Search Results (162)

Early-age cracking limits the structural use of steel fiber-reinforced lightweight aggregate concrete (SFLWAC), and robust experimental evaluation methods are still needed. This study examines the influence of steel fiber volume fractions (i.e., 0%, 0.5%, 1.0%, and 2.0%) on the cracking performance of SFLWAC through mechanical testing, autogenous shrinkage measurements, and two types of partially restrained ring tests, with and without a clapboard. The performance of three crack resistance indices is compared: the strain-based ASTM C1581 index, a stress-based area index, and a newly proposed energy-based index defined as the strain energy accumulation degree (SEAD), i.e., the ratio between the accumulated and critical strain energy density. The 28-day splitting tensile strength was improved by 77.9% and autogenous shrinkage was diminished by 30.7% as steel fiber volume content increased from 0 to 2.0%, thereby improving the resistance to shrinkage-induced cracking. In the partially restrained ring tests, SEAD decreased with increasing fiber content, and crack initiation occurred when SEAD reached an approximately constant threshold, whereas ASTM C1581 and the area index could not consistently rank mixtures when some rings cracked and others remained intact. These results demonstrate that SEAD provides a physically meaningful and unified measure of cracking risk for SFLWAC under partially restrained shrinkage and has the potential to be extended to other fiber-reinforced concretes and shrinkage-related cracking problems. Full article

This study investigates the synergistic influence of recycled concrete aggregate (RCA) and recycled steel fibers (RSF) on the fresh and hardened performance of eco-efficient fiber-reinforced self-compacting concrete (SCC). Twelve C30/37.5 mixtures were produced using demolition waste as coarse RCA at replacement levels of 25, 50, 75, and 100% by mass, combined with RSF recovered from scrap tires at volume fractions of 0.25, 0.50, and 0.75%. Fresh properties were assessed in accordance with EFNARC guidelines using slump-flow (T500), V-funnel, L-box, and J-ring tests, while hardened performance was evaluated through compressive, splitting tensile, and flexural strengths at 28 days, together with density and ultrasonic pulse velocity (UPV). Increasing RCA and RSF contents reduced workability, reflected in lower slump-flow diameters and higher T500 and V-funnel times, although most mixtures maintained satisfactory self-compacting behaviour. Compressive strength decreased with RCA content and, to a lesser extent, with higher RSF, with a maximum reduction of about 39% at 100% RCA relative to the control mix, yet values remained structurally acceptable. In contrast, RSF markedly enhanced tensile and flexural responses: at 25% RCA, 0.75% RSF increased splitting tensile and flexural strengths by approximately 41% and 29%, respectively, compared with the corresponding fiber-free mix. RCA reduced density and UPV by about 10–14%, but these reductions were partially mitigated by RSF addition. Overall, the results demonstrate that SCC with moderate RCA (25–50%) and RSF (0.50–0.75%) can achieve a favourable balance between rheological performance and enhanced tensile and flexural behaviour, offering a viable composite solution for sustainable structural applications. Full article

The mechanical behavior of fiber-reinforced concrete largely depends on the fiber morphology, geometry, and distribution. However, current numerical models do not take into account the stochastic properties of fibers with a spatial distribution, which limits their prediction accuracy and overlooks the critical impact of microstructural effects on macroscopic properties. To address this issue, a comprehensive numerical framework is developed using the Concrete Damage Plasticity (CDP) model for the concrete matrix, an elastoplastic model for steel fibers, and with cohesive zone elements applied to describe fiber–matrix interfacial debonding. Random fiber configurations are generated to represent statistical variability, and their effects on the elastic modulus, compressive strength, and tensile strength are systematically examined. A wide range of fiber parameters—including dimensions, volume fractions, stochastic orientation, and spatial distribution—is investigated to reveal microstructure-dependent mechanical behavior at the macroscale. The results highlight the critical roles of the fiber volume fraction and orientation control in enhancing mechanical behavior and provide practical guidelines for optimizing fiber incorporation strategies in concrete design. Full article

With the development of infrastructure construction, seawater sea–sand concrete (SWSSC) is expected to solve the shortage of freshwater and river sand. Polyoxymethylene (POM) fiber, owing to its excellent corrosion resistance, provides a novel approach to enhancing the bond performance of SWSSC. This study systematic study of the bond properties of bimetallic steel bars (BSBs) in POM fiber-reinforced SWSSC and develops a predictive model. Mechanical property tests of SWSSC and pull-out tests of BSB and SWSSC were conducted with various POM fiber contents. The results showed that the optimal volume fraction of POM fibers was 0.6%. At this fraction, the compressive strength and splitting tensile strength of SWSSC were improved by 17.7% and 20.3%, respectively, compared with the group without fibers. All pull-out specimens experienced splitting failure. The bond strength increased monotonically with the increase in relative cover thickness and exhibited a trend of first increasing and then stabilizing with rising POM fiber volume fraction. In addition, a bond stress–slip prediction model between BSBs and POM fiber-reinforced SWSSC was established based on the test results, which can provide theoretical support for the numerical simulation and design of BSB-SWSSC structures. Full article

This study aims to investigate the synergistic effect of hybridizing steel fibers on the shear behavior of I-shaped reinforced concrete beams (I-beams) produced with Ultra-High-Performance Concrete (UHPC) without shear reinforcement. For this purpose, five I-beams were prepared using UHPC mixtures with three fiber volume fractions (0%, 1% and 2%), incorporating either straight micro steel fibers alone or an equal combination of straight micro and hooked-end macro steel fibers, and tested under three-point loading. In addition, the experimental program evaluated the effects of hybridization on the compressive strength, splitting tensile strength and fracture behavior of UHPC. The test results showed that beams with 1% microfibers and hybrid fibers demonstrated substantial improvements in shear resistance, achieving 2.7 and 2.0 times higher shear strength than the reference beam without fibers, respectively. Moreover, the beam reinforced with only microfibers exhibited 37% greater shear strength than the beam with hybrid fibers, indicating that the synergistic effect was limited at this dosage. At a 2% fiber volume, the failure mode shifted from shear to flexure. These findings highlight the critical influence of fiber type and dosage on the shear behavior of UHPC I-beams. Full article

This study investigates the subsequent stages of recrystallisation in Interstitial-Free (IF) steel subjected to an unconventional continuous annealing process with a controlled thermal gradient. A cold-rolled steel strip was exposed to varying annealing temperatures along its length, enabling the analysis of microstructural evolution during the course of recrystallisation. The microstructure and stored energy were assessed at various positions along the strip using Electron Backscatter Diffraction (EBSD). The results underscore the significant influence of local misorientation and structural inhomogeneity on orientation selection during recrystallisation. The remaining non-recrystallised volume fraction (NRF) strongly correlates with the average misorientation gradient, obeying a phenomenological power-law correspondence with an exponent of ~3.7. This indicates that the recrystallisation process is highly sensitive to small changes in local orientation gradients. These findings highlight the crucial role of stored energy distribution for texture evolution, particularly during the early stages of recrystallisation in continuous annealing. It is observed that g-fiber grains, in comparison to a-fiber grains, are much more susceptible to grain fragmentation and therefore develop more robust intra-granular misorientation gradients, allowing for successful nucleation events to occur. In the present study, these phenomena are documented in a statistically representative manner. These insights are valuable for optimising thermal processing in interstitial-free (IF) steels. Full article

In this study, steel fibers were used to improve the mechanical properties of high-strength self-compacting concrete (HSSCC), and its effect on the fracture mechanical properties was investigated by a three-point bending test with notched beams. Coupled with the digital image correlation (DIC) technique, the fracture process of steel-fiber-reinforced HSSCC was analyzed to elucidate the reinforcing and fracture-resisting mechanisms of steel fibers. The results indicate that the compressive strength and flexural strength of HSSCC cured for 28 days exhibited an initial decrease and then an enhancement as the volume fraction (V_f) of steel fibers increased, whereas the flexural-to-compressive ratio linearly increased. All of them reached their maximum of 110.5 MPa, 11.8 MPa, and 1/9 at 1.2 vol% steel fibers, respectively. Steel fibers significantly improved the peak load (F_P), peak opening displacement (CMOD_P), fracture toughness (K_IC), and fracture energy (G_F) of HSSCC. Compared with HSSCC without steel fibers (HSSCC-0), the F_P, K_IC, CMOD_P, and G_F of HSSCC with 1.2 vol% (HSSCC-1.2) increased by 23.5%, 45.4%, 11.1 times, and 20.1 times, respectively. The horizontal displacement and horizontal strain of steel-fiber-reinforced HSSCC both increased significantly with an increasing V_f. HSSCC-0 experienced unstable fracture without the occurrence of a fracture process zone during the whole fracture damage, whereas the fracture process zone formed at the notched beam tip of HSSCC-1.2 at its initial loading stage and further extended upward in the beams of high-strength self-compacting concrete with a 0.6% volume fraction of steel fibers and HSSCC-1.2 as the load approaches and reaches the peak. Full article

Polypropylene (EPP) concrete offers advantages such as low density and good thermal insulation properties, but its relatively low strength limits its engineering applications. Waste steel fibers (WSFs) obtained during the sorting and processing of machining residues can be incorporated into EPP concrete (EC) to enhance its strength and toughness. Using the volume fractions of EPP and WSF as variables, specimens of EPP concrete (EC) and waste steel fiber-reinforced EPP concrete (WSFREC) were prepared and subjected to cube compressive strength tests, splitting tensile strength tests, and four-point flexural strength tests. The results indicate that EPP particles significantly improve the toughness of concrete but inevitably lead to a considerable reduction in strength. The incorporation of WSF substantially enhanced the splitting tensile strength and flexural strength of EC, with increases of at least 37.7% and 34.5%, respectively, while the improvement in cube compressive strength was relatively lower at only 23.6%. Scanning electron microscopy (SEM) observations of the interfacial transition zone (ITZ) and WSF surface morphology in WSFREC revealed that the addition of EPP particles introduces more defects in the concrete matrix. However, the inclusion of WSF promotes the formation of abundant hydration products on the fiber surface, mitigating matrix defects, improving the bond between WSF and the concrete matrix, effectively inhibiting crack propagation, and enhancing both the strength and toughness of the concrete. Full article

To develop cement-based composite materials with exceptional impact resistance, this study investigates the impact resistance performance of steel fiber- and glass fiber-reinforced specimens, as well as steel fiber and glass fiber textile-reinforced specimens, through drop weight impact tests. The results showed that the impact resistance of specimens increases with the number of glass fiber textile layers, glass fiber volume fractions, and glass fiber lengths, with 36GF1.5SF1.0 exhibitinh ultra-high impact resistance with a failure impact energy of 114 kJ. Compared to the specimens reinforced with glass textiles, the specimens with glass fiber showed better impact resistance at the same volume fraction. The failure mode of unreinforced specimens is divided into several pieces, while fiber-reinforced specimens have local punching shear failure at the impact site, maintaining better integrity. An impact damage evolution equation and life prediction model based on a two-parameter Weibull distribution are developed. The research results will provide a reference for the selection of fibers for engineering applications. Full article

Engineered cementitious composites (ECCs), known for their superior ductility and strain-hardening behavior compared to conventional concrete, have been predominantly studied with polyvinyl alcohol (PVA) fibers. However, the potential economic and technical advantages of incorporating steel fibers into ECCs have been largely overlooked in the literature. This study investigates the mechanical performance of ECC reinforced with different types of steel fibers, including straight, twisted, hooked, and hybrid fibers of different lengths, as compared to PVA. The inclusion of various supplementary cementitious materials (SCMs) such as slag and fly ash with each type of steel fiber was also considered at a constant fiber volume fraction of 2%. The mechanical properties were assessed through compressive strength, splitting tensile strength, and four-point flexural tests along with calculations of toughness, ductility, and energy absorption capacity indices. This study compares the mechanical properties of different ECC compositions, revealing that ECCs with hybrid steel fibers (short and long) achieved more than twice the tensile strength, 12.7% higher toughness, and 36.4% greater energy absorption capacity compared to ECCs with PVA fibers, while exhibiting similar multiple micro-cracking behavior at failure. The findings highlight the importance of fiber type and distribution in enhancing an ECC’s mechanical properties, providing valuable insights for developing more cost-effective and resilient construction. Full article

To develop an ultra-high performance concrete (UHPC) wall structure suitable for nuclear power plant applications, this study establishes a finite element model to evaluate the ultimate bearing capacity of UHPC walls under eccentric compression with single-sided thermal loading during accident conditions. The accuracy and reliability of the finite element analysis (FEA) method were rigorously validated by simulating and replicating experimental results using the same modeling approach adopted in this study. Based on the validated model, the influence of single-sided thermal loading on the ultimate bearing capacity of UHPC walls under nuclear power plant accident conditions was thoroughly investigated. Key parameters—including the reinforcement ratio, steel fiber volume fraction, temperature, eccentricity, and concrete strength grade—were systematically analyzed to determine their effects on the ultimate bearing capacity of UHPC wall specimens. The results demonstrate that the reinforcement ratio, steel fiber volume fraction, temperature, eccentricity, and concrete strength grade significantly affect the degradation rate of the ultimate load of UHPC walls as the temperature increases. Additionally, this paper proposes a calculation method for the normal section bearing capacity of rectangular cross-sections in UHPC large eccentric compression members under single-sided thermal loads. These findings provide theoretical support and scientific evidence for the design of new UHPC structural specimens in nuclear power plants. Full article

The first-cracking behavior of ultra-high-performance concrete (UHPC) is critical for the functionality and durability of its structures. However, determining the first-cracking strength by the linear limit point is challenging due to the nonlinear behavior before the initial crack. This study utilizes an improved Digital Image Correlation (DIC) technique to detect cracks and directly determine the first-cracking strength. The effect of steel fiber length, volume fraction, diameter, and shape on the first-cracking behavior was evaluated through direct tensile testing. Results indicate that incorporating steel fibers can enhance the first-cracking strength of UHPC to varying extents, ranging from 26.07% to 121.31%. Specifically, the length and volume fraction of steel fibers significantly affect the first-cracking strength, whereas the diameter and shape have minimal impact. The shape of steel fibers can influence the initial crack pattern due to stress concentration in deformed fibers. On the other hand, the inclusion of steel fibers can also negatively impact the first-cracking strength due to the introduction of air voids. Finally, considering both the positive and adverse effects of steel fibers, an updated predictive model for the first-cracking strength is proposed based on regression analysis of the experimental data. The proposed model can accurately predict the first-cracking strength of UHPC, fitting well with the existing data. Full article

Geopolymer recycled aggregate concrete (GRAC) is an eco-friendly material utilizing industrial byproducts (slag, fly ash) and substituting natural aggregates with recycled aggregates (RA). Incorporating steel-polypropylene hybrid fibers into GRAC to produce hybrid-fiber-reinforced geopolymer recycled aggregate concrete (HFRGRAC) can bridge cracks across multi-scales and multi-levels to synergistically improve its mechanical properties. This paper aims to investigate the mechanical properties of HFRGRAC with the parameters of steel fiber (SF) volume fraction (0%, 0.5%, 1%, 1.5%) and aspect ratio (40, 60, 80), polypropylene fiber (PF) volume fraction (0%, 0.05%, 0.1%, 0.15%), and RA substitution rate (0%, 25%, 50%, 75%, 100%) considered. Twenty groups of HFRGRAC specimens were designed and fabricated to evaluate the compressive splitting tensile strengths and flexural behavior emphasizing failure pattern, load–deflection curve, and toughness. The results indicated that adding SF enhances the specimen ductility, mechanical strength, and flexural toughness, with improvements proportional to SF content and aspect ratio. In contrast, a higher percentage of RA substitution increased fine cracks and reduced mechanical performance. Moreover, the inclusion of PF causes cracks to exhibit a jagged profile while slightly improving the concrete strength. The significant synergistic effect of SF and PF on mechanical properties of GRAC is observed, with SF playing a dominant role due to its high elasticity and crack-bridging capacity. However, the hydrophilic nature of SF combined with the hydrophobic property of PF weakens the bonding of the fiber–matrix interface, which degrades the concrete mechanical properties to some extent. Full article

Recycled aggregate concrete (RAC) holds significant promise for reducing the environmental impact of the construction industry. However, the poor mechanical properties of RAC compared to conventional concrete are mainly due to the porous and soft nature of recycled aggregates. While fiber reinforcement has been proposed as a promising method to address this issue, existing studies primarily focus on steel and polypropylene fibers, with limited systematic comparison of alternative fiber types and dosages. In particular, the mechanical enhancement mechanisms of basalt and glass fibers in RAC remain underexplored, and there is a lack of predictive models for strength behavior. This study evaluates the effects of basalt and glass fibers on RAC through uniaxial compression, splitting tensile, and three-point bending tests. Nine mixtures with varying fiber types and volume fractions (1.0–2.5%) were tested, and results were compared to plain RAC. Key properties such as strength, energy absorption, toughness, and flexibility were analyzed using load–displacement curves and advanced toughness indices. Both fiber types improved tensile and flexural properties, with glass fibers showing superior performance, particularly at 1.5% content, where the splitting tensile strength increased by up to 40% and the flexural strength improved by 42.19%. Basalt fibers dispersed more uniformly but were less effective in enhancing toughness and crack resistance. Excessive fiber content reduced matrix homogeneity and mechanical performance. Optimal fiber dosages were identified as 1–1.5% for glass fibers and 1–2% for basalt fibers, depending on the targeted property. Predictive formulas for the flexural strength of fiber-reinforced RAC are also proposed, offering guidance for the design of structural RAC elements. Full article

The utilization of coral aggregates in the preparation of Ultra-High Performance Concrete (UHPC) effectively addresses the material scarcity challenges in island and reef construction environments, thereby advancing the sustainable development of building materials technology. This research systematically investigates the physical and mechanical properties of Coral Sand UHPC (CSUHPC) with varying fiber contents through uniaxial compression tests, splitting tensile tests, and stress–strain curve tests under compression. The experimental results demonstrate that the incorporation of fibers significantly enhances both the mechanical strength and ductility of CSUHPC. The test data indicate that CSUHPC specimens with a steel fiber volume fraction of 3% exhibit the highest performance, attaining a compressive strength of 131.9 MPa and a splitting tensile strength of 18.5 MPa. The compressive stress–strain curve tests reveal that the incorporation of fibers induces a failure mode transition in CSUHPC specimens from brittle to ductile. Furthermore, a constitutive equation for CSUHPC was proposed, and a multi-dimensional assessment system based on the radar chart, which encompasses compressive strength, splitting tensile strength, peak strain, compressive toughness, and an energy dissipation coefficient. The optimal fiber combination was determined as a hybrid fiber system comprising 2% steel fibers and 1% polyethylene (PE) fibers, which demonstrates superior comprehensive performance. Full article