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Abstract The adoption of ultra-high-performance fibre-reinforced concrete (UHPFRC) in modern-day construction has gradually increased because of its high compressive and flexural strength at the early production ages. The UHPFRC's high initial cost discourages its production and widespread use. This cost can be reduced by incorporating waste materials in UHPFRC production. This study aims to design, model, and optimise a three-day heat-cured UHPFRC incorporating waste materials. To produce a sustainable UHPFRC mixture, this study combines rice husk ash (RHA) and recycled tire steel fibre (RTSF) as the primary waste components, with Portland limestone cement, river sand, superplasticizer, and water. The concrete is heat-cured in a hot water bath at 90 °C for 3 days, resulting in rapid strength development. A D-optimal mixture design technique was used to optimise the mix proportions, and mathematical models were developed to predict the compressive and flexural strengths. The non-significant lack-of-fit results and the high values of coefficient of determination (R2) revealed the accuracy of the models in predicting the compressive and flexural strengths of the UHPFRC. The low values of standard deviation (SD) and coefficient of variation (CV) confirmed the consistency and reliability of the prediction models. A numerical optimisation revealed that it is possible to design a UHPFRC mixture with a lower cement content of 38.29% of the UHPFRC mix, resulting in a compressive strength of 117.62 MPa and a flexural strength of 36.32 MPa. The study reveals that integrating RHA and RTSF into UHPFRC can yield high-performance properties, offering innovative solutions for resource scarcity and environmental sustainability in the construction sector.

Abstract This study was conducted to evaluate the seismic performance of an exterior precast concrete (PC) beam–column joint with ultra-high-performance fiber-reinforced concrete (UHPFRC). Currently, 45 MPa non-shrinkage mortar is used as grouting for the connection between PC beams and columns. In this study, PC joint specimens were designed using 45 MPa non-shrinkage mortar and 120 MPa UHPFRC as a grouting agent for connecting PC members. The shear reinforcement effect of UHPFRC was confirmed to reduce shear cracks in the joint core; this trend was similar in the specimens with reduced shear rebars. The maximum moment of the test specimen with the corbel was slightly increased, but there was no significant difference, and the failure pattern also showed similar results to the specimen without the corbel. In the test specimen to which the U-shaped beam was applied, the attachment surface of ultra-high-performance concrete and normal concrete were separated, and a large decrease in strength was observed. Considering workability, U-shaped beam do not seem to have any major merits in general, such as increased strength and difficulty in manufacturing, and it was judged that it was effective to separate the PC beams from the column face through corbels. Shear reinforcement through UHPFRC is effective in relieving congestion by reducing shear reinforcement bars at the joint, and it is judged that it can be used as PC joint grouting due to its excellent fluidity.

This study was conducted to evaluate the seismic performance of an exterior precast concrete (PC) beam–column joint with ultra-high-performance fiber-reinforced concrete (UHPFRC). Currently, 45 MPa non-shrinkage mortar is used as grouting for the connection between PC beams and columns. In this study, PC joint specimens were designed using 45 MPa non-shrinkage mortar and 120 MPa UHPFRC as a grouting agent for connecting PC members. The shear reinforcement effect of UHPFRC was confirmed to reduce shear cracks in the joint core; this trend was similar in the specimens with reduced shear rebars. The maximum moment of the test specimen with the corbel was slightly increased, but there was no significant difference, and the failure pattern also showed similar results to the specimen without the corbel. In the test specimen to which the U-shaped beam was applied, the attachment surface of ultra-high-performance concrete and normal concrete were separated, and a large decrease in strength was observed. Considering workability, U-shaped beam do not seem to have any major merits in general, such as increased strength and difficulty in manufacturing, and it was judged that it was effective to separate the PC beams from the column face through corbels. Shear reinforcement through UHPFRC is effective in relieving congestion by reducing shear reinforcement bars at the joint, and it is judged that it can be used as PC joint grouting due to its excellent fluidity. [ABSTRACT FROM AUTHOR]

Curvature-based damage detection has been previously applied to identify damage in concrete structures, but little attention has been given to the capacity of this method to identify distributed damage in multiple damage zones. This study aims to apply for the first time an enhanced existing method based on modal curvature analysis combined with wavelet transform curvature (WTC) to identify zones and highlight the damage zones of a beam made of ultra-high-performance fiber-reinforced concrete (UHPFRC), a construction material that is emerging worldwide for its outstanding performance and durability. First, three beams with a 2 m span of UHPFRC material were cast, and damaged zones were created by sawing. A reference beam without cracks was also cast. The free vibration responses were measured by 12 accelerometers and calculated by operational modal analysis. Moreover, for the sake of comparison, a finite element model (FEM) was also applied to two identical beams to generate numerical acceleration without noise. Second, the modal curvature was calculated for different modes for both experimental and FEM-simulated acceleration after applying cubic spline interpolation. Finally, two damage identification methods were considered: (i) the damage index (DI), based on averaging the quadratic difference of the local curvature with respect to the reference beam, and (ii) the WTC method, applied to the quadratic difference of the local curvature with respect the reference beam. The results indicate that the developed coupled modal curvature WTC method can better identify the damaged zones of UHPFRC beams. [ABSTRACT FROM AUTHOR]

This study evaluated the effects of volume fraction, aspect ratio, and shape of steel fibers on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) and the structural behavior of reinforced (R-) UHPFRC beams. The tensile strength and energy absorption capacity of ultra-high-performance concrete (UHPC) are improved by adding steel fibers and increasing its volume contents by up to 3.0 %. Compared with short straight steel fiber, medium-length straight and twisted fibers at a volume fraction of 2.0 % result in twice higher energy absorption capacity and higher flexural strength of R–UHPFRC beams. The flexural strength of R–UHPC beams increases by increasing the fiber content up to 3.0 %. However, the strain-hardening characteristics of UHPFRC negatively influence the cracking behavior and stress redistribution in structural beams, causing 48.2–54.1 % lower ultimate ductility indices. The small amounts of steel fibers with volume fraction of ≤1.0 % that exhibit strain-softening behavior only improve the peak ductility.

In this study, an orthotropic steel bridge deck overlaid with ultra-high-performance fiber-reinforced concrete (UHPFRC) is investigated using the finite element analysis. The composite bridge deck which is undergone moving-wheel load is examined under environmental surface water conditions. Two phases, i.e., Phases 1 and 2, are considered for the material model of the UHPFRC with stagnant water. In Phase 1, mechanical recoveries of the tensile strength and reloading stiffness are considered for the cracked UHPFRC caused by the autogenous self-healing behavior. In Phase 2, under the moving-wheel load, the crack bridging stress degradation in reinforced overlayer accelerates due to the closing–opening actions of surface cracks in water. In both phases, the deformation behaviors of the steel deck plate and UHPFRC overlayer are numerically examined. The results of the current numerical model agree with the experimental data in terms of the strain tendency, wherein the strain range of the steel deck plate and UHPFRC overlayer decreases in Phase 1 and progressively increases in Phase 2. Therefore, it can be asserted that, under the surface water condition, scenarios considering two phases of the material model of cracked UHPFRC, have governed the strain behaviors of the tested composite bridge deck. [ABSTRACT FROM AUTHOR]

UHPFRC Precast Segments for Sustainable and Resource‐Efficient Concrete Bridge Construction Many of the concrete bridges currently in use in Germany are in a deficient condition due to their age or design. However, increased traffic loads also highly contribute to the fact that many of the existing bridges will have to be replaced in the medium term. Due to the many advantages of concrete structures, it can be assumed that concrete bridges will continue to be designed and built in the future. In the light of the increasing consequences of climate change and the pressing need to reduce CO2 emissions also in the building industry as well, there is an urgent need for research into climate‐ and resource‐friendly as well as sustainable but also economical concrete construction methods. A promising approach to fast, effective and resource‐optimized as well as CO2‐efficient construction is the use of high‐performance materials such as UHPFRC in combination with precast segmental construction. In this paper, starting from a monolithic box girder bridge made of normal strength concrete, numerical calculations are used to investigate how much material can be saved in segmented box girder bridges by varying the concrete compressive strength (normal and high‐strength concrete as well as UHPFRC). The life cycle assessment subsequently carried out on this basis for life cycle phases A1 to A3 showed that, when the material is fully utilized, the use of UHPFRC leads to very resource‐efficient and sustainable structures compared with normal‐ and high‐strength concretes. [ABSTRACT FROM AUTHOR]

Summary: Concrete‐filled steel tubes (CFSTs) have received growing attention, owing to their rapid construction, reduced labor requirement, and reasonable material cost. While in service, the CFSTs can be subjected to unexpected impact loads, originating from vehicle and vessel collision, as well as water‐ and wind‐borne debris impact. Such extreme loading events often cause a partial or complete failure of conventional CFSTs, risking the safety and performance of the entire structural systems that rely on them. To address this issue, the current study explores how two advanced composite materials, including ultra‐high‐performance fiber‐reinforced concrete (UHPFRC) and carbon fiber‐reinforced polymer (CFRP), can be utilized to provide superior mechanical properties and minimize the vulnerability of CFSTs to impact loads. The composite materials under consideration are appropriate for both new and existing structures, in which normal‐strength concrete can be replaced with UHPFRC, while CFRP sheets can further strengthen the CFSTs. For obtaining in‐depth insights, a computational framework validated with the experimental tests was developed in the current study. Using a set of representative impact scenarios, various response measures, such as internal forces and deflections, as well as the energy absorbed by the CFSTs, were recorded during impact simulations. The investigations were then further extended to capture the influence of the main design parameters related to concrete, CFRP, and steel tube. From the conducted investigations, an energy absorption index was introduced, as a measure to evaluate the performance of CFSTs under impact loads. [ABSTRACT FROM AUTHOR]

The development of sustainable construction methods can be achieved by improving the performance of reinforced concrete elements, resulting in an increase in structural life expectancy. This paper presents a study of the structural performance of shear-deficient ultrahigh-performance concrete (UHPC) concrete beams to produce sustainable construction materials. In the first phase of the experimental campaign, performance-based optimizations were implemented for UHPC. The characteristic compressive strength of all mixes was kept at 130 ± 10 MPa. The elastic modulus of plain UHPC was obtained at 8 GPa, and for the fiber-reinforced one was 40 GPa. Additionally, 18 sets of reinforced UHPC beams were investigated for their structural behavior based on the overall depth, reinforcement ratio (ρ), and the shear-span-to-depth ratio (λ) as key variables. Here, λ was varied between 1 and 2 and ρ was varied between 0.56% and 3.15%. The experimental study determined the lowest shear strength as 4.56 MPa, and the highest shear strength was calculated as 11.34 MPa. The database of the current shear strength results and similar literature results were used to develop models for predicting shear capacity. This research focused on applying a statistical approach using neuro-fuzzy logic, the robust analytical model. The ratio of the experimentally calculated shear strength and the predicted shear strength for different values of λ and ρ was obtained between 0.75 and 1.25, which was in good agreement with the results of similar literature. The results of this study suggest that high-strength fiber may extend structural lifetimes in UHPC applications. [ABSTRACT FROM AUTHOR]

This paper presents the results of tests on flat slabs with partial application of ultra-high-performance fiber-reinforced concrete (UHPFRC) mixtures to improve the punching shear capacity of slab-column connections. One normal strength concrete (NSC) slab and two UHPFRC slabs were tested as a reference for tests on 12 UHPFRC-NSC composite slabs with full depth UHPFRC in the center area of the NSC slab. The main test parameters were the steel fiber volume (0.8 % and 1.6 % volume fractions) and the dimensions of UHPFRC mixtures. The load-column displacement curves, crack patterns, slab rotations at failure, and failure loads are presented and discussed. The failure loads are compared with estimates obtained following the yield line analysis, design codes, and other theoretical models. Test results showed that the use of UHPFRC mixtures led to an increase in initial stiffness, punching shear strength, and deformation capacity, along with a change in the slab failure mode. The shear capacity of the slab improved proportionally to the increase in the dimension of the UHPFRC. These tests confirmed the effectiveness of the UHPFRC mixtures as an alternative to enhance the punching shear capacity for slab-column connections. ● Tested the punching shear capacity of 15 UHPFRC-NSC composite two-way slabs. ● Evaluated the effects of the fiber volume fraction and UHPFRC dimension on the punching shear capacity. ● Observed punching shear failure for slabs made from entirely NSC and UHPFRC. ● Partial use of UHPFRC can effectively improve the punching shear capacity and change the failure mode. ● Assessed the punching shear strength by using theoretical models and design codes for tested slabs. [ABSTRACT FROM AUTHOR]

Ultra-high-performance fiber-reinforced concrete (UHPFRC), wherein steel fibers are a primary component, is a new cementitious material with high tensile strength and impact resistance. However, steel fibers are susceptible to corrosion in the alkaline environment of concrete matrices. By contrast, alkali-resistant glass fiber (ARGF) exhibits better corrosion resistance. However, few studies have explored the effects of ARGF on UHPFRC, leaving the optimum ARGF content and its enhancement mechanism unclear. Therefore, this study proposes a UHPFRC design that utilizes AR-GF in place of steel fibers. The effects of different types, lengths, and admixtures of AR-GF are investigated using mechanical tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that the splitting tensile strength and flexural strength of the UHPFRC increase with fiber length and fiber dosage. The optimum fiber mixing ratio is 30 kg/m3 of 12 mm-long Anti-Crak® 62.4 combined with 0.05 kg/m3 of 6 mm long Anti-Crak® HD, leading to a 23.3 % increase in splitting tensile strength and 15.8 % increase in flexural strength compared to those of undoped concrete. By analyzing the ARGF dispersion pattern at the fracture surface of the flexural test, the ARGF dispersion analysis method was proposed. SEM shows that the ARGF is coated with C-S-H, which increases its adhesion to the concrete matrix. XRD confirms that ARGF does not affect the hydration reaction of the cement in the UHPFRC. Finally, a model of ARGF-reinforced UHPFRC is established to elucidate the reinforcing mechanism. This study provides guidance and a reference for the application of UHPFRC in engineering projects. • The optimal fiber mix ratio for ARGF-HUPFRC was obtained. • AR-GF is randomly distributed in concrete. • Based on SEM, AR-GF is mainly composed of C-S-H with pull-out as the main component and increased adhesion on the surface. • Based on XRD, the C-S-H peak of ARGF-UHPC is slightly higher than that of UHPC, indicating a higher density. • An AR-GF reinforced UHPFRC model was established, revealing the mechanism of fiber reinforcement. [ABSTRACT FROM AUTHOR]

This study aims to evaluate the influence of crack repair using epoxy sealing on the tensile response of ultra-high-performance fiber-reinforced concrete (UHPFRC) under corrosive environments. Three different crack widths, i.e., 0.1, 0.3, and 0.5 mm, and two different corrosion durations, i.e., 4 and 10 weeks, were considered. The test results indicated a minor change in the tensile performance of UHPFRC with the smallest crack width of 0.1 mm under corrosive environments for up to 10 weeks. This is due to the restriction of ferric oxide formation at the densified fiber–matrix interface. A wider crack width accelerated the steel fiber corrosion and noticeably influenced the post-cracking tensile behavior. Considering the corrosion duration of 4 weeks, the tensile strength of cracked UHPFRC with a width of 0.3 mm or greater increased by approximately 12–17% owing to the moderate steel fiber corrosion. However, the tensile strength decreased during the longer corrosion duration of 10 weeks by ruptures of excessively corroded steel fibers. Crack repair using epoxy sealing increased the tensile strength of cracked UHPFRC by approximately 10% and effectively prevented further corrosion of steel fibers at the crack location, leading to higher tensile strength even after 10 weeks of corrosion.

Abstracts: Recently, Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) has offered notable advantages over other types of concrete. Therefore, a comprehensive investigation of the latest developments in Ultra-High-Performance Concrete (UHPC) is necessary to provide essential information for materials testing requirements and procedures and to expand its practical applications. The present work is a comprehensive four-part review of the UHPFRC. The current first part of the review focuses attention on the developments, principles, and raw materials of the UHPFRC. Part II covers the hydration and microstructure of the UHPFRC. Part III reviewed the fresh and hardened properties of the UHPFRC. Part IV covers the durability properties, cost assessment, applications, and challenges of the UHPFRC. This review is expected to advance the fundamental knowledge of UHPC and promote further research and applications of UHPC.

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The direct tensile behavior of ultra-high-performance surface-modified steel fiber-reinforced concrete was evaluated in this study. Various chemical modifications were applied to steel fibers, including acetone washing, hydrochloric acid washing, zinc phosphating, silica coating, and chelation using an ethylenediaminetetraacetic acid (EDTA) electrolyte solution, which improved the tensile strength, tensile strain, and g-value to a maximum of 17.76 MPa, 1.22%, and 144.51 kJ/m3, respectively. Acetone washing and EDTA chelating were the most effective methods for improving the tensile strength, whereas silica coating was the best for improving the strain capacity and energy absorption capacity. The optimal treatment time for EDTA chelation treatment was approximately 6 h, and the tensile performance decreased considerably after 12 h of treatment. Comparing these results with those of pullout experiments revealed that a high shear stress should be maintained after the fiber has fully debonded to effectively enhance the post-cracking tensile performance.

This paper presents a study on the flexural behavior of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) beams, which included conventional reinforcing bars. The study focuses on critical design aspects, such as concrete shrinkage and cracking implications on the tension-stiffening phenomenon. An experimental program with two different sized flexural reinforced UHPFRC beams was run. Beams were cast and tested in a four-point bending test (4PBT) using UHPFRC with different amounts of fibers: 130 and 160 kg/m3 (1.66% and 2.00% in vol.) to cover a wide range of strain-softening and strain-hardening constitutive UHPFRC behaviors. A non-linear finite element model (NLFEM) was developed to validate the mechanical tensile characterization of UHPFRC when applied to reinforced elements. Both shrinkage and tension-stiffening effects were considered to improve the model. After the NLFEM simulation, very reliable results were obtained at both the service and ultimate load levels compared to the experimental ones. Finally, some aspects about the design of reinforced UHPFRC cross-sections under bending forces are addressed and satisfactorily compared to the experimental results.

In this study, we investigated the effects of steel fiber type and specimen thickness on the uniaxial and biaxial flexural behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). For this purpose, three types of steel fibers (straight, three-times twisted, and six-times twisted) and three thicknesses of specimen (24, 48, and 72 mm) were used. The test results indicated that, owing to the larger perimeter of the triangular shape and mechanical anchorage effect, the twisted steel fibers exhibited better pullout resistance than the straight steel fiber with a circular shape, and its effectiveness increased with the number of ribs. In contrast, the best flexural behavior of UHPFRC was observed when the straight steel fiber was used under both uniaxial and biaxial stress states, and the six-times twisted steel fiber exhibited the worst flexural performance owing to the excessive bond strength of the composites. The uniaxial and biaxial flexural strengths of UHPFRC were insignificantly influenced by the sample thickness; however, the normalized toughness decreased with an increase in the thickness. A higher flexural strength, normalized toughness up to the peak, and deformability were observed under the biaxial flexural stress state than those under the uniaxial flexural stress state. The use of twisted steel fibers was more effective for slabs subjected to biaxial flexural stress than that for uniaxial beams.

Evaluation of the ultimate strength for the UHPFRC (ultra-high-performance fiber-reinforced concrete) flexural members was conducted. In this study, an experimental program about UHPFRC beams was conducted with the effect of fiber volume fraction, shear span to depth ratio, and compressive strength of matrix as the main variables. Among them, it was found that fiber volume fraction was the variable that had the greatest influence on the ultimate strength. The inclusion of 2% volume fraction steel fiber increases the shear and flexural strength of UHPFRC beams significantly. In particular, steel fiber inclusion changed the mode of failure of beams from diagonal shear failure into flexural failure. For the classification of failure patterns, the ultimate flexural strength and shear strength of UHPFRC members were evaluated using the current design code and UHPC guidelines. Flexural ultimate strength was affected by the size and shape of the stress block and consideration of the matrix's tensile strength. For the accurate shear strength prediction of UHPFRC beams, the tensile strength of the high strength matrix and the effect of steel fiber should be considered. [ABSTRACT FROM AUTHOR]

Abstract This study aimed to investigate the bond properties of prestressing strands embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC). Toward this end, two types of prestressing strands with diameters of 12.7 and 15.2 mm were considered, along with various concrete cover depths and initial prestressing force magnitudes. The average bond strength of the strands in UHPFRC was estimated by using pullout tests, and the transfer length was evaluated based on a 95% average maximum strain method. Test results indicated that the average bond strength of the pretensioned strand reduced as the diameter of the strand increased, and was between the bond strengths of round and deformed steel rebars. Higher bond strength was also obtained with a lower embedment length. Based on a comparison of p value, the bar diameter and embedment length most significantly influenced the bond strength of strands in UHPFRC, compared to a ratio of cover depth to diameter and initial prestressing force. Pretensioned strands in UHPFRC exhibited much higher bond strength and shorter transfer length compared with strands embedded in ordinary high-strength concrete. Lastly, ACI 318 and AASHTO LRFD codes significantly overestimated the transfer length of the strands embedded in UHPFRC.

Normal functionality of common concrete structures such as bridges and buildings relies heavily on the structural resistance under accidental or anthropogenic blast events. As one of the widely used structural types, reinforced concrete columns need to be highly considered when blast events occur to avoid severe socio-economic losses. To improve the blast–impact resistance of conventional reinforced concrete columns, this article makes the following contributions: (1) proposes to adopt the advanced ultra-high-performance fiber-reinforced concrete to strengthen the columns as a protective layer; (2) validates the superiority of ultra-high-performance fiber-reinforced concrete–strengthened columns through comparative study and specifies the controlling design parameters through sensitivity analysis; (3) implements and compares various ultra-high-performance fiber-reinforced concrete reinforcement methods; and (4) develops a numerical formula to predict the residual capacity of ultra-high-performance fiber-reinforced concrete–strengthened columns under blast impacts as a suitable alternate of the complicated and time-consuming finite element simulations. [ABSTRACT FROM AUTHOR]

This study assesses the effects of elevated temperatures (100, 200, 300, 400, and 500°C) on the bonding behavior of normal concrete (NC) substrate as old concrete and the new Green Universiti Sains Malaysia Reinforced Concrete (GUSMRC) as a repair material through slant shear, pull‐off, splitting tensile, and flexural tests. Sandblasting (SB) and grinding (GR) surface treatments were employed to enhance the bond strength of the NC/GUSMRC composite. The research also evaluates the mechanical characteristics of the GUSMRC mix which 50% of its content is ultrafine palm oil fuel ash prior to and after the exposure to elevated temperatures. The results showed degradation in the mechanical properties of the monolithic GUSMRC and the bonding strength of the NC/GUSMRC composite after exposure to elevated temperatures; however, the bonding quality is excellent. Moreover, the SB surface treatment enhanced the interfacial bonding more than the GR surface treatment before and after elevated temperature exposure. [ABSTRACT FROM AUTHOR]

Highlights • Tensile performance of UHP-FRC is improved by using long straight and twisted steel fibers. • Strain capacity and energy absorption capacity of UHP-FRC is enhanced by using PE fibers. • Hybrid use of short straight steel and PE fibers is most effective in terms of tensile performance. • A high energy absorbent UHP-HFRC with a g -value of 251 kJ/m3 is developed. Abstract To improve the tensile performance of ordinary ultra-high-performance fiber-reinforced concrete (UHP-FRC) with single steel fibers, a portion of the steel fibers was replaced with polyethylene (PE) fibers, showing a slip hardening response. For this study, three different types of steel fibers, i.e., short straight, long straight, and twisted steel fibers, and four different replacement ratios of steel fibers to PE fibers, i.e., 0, 0.5, 1.0, and 1.5%, were considered. In addition, fiber pullout patterns at localized cracks were examined under tensile loading using a camera installed with a magnifier. Test results indicate that twisted steel fibers were the most effective in improving the tensile strength and cracking performance of ultra-high-performance concrete, but led to reduced post-peak ductility due to matrix fragmentation during the pullout process. The compressive and tensile strengths and crack performance of UHP-FRCs were deteriorated when the PE fibers were added and when their replacement ratio increased. However, the strain capacity and energy absorption capacity of UHP-FRCs significantly improved when the PE fibers were incorporated and the replacement ratio was increased. The effectiveness of PE fibers on the strain and energy absorption capacities was higher for the short straight steel fibers than for the long straight and twisted steel fibers. Finally, a high energy absorbent ultra-high-performance hybrid steel and PE fiber-reinforced concrete was successfully developed at a 2% fiber volume fraction. It exhibited an energy absorption capacity of 251 kJ/m3 at peak strength, which is about 2.2–4.1 times higher than that of the ordinary UHP-FRCs with only steel fibers at the identical 2% volume fraction. [ABSTRACT FROM AUTHOR]

In this research, the flexural and shear behavior of five locally developed ultra-high-performance fiber-reinforced concrete beams was experimentally investigated. Four-point loading tests were carried out on concrete specimens which were further compared with five normal-strength concrete beams constructed at the laboratory. The objective of this study is to assess the flexural and shear behavior of ultra-high-performance fiber-reinforced concrete beams and compare them with that of normal-strength beams and available equations in the literature. Results indicate underestimation of shear (up to 2.71 times) and moment capacities (minimum 1.27 times, maximum 3.55 times) by most of the equations in beams with low-reinforcement ratios. Finally, results reveal that the experimental flexural and shear capacities of ultra-high-performance fiber-reinforced concrete specimens are up to 3.5 times greater than their normal-strength counterpart specimens. [ABSTRACT FROM AUTHOR]

Evaluation of the ultimate strength for the UHPFRC (ultra-high-performance fiber-reinforced concrete) flexural members was conducted. In this study, an experimental program about UHPFRC beams was conducted with the effect of fiber volume fraction, shear span to depth ratio, and compressive strength of matrix as the main variables. Among them, it was found that fiber volume fraction was the variable that had the greatest influence on the ultimate strength. The inclusion of 2% volume fraction steel fiber increases the shear and flexural strength of UHPFRC beams significantly. In particular, steel fiber inclusion changed the mode of failure of beams from diagonal shear failure into flexural failure. For the classification of failure patterns, the ultimate flexural strength and shear strength of UHPFRC members were evaluated using the current design code and UHPC guidelines. Flexural ultimate strength was affected by the size and shape of the stress block and consideration of the matrix’s tensile strength. For the accurate shear strength prediction of UHPFRC beams, the tensile strength of the high strength matrix and the effect of steel fiber should be considered.

This study investigates the flexural and cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC) before and after exposure to cryogenic temperatures for liquefied natural gas (LNG) storage tank applications. Normal concrete (NC), which has been used to make LNG storage tanks in Korea, was also considered for comparison. In order to evaluate the cracking resistance of NC and UHPFRC, several edge-type slabs were fabricated and tested by restraining their thermal deformation. Four-point bending tests were also performed to estimate the flexural performance before and after cryogenic cooling. Test results indicate that UHPFRC exhibited higher resistance to microcrack formation under these conditions. UHPFRC also showed substantially better flexural performance, both before and after exposure to cryogenic cooling, compared to NC. In addition, the microcracks in UHPFRC that were induced by the pre-cracking load were suddenly and effectively filled with calcium carbonate (CaCO 3 ), which was formed by a chemical reaction between melting water and calcium ions. This was verified by energy dispersive X-ray spectroscopy analysis. CaCO 3 formation resulted in enhanced flexural performance, including higher strength, deflection capacity, and energy absorption capacity, as compared to the virgin UHPFRC specimens without any cracks. [ABSTRACT FROM AUTHOR]

This study examined the effect of steel fiber geometry on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) under cryogenic conditions (approximately −162 °C). For this, compressive and tensile tests were performed using UHPFRCs containing three types of straight steel fibers and one type of twisted steel fiber. To investigate the mechanical properties of UHPFRCs under various temperatures, mechanical tests were performed in three different conditions: ambient temperature, cryogenic temperature, and recovered ambient temperature. The test results demonstrated considerable increases in both the compressive strength and tensile performance, including strength and fracture energy, for UHPFRCs with straight fibers at the cryogenic temperature, whereas that containing the twisted fibers demonstrated the poorest energy absorption capacity at the cryogenic temperature, due to the fiber fracturing. Finally UHPFRCs containing longer straight fibers most effectively achieved excellent mechanical properties at the cryogenic temperature, compared to those with short straight and twisted fibers. [ABSTRACT FROM AUTHOR]

This paper presents results from the uniaxial tests of six large-scale ultra-high-performance fiber-reinforced concrete (UHPFRC) circular columns confined by spirals. The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5 mm fibers and 0.5% of 16.3 mm fibers) in the mixture and had compressive strengths varying from 163 to 181 MPa. The effects of the volumetric ratio of spiral reinforcement, compressive strength of concrete, and presence of hybrid micro-steel fibers on the axial load responses, including post-peak deformability, were investigated. In addition, the ductility level reached by circular UHPFRC columns designed according to the minimum spiral reinforcement of current design provisions of the CSA A23.3-14 Standard and the ACI 318-14 Code were evaluated to investigate the applicability of these equations to UHPFRC columns. Test results showed that the combined effect of the minimum spiral reinforcement and steel fibers resulted in sufficient post-peak ductility of the UHPFRC columns. To investigate the efficiency of the shape of the confinement reinforcement, the test results of the circular UHPFRC columns confined by spirals were compared with those from equivalent-sized square UHPFRC columns confined by hoops. The test results demonstrate the superior performance of circular spirals for developing the ductile behavior of UHPFRC columns than the same volumetric ratio of rectilinear hoops. A design recommendation for spiral reinforcement that ensures the ductile behavior of UHPFRC columns in moderate seismic regions is proposed. [ABSTRACT FROM AUTHOR]

Full-scale reinforced concrete (RC) and ultra-high-performance fiber-reinforced concrete (UHP-FRC) columns subjected to reversed cyclic loads are imaged using ultrasonic tomography before, during, and after loads are applied. Concrete columns with varying geometries and detailing, loading protocols, and material types are considered. The columns contain internally embedded concrete strain gauges, the data from which are used to establish extent of accumulated damage caused by the applied loads at certain locations within the columns. A newly developed hybrid air-coupled ultrasonic system is used to collect a large volume of through-thickness ultrasonic data across the cross section at the plastic hinge region of the column. Ultrasonic monitoring was carried out at increasing load levels up to a drift ratio of 1%. The ultrasonic data are used to back-calculate wave velocity tomographic images (tomograms) across the cross section. A comparison of ultrasonic and strain gauge data shows that ultrasonic tomograms indicate damage progression within the columns, considering both global and local spatial scales. The results also confirm that the UHP-FRC column exhibited much less internal damage at a given load level compared to conventional reinforced concrete columns. [ABSTRACT FROM AUTHOR]

This study aims to investigate the effect of curing conditions on the free shrinkage behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). For this study, a number of exposed and sealed prismatic UHPFRC samples for drying and autogenous shrinkage measurements were fabricated and tested using two different types of embedded strain gauges. Several other tests, including mechanical tests, X-ray diffraction (XRD), and mercury intrusion porosimetry analyses, were also performed. Test results indicate that steam curing with heat (90 °C, referred to as heat curing) was effective to improve the mechanical properties of UHPFRC at an early age in terms of strength, elastic modulus, and fracture energy absorption capacity. The larger quantities of C-S-H and much smaller total cumulative pore volume were obtained for the steam-cured specimens, compared to those for the ambient-cured specimens. The ultimate autogenous shrinkage of UHPFRC was insignificantly affected by the curing conditions, whereas heat curing accelerated the shrinkage development as compared to ambient curing. In particular, there was no increase of shrinkage strains for UHPFRC after heat curing was finished. The ultimate drying and autogenous shrinkage of UHPFRC were found to be approximately −45 με and −450 με, respectively. Based on literature review, an optimized model was suggested, and the autogenous shrinkage developments of UHPFRC at both ambient and heat curing conditions were successfully predicted based on the equivalent age method. [ABSTRACT FROM AUTHOR]

• Nanoparticles were added to ultra-high-performance fiber-reinforced concrete. • Fracture resistance enhancement in UHPRC by nanoparticle addition was examined. • Rate-sensitive fracture resistances were evaluated using tensile tests. • High fracture parameters and rate sensitivities are observed at high strain rates. • Concrete with 3 wt% nano-CaCO 3 exhibited the highest dynamic increase factor. The fracture resistance of ultra-high-performance fiber-reinforced concrete (UHPFRC) containing various types of nanoparticles (NPs) under various strain rates (0.000333–156.55 s−1) was investigated. Four matrices were examined: UHPFRC without NPs (UM), UM containing 3% nano-CaCO 3 (UC), UM containing 1% carbon nanotubes (UCNT), and UM containing 1% nano-SiO 2 (US). The effects of the strain rate on the fracture resistance of the matrices, including fracture strength, fracture energy, and specific work-of-fracture (W S), were evaluated. All matrices containing NPs demonstrated a higher rate-sensitive fracture resistance than that of the UM matrix. The dynamic increase factors (DIF) for the W S of UC, US, and UCNT were 4.50, 4.30, and 3.68, respectively, whereas that of UM was the lowest at 3.44. Of the NP-containing UHPFRCs, at high strain rates between 140.6 and 156.55 s−1, the matrices arranged in order of decreasing W S magnitude are UC > US > UCNT. The fracture resistance enhancement in UC was attributed to the improvement in the C-S-H content of the fiber–matrix zone due to the added nano-CaCO 3 , which increased the interfacial bond strength of the smooth steel fibers embedded in the UHPFRC. [ABSTRACT FROM AUTHOR]

Six large Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) slabs were fabricated and tested to investigate the restrained shrinkage and cracking behaviors. The use of expanded polystyrene and Teflon sheets with two different slab thicknesses was considered to improve the shrinkage crack resistance. Free shrinkage was simultaneously measured to evaluate the degree of restraint according to the above test parameters. The test results showed that free shrinkage strains of -689 με to -723 με were obtained after 9 days, and prismatic specimens with a higher exposed surface area-to-volume ratio (S/V) had slightly higher free shrinkage strains than those with a lower S/V. Increasing the concrete slab thickness and using expanded polystyrene and Teflon sheets were effective at reducing the degree of restraint and improving the shrinkage crack resistance of the UHPFRC slabs. Among the various specimens, the slabs with the expanded polystyrene exhibited the lowest degree of restraint by 0.45 after 9 days. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the influence of exposure to cryogenic temperatures using liquid nitrogen on the mechanical properties of normal concrete (NC) and ultra-high-performance fiber-reinforced concrete (UHPFRC), which is commercially available. This research was carried out to examine the feasibility of using UHPFRC for a liquefied natural gas storage tank. For this, both compressive and direct tensile tests were performed at three different testing conditions: ambient temperature, cryogenic temperature (−170 °C), and recovered ambient temperature after experiencing the cryogenic temperature. The test results showed that the compressive strengths of both NC and UHPFRC were noticeably increased at the cryogenic temperature compared with those at ambient temperature. However, there was no improvement in the tensile strength of NC at the cryogenic temperature, and its tensile strength was deteriorated after exposure to the cryogenic temperature. In contrast with NC, the tensile performance of UHPFRC significantly increased, including improvements in strength, post-cracking stiffness, and energy absorption capacity. Given the superior mechanical properties, it was concluded that UHPFRC is suitable for liquefied natural gas storage tanks. [ABSTRACT FROM AUTHOR]

The bond behavior of glass fiber-reinforced polymer (GFRP) and steel bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated according to embedment length and bar diameter. Post-peak bond stress-slip softening curve of the GFRP bars was obtained, and a wedging effect was quantitatively evaluated. Test results indicated that a normalized bond strength of 5 was applicable for steel bars embedded in UHPFRC, and the development lengths of normal- and high-strength steel bars were determined to be 2 and 2.5 times the bar diameter, respectively. The GFRP bars exhibited approximately 70% lower bond strength than the steel bars, and the bond stress additionally applied by the wedging effect increased almost linearly with respect to the slip. Based on dimensionless bond stress and slip parameters, an appropriate theoretical model for the bond stress and slip relationship of steel bars in UHPFRC was suggested, and it was verified through comparison with the test data. [ABSTRACT FROM AUTHOR]

This study examined the electrical and self-sensing capacities of ultra-high-performance fiber-reinforced concrete (UHPFRC) with and without carbon nanotubes (CNTs). For this, the effects of steel fiber content, orientation, and pore water content on the electrical and piezoresistive properties of UHPFRC without CNTs were first evaluated. Then, the effect of CNT content on the self-sensing capacities of UHPFRC under compression and flexure was investigated. Test results indicated that higher steel fiber content, better fiber orientation, and higher amount of pore water led to higher electrical conductivity of UHPFRC. The effects of fiber orientation and drying condition on the electrical conductivity became minor as sufficiently high amount of steel fibers, 3% by volume, was added. Including only steel fibers did not impart UHPFRC with piezoresistive properties. Addition of CNTs substantially improved the electrical conductivity of UHPFRC. Under compression, UHPFRC with a CNT content of 0.3% or greater had a self-sensing ability that was activated by the formation of cracks, and better sensing capacity was achieved by including greater amount of CNTs. Furthermore, the pre-peak flexural behavior of UHPFRC was precisely simulated with a fractional change in resistivity when 0.3% CNTs were incorporated. The pre-cracking self-sensing capacity of UHPFRC with CNTs was more effective under tensile stress state than under compressive stress state. [ABSTRACT FROM AUTHOR]

This study investigates the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) with single and hybrid steel fibers. To do this, three different types of steel fibers, i.e., hooked, twisted, and straight fibers, were considered, and a UHPFRC commercially available in North America was used as a comparison. To suggest a low-cost UHPFRC exhibiting the best flexural performance, test data and cost of fibers were analyzed based on a literature review. Test results indicate that straight steel fibers provide the best flexural performance, including strength, deflection capacity, energy absorption capacity, and cracking behavior, compared with hooked and twisted fibers, especially when many fibers (2% by volume) were incorporated. Hybrid reinforcement (hooked + straight fibers) efficiently improved the flexural performance of the UHPFRC with single hooked fibers, but the twisted + straight fibers were less effective than the UHPFRC with single twisted fibers. The optimum UHPFRCs contained 2 vol% single straight steel fibers (l f /d f of 19.5/0.2) or hybrid 0.5 vol% long (l f /d f of 30/0.3) and 1.5 vol% medium-length (l f /d f of 19.5/0.2) straight steel fibers; they showed better flexural strength and cost effectiveness than other types of UHPFRCs. [ABSTRACT FROM AUTHOR]

This study investigates the axial load response of ultra-high-performance fiber-reinforced concrete (UHPFRC) columns with compressive strengths of 163 and 181 MPa (design strengths of 150 and 180 MPa). The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5 mm fibers and 0.5% of 16.3 mm fibers) and did not contain coarse aggregate. A total of nine UHPFRC columns confined by transverse reinforcement with volumetric ratios of 0.9–9.9% and two different configurations (Types A and C) were tested under pure axial load to investigate the influence of these variables. The overall behavior of the UHPFRC columns was compared with the response of similar strength ultra-high-strength concrete (UHSC) columns having coarse aggregate. Test results showed a pronounced effect of the volumetric ratio of the transverse reinforcement on the confinement. Hybrid micro-steel fibers controlled brittle cover spalling very well and assisted the transverse confinement reinforcement after the peak load. Applicability of the confinement reinforcement equations in the current seismic design provisions for developing ductile behavior of the UHPFRC columns was investigated. The analytical study examined the ability of the existing high-strength concrete (HSC) confinement models for predicting the axial load response of the UHPFRC columns, and a prediction model that accounts for the effects of steel fibers and the stress-strain relationship of UHPFRC is proposed. [ABSTRACT FROM AUTHOR]

A nonlinear finite element analysis was performed to simulate the flexural behaviors of ultra-high-performance fiber-reinforced concrete beams. For this, two different tension-softening curves obtained from micromechanics-based analysis and inverse analysis were incorporated. For micromechanics-based analysis, two-dimensional and three-dimensional random fiber orientations were assumed to obtain the fiber-bridging curve, and a softening curve of matrix in ultra-high-performance fiber-reinforced concrete was used. The use of tension-softening curves obtained from inverse analysis and micromechanics-based analysis using two-dimensional random fiber orientation exhibited fairly good agreement with the experimental results, whereas the use of tension-softening curve from micromechanics-based analysis using three-dimensional random fiber orientation underestimated the experimental results. [ABSTRACT FROM AUTHOR]

This study examines the rate dependent flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams with three different sizes. Two different loading rates (static and impact), fiber aspect ratios (l f /d f of 65 and 100), and fiber types (straight and twisted) were considered. Test results indicated that the static flexural performance, including the flexural strength and toughness, were improved by increasing the fiber aspect ratio or through the use of twisted steel fibers. The static flexural strength clearly decreased with an increase in specimen size due to a decrease in the number of fibers at the crack surface. The use of straight steel fibers with a higher aspect ratio of 100 provided the best impact resistance in terms of the highest post-cracking flexural strengths and the largest normalized energy dissipation rates, compared to those of twisted steel fibers and straight steel fibers with a reduced aspect ratio of 65. Thus, the use the straight steel fibers with high aspect ratios was recommended to improve the impact resistance of UHPFRC. Dynamic increase factor (DIF) on the flexural strength of UHPFRC beams was properly investigated with strain-rate, regardless of specimen size. In addition, there were no effects with regard to the fiber aspect ratio and type on the relationship between the DIF of the first-cracking flexural strength and the stress- (or strain-) rate. [ABSTRACT FROM AUTHOR]

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is a relatively new construction material. In comparison with conventional high-strength concrete, UHPFRC does not usually contain coarse aggregates larger than 6-7 mm in size. This paper presents the outcomes of an experimental study of UHPFRC beams subjected to four-point loading. The effect of two parameters was studied, namely, the fiber content and the temperature of the curing water. Eight UHPFRC beams were tested, comprising six beams reinforced with rebars and two beams without rebars. Three fiber contents were investigated in this study (1, 2, and 4% in volume). The study investigated two curing temperatures of water, 20 and 90°C. The results presented in this paper include deflections, toughness energy, and moment capacity and also includes a comparison with calculations according to EC2 provisions. A minor difference was observed in the deformation and flexural behavior of beams with fiber contents of 1 and 2% (in volume). However, beams with 4% (in volume) fibers exhibited a higher flexural capacity. Only flexural failure was observed and no shear-related failure was recorded. Beams with 1% (in volume) fibers for both curing regimes had the highest peak load toughness energy. Beams reinforced with rebars and cured at 20°C had a significantly higher bending resistance. [ABSTRACT FROM AUTHOR]

In this study, the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) is examined as a function of fiber length and volume fraction. Straight steel fiber with three different lengths (lf) of 13, 19.5, and 30 mm and four different volume fractions (vf) of 0.5%, 1.0%, 1.5%, and 2.0% are considered. Test results show that post-cracking flexural properties of UHPFRC, such as flexural strength, deflection capacity, toughness, and cracking behavior, improve with increasing fiber length and volume fraction, while first-cracking properties are not significantly influenced by fiber length and volume fraction. A 0.5 vol % reduction of steel fiber content relative to commercial UHPFRC can be achieved without deterioration of flexural performance by replacing short fibers (lf of 13 mm) with longer fibers (lf of 19.5 mm and 30 mm). [ABSTRACT FROM AUTHOR]

The cyclic behavior of six full-scale reinforced concrete (RC) beams with a deficient lap splice strengthened with ultra-highperformance fiber-reinforced concrete (UHPFRC) is experimentally investigated. The experimental program is based on the findings of previous test series carried out in the same research program that demonstrated the ability of UHPFRC to eliminate bond failure in deficient lap splices of beams and wall-type bridge columns. The objective of this experimental work is to determine the efficiency of this strengthening technique on wide flexural elements (beams, slabs, walls, or wall columns) subjected to reverse cyclic loading. Specimen reinforcement consists of two pairs of deformed bars spliced at midspan on both tension and compression faces. The strengthening technique consists of replacing normal concrete around lapped bars in the splice region by UHPFRC, which allows for keeping the original member geometry. One type of fiber, three fiber contents, two bar diameters, and two bar arrangements are considered. For isolating the UHPFRC contribution, the splice regions are free of any confinement. The beam specimens are tested at four points, bending with a constant-moment region along the splice length. The result indicates that UHPFRC with a fiber content of 2 or 3% can significantly increase the bond strength of splice bars without confinement. The levels of ductility reached for the highest fiber content meet the requirements for high ductility demand, such as in seismic design. The results demonstrate that an appropriate casting method combined with a self-compacting UHPFRC with an appropriate fiber content ensure the efficiency of the strengthening technique for providing for the continuity of lapped bars and for enabling a high ductility capacity under monotonic or cyclic loading. The results also confirm the applicability of the method for strengthening lap-spliced regions of wide elements--such as slabs, shear walls, and wall-bridge piers--without having to provide any confinement. [ABSTRACT FROM AUTHOR]

• The interfacial bond behavior between ribbed CFRP bars and UHPFRC is investigated. • The failure modes observed from the pull-out specimens of the ribbed CFRP bar embedded in UHPFRC are pull-out failure, splitting failure, and splitting-pull-out failure. • The bond strength improves with the increase of concrete lug ratio and relative rib area. • The previous bond strength predictions are compared with the experimental data, and a novel bond strength prediction model considering anchorage length, cover. thickness, and CFRP bar surface geometric ratio is established. Carbon fiber-reinforced polymer (CFRP) bars and ultra-high-performance fiber-reinforced concrete (UHPFRC) feature high strength, high elastic modulus, and excellent durability, which can be combined and applied to the high-load or corrosive environments. To comprehend the bond behavior between the ribbed CFRP bar and UHPFRC, 30 cubic specimens for axial pull-out tests and 12 cubic specimens for eccentric pull-out tests were tested in this study. The effects of CFRP bar diameter, anchorage length, and cover thickness on the bond behavior were investigated. The results indicated that three different failure modes, which were pull-out failure, splitting failure and splitting-pull-out failure, were detected. The delamination of fibers and resin in the specimens with pull-out failure caused interfacial bond failure between the ribbed CFRP bar and UHPFRC. The bond strength increased from 18.41 MPa to 36.41 MPa when the diameter increased from 10 mm to 12 mm, owing to the increase in the geometric ratios (concrete lug ratio and relative rib area) of the CFRP bar surface. Additionally, the increase of cover thickness was beneficial for improving the interfacial bond strength, and the critical cover thickness was about three times the bar diameter (3 d). The residual stage of pull-out specimens still exhibited a high-stress level in the case of splitting failure owing to the fiber bridging effect of UHPFRC. Finally, a bond strength prediction model related to anchorage length, cover thickness, and the geometric ratio of CFRP bar surface was proposed. [ABSTRACT FROM AUTHOR]

Secondary hydration is the main driving force of autogenous self-healing and directly impacts the closure of cracks in damaged cementitious materials. In fiber-reinforced cement composites, secondary hydration also dominates the regeneration of the fiber-matrix interface during the self-healing processes. As an important factor influencing the cement hydration, silica fume (SF) addition can be crucial to the self-healing performance of cementitious materials and composites. In this paper, the effect of SF on the autogenous self-healing of ultra-high-performance fiber-reinforced concrete was investigated. With a ≥20% SF addition, the secondary hydration the cement matrix adjacent to the cracks was accelerated. The dissolution of amorphous silica and the formation of Si-containing healing phases was observed. These effects predominated the closure of the mouth and the inner part of the cracks. SF addition also enhanced the regeneration of fiber-matrix interface by promoting the growth of self-healing products in these areas. The closure of cracks and the regeneration of fiber-matrix interface are both responsible for the restoration of flexural properties. • The secondary hydration acceleration of the cement matrix after cracking was found with a ≥20% SF addition, which promoted the self-healing of the cracks in UHPFRC. • The effect of SF on the structural regeneration of the cracks in UHPFRC was elucidated with a self-healing mechanism of the amorphous Si dissolution. • The self-healing of the deep matrix crack and fiber-matrix interface together induced the flexural property restoration of the SF-containing UHPFRC. [ABSTRACT FROM AUTHOR]