Single Fiber Pull-out Test Research Articles

Effect of expansive agent on the mechanical properties of carbon-fibre-reinforced cement-based composites

Improving the flexural strength of a cement-based composite (CBC) reinforced with chopped carbon fibre (CF) while maintaining good workability is highly attractive for building high-rise large-span structures. A sulfoaluminate-based expansive agent (EA) was used to optimise the mechanical properties of CBCs reinforced with chopped CF by improving the fibre–matrix interfacial properties. The results of single-fibre pull-out tests showed that the interfacial frictional bond strength was improved by up to 51% owing to the addition of the EA, whereas the chemical debonding energy remained nearly unchanged. The effects of various concentrations of EA on the strength of cement pastes containing various lengths and volume fractions of CF were then investigated. The results indicated that, benefitting from the EA-induced high interfacial bond strength, the flexural strength and fluidity of the CF-reinforced cement paste could be further optimised. For example, utilising the EA, the flexural strength could be improved by 28% for a cement paste containing 0.5% CF of length 15 mm.

Micromechanical behavior of single fiber-asphalt mastic interface: Experimental studies by self-designed innovative pullout test

Micromechanical behavior of the interface between fiber and asphalt is a crucial factor in the performance of fiber reinforced asphalt concrete, and a thorough understanding of this behavior is essential for the design and optimization of these materials. To evaluate the micromechanical behavior of the interface, an innovative single fiber pullout test platform was designed, and the experimental parameter conditions were systematically studied. Three types of fibers including basalt fiber (BF), glass fiber (GF) and polyester fiber (PF) were adopted, and the pullout test was performed at 60 ℃, 25 ℃ and −10 ℃, respectively, where the micromechanical behavior at the interface under different experimental conditions was obtained. The results showed that the recommended loading rate of the fiber pullout test is 1–3 mm/min, and the measurement of variability is satisfactory. The single fiber pullout behavior was observed and the load-displacement curve was recorded real-time. Three failure modes (fiber pullout, fiber fracture and matrix failure) at the interface between the single fiber and asphalt mastic were observed. Furthermore, a bond-slip model which can accurately characterize the bonding-debonding behavior of the interface between the single fiber and asphalt mastic was established. It was found from the study that micromechanical behavior of the interface between the single fiber and asphalt mastic is influenced by the type of fiber and temperature, the peak load and displacement are significantly different. These findings presented in this study can play a significant role in revealing the reinforcement mechanism of fiber and providing a guidance for the efficient application and development of fiber reinforced asphalt mixture.

Effect of Fiber Sizing Levels on the Mechanical Properties of Carbon Fiber-Reinforced Thermoset Composites.

Fiber sizing is one of the most important components in manufacturing composites by affecting mechanical properties, including strength and stiffness. The sizing of manmade fibers offers many advantages, such as improving fiber/matrix adhesion and bonding properties, protecting fiber surfaces from damage during the processing and weaving stages, and enhancing the surface wettability of polymer matrices. In this work, the influence of fiber sizing levels on carbon fibers' (CFs) mechanical properties is reported at room temperature using single fiber tensile testing (Favimat+), single fiber pullout testing (SFPO), and interfacial elemental analysis by X-ray photoelectron spectroscopy (XPS). Standard modulus CFs (7 ± 0.2 μm in diameter) were sized using two commercially available Michelman sizing formulations. The average solid content for each sizing formulation was 26.3 ± 0.2% and 34.1 ± 0.2%, respectively. HEXION RIMR 135 with curing agent RIMH 137 was used as a model thermoset epoxy matrix during SFPO measurements. A predictive engineering fiber sizing methodology was also developed. Sizing amounts of 0.5, 1, and 2 wt.% on the fiber surface were achieved for both sizing formulations. For each fiber size level, 50 single-fiber tensile testing experiments and 20 single-fiber pull-out tests were conducted. The ultimate tensile strength (σult) of the carbon fibers and the interfacial shear strength (τapp) of the single fiber composite were analyzed. The sizing levels' effect on interfacial shear stress and the O/C (Oxygen/Carbon) surface composition ratio was investigated. Based on our experimental findings, an increase of 6% in fiber performance was recorded for ultimate tensile and interfacial shear strengths. As a result, generalized fiber sizing and characterization methods were established. These developed methods can be used to characterize the strength and interfacial shear strength of manmade fibers with different sizing formulations and solid contents irrespective of the matrix, i.e., thermoset or thermoplastic.

Development of low carbon engineered cementitious composite (ECC) using nano lime calcined clay cement (nLC3) based matrix

The construction industry is responsible for around 5% of total CO2 emissions globally. As a result, recent research is focused on the sustainability aspects of construction materials. Engineered cementitious Composites (ECC) are high-performance fiber reinforced composites with improved ductility and tensile strength but utilize higher cement content, raising concerns about their sustainability. In this study, nano-lime and calcined clay are combined with cement to develop a high strength sustainable nano lime calcined clay cement (nLC3) based Engineered Cementitious Composite (ECC). Initially, the packing density model was employed to develop a high strength nLC3-based matrix by combining the particle packing models and chemical compatibility while keeping costs to a minimum. Micromechanical modeling was then applied to determine the critical volume of fibers for the novel matrix using single fiber pullout tests and matrix toughness tests. According to the micromechanical model, the critical volume fraction is found to be 1.93% for the nLC3 mix. This was further confirmed by casting dog bone samples containing 2% fibers by volume which confirmed strain hardening response. The uniaxial test results indicated a strength of around 5.85 MPa in tension and 51 MPa in compression with about 3% tensile strain. These results are comparable to conventional ECC and better than previously developed LC3 based ECC. This study reveals that nLC3-based ECC is a more sustainable composite as compared to conventional ECC mix without any compromise on strength and cost.

Peridynamic study of fatigue failure of engineered cementitious composites

Fatigue crack propagation is crucial in studying fatigue in engineered cementitious composites (ECC). This paper established a peridynamics (PD)-based fatigue crack propagation model for ECC, incorporating the “cycle broken bonds number” to enhance calculation efficiency. First, the PD fatigue crack propagation model was constructed, and the validity of the model was verified by simulating the fatigue crack propagation path of an inclined edge crack in the plate. Secondly, an improved PMB model considering the strain softening stage was used to model the ECC cement matrix, and the fibers were modeled as one-dimensional cyclic dynamic rods to establish a PD fully discrete model of ECC. At the same time, an interface exponential friction attenuation model was introduced. Then, the effectiveness and validness of the fully discrete model were verified by simulating single fiber pull-out tests and uniaxial tensile tests of ECC specimens. Finally, the fatigue crack propagation forms of ECC beams with mid span Type I cracks under three-point bending fatigue loads were studied, and the effects of fiber volume fraction, stress level, and fiber length on the crack propagation forms and structural deflection of ECC beams under four-point bending fatigue loads were investigated.

Development of Engineered Cementitious Composites/Strain-hardening Cementitious Composites (ECC/SHCC) with waste granite fine powders

In this study, a sustainable Engineered Cementitious Composites/Strain-hardening Cementitious Composite (ECC/SHCC) mixture with the replacement of standard sand (SS) by an industrial waste material granite fine (GF) was developed. A thorough evaluation of mechanical properties, including compressive strength, tensile strength, and matrix toughness, was conducted to examine the feasibility and potential of the newly developed GF-ECC/SHCC. The introduction of GF is critical to enhance the tensile performance of ECC/SHCC, revealing a significant advancement over the existing formulations. Additionally, the intricate interplay between the rheological and mechanical properties has been revealed in this research, effectively linking material flow behavior to its tensile performance. By characterizing the morphology and microstructure of raw materials and hydration products, this study illustrates GF’s complex impacts at both macro and micro levels. The single-fiber pullout test and the fiber-bridging constitutive model of ECC/SHCC were applied to evaluate the effect of GF on the fiber-bridging behavior of the developed GF-ECC/SHCC mixtures. Although with up to 10% matrix compressive strength loss and higher superplasticizer dosages, the replacement of SS by GF efficiently improved the strain-hardening capacity and the ultimate tensile strength of the mixtures. This critical insight confirms GF is an effective substitute for SS in ECC/SHCC materials, ultimately enhancing performance across various parameters.

Micromechanical Characterization of Continuous Fiber Date Palm Composites

ABSTRACT Date palm fiber composites with aligned, continuous fibers and epoxy matrix were prepared and mechanically characterized using micromechanical models. A hydrothermal process with microwave heating was utilized to extract the date palm frond (midrib) fibers, making the process eco-friendly as opposed to the commonly reported chemical extraction method. Single fibers were then characterized and found to have an average elastic modulus and ultimate tensile strength of 17.1 ± 1.5 GPa and 333 ± 27.4 MPa, respectively. Interfacial shear strengths ranging between 6 and 12 MPa were determined using a single-fiber pull-out test. Fibers were aligned to make uniaxial tensile samples with volume fraction ranging from 11% to 56%, elastic modulus ranging from 1.72 GPa to 9.52 GPa and ultimate tensile strengths ranging from 16.9 MPa to 43.8 MPa. All the results were found to obey “Rule of Mixtures” theory. The composite’s mechanical properties are similar to or exceeding those of the commercially available wood products such as medium density fiberboard, oriented strand board, thick particle board, hard fiber board, and plywood, making the hydrothermal approach with microwave heating as a promising approach for preparation of date palm composites and further opens up similar process pathway for other natural fibers.

A 3D cohesive‐frictional coupled interface model for mesoscale simulation of steel fibre‐reinforced concrete

AbstractA 3D mesoscale finite element modelling approach is developed for simulating complicated damage and fracture behaviour in steel fibre‐reinforced concrete (SFRC) with explicit modelling of fibre–matrix interfaces. In this approach, a new 3D four‐noded cohesive‐frictional coupled interface element is developed to model the nonlinear interfacial bond‐slip behaviour, supplemented by a kinematic multiple‐point‐constraint (kMPC) algorithm to simulate the wrapping effect of the mortar around the fibres. They are implemented as a user‐defined element (UEL) and a user‐defined MPC subroutine in ABAQUS, respectively. Three cohesive‐frictional constitutive relationships are proposed to describe the nonlinear bond‐slip behaviour of different fibre–matrix compositions. The new approach is validated by single fibre pullout tests, direct tensile tests and three‐point bending tests of SFRC specimens with randomly distributed fibres. The results show that the new approach is capable of effectively capturing typical failure mechanisms in SFRC, such as fibre yielding, matrix failure, and fibre–matrix debonding and slipping.

Pull-out behaviour of steel fibres embedded in ultra-high-performance concrete after exposure to high temperatures

The paper examined the bond-slip behaviour of steel fibres embedded in ultra-high-performance concrete (UHPC) after exposure to elevated temperatures. Different types of steel fibres were cast into the halved UHPC dog-bone-shaped samples, including straight fibres, three types of hooked-end fibres and corrugated fibres. The bond response was investigated through single fibre pull-out tests on the samples unheated and heated to 200, 400 and 600 °C. In addition, the effects of polypropylene (PP) fibre addition and heating rate on steel fibre’s bond behaviour were also investigated. The results revealed an increase in bond strength when the straight and hooked-end fibre samples were heated to 200 °C, while the bond strength of the corrugated fibre decreased at the same temperature. Bond strength reductions were observed at higher temperatures. In addition, the straight fibres led to the lowest pull-out resistance as both the hooked-end and the corrugated fibres significantly enhanced the bond strength. With the hooked-end or corrugated fibres, the failure mode gradually shifted from pull-out failure to fibre rupture. Besides, adding PP fibres increased the bond strength of straight fibres at 400 °C but reduced the bond strengths of hooked-end fibres at ambient and high temperatures. High heating rate also led to a greater bond strength for the straight fibres at 200 °C and 400 °C.

Studies on interfacial shear strength of pineapple leaf fibre from agro-waste reinforced polypropylene composites: Influence of fibre length and carding parameters

This work investigates the potential utilisation of the agro-waste pineapple leaf fibre (PALF) as a green reinforcement in polypropylene (PP) composites. Generally, chemical treatment is used to clean the scratched PALF, and the effluents of those chemicals are discharged into nature. A new mechanical approach (carding) was used in this work to remove the remaining extraneous substances from the scratched and sun-dried PALF instead of using any chemical treatment. The percentage of sticky fibres and dust content decreased, and fibre breakage increased with the increasing number of carding passages. The individualized fibre properties were also influenced by the number of carding passages. The carding process reduced the diameter of PALF due to removal of extraneous materials. The tensile properties of raw PALF improved significantly until the fourth carding passage and further increase in the number of passages did not show any significant influence on the tenacity and modulus. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) confirmed the elimination of hemicellulose along with other gummy and waxy substances from the individualized PALF due to the carding action. The crystallinity % also increased after the carding process due to the removal of amorphous substances. As the reinforcement-matrix interfacial shear strength (IFSS) strongly influences the physico-mechanical properties of composites, IFSS of carded PALF reinforced finer and coarser PP fibre micro-bonded composites was also studied using the single fibre pull-out test (SFPT). Among different composites, the fourth carding passed PALF-reinforced finer PP fibre micro-bonded composite showed the highest IFSS. Additionally, the load- extension curves and SEM images of the tensile pulled-out surface were investigated to study the interfacial adhesion between PALF and PP micro-droplets. The increased surface roughness of PALF due to the carding action resulted in an improvement in the IFSS of PALF-reinforced PP micro-bonded composites.

Pull‐out behavior of polymer fibers in concrete

AbstractThe bond between polymer fibers and the surrounding cementitious matrix is essential for the development of concrete reinforcement. The single fiber pull‐out test (SFPT) is the standard characterization technique for testing the bond strength. However, the different phases of debonding cannot be distinguished by the SFPT. This study investigates the debonding of different polymer fibers from the surrounding cementitious matrix with a modified SFPT and proposes methods to change the SFPT setup to generate more valuable information on the debonding mechanism. The SFPT was equipped with linear variable differential transformers (LVDT), digital image correlation (DIC) and acoustic emission (AE) analysis. The results demonstrate that the modified SFPT allows a better understanding of the different phases of debonding during fiber pull‐out. Furthermore, bond strength values calculated by different methods reveal that the chemical bond of the investigated polymers is not different as reported by previous studies. Deformation measurements performed using LVDTs and DIC are suitable measuring techniques to characterize the debonding mechanism in SFPT. A correlation between recorded AE and debonding phases was not found.

Interfacial strength characteristics between modified iron tailings and profiled fibers under dry–wet and freeze–thaw environments

The interfacial properties of fibers and matrix materials have an important effect on the mechanical properties of fiber-reinforced materials. To study the role of fiber morphology and the environmental effects on the interfacial properties of fibers and matrix materials, the interfacial strength characteristics of undulation polypropylene fiber (UPF) and serrated polypropylene fiber (SPF) with cement-modified iron tailings (CIT) were analyzed through single-fiber pullout tests. The effects of curing age, freeze–thaw cycles, and dry–wet cycles on interfacial strength were analyzed. The energy dissipation during fiber pullout under different environmental conditions was verified, and the microscopic mechanism of interfacial strength change was determined. The results show the following: (1) The interfacial strength of UPF and SPF is positively correlated with curing age, while freeze–thaw and dry–wet cycles can impair their interfacial properties; also, the interfacial strength decreases substantially and tends to flatten out after 3 cycles. (2) The dissipation energy of UPF and SPF increases with curing age, but decreases as the number of dry–wet cycles and freeze–thaw cycles increases. The critical reinforcement length of UPF is better than that of SPF by 363% and 50% under freeze–thaw and dry–wet cycles, respectively. Also, its performance and reinforcement effect in reducing crack development under freeze–thaw and dry–wet environments are better compared to SPF. (3) The interfacial strength of the profiled fibers consists of adhesion and friction. Freeze–thaw cycles cause a significant damage to adhesion and friction, while dry–wet cycles mainly affect adhesion. Interfacial strength is related to the curvature of the fiber and the unconfined compressive strength of CIT, which follows a linear function.

Interface bonding properties of polyvinyl alcohol (PVA) fiber in alkali-activated slag/fly ash

This paper presents an experimental study on the interface bonding properties of polyvinyl alcohol (PVA) fiber in alkali-activated slag/fly ash (AASF) pastes. Three interface bonding properties (i.e., the chemical bonding energy Gd, the initial frictional bond strength τ0, and slip-hardening behavior) were determined using single-fiber pullout tests. The microstructure and chemical composition of the reaction products in the fiber/matrix interfacial transition zone (ITZ) and the nearby matrix were also characterized to reveal the influence of PVA fiber to its surrounding matrix. It is found that Gd increases primarily with increasing Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Unlike that in cementitious materials, the inclusion of PVA fiber in AASF pastes promotes the formation of a high-Ca C-(N-)A-S-H phase rather than crystalline portlandite near the fiber surface. This study provides useful guidance for tailoring the interface bonding properties of AASF and also the development of high-performance composites such as strain-hardening geopolymer composites.

Effect of multi-scale reinforcement on fracture property of ultra-high performance concrete

This paper aims to achieve multi-scale crack control of concrete by using graphene oxide (GO) and macroscopic steel fiber in combination, and to reveal the multi-scale collaborative reinforcement mechanism of GO and steel fiber. A three-point bending fracture mechanics test was employed to analyze the macroscopic fracture properties of ultra-high performance concrete (UHPC) reinforced by GO, and a single fiber pull-out test was utilized to assess the impact of GO on the fiber–matrix interface properties. The results showed that 0.04% GO reinforced UHPC had the best fracture property, and steel fiber and GO showed good synergistic crack control effect. GO directly acted on controlling the cracking of concrete matrix at the nanoscale, thus improving the fracture energy of UHPC matrix by 14% from 73.08 N/mm to 83.39 N/mm. This micro-scale matrix reinforcement provided by GO further improved the pull-out property of steel fibers, and the pull-out energy dissipation of straight steel fibers and hook-end fibers was increased by 43.7% and 31.5%. A micromechanical prediction model for fracture energy of straight steel fiber UHPC was established, which could construct the quantitative relationship between UHPC fracture energy and micromechanical parameters. The model showed that the initial fiber–matrix bond strength (τ0) and slip hardening coefficient (k0) had a powerful influence on the UHPC fracture energy.

Micromechanical study on polypropylene-bicomponent fibers to improve mechanical interlocking for application in strain-hardening cement-based composites

Polyproplylene (PP) fibers find application in strain-hardening cement-based composite to enable the formation of multiple fine cracks for high energy absorption. Often, however, the mechanical performance of such composites suffers from insufficient fiber-matrix interaction. In the research at hand, bicomponent PP single fibers with rough surfaces for improved mechanical interlocking are produced using an IPF (Leibniz-Institut für Polymerforschung Dresden e. V.), an in-house designed and built, laboratory-scale piston fiber spinning device. The melt-spun fibers consist of a shell component composed of PP and various volume percentages of different inorganic particles of calcium carbonate (CaCO3), aluminum oxide (Al2O3), and of a core component made of the same polymer as in the shell. The bicomponent fibers, with shell diameters between 20 and 45 μm, were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to understand their morphology and to study the fiber surfaces after composite failure. Tensile strength and Young's modulus of the fibers were evaluated using tension tests. Furthermore, the single fiber pullout (SFPO) test was used to investigate the interfacial interaction between fiber and a cement-based matrix. Significant improvements in fiber-matrix bonding were achieved due to the rough surface employed in connection with the particles incorporated in the outer shell. Still further, the fiber's strength, attained using an offline step in the drawing process, contributes to enhanced energy adsorption under dynamic pullout loading. To evaluate the performance of the newly developed bicomponent fibers, they were compared to a self-spun monocomponent PP fiber and a commercial PP fiber. This comparison revealed slip-hardening induced by increasing surface roughness for enhanced mechanical interlocking and plastic polymer deformation in the fiber-matrix contact zone.

Strain hardening magnesium-silicate-hydrate composites with extremely low fiber dosage of 0.5% by volume

Formulation of strain hardening cementitious composites typically engage 2% or more fiber by volume, resulting in higher cost and difficult processing. This study presents the development of strain hardening magnesium-silicate-hydrate composite with an extremely low fiber volume fraction of 0.5% via micromechanics-guided design approach. The developed composite demonstrated a tensile strain capacity of 7.2% with a tensile strength of 2.24 MPa, and a compressive strength of 86.1 MPa. The fiber/matrix interfacial bond was characterized using single fiber pullout test. The microstructural characterization of fiber surface and fiber tunnel in the matrix was carried out to understand the fiber/matrix interface properties. The micromechanics-based assessment of critical fiber volume fraction required to achieve strain hardening was also conducted. The material sustainability of the developed composite was evaluated and compared with existing Portland cement-based strain hardening cementitious composites, and strategies to further reduce embodied carbon and primary energy were proposed.

Mechanical strength enhancement of natural fibre composites via localized hybridization with stitch reinforcement

Abstract Stress concentration is an unavoidable phenomenon during the fabrication of composite structures. This research focuses on reducing stress concentration. Circular holes made on the composite structural member induce stress-concentrated regions. These stress-concentrated regions around the circular hole (considered as a weak section) were strengthened by adding synthetic fibre (local hybridization) in the form of fibre stitches (locked loop stitch). Through conducting a single fibre pull-out test, the present study analysed the impact of incorporating synthetic fibres in stitch form and the enhancement of Interfacial Shear Strength (IFSS). The load acting on the broken warp fibres is distributed to the broken weft fibres when the number of concentric stitches increases. The increment of concentric stitches resulted in the increase of IFSS due to localized hybridization. The tensile test results also show a significant improvement with 110 Nmm axial fastening force, from 6.749 MPa (for the natural fibre composite) to 76.91 MPa (for locally hybridized with eight concentric stitches). A simple bolted lap joint with local hybridization around the hole has been evaluated for different clamp-up forces. The best combinations of the clamp-up force and the extent of hybridization have been identified. SEM images illustrate the reduced voids and the reduced fibre pull-out due to the local hybridization around delamination-prone areas.

Effect of moisture content on mechanical behavior of ultra-high toughness cementitious composites

This study presented an investigation into the basic mechanical properties of Ultra-high toughness cementitious composites (UHTCC) under varying moisture levels (0%, 56.5%, and 100%). UHTCC have been recognized for their excellent deformation and crack control abilities, making them ideal for repairing and reinforcing hydraulic structures. In this study, static compressive test and direct tensile test were conducted to determine the influence of free water on compressive strength, tensile stress and strain relationship, elastic modulus and energy absorption ability of UHTCC. Meanwhile, the notched beam fracture test and single fiber pull-out test were also performed to investigate the mechanism behind such phenomenon. The results of the study revealed that as the relative moisture content of UHTCC increased, the compressive and tensile strengths decreased to varying degrees. The softening effect of water on compressive strength of UHTCC was found to be more significant than that on its tensile strength. Furthermore, the direct tensile deformation capacity of UHTCC increased from 2.4% to 4.0% as the relative moisture content increased. Water infiltration into UHTCC was found to have a minimal effect on the number of tensile cracks but significantly increased their width. The results of notched beam fracture and fiber pull-out tests demonstrated that water infiltration led to a 30.4% decrease in matrix fracture toughness, a 37.3% decrease in chemical bonding strength, and a 23.4% decrease in frictional bonding strength between fibers and the matrix. The increased deformation ability of UHTCC in water environments was attributed to the increase in strain hardening energy index (Jb’/Jtip).

INGREDIENTS DEGRADATION IN STEEL FIBER REINFORCED CONCRETE AFTER THERMAL LOADING

The steel fiber reinforced concrete in structures provides good mechanical and physical characteristics and such material applications are increasing. At the same time, if structure is subjected to thermal load it is affecting both properties of reinforcement and matrix as well. The aim of the present research is to investigate the degradation of the characteristics of fibers, randomly distributed in the body of concrete block. A short steel fiber reinforced concrete slabs (400x300x18cm) [1], [2] with homogeneously distributed fibers was undergone to calefaction process from one side. dimension of 16.5x14.5x15 cm) Fiber fracture process was performed based on the single fiber pull-out test by preparing the specimens with fiber from calefacted SFRC block. Experimental curves from each layer were compared with different layer of fibers and investigation had shown that higher load-bearing capacity is there over the thermal loaded fiber. The conclusion were made with the microscopic results and the experimental data.

Rate-sensitive pullout resistance of smooth-steel fibers embedded in ultra-high-performance concrete containing nanoparticles

Single-fiber pullout tests for smooth-steel fibers embedded in ultra-high-performance concrete (UHPC) containing nanoparticles (NPs) were conducted at various pullout speeds (0.0167–500 mm/s). Three types of NPs, namely, nano-CaCO3, nano-SiO2, and nano-CNT, were added to UHPC at dosages between 1 and 3 wt % of cement. The addition of NPs to UHPC notably increased the rate sensitivity of fiber pullout resistance. Among the NPs, the addition of nano-CaCO3 (3 wt %) produced the highest rate sensitivities for the dynamic increase factors (DIFs) of both τeq (1.95) and τpeak (2.06), whereas UHPC without NPs obtained the lowest rate sensitivity of τeq (DIF = 1.18), and UHPC containing 3 wt % nano-CNT had the lowest rate sensitivity of τpeak (DIF = 1.62). Nanoindentation, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests revealed that the higher amount of C–S–H at the fiber–matrix zone produced higher pullout resistance and rate sensitivity.