Ultra-high-performance Fiber-reinforced Concrete Specimens Research Articles

Transport Characteristics and Corrosion Behavior of Ultra-High Performance Fiber-Reinforced Concrete with the Key Mix Parameters

The presence of low-quality coarse aggregates and exposure to aggressive conditions are the two major problems with the durability of concrete. Therefore, an alternative concrete with enhanced properties to prevent fluid and ionic mobility compared to conventional concrete is needed. This study investigated the effects of main mix parameters on the transport characteristics and corrosion behavior of ultra-high performance fiber-reinforced concrete (UHPFRC). A set of 27 UHPFRC mixtures with different combinations of w/b ratio, cement, and silica fume contents, based on a 33-factorial experiment design, were prepared and tested for water permeability, chloride penetrability, electrical resistivity, chloride profile, and corrosion current density. The results showed that UHPFRC mixtures exhibited excellent durability properties characterized by negligible water penetration (< 15 mm), negligible and very low chloride permeability when the w/b ratio was 0.15 (< 100 Coulombs) and up to 0.2 (< 300 Coulombs), respectively, and very low chloride concentrations at the rebar level (0.03–0.18 wt.%). All resistivity values were within the range of 26.7–78.8 kΩ cm (> 20 kΩ cm) and pH values were 12.41–13.01, indicating the implausible likelihood of corrosion in the UHPFRC mixtures. This was confirmed through the corrosion current density measurements of reinforced UHPFRC specimens after 450 days of chloride exposure, which were below the critical limit for the corrosion initiation of reinforcing steel. Finally, the experimental data were statistically analyzed and fitted for all the listed tests, and models were developed for them using the regression analysis such that regression coefficients were within 0.90–0.99.

Corrosion-induced deterioration and fracture mechanisms in ultra-high-performance fiber-reinforced concretet

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is an excellent material for harsh environments, but corrosion will change its internal microstructure and complicate the fracture evolution, bringing great difficulties in evaluating the long-term service life. Limited attention has been paid to the fracture mechanism of the UHPFRC upon corrosion. In the present study, integrating acoustic emission (AE) and digital image correlation (DIC) techniques are used to assess the micro/macrocracking characteristics of the specimens upon various corrosion degrees. Results show that the 56-day corroded UHPFRC with 2 vol% presents a remarkable decrease rate of 32%, 29% and 30% in the flexural stiffness, flexural strength and compressive strength. During the loading process, compaction of the original defects induced by fiber corrosion is concentrated in the elastic stage, the newborn cracks triggered by loading mainly occur in the strain-hardening stage, and the expansion of cracks mainly lies in the strain-softening stage. Corroded UHPFRC specimens with higher corrosion damage have a greater maximum strain value at the crack. In addition, the failure mode changes from shear crack failure to a brittle failure of tensile crack as corrosion damage increases. The macroscopic destruction of the corroded UHPFRC is a manifestation of internal microdamage evolution in fiber corrosion and matrix deterioration.

Tensile behavior of UHPFRC made by a fiber orientation device with adjustable vibrational frequency

Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) not only has high compressive and tensile strength, but also exhibits strain-hardening characteristics in tension due to the addition of steel fibers. In this study, a new-style fiber orientation device equipped with a vibration motor of adjustable vibrational frequency was used to prepare UHPFRC. The wall effect and fiber length were considered in the manufacturing process of the device. Uniaxial tensile tests were conducted on fiber-oriented UHPFRC to explore the effects of vibrational frequency and corresponding vibrational force on the tensile behaviors. The test results showed that the elastic ultimate tensile strength, elastic ultimate tensile strain, tensile strength, peak tensile strain, and tensile elastic modulus of UHPFRC specimens cast by fiber orientation process at various vibrational frequencies were different. The fiber orientation parameters were quantitatively evaluated by the image analysis technique to explain the differences in tensile properties of the specimens. The effect of different vibrational frequencies on the tensile strength of UHPFRC matrix and its porosity was also considered. Therefore, this study provides a reference for how to adjust the vibrational frequency of the vibrating platform under the directional channel when pouring relevant components.

Quantitative analysis of steel fibers on UHPFRC uniaxial tensile behavior using X-CT and UTT

Steel fiber is usually used to improve ultra-high-performance fiber-reinforced concrete (UHPFRC) tensile properties and remedy its brittle failure. X-ray CT scanning (X-CT) was used to observe the orientation and distribution of steel fibers inside UHPFRC. It revealed that the steel fiber inside UHPFRC distribution angle is not equal probability, and the probability is higher along the pouring direction in uniaxial tensile specimen mold. The uniaxial tensile tests (UTT) of 19 UHPFRC specimens with different fiber volume fractions and aspect ratios were performed to analyze the steel fiber on the tensile response and tension failure patterns of UHPFRC. Test results indicated that with an increase in the volume fraction or aspect ratio of steel fiber, the first-cracking strength, tensile strength and peak tensile strain of UHPFRC increase; while they have different growth rates. In addition, with the increase of volume fraction and aspect ratio, the failure patterns of UHPFRC tension specimens gradually changed from brittle failure with single crack to ductile failure with multiple cracks. The stress superposition compositions of tensile strength were different under different failure patterns. Combining mechanical analysis and numerical statistical analysis, prediction equations of the tensile strength of strain softening UHPFRC and pseudo-strain hardening UHPFRC considering the effect steel fiber parameters were respectively proposed. Through the mesoscopic equivalent model analysis of UHPFRC uniaxial tensile specimens, the functional relationships between the effect of steel fiber parameters on the first-cracking strength and peak tensile strain were analyzed, and then their prediction equations were proposed, combined with numerical statistical analysis. The calculation accuracy of the series of prediction equations proposed in this paper were verified by the collected literature data. Meanwhile, comparing the prediction equations proposed in this paper with the existing equations in the literature, the equations proposed in this paper were better.

Investigation of long-term tension stiffening mechanism for ultra-high-performance fiber reinforced concrete (UHPFRC)

Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) is promptly emerging concrete that has ultra-high compressive strength, and its ductile properties are ensured in the addition of steel fibers. Steel fibers in UHPFRC feature strain-hardening characteristics and exert the damage pattern as intimately spaced narrow cracks. The tensile performances of reinforced concrete elements are closely interrelated with the bonding interactions between the reinforcing rebars and concrete. Since the UHPFRC exhibits different tensile behavior and cracking behavior from traditional strength concrete, it is crucial to analyze the tension stiffening mechanism and cracking propagation of UHPFRC elements in long-term serviceability limit. This study presents the time-dependent tensile behavior of reinforced UHPFRC, especially on the instantaneous and sustained tension stiffening mechanism and post-cracking behavior for the long term. A newly designed testing rig was adopted to introduce the novel testing method. The testing rig was designed in a way to overcome the limitations in applying the sustained tensile loads found in the critical literature review. This novel testing method includes the design of the testing frame for intensive sustained tensile loads application, specimen preparations, and installations. To investigate the long-term tension stiffening mechanism, different types of sustained tensile loads, i.e., 20 kN, 40 kN, 60 kN, and 80 kN, were considered. The applied sustained tensile loads were ascertained from different percentages of cracking load (40 kN) of reinforced UHPFRC derived from the 28-day instantaneous tension stiffening test. Crack width and crack spacing of reinforced UHPFRC specimens were determined for both instantaneous and sustained tensile loads. Notably, crack propagation was considerably affected by the long-term sustained tensile loads compared to instantaneous tensile loads. The autogenous and drying shrinkage strains were 392 micro-strains (µɛ), and 170 µɛ, respectively, at 180-day, where the total shrinkage strain of 562 µɛ significantly influenced the time-dependent tension stiffening behaviors of UHPFRC. A maximum total average concrete strain of 3500 µɛ was found for the sustained tensile load of 80 kN at 180-day. In addition, the material properties of UHPFRC, such as compressive strength, splitting tensile strength, and modulus of elasticity tests, were also conducted, and the maximum compressive strength of 160 MPa was reported at 120-day.

Cracking analysis in Ultra-High-Performance Fiber-Reinforced Concrete with embedded nanoparticles via a diffuse interface approach

Several recent experimental investigations have clearly demonstrated that the embedding of a little quantity of nanoparticles within the concrete mixture can lead to a notable increase in strength and toughness properties, especially in the case of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC), for which nanoparticles are adopted in a synergistic combination with randomly distributed short-fiber reinforcements. Common fracture and damage models often are not able to capture the complex failure behavior of such structures when subjected to general loadings, which usually promote multiple crack propagation under mixed-mode conditions. In the present work, a crack propagation analysis in nano-enhanced UHPFRC structures is performed, by using a refined diffuse interface approach, according to which a standard finite element mesh is enriched with cohesive interface elements placed along all the internal boundaries and representing potential crack segments. A trilinear traction-separation law has been proposed and properly calibrated to incorporate the toughening effects provided by the embedded nanoparticles. The reliability of the proposed fracture model is assessed by performing complete failure simulations of UHPFRC specimens containing different fractions of nanoparticles (i.e., graphite nanoplatelets) and by comparing the related numerical outcomes with the available experimental data. The present results have shown the numerical accuracy of the proposed approach, also highlighting the role of the embedded nanoparticles in the overall load-bearing capacity as well as in the crack width control for UHPFRC structures.

Self-healing efficiency of Ultra High-Performance Fiber-Reinforced Concrete through permeability to chlorides

This study presents a novel methodology to evaluate the self-healing capability of Ultra High-Performance Fiber-Reinforced Concrete (UHPFRC) designed to compare conventional concrete types. The procedure used combines loading reinforced concrete elements until a fixed strain level to have a comparable total crack opening. Afterwards, water penetration to chlorides is used as an indicator of permeability. This work compares autogenous healing efficiency of a conventional concrete, a high-performance concrete, and two types of UHPFRCs with and without 0.8% of a crystalline admixture (CA) by the binder weight. The results show that all UHPFRC specimens exhibited excellent autogenous healing, higher than conventional concretes for an equivalent total crack. The self-healing results depended greatly on the crack size and the fiber content. Additionally, UHPFRCs with CA obtained the lowest water permeability after promoting self-healing for one month in water immersion and presented almost complete healing against chloride penetration.

Mechanical and durability properties of ultra-high performance steel FRC made with discarded materials

This study investigates the mechanical and durability properties of 12 ultra-high performance fiber reinforced concrete (UHPFRC) mixes manufactured using alternate aggregate sources. Natural gabbro aggregates (GA), steel slag aggregates (SSA), and recycled concrete aggregates (RCA) were employed as coarse aggregates. The volume of the coarse aggregates was limited to 12.5% of the total UHPC volume. Three types of steel fibers were employed: macro hooked-end fibers, micro straight fibers, and a hybrid combination of these two fibers. The fiber volume fractions tested were 0% and 1%. Properties investigated were compressive and flexural tensile strengths, electrical resistivity, porosity, and rapid chloride permeability. Test results revealed that the effect of aggregate type was more pronounced on the flexural strength than compressive strength. UHPFRC with steel micro-fibers or a hybrid combination of both steel macro and micro-fibers had a more noticeable effect on the compressive strength than that with steel macro-fibers, irrespective of aggregate type. Moreover, UHPFRC specimens with steel macro-fibers showed the highest flexural strength, regardless of the aggregate type. The SSA improved all durability properties of UHPFRC, whereas RCA noticeably deteriorated the resistivity and chloride permeability. The incorporation of steel micro-fibers improved the porosity and chloride permeability of all UHPFRC mixes.

Flow distance induced variation analysis of digitally segmented steel fibers in UHPFRC

Ultra-high performance fiber-reinforced concrete (UHPFRC) is a construction material with outstanding mechanical properties and durability. The degrees of dispersion and orientation of included steel fibers are known to have a decisive effect on UHPFRC’s material properties. In this study, UHPFRC specimens with different volumes of fibers were prepared to investigate the variation of spatial information of steel fibers in UHPFRC. Each sample was scanned individually through micro-computed tomography (micro-CT) to analyze the three-dimensional fiber statistics. To enhance the accuracy of the analysis, bundled fibers were digitally segmented through reliability-based maximum likelihood estimation (MLE) and a watershed algorithm. It was quantitatively found that the number of contact areas of bundled fibers decreased and fibers tended to be aligned more horizontally as flow distance increased. However, at the casting position and the opposite region, there was irregular distribution, which has the potential to affect the designed material properties of UHPFRC.

Mechanics-based formulation for modeling of ultra high-performance fiber reinforced concrete material using bonded discrete elements

Ultra High-Performance Fiber Reinforced Concrete (UHPFRC) possesses high strength and ductility relative to traditional concrete. As a relatively novel material, utilization of UHPFRC generally requires comprehensive studies on feasibility and applicability, where numerical modeling techniques become more convenient and cost-effective as compared to laboratory or field testing. In this study, a numerical model based on the bonded discrete element method is developed to simulate mechanical behaviors of UHPFRC materials. The formulation consists of three major components, which correspond to physical counterparts of fine aggregates, the cementitious matrix, and embedded steel fibers. A mechanics-based derivation is presented that facilitates calibration of the associated constitutive parameters. As validation of the numerical modeling and demonstration of the calibration procedure, results from laboratory experiments conducted on UHPFRC specimens are reproduced via simulation. In addition, parametric studies targeting the effect of particle size and packing are carried out to assess framework robustness.

Influence of steel fiber on compressive properties of ultra-high performance fiber-reinforced concrete

The compressive properties (include compressive strength and modulus of elasticity) of Ultra-high performance fiber-reinforced concrete (UHPFRC) are the most important performance index in structural design. This paper presents experimental results from tests conducted on 36 UHPFRCs with different volume fractions and aspect ratios of steel fiber to investigate the effect of steel fiber on the compressive properties of UHPFRC. The test results indicated that the compressive strength and modulus of elasticity of the hardened UHPFRC increase as the fiber volume fraction or aspect ratio increases. However, the increase trend of compressive strength and modulus of elasticity slowed down when the volume fraction exceeded 2%. It was observed that the steel fiber can restrain the occurrence and development of cracks when UHPFRC specimens are compressed, provided a positive effect for reinforcing UHPFRC, but it also reduced the flowability of fresh UHPFRC, which is negative for reinforcing effect. X-ray CT scanning revealed that the porosity and pore size of hardened UHPFRC increased with the increase of the fiber volume fraction due to its weakened flowability. A prediction model was established based on the analysis of the positive and negative effects of the steel fiber. Semi-empirical prediction formulas for the compressive strength and modulus of elasticity were proposed by regression analysis of the test data in this paper, which were verified and revised by the experimental database of 155 tests from literature. In addition, a relationship formula between modulus of elasticity and compressive strength of UHPFRC was presented, and was verified and revised by the experimental database of 320 tests conducted around the world.

Direct procedure to characterize the tensile constitutive behavior of strain-softening and strain-hardening UHPFRC

There is a need to establish a complete process to characterize Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) in both strain-hardening and strain-softening tensile behavior. This process should be simple and easy to apply so that its application is direct. Therefore, this paper presents the development of a complete process to obtain the tensile constitutive parameters of UHPFRC. A simplified inverse analysis based on four-point bending tests (4PBT) to derive the tensile material properties of strain-hardening UHPFRC was adapted to be applied in the event of strain softening using a nonlinear finite element model (NLFEM). To fulfill this objective, an extensive experimental program was run with 227 UHPFRC specimens tested in 4PBT that exhibited strain-softening and strain-hardening tensile responses. As a reference, the characteristic UHPFRC tensile constitutive behavior was obtained. Finally, a predictive application capable of predicting tensile behavior using the experimental 4PBT curve as input was developed with the experimental database.

Failure Analysis of Ultra High-Performance Fiber-Reinforced Concrete Structures Enhanced with Nanomaterials by Using a Diffuse Cohesive Interface Approach.

Recent progresses in nanotechnology have clearly shown that the incorporation of nanomaterials within concrete elements leads to a sensible increase in strength and toughness, especially if used in combination with randomly distributed short fiber reinforcements, as for ultra high-performance fiber-reinforced concrete (UHPFRC). Current damage models often are not able to accurately predict the development of diffuse micro/macro-crack patterns which are typical for such concrete structures. In this work, a diffuse cohesive interface approach is proposed to predict the structural response of UHPFRC structures enhanced with embedded nanomaterials. According to this approach, all the internal mesh boundaries are regarded as potential crack segments, modeled as cohesive interfaces equipped with a mixed-mode traction-separation law suitably calibrated to account for the toughening effect of nano-reinforcements. The proposed fracture model has been firstly validated by comparing the failure simulation results of UHPFRC specimens containing different fractions of graphite nanoplatelets with the available experimental data. Subsequently, such a model, combined with an embedded truss model to simulate the concrete/steel rebars interaction, has been used for predicting the load-carrying capacity of steel bar-reinforced UHPFRC elements enhanced with nanoplatelets. The numerical outcomes have shown the reliability of the proposed model, also highlighting the role of the nano-reinforcement in the crack width control.

Influence of production parameters on the fiber geometry and the mechanical behavior of ultra high performance fiber‐reinforced concrete

AbstractIn this paper, the relationship between production parameters of ultra high performance fiber‐reinforced concrete (UHPFRC) and the spatial distribution and orientation of the steel fibers is investigated. UHPFRC specimens with varying fiber diameter, fiber volume fraction, and rheology of the mixture are produced. Additionally, casting is performed from the side or the middle of the formwork. Imaging by micro computed tomography allows for a statistical analysis of the spatial arrangement of the fibers in the test specimens. The flexural behavior and the load capacity of the specimens are analyzed by four point bending tests. The results of the bending tests are well explained by characteristics of the fiber systems determined from the image data.

Flexural and compressive creep behavior of UHPFRC specimens

The long term behavior of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is analyzed in this study. The experimental campaign covered creep in compression and creep in flexure in cracked state. Three types of specimens were cast: cylindrical specimens (Ø100 × 200 mm) for compressive creep and shrinkage, and prismatic specimens type regular “R” (150 × 150 × 600 mm) and type slim “S” (150 × 40 × 600 mm) for flexural creep in cracked state. Specimens R were notched up to 50 mm in depth to weak the central section and then pre-cracked until 0.65 mm of Crack Mouth Opening Displacement (CMOD). Specimens S were pre-cracked unnotched until a loss of 50% of stiffness was observed. Flexural creep tests were performed during 270 days under load, and until 360 days the compressive tests. Measurements from three experimental sources were obtained: CMOD, compressive strains on top of prismatic specimens and longitudinal compressive strains in cylindrical specimens. Creep coefficients and parameters related with deferred deformations velocity were obtained from all three sources. Creep coefficients under flexure at 270 days ranged from 0.62 to 1.20 in the tensile zone, and from 0.72 to 0.90 in the compressive zone. Creep coefficient in compression at one year was 1.07, which is consistent with values found in the literature. Deferred deformations velocities at early ages were greater in specimens R than in specimens S, and a secondary creep stage was achieved in all specimens after 210 days of sustained loading.

Direct tensile behaviors of steel-bar reinforced ultra-high performance fiber reinforced concrete: Effects of steel fibers and steel rebars

Based on strengthening the structural performance of ultra-high performance fiber reinforced concrete (UHPFRC), a steel-bar reinforced ultra-high performance fiber-reinforced concrete (R-UHPFRC) is presented in this study. To provide more information about the direct tensile behaviors of R-UHPFRC member, several dogbone-shaped R-UHPFRC specimens are tested, with the experimental variables including the volume content of steel fiber and the reinforcement ratio of steel rebar. The effects of the fiber volume fraction and reinforcement ratio of steel bars on the direct tensile properties of R-UHPFRC are evaluated using multiple performance parameters, including load-deformation relationship, tension-stiffening behavior, tensile stiffness and damage pattern. Test results indicate that there is minimal difference in overall load-elongation responses of R-UHPFRC specimens with an increasing fiber volume fraction. The variation in the reinforcement ratio of the steel bar, by contrast, has significant influence on the tensile responses of R-UHPFRC specimen, especially the peak strength and hardening modulus increases considerably with a growing reinforcement ratio as compared to non-reinforced UHPFRC specimens. Importantly, the tensile strain values of R-UHPFRC with rebars are significantly higher than the counterparts of UHPFRC without rebars, and this rising trend is even more pronounced when the reinforcement ratio increases. Further, the use of R-UHPFRC with combined reinforcements is beneficial to extend the strain hardening capacity of UHPFRC itself. Even, a desired post-peak ductility can be achieved by a rationally designed reinforcement ratio of steel bars. The increased reinforcement ratio of steel rebars transfers the tensile failure pattern of the R-UHPFRC members from a single, localized crack into multiple localized cracks.

Research on dynamic constitutive model of ultra-high performance fiber-reinforced concrete

A dynamic constitutive model for ultra-high performance fiber-reinforced concrete (UHPFRC) was investigated in this paper. In view of the particularity of UHPFRC material, two improvements including the modifications of yield surface and the change of strain rate effect due to the adjunction of steel fibers were made based on the Holmquist-Johnson-Cook model (HJC model). Further, on the purpose of model parameter determination, a series of mechanical tests for UHPFRC material, including uniaxial compression test, bending test, uniaxial cyclic compression test and SHPB test were carried out. And the 22 calculation parameters required in the improved HJC model such as strength parameters, damage parameters and pressure parameters were obtained through data analysis and fitting. Finally, the numerical simulation of the SHPB test performed in this study and the drop hammer impact test in the literature were carried out in order to verify the rationality and practicability of the improved HJC model. The numerical simulation results of SHPB test show that the failure morphology and stress-strain curves of UHPFRC specimens are in good agreement with the test results. When the improved HJC model and the appropriate corrosion strain value (1.0% in this work) were selected, the mid-span deflection-time curve obtained by numerical simulation is consistent with the result of the drop hammer impact test.

Flexural Response of UHPFRC Beams Reinforced with Steel Rebars

Abstract This article compares the flexural responses of ultrahigh-performance fiber-reinforced concrete (UHPFRC) specimens and their normal-strength concrete (NSC) counterparts through an experimental study. Four UHPFRC specimens reinforced with 2 % steel fiber (by total volume of concrete) with a length of 13 mm and a diameter of 0.2 mm were used with longitudinal steel rebars at different reinforcement ratios to determine their flexural responses. For comparative purposes, three NSC beams were also tested. Results were compared with relevant equations in the literature. Moreover, the fracture energy of the specimens was compared to provide a better understanding of the ductility in the two types of beams. Results showed better performance of UHPFRCs in terms of peak load, fracture energy, and moment capacity as compared with their NSC counterparts. Failure in UHPFRC specimens with high reinforcement ratios was dominated by shear-flexure patterns, while flexure patterns were dominant in specimens with low reinforcement ratios. Failure of NSC specimens, on the other hand, was characterized by shear, regardless of reinforcement ratio. Additionally, available equations for moment capacity of ultrahigh-performance concretes (UHPCs) appear too conservative, especially for higher reinforcement ratios. Lastly, statistical models were proposed to predict the load–displacement curves of UHPFRC dogbone and beam specimens, which fitted well with experimental results with correlations over 0.90.

Implication of calcium sulfoaluminate-based expansive agent on tensile behavior of ultra-high-performance fiber-reinforced concrete

This study aims to investigate the effects of a calcium sulfoaluminate (CSA) based expansive agent (EA) on the tensile behaviors of ultra-high-performance concrete (UHPC) reinforced with straight and twisted steel fibers. For this, five different CSA EA amounts ranging from 0% to 8% were considered. Because the major purpose of adding a CSA EA into a cementitious matrix is to reduce the amount of shrinkage, its implication on the free-shrinkage response of UHPC was also evaluated. The test results indicate that approximately 7% and 10% smaller shrinkage strains were obtained in UHPC matrices with 6% and 8% CSA EAs than that of plain UHPC, respectively. Most of the tensile parameters, including the tensile strength, strain capacity, and g-value, of UHPCs with straight and twisted steel fibers were deteriorated when including a CSA EA and increasing its amount, except for the tensile strength of straight fiber specimens. Thus, the CSA EA needs to be carefully applied for UHPC mixture, despite its positive effect on reducing the amount of shrinkage strains by greater than 6%. There was no clear trend regarding the effects of the amount of CSA EA on the number of fibers located near the localized crack of ultra-high-performance fiber-reinforced concrete (UHPFRC), and the unit number of fibers in UHPFRC specimens with 2% (by volume) of 0.3-mm diameter steel fibers ranged from 25 to 35 ea/cm2. The tensile strengths of UHPC reinforced with straight and twisted steel fibers were both improved when increasing the number of fibers, although the former showed a much higher rate of increase and degree of correlation than the latter. The strain capacity and g-value of UHPC with straight and twisted fibers showed only marginal correlations with the number of fibers regardless of the amount of CSA EA applied.

Cyclic behavior of ultra‐high performance fiber reinforced concrete beam‐column joint

Ultra‐high performance fiber reinforced concrete (UHPFRC) is a unique class of fiber‐reinforced concrete featuring ultra‐high compressive strength and ductile tensile strain hardening behavior, accompanied by multiple narrow cracking. Although many studies have confirmed its superior mechanical and damage tolerance properties under monotonic or blast loading, limited research has been carried out on the cyclic performance of UHPFRC structural members. This paper proposes the use of UHPFRC to improve the cyclic performance of structural elements. An experimental program was carried out on a large number of UHPFRC beam‐column joint specimens under a cyclic lateral load. After the cyclic loading test, the following results were obtained: (a) hysterical performance, (b) maximum load, (c) maximum displacement, (d) the maximum energy dissipation (measured by hysteresis cycles), (e) stiffness degradation, (f) the cracking pattern, and (g) the cracking area. The analysis showed that UHPFRC specimens have at least 157% higher energy dissipation than nonfiber conventional reinforced concrete (RC). The initial stiffness of UHPFRC specimens without fibers (NF) was at least 23% higher than the RC specimen. For UHPFRC with fiber, initial stiffness was at least 45% higher than RC specimen.