Ultra-high-performance Fiber-reinforced Concrete Research Articles

Incorporating Limestone Powder and Ground Granulated Blast Furnace Slag in Ultra-high Performance Concrete to Enhance Sustainability

While ultra-high performance concrete (UHPC) offers numerous advantages, it also presents specific challenges, primarily due to its high cost and excessive cement content, which can pose sustainability concerns. To address this challenge, this study aims to develop cost-effective and sustainable UHPC mixtures by incorporating ground granulated blast furnace slag (GGBFS) and limestone powder (LP) as partial replacements for portland cement. Eight fiber-reinforced UHPC mixtures were investigated, with a water-to-cementitious materials (w/cm) ratio of 0.15. In four of the UHPC mixtures, 25% of the cement was replaced with GGBFS, and further, LP was added as a mineral filler, partially substituting up to 20% of the cement. In the remaining four mixtures, cement was replaced with only LP up to 20% (without GGBFS). The 28-day compressive strength of the UHPC mixture with 25% GGBFS and 20% LP was 149 MPa, 3.50% lower than the mixture without GGBFS. Its 28-day flexural strength decreased by 30%. Increasing LP replacement reduced drying and autogenous shrinkage, with a 29% shrinkage reduction at 20% LP replacement. Moreover, UHPC mixtures with GGBFS exhibited lower shrinkage compared to those without GGBFS for all LP replacements up to 20%. For evaluating the sustainability of UHPC mixtures, the cement composition index (CCI) and clinker to cement ratio (CCR) were determined. For 20% LP replacement with 25% GGBFS, CCI was 3.6 and the CCR was 0.5, 38% decrease from the global clinker to cement ratio. Overall, 20% LP replacement UHPC mixtures with and without GGBFS can produce UHPC class performance and reduce the environmental impact.

Structural Behavior of Full-Depth Deck Panels Having Developed Closure Strips Reinforced with GFRP Bars and Filled with UHPFRC

The adoption of prefabricated elements and systems (PBES) in accelerating bridge construction (ABC) and rapidly replacing aging infrastructure has attracted considerable attention from bridge authorities. These prefabricated components facilitate quick assembly, which diminishes the environmental footprint at the construction site, alleviates delays and lane closures, reduces disruption for the traveling public, and ultimately conserves both time and taxpayer resources. The current paper explores the structural behavior of a reinforced concrete (RC) precast full-depth deck panel (FDDP) having 175 mm projected glass-fiber-reinforced polymer (GFRP) bars embedded into a 200 mm wide closure strip filled with ultra-high-performance fiber-reinforced concrete (UHPFRC). Three joint details for moment-resisting connections (MRCs), named the angle joint, C-joint, and zigzag joint, were constructed and loaded to collapse. The controlled slabs and mid-span-connected precast FDDPs were statically loaded to collapse under concentric or eccentric wheel loading. The moment capacity of the controlled slab reinforced with GFRP bars compared with the concrete slab reinforced with steel reinforcing bars was less than 15% for the same reinforcement ratio. The precast FDDPs showed very similar results to those of the controlled slab reinforced with GFRP bars. The RC slab reinforced by steel reinforcing bars failed in the flexural mode, while the slab reinforced by GFRP bars failed in flexural-shear one.

Improved mechanical and microscopic properties of ultra-high-performance concrete with the addition of hybrid alkali-resistant glass fibers

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.

Effects of specimen thickness and fiber length on tensile and cracking behavior of UHPFRC: Uniaxial tensile test and micromechanical modeling

This study aims to investigate the effects of specimen thickness and fiber length on the tensile and cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). To this end, a uniaxial tensile test was conducted with three specimen thicknesses (30, 50, and 100 mm) and two fiber lengths (13 and 20 mm), and the fiber orientation, dispersion and specimen void were quantitatively evaluated based on image recognition. The test results indicated that fiber orientation was improved with the decreased specimen thickness and increased fiber length. Meanwhile, the initial cracking and peak stress, capacity to limit cracking were enhanced with the decreased specimen thickness. A modified prediction model considering the wall effect, flattening and squeezing effect was developed to predict the probability density function p(θ) of fiber orientation angle. Additionally, the uniformity factor μ2 was introduced to predict crack number, and the relationship between the μ2 and parameter ψ = (Vf × lf/df)/t was determined. Furthermore, a model was developed to convert the main crack width into uniaxial tensile strain. All models and relationships were validated using test data. A micromechanical model that considered the predicted p(θ) and conversion model was established to predict the uniaxial tensile response of UHPFRC, which was also successfully validated using test data.

Triaxial compressive behavior of 3D printed PE fiber-reinforced ultra-high performance concrete

The layered deposition process of 3D concrete printing can lead to reduced mechanical properties at the interfaces between filaments. To address this limitation, external confinement devices, such as fiber-reinforced polymer (FRP) wrapping, have been proposed to enhance the strength of 3D-printed concrete concrete. Achieving this requires a solid understanding of the triaxial mechanical performance of 3D-printed concrete. This study presents an experimental investigation of the triaxial compressive behavior of 3D-printed PE fiber-reinforced ultra-high performance concrete (3DP-PEUHPC). A total of 16 pairs of concrete cubes were prepared, including mold-cast and 3D-printed specimens, and subjected to uniaxial and triaxial compression tests. The results revealed that the 3D-printed specimens exhibited either column-type or diagonal shear failures under triaxial compression. Weak bonding was observed at both filament-fusion and layer-fusion interfaces, with these weaker bonding interfaces, particularly when aligned parallel to the axial load, showing susceptibility to stress concentration and crack initiation. This led to a reduction in load-bearing capacity of the 3D-printed specimens compared to the mold-cast specimens. Importantly, as confining stresses increase, the difference in compressive strength between 3D-printed and mold-cast specimens decreases, highlighting the effectiveness of confinement in mitigating the directional weaknesses inherent in 3D-printed concrete. This paper also presents a modified model for predicting the axial stress-strain relationship of 3DP-PEUHPC under confinement, providing insights into the mechanism of FRP confinement on the compressive strength of 3D-printed concrete structures.

Two-lift concrete pavement with advanced fibre reinforced concrete as the top lift and roller compacted concrete as the bottom lift

Roller Compacted Concrete (RCC), developed for pavements and dams due to its low cement content, is constrained by limited fatigue resistance, making it suitable only for low-traffic pavements. This study aims to develop a high-fatigue-resistant Two-lift Concrete Pavement (TLCP) incorporating RCC, with three primary objectives: (i) constructing TLCP with a two-thirds RCC bottom layer and a one-third ultra-high-performance fiber-reinforced concrete (UHPFRC) or engineered cementitious composites (ECC) top layer, (ii) conducting fatigue testing on 225 specimens, and (iii) analyzing fatigue data using statistical methods to determine the best approach. Results showed that TLCP specimens had higher compressive and flexural strengths than RCC, though lower than UHPFRC or ECC. A graphical method was found most accurate for predicting fatigue life. TLCP with UHPFRC achieved 3.85–7.5 times greater fatigue life, while TLCP with ECC showed 2.63–5.96 times higher fatigue life than RCC, with ECC offering significant cost savings.

Punching shear behavior of ultra-high-performance fiber-reinforced concrete and normal strength concrete composite flat slabs

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.

Fire resistance of 3D printed ultra-high performance concrete panels

Ultra-high performance fiber-reinforced concrete (UHPFRC) is highly suitable for 3D concrete printing (3DCP) due to its high flexural strength, thereby reducing the need for reinforcements. However, UHPFRC is susceptible to spalling under exposure to fire, limiting its application as structural members. In this paper, the effect of 3D printing process on the fire behavior of UHPFRC is studied, benchmarking against mold-cast panels. The insulation, integrity, and structural adequacy of the panels are investigated using the heat transfer mechanisms, failure modes, and the post-fire compressive strength of the panels, respectively. The presence of interlayers reduced the spalling of UHPFRC under fire and improved the structural integrity of 3D printed specimens compared to mold-cast specimens. Further, the addition of 0.5 % polypropylene (PP) fibers eliminated the interlayer delamination and spalling in 3D printed UHPFRC panels. Similarly, 3D printed panels showed improved structural adequacy than mold-cast specimens. The residual compressive strength of 3D printed UHPFRC panels after being exposed to fire was observed to be above 50 % of the initial mean compressive strength. However, the insulation property of 3D printed panels was reduced compared to that of the mold-cast counterparts due to the high rate of heat transfer via the porous interlayers. The addition of PP fibers improved the insulation resistance of the interlayer region and surface of the 3D printed panels. The strength anisotropy of the 3D printed UHPFRC reduced significantly following the fire exposure. Further, the thermal bowing of 3D printed panels was higher with an increased dosage of PP fibers due to the increase in the thermal strain of UHPFRC. Therefore, adequate care must be given in the design of 3D printed structures for potential fire resistant wall, slab, and façade elements.

Surface electrical properties of UHPFRC with graphene

The role of graphene inclusion in ultra-high-performance concrete (UHPC) and in ultra-high-performance fiber reinforced concrete (UHPFRC) and the role of steel fibres (used in UHPFRC) have been studied. Surface electrical properties were shown as useful means to deeply investigate the properties of UHPFRC including graphene. Due to the inclusion of fibers a marked increasing of surface conductivity was shown in UHPFRC respect to UHPC. Surface conductivity has been estimated ∼20 % lower than bulk conductivity in mature UHPFRC. A method for assessing the dispersion grade of graphene in concrete, based on surface electrical measurements and numerical modelling, is presented. In case of mature UHPC, increasing surface conductivity with increasing the graphene content was observed. In case of UHPFRC, graphene within the investigated range of contents (≤1 % bwoc) was found not to greatly affect surface conductivity. With including graphene, a reduction of microstructural disorder was estimated, whereas the inclusion of fibers was estimated to increase the microstructural disorder.

Experimental characterization of indented dry connections for precast double‐curved concrete shells

AbstractThis paper presents the experimental validation of a connections system to connect segmented precast long‐span thin‐shell structures made with lightweight ultra‐high‐performance fiber‐reinforced concrete (UHPFRC). The work comprises the segmentation of the large‐span ultra‐thin shell to meet the requirements of prefabrication (construction and transport limitations), and the design and characterization of the connections. Once the assembling of the shell required the use of post‐tensioned unbonded prestressing, an indented dry connection was chosen. The connections are experimentally characterized with in‐plane and out‐of‐plane shear tests for different prestressing levels. The results are presented and discussed, demonstrating the balanced behavior of the indented dry connection under shear loading. The connections exhibited high ductility in shear, capitalizing on the use of a UHPFRC to fabricate the shell. The prestressing also demonstrated its efficacy, significantly enhancing both the stiffness and the capacity of the indented dry connection.

Design, modelling and optimisation of ultra high-performance fibre reinforced concrete incorporating waste materials

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.

Investigation of the dynamic mechanical response of corroded ultra-high performance fiber reinforced concrete (UHPFRC) with initial defects

This study addresses the dynamic mechanical response of the corroded ultra-high-performance fiber-reinforced concrete (UHPFRC) with initial defects, considering the possibility of corrosion deterioration induced by various pre-existing cracks during the long-term service life. For this purpose, an integrated accelerated corrosion method and Split Hopkinson Pressure Bar (SHPB)/high-speed camera etc. techniques are employed. Results show that increasing pre-impacting damage promotes the crack density and maximum width by 32.4%–62.3 % and 1.11–1.8 times, respectively. In terms of mechanical properties, coupling damages of initial defects and corrosion have adverse effects on the dynamic mechanical response. Typical fib Model Code 2010 applies to predict the DIF evolution of the corroded UHPFRC with initial defects. Numerous shear cracks are created at an angle along the weak interface as the corroded specimens with initial defects are again subjected to axial loading, revealing the associated failure mechanism. These results shed light on the dynamic response of corroded UHPFRC containing various initial defects and the failure mechanism gives some reference to service status evaluation.

Development and properties of cost-effective self-sensing Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) incorporating steel slags

This study examined the electrical and self-sensing capacities of ultra-high performance fiber-reinforced concrete （UHPFRC） with and without steel slag powder. The inclusion of steel slags provides a reliable way to enhance the electrically conductive and self-sensing capabilities of UHPFRC, which is composed of cement, silica fume, and steel fiber. The conductibility of UHPFRC and its response to external loading were investigated. Additionally, through 3D analysis of fiber structure and pore distribution, the conductive mechanisms were revealed. The results show that the addition of steel slags powder (SSP) can improve both mechanical properties and flowability. The maximum fractional change in electrical resistivity under compressive and flexural loads reached 37.6 % and 54.1 %, respectively. However, the conductive model under compressive loading differs from that under flexural loading. Furthermore, an analysis of carbon emissions and material costs revealed that using SSP reduced carbon emissions by 45.2–96.4 % and lowered costs by 41.8–88.8 %, making this approach both economically and environmentally advantageous.

Fatigue and Ultimate Strength Evaluation of GFRP-Reinforced, Laterally-Restrained, Full-Depth Precast Deck Panels with Developed UHPFRC-Filled Transverse Closure Strips

A depth precast deck panel (FDDP) is one element of the prefabricated bridge element and systems (PBES) that allows for quick un-shored assembly of the bridge deck on-site as part of the accelerated bridge construction (ABC) technology. This paper investigates the structural response of full-depth precast deck panels (FDDPs) constructed with new construction materials and connection details. FDDP is cast with normal strength concrete (NSC) and reinforced with high modulus (HM) glass fiber reinforced polymer (GFRP) ribbed bars. The panel-to-girder V-shape connections use the shear pockets to accommodate the clustering of the shear connectors. A novel transverse connection between panels has been developed, featuring three distinct female-to-female joint configurations, each with 175-mm projected GFRP bars extending from the FDDP into the closure strip, complemented by a female vertical shear key and filled with cementitious materials. The ultra-high performance fiber reinforced concrete (UHPFRC) was selectively used to joint-fill the 200-mm transverse joint between adjacent precast panels and the shear pockets connecting the panels to the supporting girders to ensure full shear interaction. Two actual-size FDDP specimens for each type of the three developed joints were erected to perform fatigue tests under the footprint of the Canadian Highway Bridge Design Code (CHBDC) truck wheel loading. The FDDP had a 200-mm thickness, 2500-mm width, and 2400-mm length in traffic direction; the rest was over braced steel twin girders. Two types of fatigue test were performed: incremental variable amplitude fatigue (VAF) loading and constant amplitude fatigue (CAF) loading, followed by monotonically loading the slab ultimate-to-collapse. It was observed that fatigue test results showed that the ultimate capacity of the slab under VAF loading or after 4 million cycles of CAF exceeded the factored design wheel load specified in the CHBDC. Also, the punching shear failure mode was dominant in all the tested FDDP specimens.

Axial compressive behavior of circular RC stub columns jacketed by UHPC and UHPFRC

This study investigated the axial compressive behavior of circular reinforced concrete (RC) stub columns jacketed by ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHPFRC). Eighteen composite columns were tested under compressive loadings on the entire section and only the RC section, while three control RC columns were tested under axial compression. Loading on the entire section exhibited a higher maximum compressive load and better expansion restraint than loading on the RC section. Adding 1 % and 2 % steel fibers by volume significantly increased the maximum compressive load and ductility index. However, the difference between 1 % and 2 % fiber content was not substantial. Maximum compressive load when loading on the entire section surpassed that when loading on the RC section, increasing by 75.6 %, 76.18 %, and 76.78 % for 0 %, 1 %, and 2 % steel fiber volume, respectively. Loading on the RC section led to faster cracking and failure, resulting in more tensile stress and lateral strain in the UHPC and UHPFRC jackets. Conversely, loading on the entire section maintained the expansion restraint even after reaching the maximum load. Columns with higher steel fiber volume demonstrated a greater enhancement in strength and ductility. Furthermore, a finite element model (FEM) using ABAQUS software was built to identify the experiments. Material models for UHPC, UHPFRC, and NSC core were proposed. The comparison between FEM and the test results shows that the FEM can predict accurately the behavior of tested columns. Based on developed FEM, a parametric study with the change of two variables, including the thickness and the compressive strength of the UHPC and UHPFRC layers, was conducted. The increase in the thickness of the UHPC or UHPFRC layer led to a remarkable increase in the maximum compressive load and ductility, indicating the importance of the thickness of the UHPC and UHPFRC jackets in restraining the expansion of the RC core. However, increasing the compressive strength of UHPC and UHPFRC had minimal impact on the maximum compressive load and ductility. Derived from these findings, UHPC and UHPFRC can be considered alternative material to provide lateral confinement to RC columns in addition to the traditional materials, indicating the potential application in strengthening or repairing damaged RC columns

Study of the mechanical properties, durability, and microstructure of an ultra-high-performance fiber-reinforced concrete containing recycled fillers

Abstract In this article, a specific mix design of the ultrahigh-performance fiber-reinforced concrete (UHPFRC) was proposed and applied for Algerian materials that are currently in use. In the absence of a general mix design that limits their use and reuses waste materials, the present mix design can help reduce the high manufacturing cost of this type of concrete and make it more environmentally friendly. Here, we have looked for a reference mix design based on local ordinary aggregates, and then, we introduced the recycled fillers from ceramic waste and granulated blast furnace slag into our mix design, and we studied their effects on the properties of the fresh state (density, workability, and air content) and in the hardened state, namely compressive and flexural tensile strength, modulus of elasticity, ultrasonic and sclerometer resistance, as well as durability tests (capillary and immersion absorption, porosity, and chemical resistance). The microstructure analysis was carried out using scanning electron microscopy, complemented by energy-dispersive X-ray spectroscopy. Comparison of the results of UHPFRC using ceramic waste and slag fillers with the control UHPFRC after 28 days shows an increase in compressive strengths of approximately 8 and 7%, respectively, as well as an increase in flexural tensile strengths of 17 and 2%, respectively. In addition, there was a decrease in porosity of 17 and 42%, respectively. The results reveal that it is possible to produce UHPFRC based on local materials and improve its performance, durability, and microstructure with recycled fillers.

Experimental study on bond performance of interface between concrete and UHPFRC incorporating recycled tyre fibers

This paper conducted single-side shear tests on 27 groups specimens with dimensions of 100 mm × 100 mm × 40 mm to investigate the shear bond performance of the interface between concrete and ultra-high performance fiber reinforced concrete (UHPFRC) incorporating recycled tyre fibers, as an eco-friendly and cost-effective material. The effects of concrete strength, interface roughness, interface adhesives, and the substitution rates of recycled tyre fibers in UHPFRC were analyzed by Response Surface Methodology model. The findings revealed the existence of two failure modes, specifically interface failure and concrete failure, and it was observed that the shear load-slip curves exhibited a similar trend across various specimens, characterized by a gradual increase accompanied by an escalating growth rate. The higher concrete strength and interface roughness as well as the lower substitution rates of recycled tyre fibers would enhance the shear bond strength, while interface adhesives had minor effects. Based on the micro-bonding mechanism and experimental data, two theoretical models for shear bond strength and shear load-slip curves were established, demonstrating good agreement between prediction and experimental results. Considering the shear bond strength, the environmental and economic analysis was carried out, and UHPFRC with 25 % substitution rates of recycled tyre fibers exhibited the best benefits.

Shear behavior of pre-damaged RC beams strengthened with UHPFRC-CFRP grid layer

Ultra-high performance fiber reinforced concrete (UHPFRC), incorporating recycled tyre fibers, and carbon fiber-reinforced polymer (CFRP) grid were utilized to strengthen the pre-damaged reinforced concrete beams caused by coupling action of sustained loads and marine environment. The failure modes, characteristic loads, deflection features, and strain behavior of the strengthened beams with various types of UHPFRC, thicknesses of UHPFRC layers, CFRP grid ratios, and CFRP grid layout angles were investigated. The results indicated that they exhibited the typical shear failure, and their characteristic loads increased by 102 %–205 % compared to the undamaged beam. By increasing the thickness of the UHPFRC layer and the CFRP grid ratio, and adopting 45° CFRP layout angle, the cracking stiffness, energy absorption capacity, and shear force sharing ability would be further improved, while adopting the UHPFRC incorporating recycled tyre fibers would decrease the ductility. Moreover, analytical models were developed to calculate the ultimate loads, manifesting reasonable accuracy.

Enhancing the pull-out behavior of ribbed steel bars in CNT-modified UHPFRC using recycled steel fibers from waste tires: a multiscale finite element study

In the current investigation, the effect of recycled steel fibers recovered from waste tires on the pull-out response of ribbed steel bars from carbon nanotube (CNT)-modified ultrahigh performance fiber reinforced concrete (UHPFRC) was considered using the multiscale finite element method (MSFEM). The MSFEM is based on three phases to simulate CNT-modified UHPC, recycled steel fibers (RSFs), and ribbed steel bars. For the first time, a bar ribbed has been simulated to make more realistic assumptions, and RSFs have been distributed in the form of curved cylinders of different lengths and with a random distribution within a concrete matrix. The interaction of the steel bar and the RSFs with the concrete is applied by the cohesive zone model (CZM). After confirming the simulation outcomes with the experimental results, the steel bar pull-out response is investigated using load-slip curves. The impact of the CNT content, RSFs and their aspect ratio on the bond strength of steel bars and CNT-modified UHPFRC was assessed. The results show that using RSFs with a lower aspect ratio (steel microfibers) significantly improves the pull-out characteristics of steel bars from concrete. Accordingly, the proposed MSFEM is considered for simulating the effects of different parameters on the pull-out response of ribbed steel bars from concrete without causing complex, time-consuming, or costly experiments. The results indicated that waste fiber or RSF can be used as a toughening component in CNT-modified ultrahigh-performance concrete and as a replacement for industrial steel fibers.

Enhanced impact resistance of RC beams using various types of high-performance fiber-reinforced cementitious composites

This study investigates the impact resistance of reinforced concrete (RC) beams using various types of high-performance fiber-reinforced cementitious composites (HPFRCCs). A total of ten large-sized RC beams were fabricated and tested under static and impact loading conditions. Four different types of HPFRCCs with 2% by volume of steel, polyvinyl alcohol (PVA), and polyethylene (PE) fibers were considered. Ultra-high-performance fiber-reinforced concrete (UHPFRC) based on steel fibers demonstrated superior compressive and tensile strengths, while high-performance strain-hardening cementitious composite (HP-SHCC) based on PE fibers exhibited the highest strain and energy absorption capacities. The steel-bar reinforced (R-) UHPFRC and HP-SHCC beams showed superior flexural load capacity compared to reinforced normal strength concrete (R-NSC) beams. The highest ductility of 8.02 was found in the R-HP-SHCC beam, almost 5 times higher than that of the R-NSC beam. By drop hammer impact with a kinetic energy of 30 kJ, the R-UHPFRC beam completely failed due to its higher reaction load and lower structural ductility, while other RC beams made of NSC and HPFRCCs withstood the identical impact load. The R-HP-SHCC beam exhibited the best impact resistance, with a 29.1% lower maximum deflection compared to the R-NSC beam, and the largest residual load capacity after the impact damages. The R-HP-SHCC beam, despite being damaged, showed no reduction in flexural stiffness, yet it demonstrated an impressive 5.4% increase in load capacity compared to the undamaged beam.