Fiber Reinforced Concrete Structures Research Articles

Исследование свойств двухслойных бетонно-фибробетонных конструкций при различных соотношениях толщины слоев

Introduction. The article presents the results of a study of the strength properties of concrete, fiber-reinforced concrete and combined structures (two-layer concrete-fiber-reinforced concrete structures, where one layer is made of concrete without reinforcement, the second is made of fiber-reinforced concrete). Materials and Methods. Two types of concrete mixture were prepared, differing only in that polypropylene fiber was added to one (dosage 5 kg/m3), and to the other not. From these mixtures 4 series of samples were made, three pieces in each one with dimensions of 100×100×400 mm of various compositions: 1st series is completely made of a mixture with fiber; 2nd series is completely without fiber; 3rd series is made of two layers: the first half (50 mm in height of the sample) consists of a mixture with fiber, and the second half without it; 4th series is one third (33 mm in height of the sample) from a mixture with fiber, and two thirds part without fiber. Results. After the standard hardening period all specimens were tested for tensile flexural strength (three-point bending). On the basis of these tests, conclusions were drawn about the possibility of using fiber reinforcement of structures not for the entire volume, but only within the stretched zone. Discussion. Based on these tests, conclusions were drawn about the possibility of using fiber reinforcement structures not for the entire volume, but only within the stretched zone. Conclusion. The results of laboratory studies have shown that samples with fibroblast have residual strength, while samples without fibroblast have a sharp decrease in strength after reaching a peak value, and the processes of changing the limit of proportionality (strength) when testing combined samples of concrete and fibroblast structures are described. Resume. The results can be used in the design of fiber-reinforced concrete structures experiencing tensile loads, as well as in the development of research in this area. The principal possibility of using combined (two-layer) structures in the design of sprayed concrete supports with a complex geometry of the production contour is proved. The further development of this research topic is relevant when considering multilayer concrete and fiber-reinforced concrete structures. For practical purposes, these results can be useful, especially when designing sprayed concrete supports of mine workings.

Design of Fibre-Reinforced Concrete Structures

Different types of concrete are invented in the past years, in order to improve it`s behaviour for different loading and environmental situations. Among them, fibre reinforced concrete (FRC) is one of the most recently developed concrete types. Design procedures for the FRC elements are included in only a few existing code provisions. Among them, fib Model Code 2010 (MC 2010) provides the biggest amount of necessary information and recommendations in order to design FRC elements. Yet, a lack of the guidelines for the design of elements loaded with different combinations of bending moment and axial force is noticed in the existing code provisions. Therefore, in the scope of this paper, interaction curves for FRC are developed. The interaction curves are developed for both, ultimate limit state (ULS) and serviceability limit state (SLS). Furthermore, example of the use of such interaction curves is given. Comparison in the design of reservoir made of reinforced concrete (RC) and FRC is performed. It is shown that the use of FRC especial benefits design according to SLS, as FRC provides better behaviour regarding crack spacing and crack width of element, while developed interaction curves for SLS significantly decrease the time necessary for such a design.

Bond Behavior of Recycled Tire Steel-Fiber-Reinforced Concrete and Basalt-Fiber-Reinforced Polymer Rebar after Prolonged Seawater Exposure

The integration of basalt-fiber-reinforced polymer (BFRP) rebars into concrete design standards still remains unrealized due to limited knowledge on the performance of the rebars in concrete, particularly in terms of bond durability in harsh conditions. In this work, we investigated the bond durability characteristics of BFRP rebars in fiber-reinforced self-compacting concrete (FRSCC) structures. To this aim, a number of 24 FRSCC pullout specimens reinforced with either BFRP rebar or glass-fiber-reinforced polymer, GFRP, rebar, which is a commonly used type of FRP, were fabricated. Half of these specimens were submerged in simulated seawater for a two-year span, while the other 12 similar specimens were maintained in standard laboratory conditions for comparative purposes. Subsequently, all 24 specimens underwent monotonic and fatigue pull-out tests. The exploration in this study focused on investigating the influence of the environmental condition, reinforcement type, and loading type on the bond stress versus slip relationship, maximum bond stress, and failure mode of the specimens. Based on the results obtained and by adopting the durability approach of industry standards for prediction of the bond retention of FRP-reinforced concrete, the bond strength retention between BFRP/GFRP and FRSCC after 50 years of exposure to seawater was estimated. The outcomes of the study are expected to enhance engineers’ confidence in the use of FRP, especially BFRP, for constructing durable and sustainable reinforced concrete structures in aggressive environments.

The Efficacy of Electromechanical Impedance Monitoring of Embedded Piezoelectric Ceramic Lead Zirconium Titanate Sensors in Different Orientations for Hybrid Fiber-Reinforced Concrete Structures Hydration

Abstract In this study, the influence of the orientation of embedded piezoelectric ceramic lead zirconium titanate (PZT) on the mechanical performance of hybrid fiber-reinforced (polypropylene and glass fiber) concrete beams was evaluated. The performance of concrete was evaluated under self-weight, followed by assessing the mechanical property using the electromechanical impedance (EMI) technique after optimization of M30 grade concrete with polypropylene fiber and glass fiber. PZT patches are embedded at different orientations, i.e., 0 deg, 45 deg, and 90 deg, with the axis of the structure for monitoring the hydration of the RC beam. The change in stiffness due to heat hydration in the concrete structure after 5, 7, 14, 21, and 28 days was observed by curing hybrid concrete beams and examining them after 5, 7, 14, 21, and 28 days. On the fifth day, beams were simply supported and allowed to deflect under their weight, and measurements of heat hydration in terms of conductance at frequencies ranging between 1 and 1000 kHz were done. Similarly, days 7, 14, 21, and 28 were done. Day 5 was considered the baseline. It is noted that the PZT sensor placed at an angle of 45 deg is the least effective in recording the incremental changes in hydration that occurred in the concrete beam. The highest quality results were obtained at 90 deg, which is further demonstrated by statistically quantifying the changes using the root-mean-square deviation (RMSD) percentage method and proves to be the most optimized orientation to obtain the stiffness of the hybrid reinforced beam in terms of conductance.

An integrated topology optimization method including manufacturing constraints for 3D printed fiber-reinforced concrete structures

The difficulty of achieving overall optimization design of concrete materials, structures, and performances has always been a bottleneck restricting the safety and comprehensive performance improvement of civil engineering structures throughout their life cycle. Driven by digital design, 3D concrete printing (3DCP) with a high degree of freedom and flexibility can achieve accurate distribution of materials and flexible construction, which is a favorable guarantee for the high-quality construction of high-performance structures. In this paper, an integrated design method of material-process-performance is developed by introducing both concrete material properties and printing process parameters into the optimization design. The nodal volume fraction and material orientation are selected as design variables to obtain lightweight structures, meanwhile the tensile capacity enhanced through fiber alignment is fully utilized. The minimum size control combined with the central axis algorithm is introduced to achieve integer multiple of nozzle size and also to improve the manufacturing quality. Structural compliance and stress are adopted as the objective function and constraint to improve structural stiffness and strength, respectively. Test results show that the strength-to-weight ratio of the optimized beam is 42 % higher than that of the cast-in-place one, and therefore the effectiveness of the proposed method is validated.

Multi-Parameter Methodology for Assessing Quality Indicators of Nanomodified Fiber-Reinforced Concrete for Construction Site

Nanomodified fiber-reinforced concrete is a building material for which the required characteristics of fracture toughness are a distinctive feature. Determination of the stress intensity factor of fiber-reinforced concrete makes it possible to correctly assess the resistance of the material during the formation and development of cracks. The proposed multi-parameter methodology for assessing the quality indicators of nanomodified fiber-reinforced concrete makes it possible to evaluate the quality of a fiber-reinforced concrete structure in construction and laboratory conditions. To carry out control at the construction site, modern and long-used methods of non-destructive testing are used: ultrasonic sounding, ultrasonic tomography, elastic rebound, separation with chipping. For laboratory studies, the technique provides for the manufacture of prism samples that can be molded or cut from the body of the structure. This methodology makes it possible to obtain in laboratory conditions such material parameters as tensile strength in bending, tensile strength in splitting, critical stress intensity factor for normal separation, critical stress intensity factor for transverse shear, energy consumption for individual stages of deformation and destruction of the sample, as well as to evaluate the uniformity of distribution fibers. Moreover, it is provided to obtain all the parameters on one sample from the series, which eliminates errors and inaccuracies in the quality indicators of the material associated with different conditions of hardening, molding, inaccuracies in duplicating the composition.

Uniaxial constitutive model for fiber reinforced concrete: A physics-based data-driven framework

Fiber reinforced concrete (FRC) has improved strength and ductility, making it suitable for a wider range of engineering structures. The development of a constitutive model is crucial for analyzing mechanical behavior of these structures, while this still remains challenging as the complex composition of FRC makes it difficult to formulate explicit relationships among a range of critical material parameters. With the latest development of data and digital technologies, data-driven approaches have emerged as a powerful alternative that are capable of solving advanced and complex engineering problems. Developing data-driven methods based on objective data analysis and decision-making to predict mechanical behavior of engineering structures is an important direction of such development. In this paper, a uniaxial constitutive model of fiber-reinforced concrete is developed by a physics-based data-driven framework. The framework consists of three important parts, including construction of experimental database, parameters calibration of physical models, and implementation of neural network. From the experimental data collected from published literature, an experimental database is built first. A physical model is proposed by modifying the uniaxial constitute model for normal concrete, so that it is more convenient and straightforward to consider the effect of fibers on fiber reinforced concrete subjected to compression, tension and repeat loading conditions. The parameters of the model are calibrated against experimental data by the swarm intelligence optimization algorithms. With the calibrated parameters of the physical model, a Fully Connected Neural Network (FCNN) is trained to be used to predict the physical parameters of fiber reinforced concrete. By comparing with independent experimental data, the proposed fiber-reinforced concrete uniaxial constitutive model constructed using the physics-based data-driven framework can accurately predict the stress–strain relationships of a range of FRC, which suggests that the FRC material model can be used in the numerical simulation and design of FRC components and structures. In addition, the proposed model is applicable to multiple loading conditions.

Size Effect of Synthetic Fibre Reinforced Concrete – Investigation using a Semi-Discrete Analytical Beam Model

Abstract The size effect is a well-known characteristic of concrete structures. However, in the case of fibre-reinforced concrete (FRC), this issue is not thoroughly explored. Most design recommendations of FRC neglect the size effect or handle the behaviour of FRC structures in case of different structural sizes similar to plain concrete structures (assuming FRC is a homogeneous material). The aim of this paper is to show that the size effect of FRC can be divided, the share of the concrete matrix and the fibres in the size-dependent properties is separable. For the size effect research fifteen synthetic macro fibre reinforced concrete and six plain concrete beam specimens were prepared and tested in three different sizes and then evaluated with the semi-discrete analytical (SDA) model. The analysis of the experimental specimens has shown that the size effect significantly influences the concrete material in the case of FRC with softening material behaviour, but the residual loadbearing capacity which mainly arise from the local bridging effect of fibres is essentially independent of the structural size. It is also shown in this paper that the two defining parameters of the SDA model is independent of the structural size, so the model provides an excellent tool in case of the design of real-sized FRC structures.

Electro-Mechanical Impedance-Based Structural Health Monitoring of Fiber-Reinforced Concrete Specimens under Four-Point Repeated Loading

Fiber Reinforced Concrete (FRC) has shown significant promise in enhancing the safety and reliability of civil infrastructures. Structural Health Monitoring (SHM) has recently become essential due to the increasing demand for the safety and sustainability of civil infrastructures. Thus, SHM provides critical benefits for future research to develop more advanced monitoring systems that effectively detect and diagnose the damage in FRC structures. This study investigates the potential of an Electro-Mechanical Impedance (EMI) based SHM system for detecting cracks in FRC prisms subjected to four-point repeated loading. For the needs of this research, an experimental investigation of three FRC specimens with the dimensions 150 × 150 × 450 (mm) were subjected to three different loading levels where no visual cracks formed on their surface. Next, prisms were subjected to reloading until they depleted their load-carrying capacity, resulting in pure bending fracture at the mid-span. A network of nine cement paste coated Piezoelectric lead Zirconate Titanate (PZT) transducers have been epoxy bonded to the surface of the FRC prisms, and their frequency signal measurements were utilized for quantitative damage assessment. The observed changes in the frequency response of each PZT sensor are evaluated as solid indications of potential damage presence, and the increasing trend connotes the severity of the damage. The well-known conventional static metric of the Root Mean Square Deviation (RMSD) was successfully used to quantify and evaluate the cracking in FRC specimens while improving the efficiency and accuracy of damage detection. Similarly, the dynamic metric of a new statistical index called “moving Root Mean Square Deviation” (mRMSD) was satisfactorily used and compared to achieve and enhance accuracy in the damage evaluation process.

Microcrack monitoring and fracture evolution of polyolefin and steel fibre concrete beams using integrated acoustic emission and digital image correlation techniques

The use of polymer and steel fibres in plain concrete appears to be an excellent solution for limiting crack propagation and improving the post-ductility performance of concrete structures. Based on this premise, this study investigated the fracture evolution of polyolefin fibre-reinforced concrete (PFRC) and steel fibre-reinforced concrete (SFRC) specimens through the integrated application of two diagnostic techniques, acoustic emission (AE) and digital image correlation (DIC), under three-point bending tests. Based on the processing of AE signals, different AE statistical parameters such as the cumulative number of hits, amplitude distribution, and some representative analysis methods including the b-value method, Ib-value method, and AE intensity analysis methods were selected to analyse the early detection of cracking and post-cracking behaviour in PFRC vs SFRC specimens during mechanical degradation. Simultaneously, the DIC technique was used to validate the fracture evolution of the AE results. Furthermore, to verify the reliability of the AE and DIC results, the damage localisation and fracture evolution of the PFRC versus SFRC specimens were confirmed by integrating the AE fracture energies and DIC outcomes. The tests and analysed results showed that the addition of steel fibres to plain concrete significantly improved the ability to restrict crack propagation and provided higher post-cracking resistance compared to PFRC specimens owing to their stronger fibre–matrix bonding, effective fibre bridging, and crack-arresting mechanism. The present study indicates that the combined AE and DIC techniques are highly effective for the early detection of damage and ductility performance in fibre-reinforced concrete structures.

Two-dimensional integrated mixed-mode smeared crack model for simulating FRC structures

This paper describes the implementation and assessment of the predictive performance of a two dimensional mixed-mode fracture smeared crack model (MMFSCM) in a finite element method (FEM) based software. The referred constitutive model is based on the integration of the aggregate interlock and fibre pullout resisting mechanisms to simulate the behaviour of Fibre Reinforced Concrete (FRC) in both fracture mode I and II. The models used to simulate the contribution of each resisting mechanism predict the normal and shear stresses at the crack level based on the crack opening and sliding displacements. For this purpose, two aggregate interlock models and three fibre pullout models are used. The appraisal of this approach is conducted by simulating two numerical case studies consisting of a three-point notched beam bending test (3PNBBT) for assessing the structural behaviour in mode I, and shear panel tests to appraise the predictive performance of the model in mode II. In addition, modifications to the original formulation of the fibre pullout models are proposed, and a multi-linear branch model is also developed and implemented in order to facilitate the use of inverse analysis tools for defining the mode I behaviour. The numerical simulations have predicted with good accuracy the experimental responses, both in mode I and mode II, capturing not only the peak loads but also the evolution trend of the post-peak responses of the analysed scenarios. The modifications proposed to the fibre pullout models enhanced the overall predictive performance of the MMFSCM. The multi-linear branch model exhibited excellent performance, capturing the experimental response with very good accuracy in all 3PNBBT simulations.

Eco-Friendly Design and Sustainability Assessments of Fibre-Reinforced High-Strength Concrete Structures Automated by Data-Driven Machine Learning Models

In recent years, adding fibres into brittle concrete to improve ductility has gained momentum in the construction industry. Despite the significant momentum, limitations do exist in design and industrial applications, contributing to the complexity of shear behaviours in fibre-reinforced concrete and the existing empirical models that can hardly provide a reasonable prediction, especially for high-strength concrete applications. A critical review reveals that current research mostly focuses on single eigenvalue analysis and pay less attention to the different synergetic effect of fibres on high-strength concrete and normal-strength concrete. This study aims to fill the research gap by the unprecedented use of reliable models for the prediction and evaluation of structural and sustainable properties of high-strength fibre-reinforced concrete beams. To this end, this study establishes three novel deep learning (ANN, BNN, and Xgboost) models for designing and optimising the shear capacity of ‘high-strength’ fibre-reinforced concrete beams towards the circular economy. In addition to introducing a new type of novel machine learning (BNN) model, which is capable of structural design and takes into account complex design features, our study also enhances sustainability by reducing greenhouse gas (GHG) emissions. The novel prediction models unprecedentedly elicit flexural capacity, structural stiffness, carbon emission, and price, together with the shear strength for high-strength fibre-reinforced structures. Firstly, this study focuses on multiple parameters for forecasting high-strength fibre-reinforced concrete beams. In addition, the models provide more comprehensive insights into the design and manufacture of high-strength steel fibre-reinforced concrete structures in a more environmentally friendly manner. With the help of the proposed models, it will be more cost-benefit and time-efficient for the researchers to obtain the optimum design with the consideration of both structural and sustainable performance. The established models exhibit excellent prediction accuracy, and the Bayesian neural network (BNN) is found to have the best performance: R2 is 0.937, MSE is 0.06 and MAE is 0.175 in shear strength prediction; R2 = 0.968, MSE is 0.040, and MAE is 0.110 in flexural capacity prediction; R2 is 0.907, MSE is 0.070, and MAE is 0.204 in shear stiffness prediction; R2 is 0.974, MSE is 0.022, and MAE is 0.063 in carbon emission prediction; and R2 is 0.977, MSE is 0.020, and MAE is 0.082 in price prediction.

Design of Steel Fibre Reinforced Concrete Structures According to the Annex L of the Eurocode-2 2023

The Annex L of the new Eurocode – 2 gathers -for the first time- provisions for the design of Steel Fibre Reinforced Concrete (SFRC) structures. This response to the necessity of the construction sector for provisions and regulations of this composite material within the European space and this Annex is supported by the solid background derived from consistent research and experiences carried out along the last two decades. The existence of these design provisions could both favour competitiveness and enhance sustainability performance within the construction sector since new concrete reinforcement alternatives could be considered while ensuring the structural reliability level accepted for traditional reinforced concrete structures. This paper is aimed to cover -not exhaustively- all those features concerning SFRC included into the EC-2 and complement those -when considered necessary- with scientific literature.

New amendment to the Code of Practice on the design of fiber-reinforced concrete structures with non-steel fibers

Introduction. The acting version of the Code of Practice on the design of fiber-reinforced concrete structures with non-steel fibers SP 297.1325800.2017 “Fiber-reinforced concrete structures and precast products with non-steel fibers. Design rules” contain no instructions for the strength calculation of circular cross-sections with and without reinforcement under the action of bending moments and longitudinal forces. In addition, the current version provides no instructions for determining the subclass index of fiber-reinforced concrete with non-steel fibers using the residual axial tensile strength, as well as those for calculating the fire resistance of fiber-reinforced concrete structures. The absence of these provisions leads to significant difficulties in the design of individual structures made of fiber-reinforced concrete, which increases material and time expenditures.Aim. To improve the system of urban planning activities in terms of clarifying and supplementing the current regulatory documents for the design of fiber-reinforced concrete structures with non-steel fibers.Methods and materials. A draft of amendment No. 2 to SP 297.1325800.2017 was developed taking into account contemporary requirements, as well as the latest scientific research. In particular, the results of the research work concerning the fire resistance of structures made of fiber-reinforced concrete with non-steel fibers, performed by the Research Institute of Concrete and Reinforced Concrete (NIIZHB) named after A.A. Gvozdev in 2020, were taken into account. In addition, the experience of the authors and design organizations, gained during the practical application of this Code of Practice, was analyzed in order to clarify its provisions for the convenience of users.Results. The results of the work involve the draft of the amendment No. 2 to SP 297.1325800.2017, whose positions include new calculation techniques, as well as the refinements of existing calculation and construction methods. The developed amendment has passed public discussions, as well as the necessary expertise, and is being prepared for approval and enactment.Conclusion. Positions of amendment No. 2 to SP 297.1325800.2017 “Fiber-reinforced concrete structures and precast products with non-steel fibers. Design rules” can be used to develop reliable design solutions for these structures, including the achievement of a certain economic effect.

A numerical failure analysis of nano-filled ultra-high-performance fiber-reinforced concrete structures via a moving mesh approach

This work proposes a novel FE-based model for simulating crack propagation mechanisms in Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) structures (typically exhibiting compressive strength over 150 MPa and tensile strength around 10 MPa) enhanced with nanofillers under general loading conditions. This model combines synergistically a Moving Mesh (MM) technique with the Interaction Integral (M-integral) method, and the R-curve concept. In particular, the MM technique, based on the Arbitrary Lagrangian-Eulerian formulation, is used for tracking geometry changes of the computational domain caused by crack advance, thus avoiding massive re-meshing operations. In particular, a mesh regularization technique based on proper rezoning equations is used to ensure that the mesh points move consistently. The M-integral method is used for extracting the crack front variables (i.e., the Stress Intensity Factors (SIFs)), required for the computation of the mixed-mode crack growth direction. Finally, the well-known R-curve concept allows the ductile behavior of nano-enhanced UHPFRC beams to be captured in a LEFM setting. In particular, the crack growth resistance, regarded as a function of the crack extension, is numerically derived via a calibration procedure starting from the available experimental data. In addition, to assess the reliability and effectiveness of the proposed approach, the results of numerical simulations are compared with experimental outcomes reported in the literature.

Lessons learnt from blind competition of shear behavior of fiber‐reinforced concrete T‐beam

AbstractThe positive effect of fiber‐reinforced concrete (FRC) on shear capacity of reinforced concrete beams is well presented in many experimental studies since 1970s. However, as the main effect of fibers is in the cracking stage, it increases the complexity to analyze such structures numerically. With the increased availability of nonlinear finite element analysis software, structural designers are encouraged to use them to solve complex structural problems. In such cases, understanding and correct definition of material laws in cracked stage of FRC structures are crucial. A blind competition was organized by fib WG 2.4.2 to address this issue. It revealed that the numerical predictions of almost all of the participants overestimated the shear capacity of a T‐beam subjected to bending. The aim of the current study is to demonstrate critical aspects in the definition of input parameters of FRC analysis. A comprehensive parametric analysis is performed considering tensile softening law, shear stiffness, and shear strength definition in cracked stage as well as shrinkage effects. The results suggest that most important aspect is the correct definition of tensile softening law, which should correspond to the lower bound of residual strength obtained by standard notched prisms tests according to EN 14651 or even smaller.

Experimental Study on Impact Behavior of Concrete Panel with and without Polypropylene Macrofibers

Macrofibers have often been used to increase the tensile strength, durability, crack resistance, spalling, impact resistance, and toughness of concrete. However, the impact behavior of fiber-reinforced concrete (FRC) structures is quite different from their static behavior, and the effectiveness of macrofibers in improving impact resistance should be carefully evaluated. In this study, the impact behavior of FRC with polypropylene (PP) macrofibers was studied through a series of drop-weight impact tests. First, the material characteristics of the FRC with PP fibers under static conditions were evaluated. Test specimens were constructed and drop-weight impact tests were performed. The main parameters were the presence or absence of PP fibers and the drop height, which is related to the magnitude of the impact energy. From the results, it can be found that the crack width of the FRC specimen was smaller than the normal concrete specimen for a similar residual deflection after the impact test due to the bridging effect of the macrofibers. However, the effect of PP fibers on the impact resistance was not significant, even though there was a considerable increase in tensile and flexural performance under static conditions, since the hardening effect after the sharp reduction in strength shown in the static test of FRC is not effective in the impact test.

Tensile behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) structure with cold joints

Ultra-high-performance fiber-reinforced concrete (UHPFRC) structures comprise connection joints, which are typically the weakest part. Appropriate joint construction can improve the force-transfer performance of UHPFRC structures. In this study, the effects of the interfacial roughening method, steel fiber content and length, interfacial reinforcement ratio, and embedment length on the tensile behavior of UHPFRC cold joints are investigated via direct tension tests. The test results indicate that the roughening methods using high-pressure water jet and uneven plastic formwork significantly improve the bond performance of UHPFRC cold joints. Moreover, the steel fibers in the cold-joint interface enhance the tensile strength and toughness of these joints. The typical failure mode of the unreinforced UHPFRC cold joint is found to be brittle failure. By contrast, the cracking and failure surfaces shift from the cold joints to the UHPFRC matrix when the UHPFRC cold joint is well-reinforced. Furthermore, an increase in the interfacial reinforcement ratio and embedment length improves the ultimate tensile strength of the reinforced UHPFRC cold-joint specimens, whereas the minimum embedment length decreases with the interfacial reinforcement ratio (should be at least 40 mm). In addition, an equation for the minimum embedment length of reinforcing steel rebars is proposed. The proposed method can be applied to determine the effects of the parameters, tensile strengths of the UHPFRC matrix and cold joint interface, reinforcement ratio, and rebar diameter on the tensile behavior of UHPFRC cold joints.

Microstructure, Shrinkage, and Mechanical Properties of Concrete with Fibers and Experiments of Reinforced Concrete Beams without Shear Reinforcement.

The current findings on concrete with fibers show that research has focused primarily on individual aspects, especially in terms of mechanical properties and structural uses. However, no broader view of the problems solved has been provided. In this study, we present a conceptual overview of a new, comprehensive experimental program for the assessment of fiber-reinforced concrete, which includes the analysis of microstructural and structural elements, as well as specific features such as shrinkage and resistance to pressurized water. The proposed experimental program included several variants of schemes for the dosing of fibers into concrete, using steel fibers that were short and straight. Fiber dosing was performed up to 110 kg/m3. The basic tests performed included tests of the compressive strength of concrete, and of the split and flexural tensile strength for different dosing amounts. Within the structural tests of reinforced concrete beams without shear reinforcement, two variants of spans with different degrees of reinforcement were implemented. Herein, the test results are evaluated graphically with a detailed analysis of the positive effect of fibers, and we also provide general recommendations for the structural uses of the fibers used and the design of fiber-reinforced concrete structures. Among the important results of this experimental program was the observation of a significant increase (of the order of tens) of the percentage of the split tensile strength and an increase of the overall load-bearing capacity of the reinforced concrete beams without shear reinforcement. Among the important aspects of our findings is the fact that a fine-grained concrete mixture was used, which increased resistance to pressure water seepage, and therefore, the effect of shrinkage can be influenced by the method of production and the treatment of the concrete used. We also provide detailed figures of the microstructure.

ДЕФОРМИРОВАНИЕ ФИБРОБЕТОНА ПРИ ОДНОКРАТНОМ ДИНАМИЧЕСКОМ ВОЗДЕЙСТВИИ С УЧЕТОМ ВЛИЯНИЯ НАЧАЛЬНЫХ НАПРЯЖЕНИЙ ОТ СТАТИЧЕСКОЙ НАГРУЗКИ

An analysis of the experimental studies presented in the scientific literature indicates that the strength and deformability of fiber-reinforced concrete under dynamic additional loading may depend on the initial level of stresses caused by a preliminarily applied static load. In this regard, the purpose of this study was to build improved models of fiber-reinforced concrete deformation of operated fiber-reinforced concrete structures of bearing systems of buildings and structures during their dynamic additional loading, taking into account the nonlinearly elastic law of material deformation at the stage of loading with a static load, as well as changes in the parameters of viscous resistance during loading. As a model of the force resistance of fiber-reinforced concrete of loaded fiber-reinforced concrete structures during their dynamic additional loading caused by an emergency situation, the Kelvin-Voigt model for an elastic-viscous body was adopted. On the basis of the adopted model, an analytical solution of the differential equation of uniaxial compression of fiber-reinforced concrete was obtained under its single dynamic additional loading from an arbitrary level of initial stresses acting in it from a previously applied static load. A step-iterative approach to determining the parameters of state diagrams of fiber-reinforced concrete under static-dynamic loading conditions is proposed. An analysis of the obtained dependences between stresses and strains shows that an increase in the level of initial stresses acting in a compressed concrete element at the moment a dynamic load is applied to it leads to a decrease in the dynamic strength of concrete with the same impact parameters.