Pull-out Tests Research Articles

Features of the Reinforcement–Soil Interfacial Effect in Fiber-Reinforced Soil Based on Pullout Tests

To investigate the reinforcement–soil interfacial effects in fiber-reinforced soil, this study developed a novel horizontal pullout tester and conducted pullout tests on coarse polypropylene fibers in plain soil, cemented soil, and fine fiber-reinforced cemented soil. Three soil types were analyzed: low liquid limit clay, high liquid limit clay, and clay sand. The pullout tester proved to be both scientifically robust and efficient. Depending on the soil properties, coarse polypropylene fibers were pulled out intact or fractured. The pullout curves displayed distinct multi-peak patterns, with wavelengths closely linked to the fiber’s intrinsic characteristics. The pullout curve wavelength for plain soil matched the fiber’s intrinsic wavelength, while it was slightly greater in cemented soils. The peak pullout force increased with extended curing periods, higher cement content, more excellent compaction, and the addition of fine polypropylene fibers. Among these factors, compaction had the most significant impact on enhancing the soil–fiber interfacial effect. Friction, cohesion, and fiber interweaving created interlocking effects, inhibiting fiber sliding. Cement hydration processes further deformed the fiber, increasing its friction coefficient and sliding resistance. Hydration products also fill soil voids, improving soil compactness, enlarging the fiber–soil contact area, and enhancing frictional and occlusal forces at the interface.

A Modified Bearing Capacity Model for Inclined Shallow Anchor Cable with Experimental Verification

Most theoretical models of shallow anchor cables do not take the effect of anchor inclination into consideration, which is an important factor influencing load distribution, stress concentration, and failure mechanisms. In this paper, a modified bearing capacity was developed for a single anchor cable, taking the anchor inclination into consideration, based on the principle of limit equilibrium. Then, a series of indoor pull-out tests of single anchors with different inclinations were performed, where the effects of the anchor inclination on the bearing capacity and failure mechanisms were carefully analyzed. The experimental bearing capacities were compared to the predicted data of the proposed modified model, as well as other existing experimental results, aiming to verify the applicability and accuracy. The results show that the bearing capacity increases with decreasing anchor inclination because the vertical component of the force acting on the anchor cable increases. The failure models of the anchor cables, pulled out at different angles, exhibit an asymmetric “inverted trumpet” shape, which is caused by the varying stress distributions around the anchor cable during pull-out. In addition, the bearing capacities of the theory differ very little from the experimental and previous results, with a max error of nearly 10%. This study confirms that the proposed model reliably captures the effects of anchor inclination, providing valuable insights for designing inclined anchors in engineering practice.

Numerical Simulation of Anchor Pullout and Shear Tests Using a Regularized Damage Model

The prediction of the mechanical behavior of anchorage plates in reinforced concrete is crucial for ensuring equipment reliability, particularly in sensitive installations. This paper employs finite element analysis to forecast the experimental outcomes of anchor pullout and shear tests. In this article, a regularized damage model is proposed to simulate the effects of loads transmitted to the concrete by the anchor rod. Specifically, a modified Mazars model is introduced, incorporating an “energetic” regularization in both tension and compression. The model is validated for a single anchor rod under tension and subsequently for a complete anchorage system subjected to both tension and shear forces. Various failure modes, such as concrete cone cracking or steel rupture, are accurately represented, alongside the overall anchorage strength. This approach, thus, faithfully reproduces the mechanical behavior of anchorage plates, ensuring equipment robustness under diverse loading conditions.

The Effect of Adhesive Quantity on Adhesion Quality and Mechanical Characteristics of Woven Kevlar Fabric-Reinforced Laminated Structures

This study investigated the adhesion and mechanical properties of woven fabric-reinforced laminates (FRLs) made with four distinct Kevlar fabrics of varying areal densities (36 g/m2, 60 g/m2, 140 g/m2, and 170 g/m2) under different fabric-to-adhesive weight ratios (1:0.5, 1:1, and 1:1.5) in both the warp and weft directions. A novel aspect of this research lies in our systematic study of the effect of adhesive quantity on FRLs, a topic that has received limited attention despite its critical role in laminate performance. Additionally, the application of a newly developed yarn pullout test alongside the standard T-peel test provides unique insights into the interfacial behavior of laminates. The results show that in lower areal density fabrics (36 g/m2 and 60 g/m2), adhesive quantity minimally affects the pullout and T-peel forces or tear strength, indicating that structural integrity can be maintained with reduced adhesive application. In contrast, higher areal density fabrics (140 g/m2 and 170 g/m2) benefit from an increased adhesive ratio, with a transition from 1:0.5 to 1:1 significantly enhancing the pullout resistance, while further increases to 1:1.5 yielded diminishing returns. Tensile strength remained consistent across all samples, highlighting that it is largely dictated by the inherent properties of the fibers and fabric structure rather than the adhesive. This study concludes that a 1:1 fiber-to-adhesive ratio offers an optimal balance of adhesion quality and mechanical performance for FRLs. By addressing the understudied impact of adhesive quantity on FRLs and introducing the yarn pullout test, this research provides novel and practical guidelines for optimizing FRLs in applications demanding high structural integrity and adaptability under challenging conditions.

Synthetic conduits efficacy in neural repair: a comparative study of dip-coated polycaprolactone and electrospun polycaprolactone/ polyurethane conduits.

Peripheral nerve injuries represent the most common type of nervous system injuries, resulting in 5 million injuries per year. Current gold standard, autografts, still carry several limitations, including the inappropriate type, size, and function matches in grafted nerves, lack of autologous donor sites, neuroma formation, and secondary surgery incisions. Polymeric nerve conduits, also known as nerve guides, can help overcome the aforementioned issues that limit nerve recovery and regeneration by reducing tissue fibrosis, misdirection of regenerating axons, and the inability to maintain long- distance axonal growth. Polymer-based double-walled microspheres (DWMS) are designed to locally and in a sustainable fashion deliver bioactive agents. Lysozyme is a natural antimicrobial protein that shares similar physical and chemical properties to glial cell line-derived neurotrophic factor (GDNF), making it an ideal surrogate molecule to evaluate the release kinetics of encapsulated bioagent from polymeric biodegradable microspheres embedded in Polycaprolactone and Polycaprolactone/Polyurethane blend nerve conduits.
Approach: Lysozyme was encapsulated in poly(lactic-co-glycolic acid)/poly(L-lactide) (PLGA/PLLA) double-walled microspheres fabricated through a modified water-oil-water emulsion solvent evaporation method. Lysozyme-loaded DWMS were further embedded in PCL and PCL-PU based nerve guides constructed via polymer dip-coating and electrospinning method respectively. Lysozyme DWMS and nerve guides were imaged using scanning electron microscopy (SEM). Released lysozyme concentration was determined by using a colorimetric micro-BCA protein assay and spectrophotometric quantitation. Tensile and suture pull-out tests were utilized to evaluate the mechanical properties of both dip-coated and electrospun nerve guides, embedded and free of lysozyme DWMS.
Main Results: The study revealed significant distinctions in the lysozyme release profiles, and mechanical properties of the manufactured polymer nerve guides. Both PCL dip-coated and PCL/PU electrospun DWMS-embedded nerve guides revealed biphasic protein release profiles. PCL/PU electrospun and PCL dip-coated nerve guides released 16% and 29% of the total protein concentration within 72 hours, plateauing at week 16 and week 8, respectively. SEM analysis of the nerve guides confirmed the homogeneity and integrity of the polymer nerve guides' structures. The electrospun guides were found to be more flexible with a higher extension under stress bending, while the dip-coated PCL nerve guides displayed more rigid behavior.
Significance: This study provides useful insights on how to optimize nerve guide design and fabrication to enhance recovery progress of peripheral nerve injuries.&#xD.

Behavior of Hybrid Natural Fiber Reinforced Polymers Bars Under Uniaxial Tensile Strength and Pull-Out Loads with UHPC

This study evaluated the uniaxial tensile strength and bond performance of natural hybrid reinforcement bars. Hybrid FRP combines multiple fibers and matrixes, resulting in a desirable performance. Two types of hybrid bars were tested: one with natural fibers surrounded by glass fiber, and the other with a steel core surrounded by natural fiber and then glass fiber. Thirteen samples were used to assess tensile behavior, with four groups including glass fiber, flax, sisal, and jute fibers. Pull-out behavior testing was conducted on twelve samples, divided into four groups of fiberglass, flax, sisal, and jute. Each group used three types of concrete: normal, high strength, and ultra-high-performance concrete (UHPC). The results refer to flax samples that had a higher tensile strength and elastic modulus of 143MPa and 38GPa, respectively, than samples made of sisal and jute fibres. The hybrid bars with a steel core exhibited a significant improvement in elastic modulus of 206% in compared to samples made solely from glass, sisal, and jute fibers. On the other hand, the samples with UHPC showed the highest bond strength. The sample U-GFRP with ultra-high-performance concrete showed the highest bond strength 9.32MPa, while the sample N-GFRP with normal concrete showed the lowest bond strength 5.87MPa, respectively. However, this study suggests that hybridizing natural fibers can be a cost-effective and eco-friendly alternative to synthetic fibers.

Tensile behaviour of a novel grouted sleeve splice for high-strength rebars

This study introduces a novel type of grouted sleeve fabricated from a standard low-alloy seamless steel pipe using cold-rolling techniques. The main goal is to achieve an effective connection between the high-strength steel (HSS) reinforcements in the joints of precast concrete structures. To investigate the structural performance of the developed splice under uniaxial tensile loading, 24 coupler specimens configured with 28 mm diameter HRB600 reinforcements were prepared. Two primary parameters, namely the embedded length of the spliced bar and number of inner concentric ribs rolled on the sleeve, were considered. By conducting direct pullout tests, the performance of the developed splices was thoroughly assessed based on the failure mode, bond stiffness, bond ductility, and strength. The test results demonstrated that increasing the bar embedded length and number of inner ribs enhances the initial bond stiffness and ductility of the splice. Increasing the quantity of inner ribs only in the inelastic region of the splice is effective for improving its tensile capacity. For grout splices with large-diameter HSS bars, an embedded length of 8db is recommended to guarantee bar fracture failure and satisfy the specification requirements. This length exceeds that of splices with normal-strength reinforcements. Finally, a formula was proposed to estimate the inelastic region length and tensile strength of the proposed splice.

Enhanced carbon fiber interface with thermoplastics via nanostructure surface modification: failure, morphology and wettability analysis

Improving the fiber-matrix adhesion in thermoplastic composites remains a significant challenge due to the lack of chemical bonding between thermoplastics and common reinforcing fibers. This study investigates the effectiveness of carbon fibers enhanced with nanostructure surface modification for strengthening the interfacial adhesion to thermoplastic matrices. The fiber surface was modified with graphene nanoplatelets (GNP) through the facile coating method, and the apparent interfacial shear strength (IFSS) was determined by the single-fiber pullout test. GNP-coated fiber improved IFSS by 74% with neat high-density polyethylene (HDPE-Neat) and 28% with maleic anhydride-grafted HDPE (HDPE-8MA), while reduced by 27% with polyamide 6 (PA6) due to different failure mechanisms. Morphology, chemical, and wettability analysis were conducted on the nano-enhanced carbon fibers to quantitatively elucidate these findings on micro/nanoscale, combining machine learning-based image segmentation, X-ray photoelectron spectroscopy (XPS), and contact-angle measurements of intermittent beading on fibers.

An organic-inorganic interface structure for CFRP and the enhancement of mechanical properties at room/high temperature

The performance of carbon fiber reinforced polymers (CFRP) depends on various factors, with particular emphasis on the intricate microscopic interface. Through the combined interface modification by nanomaterials (ZrO2) and polyimide (PI), an organic-inorganic interface enhanced structure of CFRP with phenolic resin (PR) have been successfully devised and fabricated. The analysis encompassed the surface morphology, contact angle and surface energy of carbon fibers (CF), aimed at characterizing the interaction of grafted ZrO2/PI on interfacial properties. Single fiber pull-out testing was employed to ascertain both the failure mode of the interface and the interfacial shear strength (IFSS). The results revealed that the surface modification augmented the IFSS by 70 %. The fortified organic-inorganic interface layer resulted in enhancements of 14 % and 15 % in tensile strength and flexural strength, respectively, in comparison to untreated CFRP. Moreover, even under high-temperature condition (300 °C), the tensile properties of modified CFRP exhibited only 22 %–26 % reduction, which demonstrated the advantages of composites in harsh environments. This can be primarily attributed to the strengthened layers of ZrO2/PI, which securely anchored the matrix and reinforcement, thereby mitigating stress concentration in CFRP under extreme conditions.

Performance evaluation of innovative self-healing corrosion protection coatings for prestressing strands

This study presents the design and experimental evaluation of advanced corrosion protection coatings for application on prestressing strands which are the core constituents of prestressed concrete structures such as bridges. Variety of self-heal coatings embodying corrective and protective phenomena in response to the degrading effects of corrosion have been designed and tested in simulated aggressive weathering conditions. Standard 7-wire prestressing strands coated with self-heal epoxy, self-heal toughened epoxy and hybrid epoxy coating systems were subjected to salt fog spray up to a duration of 2500 h, and 3M CalCl2, 3M NaOH, saturated Ca(OH)2 solutions and distilled water up to 45 days duration. Furthermore, rust creepage of the coated prestressing strands was measured after extended exposure to aggressive corrosive environment. Bond strength of the self-heal epoxy coated prestressing strands was evaluated through pullout test using high-strength concrete.Significant improvement in corrosion resistance, hydrolytic stability, marked reduction in rust creepage, and improved bond strength, brought about by the innovative self-heal epoxy coatings were recorded in laboratory tests. Furthermore, electrochemical impedance spectroscopy (EIS) data proved excellent corrosion protection qualities of self-healing coatings. The ATR-FTIR spectra of various self-heal epoxy coating systems and optic microscopic images of the coatings further verified these findings.

Mechanical properties of a novel fiber-bridging interface for connecting FRP profile and concrete

The connection between FRP profile and concrete is critical for structural performance. This study introduces a novel fiber-bridging interface to enhance the FRP-concrete interfacial behavior. The interface comprises an epoxy resin adhesive layer, a carbon fabric layer, a mixture of adhesive and sand layer, and U-shaped steel fibers. Central pull-out tests were conducted to investigate the mechanical performance of this novel interface. The investigated variables included bond length and fiber volume fraction. Test results indicate that all specimens failed in a brittle mode at the adhesive layer, with load plateauing after reaching the peak. The number of steel fibers had limited influence on the interfacial behavior. Based on the load-slip curves, a bond stress-slip model for the tangential behavior was developed. An interfacial expansion model was further developed by means of FE analysis and machine learning. The three most widely used machine learning models, i.e., the BP neural network model, the random forest model, and the XGBoost model were selected. Comparisons show that all three models provide reasonable predictions, with the XGBoost model demonstrating the best performance. These models for the tangential and normal behavior of FRP-concrete interface were implemented into FE models for numerical analysis. Comparisons between numerical and experimental results show that the proposed models accurately describe the interfacial behavior of the fiber-bridging interface under brittle failure mode. The innovative interface proposed in this paper can be used for connecting concrete and FRP in various scenarios, and the proposed methodology of calibrating local bond behavior parameters from global response offers a new approach for establishing interfacial bond models.

Effects of self-expandable pedicle screws with shape memory alloy structures on spinal fixation strength: A finite element study.

In many spine surgeries, pedicle screws are commonly used to stabilize vertebrae, however, loosening can be a complication. Different designs have shown improvements in fixation strength, with self-expandable screws featuring shape memory alloy (SMA) structures being of particular interest. This study aimed to assess the fixation strength of self-expandable pedicle screws made with SMA (specifically Nickel-Titanium) sheets. Three types of screws were evaluated: self-expandable screws with a smooth SMA surface, self-expandable screws with a porous SMA surface, and standard design screws. Each screw underwent pullout tests for comparison. Following the tests, the self-expandable screw with a porous surface exhibited the highest pullout force (1141.83 N), compared to 1056.86 N for the smooth self-expandable screw and 1104.25 N for the standard screw. The dissipated plastic strain energy differed among the screws, with values of 0.073 J for the porous self-expandable screw, 0.065 J for the smooth self-expandable screw, and 0.089 J for the standard pedicle screw. Notably, the porous self-expandable screw showed reduced stress on the bone-screw interface. Improving the mechanical design of pedicle screws could significantly enhance screw-bone fixation strength. The utilization of self-expandable pedicle screws with porous surface SMA sheets demonstrates superior performance, potentially mitigating complications like loosening.

Investigation on bond behavior of GFRP bar embedded in ultra-high performance polyoxymethylene fiber reinforced concrete

The combination of glass fiber reinforced polymer (GFRP) bars and polyoxymethylene (POM) fiber reinforced ultra-high performance concrete (UHPC) can create a structural system with excellent service performance and exceptional durability. Bond is a critical factor affecting the service performance of structures constructed using GFRP bars and POM fiber reinforced UHPC. Therefore, this paper investigates the bond behavior between GFRP bar and POM fiber reinforced UHPC by using a direct pull-out test. The test variables were reinforcement type, volume fraction of POM fiber, embedment length, diameter of reinforcement, thickness of concrete cover, and compressive strength of concrete. The effects of these test variables on the failure mode, bond stress-slip curve and bond strength of GFRP bar with POM fiber reinforced UHPC were discussed. The test results revealed that the volume fraction of POM fibers and the reinforcement diameter have barely effect on the bond strength. The bond strength initially increases with increasing concrete cover thickness, and then plateaus after the concrete cover thickness reaches 3.35 times the reinforcement diameter. Furthermore, a predictive model for bond strength of GFRP reinforced UHPC specimens was proposed and validated using test data from other literature. A general bond-slip constitutive model was established to accurately describe the bond-slip relationship of GFRP bar and UHPC.

Coated sisal/PET hybrid braided textiles interface behavior into cementitious matrix

ABSTRACT Intra-ply hybridization of two different braided yarns can enhance flexural and shear strength compared to conventional yarn properties. Hybrid braids offer the potential to reduce costs and boost industrial productivity. An impregnated textile with resin coating exhibits more cohesive behavior, ensuring stress distribution throughout the yarns and preventing a reduction in the composite’s ultimate tensile capacity due to telescopic failure. This study aimed to develop new coated hybrid braided textiles made from sisal/PET for reinforcement employment in cementitious composites and to evaluate their bonding effect with the matrix through pullout tests. Materials were selected for tensile strength properties, thickness, and bonding characteristics. The braided fibers were chosen based on their strength and elongation (549N and 29.2%) and diameter (2.41 mm). They were coated with green epoxy resin and subjected to the pullout method for further evaluation. The results indicated that the best sample of the developed sisal/PET braided textile comprised 43% sisal and 57% PET, due to its higher elongation (29.17%), tenacity (216 cN/Tex), and tensile strength (549 N). The pullout properties demonstrated high energy absorption, reaching 2100.0 N.mm, which is 2.8 times higher than steel, exhibiting a pseudo-ductile behavior. This indicates its potential for use in cementitious composite reinforcement.

Machine learning prediction of interfacial bond strength of FRP bars with different surface characteristics to concrete

Fiber-reinforced polymer (FRP) bars have been implemented in civil infrastructures as internal reinforcement. The bond strength of FRP bars to concrete depends on the surface characteristics considerably. This study used machine learning (ML) techniques to explore the influence of bar surface types on the bond properties quantitatively. A database including 158 FRP bars-concrete pull-out testing results was compiled. The geometric factors, including rib spacing (wc), rib width (wf) and rib height (rh), were proposed to quantify the surface configurations of FRP bars. Twelve ML models were trained to predict the interfacial bond strength. The ML models demonstrated higher accuracy in predicting the bond strength compared to eight existing equations from the literature. Among these models, CatBoost exhibited the highest accuracy, with an RMSE 58.3 % lower than the most accurate existing equation. CatBoost was utilized in parametric research on the influencing factors, and demonstrated that wf had the highest weight contribution to interfacial bond strength. Additionally, this study effectively combines ML models with physical meaning-driven analysis methods, resulting in a practical and interpretable equation for calculating FRP bars-concrete interfacial bond strength.

Bond and tensile properties of flax textile reinforced recycled aggregate concrete: Strategies for interfacial enhancement and corresponding mechanisms

This study investigated the bond properties between flax fibre reinforced polymer (FFRP) mesh and various cementitious matrices, examining the impact of incorporating medium-sized recycled aggregates (2 – 8 mm). Enhancement strategies included modifying the FFRP surface with rice husk ash (RHA) and incorporating RHA into the cementitious matrix. Pull-out and uniaxial tensile tests were performed on FFRP reinforced mortar, FFRP reinforced recycled aggregate concrete (RAC), RAC reinforced with RHA-modified FFRP and RAC (containing RHA) reinforced with RHA-modified FFRP. The results showed that FFRP surface modification with RHA significantly improved bond strength, by 32 % compared to pulling normal FFRP from RAC. This was attributed to increased surface roughness of FFRP, enhanced chemical bonding due to pozzolanic reaction of RHA, and improved interfacial transition zone resulting from increased wettability and decreased water absorption ratio of the treated FFRP. Additionally, the inclusion of medium-sized recycled aggregates mitigated debonding by enhancing mechanical interlocking with the FFRP mesh. Compared to FFRP reinforced mortar, FFRP reinforced RAC and RHA-modified FFRP reinforced RAC exhibited increased strain capacity and refined crack propagation, indicating improved stress distribution. However, the enhanced anchoring reduced tensile strength by restricting the full utilisation of strain capacity in the post-cracking phase. These findings highlight the potential of surface modification of reinforcement with supplementary cementitious materials and the incorporation of medium recycled aggregates in the matrix to enhance bond performance and optimise the mechanical behaviour of cementitious composites.

Large deformation simulation of uprooting of trees with complex root system architectures using material point method with embedded truss elements

Abstract Background and aims Urban trees in coastal cities like Hong Kong may suffer from an uprooting failure when subjected to extreme winds. A proper numerical model for tree uprooting simulation can help to select tree species or soil types that better resist uprooting failure. However, modeling tree uprooting is challenging as it is a cross-disciplinary problem involving complex root system architectures (RSAs) and large deformation of both roots and soils. This study aims to develop a hybrid numerical model that combines truss elements and material point method (MPM) to simulate the entire large-deformation uprooting process of trees with complex RSAs. Methods The tree uprooting model is developed by coupling truss elements in finite element method (FEM) with MPM. Laboratory pull-out tests using artificial roots and real root cuttings are adopted to validate the developed model. A comparative study is performed to investigate the difference between using complex and simplified RSAs in tree uprooting simulations. Results The developed model provides consistent predictions of peak load, critical displacement and failure mode when compared with results from laboratory tests. Moreover, the comparative study shows that the uprooting resistance obtained with a complex RSA is higher than that with a simplified RSA. The difference varies with the soil and root mechanical properties. Conclusion The developed hybrid model offers a novel way for simulating an entire tree uprooting process involving large deformations and complex RSAs. The study shows that using a simplified RSA to approximate the complex RSA might result in misleading failure modes.

Optimizing superelastic shape-memory alloy fibers for enhancing the pullout performance in engineered cementitious composites

Abstract This study explores the effect of integrated superelastic shape-memory alloy fibers (SMAFs) on the mechanical performance of engineered cementitious composites (ECCs). Various SMAF configurations – linear-shaped SMAFs (LS-SMAFs), hook-shaped SMAFs (HS-SMAFs), and indented-shaped SMAFs (IS-SMAFs) – with diameters of 0.8 and 1.0 mm were incorporated into ECC matrices, and surface texturization was achieved through abrasive paper treatment. Their mechanical properties were assessed through single fiber pullout tests on ECC mixtures containing 1.5 and 2.0% polyvinyl alcohol (PVA), subjected to both monotonic and cyclic loading conditions. Qualitative analysis, employing scanning electron microscopy, demonstrated that the IS-SMAF configuration provided superior mechanical interlocking and fiber–matrix adhesion, with a distinct flag shape observed during tensile testing. Quantitative data indicated that IS-SMAFs significantly improved the tensile strength and pullout resistance, with slip distances of ≥5 mm and average pullout loads ranging from 263 to 403 N. LS-SMAFs demonstrated better performance compared to HS-SMAFs and LS-SMAFs in terms of tensile and pullout characteristics. Additionally, ECCs with increased PVA content exhibited enhanced withdrawal performance. Thermogravimetry analysis and X-ray diffraction provided insights into the high-temperature stability and crystalline structure of the composites. These results underscore the effectiveness of IS-SMAFs in enhancing ECC properties, offering significant implications for the development and optimization of high-performance composite materials in civil engineering applications.

Performance augmentation of fiber reinforced concrete through in situ mineralization of polycrystalline calcium carbonate on fiber surfaces

Enhancing the performance of fiber-reinforced concrete through the meticulous regulation of interfacial microstructure and interaction mode stands as a pivotal area of research. Notably, there are significant differences in the microstructure of several polycrystalline forms of calcium carbonate, which can profoundly influence the interaction behavior between them and the matrix. However, the tailored modulation of calcium carbonate polymorphism for the purpose of fiber surface modification remains unreported. In this paper, polycrystalline mineralization was induced by polydopamine on the surface of polyvinyl alcohol fiber by biomimetic method, and the fiber surface was modified by calcite and aragonite. The cubic calcite and acicular aragonite minerals notably roughened the fiber surface, enhancing interfacial properties between PVA fibers and cement matrix. The damaged form of the interface changed from adhesion failure to cohesive failure. Quantitative assessment of fiber-matrix interfacial interactions via single-fiber pullout tests revealed aragonite's unique morphology and exceptional mechanical attributes yielding higher frictional resistance against pullout loads. The good bonding between calcite and the cement matrix improves the strain-hardening behavior of fiber pullout and significantly enhances energy dissipation. In addition, enhanced interfacial properties bolster composites' mechanical strength. The acicular and cubic mineralized layers increased the flexural strength of the fiber cementitious materials by 35 % and 41 %, respectively. The energy absorbed in resisting the impact of a falling ball increased by 25 % and 36 %, respectively. Analysis reveals calcite promotes hydration more significantly at comparable particle sizes, bolstering interfacial bond strength with cement, and offering superior reinforcement over aragonite for fiber matrix bridging. This research provides a theoretical basis for promoting the application of polycrystalline CaCO3 and the sustainable development of high-performance fiber-reinforced concrete.

Hybrid fibre-reinforced cementitious composites with short polyethylene and continue carbon fibres: Influence of roving impregnation on tensile and cracking behaviour

Hybrid externally-bonded reinforcements are considered a viable technique for strengthening existing concrete structures. They combine high-performance impregnated textiles with matrices containing dispersed microfibres to foster the ductility and toughness of the composite system. In this paper, the mechanical performance of textile-reinforced strain-hardening cement-based composites (TR-SHCC) is investigated in detail. A novel high-performance inorganic binder based on limestone calcined clay cement (LC3) is reinforced with both polyethylene (PE) dispersed microfibres and carbon fibre (CF) textiles as continuous biaxial reinforcement. The CF yarns are impregnated by an automated process to improve the monolithic response under uniaxial tensile loading and to ensure high production consistency. Fully inorganic suspensions, i.e., geopolymer and cement-based, are being investigated, as they can provide superior thermal stability compared to traditional polymeric impregnating agents. Interphase adhesion is investigated by single-yarn pull-out tests, microscopy and µCT at various micro scales. On the one hand, the improved adhesion promoted by cement impregnation resulted in the finest and most diffuse crack pattern. Conversely, the strength of the overall composite is mainly governed by the tensile failure of the yarns, irrespective of the bond, and dispersed fibres consistently improve the post-cracking stage and the strength of the hybrid composites.