventional prestressed concrete sleepers, critically comparing the residual capacity, stiffness and toughness.

Thanks to the use of steel fibers, concrete can enhance its toughness, achieving a better control of the post-cracking phase behaviour. Steel Fiber Reinforced Concrete (SFRC) has been largely employed by practitioners in different applications, including tunnel linings and precast tunnel segments. Working mainly under compression, tunnel linings do not require high percentages of steel reinforcement, which can be entirely replaced by the addition of steel fibers in the concrete matrix. As a matter of fact, fibers will help in the control of the cracking of tunnel structures which is more difficult to obtain for ORC structures because of the design method, which is mainly concentrated on the ultimate limit state. This paper will focus on a new methodology for the regeneration of existing tunnels based on the extrusion of SFRC new lining against the existing tunnel. In addition to small-scale tests performed to identify the material properties of the proprietary SFRC mix, an experimental campaign is currently undergoing at the Elsa Laboratory of the EC Joint Research Center in Ispra (Italy), on real-scale extruded FRC tunnel segment mock-ups in order to assess their behaviour in service under different loading combinations, and to evaluate their ultimate load bearing capacity. The paper is going to present the results of two nominally identical tests, performed so far, together with their analysis and validation according to the fib Model Code 2010 design criteria.

The construction industry is a key sector worldwide thanks to its huge social and economic impact. Considering that concrete is the most used construction material all over the world, its role is of paramount importance, with the demand of more efficient concrete structures obviously promoting an increase in terms of sustainability in order to accompany the social and economic transition. This makes (High-Performance) Fiber-Reinforced Concrete – (HP)FRC a promising solution, allowing for more efficient harvesting and storage processes with the possible implementation of digital fabrication, and for structures with improved durability and increased service life. FRC systems represent a promising solution not also towards ordinary reinforced concrete systems, but also with respect to steel structures thanks to the lower maintenance demand for both durability and fire performance. In the present paper, the adoption of HPFRC for an innovative Gravity Energy Storage System is briefly described, starting from the material mechanical characterization, following with full-scale tests and structural design. The results highlight the efficiency of the structural system adopted, enabling significant reductions in traditional reinforcement, and the reliability of Model Code 2010 as a basis for design.

The use of conventional reinforced concrete for building partition walls has proven several benefits in terms of acoustic performance and robustness. Horizontal reinforcement is typically governing in their design, and arranged to control cracking due to shrinkage and imposed deformations. In view of reducing material consumption and CO2eq, an alternative reinforcement concept based on the use of macro-synthetic fibres (MSFRC) is proposed in this paper and applied in practice within an actual building recently completed in Switzerland. This paper collects the research and practical experiences of the authors and aims at addressing questions that may arise during the construction and design stage of such members. To this end, a real-scale cast-in-situ MSFRC prototype wall of h/b/t = 2.8/1.2/0.15 m was cast. The objective is to shed some light on the casting procedure for wall elements and to assess the mechanical and rheological properties of the mix and fibre distribution and orientation.

In the context of an experimental programme on the use of macro-synthetic fibre reinforced concrete (MSFRC) for casting building partition walls, three walls from a residential building in Switzerland were selected for an on-site application (with lengths varying between 2.8 and 13 m). The walls had an amount of fibres equal to 4 kg/m3 and only conventional reinforcement at the construction joints with adjacent walls. In addition, specimens were cast in order to measure the strength, rheological properties and characterise the post-cracking behaviour of MSFRC. A crack monitoring protocol was conducted on the walls for crack detecting and tracking their evolution over time, with no noticeable cracks in the short walls. The cracks detected for the 13 m-long wall were measured and the results summarized in this document. This research contribution aims at showcasing the use of MSFRC as an alternative to ordinary reinforcement for building partition walls.

The utilization of Ultra-High-Performance Concrete (UHPC) for connecting bridge elements in accelerated bridge construction has become increasingly prevalent. Among their various applications, connecting bridge decks using UHPC stands out due to its ability to leverage key properties, including enhanced shear strength, tensile strength, compressive strength, and fatigue resistance. This study focuses on extensive testing and material modeling using an approach to use the flexural test results of small beams for back-calculation and validation of material constitutive response and using these properties in the context of a hybrid reinforcement strategy for typical UHPC bridge deck connection elements. Experimental programs were conducted using a 4-point bend test at both the materials and structural levels. After the assessment of the material data obtained from closed-loop tests, the results were evaluated using an actual-size reinforced concrete section tested under flexure. Results reveal multiple cracking mechanisms induced by the fibers, with the fibers exhibiting substantial capacity to withstand loads at large deflections. The analysis was used to characterize the contribution of the matrix, fiber, and reinforcement rates, thus providing insight into the tension-stiffening behavior of Hybrid section composites.

Concrete with the addition of fibres is increasingly being used in the construction industry instead of the usual steel bar reinforcement. In the meantime, intensive research work and investigations have led to a sound knowledge of the material characteristics. This makes it possible to assess the advantages and disadvantages of this innovative building material in terms of practical and economic construction. Compared to reinforced concrete, however, this area is still under-researched; particularly with regard to the load-bearing behaviour of fasteners in fibre-reinforced concrete, science is still lagging behind, and anchor certifications for this relatively new building material as an anchoring base are not adequately defined. In order to investigate the question of whether and how the addition of fibres to normal concrete improves the load-bearing capacity of fasteners, axial tensile tests were carried out on product-neutral headed stud anchors. The results should provide new knowledge in this field of research. For this work, fasteners were tested in polymer fibre concretes with different mix designs and different fibre contents and compared to a standard plain concrete.

Recently performed full scale tests of steel fibre reinforced concrete (SFRC) and self-stressing SFRC (SFRSSC) slabs on piles showed a high load bearing capacity significantly exceeding the design ultimate limit state loads. Two flat slabs with dimensions of 15 $$\times $$ × 16 m were constructed and tested during the summer and autumn in 2023 in Jelgava, Latvia. The slabs had thickness of 150 mm and spans of 3.0 m having randomly distributed steel hooked end fibres (50 kg/m $$^3$$ 3 ) as the only reinforcement. In the first stage of the test, a uniformly distributed load was gradually applied till serviceability limit state (SLS) load level was obtained. The load was kept for 7 days and then unloaded. During this period the development of strains, crack widths and deflections were monitored. Although the measurements were stable after the application of the load, the crack widths and the deflections drastically increased throughout the week in a step wise manner. The total increase of a dominant crack for the SFRC slab was from 0.4 mm to 1.1 mm during a week of constant loading. In the case of the SFRSSC slab, the increase was from 0.2 mm to 0.6 mm during the week. The moment, when the increase occurred was observed at the middle of each day corresponding to the increase of the air temperature. The objective of this presentation is to show the observed huge effect of the relatively small temperature deviations and propose an explanation of the phenomena.

Reactive magnesium cement (RMC) heavily relies on the elevated CO2 curing to gain strength, which limits its productivity and increases the curing cost. To address this problem, a hollow natural fibers reinforced MgO-SiO2 (HNFs-MS) composite was proposed in this study, in which the formation of magnesium-silicate-hydrate (M-S-H) provided sufficient early strength, and subsequent carbonation of residual MgO/Mg(OH)2 enabled continuous strength development and CO2 sequestration. The results showed that the strength development under moisture curing depends on the formation of M-S-H, while the carbonation of residual MgO/Mg(OH)2 dominates under carbonation curing. The incorporation of HNFs in MS composites not only accelerates the strength gain under moisture curing and subsequent carbonation curing, but also effectively improves the volume stability and CO2 sequestration, especially in deep regions. This is relevant to the porous microstructure of HNF, which can be the pathway for free CO2 diffusion inwardly and offset the surrounding matrix expansion.

Water ingress in porous concrete structures occurs frequently due to rain, humidity or even water pipe leakages. It can influence the long-term durability properties of structures, since it may contain damaging ions such as chloride which may lead to corrosion of accommodated steel reinforcements. Furthermore, in the case of frost, water expands inside the concrete and causes microcracks. Hence, monitoring of water ingress is highly important for maintaining concrete structures and prolonging their service life. On this basis, self-sensing concrete is a promising candidate for this purpose.Fiber reinforced cementitious composites are fabricated broadly by utilizing e.g. polyethylene (PE) fibers. However, those are intrinsically non-conductive and chemically inert. Imparting electrical conductivity to PE fibers gives an opportunity to use them as smart and durable reinforcements for producing self-sensing concrete. Herein, PE fibers were coated via tannic acid modified carbon nanotubes (CNTs) to fabricate highly electrically conductive reinforcing fibers embedded in different concrete matrices. Self-sensory concrete composites were obtained enabling to monitor water ingress.Electrical resistance measurements of smart high-strength (HS) or normal-strength (NS) concrete specimens were continuously recorded during soaking process in deionized water solution. The results demonstrated appropriate sensory properties with relative resistance changes of around 36.1% and 2.5% against deionized water for NS and HS matrices, respectively. Apart from changes in electrical resistance, different graph shapes were found with respect to the types of matrix under investigation. All in all, it suggests that the developed strategy in this paper can be used according to both “pattern” and “intensity” analyses for timely detection of water seepage inside concrete structures.

The protection of new structures and the necessary evaluation/intervention measurements in the case of existing structures subjected to fire damage are research topics of importance in a worldwide scale. It is known that the use of micro-synthetic fibers reduces the occurrence of explosive spalling, and that its use with steel fibers promotes beneficial effects in terms of fire resistance. However, the current standards and recommendations do not provide an adequate approach regarding the use of steel fiber reinforced concrete (SFRC) for structural applications under fire. In this context, the present work evaluates the mechanical behavior of the SFRC subjected to an actual fire simulation, as it assesses the reinforcement capacity provided by steel fibers. The heating procedure was conducted using a vertical fire simulator that induced a single-surface fire exposure following the hydrocarbon fire curve. The post-crack mechanical behavior of samples produced with fiber contents of 0.26%, 0.45%, and 0.90% in volume (i.e. 20, 35 and 70 kg / m3, respectively) were evaluated. The post-crack tensile properties were assessed by means of the Double Edge Wedge Splitting (DEWS) test and the standard three-point bending test. Results shows that the post-crack flexural tensile strength is greatly affected by fire in both service and ultimate limit states. The results are of importance for the implementation and validation of recently developed numerical models focused on assessing the bearing capacity of tunnel structures built with SFRC.

Macro-Synthetic Fiber Reinforced Concretes (MSFRCs) have emerged not long ago if compared with other types of Fiber Reinforced Concretes (FRCs). They have mainly used in non-structural works but in nowadays, MSFRCs are becoming gradually accepted in the structural field. To reach this situation, a characterization of this complex material is needed and although several authors have contributed to the study of their behaviour, many aspects require further investigation in this area. This experimental study addresses the effect of environmental temperatures on the tensile behavior of MSFRC after a short-period of exposure. To carry out this, one type of concrete matrix is selected and reinforced with 8 kg/m3 of a commercial Class II macro-synthetic fiber. Uniaxial tension tests are conducted on notched cylinders of MSFRC obtained by coring from standard prismatic beams. The tests are performed at the temperatures ranging from –30 ℃ to 40 ℃ using an environmental chamber to maintain controlled temperature conditions throughout the experiments. The results show that temperature variations influence on the tension behavior of MSFRC. High temperatures lead to a decline in MSFRC performance, whereas an increase is noted at temperatures below zero.

This paper delves into the influence of freeze-thaw (FT) cycles on the mechanical and cracking behavior of strain-hardening cementitious composites (SHCC). These composites were made using a low carbon cementitious matrix – limestone calcined clay cement (LC3) – and reinforced with 2 wt.% ultra-high molecular weight polyethylene (PE) fibers. Performance evaluation was conducted through uniaxial tension tests at a quasi-static deformation rate. Some specimens were preloaded to 1% strain before FT exposure, while the others were investigated without any damage (virgin). All the specimens were then subjected to standard FT cycles as per RILEM recommendations (TC 117-FDC) and exposed to both de-icing salt solution and distilled water for a total of 180 cycles. Following this exposure, further uniaxial tensile tests until ultimate failure were carried out. Analysis of mechanical properties unveiled a marginal deterioration in tensile strength post-FT cycles compared to reference samples. However, all composites sustained a reasonable strain capacity of a minimum of 2.5%, with an average crack width controlled at around 100 µm. From a physical degradation standpoint, evident scaling manifested only when specimens encountered FT cycles in de-icing salt solution. In general, SHCC made of the LC3 matrix showed exceptional resilience to such challenging environments, maintaining a level of performance deemed satisfactory.

The present study analyzes, from a statistical point of view, the effects of low and high temperatures on the flexural behavior of the different Fiber Reinforced Concretes (FRCs). The data considered are the experimental results obtained in a previous extensive campaign with 434 specimens in total (size 150 × 150 × 600 mm). Th e temperatures considered are extreme environmental conditions with values from –15 to 60 ℃ that are applied during 3, 90 and 180 days and kept during the flexural test. The FRCs studied are three types of Macro Synthetic Fiber Reinforced Concretes (MSFRC) and one Steel Fiber Reinforced Concrete (SFRC).To identify significant factors and differences among the groups, several analyses of variances (ANOVAs) were performed using the software Stat graphics Centurion. These analyses allowed to stablish relationships between the selected dependent variables (flexural strengths at limit of proportionality and residual flexural strengths at different Crack Mouth Opening Displacement (CMOD) of 0.5 and 2.5 mm) and factors as temperature, time of exposure and state (pre-cracked or non-pre-cracked). These relationships, in turn, provide insights into the practical implications of FRCs, expanding the existing knowledge. As result, simple numerical equations generated by the same software, are presented and proposed as a first approach for future numerical models by each FRC type.

Ultra High-Performance Fiber Reinforced Concrete (UHPFRC) has enhanced self-healing capability thanks to its reduced water-to-cement ratio and the presence of unhydrated cement particles, as well as its crack pattern with multiple cracks. Normally, this concrete type is reinforced with high contents of fine steel fibers to confer ductility and high tensile strength.This study examines the impact of two fiber dosages and types on the self-healing ability of a UHPFRC. The combinations compared are UHPFRC with 40 kg/m3 of 65/35 3D steel fiber and UHPFRC with 160 kg/m3 short straight-shaped steel fibers (13/0.2). Self-healing is assessed through crack closure, water permeability, and protection against chloride penetration. Disks of size Φ100 × 50 mm were pre-cracked at the age of 21 days to produce cracks between 100–450 μm. Crack width and water permeability in cracked conditions were studied before and after the healing process. At 28 days old, self-healing was promoted through two different conditions: continuous immersion in water at a temperature of 20 ℃ and exposure to a high-humidity environment at 20 ℃ with 95% relative humidity. After healing and performing the final crack width and water permeability tests, chloride permeability through healed cracks and the matrix were also evaluated to evaluate the protection against the penetration of chlorides.

This paper aims to provide a comprehensive examination of shrinkage behaviour in fibre-reinforced concrete integrating post-consumed textile synthetic fibres. The effects of fibre volume fraction and fibre length on mechanical properties and shrinkage were studied to determine the optimum textile parameters. The microstructure, pore-structure and fibre-matrix interfacial properties of the optimised mixtures were then characterised. High tensile strength and flexibility in textile fibres enhance bridging capabilities and adherence between fibres and the cement matrix. Pore refinement was pronounced in the case of hydrophilic textile fibres. This correlates to inferior performance in shrinkage resistance of concrete compared to hydrophobic Nylon. Hydrophilic fibres exhibit strong bonds with the cement matrix, owing to their capacity to facilitate further hydration through water absorption on the fibre surface. Conversely, hydrophobic fibres create weaker bonds within the cement matrix, compromising the concrete's ability to absorb energy and control crack propagation, particularly in interfacial regions. The fibre-matrix ITZ thickness was dependent on fibre size, while the wettability of fibres was observed to affect the phase distribution in the vicinity of the fibre surface. With the increment of curing age, the microstructure at the fibre interface becomes denser due to the hydration of the clinker phase facilitating the growth of CSH and CH phases.

Civil structures and strategic infrastructures are frequently subject to deterioration due to cyclic actions throughout their service life, which can jeopardise their structural stability even at low stress levels. A remarkable enhancement in the fatigue performance of concrete structures can be achieved thanks to the introduction of fibres, since they provide higher toughness to structural elements, with specific regards to the mechanical response at stress levels exceeding the cracking and damage thresholds in compression and bending. The synergy between a high-performance concrete matrix and fibre reinforcement can be particularly conducive to the improvement of fatigue performance. One of the most promising fields for the application of the resulting High-Performance Fibre Reinforced Concrete (HPFRC) materials is represented by structures and infrastructures - including bridges, pavements, infrastructures for energy harvesting - whose service condition is governed by cyclic loading throughout their service life. The characterisation of the fatigue behaviour of the materials hence represents a crucial aspect for structural design of the aforesaid engineering artefacts. To this aim, in the present study, a HPFRC mix has been tested under compression and bending fatigue loadings at different stress levels and number of cycles. With reference to flexural tests, fatigue resistance has been then compared with the estimation of fib Model Code, which proposes, for constant stress amplitudes, numerical correlations - the $$S-N$$ S - N curves - to evaluate the number of cycles to failure based on the type of actions and the stress levels. These correlations, however, are based on empirical experience on plain concrete and adjustments are required for a proper implementation in the fatigue design of new cementitious materials. The present work provides a tailored $$S-N$$ S - N curve formulation by calibrating the existing coefficients based on the experimental characterisation conducted on a performance-based HPFRC mix design.

There is still a major gap in the literature when it comes to the study of the flexural fatigue degradation of fiber reinforced concrete (FRC). Although relevant papers have already brought scientific breakthrough in the analysis of fiber reinforced concrete under compressive fatigue loads, the application of cementitious composite members demand that the models and the design mathematical foundation to be elaborated in accordance to the flexural mechanical response. Therefore, the present research brings an overview of the influence of two key parameters on the flexural fatigue life: fatigue load level and load ratio. Moreover, the three parameter Weibull distribution is proposed as alternative to the more traditional linear Wöhler curves to verify the fatigue life of the cement composite. The experimental campaign encompassed flexural fatigue tests performed on notched prisms, which were first monotonically pre-cracked until reaching a crack mouth opening displacement of 0.50 mm. Thereafter, the specimens were subjected to fatigue loading until reaching 1,000,000 cycles or the material failure. The fatigue tests were performed under three distinct load levels (70%, 80% and 90% of fR,1) and stress ratios (0.20, 0.30, 0.40). Based on the Weibull curves, it was possible to determine the flexural fatigue life equations in terms of 5% of failure probability for all studied fatigue parameters. The Weibull curves could bring a more accurate analysis of the fatigue life of steel fiber reinforced concrete and may be used as major references for future design guidelines of FRC members.

The fatigue response of high-performance fiber-reinforced concrete (HPFRC) varies with fiber content and specimen size. Due to the heterogeneous nature of concrete, it is possible that these different behaviors can be explained by variations in fiber distribution. Therefore, in this work the correlation between fiber orientation and the flexural fatigue strength of HPFRC is studied, considering different fiber volumes (0.3%, 0.6% and 1%) and specimen sizes (75 × 75 × 300 and 150 × 150 × 600 mm). To analyze the fibers, the specimens were scanned with micro-computed tomography (microCT). The results show a clear correlation between the fiber orientation index and fatigue life, such that the more orthogonal the fibers are to the failure plane, the longer the fatigue life. Moreover, it is observed that this relationship is independent of fiber content, although it changes with specimen size.

The research presented here analyzes the relation between fiber orientation and fatigue life under tension forces through the combined use of computed tomography (CT), digital image processing (DIP) software and wedge splitting test (WST). To achieve this goal, eight conventional concrete prisms 150 mm × 150 mm × 600 mm were cast and next, sixteen cubes 150 mm edge were extracted, and a groove and notch were carved on different faces in order to force a clearly different fiber orientation with respect to the cracking plane. However, the exact fiber orientations were obtained from a miniprism extracted from the previously mentioned concrete prisms using a CT-scan device and a DIP software. The results show that there is a strong correlation between the crack-sewing fiber orientation on the one hand and fatigue life on the other hand. Cubes with a higher percentage of fibers perpendicular to the crack surface (i.e. with a higher efficiency index) show a longer fatigue life, while cubes with a lower efficiency index show a shorter fatigue life and a higher crack opening rate per cycle.

Fatigue failure results from the propagation of cracks within material as damage accumulates over successive loading cycles. In this context, this contribution presents results and advances derived from an experimental program on the flexural fatigue behaviour of pre-cracked macrosynthetic fibre reinforced concrete carried out on notched 150 × 150 × 600 mm3 beams. Pre-crack considered in this study was 2.5 mm, this adopted as maximum permissible crack width for the ultimate limit state in Model Code 2010 and related to fR3. The upper load value was varied as a percentage of residual flexural strength, fR3. Throughout the testing procedure, evolution of the crack mouth opening displacement (CMOD) alongside various design-sensitive parameters were monitored. The hysteresis loop area was used to analyse the energy dissipation along the fatigue test. An energy dissipation ratio was established to provide a comprehensive understanding of the process up to failure. Three stages of energy dissipation were identified and for each one, a regression equation to describe the behaviour is provided. By using the equation, the energy dissipation ratio can be estimated with a prediction error of 13%. Failure occurred when the dissipated energy surpasses the initial value at the beginning of the test.

The impact on microcracking and strain behavior of two different high-strength microfibers (steel and carbon) in compressive fatigue tests of high-strength concrete was assessed through systematic experimental investigations and microscopic analysis. To examine and compare the effects of the fibers on fatigue behavior and damage progression, the study involved determining the number of cycles to failure, axial strain, and a detailed microscopic evaluation of microcrack behavior. For this purpose, sections were taken from the specimens according to defined numbers of load cycles and thick sections were prepared. These thick sections were examined microscopically and evaluated regarding the amount, size and position of the microcracks. The number of load cycles to failure tended to be slightly lower for the fiber-reinforced specimens compared to the fiber-free specimens. The strains in the axial direction of the fiber-free specimens are lower until fracture. The microscopic analysis shows that microcracking primarily takes place in the interfacial transition zone (ITZ). The crack width changed only marginally during the applied load cycles and predominantly newly formed microcracks appeared with increasing degradation. This was found in both the fiber-free and fiber-reinforced specimens. The fiber-reinforced specimens show a reduced total crack area compared to the fiber-free specimens. Additionally, it can be concluded that the increasing degradation essentially is caused by the formation of new microcracks.

Due to the superior mechanical characteristics and considerable durability strain-hardening cement-based composites (SHCC) represents a highly promising class of materials for addressing the contemporary challenges in the construction industry. However, the safe application of this material necessitates a comprehensive understanding of the material’s performance under various loading scenarios such as cyclic loading. Furthermore, the assessment of the composite’s behavior mandates consideration of ambient conditions, as the mechanical properties of polymeric fibers are notably influenced by temperature variations. Consequently, the study at hand focuses on the synergistic impact of cyclic loading and varying temperatures on the degradation of SHCC. For this, a cyclic tension-swelling load was applied to pre-damaged dumbbell-shaped specimens at a loading frequency of 1 Hz for 100,000 loading cycles before the residual load-bearing capacity was quantified. The force-controlled upper and lower load levels were set to 80% and 10% of the mean first-crack stresses (ơt,e) of the testing series at the various temperatures. The targeted temperature regimes during the tests were set to −20 °C, 40 °C, and 80 °C, respectively. Subsequently, the fracture surfaces were investigated by means of electron microscopy to determine the degradation caused by the load at the specific temperatures. Finally, the outcomes derived from the experimental tests and microscopic investigations are deliberated to elucidate the underlying mechanisms of deterioration.

Fibre-reinforced concrete is a material under continuous research even though it has been in use for many years. Much effort continues to be devoted to quantifying its contribution to serviceability limit state response. The effects of fibres in the calculation of crack opening and mean crack spacing have been included in the fib Model Code for Concrete Structures 2010 and in Annex L of the new Eurocode 2. The aim of this paper is to analyse the influence of different factors on the occurrence and pattern of cracks. A statistical design of experiments has been proposed to rigorously analyse the effect of the variables normally considered. The experimental programme is carried out on fibre-reinforced concrete beams tested in bending. In this study, the results of 18 beams, each 3.70 m long, are presented. These beams were tested until reaching the service deformation in the main reinforcements, and their condition of cracking was maintained for one year. The beams were retested to the same load level, sustained for 24 h, to assess short-term delayed deformations. Finally, they were loaded until failure. With the results obtained for crack spacing and crack opening, the influence of the different factors was determined by statistical analysis. Results have been obtained for an instantaneous load, for a prolonged load and for failure. Finally, using the statistical results obtained, a formulation has been developed to calculate the average crack spacing and has been compared with the existing formulation in the draft Eurocode 2, Annex L and Model Code 2010.

Macro-synthetic fibres are increasingly gaining acceptance as a type of reinforcement for structural applications, particularly in the form of polypropylene fibre-reinforced concrete (PPFRC). However, there is a need for additional understanding of the structural behaviour of PPFRC, particularly in relation to its time-dependent properties such as shrinkage and creep. To address this, an extensive experimental program was conducted on the flexural load-bearing capacity of full-scale PPFRC beams. Beams with spans of 3 m and cross-section sizes of 200 × 250 mm2 were cast from C40/50 concrete with varying amounts of two polypropylene fibre types (0, 3, and 9 kg/m3 and aspect ratios of 56 and 67). Four beams were cast for each of the five types of concrete. After 28 days, two beams from each concrete type were subjected to three-point bending tests until failure. Subsequently, one beam from each concrete was subjected to sustained load for one year in a four-point bending configuration, while another beam from each concrete was left unloaded for one year under the same conditions. After the one-year period, both remaining beams from each concrete were tested until failure in bending. The test results facilitated the assessment of the contribution of fibres to the flexural strength of beams and the impact of sustained load on changes in flexural capacity over a one-year period. These findings were complemented by a comprehensive physical-mechanical characterization of the concretes. The outcomes of this study can offer additional support for the safe and reliable structural use of PPFRC.

Strain hardening cementitious composites (SHCC) possess exceptional crack width control, which has been utilized to improve the durability of various concrete structures. Recently, several studies have reported the development of 3D printable SHCC. However, the performance-based design of a 3D printable SHCC, specifically for a bridge-related application, has not been reported. This paper describes a framework for developing an SHCC suitable for extrusion-based 3D printing while achieving the mechanical properties and crack width control needed to mitigate end-cracking in a prestressed concrete bridge girder. The framework proposes the use of finite element analysis to estimate the properties of SHCC needed for the given application and iterative modification of the mix to fulfill the strength and printing requirements. The proposed framework consists of various stages, and the initial stage of rheological modification of a baseline SHCC is discussed in this article. Sixteen variations of the baseline mix were developed by varying the proportions of viscosity modifying agent (VMA) and high range water reducing admixture (HRWRA). The flow properties of trial mixtures were evaluated using the flow table test, fiber dispersion was evaluated through manual inspection of the mix, and extrudability and buildability were examined qualitatively using a hand-operated extruder. Four suitable mixtures were identified based on preliminary examination which will be further optimized and evaluated in future work.

Recently, extrusion-based 3D Concrete Printing technology has seen significant development. It is utilized to create individual modules in factories and to print entire structures on construction sites. Strain Hardening Cementitious Composites (SHCC) have been demonstrated by various research groups as a suitable printing material. This material, reinforced with short fibers, addresses the brittleness issue inherent in conventional concrete, which negatively impacts both the strength and durability of concrete structures. Using SHCC to print the external contour of individual modules can prevent damage during transportation or assembly.To create continuous reinforcement in the printed structure, conventional steel reinforcement can be used. However, its application has limitations, such as the complexity of automating reinforcement integration and the need for a sufficient protective layer. These limitations can be overcome by using freshly impregnated carbon yarns for reinforcement. The initial flexibility of these yarns allows for the automation of the reinforcement process, while the cementitious impregnation enhances its bond with the material of the reinforced structure.Measuring the bond strength of printed SHCC with carbon yarn reinforcement placed between the printed layers is challenging. Fabricating specimens directly from the printed structure is complicated due to the presence of fibers in SHCC and the high probability of damaging the textile yarn. Creating samples by casting does not account for the layering inherent in 3D printing, leading to an inaccurate assessment of the actual bond strength.This publication presents a method for measuring the bond strength of printed SHCC with carbon yarn reinforcement directly on 3D printed samples. This approach allows for more accurate measurements by considering the layered structure of the printed material. The publication provides a detailed description of the method and test results for the obtained samples.

With increasing number of 3D-printed concrete structures, the demand for adequate reinforcement concepts for this modern construction method has been also increasing. From a technical perspective, the new technology can only be successfully used if the 3D printable materials fulfill the high requirements in terms of their rheological properties in a fresh state and mechanical properties in a hardened state. In this context, the integration of reinforcement presents major challenges for both industry and research. One promising approach to reinforcement is the addition of steel fibers to the concrete. These fibers enhance the ductility, durability, and robustness of 3D-printed structures. While the addition of fibers offers some manufacturing advantages, it considerably complicates the adjustment of the concrete mix’s rheology and requires a deep understanding of its behavior in the fresh state. Thus, this article addresses the time-dependent development of the static yield stress and the structural build-up rate of several cement-based materials containing steel fibers. In the printable cement-based materials with a maximum grain size of 8 mm, 0.5% and 1.0% by volume of straight steel fibers are added, each fiber measuring 25 mm in length and 0.4 mm in diameter. The test methods applied include the observation of the spread flow over time and the uniaxial compression tests. The results obtained provide a basis for the future efficient use of steel fibers in the context of 3D printing with concrete.

Using a material extrusion 3D printing technology, we fabricate beams with axial rebar and subject them to four-point flexural tests. Beams were fabricated using fiber-reinforced cementitious composite by both 3D printing and formwork casting methods, and their structural performance were compared. The printed specimens demonstrate load bearing and deformation capabilities equal to or greater than those of cast specimens, with maximum loads exceeding values calculated by existing design formulas. The printed specimens also show good crack dispersion and confirm that the proposed manufacturing method adequately ensures bond characteristic between rebar and printed element.

The paper is based on results from an ongoing Norwegian R&D project with the overall goal to generate data that can be used to overcome remaining obstacles for a wider use og fibre reinforced concrete (FRC). One such obstacle is the unsafety concerning fibre distribution and orientation. A specific goal is therefore to provide data that can be used to recommend casting procedures and belonging concrete rheology limits that give reliable and predictable fibre distribution in wall elements. A number of walls of 2.5 x 7 m, were cast of both self-compacting concrete (SCC) with various rheology and vibrated concrete (VC) and placed using pump and hose with 1–2 drop zones. The results indicate a rather narrow SCC rheology window for success in terms of fibre distribution in a wall, and that pouring from two positions improved the fibre distribution only slightly. The results confirm also that the influence of mixing and transport on fibre distribution is far less important than concrete rheology and casting method.

In Latin America, fiber reinforced shotcrete (SRF) is widely used for tunnels linings in road, hydraulic and mining projects. In the most of these projects, the specifications commonly establish (1) the energy absorption capacity that the FRS must achieve and (2) the fiber content to be used to reinforce it. Although there are adequate standards to determine the fiber content in the fresh state and the energy absorption capacity by testing square or round panels, the criteria to evaluate compliance with the specifications given by different recommendations are based on deterministic analyzes of the test results, which do not consider the conditions of the execution of the works or a certain level of confidence, nor outlier values are identified and treated.On the other hand, due to the random distribution of fibers in the concrete mass during mixing and spraying, the results of the fiber content tests present high scatter, provoking claims between principals and contractors.Considering the above, in this paper a probabilistic method to evaluate compliance with the fiber content specified in a project is proposed, and expressions to determine the minimum number of fibers necessary to comply with the nominal content established in the specifications are given, which are applied to evaluate the FRS used in a real mining project developed in Chile.

The paper presents a theoretical method to optimize the tensile fatigue performance of steel fibre reinforced concrete. The approach bases on new SN curves and mathematical optimization techniques. The new SN curves take into account stress level and stress ratio as well as governing fibre parameters like fibre type, dosage, orientation and bond behaviour to the concrete’s matrix. Effects of these interacting parameters are captured by a dimensionless ductility index contrasting pre- and post-cracking load bearing capacities. The ductility index relies on the envelope concept and thus establishes equivalence of stress level and crack width dependent fibre effects at static and cyclic loadings. The SN approach is validated by experimental data taken from the literature. Results are in good accordance on average and underline the beneficial effect of steel fibres on the tensile fatigue performance of normal strength concrete. Based on the derivations mathematical optimization techniques are used to theoretically optimize fibre length, shape and crack-width dependent pullout behaviour with respect to fatigue lifetime. In detail, a scalar-valued objective function exemplarily derived from the SN-curves is minimized. In the convex optimization task the Karush-Kuhn-Tucker conditions are both, necessary and sufficient conditions to fulfill minimum requirement. The system of equations is numerically solved with Newton-Raphson Approach and Line Search Algorithm. In doing so the experimental observation of beneficial effects of enhanced fracture energy, fibre length and bond behaviour on numbers of cycles to failure can be theoretically proven.

The time-dependent pull-out behavior of fibers in concrete exhibits wide scattering, even for nominally identical specimens in a controlled environment. This variation limits the capabilities and insights provided by deterministic modeling approaches. Nevertheless, little work has been done toward understanding the stochastic nature of single-fiber pull-out under sustained loading. Such understanding is necessary for modeling the performance of load-carrying members made of fiber-reinforced concrete. For this research study, pull-out creep under sustained loading is modeled using a discontinuous Markov process. Along with damage development, the creep process affects progressive debonding along the fiber–matrix interface. Therefore, for physical consistency, the transition probability to the debonded state is quantified through an energy balance approach within the framework of fracture mechanics. Considering interaction between the energy release rate and the corresponding fracture resistance, models are useful to assess debonding propagation over time until complete pull-out. Results demonstrate that the probabilities of the pull-out time deviate from a normal distribution.

It is well known that fiber orientation and fiber distribution in FRC strongly affect mechanical behavior. In particular, fiber orientation depends on casting manner during execution. This research presents numerical results on flow analysis of flowable FRC which gives visualized fiber orientation. Three dimensional Distinct Element Method (DEM) was applied to the analysis. Mortar matrix was modeled using spheres having 5 mm in diameter, and each fiber having 30 mm in length and 0.5 mm in diameter was discretized by using 60 small spheres. Fiber orientation within a member was roughly simulated, and there was an area without any fiber at confluence part in a member. In addition, it was also presented that fibers aligned in one direction along reinforcing bars.

The verification of the fatigue performance of concrete often involves intricate and time-consuming experimental programs. The inherent complexities arising from variations in fibre distribution and orientation within fibre-reinforced concrete is evident when interpreting residual flexural performance results. These variations are particularly pronounced under fatigue loading, given the considerable scatter associated with the phenomenon. Addressing this challenge requires either the development of models that incorporate a logical basis for analysing design uncertainties to ensure a robust evaluation of failure probability, and to calibrate material partial safety coefficients that cover the uncertainties associated to the material performance and the inaccuracies of the models. In this regard, with the aim of proposing a model capable of predicting fatigue behaviour and thereby reducing the time required for fatigue tests, a conceptual model is presented herein. This model, formulated in a generalized form equation, considers the slope of the crack mouth opening development evolution versus the number of cycles curve for a cracked section subjected to a fatigue load that mobilizes the residual flexural tensile strength for CMOD = 0.5 mm. To validate the regression equation, a database of pre-cracked 5C strength class SFRC notched beam subjected to flexural fatigue extracted from existing literature is considered. By adopting mean values of CMOD and known strength at pre-crack, the equation proves to be capable to predict CMOD for an estimated number of cycles.

For a sustainable design, the equivalent reuse of reinforced concrete (RC) components helps to significantly reduce waste and $$\hbox {CO}_2$$ CO 2 -emissions from construction. A key task is the development of new connections for the modules transmitting internal forces and shaping the new structure.In this contribution, the derivation of an additive joint for RC beams using steel-fiber reinforced concrete (SFRC) and additional steel reinforcement is presented. The geometry of the cut-out for jointing thereby relies on the maintenance of load-bearing capacity and serviceability with respect to a complete beam, which is experimentally investigated. The tests were performed in full scale and holistically measured, e.g., using digital image correlation to monitor deformations and growth of crack widths. The experiments revealed that the additive beam exhibits a 20% higher load-bearing capacity than the complete beam due to the SFRC joint. In numerical investigations a finite element (FE) model is built up and validated with the experiments. The FE simulation bases on cohesive interface elements, employing a traction-separation law. The numerical investigation possesses good accordance with experimental results for both, the load-bearing behavior and even more for the crack distribution. By means of a global sensitivity analysis (GSA) based on the FE model the influence of the additional reinforcement, material parameters, and joint roughness are quantified. It reveals that the fibre content and the length of the additional reinforcement are decisive for the load-bearing capacity.

The application of FRC in slabs supported on columns or piles has shown technical and economic benefits that can be obtained by combining properly fibre reinforcement mechanisms to those provided by conventional reinforcement placed, as a strip, in the alignment of columns/piles. Therefore, to investigate the influence of steel fibre reinforcement for the serviceability and ultimate limit design state conditions (SLS and ULS, respectively) of statically indeterminate shallow structures that includes conventional steel reinforcement (herein abbreviated by HSFRC), representing a region in the alignment of the columns/piles of an R/FRC slab, six continuous shallow beams of 400 × 125 mm2 cross section, and 6500 mm of length (two equal continuous spans of 3000 mm each) were experimentally tested. The variables investigated in this experimental program considered the fibre content and the conventional flexural reinforcement ratio. The results show that in ULS conditions, the HSFRC shallow beams performed better than conventional RC shallow beams in terms of load carrying capacity, providing warnings of imminent collapse and stable post-peak responses. At SLS conditions, the HSFRC shallow beams presented an average crack width and spacing up to 50 and 48% lower than their counterparts in conventional RC.

A numerical tool for characterizing the tensile properties of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) is presented. The application is based on a model capable of reproducing the load-deflection behavior of prismatic unnotched UHPFRC specimens under the Four-Point Bending Test (4PBT) scheme, which enables the implementation of numerical methodologies for the characterization of UHPFRC tensile properties based on inverse analysis. The constitutive behavior considered takes into account three distinct branches, encompassing both strain hardening (SH) and strain softening (SS) behaviors. The constitutive behavior is implemented in a non-linear section analysis to consider the crack development. Moreover, the sectional behavior is implemented at a structural level through a non-linear hinge model. Therefore, the numerical model allows for an immediate analysis of the load-deflection response behavior with respect to those variables that influence the 4PBT. This includes those specific parameters that define the UHPFRC material behavior as well as those ones that determine the test typology and the geometry of the specimens. The numerical application also includes an iterative inverse analysis method enabling the automatic search for the tensile properties given a load-deflection experimental response. Both the numerical model and the iterative method have been coupled in a web application allowing the UHPFRC analysis and characterization process to be carried out quick and easy in any field of work. The numerical model and the iterative inverse analysis method have been validated by comparing their response with previous experimental works of characterization. Overall, the developed application proved to be a simple and reliable tool that enables an intuitive graphical analysis of the behavior of UHPFRC 4PBTs.

Many concrete structures might be exposed to accidental or induced impacts during their service life. In case of moderate-rate impacts (e.g. rock falls, vehicle hits, etc), reinforced concrete (RC) elements may suffer significant damage even though a complete collapse does not necessarily occur. In such cases, the remaining load carrying capacity or residual performance needs to be assessed in order to ensure the users’ safety or make a decision on the necessity of repair or demolish. The use of fiber-reinforced concrete (FRC) for structures subjected to impacts has been proven to increase the structural strength. Though impact events may lead to severe cracking, spalling or other damage mechanisms, the residual capacity of FRC structures might still be considerable. However, very few specific studies have dealt with the evaluation of impact-damaged structures. In the present contribution, a discussion on the residual capacity and failure mode of FRC beams is presented based on experimental results of beams subjected to a drop-weight impact and a subsequent quasi-static test to failure. Interestingly, it is found that the failure mode of impact-damaged elements can differ from the one of comparative elements, which were not subjected to previous impacts. Also the residual capacity is dependent on the damage level caused by the impact.

The Ultra high-performance steel fibres reinforced concrete (UHPFRC) investigated in this paper is a fine-grained cement-based composite material with outstanding mechanical properties. Its key attributes include an ultra-high compressive strength in excess 150 MPa and a permanent post-cracking strength in excess 5 MPa. To increase its structural integrity, steel fibres 13 mm long and 0.2 mm in diameter are added to the matrix to reinforce it. In order to assess the properties of the UHPFRC under varying loading conditions, the prism-shaped specimens are subjected to three-point bending tests over a range of loading rates from quasi-static regime to dynamic impacts at intermediate strain rates. The experiments are performed using an in-house developed testing machine based on linear motors and are conducted at 4 different loading velocities with at least 5 specimens tested at each strain rate. The tests are observed using a high-speed camera. For a better understanding of the material behaviour, the testing equipment is combined with a laboratory high power X-ray imaging set-up that allows internal inspection of the samples to analyze the effect of imperfections, inhomogeneities, voids and dominant fibre orientation. X-ray imaging is performed before and after mechanical testing and also in-situ during the loading using a high-speed X-ray imaging camera. A significant dynamic increase factor is observed between the individual strain rates, while the dominant fibre orientation is identified as a crucial aspect causing the differences between the specimens. This innovative experimental approach provides invaluable insights into the material response to dynamic loading conditions and offers a comprehensive understanding that is crucial for optimizing its performance in a variety of real-world applications.

The current work investigates the performance of sustainable fiber-reinforced composites (FRCs) made of limestone calcined clay (LC3) binder systems with added superabsorbent polymer (SAP). These composites were reinforced with novel bicomponent polypropylene (PP) fibers compared to standard monocomponent PP fibers. Focus is set on bicomponent PP fibers where the smooth outer surface is modified by including calcium carbonate (CaCO3) particles in the outer shell. Fibers are manufactured at IPF by a pilot melt spinning line. The dynamic single fiber pullout test is used to study the fiber/matrix interaction. The results revealed that incorporating CaCO3 particles in the outer shell significantly improved the mechanical interlocking, leading to increased energy absorption capabilities during fiber pullout compared to the smooth monocomponent PP fibers. The dynamic tensile response of LC3-based FRCs with different PP fibers was investigated using a gravitational Split-Hopkinson Tension Bar (SHTB) setup to assess the performance at high strain rates. It was evident that the bicomponent PP fibers with CaCO3 facilitated better stress transfer mechanisms in the crack flanks due to improved fiber-matrix interface, resulting in multiple crack formation with strain hardening behavior. Overall, the findings suggest that combining FR-LC3 with SAP and bicomponent PP fibers is promising for structural strengthening under dynamic loading scenarios while offering reduced carbon footprint and cost advantages.

Understanding the mechanical behaviour of UHPFRC materials under extreme loads is crucial for designing safer structures. Under severe loading cases such as impacts and explosions, structural strength and reliability has become increasingly important due to increased accident frequency and global terrorist threats. It should be noted that many critical infrastructures, such as government buildings (e.g., embassies, checkpoints), tunnels, and bridges, are susceptible to improvised explosive devices (IEDs). The protection of infrastructure from the effects of IEDs often requires the construction of protective structures, such as cementitius composites shields. A wide experimental campaign on several types of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) under dynamic loading is summarised in this paper as well as the different experimental techniques are described.

The influence of hybrid fiber reinforcement on the ballistic and heat resistance of reactive powder concrete was evaluated through an experimental program. Prismatic and cylindric test specimens were cast using different binary combinations of steel, polypropylene, polyvinyl alcohol, glass and aramid fibers. The resistance to elevated temperatures was evaluated by determining the residual strength after exposure to 200, 400, 600 and 800 ℃ (2 h isothermic dwell time). The ballistic resistance of hybrid fiber-reinforced cylindric samples was investigated experimentally by the DoP test. Specimens were subjected to the 7.62 × 54 R B32 API projectile impact load using the striking velocity of 850 m/s. The influence of different hybridization concepts on the projectile impact resistance represented by differential efficiency factor (DEF), height of impact crater and damaged surface area was evaluated. It was found that by appropriate hybridization of the fiber reinforcement, superior ballistic and heat resistance of RPC can be achieved compared to only steel fiber-reinforced RPC. Steel and PVA mix (3:1 Vol.) was found the most effective combination to achieve a high level of dual heat and ballistic protection.

The number and intensity of attacks has steadily increased worldwide in recent years, requiring more effective building protection elements and structures to minimise the personal injury and anticipated economic damage. Threats that can be expected to affect structural protection systems include bullets from standard firearms, explosions from explosives, vehicle impacts, etc. Targets of attack are not only military and government facilities, but also civilian and industrial facilities, such as those providing essential services, which should be specially protected. In order to ensure the protection of critical infrastructure, the Institute of Structural Engineering, Chair of Concrete Construction at the University of the Federal Armed Forces in Munich is investigating the use of steel fibre concrete as a protective material during reinforcement work on the above structures.As part of this investigation, various reinforced concrete and fibre reinforced concrete slabs of concrete grade C40/50 with different fibre contents (0, 0.5, 1.0 and 2.0 vol%) were produced and subjected to contact detonation with a variable charge (SEMTEX10). After detonation, the plates were scanned using 3D hand scanners (Zeiss systems). The 3D plates were first meshed from the point cloud and visually detectable damage parameters (craters/bursts) were determined using Geomagic-Design X software. Missing crater geometry, damage area, flaking depth, volume and weight of flakes could be accurately displayed and determined. By comparing the results obtained, a positive influence of the steel fibres on the damage to the concrete slabs could be demonstrated. The article presents the results of the tests carried out and the evaluation of the parameters described above.

The addition of fibers, such as steel, as randomly dispersed reinforcement in concrete is effective in improving the behavior of structural elements subjected to impact loads, reducing the formation and propagation of cracks, spalling, and mass loss. Moreover, it is possible to consider the use of external thin strengthening layers as strain-hardening cementitious composites (SHCC), where their high energy dissipation through the formation of multiple cracks further contributes to the impact resistance and structural integrity maintenance after high strain rate loadings. In the present work a drop-weight impact test machine with energy up to 2.5 kJ was designed and instrumented with piezoelectric loading cells, piezoelectric impact accelerometers, and a data acquisition system with a capacity of 2 million samples per second. This impact machine, allied with a digital image correlation (DIC) system with two high-speed cameras, was used to acquire the strain and displacement data to understand the steel fiber reinforced concrete (SFRC) behavior under impact loading. The addition of steel fibers to concrete reduces crack formation and deflections after impact loads, reducing damage and maintaining the SFRC specimen’s integrity when compared with ordinary concrete’s impact resistance. Short-beam SFRC specimens were molded and subjected to an impact three-point bending test using state-of-the-art drop-weight equipment with a strain rate of 4.2 s−1. Poly-vinyl-alcohol (PVA) SHCC thin strengthening layers were added to undamaged and pre-cracked short-beam SFRC at the top or the bottom face of the specimens to evaluate their contribution to reduce the crack formation and propagation under impact loading.

While the integration of fibers in concrete is not novel, the application and use of continuous fabric-like textiles represent a more recent development. Carbon-based textiles stand out for their superior mechanical properties, demonstrating high strength and durability when combined with cementitious matrices. Coatings applications in multifilament yarn textile reinforcements are crucial to enhance adhesion and facilitate effective stress transfer between the fabric and the cementitious matrix, ensuring long-term durability and structural integrity for the Textile Reinforced Concrete (TRC). Although epoxy resin is commonly employed as it results in excellent TRC mechanical performance, its hydrophobic nature poses challenges in achieving strong chemical adhesion, potentially leading to issues such as delamination between the textile reinforcement and the matrix. In this context, water-dispersed epoxy emerges as a promising alternative, mitigating hydrophobicity and enhancing chemical adhesion, thus improving the overall integrity of the composite. Continued research on the use of this alternative coating at a material level regarding their suitability for a structural application remains to be investigated. Thus, this study aims to assess the impact of a water-dispersed epoxy resin compared to a commercially available epoxy resin on the mechanical behavior of carbon TRC through direct tensile tests. The evaluation focuses on the assessment of its viability for future practical applications.

Strain-hardening Cementitious Composites (SHCC) exhibit unique tensile strain-hardening behavior accompanied by closely spaced cracks. Recent research has shown that incorporating Carbon Fiber-Reinforced Polymer (CFRP) laminate strips together with SHCC in the tensile zone can effectively enhance/recover the load-bearing capacity of RC beams. Additionally, the widely-adopted Near Surface Mounted (NSM) technique has proven effective in retrofitting and rehabilitating underperforming RC structures. The bond behavior between concrete and the NSM-CFRP system has been extensively evaluated, but information on the bond behavior between SHCC and NSM-CFRP is very limited. This study aims to investigate the bond behavior between SHCC and NSM-CFRP laminate strips under direct tension, which is crucial for optimizing the design and applications of this system in practical scenarios. High-strength SHCC with a compressive strength of 112 MPa, tensile strength of 8.6 MPa, and tensile strain capacity of 5.5% was used. The effects of different sand coatings, number of CFRP layers, cover thickness, and bond lengths were discussed. The findings show that the bond behavior between SHCC and CFRP can be improved by utilizing proper sand size. Additionally, the effective bond strength decreases with increasing bond length, but increases with increasing strip width and strip layers. The findings of this study can contribute to developing innovative and reliable techniques for reinforcing and rehabilitating RC structures.

Non-metallic reinforcement not only facilitates a substitution of conventional reinforcing steel, but also allows for slender members with a reduced concrete cover or an increase in structural capacity, significantly improving the sustainability of concrete buildings. The ability of non-metallic reinforcement in advancing the ultimate limit state for concrete members directs attention to serviceability criteria, such as crack width limitation, which become decisive for design. These criteria need to be accounted for by adequate models, especially regarding the bond behaviour and tension stiffening effects. To characterise the bond behaviour and derive a crack width related value of bond stress, pull-out tests were conducted on a carbon and a glass fibre grid. The crack development under cyclic loading was focused using uniaxial tensile tests in which the reinforcement strain was detected with fibre optic sensors to approximate a mean strain value along the transfer length to account for tension stiffening. The results were used for a prediction of the crack width obtained in the uniaxial tests.

In recent years, numerous efforts have been made to find an alternative to reinforcing steel in order to achieve improved properties such as corrosion resistance and a slender design of concrete elements. A promising approach is the use of non-metallic fibre-reinforcement polymers (FRP) for reinforcement of concrete structures. Basalt fibres have great potential in this respect due to their excellent properties such as high tensile strength, corrosions resistance and low weight. These fibres from natural rock can be produced in an environmentally friendly way and have excellent recyclability compared to glass or even carbon fibres. In this study, two different bar diameters made of basalt fibre reinforced polymer (BFRP) are evaluated with regard to their mechanical properties depending on their structure.

Non-corrosive carbon fiber-reinforced plastic (CFRP) reinforcements enable significant reduction in the concrete cover dimensions of concrete components. The high tensile strength of CFRP furthermore allows thinner structures. This results in savings of primary resources, thus leading to more CO2-efficient structures. Therewithal, the use of eco-concretes can further enhance this saving effect: Due to their cement-reduced mixture compositions, they exhibit a more favorable CO2 balance compared to standard concretes. Therefore, construction elements made of a novel CFRP-reinforced eco-concrete (CFRP-EC) combination introduce an innovative two-track approach for diminishing CO2 emissions. On this basis, the research project “Congreen.Carbon.System” is developing precast frame components made of CFRP-EC for railway construction. In this paper, an eco-concrete and CFRP reinforcement to be used for this purpose are presented. Results on the eco-concrete’s strength development as well as on CFRP rebar pull-out tests are presented. Thus, an initial analysis of the bond behavior of eco-concrete with CFRP rebars is performed.

Modern high-performance composite materials, such as carbon textile-reinforced concrete, offer a new potential for developing structural systems that promise a significantly higher sustainability compared to state-of-the-art. However, this promise can only be fulfilled if both material components are fully utilized. Indeed, without proper selection of design parameters, a significant portion of the high-performance material remains wasted, raising concerns about the environmental impact particularly related to the carbon reinforcement material with considerable high specific CO2 footprint. Therefore, it is crucial to evaluate the environmental impact in direct relation to the functional performance of structural elements. Our contribution sketches the chain of mathematically formulated criteria that enables us to link the functional performance, defined by serviceability and ultimate limit states, with the material efficiency of the design. In simple terms, we enhance the recently published closed-form model identifying the design parameters of fully utilized cross-sections with the direct evaluation of the equivalent carbon footprint related to the functional unit represented by one square meter of a floor area. Our goal is to stimulate further refinements of the performance-based sustainability metrics that can be directly embedded in the numerical design models and assessment rules for emerging construction materials.

Knowledge of the bond behaviour of reinforcing elements in concrete structures is crucial, as it has a particular influence on the load-bearing behaviour of components. Realistic mathematical modelling is therefore important in order to be able to make statements about, for example, deformations, cracking or anchoring. For reinforced concrete, this has been studied and described in detail, so that today’s design can be calculated with the sufficiently accurate, simple assumption of a constant, average bond stress as a function of the concrete tensile strength. For concrete with non-metallic reinforcement, such a formulation has yet to be derived. The large number of different types of reinforcing fibers and their impregnation, together with the desired slender design, result in different bond load mechanisms to be considered. This paper presents investigations on the characterization of the local bond behaviour of non-metallic, textile reinforcing elements. The derivation of a material-specific parameter is investigated by means of tests on the composite material and with digital image correlation (DIC) and distributed fiber optic measurement (dFOS). In the still valid mechanical idealizations of the bond behaviour, their applicability is investigated.

The use of Textile Reinforced Concrete is becoming more and more important both in retrofitting of existing structures and in new constructions. This asks for reliable approaches for the design of structural members. In particular, referring to bending behavior, traditional approaches for Reinforced Concrete structures that account for a perfect bond between concrete and reinforcement cannot be adopted for these kinds of composites where the interaction between reinforcement fabric and concrete plays a key role in the global tensile response of the solution. General characterization approaches refer to the composite as a whole providing a tensile behavior that already takes into account the specific interaction between reinforcement and concrete. The present paper aims at presenting a design approach for bending behavior of these composites that propose the use of an equivalent reinforcement layer of reinforcement, whose mechanical properties are defined by tensile tests on the composite, and that is considered as embedded in the cross section under design. The paper discusses the reliability of the approach, also discussing the dimension and definition of the equivalent reinforcement layer comparing the prediction of the model with experimental results.

In the last three decades, extensive research and development have propelled carbon reinforcement as a compelling alternative to conventional steel in concrete, leading to an increased application in construction in recent years. This innovative material, resistant to corrosion, allows the concrete cover to be reduced to a minimum, enabling the construction of very filigree components and members. Against this background, the stability behavior is a theme of growing importance for the design of such structures. Moment-normal force interaction diagrams can help to assess the failure mode of slender compression members. Hence, an analytical failure envelope for carbon-reinforced concrete under combined bending and longitudinal force is derived, using an adapted, linear-elastic material model for the non-metallic reinforcement. Experimental investigations on carbon-reinforced concrete components with different slenderness ratios under simultaneous axial and flexural loads unveil insights into load-bearing and deformation behavior. This research contributes to the development of sustainable and resource-efficient concrete structures, emphasizing the significance of stability and slenderness ratios in design considerations.

Externally bonded reinforcement (EBR) systems are widely used for strengthening and repairing existing structures, since they can provide significant performance enhancements without significantly affecting geometry, mass and stiffness of the considered structure. In the last years, among others, textile/fabric reinforced cementitious composites (Textile Reinforced Mortar (TRM), Fabric Reinforced Cementitious Matrix (FRCM), Textile Reinforced Concrete (TRC), etc.) are getting more popular as a retrofitting material. However, the effectiveness of EBR systems depends not only on the reinforcement strength but also on several bond related factors that can affect the reinforcement exploitation and their failure modes. Understanding these failure modes is important both for the design and for the assessment and monitoring of retrofitting interventions. To address this issue this paper will present some preliminary results on textile reinforced composites-to-concrete substrate bond behavior, investigated for different failure modes, and monitored through nondestructive techniques (NDTs) including digital image correlation and acoustic emission.

Carbon-reinforced concrete is a construction technique that involves reinforcing concrete elements using carbon textile reinforcement. Such a technique is relatively new, and thus there are still some unknowns regarding the long-term behavior of carbon concrete. Therefore, the distributed fiber optic sensor technology DFOS was utilized in several experimental investigations to have an in-depth understanding of the long-term behavior of carbon concrete and its structural behavior under loading. These experimental investigations involved studying the shrinkage behavior of carbon concrete, the long-term behavior of self-stressing carbon concrete, as well as their structural behavior under loading. This paper describes the different DFOS techniques and procedures used to monitor carbon textile-reinforced concrete. Additionally, some of these experiments’ results are also reported and discussed herein. The results have shown that using the DFOS technology can provide a comprehensive overview of the strains developed along the concrete member, which can be interpreted into stresses. Thus, DFOS technology was able to provide an in-depth understanding of the long-term and structural behavior of carbon concrete.

Amid the growing concern surrounding the environmental impact and carbon footprint of concrete materials, reducing the carbon footprint of the matrix phase needs to be complemented with finding alternatives to steel rebars as the primary reinforcement. In this study, various plates and shapes were manufactured using TRC to explore its structural capabilities. These components were then subjected to compression testing to assess their performance under load. Additionally, the deformation and failure processes were meticulously recorded using Digital Image Correlation (DIC) techniques, allowing for a precise digital analysis of the material behavior under stress. The critical load—the maximum stress the material can withstand before failure—was a key focus of the investigation, providing valuable insights into the potential applications and limitations of TRC in construction. This comprehensive approach not only highlights the mechanical properties and performance of TRC but also reinforces its viability as an eco-friendly alternative in modern engineering solutions.The Finite Strip Method is a suitable technique for the buckling analysis of plates. By employing strips along the length of the specimen, the critical local buckling stress can be computed. Notably, the FSM is computationally efficient, as it avoids the need for the extensive use of small finite elements with numerous nodal points,. This paper aims to shed light on the manufacturing and optimizing TRCs for eventual compression applications such as truss members subjected to load reversals. The primary focus is to explore the modeling of structural sections in plates, stiffened members, panels, and trusses which may require load reversal, in both tension and compression zones. The study involves the testing of plate coupons and sections subjected under compression and comparing the experimental data and the compression behavior of angle shapes in TRC with the semi-analytical models.

Carbon reinforced concrete (CRC) offers environmental benefits and enables the construction of slender, thin-walled, and geometrically flexible structural elements. Nevertheless, extra caution is essential for thin components when requiring the incorporation of fasteners through precast or post-cast methods. The behavior of fasteners in thin CRC elements is of most importance, as it represents a pioneer exploration into the future of construction. However, the behavior of fasteners in CRC differs from that in conventional steel-reinforced concrete (RC) components due to the reduced thickness. This paper investigates the maximum strength and failure behavior of thin concrete plates with cast-in-place fasteners, examining both plain and textile-reinforced concrete. The overall cracking behavior of plain concrete plates was decisively influenced by the occurrence of splitting cracks. The introduction of textile reinforcement shows a significantly higher ultimate strength of the concrete plate and shifts the failure mode to pullout failure than splitting failure in plain concrete components.

This work describes the experimental research activity executed to develop and evaluate the potential of new reinforcement solutions for concrete structures in marine environment conditions. For this purpose, four series of small-scale beams were tested in three-point loading configurations to determine the influence of polypropylene fibres (PP) on the shear capacity of these beams, and the flexural reinforcement efficiency of glass fibre reinforced polymer (GFRP) bars. The partial use of recycled aggregates, as well as the use of seawater on the production of the concrete was also explored in an attempt of developing a more sustainable material, by taking the advantage of the corrosion-immunity of the adopted reinforcements. Reinforced concrete (RC) beams were made by two types of concrete reinforced with PP fibres (PPFRC1 and PPFRC2), and one plain concrete (PC). The mechanical properties at different ages in the hardened state of the developed FRC and PC were determined to derive the fundamental information for modelling the structural behaviour of the tested RC beams. Besides the force mid-span deflection, the crack patterns and failures modes were registered. Reference beams flexurally reinforced with steel bars were also tested for comparing the efficiency of GFRP and steel bars in terms of stiffness, load carrying capacity and deflection performance. Additionally, the shear resistance of RC beams was estimated by using the formulations of RILEM TC 162-TDF and fib Model Code 2010. The relevant results are presented and discussed in this paper.

Externally bonded (EB) fabric-reinforced cementitious matrix (FRCM) composites, also referred to as textile reinforced concrete (TRC), are getting increasingly popular in the field of structural retrofit- ting as they combine good mechanical properties with important advantages in terms of cost effectiveness, ease of intervention, and reversibility. In addition, FRCMs have good resistance to relatively elevated temperature, also by vitue of the protecting role exerted by the inorganic matrix on the embedded textile. While polymeric impregnation of multifilament textiles maximizes the mechanical response of FRCM, it also introduces an organic element into an otherwise fully inorganic composite and, accordingly, raises some concerns in terms of high temperature vulnerability of the material. In this paper, the influence of thermal preconditioning on the tensile properties of carbon FRCM in a cementitious mortar is investigated, with special regard to the role of epoxy-impregnation of the open-mesh textile. Eight FRCM specimens are subjected to a 250-minute-long thermal preconditioning up to $$300\,^{\circ }\text {C}$$ 300 ∘ C , and their mechanical behavior is assessed and compared with eight specimens in the control group. It is found that, unexpectedly, the greatest performance loss is associated with the control, as opposed to the epoxy-coated, group.

In the last few years, externally bonded fabric-reinforced cementitious matrix (FRCM) composites have been increasingly employed as externally bonded (EB) reinforcement of existing concrete and masonry structures. Failure of EB FRCM reinforcement is generally caused by composite debonding at the matrix-fiber interface. The stress transfer mechanism at the joint interface is analytically described within the fracture mechanics framework assuming a pure Mode-II loading condition and a zero-thickness interface, which allows for formulating the bond differential equation. Its solution requires the knowledge of the cohesive material law (CML) of the specific interface studied, often obtained by calibration of direct shear (DS) test results. In this paper, a rigid-trilinear CML is used to analytically describe the bond behavior of four control and four thermally preconditioned carbon FRCM-masonry joints subjected to single-lap direct shear test. Thermal preconditioning consists of 250-min-long exposure up to 300 ℃. Stress responses obtained from the DS tests are used to calibrate the CML of the matrix-fiber interface, which is then used to solve the bond differential equation. Comparison between the analytical and experimental stress responses of control and conditioned specimens sheds light on the effect of temperature exposure on the bond behavior of carbon FRCM-masonry joints.

In the presented research, short microfibres are integrated into 3D Textile Reinforced Cements (3D TRCs). 3D textiles provide significant advantages compared to 2D textiles, such as more straightforward manufacturing and improved anchorage, leading to enhanced flexural properties and cracking behaviour. The short fibres provide a further improvement of the mechanical performance, along with advanced crack control and narrower crack widths. The influence of the 3D textile’s architecture on the manufacturing of microfibre-enhanced TRCs should, however, be investigated as the presence of the 3D textile’s spacer yarns could significantly affect the fibre-mortar mixture’s penetration through the textile. In this paper, the influence of the spacer yarns on the manufacturing and mechanical performance of microfibre-enhanced 3D TRCs is investigated, comparing equivalent 2D and 3D TRCs with integrated short fibres, as well as different casting directions for microfibre-enhanced 3D TRCs and their influence on the material’s flexural behaviour. 6 mm length polypropylene (PP) fibres were added to the fresh cement matrix with a volume fraction of 1.0 v%. Although the different casting directions did not lead to significant differences in flexural mechanical response and crack formation, casting in the spacer yarn direction did generate more repeatable behaviour. Furthermore, the comparison between microfibre-enhanced 2D and 3D TRCs showed little difference in the macroscopic mechanical response. A higher energy absorption was, however, observed for the microfibre-enhanced 3D TRCs.

Material-efficient building requires construction designs that do justice to the special properties of the construction materials. This applies in particular to the innovative material of textile reinforced concrete, which requires an interplay between the requirements of the material and the construction design and vice versa. Furthermore, advanced concrete production methods, such as additive manufacturing or concrete folding, require novel, adapted textile reinforcements. Within this paper, the authors present the potential of two approaches for the production of such textile reinforcements: tessellated textiles and branched 3D textiles. Tessellated textiles are produced from flat textiles by realizing a periodic deformation orthogonal to the textile plane, similar to corrugated cardboard. Branched 3D textiles are produced using two industrial robots working in close concert to produce a biomimetic textile reinforcement that can support itself immediately after manufacturing. The study discusses the respective production mechanisms as well as potential reinforcement geometries and component designs. Initial analysis suggests that the use of tessellated or 3D branched textile reinforcements makes it possible to realize a load path compatible and material-minimized reinforcement geometries. Tessellated structures promise the realization of large-area load-bearing elements, while 3D branched textiles are suitable for branched elements with high vertical expansion. What both approaches have in common is that if the processes are successfully implemented on an industrial scale, they enable the production of reinforced concrete elements that are impossible to produce using current methods.

Concrete as one of the most widely used materials has a significant impact on the emission of CO2 and resource consumption. To reduce these negative aspects, concrete structures need to become more efficient and lightweight. Textile-reinforced concrete (TRC) as a novel material promotes new possibilities in the design of filigree load-bearing structures. In turn, such a shift demands new methods in the manufacturing of concrete elements to ensure an appropriate level of quality compared to common methods. As one possibility it was proposed to rethink a vacuum-assisted die-casting method, which is widely used in other industries such as automotive, for the production of thin-walled TRC components. In this regard, the question arose, if the specific manufacturing conditions result in different material properties of the high-performance concrete matrix. Thus, current research focused on detecting differences and similarities in the material properties of concrete and TRC samples cast under standard conditions and under reduced air pressure conditions or how it is referenced further in the text negative air pressure condition (APC).On the meso scale, no noteworthy differences were determined. To dive deeper into understanding how the altered environmental conditions impact the high-performance concrete matrix, a series of experiments were conducted to scrutinize the microstructure of samples that were extracted from large-scale shell elements cast under different grades of air pressure using a computed tomography (CT) and X-ray diffraction (XRD) method. The results are presented in the paper.

Textile reinforced concrete (TRC) has emerged as valuable and high-performance construction material. However, modern polymer-based textiles often encounter issues due to poor compatibility with concrete and inadequate temperature resistance. Mineral-impregnated carbon-fiber (MCF) reinforcement represent an innovative class that integrates high-performance fibers with robust inorganic matrix materials. This approach offers numerous advantages, including superior temperature resistance, cost-effectiveness, compatibility with cementitious substrates and technological flexibility in production and application. Before entering key markets, the challenge related to the lengthy hydration time required for cementitious materials to attain sufficient strength hinders economic production and flexible applications. To address this concerns and pave the way for commercialization, the present work focuses on the development of a quick and cost-effective curing methodology for cement-based MCF. Engineering with automated inline processing, low temperature activation was exploited in the curing process to expedite functional robustness within the initial few hours. Mechanical evaluation showcased the advancement in the strength evolution of as-produced MCFs with extended curing at 40 ℃ and 60 ℃. By adjusting curing parameters, exceptional early-age strengths can be achieved for MCFs, making them promising candidates for conventional polymer-impregnated composites.

Fibre-reinforced concrete is a better option for reinforcement than conventional steel-reinforced concrete in many areas, such as tunnel construction, flooring, or bridge building in earthquake zones. This preference is due to the fact that steel, typically used in bars or mats, is associated with large crack widths and is vulnerable to corrosion. Therefore, this research focuses on the use of fibers made from corrosion-resistant materials like carbon. We present a continuous carbon fiber reinforcement, the individual filaments of which are bound together by a cement-based, mineral impregnation. This heat-resistant, high-performance impregnation remains flexible in its freshly impregnated state and offers a strong bond with concrete when hardened. This flexibility makes mineral-impregnated carbon fibers (MCF) particularly suitable for continuous reinforcement in concrete extrusion processes, especially in the context of 3D concrete printing (3DCP). MCF can be integrated into large concrete cross-sections deposited with a portal printer, as well as into finer concrete filaments deposited with an industrial robot arm, or in a stationary extruder where concrete is continuously molded horizontally into elements. The type and method of integration vary significantly depending on the respective extrusion process. This paper presents an overview of these methods, along with their respective advantages and disadvantages. Subsequently, the effects of the individual manufacturing processes on the mechanical performance of the composite and the crack development under tensile load are discussed. In combination with mineral-impregnated carbon fibers, these new manufacturing techniques offer enormous potential for specialised, individual elements.

The attractiveness of strain-hardening cement-based composites (SHCC) in civil engineering applications results from their pseudo-ductile response and multiple fine cracks under tensile loading. Previous research has demonstrated that SHCC show high potential as a repair material for concrete structures. They also hold promise to overcome the current limitations of traditional application techniques, paving the way for advanced automated systems. In this contribution, the effectiveness of a novel SHCC formulation for retrofitting concrete elements is investigated. This formulation is based on limestone calcined clay cement (LC3) and polyethylene fibres. The focus is on the influence of different technological parameters, such as the volume fraction of fibres and the application technique, on the mechanical performance of SHCC. Two different SHCC strengthening systems are developed and applied to the tensile side of concrete beams, either with or without a biaxial carbon textile reinforcement. In total, 18 beams are prepared and subjected to three-point bending (3PB) tests. The influence of the repair systems on the flexural behavior of the retrofitted concrete beams is demonstrated and discussed. The test results reveal that the strengthening layers under investigation significantly improve the flexural performance of concrete beams.

Investigating the behavior of structures under dynamic loads like impact is crucial since they can induce severe damage to structures and people’s lives. This paper reviews studies on enhancing the behavior of axially-loaded reinforced concrete members under impact loading including various strengthening methods like fiber-reinforced polymers (FRP) and fiber-reinforced composites (FRC). Additionally, it discusses other materials like strain-hardening cementitious composites (SHCC) and textile-reinforced concrete (TRC) as potential strengthening methods for axially loaded members against impact loading.

Due to their pronounced brittleness, existing RC structures need impact strengthening. Therefore, it was targeted to design a strengthening structure for the impact-facing side consisting of a cover layer distributing the impact load and a damping layer that aims to absorb impact energy and reduce the contact force between the impactor and the structure. The investigated cover layer materials were short-fiber concrete and distinct textile-reinforced concretes, which benefit from the high strength and ductility achieved by adding fibers and textiles to the concrete matrix and the internal friction between the fibers or textiles and the matrix, respectively. The examined damping layer materials were elastic concrete with waste tire rubber aggregates and lightweight concretes of various strengths and stiffnesses, which allow for densification upon the impact. The different material combinations were explored as strengthening layers against impact loading. Drop tower experiments with a solid steel impactor of $${21.66}\,{\text {kg}}$$ 21.66 kg , inducing a hard impact, were conducted to validate the performance of the strengthening layers. The deployed specimens were fully supported RC cuboids. The experiments proved the outstanding performance of a strengthening layer consisting of an infra-lightweight concrete damping layer and a short-fiber reinforced concrete cover layer, preventing damage until an impactor velocity of $${54}\,{\text {m}/\text {s}}$$ 54 m / s .

Throughout their service life, concrete structural elements may be exposed to a wide range of loading events, from low strain rates associated resulting dead and live loads to extreme dynamic loads like blast explosions and earthquakes. Most cement-based components lack the necessary strength, toughness, and ductility to maintain their integrity when subjected to impact and other dynamic loads. Consequently, concrete structures are prone to significant damage from high-velocity impact loads. The present study aims at evaluating the enhancement to the impact performance of concrete panels under high-velocity projectile impact loads associated with the use of textile fibres as a reinforcing material. Control and TRC panels with dimensions 280 mm × 280 mm × 25 mm reinforced with steel and carbon textiles were prepared and tested under high-velocity impact loading. All panels were subjected to high-velocity impact loads from a hemispherical steel projectile with weight of 68 g fired from a compressed air gun, with impact velocities ranging from 60 m/s to 90 m/s. The high-velocity impact loading test was simulated numerically and compared with the corresponding experimental results. The numerical findings generally align well with the test results. In comparison with the control panels, both the numerical and experimental outcomes from the tests indicate that the steel textile significantly improves the panels’ impact resistance by reducing crack propagation, local damage, and penetration depth by 80%, 60%, and 8% at impact velocities of 60, 80, and 90 m/s respectively.

When reinforced concrete slabs are subjected to impact loads such as rockfall impact or accidental load, collapse or perforation can occur. In order to protect people’s lives, reinforced concrete structures can be strengthened with textile-reinforced concrete (TRC). To develop an engineering model to describe the behavior of reinforced concrete slabs strengthened with textile-reinforced concrete, quasi-static and impact loads were applied to reinforced concrete plates which were strengthened with a thin layer of fine concrete with two embedded carbon textiles. An engineering model for these strengthened reinforced concrete plates under impact loading is described in this paper. The model is based on a multi-degree-of-freedom system with springs and viscous dampers. To evaluate the accuracy of the spring curves of the model as well as the calculated deformations, experimental results of the mentioned loading regimes are presented. While the comparison between the calculation and the experiment already shows good results, further improvements for plates with extensive damage are sought.