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Article

Influence of Coated Steel Fibers on Mechanical Properties of UHPC Considering Graphene Oxide, Nano-Aluminum Oxide, and Nano-Calcium Carbonate

by
Seyed Sina Mousavi
1,*,
Khatereh Ahmadi
1,
Mehdi Dehestani
1 and
Jung Heum Yeon
2
1
Department of Civil Engineering, Babol Noshirvani University of Technology, P.O. Box 484, Babol 47148-71167, Iran
2
Civil Engineering Program, Ingram School of Engineering, Texas State University, San Marcos, TX 78666, USA
*
Author to whom correspondence should be addressed.
Fibers 2025, 13(4), 37; https://doi.org/10.3390/fib13040037
Submission received: 28 January 2025 / Revised: 2 March 2025 / Accepted: 26 March 2025 / Published: 29 March 2025
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Abstract

:

Highlights

What are the main findings?
  • The use of poly(vinyl alcohol) as a coupling agent proved to be effective in uniformly coating graphene oxide, nano-aluminum oxide, and nano-calcium carbonate on the surfaces of micro steel fibers within the UHPC mixture;
  • Applying a coating of graphene oxide, nano-aluminum oxide, and nano-calcium carbonate to the surface of micro steel fibers significantly improved the mechanical characteristics of UHPC.
What is the implication of the main finding?
  • Utilizing coated micro steel fibers presents a promising method to reduce the high volume fractions of fibers in UHPC mixtures while maintaining comparable, or even superior, mechanical properties, thereby enhancing the workability of UHPC;
  • The suggested coating technique greatly decreased the thickness of the ITZ surrounding the fibers, especially for the coating that contains nano-calcium carbonate, which addresses the durability concerns associated with UHPC.

Abstract

The addition of high volume fractions of fibers in ultra-high-performance concrete (UHPC) presents specific durability-based challenges due to the high content of interfacial transition zones (ITZ) between the fibers and surrounding mortar, along with the production cost. Hence, this study explored a novel coating approach on the surface of micro steel fibers, considering various nanomaterials, including graphene oxide (GO), nano-aluminum oxide, and nano-calcium carbonate. Poly(vinyl alcohol) (PVA) was employed as a coupling agent. UHPC mixtures containing coated fibers were compared with reference uncoated fiber-reinforced UHPC and UHPC containing GO. The proficiency of the proposed technique was measured through compressive strength, direct tensile, and flexural tests. A microstructure analysis was conducted using scanning electron microscope (SEM) images to determine the ITZ depth surrounding the coated fibers. Findings indicated improvements ranging from 10.7% to 21% for compressive strength, 11.2% to 38% for tensile strength, and 26.6% to 60% for flexural capacity.

1. Introduction

Fibers have been used in various industries, including civil, mechanical, biomedical, textile, aerospace, etc. The low fiber/matrix interfacial bond strength and tensile properties of fibers have been critical for researchers [1,2]. Researchers have proposed various approaches to compensate for some percentage of the bonding reduction [3]. One of the main promising approaches was coating nanomaterials on the surface of fibers to repair or strengthen the fiber’s bonding and mechanical properties [4]. Although extensive experimental works concentrated on the mechanical and environmentally resistant properties of single fibers, very limited studies worked on the coated fiber-reinforced concrete, especially ultra-high performance (HPC) mixtures containing high volume fractions of fibers.
In the field of single fiber works, Gao et al. [5] introduced a nanoscale hybrid coating using styrene–butadiene copolymer with single and multi-walled carbon nanotubes (CNTs) along with nanoclays to enhance the mechanical properties of glass fibers. They found that coated glass fibers had meaningfully enhanced mechanical characteristics and corrosion resistance. Furthermore, they reported that the coating technique improved single glass fibers’ tensile and bending strength. Zhang and Mi [6] used a surface treatment of shape-memory alloy (SMA) fiber using nano-silica. Based on pull-out tests, they found that coating can efficiently promote interfacial shear strength. Wei et al. [7] applied a coating consisting of epoxy and SiO2 nanoparticles onto the basalt fiber. They reported that the addition of the nano SiO2–epoxy composite coating considerably increased the tensile strength along with interfacial properties of the basalt fibers in the basalt fiber-reinforced resin matrix composite. Siddiqui et al. [8] proposed a healing coating consisting of neat epoxy and CNTs on the brittle surface of glass fibers to control the stress concentrations and accordingly to advance the reinforcing efficiency. They found that using the CNT–epoxy nanocomposite coating caused a noteworthy improvement in the tensile strength of single glass fibers. Moreover, they reported that CNTs used in the supplementary epoxy coating improved the influence of crack healing. Siddiqui et al. [9] reported similar results where CNT nanocomposite coatings applied on the glass fiber surface enhanced the fiber tensile strength and the crack healing efficiency. Ma et al. [10] worked on introducing polymer-based coating for glass fibers, consisting of a composite of epoxy, CNTs nanomaterials, and graphene nanoparticles. Their results indicated that the mechanical and electrical properties of single glass fibers were meaningfully improved by using the proposed nanocomposite coating. Furthermore, they noted that applying a coating technique for fiber using plate-like graphene effectively prevented the intrusion of moisture and alkali ions through the coated layer. This resulted in safeguarding the fibers and consequently enhanced their ability to resist environmental damage. Liu et al. [11] applied coatings containing neat vinyl ester and nano clay onto the single fibers against alkali and moisture attack. After their investigation, they found that glass fibers coated with nanomaterials exhibited less reduction in tensile strength following exposure to an alkaline environment. This was attributed to the barrier properties of the coating, specifically the nano clay. Asadi et al. [12] experimentally studied the bond behavior and mechanical characteristics of cellulose nanocrystals (CNC)-coated glass fiber/epoxy composites. They reported an improvement in bond shear strength up to around 69% in coated glass fiber-reinforced composites. Özen et al. [13] carried out experimental investigations into the possible application of jute fibers as natural fibers in concrete mixtures that were treated with a graphene oxide (GO) coating. Their results showed that applying GO to the surfaces of the natural fibers caused greater slump values. Moreover, the mechanical findings demonstrated a notable improvement in flexural strength when both GO-coated and uncoated fibers were included in the mixtures. This enhancement was attributed to the improved interfacial bond between the coated fibers and surrounding matrix. However, their research concluded that the coating did not influence the freeze–thaw resistance of the concrete. Chen and Yu [14] developed a novel surface treatment technique for miscanthus, converting it into a bio-fiber by employing hydrophobic silica aerogel. Their findings revealed that the addition of silica aerogel-treated miscanthus fiber improved mechanical properties along with insulating capabilities. Moreover, incorporating superhydrophobic aerogel particles decreased the leaching of sugar from the miscanthus, and the peak hydration heat is merely marginally delayed when aerogel-coated miscanthus fibers are utilized. Xin et al. [15] employed a nanocomposite based on epoxy resin reinforced with CNTs as a surface coating for recycled GFRP fibers. They discovered that this coating magnificently prevented the interfacial interaction between the fibers and the alkaline conditions in the mortar. Consequently, it diminished the corrosion of alkali ions on the fibers, which improved their resistance to alkalinity. Han et al. [16] conducted experimental tests to encourage the formation of polycrystalline mineralization of polydopamine on PVA fiber surfaces using a biomimetic technique. They observed that the fiber surface was effectively altered with calcite and aragonite. Their findings suggested that the cubic calcite and acicular aragonite minerals notably increased the roughness of the fiber surface, thereby improving the interfacial properties between the PVA fibers and the cement matrix. Moreover, they noted that the presence of both acicular and cubic mineralized layers enhanced the flexural strength of the fiber-reinforced cementitious materials by 35% and 41%, respectively. You et al. [17] utilized a GO coating on the surface of recycled GFRP fibers to mitigate the decreases in compressive and tensile strength commonly seen with untreated recycled GFRP fibers. Their results demonstrated that, although the GO coating had a slight effect on the fluidity and autogenous shrinkage of ultra-high-performance fiber reinforced concrete (UHPFRC), it significantly improved both compressive strength and flexural strength, achieving enhancements of as much as 6.6% and 32.4%, respectively. Because of the insufficient interfacial transition zone (ITZ) associated with natural fibers in cement-based composites, Zhao et al. [18] introduced a surface treatment for sisal fibers that includes a silane coupling agent along with SiO2 nanoparticles. They showed that this treatment established a hydrophobic barrier on the fiber surface, which minimizes water absorption and improves resistance to Ca2+ penetration. Their results revealed that the interfacial bond strength of the treated sisal fibers increased by 54.55–62.32%, a change attributed to the enhanced dimensional stability offered by the hydrophobic silane coupling agent and the improved chemical bonding resulting from the pozzolanic activity of SiO2 nanoparticles. Du et al. [19] suggested using cellulose nanocrystals (CNCs) as a coating for steel fibers to enhance and reinforce the weaknesses found in the ITZ surrounding the fibers embedded within ultra-high-performance concrete (UHPC). They noted improvements in flexural strength and toughness of UHPC with CNCs-coated steel fibers, enhancing by up to 14% and 18%, respectively. These enhancements can likely be linked to the densification of the ITZ. Deng and Li [20] investigated how zinc phosphate-coated steel fibers affect the corrosion behavior of concrete. Their findings indicated that the suggested fiber coating method not only enhanced corrosion resistance but also boosted bond strength by 19.3%. Similar coating approaches were studied by literature using various coupling agents, nanomaterials, and fiber types [21,22,23,24,25]. A primary issue highlighted in these earlier studies was that they relied solely on a limited range of chemical and mechanical tests to evaluate the performance of coated fiber-reinforced specimens, whereas non-destructive inspection methods could effectively assess the porosity surrounding the fibers [26,27].
Although coating has been severally studied in the field of cementitious composites and concrete [28,29], only limited works concentrated on the influence of coated fibers on various characteristics of cementitious composites. In this field, Pi et al. [30] employed nano-SiO2-coated steel fiber to develop the bonding strength between the fiber and surrounding matrix within a cement-based composite through a sol-gel modification method. They considered an 8% solvent mass fraction of ethyl orthosilicate (TEOS) along with a 1% solvent mass fraction of ionic surfactant cetyltrimethylammonium bromide in the coating procedure. They reported that coated steel fiber using SiO2 developed the ITZ microstructure, particularly the region surrounding fibers, resulting in higher fiber/matrix bond strength. Zhang et al. [31] used grafting graphite oxide (GO) on the surface of basalt fiber to improve its bonding strength with the surrounding cement matrix. They found that coating using grafting GO can improve hydrophilicity along with the surface roughness of BF fibers. Their findings discovered that coated fibers had greater interfacial shear strength along with higher cement stone compressive strength (up to of 26.91%). They also showed that using coated BF fibers caused around 11% reductions in the porosity of the specimens. Ahmadi et al. [4] used polyvinyl alcohol (PVA) powder for playing the role of coupling agent to bond GO on the surface of micro steel fibers embedded in 3D printable concrete mixtures considering various coating approaches. Their experimental observations showed that using this novel coating technique resulted in 14.2% and 21.2% greater compressive and tensile strengths, respectively, while a comparable result was found for flexural strength. They also demonstrated that using coated steel fibers caused a 73.7% reduction in crack width. They also reported that using coated fibers enhanced the self-healing capabilities and strength, regaining up to 43.9% and 34.3% improvement in flexural and tensile strengths, respectively. Musale et al. [32] proposed a novel coating technique to cover the surface of treated clustered coir fiber embedded within a cement composite beam. They used epoxy resin to attach CNTs on the surface of the coir fiber. Their findings revealed that using the proposed coated coir fiber increased the bond capacity with the surrounding matrix, so the addition of 0.1 wt.% epoxy/CNTs coated coir fiber improved the flexural strength along with beam deformations.
Although GO was severally used as filler or coating material within cementitious composite that researchers have extensively studied [4,33,34,35,36,37,38], very few studies concentrated on using nano-aluminum oxide and nano-calcium carbonate as coating materials in cementitious composites. In the case of nano-aluminum oxide, Muzenski et al. [39] investigated the effects of aluminum oxide nanomaterials in mortars made from oil well cement. They discovered that incorporating 0.25% aluminum oxide led to a remarkable 30% enhancement in compressive strength. Experimental findings presented by Younus et al. [40] indicated that incorporating nano-alumina into the alkali-activated self-compacting concrete (SCC) at an optimal dosage of 1.0–1.5% improved mechanical properties. Similarly, Moghaddam et al. [41] demonstrated that the inclusion of aluminum oxide nanoparticles alongside glass fibers in SCC resulted in an enhancement of mechanical properties ranging from 61% to 107.6% for the optimal dosage of 2.0% aluminum oxide nanomaterials. Concerning nano-calcium carbonate, previous studies indicated that using nano-calcium carbonate can enhance the hydration process due to its nucleation effect and chemical influence [42]. Sldozian et al. [43] demonstrated that using a mixture of 4% nano-CaCO3 as a replacement for a percentage of the weight of cement resulted in the highest compressive and splitting tensile strength. Experimental observations presented by Salih et al. [44] revealed that incorporating 1% nano-CaCO3 into mixtures with water-to-cement ratios of 0.35 and 0.45 enhanced the compressive strength of the cement by 40% and 50%, respectively, by minimizing the size of large pores. Li et al. [45] investigated the effects of incorporating different percentages of nano-CaCO3 into autoclaved concrete mixtures. Their findings showed that an optimal level of 3% nano-CaCO3 enhances filling and accelerating effects, improves the generation of cement hydration products, reduces porosity, and refines the micropores in autoclaved concrete. Similar results were reported for nano-calcium carbonate-containing concrete with higher mechanical properties as compared to the normal mixtures [46,47,48,49]. After a thorough review, it is evident that while there has been significant research on the use of nano-aluminum oxide and nano-calcium carbonate in concrete mixtures, there are no targeted studies that investigate the potential and impact of employing these nanomaterials as coatings to rectify surface imperfections of fibers in concrete and enhance the microstructure of the ITZ at the fiber/matrix junction. Therefore, this study introduces the novel use of nano-aluminum oxide and nano-calcium carbonate as nanocoating materials for steel fibers for the first time.
Although researchers have carried out critical studies on coating fibers’ surface to enhance ITZ surrounding the fibers and reducing porosity, the area of coated fibers needs additional focus to fill current research gaps related to various concrete types, fiber varieties, different coupling agents, and a range of nanomaterials. Furthermore, coated fibers have considerable potential for application in UHPC to minimize the fiber volume fraction. Using a high fiber volume can create difficulties regarding the fresh properties of UHPC and may impair its durability due to the reduced ITZ quality surrounding the fibers. Additionally, using UHPC in areas with dense steel requires a specific level of workability, which a high volume fraction of fibers cannot adequately provide. More investigation is needed to develop methods that decrease the fiber volume fraction in UHPC while sustaining similar mechanical and durability properties. Accordingly, thorough experimental work was conducted in this research aimed at achieving the following specific goals:
  • What is the effectiveness of the coating technique in improving the mechanical characteristics of HPC?
  • How does the type of coating materials impact the efficiency of coated fibers embedded in UHPC?
  • What influence do coated fibers have on controlling the thickness of ITZ at the fiber/matrix interaction?
To follow these aims, extensive experimental work was considered in the present investigation, including various types of micro and nanomaterials for coating, including GO, nano-aluminum oxide, and nano-calcium carbonate. Different mechanical tests were conducted to examine the efficiency of the proposed coating technique, such as compressive, direct tension, and flexural tests. UHPC mixture with 2.0% micro steel fiber was considered as the reference concrete. Regarding coupling agents, polyvinyl alcohol (PVA) powder was used to bond micro and nanomaterials onto the surface of steel fibers. This coupling agent was efficiently proposed by Ahmadi et al. [4] for coated steel fibers embedded in 3D printable concrete. The present study also used scanning electron microscope (SEM) images prepared from the fiber/matrix interfacial zone to measure the thickness.

2. Experimental Program

2.1. Materials, Mixtures, and Methods

Along with cement with a density of 3.1 gr/cm3, ground granulated blast-furnace slag (GGBFS) and silica fume (SF) were used in UHPC mixtures. The chemical composition of the powders is mentioned in Table 1. Particle size distribution curves of silica sand used as fine aggregate are illustrated in Figure 1, which is much finer than similar sand considered by Ahmadi et al. [4]. Micro steel fiber with a length of 16 mm, an aspect ratio of 64, a tensile strength of 2720 MPa, and a volume fraction of 2.0% were used in the present study (Table 2). A sand-to-binder ratio (s/b) of 0.50 and water-to-binder (w/b) ratio of 0.19 were selected for the UHPC mixture. As mentioned in Table 3, GGBFS and SF with around 45% and 10% of binder weight were considered for the UHPC mixture, respectively.
As stated in Table 4, different commercial micro and nanomaterials were used for the coating technique, including GO with a particle size of 2–7 µm, nano-calcium carbonate with a particle size of 15–40 µm, and nano-aluminum oxide with 50–80 µm particle size. GO also had an average flake thickness of less than 2 nm. Other details of micro and nanomaterials are listed in Table 4. As a coupling agent to bond micro and nanomaterials onto the surface of steel fibers, PVA powder was also utilized during the coating procedure, which has the chemical formula of (C2H4O)n, a 1.19 g/cm3 density, and a 450 °C ignition temperature. As mentioned in Table 5, five different specimen types were prepared for the present study, including (1) UHPC mixture without using coated fibers and materials; (2) UHPC-GO containing uncoated steel fibers along with utilizing GO as fillers within mixtures; (3) UHPC-NGO mixture reinforced with coated fibers produced from GO/PVA composite; (4) UHPC-NCaCO3 mixture reinforced with coated fibers produced from nano-calcium carbonate/PVA nanocomposite; and (5) UHPC-NAl2O3 mixture reinforced with coated fibers produced from nano-aluminum oxide/PVA nanocomposite. Among the studied mixtures, only the “UHPC-GO” mixture used GO within the mixture without coating the fibers to compare the results with others. Generally, there are two separate scenarios of using micro and nanomaterials in cementitious composites or concrete, including (A) using micro and nanomaterials directly within the mixture, acting as a nucleus and nanofilling to generate an enhanced C-S-H pattern and organized crystallization; and (B) considering the same behavior but locating at the fiber/matrix interfacial zones to control the ITZ thickness and weakness. Accordingly, to consider and compare both mechanisms, the mixture “UHPC-GO” was added to the experimental program, where 0.03% GO (by weight of cement) was used in the UHPC mixture, recommended by the literature [33,34]. The coating approach was followed, similar to the recent paper published by the authors [4]. Several steps were followed during the coating process in this study, as illustrated schematically in Figure 2. Initially, micro steel fibers were cleaned using a mixture of 96% ethanol and distilled water. After cleaning, the fibers were dried in an oven maintained at 80 °C. Concurrently, a separate preparation was conducted for the polyvinyl alcohol (PVA) as a coating agent and the dispersion of coating materials (micro and nanoscales). PVA powder (5.0 g) was dissolved in water (500 mL) at 90 °C for 4 h while being stirred at a speed of 700 rpm. At the same time, the micro and nanomaterials (1.04 g of GO, 1.5 g of CaCO3, and 1.8 g of Al2O3) were dispersed in water (200 mL) using an ultrasonic probe for 30 min until a completely homogeneous mixture was achieved. The different coating materials were attributed to the varying densities outlined in Table 4. The dosage of GO used in the “UHPC-GO” mixture and coated on the fibers within the “UHPC-NGO” was identical (0.03% by weight of cement) to allow for a comparison of the efficiency of GO’s presence in the matrix versus surrounding the fibers. This comparison aimed to evaluate which scenario better enhances the nucleation influence of GO particles. To maintain a similar volume of coating material dispersed in the water, 1.5 g of CaCO3 and 1.8 g of Al2O3 were used instead of 1.04 g (equivalent to the GO dosage). After that, the micro and nanomaterials were stirred using a magnetic stirrer for 10 min to ensure dispersion and prevent settling and then slowly added to the prepared PVA solution. The PVA and dispersed nanomaterial solution were continuously stirred on a magnetic stirrer for 30 min until the mixture was completely uniform. Subsequently, the organized solution was poured within a horizontal container, including the dried fibers. The container with the final combination of solution and fibers was then placed in an oven at 80 °C for 24 h to perform the coating process. The same method was followed for coating Al2O3 nanomaterials as for GO nanomaterials. For coating CaCO3 nanomaterials on the surface of microfibers using PVA, the steps were similar to those described above, with the only difference being that the CaCO3 nanomaterials were dispersed in methanol solution using an ultrasonic probe and then added to the prepared PVA solution.

2.2. Test Setups

Similar to the approach followed by the literature [4,33,50], three different mechanical tests were selected in the current experimental investigation to determine how much the coating and nanomaterials can affect the hardened properties of UHPC. Compressive strength, direct tensile, and flexural are the main tests performed in the present study, as shown in Figure 3. To assess the compressive strength, we used three 50 mm cubes for each mixture. We measured the direct tensile strength of UHPC using briquette-type specimens that adhered to the standards of AASHTO T 132, as well as ASTM documents C307 and C190. The tensile specimen used was bone-shaped, with a length of 76.2 mm, a thickness of 25.4 mm, and a cross-section of 645 mm2 at its midpoint. Additionally, we conducted a three-point bending test on UHPC specimens that had been cured for 28 days in a 40 × 40 × 160 mm prism mold. The UHPC samples were cured under controlled conditions, ensuring a relative humidity of 95% and a steady temperature of 23 °C. The UHPC samples were fully submerged in water for a prolonged curing duration of 56 days to ensure ideal hydration and strength development, which is crucial for evaluating their mechanical properties. For both the direct tensile and bending tests, we employed a displacement rate of 1.27 mm/min. Three repetitions were considered for each test, so the average value along with the standard deviation was reported in the present study. In addition to conducting mechanical tests, SEM images were included for each UHPC series to investigate how coating and nanomaterials can influence the ITZ thickness. For the SEM analysis, a series of precise steps were meticulously followed. Small specimens were carefully extracted from the internal regions of the UHPC samples to ensure a representative evaluation of the fiber–matrix interface. These specimens included fibers embedded within the surrounding cement-based matrix. The collected samples were then sliced into smaller, manageable pieces and thoroughly cleaned to eliminate any loose particles or contaminants. To enhance conductivity and improve the quality of SEM imaging, the samples were coated with a thin layer of gold using a sputter coater. This procedure was crucial in preventing charging effects during the SEM analysis, thereby ensuring the clear and accurate imaging of the non-conductive cementitious matrix. Subsequently, the gold-coated samples were examined using a scanning electron microscope, capturing high-resolution images to investigate the microstructure, with particular emphasis on the fiber–matrix interface and the overall morphology of the UHPC.

3. Results

3.1. Compressive Strength

The compressive strength of all the UHPC mixtures is illustrated in Figure 4. Generally, the overall results indicate that using nanomaterials and coated fibers improved the compressive strength of UHPC specimens, showing that a practical reduction in the fiber content can be applied without changing the compressive strength. This could efficiently reduce the cost of UHPC and help provide proper fresh properties. The findings showed that using 0.03% GO within the UHPC mixture caused a +22.6% increase in compressive strength, which can be attributed to the microstructural enhancement due to the nanofilling and nucleus effects of nanoparticles dispersed within the matrix. It causes a densified C-S-H microstructure along with refined hardened interlocking at the nan level. Similarly, previous work confirmed that using 0.03% of Go improved the hardened properties of cementitious composites. Bhojaraju et al. [34] noted that employing 0.03% of GO within mixture of cementitious composites with w/c = 0.35 resulted in about a +6.1% improvement in compressive strength. Wu et al. [51] reported that using 0.02–0.08% GO nanosheets caused around a 12.8–34.1% enhancement in the compressive strength of concrete with w/c = 0.50.
A comparison between the compressive strength of the UHPC-GO mixture with previous studies [34,51,52,53,54,55,56,57] is shown in Figure 5. Generally, the comparison analysis indicates improvement percentages ranging from 5.0% to around 85%. The dosage of GO used in the mixture, dispersion techniques considered in these studies, w/c ratio of concrete composition, various types of binders and SCMs used in the mix, and different kinds of GO commercial products can significantly affect the improvement percentage. Details of literature that used coating for fiber-reinforced cementitious composites are summarized in Table 6. As illustrated in Figure 5, the comparison analysis shows that the compressive strength improvement percentage of UHPC-GO is in the range of values reported by Lv et al. [54] with w/c = 0.37 and Devi and Khan [57] with w/c = 0.45–0.50. Compressive strength results of UHPC mixtures containing coated fibers are shown in Figure 4. Regarding GO, findings revealed that although GO-coated fibers caused a +18% increase in compressive strength, its effect is slightly lower (−3.8%) than using GO directly within the mixture. Among the coating materials used in the coating process, the results of the present study showed that nano CaCO3 has a comparable effect with GO on compressive strength. Also, the UHPC-NCaCO3 mixture has +2.6% and +8.9% higher compressive strength than the UHPC-NGO and UHPC-NAl2O3 mixtures, respectively. Generally, all the coated fibers used in the UHPC mixture caused a higher compressive strength.
A comparison between the results of coating fibers and the literature is shown in Figure 6. Nano-SiO2 and GO are the nanomaterials used in previous studies. Steel and basalt fibers are the only fiber types considered in these investigations. Compressive strength enhancement in the 4.4–14.2% range was observed in these experimental works. Growth of compressive strength (%) was selected to perform this comparison. The analysis shows that the nano-SiO2 coating reported by Pi et al. [30] has the lowest improvement in compressive strength compared to other nanomaterials considered for coating. Similar to the present experimental work, the previous studies reported by Ahmadi et al. [4] and Zhang et al. [31] also confirmed that the GO-coating has a lower compressive strength improvement as compared to the CaCO3-coating, while performing better than the Al2O3-coating. Moreover, the comparison results showed that the GO-coating used in the UHPC mixture performed better than mixtures considered by Ahmadi et al. [4] and Zhang et al. [31], showing that using mineral fillers along with SCMs can considerably affect the efficiency of the GO-coating technique. It is important to note that, for the purpose of comparison, the mechanical properties of the samples with coated fibers were evaluated against those with uncoated fibers, and the parameters of “strength growth” used in Figure 6 were taken from each referenced study.

3.2. Tensile Strength

Direct tensile strength results and normalized tensile strength are depicted in Figure 7. The results show that using GO nanomaterials directly within the mixtures resulted in a +33.8% increase in the direct tensile strength. Among the coated techniques, the findings revealed that +38.5%, +24.6%, and +10.8% improvements were found for GO-, CaCO3-, and Al2O3-coating, respectively, showing that the GO-coating has the highest performance among other techniques. Comparing GO results indicates that both using directly within the UHPC mixture (as nanopowders) or coating on the fiber’s surface caused almost comparable tensile strength. Among all the UHPC mixtures, the results showed that the Al2O3-coating had the lowest improvement. Direct tensile strength was also normalized with the square root of compressive strength to precisely compare the normalized tensile results (dashed line in Figure 7). A comparable tendency was also detected for the normalized tensile strength, where the GO-coating technique has the highest normalized tensile capacity with +19%, +12.5, and +20.9% higher normalized tensile strength as compared to UHPC-GO, UHPC-NCaCO3, and UHPC-NAl2O3, respectively. A comparison between the tensile strength growth of the present study regarding the direct tensile strength of coated-steel fiber-reinforced UHPC and previous investigations is illustrated in Figure 8. Only the GO-coating was considered for the direct tensile strength test by the literature, with the improvement percentage ranging from 5.1% to 21.2%. The comparison analysis shows that GO- and CaCO3-coating techniques perform better than the previous studies, while a comparable and lower trend was found for the Al2O3-coating technique, especially as compared to work conducted by Ahmadi et al. [4].

3.3. Flexural Strength

Flexural capacity, along with the normalized flexural force of all the UHPC mixtures, is shown in Figure 9. Findings discovered that the addition of GO powders within UHPC mixtures resulted in a +36.7 improvement in flexural capacity, while some deviation in repetitions of results was observed for this mixture. Although using GO-coating fibers caused a +26.6% development in the flexural capacity, UHPC-NGO specimens have a slightly lower flexural strength (−7.4%) than UHPC-GO ones. Among the coated specimens, UHPC-NAl2O3 samples showed the highest flexural capacity, which differs from the compressive and tensile strength results. Regarding the CaCO3-coating technique, although a lower strength was observed for direct tensile strength (Figure 7), a considerably greater flexural capacity (+18.4%) was found as compared to the GO-coating technique (Figure 9). The flexural results were also normalized with the square root of compressive strength, denoted as normalized flexural capacity (dashed line in Figure 9). A similar trend was observed for the normalized flexural capacity, where Al2O3-coated specimens had a +29.5% and +10.8% higher normalized flexural capacity as compared to the GO-coating and Al2O3-coating techniques, respectively.
A comparison between flexural strength improvement reported by previous investigation and the present study is described in Figure 10. It can be deduced from the comparison results that all the coated-UHPC specimens of the present study have significantly higher flexural strength growth; a range of 0.0–15.3% improvements were reported by the literature, which is considerably lower than the range of 26.6–60.1% found by the present study. It seems that the coating technique, especially PVA as a coupling agent, performs better in UHPC mixtures than other types of concrete or cementitious composites. Another comparison analysis between different mechanical tests is conducted in the present study, presented in Figure 11. Generally, findings showed that the concrete compressive strength was less affected than other mechanical properties. Also, the flexural strength was highly influenced by the coating technique, where the highest deviation with the trend lines of other mechanical properties is illustrated in Figure 11. Regarding the addition of GO powder dispersed in a UHPC mixture, the analysis shows that tensile and flexural strengths affected better as compared to the compressive strength. The highest difference between the patterns of mechanical tests was observed for the CaCO3 and Al2O3-coating techniques. The failure of compressive, direct tensile, and flexural UHPC samples is shown in Figure 12a, Figure 12b, and Figure 12c, respectively.

4. Discussion

SEM images of uncoated and coated fibers embedded in UHPC specimens are illustrated in Figure 13. The general view depicts that using uncoated steel fibers in the reference and GO-contained UHPC causes considerable ITZ thickness, as compared to the coated samples. No significant ITZ change was observed for UHPC-GO specimens compared to the reference UHPC sample. It may be because GO powders within the UHPC can only react with hydration products in the matrix to generate additional and supplementary C-S-H without affecting the porosity at the rebar/matrix interface. As shown in Figure 14, it can be seen that using coated fibers promisingly enhances the microstructure of the matrix surrounding the fibers and accordingly reduces the interfacial porosity. This phenomenon was assessed quantitatively through SEM images, which is depicted in Figure 15, from which it can be inferred that the coated UHPC samples had a significantly lower ITZ thickness as compared to other UHPC specimens. Adding GO powders within the UHPC mixture could not reduce the ITZ thickness, so an almost comparable ITZ width was recorded. However, using coated fibers resulted in −69.3%, −91.6%, and −73.8% reductions in the ITZ width of GO-, CaCO3-, and Al2O3-coated UHPC specimens, respectively.
It can be comprehended from the findings that the CaCO3-coating showed the best performance among other nanomaterials used in the present study to reduce fiber/matrix porosity. Along with the thickness of ITZ, the strength of the products filling the porosity plays a major role in the mechanical properties, which can be considered as one of the main leading factors for justifying the results mentioned in Figure 4, Figure 7 and Figure 9. Generally, nanoparticles play a vital part in optimizing the structure of C-S-H and improving the microstructure of both the matrix and the multifaceted ITZ. These nanoparticles have the capability to suppress micro-cracks and improve the contact between hydration products, leading to overall improvements in the material’s performance [59,60]. The main mechanism in nanoconcrete is the supplementary consumption of CH and, accordingly, the generation of extra C-S-H [61]. Nanomaterials play another important role as they act as a starting point for forming hydration products directly on their surfaces. This process, known as nucleation locates for C-S-H growth, becomes increasingly significant as the size of the particles decreases [62]. This mechanism may be the primary reason behind the presence of additional C-S-H in coated fibers, where the formation of this compound can be specifically observed in the ITZ. In the current study, it was observed in Figure 16 that the use of PVA and coating techniques contributed to the presence of nanoparticles. These nanoparticles play a crucial role in filling the damaged surface of fibers and promoting the proximity of hydration products to the fiber surface, thus reducing the thickness of the ITZ. In simpler terms, the well-dispersed nanoparticles on the fiber surface facilitate the nucleation and production of C-S-H in the voids, leading to a more uniform and denser ITZ zone. Moreover, the presence of aluminates in the pore solution of the matrix containing certain nanoparticles allows for the formation of a complex 3-D bonding scheme, potentially enhancing the overall strength through the substitution of silicon or calcium in C-S-H to form calcium–aluminate–silicate–hydrate (C-A-S-H) gel.
The main mechanism of coating techniques observed in the present study is schematically illustrated in Figure 17. It can be observed that each nanomaterial used in the coating technique can produce specific types of hydration products for fulfilling the matrix/fiber interfacial zones. Generally, using nanomaterials within cementitious composites can significantly impact its structural properties. Nanomaterials have the potential to create tiny pores within the cement matrix, which can decrease porosity and enhance the structural organization of calcium hydroxide (CH), as can be seen in Figure 17 [63]. In particular, adding CaCO3 nanomaterials alters the hydration reaction with C3A in cement clinker, resulting in enhanced microhardness and elastic modulus of the early hydration products [42]. An investigation by Meng et al. [63] showed that increasing the content of calcium carbonate nanomaterials initially increased and then decreased the consumption of C3S and C2S. This resulted in the formation of more ettringite and less CH, resulting in a decrease in the amount of C–S–H gel in the cement paste. The observed trend in Figure 15 suggests that the high activity of nano CaCO3, owing to its large specific surface area and rapid reaction with Ca(OH)2, contributes to these promising outcomes. Regarding GO powders, extensive discussion was explained for mixtures containing GO powders [33,34], which can be extended to the observations found in the present study. As the GO content increased gradually, there was a relatively high consumption of C3S and C2S, and this consumption increased gradually. A significant amount of ettringite was formed as a result. Regarding Al2O3 nanomaterials, in a study by Joshaghani et al. [64], it was found that nano Al2O3 particles played a critical part in improving the properties of the mix. These particles effectively filled the pores in the ITZ, thereby decreasing porosity and refining the overall structure of the mix. Additionally, as the hydration reactions progressed, the aluminum from Al2O3 nanoparticles was incorporated into the C-S-H and C-A-S-H gel phases. This incorporation contributed significantly to the development of compressive strength in the material. In the study by Leon et al. [65], it was noted that incorporating nano-Al2O3 into cement mortars resulted in changes to the distribution of pore sizes. Specifically, there was a reduction in the quantity of large capillary pores and a rise in the number of medium-sized capillary pores. In specimens containing nano Al2O3, researchers observed a significant number of portlandite crystals taking the form of plates or displaying hexagonal morphology. Their findings suggested that the presence of Al2O3 led to a lower gel/total portlandite relationship, indicating that Al2O3 nanomaterials may accelerate early-age hydration reactions without impacting the pozzolanic reaction of silica. Additionally, the study revealed that mortars with Al2O3 nanomaterials exhibited a smaller relationship between C-S-H gel loss and Ca(OH)2 loss compared to the reference specimen. This suggests that gels shaped in the existence of alumina experience stoichiometric modifications lacking consuming the portlandite generated by any subsequent reaction. It is worth mentioning that in some nanoconcrete investigations, it was found that the particle size of nanomaterials can affect the findings [66]. As mentioned in Table 4, nanomaterials used in the present experimental program had different ranges of particle sizes, which can be the main reason for some findings. In their research, Oltulu and Sahin [67] used Al2O3 particles with a small size of 13 nm, while Leon et al. [65] worked with Al2O3 particles ranging from 260 to 550 nm. This supports the findings of Gaitero et al. [68], who argue that the beneficial effects of nanoparticles are more pronounced in colloidal dispersions than in dry forms. Additionally, Li et al. [69] noted that high surface energy nano-alumina can speed up the cement hydration process through actively contributing to the hydration reaction along with secondary hydration. This results in a more even distribution of hydration products and a denser overall structure. Accordingly, in alignment with the results of the present study and the suggestions from earlier research, the main mechanism of the experimental observation can be listed as follows (Figure 17): (1) UHPC-NGO specimens contain C-S-H crystals, portlandite, and Afm at the tip of layers located at ITZ zones; (2) UHPC-NCaCO3 contains C-S-H crystals, portlandite, Aft, and C-A-S-H; and (3) UHPC-NAl2O3 contains C-S-H crystals, portlandite, and C-A-S-H gel.

5. Conclusions

In this study, an innovative experimental program was conducted to investigate how using coated fiber affects the mechanical properties of UHPC. Micro steel fibers were coated employing PVA as coupling agent and three various types of materials, including GO, nano alumina, and nano calcium carbonate. Compression, direct tensile, and flexural tests were conducted in the present study. After analyzing the results of the experiments, we can draw the following conclusions:
-
Coating techniques showed very promising findings concerning compressive strength; so, using GO, nano calcium carbonate, and nano alumina caused +18.0%, 21.1%, and 11.2% improvements in the compressive strength of UHPC, respectively. Using PVA as a coupling agent showed better performance in placing homogenous nanomaterials on the surface of steel fibers;
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Among coating techniques, findings showed that GO/PVA composite had the best performance regarding direct tensile strength with about a +38.5% enhancement compared to the reference UHPC;
-
In the case of the flexural test, the results indicated that using nano alumina/PVA nanocomposite coating on the steel fibers resulted in a +60.0% increase in bending strength, which is the optimum coating among the studied nanomaterials. Moreover, the proposed coating technique best influences flexural strength among the mechanical properties;
-
The SEM analysis of coated fibers showed that all the proposed coating techniques significantly reduced the ITZ thickness surrounding the fibers, which was considerably better than using nanomaterials dispersing within the matrix. Among the nanomaterials used, nano calcium carbonate showed the best performance with 91.6% reductions in porosity width at the fiber/matrix interface.
The present study highlights the need for further experimental research to build upon and broaden the existing findings by using new nanomaterials, new coupling agents, different types of fibers, various generations of concrete composition, and chemically testing the dispersion of nanomaterials on the surface of fibers. Moreover, the tensile strain capacity of UHPC represents a crucial factor that warrants thorough investigation, particularly in relation to the application of fiber coatings. It is essential for future work to assess whether the implementation of these coatings can effectively improve the tensile capacity of UHPC. Furthermore, large-scale UHPC beams may be fabricated by future investigations both with and without coated fibers to assess the extent to which the coating method influences load–displacement curves, post-cracking behavior, and the fracture energy of the samples. Additionally, it is advisable for future studies to evaluate the environmental aspects of the proposed coating methods through life cycle assessment (LCA) and to include a cost comparison.

Author Contributions

Conceptualization, S.S.M., K.A. and M.D.; methodology, S.S.M., K.A. and M.D.; validation, S.S.M., K.A., M.D. and J.H.Y.; formal analysis, S.S.M. and K.A.; investigation, S.S.M. and K.A.; data curation, S.S.M., K.A., M.D. and J.H.Y.; writing—original draft preparation, S.S.M. and K.A.; writing—review and editing, S.S.M., K.A., M.D. and J.H.Y.; visualization, S.S.M. and K.A.; supervision, M.D.; and project administration, M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data supporting the findings of this study are available upon simple request from the corresponding author.

Acknowledgments

The authors thank the concrete laboratory of Babol Noshirvani University of Technology in Iran for their support throughout the experimental work conducted in the present study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Particle size distribution curves for silica sand used in the present UHPC mixtures [4].
Figure 1. Particle size distribution curves for silica sand used in the present UHPC mixtures [4].
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Figure 2. Schematic representation of the coating technique proposed in the present study.
Figure 2. Schematic representation of the coating technique proposed in the present study.
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Figure 3. Experimental tests considered in the present study: (a) compressive test; (b) direct tension test; and (c) flexural test.
Figure 3. Experimental tests considered in the present study: (a) compressive test; (b) direct tension test; and (c) flexural test.
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Figure 4. Compressive strength results for various materials used in fiber coating.
Figure 4. Compressive strength results for various materials used in fiber coating.
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Figure 5. Comparing compressive strength of “UHPC-GO” mixture with previous studies [34,52,53,54,55,56,57].
Figure 5. Comparing compressive strength of “UHPC-GO” mixture with previous studies [34,52,53,54,55,56,57].
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Figure 6. Comparing the compressive strength of UHPC mixture containing coated fibers with previous investigations [4,30,31].
Figure 6. Comparing the compressive strength of UHPC mixture containing coated fibers with previous investigations [4,30,31].
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Fibers 13 00037 g007
Figure 7. Direct tensile strength along with normalized results for various nanomaterials used in fiber coating.
Figure 7. Direct tensile strength along with normalized results for various nanomaterials used in fiber coating.
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Fibers 13 00037 g008
Figure 8. Comparing the direct tensile strength with the previous findings for fiber coating using other nanomaterials and fiber types [4,31].
Figure 8. Comparing the direct tensile strength with the previous findings for fiber coating using other nanomaterials and fiber types [4,31].
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Figure 9. Flexural force results for various nanomaterials used in fiber coating.
Figure 9. Flexural force results for various nanomaterials used in fiber coating.
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Figure 10. Comparing the flexural strength with the previous findings for fiber coating using other coating materials and fiber types [4,30,58].
Figure 10. Comparing the flexural strength with the previous findings for fiber coating using other coating materials and fiber types [4,30,58].
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Figure 11. Comparing strength growth of mechanical properties with respect to various nanomaterials used in the fiber coating.
Figure 11. Comparing strength growth of mechanical properties with respect to various nanomaterials used in the fiber coating.
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Figure 12. Failure of samples: (a) compressive test; (b) direct tensile test; and (c) flexural test.
Figure 12. Failure of samples: (a) compressive test; (b) direct tensile test; and (c) flexural test.
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Figure 13. SEM images of mortars containing coated fibers for various nanomaterials: (a) UHPC; (b) UHPC-GO; (c) UHPC-NGO; (d) UHPC-NCaCO3; and (e) UHPC-NAl2O3.
Figure 13. SEM images of mortars containing coated fibers for various nanomaterials: (a) UHPC; (b) UHPC-GO; (c) UHPC-NGO; (d) UHPC-NCaCO3; and (e) UHPC-NAl2O3.
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Figure 14. SEM images of ITZ zones for various coated fibers-contained mortars: (a) UHPC; (b) UHPC-GO; (c) UHPC-NGO; (d) UHPC-NCaCO3; and (e) UHPC-NAl2O3.
Figure 14. SEM images of ITZ zones for various coated fibers-contained mortars: (a) UHPC; (b) UHPC-GO; (c) UHPC-NGO; (d) UHPC-NCaCO3; and (e) UHPC-NAl2O3.
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Figure 15. Measurement of interfacial transition zone (ITZ) width for various nanomaterials (records were obtained from various points of samples along with interactions).
Figure 15. Measurement of interfacial transition zone (ITZ) width for various nanomaterials (records were obtained from various points of samples along with interactions).
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Figure 16. SEM images of the surface of GO-coated fibers containing coupling agent along with nanomaterials coated.
Figure 16. SEM images of the surface of GO-coated fibers containing coupling agent along with nanomaterials coated.
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Figure 17. Schematic illustration of the coating mechanism proposed in the present study.
Figure 17. Schematic illustration of the coating mechanism proposed in the present study.
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Table 1. Chemical composition of powders.
Table 1. Chemical composition of powders.
CompoundOPCGGBSSF
SiO220.7334.9285.62
CaO46.2130.650.96
Al2O33.019.480.90
Fe2O32.470.553.63
MgO1.026.301.79
Na2O0.360.610.94
K2O0.9970.962.30
TiO20.170.550.07
MnO0.0631.0570.24
P2O50.160.0010.127
LOI21.8N3.01
Table 2. Physical properties of micro steel fiber.
Table 2. Physical properties of micro steel fiber.
ShapeMaterialLength (mm)Aspect Ratio (L/d)Tensile Strength (MPa)Modulus of Elasticity (GPa)
StraightHigh-carbon steel16642720210
Table 3. Composition of UHPC mixture (kg/m3).
Table 3. Composition of UHPC mixture (kg/m3).
CementSFGGBFSSilica Sandw/b RatioMicro Steel Fiber
(Volume Fraction)
5801315886520.192.0%
Table 4. Chemical and physical properties of coating materials.
Table 4. Chemical and physical properties of coating materials.
Coating MaterialsMorphologyMolecular Mass (g/mol)Density
(g/cm3)		Average Flake ThicknessParticle Size
graphene oxide (GO)Sheet4.242.1Less than 2 nm2–7 µm
nano-calcium
carbonate		Cubic or
hexagonal		100.092.93-15–40 nm
nano-aluminum
oxide		Semi-sphere101.963.65-50–80 nm
Table 5. Mixture identification.
Table 5. Mixture identification.
MixCoating for FiberCoupling Agent for Coating TechniqueFillers Used Within the Mixture
UHPC---
UHPC-GO--Graphene oxide (GO)
UHPC-NGOGraphene oxide
(GO)		Poly(vinyl alcohol)-
UHPC-NCaCO3Nano-calcium
carbonate		Poly(vinyl alcohol)-
UHPC-NAl2O3Nano-aluminum
oxide		Poly(vinyl alcohol)-
Table 6. Details of literature used coating for fiber-reinforced cementitious composites.
Table 6. Details of literature used coating for fiber-reinforced cementitious composites.
ReferenceFiber TypeCoating MaterialsConcrete Typew/p Ratio
Signorini et al. [58]Polypropylene draw-wireSilica coatingFiber-reinforced concrete0.20
Pi et al. [30]Micro steel fibersNano-SiO2Steel fiber-reinforced cement-based composite0.35
Ahmadi et al. [4]Micro steel fibersGraphene oxide (GO)3D-printable concrete0.42
Zhang et al. [31]Basalt fiberGraphene oxide (GO)Oil well cement slurry0.44
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MDPI and ACS Style

Mousavi, S.S.; Ahmadi, K.; Dehestani, M.; Yeon, J.H. Influence of Coated Steel Fibers on Mechanical Properties of UHPC Considering Graphene Oxide, Nano-Aluminum Oxide, and Nano-Calcium Carbonate. Fibers 2025, 13, 37. https://doi.org/10.3390/fib13040037

AMA Style

Mousavi SS, Ahmadi K, Dehestani M, Yeon JH. Influence of Coated Steel Fibers on Mechanical Properties of UHPC Considering Graphene Oxide, Nano-Aluminum Oxide, and Nano-Calcium Carbonate. Fibers. 2025; 13(4):37. https://doi.org/10.3390/fib13040037

Chicago/Turabian Style

Mousavi, Seyed Sina, Khatereh Ahmadi, Mehdi Dehestani, and Jung Heum Yeon. 2025. "Influence of Coated Steel Fibers on Mechanical Properties of UHPC Considering Graphene Oxide, Nano-Aluminum Oxide, and Nano-Calcium Carbonate" Fibers 13, no. 4: 37. https://doi.org/10.3390/fib13040037

APA Style

Mousavi, S. S., Ahmadi, K., Dehestani, M., & Yeon, J. H. (2025). Influence of Coated Steel Fibers on Mechanical Properties of UHPC Considering Graphene Oxide, Nano-Aluminum Oxide, and Nano-Calcium Carbonate. Fibers, 13(4), 37. https://doi.org/10.3390/fib13040037

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