Dynamic mechanical behavior of UHPC under steam curing: effects of steel fiber

Dynamic mechanical behavior of UHPC under steam curing: effects of steel fiber | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Dynamic mechanical behavior of UHPC under steam curing: effects of steel fiber Yongsheng Zhu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7653888/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Ultra-High Performance Concrete (UHPC) demonstrates significant potential for extreme-load structures due to its ultra-high compressive strength (> 120 MPa) and exceptional durability, yet its inherent brittleness critically limits seismic and blast resistance. This study investigates the influence mechanism of steel fiber content on the dynamic mechanical behavior of steam-cured UHPC. Through Split Hopkinson pressure bar experiments and microscopic pore structure analysis, we reveal the strain rate strengthening effect, the synergistic regulation of fiber content and pore structure, and the impact damage mechanism. Results indicate that the dynamic compressive strength of UHPC increases exponentially with rising strain rates, accompanied by significantly enhanced dynamic increase factor and strain rate sensitivity. When steel fiber content increases from 1% to 2.5%, the dynamic compressive strength improvement rate rises from 12.3% to 13.8%, attributed to fiber bridging that inhibits microcrack propagation and enhances energy dissipation. The fiber content effect exhibits a nonlinear threshold characteristic: At 1% content, the fiber network remains incomplete, resulting in matrix-dominated behavior. At 2% content, an initial crack-blocking network forms, but weak interfacial zones induce large-pore defects (porosity for 10–100 µm pores exceeds that of the low-content group). At 2.5% content, the fiber network density surpasses the percolation threshold. Through combined physical blocking and chemical optimization (gel deposition), total porosity reduces to 2.98%, achieving peak dynamic compressive strength (215.1 MPa) and substantially improved toughness. Steam curing accelerates hydration and refines matrix pores (> 100 µm pores eliminated completely), yet high temperatures embrittle the fiber-matrix interfacial transition zone. The 2.5% fiber content mitigates interfacial weakening by optimizing pore distribution (26.5% reduction in 1–10 µm mesopores), ultimately achieving optimal strength-toughness balance. This study confirms that 2.5% steel fiber content under steam curing represents the optimal mix design, providing critical parameters for impact-resistant structural applications and establishing a theoretical foundation for high-toughness UHPC development. Ultra-high performance concrete Steel fiber content Steam curing Dynamic increase factor Impact damage mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Ultra-High Performance Concrete (UHPC) exhibits exceptional application potential in critical structures subjected to impact loads, such as protective engineering, nuclear power facilities, and military constructions, owing to its ultra-high strength (compressive strength > 120 MPa), superior toughness, and outstanding durability [ 1 ][ 2 ]. However, the highly dense microstructure of UHPC also leads to significant brittleness, making it prone to sudden fracture under dynamic loading. This limitation severely restricts its broader application in seismic and blast-resistant structures. To overcome this drawback, the engineering field commonly employs steel fiber reinforcement technology, which enhances material ductility and energy absorption capacity through the synergistic interaction between fibers and the matrix. Research indicates that the steel fiber content (0%–2.5%) significantly modulates the dynamic mechanical response characteristics of UHPC. Split Hopkinson pressure bar (SHPB) tests reveal that the dynamic compressive strength of UHPC exhibits a marked increase compared to its static value. Both the dynamic increase factor (DIF) and peak stress follow an exponential strengthening trend with increasing strain rate (range: 10¹–10³ s⁻¹), confirming the pronounced strain rate sensitivity of UHPC [ 3 ][ 4 ]. The dynamic compressive strength reaches its maximum enhancement rate at a fiber content of 2.0%. Beyond 2.0%, the strength increase rate diminishes due to fiber clustering effects leading to reduced fluidity and interfacial weakening [ 5 ]. Microscopic analysis reveals that fibers inhibit crack propagation through bridging mechanisms: at a 2.5% fiber content, crack suppression efficiency increases by 60%. Hooked-end fibers significantly improve toughness by reducing the slope of the post-peak stress descent stage by 35%, whereas straight fibers exhibit slightly weaker toughening effects under high strain rates due to a higher risk of debonding [ 6 ]. Furthermore, the influence of fiber content on energy dissipation is nonlinear: energy absorption increases linearly with fiber content in low-dosage groups (≤ 1.5%), but decreases in efficiency for high-dosage groups (2.0%–2.5%) due to uneven fiber distribution [ 7 ]. Wu et al. [ 8 ] through experiments with five curing regimes (standard curing, 60°C/90°C steam curing for 3d/7d) that steam curing temperature and duration are positively correlated with UHPC compressive strength. However, flexural strength and ultimate tensile strain first increase and then decrease, with significant flexural strength degradation observed under 90°C curing for 7 days. Yu Junchao et al. [ 9 ] noted that while steam curing enhances the early-age mechanical properties of UHPC, it can lead to later-age strength retrogression (e.g., a 5%~8% decrease after 28 days), necessitating a balance between curing duration and long-term performance. Steam curing improves early-age mechanical properties by accelerating hydration, refining pores, and optimizing interfaces. Nevertheless, it faces bottlenecks such as later-age strength retrogression, high-temperature interfacial weakening, and durability trade-offs. This study investigates the influence of steam curing methods and steel fiber content on the dynamic properties of UHPC using SHPB tests. The analysis encompasses failure modes, stress-strain curves, the dynamic increase factor for compressive strength, and the strain rate effect. Furthermore, the microstructure of UHPC under these curing regimes was examined via mercury intrusion porosimetry (MIP) tests. 2. Materials and methods 2.1. Materials The study utilized ordinary Portland cement (grade P.O. 52.5), grey powder fly ash, grey powder silica fume, quartz sand with a particle size range of 70–140 mesh, a polycarboxylate superplasticizer with a water reduction rate exceeding 40%, and copper-coated, hooked-end steel fibers (length 13 mm) as the primary constituents. 2.2. Specimen preparation The UHPC specimens were prepared by dry mixing sand, steel fibers, and cementitious materials (cement and silica fume) for 2 minutes, followed by wet mixing with superplasticizer-containing mixing water for 15 minutes to achieve the required workability, then cast into molds of different shapes (100 mm × 100 mm × 100 mm cubes for compressive strength, 100 mm × 100 mm × 400 mm prisms for flexural strength, and Φ100 mm × 50 mm cylinders for SHPB tests), covered with plastic film, demolded after 24 hours of curing at 20°C, and subsequently cured either in a steam curing chamber at 75°C for 6 days or under standard conditions (20 ± 1°C, relative humidity ≥ 95%) for 28 days before undergoing static and dynamic mechanical property tests. 2.3. Static strength tests To address the size effect in compression tests, this study utilized specimen dimensions consistent with those of conventional concrete in static tests, and the results were averaged from three specimens, in compliance with the Standard for Test Methods of Fiber Reinforced Concrete [ 10 ]. 2.4. SHPB testing The dynamic compressive properties of UHPC were characterized using a SHPB apparatus. This method, based on the one-dimensional elastic stress wave propagation theory, is a standardized experimental technique for evaluating the high-strain-rate mechanical behavior of concrete-like materials [ 11 ]. The core components of the system include a data acquisition unit, an incident bar, a transmission bar, and an absorption bar, as illustrated in Fig. 1 . Strain gauges (nominal resistance 120 ± 1 Ω) were precisely attached to the bar surfaces to capture the time-history data of the incident wave ( \(\:{\text{ε}}_{\text{I}}\left(\text{t}\right)\) ), reflected wave ( \(\:{\text{ε}}_{\text{R}}\left(\text{t}\right)\) ), and transmitted wave ( \(\:{\text{ε}}_{\text{T}}\left(\text{t}\right)\) ). Based on the propagation, reflection, and transmission of elastic stress waves at the bar-specimen interfaces, combined with the assumptions of dynamic force equilibrium and uniform deformation, the true dynamic stress σ ( t ), strain ε ( t ), and strain rate \(\:\dot{\epsilon\:}\) ( t ) of the UHPC specimen were derived as follows: $$\:\begin{array}{c}\text{Σ}\left(\text{t}\right)\text{=}\frac{{\text{E}}_{\text{0}}{\text{A}}_{\text{0}}}{{\text{2A}}_{\text{S}}}{\text{[}\text{ε}}_{\text{I}}\left(\text{t}\right)\text{+}{\text{ε}}_{\text{R}}\left(\text{t}\right)\text{+}{\text{ε}}_{\text{T}}\left(\text{t}\right)\text{]}\#\left(1\right)\end{array}$$ $$\:\begin{array}{c}\text{ε}\left(\text{t}\right)\text{=}\frac{{\text{C}}_{\text{0}}}{{\text{l}}_{\text{S}}}{\int\:}_{\text{0}}^{\text{t}}\left[{\text{ε}}_{\text{I}}\left(\text{t}\right)\text{-}{\text{ε}}_{\text{R}}\left(\text{t}\right)\text{-}{\text{ε}}_{\text{T}}\left(\text{t}\right)\right]\text{d}\text{t}\#\left(2\right)\end{array}$$ $$\:\begin{array}{c}\dot{\text{ε}}\left(\text{t}\right)\text{=}\frac{{\text{C}}_{\text{0}}}{{\text{l}}_{\text{s}}}\left[{\text{ε}}_{\text{I}}\left(\text{t}\right)\text{-}{\text{ε}}_{\text{R}}\left(\text{t}\right)\text{-}{\text{ε}}_{\text{T}}\left(\text{t}\right)\right]\#\left(3\right)\end{array}$$ Where, \(\:{\text{ε}}_{\text{I}}\) , \(\:{\text{ε}}_{\text{R}}\) , and \(\:{\text{ε}}_{\text{T}}\) represent the incident strain, reflected strain, and transmitted strain, respectively; A 0 is the cross-sectional area of the incident rod, A S is the cross-sectional area of the specimen, E 0 is the elastic modulus of the incident rod, l S is the length of the specimen, and C 0 denotes the longitudinal wave velocity in the incident rod. The concrete specimens used in the SHPB tests are cylindrical in shape, with a diameter of 100 mm and a thickness of 50 mm. The SHPB system and specimens used in the tests are shown in Fig. 2 . 3. Results and discussion 3.1 Static mechanical properties of UHPC Steam curing significantly enhances the compressive strength of UHPC, but the increment diminishes with increasing steel fiber content. When the steel fiber content is 1%, the compressive strength of the steam-cured group (156.1 MPa) is 37.7% higher than that of the standard-cured group (113.4 MPa); however, the increase drops to 11.7% at a steel fiber content of 2.5%. This phenomenon is attributed to the combined effects of two mechanisms: (1) positive reinforcement, where steam curing at 75°C accelerates the hydration reaction, reducing the porosity of the matrix and thereby increasing the strength (with each 1% increase in steel fiber content, the standard-cured strength increases by 18.3 MPa); and (2) mitigation of interfacial embrittlement, as the high temperature reduces the thickness of the fiber-matrix interfacial transition zone (ITZ), thereby weakening the bridging effect of the fibers. Table 1 Compressive strength of concrete. Code 1 2 3 Average B 1.0 112.9 113.2 114.0 113.4 B 2.0 123.3 135.3 152.6 137.1 B 2.5 148.2 152.6 157.6 152.8 3.2 Dynamic properties and failure modes The dynamic properties of concrete are directly related to its failure modes. The degree of impact-induced damage in specimens increases with the strain rate. This is because, at higher strain rates, the increase in the number of cracks constitutes a significant portion of the strain, and UHPC is fragmented into small pieces, thereby dissipating energy [ 12 ]. With the incorporation and increasing content of steel fibers, the failure mode of the specimens changes significantly. After fracturing, the specimens still remain in large pieces, and their deformability and toughness are greatly improved. As shown in Fig. 3 , the failure mode of UHPC specimens shows a significant gradient evolution as the steel fiber content increases from 1% to 2.5%. For the 1% fiber content group, the failure mode shifts from superficial spalling to brittle overall disintegration with increasing impact air pressure. Under low impact air pressure, edge layer spalling occurs (with debris particle size < 2 mm). At medium air pressure, a single radial main crack develops (with a width of 0.8 mm ± 0.1 mm). Under high air pressure, the specimen undergoes brittle disintegration with through-cracks and no deflection. The characteristics include straight crack propagation and sharp-edged fragments, indicating weak fiber bridging effects and a dominant failure process controlled by the matrix's resistance to impact. The 2% fiber content group shows transitional improvement: a micro-crack network at the impact point under low air pressure; multiple branching cracks at medium air pressure; and, under high air pressure, the specimen fractures but remains intact, with fibers clearly exposed, verifying the serial constraint effect of fibers on the fragments. The 2.5% fiber content group exhibits remarkable toughness: even under high air pressure, the specimen maintains its geometric integrity, with only fine, network-like cracking on the surface and no structural fragmentation in the core. This is attributed to the critical density of the fiber network distribution at the microscopic level, which effectively inhibits crack penetration. 3.3 Dynamic stress-strain curves The dynamic compressive strength of the specimens is higher than the static compressive strength. This is because, according to the principle of concrete specimen failure, one or several main cracks will preferentially propagate, and these main cracks suppress the development of surrounding smaller cracks. Under static conditions, the main crack has sufficient time to form and propagate, eventually penetrating the specimen and causing it to fracture into multiple pieces. However, during high-speed impact, the failure is not entirely due to the development of the main crack. Due to the extremely high speed, the main crack does not have enough time to propagate through the weak zones and tends to expand outward in a straight line. This process involves penetrating the concrete matrix and aggregate, accompanied by the generation of a large number of microcracks, which can absorb more energy. The experimental results show the following trends: (1) The peak impact compressive strength increases with the strain rate; (2) At high strain rates, the increase in peak impact compressive strength diminishes with further increases in strain rate, and the strain rate sensitivity gradually decreases. The stress-strain curves of all specimens under different air pressure impacts follow the same trend, initially rising approximately linearly and then rapidly declining after reaching the peak due to specimen fracture. The average peak stresses for the S1.0, S2.0, and S2.5 groups are 168.3 MPa, 189.0 MPa, and 215.1 MPa, respectively, with enhancement rates of 12.3% and 13.8%. 3.4 Relationship between the strain rate and DIF With the increase in steel fiber content, the slope of the strain rate-DIF curve continuously increases, indicating that the incorporation of steel fibers makes the dynamic compressive strength of the specimens more sensitive to the increase in strain rate, as illustrated in Fig. 5 . Even at higher strain rates, UHPC with a high fiber content still maintains good strain rate sensitivity. When specimens are subjected to impact-induced damage, strain rate sensitivity can be attributed to three aspects: viscous effects, crack evolution, and inertial effects. The improvement in the impact resistance of UHPC is mainly due to the addition of steel fibers, which enhances the viscosity of the cementitious matrix through crack-blocking and bridging effects, thereby inhibiting crack propagation and absorbing more energy. As the strain rate increases, both the aggregate and the interfacial transition zones of the specimens are damaged, and individual cracks tend to become straighter, directly shattering and penetrating through the aggregate and cement matrix. 3.5 Microstructural Analysis The pore structures of the three groups of steam-cured UHPC (S1.0, S2.0, and S2.5) exhibit significant scale-dependent heterogeneity, as shown in Fig. 6 . Based on the pore size distribution statistic, all samples have no pores in the > 100 µm range (with values of 0), indicating that the steam-curing process effectively inhibits the formation of very large pores. Pores are mainly concentrated in the meso-micro scale below 10 µm. Among them, the 10–100 µm range is the key difference zone: the S2.0 group has the highest pore distribution value (0.0243756) in this range, significantly higher than S1.0 (0.0229473) and S2.5 (0.0094689); the distribution value of 1–10 µm mesopores also follows the pattern S2.0 (0.0122306) > S1.0 (0.0110918) > S2.5 (0.0089812). Combined with the mercury injection volume–pore size relationship (Table 1 ), the unit mass mercury injection volume of S2.0 at larger pore sizes (e.g., 90.9 µm) (0.01358) is significantly higher than that of the other two groups, further confirming its large-pore aggregation characteristics; while the mercury injection volume of S2.5 at smaller pore sizes (e.g., 6.04 µm) (0.000583) is the lowest, indicating that its pores are generally finer. The regulation of pores in steam-cured UHPC by steel fibers follows a nonlinear pattern of ineffective at low dosage-aggravated at medium dosage-inhibited at high dosage. When the dosage is 1.0%, the insufficient number of fibers can only slightly reduce the mesopore volume through local filling but cannot effectively constrain the formation of large pores. When the dosage increases to 2.0%, the fibers initially form a weak network, but due to uneven distribution, “interface weak zones” are created—where fiber clusters hinder the flow of cement paste, trap air bubbles, and form large pore defects (the porosity of 10–100 µm pores exceeds that of S1.0), and the mesopore zone (1–10 µm) still has a higher porosity than S1.0 because the fiber bridging effect is not fully exerted. When the dosage reaches 2.5%, the fiber network density exceeds the percolation threshold, significantly inhibiting the development of large pores (a 61.2% reduction in 10–100 µm porosity) and mesopores (a 26.5% reduction in 1–10 µm porosity) through physical blocking (fiber interweaving and filling to cut off pore connectivity) and chemical optimization (active sites on the fiber surface promote gel deposition), while the total porosity is reduced to the lowest level (2.98%). In summary, the optimization of pores in steam-cured UHPC requires controlling the steel fiber dosage around 2.5% to balance dispersibility and pore- blocking effects and avoid large-pore defects at medium dosages. 4. Conclusions Steam curing significantly enhances the strength of the UHPC matrix, but its beneficial effect decreases nonlinearly with increasing steel fiber content. UHPC exhibits significant strain rate strengthening under dynamic impact loading: its dynamic compressive strength, peak stress, and dynamic strength increase factor (DIF) all increase continuously with increasing strain rate, and the dynamic compressive strength is significantly higher than the static value. This phenomenon is due to the instantaneous high-energy loading characteristics of impact loading, which promotes the rapid nucleation and propagation of microcracks within the material. The incorporation of steel fibers effectively inhibits the penetration of microcracks through bridging effects, significantly improving dynamic impact resistance, with performance gains increasing with fiber content. The effect of steel fiber content on the dynamic mechanical behavior of UHPC shows a three-stage nonlinear characteristic: Stage I (0%→1%): Dynamic compressive strength increases significantly (increase ≥ 25%), with fibers initially forming a crack-blocking network; Stage II (1%→2%): Material properties continue to optimize, with enhanced matrix-fiber synergy; Stage III (2%→2.5%): Compressive strength and toughness reach an optimal balance, but further increases in fiber content will lead to a ≥ 40% decrease in workability and fiber agglomeration defects, with marginal gains significantly reduced. Increasing steel fiber content can significantly enhance the strain rate sensitivity of UHPC: at the same strain rate, specimens with higher fiber content have higher DIF values; the slope of the strain rate-DIF curve increases with fiber content, indicating that fibers enhance the tortuosity of crack propagation energy dissipation paths, thereby strengthening the material's dynamic response to strain rate dependence. Declarations Author Contribution Zhu wrote the main manuscript text and reviewed the manuscript. References Ren, L., He, Y., Wang, K. Research progress on the impact resistance of ultra-high-performance concrete. Bull. Chin. Ceram. Soc. 37 (01), 146-154+165 (2018). DOI:10.16552/j.cnki.issn1001-1625.2018.01.023. Shi, C. J., Wang, D. H., Wu, L. M., et al. The hydration and microstructure of ultra-high-strength concrete with cement-silica fume-slag binder. Cem. Concr. Compos. 61, 44−52 (2015). Hu, L., Li, Y. C., Tang, Z. Q., et al. Study on the dynamic properties of ultra-high-performance concrete based on SHPB tests. J. Prot. Eng. 45 (05), 13-21 (2023). Li, Y. Y., Zhao, C. H., Li, H. X., Liu, W. D. Study on the toughness and microstructure of UHPC with a high steel fiber content. Constr. Build. Mater. 460, 139810 (2025). ISSN 0950-0618. Men, G. Y., Jia, X. L., Zhu, W. B. Research on the workability and mechanical properties of ultra-high-performance concrete. Concr. Cement Prod. (09), 64-68 (2023). DOI:10.19761/j.1000-4637.2023.09.064.05. Li, M., Cheng, Y. H., Wu, H. Effects of steel fiber content and type on projectile impacting resistance of UHPC: Mesoscale analysis. Int. J. Impact Eng. 198, 105228 (2025). ISSN 0734-743X. https://doi.org/10.1016/j.ijimpeng.2025.105228 Wu, Y. K., Yao, Y. M. A review on the dynamic damage mechanism of ultra-high-performance concrete (UHPC). Concr. Cement Prod. (04), 1-5+16 (2021). DOI:10.19761/j.1000-4637.2021.04.001.06. Wu, J. D., Guo, L. P., Cao, Y. Z., et al. Influence of steam curing regime on the early mechanical properties and microstructure of ultra-high-performance concrete. J. Southeast Univ. (Nat. Sci. Ed.) 52 (04), 744-752 (2022). Yu, J. C., Zhang, J. Q., Peng, S., et al. A review on the influence of curing on the properties of UHPC. Sichuan Cement (11), 334 (2018). CECS 13—2009. Standard for test methods of fiber-reinforced concrete [S]. Guo, R. Q., Ren, H. Q., Zhang, L., et al. Research progress on the experimental technology of large-diameter split Hopkinson rod. J. Ordnance Eng. 40 (07), 1518-1536 (2019). Hasan Şahan Arel. Effects of curing type, silica fume fineness, and fiber length on the mechanical properties and impact resistance of UHPFRC. Results in Physics 6, 664-674 (2016). ISSN 2211-3797. https://doi.org/10.1016/j.rinp.2016.09.016 Additional Declarations No competing interests reported. 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10:04:45","extension":"png","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":29065,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/80bef46fb14ad746a7ae1c33.png"},{"id":92399776,"identity":"ad3ca5df-4370-4df0-a35b-aa8cb0b2a3fa","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"png","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":7920,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/4810436bec45ff95b676200d.png"},{"id":92400914,"identity":"09f5e00f-6a5b-4c46-a475-5036acc8065f","added_by":"auto","created_at":"2025-09-29 10:12:45","extension":"png","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":56887,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/79f3f1b35620c5232a3c51f4.png"},{"id":92399772,"identity":"8dfb985d-7356-4147-84da-64faba850360","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"xml","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":47891,"visible":true,"origin":"","legend":"","description":"","filename":"4b9922b9ddab42269ce91ecf357984941structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/376ad7081b7378e1946247b6.xml"},{"id":92399771,"identity":"ef2bc31c-12f1-4658-b3c6-8d7696aabc91","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"html","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":55674,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/fde89ab80e2a8f67ba8c890b.html"},{"id":92399761,"identity":"159e6098-dae8-41d4-8178-c1a2eb955f54","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":132903,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of split Hopkinson bar.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/175ca7205967a87365376695.png"},{"id":92399764,"identity":"f72fd5db-299c-4eb5-aa5e-21471cb1659f","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1076026,"visible":true,"origin":"","legend":"\u003cp\u003eSHPB equipment diagram: (a) Cylindrical specimens; (b) Physical drawing of SHPB equipment\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/e8ec740b8859710f74f73ec0.png"},{"id":92400909,"identity":"e118ffd4-fdf2-4c7f-be24-30f24c5cdfc0","added_by":"auto","created_at":"2025-09-29 10:12:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":723564,"visible":true,"origin":"","legend":"\u003cp\u003eFailure modes of specimens at different strain rates.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/fab8a483c650f867003371d8.png"},{"id":92400910,"identity":"d9775c31-21f3-4cad-bc67-b47aab85ca59","added_by":"auto","created_at":"2025-09-29 10:12:45","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":385040,"visible":true,"origin":"","legend":"\u003cp\u003eStress and strain curve of specimens in different strain rate.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/5c5a0935e2880a77969188bc.png"},{"id":92399769,"identity":"e9af6470-ef41-4be6-918c-3770898a615d","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":75787,"visible":true,"origin":"","legend":"\u003cp\u003eStrain rate-DIF curve\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/a7a045a72102362bc5411b3b.png"},{"id":92399766,"identity":"db6413cc-acc0-408b-9fc3-2f5dc9d2abef","added_by":"auto","created_at":"2025-09-29 10:04:45","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":132050,"visible":true,"origin":"","legend":"\u003cp\u003eMIP analysis of concrete specimens.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/cfb3cb751b7e2209eaf89754.png"},{"id":92402194,"identity":"6f4e481b-53cb-44db-8017-c4228af0db92","added_by":"auto","created_at":"2025-09-29 10:36:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3032540,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7653888/v1/a211577c-17d0-4d21-9d49-c97f16565883.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Dynamic mechanical behavior of UHPC under steam curing: effects of steel fiber","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eUltra-High Performance Concrete (UHPC) exhibits exceptional application potential in critical structures subjected to impact loads, such as protective engineering, nuclear power facilities, and military constructions, owing to its ultra-high strength (compressive strength\u0026thinsp;\u0026gt;\u0026thinsp;120 MPa), superior toughness, and outstanding durability [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e][\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, the highly dense microstructure of UHPC also leads to significant brittleness, making it prone to sudden fracture under dynamic loading. This limitation severely restricts its broader application in seismic and blast-resistant structures.\u003c/p\u003e\u003cp\u003eTo overcome this drawback, the engineering field commonly employs steel fiber reinforcement technology, which enhances material ductility and energy absorption capacity through the synergistic interaction between fibers and the matrix. Research indicates that the steel fiber content (0%\u0026ndash;2.5%) significantly modulates the dynamic mechanical response characteristics of UHPC. Split Hopkinson pressure bar (SHPB) tests reveal that the dynamic compressive strength of UHPC exhibits a marked increase compared to its static value. Both the dynamic increase factor (DIF) and peak stress follow an exponential strengthening trend with increasing strain rate (range: 10\u0026sup1;\u0026ndash;10\u0026sup3; s⁻\u0026sup1;), confirming the pronounced strain rate sensitivity of UHPC [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e][\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The dynamic compressive strength reaches its maximum enhancement rate at a fiber content of 2.0%. Beyond 2.0%, the strength increase rate diminishes due to fiber clustering effects leading to reduced fluidity and interfacial weakening [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eMicroscopic analysis reveals that fibers inhibit crack propagation through bridging mechanisms: at a 2.5% fiber content, crack suppression efficiency increases by 60%. Hooked-end fibers significantly improve toughness by reducing the slope of the post-peak stress descent stage by 35%, whereas straight fibers exhibit slightly weaker toughening effects under high strain rates due to a higher risk of debonding [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Furthermore, the influence of fiber content on energy dissipation is nonlinear: energy absorption increases linearly with fiber content in low-dosage groups (\u0026le;\u0026thinsp;1.5%), but decreases in efficiency for high-dosage groups (2.0%\u0026ndash;2.5%) due to uneven fiber distribution [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWu et al. [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] through experiments with five curing regimes (standard curing, 60\u0026deg;C/90\u0026deg;C steam curing for 3d/7d) that steam curing temperature and duration are positively correlated with UHPC compressive strength. However, flexural strength and ultimate tensile strain first increase and then decrease, with significant flexural strength degradation observed under 90\u0026deg;C curing for 7 days. Yu Junchao et al. [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] noted that while steam curing enhances the early-age mechanical properties of UHPC, it can lead to later-age strength retrogression (e.g., a 5%~8% decrease after 28 days), necessitating a balance between curing duration and long-term performance. Steam curing improves early-age mechanical properties by accelerating hydration, refining pores, and optimizing interfaces. Nevertheless, it faces bottlenecks such as later-age strength retrogression, high-temperature interfacial weakening, and durability trade-offs.\u003c/p\u003e\u003cp\u003eThis study investigates the influence of steam curing methods and steel fiber content on the dynamic properties of UHPC using SHPB tests. The analysis encompasses failure modes, stress-strain curves, the dynamic increase factor for compressive strength, and the strain rate effect. Furthermore, the microstructure of UHPC under these curing regimes was examined via mercury intrusion porosimetry (MIP) tests.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eThe study utilized ordinary Portland cement (grade P.O. 52.5), grey powder fly ash, grey powder silica fume, quartz sand with a particle size range of 70\u0026ndash;140 mesh, a polycarboxylate superplasticizer with a water reduction rate exceeding 40%, and copper-coated, hooked-end steel fibers (length 13 mm) as the primary constituents.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Specimen preparation\u003c/h2\u003e\u003cp\u003eThe UHPC specimens were prepared by dry mixing sand, steel fibers, and cementitious materials (cement and silica fume) for 2 minutes, followed by wet mixing with superplasticizer-containing mixing water for 15 minutes to achieve the required workability, then cast into molds of different shapes (100 mm \u0026times; 100 mm \u0026times; 100 mm cubes for compressive strength, 100 mm \u0026times; 100 mm \u0026times; 400 mm prisms for flexural strength, and Φ100 mm \u0026times; 50 mm cylinders for SHPB tests), covered with plastic film, demolded after 24 hours of curing at 20\u0026deg;C, and subsequently cured either in a steam curing chamber at 75\u0026deg;C for 6 days or under standard conditions (20\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C, relative humidity\u0026thinsp;\u0026ge;\u0026thinsp;95%) for 28 days before undergoing static and dynamic mechanical property tests.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Static strength tests\u003c/h2\u003e\u003cp\u003eTo address the size effect in compression tests, this study utilized specimen dimensions consistent with those of conventional concrete in static tests, and the results were averaged from three specimens, in compliance with the Standard for Test Methods of Fiber Reinforced Concrete [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. SHPB testing\u003c/h2\u003e\u003cp\u003eThe dynamic compressive properties of UHPC were characterized using a SHPB apparatus. This method, based on the one-dimensional elastic stress wave propagation theory, is a standardized experimental technique for evaluating the high-strain-rate mechanical behavior of concrete-like materials [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The core components of the system include a data acquisition unit, an incident bar, a transmission bar, and an absorption bar, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Strain gauges (nominal resistance 120\u0026thinsp;\u0026plusmn;\u0026thinsp;1 Ω) were precisely attached to the bar surfaces to capture the time-history data of the incident wave (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{\u0026epsilon;}}_{\\text{I}}\\left(\\text{t}\\right)\\)\u003c/span\u003e\u003c/span\u003e), reflected wave (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{\u0026epsilon;}}_{\\text{R}}\\left(\\text{t}\\right)\\)\u003c/span\u003e\u003c/span\u003e), and transmitted wave (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{\u0026epsilon;}}_{\\text{T}}\\left(\\text{t}\\right)\\)\u003c/span\u003e\u003c/span\u003e). Based on the propagation, reflection, and transmission of elastic stress waves at the bar-specimen interfaces, combined with the assumptions of dynamic force equilibrium and uniform deformation, the true dynamic stress \u003cem\u003eσ\u003c/em\u003e(\u003cem\u003et\u003c/em\u003e), strain \u003cem\u003eε\u003c/em\u003e(\u003cem\u003et\u003c/em\u003e), and strain rate \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\dot{\\epsilon\\:}\\)\u003c/span\u003e\u003c/span\u003e(\u003cem\u003et\u003c/em\u003e) of the UHPC specimen were derived as follows:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}\\text{\u0026Sigma;}\\left(\\text{t}\\right)\\text{=}\\frac{{\\text{E}}_{\\text{0}}{\\text{A}}_{\\text{0}}}{{\\text{2A}}_{\\text{S}}}{\\text{[}\\text{\u0026epsilon;}}_{\\text{I}}\\left(\\text{t}\\right)\\text{+}{\\text{\u0026epsilon;}}_{\\text{R}}\\left(\\text{t}\\right)\\text{+}{\\text{\u0026epsilon;}}_{\\text{T}}\\left(\\text{t}\\right)\\text{]}\\#\\left(1\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}\\text{\u0026epsilon;}\\left(\\text{t}\\right)\\text{=}\\frac{{\\text{C}}_{\\text{0}}}{{\\text{l}}_{\\text{S}}}{\\int\\:}_{\\text{0}}^{\\text{t}}\\left[{\\text{\u0026epsilon;}}_{\\text{I}}\\left(\\text{t}\\right)\\text{-}{\\text{\u0026epsilon;}}_{\\text{R}}\\left(\\text{t}\\right)\\text{-}{\\text{\u0026epsilon;}}_{\\text{T}}\\left(\\text{t}\\right)\\right]\\text{d}\\text{t}\\#\\left(2\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}\\dot{\\text{\u0026epsilon;}}\\left(\\text{t}\\right)\\text{=}\\frac{{\\text{C}}_{\\text{0}}}{{\\text{l}}_{\\text{s}}}\\left[{\\text{\u0026epsilon;}}_{\\text{I}}\\left(\\text{t}\\right)\\text{-}{\\text{\u0026epsilon;}}_{\\text{R}}\\left(\\text{t}\\right)\\text{-}{\\text{\u0026epsilon;}}_{\\text{T}}\\left(\\text{t}\\right)\\right]\\#\\left(3\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{\u0026epsilon;}}_{\\text{I}}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{\u0026epsilon;}}_{\\text{R}}\\)\u003c/span\u003e\u003c/span\u003e, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{\u0026epsilon;}}_{\\text{T}}\\)\u003c/span\u003e\u003c/span\u003e represent the incident strain, reflected strain, and transmitted strain, respectively; \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the cross-sectional area of the incident rod, \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003eS\u003c/em\u003e\u003c/sub\u003e is the cross-sectional area of the specimen, \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the elastic modulus of the incident rod, \u003cem\u003el\u003c/em\u003e\u003csub\u003e\u003cem\u003eS\u003c/em\u003e\u003c/sub\u003e is the length of the specimen, and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e denotes the longitudinal wave velocity in the incident rod.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe concrete specimens used in the SHPB tests are cylindrical in shape, with a diameter of 100 mm and a thickness of 50 mm. The SHPB system and specimens used in the tests are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Static mechanical properties of UHPC\u003c/h2\u003e\u003cp\u003eSteam curing significantly enhances the compressive strength of UHPC, but the increment diminishes with increasing steel fiber content. When the steel fiber content is 1%, the compressive strength of the steam-cured group (156.1 MPa) is 37.7% higher than that of the standard-cured group (113.4 MPa); however, the increase drops to 11.7% at a steel fiber content of 2.5%. This phenomenon is attributed to the combined effects of two mechanisms: (1) positive reinforcement, where steam curing at 75\u0026deg;C accelerates the hydration reaction, reducing the porosity of the matrix and thereby increasing the strength (with each 1% increase in steel fiber content, the standard-cured strength increases by 18.3 MPa); and (2) mitigation of interfacial embrittlement, as the high temperature reduces the thickness of the fiber-matrix interfacial transition zone (ITZ), thereby weakening the bridging effect of the fibers.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eCompressive strength of concrete.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCode\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eAverage\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB\u003csub\u003e1.0\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e112.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e113.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e114.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e113.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB\u003csub\u003e2.0\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e123.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e135.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e152.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e137.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB\u003csub\u003e2.5\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e148.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e152.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e157.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e152.8\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Dynamic properties and failure modes\u003c/h2\u003e\u003cp\u003eThe dynamic properties of concrete are directly related to its failure modes. The degree of impact-induced damage in specimens increases with the strain rate. This is because, at higher strain rates, the increase in the number of cracks constitutes a significant portion of the strain, and UHPC is fragmented into small pieces, thereby dissipating energy [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. With the incorporation and increasing content of steel fibers, the failure mode of the specimens changes significantly. After fracturing, the specimens still remain in large pieces, and their deformability and toughness are greatly improved.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the failure mode of UHPC specimens shows a significant gradient evolution as the steel fiber content increases from 1% to 2.5%. For the 1% fiber content group, the failure mode shifts from superficial spalling to brittle overall disintegration with increasing impact air pressure. Under low impact air pressure, edge layer spalling occurs (with debris particle size\u0026thinsp;\u0026lt;\u0026thinsp;2 mm). At medium air pressure, a single radial main crack develops (with a width of 0.8 mm\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mm). Under high air pressure, the specimen undergoes brittle disintegration with through-cracks and no deflection. The characteristics include straight crack propagation and sharp-edged fragments, indicating weak fiber bridging effects and a dominant failure process controlled by the matrix's resistance to impact. The 2% fiber content group shows transitional improvement: a micro-crack network at the impact point under low air pressure; multiple branching cracks at medium air pressure; and, under high air pressure, the specimen fractures but remains intact, with fibers clearly exposed, verifying the serial constraint effect of fibers on the fragments. The 2.5% fiber content group exhibits remarkable toughness: even under high air pressure, the specimen maintains its geometric integrity, with only fine, network-like cracking on the surface and no structural fragmentation in the core. This is attributed to the critical density of the fiber network distribution at the microscopic level, which effectively inhibits crack penetration.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Dynamic stress-strain curves\u003c/h2\u003e\u003cp\u003eThe dynamic compressive strength of the specimens is higher than the static compressive strength. This is because, according to the principle of concrete specimen failure, one or several main cracks will preferentially propagate, and these main cracks suppress the development of surrounding smaller cracks. Under static conditions, the main crack has sufficient time to form and propagate, eventually penetrating the specimen and causing it to fracture into multiple pieces. However, during high-speed impact, the failure is not entirely due to the development of the main crack. Due to the extremely high speed, the main crack does not have enough time to propagate through the weak zones and tends to expand outward in a straight line. This process involves penetrating the concrete matrix and aggregate, accompanied by the generation of a large number of microcracks, which can absorb more energy.\u003c/p\u003e\u003cp\u003eThe experimental results show the following trends: (1) The peak impact compressive strength increases with the strain rate; (2) At high strain rates, the increase in peak impact compressive strength diminishes with further increases in strain rate, and the strain rate sensitivity gradually decreases. The stress-strain curves of all specimens under different air pressure impacts follow the same trend, initially rising approximately linearly and then rapidly declining after reaching the peak due to specimen fracture. The average peak stresses for the S1.0, S2.0, and S2.5 groups are 168.3 MPa, 189.0 MPa, and 215.1 MPa, respectively, with enhancement rates of 12.3% and 13.8%.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Relationship between the strain rate and DIF\u003c/h2\u003e\u003cp\u003eWith the increase in steel fiber content, the slope of the strain rate-DIF curve continuously increases, indicating that the incorporation of steel fibers makes the dynamic compressive strength of the specimens more sensitive to the increase in strain rate, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Even at higher strain rates, UHPC with a high fiber content still maintains good strain rate sensitivity.\u003c/p\u003e\u003cp\u003eWhen specimens are subjected to impact-induced damage, strain rate sensitivity can be attributed to three aspects: viscous effects, crack evolution, and inertial effects. The improvement in the impact resistance of UHPC is mainly due to the addition of steel fibers, which enhances the viscosity of the cementitious matrix through crack-blocking and bridging effects, thereby inhibiting crack propagation and absorbing more energy. As the strain rate increases, both the aggregate and the interfacial transition zones of the specimens are damaged, and individual cracks tend to become straighter, directly shattering and penetrating through the aggregate and cement matrix.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Microstructural Analysis\u003c/h2\u003e\u003cp\u003eThe pore structures of the three groups of steam-cured UHPC (S1.0, S2.0, and S2.5) exhibit significant scale-dependent heterogeneity, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Based on the pore size distribution statistic, all samples have no pores in the \u0026gt;\u0026thinsp;100 \u0026micro;m range (with values of 0), indicating that the steam-curing process effectively inhibits the formation of very large pores. Pores are mainly concentrated in the meso-micro scale below 10 \u0026micro;m. Among them, the 10\u0026ndash;100 \u0026micro;m range is the key difference zone: the S2.0 group has the highest pore distribution value (0.0243756) in this range, significantly higher than S1.0 (0.0229473) and S2.5 (0.0094689); the distribution value of 1\u0026ndash;10 \u0026micro;m mesopores also follows the pattern S2.0 (0.0122306)\u0026thinsp;\u0026gt;\u0026thinsp;S1.0 (0.0110918)\u0026thinsp;\u0026gt;\u0026thinsp;S2.5 (0.0089812). Combined with the mercury injection volume\u0026ndash;pore size relationship (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), the unit mass mercury injection volume of S2.0 at larger pore sizes (e.g., 90.9 \u0026micro;m) (0.01358) is significantly higher than that of the other two groups, further confirming its large-pore aggregation characteristics; while the mercury injection volume of S2.5 at smaller pore sizes (e.g., 6.04 \u0026micro;m) (0.000583) is the lowest, indicating that its pores are generally finer.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe regulation of pores in steam-cured UHPC by steel fibers follows a nonlinear pattern of ineffective at low dosage-aggravated at medium dosage-inhibited at high dosage. When the dosage is 1.0%, the insufficient number of fibers can only slightly reduce the mesopore volume through local filling but cannot effectively constrain the formation of large pores. When the dosage increases to 2.0%, the fibers initially form a weak network, but due to uneven distribution, \u0026ldquo;interface weak zones\u0026rdquo; are created\u0026mdash;where fiber clusters hinder the flow of cement paste, trap air bubbles, and form large pore defects (the porosity of 10\u0026ndash;100 \u0026micro;m pores exceeds that of S1.0), and the mesopore zone (1\u0026ndash;10 \u0026micro;m) still has a higher porosity than S1.0 because the fiber bridging effect is not fully exerted. When the dosage reaches 2.5%, the fiber network density exceeds the percolation threshold, significantly inhibiting the development of large pores (a 61.2% reduction in 10\u0026ndash;100 \u0026micro;m porosity) and mesopores (a 26.5% reduction in 1\u0026ndash;10 \u0026micro;m porosity) through physical blocking (fiber interweaving and filling to cut off pore connectivity) and chemical optimization (active sites on the fiber surface promote gel deposition), while the total porosity is reduced to the lowest level (2.98%). In summary, the optimization of pores in steam-cured UHPC requires controlling the steel fiber dosage around 2.5% to balance dispersibility and pore- blocking effects and avoid large-pore defects at medium dosages.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eSteam curing significantly enhances the strength of the UHPC matrix, but its beneficial effect decreases nonlinearly with increasing steel fiber content.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eUHPC exhibits significant strain rate strengthening under dynamic impact loading: its dynamic compressive strength, peak stress, and dynamic strength increase factor (DIF) all increase continuously with increasing strain rate, and the dynamic compressive strength is significantly higher than the static value. This phenomenon is due to the instantaneous high-energy loading characteristics of impact loading, which promotes the rapid nucleation and propagation of microcracks within the material. The incorporation of steel fibers effectively inhibits the penetration of microcracks through bridging effects, significantly improving dynamic impact resistance, with performance gains increasing with fiber content.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe effect of steel fiber content on the dynamic mechanical behavior of UHPC shows a three-stage nonlinear characteristic: Stage I (0%\u0026rarr;1%): Dynamic compressive strength increases significantly (increase\u0026thinsp;\u0026ge;\u0026thinsp;25%), with fibers initially forming a crack-blocking network; Stage II (1%\u0026rarr;2%): Material properties continue to optimize, with enhanced matrix-fiber synergy; Stage III (2%\u0026rarr;2.5%): Compressive strength and toughness reach an optimal balance, but further increases in fiber content will lead to a\u0026thinsp;\u0026ge;\u0026thinsp;40% decrease in workability and fiber agglomeration defects, with marginal gains significantly reduced.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eIncreasing steel fiber content can significantly enhance the strain rate sensitivity of UHPC: at the same strain rate, specimens with higher fiber content have higher DIF values; the slope of the strain rate-DIF curve increases with fiber content, indicating that fibers enhance the tortuosity of crack propagation energy dissipation paths, thereby strengthening the material's dynamic response to strain rate dependence.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eZhu wrote the main manuscript text and reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eRen, L., He, Y., Wang, K. Research progress on the impact resistance of ultra-high-performance concrete. Bull. Chin. Ceram. Soc. 37 (01), 146-154+165 (2018). DOI:10.16552/j.cnki.issn1001-1625.2018.01.023.\u003c/li\u003e\n\u003cli\u003eShi, C. J., Wang, D. H., Wu, L. M., et al. The hydration and microstructure of ultra-high-strength concrete with cement-silica fume-slag binder. Cem. Concr. Compos. 61, 44\u0026minus;52 (2015).\u003c/li\u003e\n\u003cli\u003eHu, L., Li, Y. C., Tang, Z. Q., et al. Study on the dynamic properties of ultra-high-performance concrete based on SHPB tests. J. Prot. Eng. 45 (05), 13-21 (2023).\u003c/li\u003e\n\u003cli\u003eLi, Y. Y., Zhao, C. H., Li, H. X., Liu, W. D. Study on the toughness and microstructure of UHPC with a high steel fiber content. Constr. Build. Mater. 460, 139810 (2025). ISSN 0950-0618.\u003c/li\u003e\n\u003cli\u003eMen, G. Y., Jia, X. L., Zhu, W. B. Research on the workability and mechanical properties of ultra-high-performance concrete. Concr. Cement Prod. (09), 64-68 (2023). DOI:10.19761/j.1000-4637.2023.09.064.05.\u003c/li\u003e\n\u003cli\u003eLi, M., Cheng, Y. H., Wu, H. Effects of steel fiber content and type on projectile impacting resistance of UHPC: Mesoscale analysis. Int. J. Impact Eng. 198, 105228 (2025). ISSN 0734-743X. https://doi.org/10.1016/j.ijimpeng.2025.105228\u003c/li\u003e\n\u003cli\u003eWu, Y. K., Yao, Y. M. A review on the dynamic damage mechanism of ultra-high-performance concrete (UHPC). Concr. Cement Prod. (04), 1-5+16 (2021). DOI:10.19761/j.1000-4637.2021.04.001.06.\u003c/li\u003e\n\u003cli\u003eWu, J. D., Guo, L. P., Cao, Y. Z., et al. Influence of steam curing regime on the early mechanical properties and microstructure of ultra-high-performance concrete. J. Southeast Univ. (Nat. Sci. Ed.) 52 (04), 744-752 (2022).\u003c/li\u003e\n\u003cli\u003eYu, J. C., Zhang, J. Q., Peng, S., et al. A review on the influence of curing on the properties of UHPC. Sichuan Cement (11), 334 (2018).\u003c/li\u003e\n\u003cli\u003eCECS 13\u0026mdash;2009. Standard for test methods of fiber-reinforced concrete [S].\u003c/li\u003e\n\u003cli\u003eGuo, R. Q., Ren, H. Q., Zhang, L., et al. Research progress on the experimental technology of large-diameter split Hopkinson rod. J. Ordnance Eng. 40 (07), 1518-1536 (2019).\u003c/li\u003e\n\u003cli\u003eHasan Şahan Arel. Effects of curing type, silica fume fineness, and fiber length on the mechanical properties and impact resistance of UHPFRC. Results in Physics 6, 664-674 (2016). ISSN 2211-3797. https://doi.org/10.1016/j.rinp.2016.09.016\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Ultra-high performance concrete, Steel fiber content, Steam curing, Dynamic increase factor, Impact damage mechanism","lastPublishedDoi":"10.21203/rs.3.rs-7653888/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7653888/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUltra-High Performance Concrete (UHPC) demonstrates significant potential for extreme-load structures due to its ultra-high compressive strength (\u0026gt;\u0026thinsp;120 MPa) and exceptional durability, yet its inherent brittleness critically limits seismic and blast resistance. This study investigates the influence mechanism of steel fiber content on the dynamic mechanical behavior of steam-cured UHPC. Through Split Hopkinson pressure bar experiments and microscopic pore structure analysis, we reveal the strain rate strengthening effect, the synergistic regulation of fiber content and pore structure, and the impact damage mechanism. Results indicate that the dynamic compressive strength of UHPC increases exponentially with rising strain rates, accompanied by significantly enhanced dynamic increase factor and strain rate sensitivity. When steel fiber content increases from 1% to 2.5%, the dynamic compressive strength improvement rate rises from 12.3% to 13.8%, attributed to fiber bridging that inhibits microcrack propagation and enhances energy dissipation. The fiber content effect exhibits a nonlinear threshold characteristic: At 1% content, the fiber network remains incomplete, resulting in matrix-dominated behavior. At 2% content, an initial crack-blocking network forms, but weak interfacial zones induce large-pore defects (porosity for 10\u0026ndash;100 \u0026micro;m pores exceeds that of the low-content group). At 2.5% content, the fiber network density surpasses the percolation threshold. Through combined physical blocking and chemical optimization (gel deposition), total porosity reduces to 2.98%, achieving peak dynamic compressive strength (215.1 MPa) and substantially improved toughness. Steam curing accelerates hydration and refines matrix pores (\u0026gt;\u0026thinsp;100 \u0026micro;m pores eliminated completely), yet high temperatures embrittle the fiber-matrix interfacial transition zone. The 2.5% fiber content mitigates interfacial weakening by optimizing pore distribution (26.5% reduction in 1\u0026ndash;10 \u0026micro;m mesopores), ultimately achieving optimal strength-toughness balance. This study confirms that 2.5% steel fiber content under steam curing represents the optimal mix design, providing critical parameters for impact-resistant structural applications and establishing a theoretical foundation for high-toughness UHPC development.\u003c/p\u003e","manuscriptTitle":"Dynamic mechanical behavior of UHPC under steam curing: effects of steel fiber","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-29 10:04:40","doi":"10.21203/rs.3.rs-7653888/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"89c4a9df-9883-4b28-a2f7-2c570b0b4110","owner":[],"postedDate":"September 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-29T10:04:42+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-29 10:04:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7653888","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7653888","identity":"rs-7653888","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}