Steel Fiber Volume Fraction Research Articles

Experimental and numerical investigation on seismic performance of SFRC shear walls with CFRP bars

In order to investigate the seismic performance of steel fiber reinforced concrete (SFRC) shear walls with CFRP bars, quasi-static tests were conducted on six full-scale shear walls with a shear span ratio of 2.0, including one steel bars reinforced ordinary concrete (RC) shear wall, one CFRP bars reinforced ordinary concrete (CRC) shear wall and four CFRP bars reinforced SFRC (CSFRC) shear walls. The variables included steel fiber volume fraction (SFVF), axial load ratio and concrete strength grade. The experimental results showed increasing the SFVF could significantly decrease the concrete damage level and improve repairability. Meanwhile, the bearing capacity, ductility, and energy dissipation capacity all improved, and the stiffness degradation slowed. Compared to RC shear wall, the peak load of the CSFRC shear wall with SFVFs of 0 %, 0.75 % and 1.5 % increased by 16.36 %, 30.06 % and 46.01 %, respectively. The residual drift ratio decreased by 54.75 %, 61.61 %, and 52.98 % at 2.2 % drift ratio, respectively. Compared to CRC shear wall, the cumulative energy dissipation of the CSFRC shear wall with SFVFs of 0.75 % and 1.5 % increased by 39.54 % and 94.10 %; and the ductility increased by 5.12 % and 18.06 %, respectively. These improvements indicated that applying SFRC with SFVFs of 0.75 % and 1.5 % to shear walls reinforced with CFRP bars could effectively improve the seismic performance and compensate for the shortcomings of resilient shear walls. In addition, increasing the axial load ratio improved the bearing capacity, but decreased deformation capacity and accelerated stiffness degradation. An increase in concrete strength significantly improved the bearing capacity, stiffness and energy dissipation but reduced ductility. Meanwhile, the stability of lateral loads at large drift ratios weakened. Additionally, a detailed finite element model (FEM) was established using DIANA and the accuracy was verified by comparing the simulation and experimental results. Subsequently, parametric analyses were carried out to study the effects of shear span ratio, CFRP bar ratio, and wall thickness on the seismic behaviour of CSFRC shear walls.

Study on the Reinforced Properties of Geopolymer Fibers with a Sustainable Development Role

Geopolymers are of great significance in reducing the consumption of mineral resources, saving energy, protecting the environment, and realizing sustainable economic and social development. This experiment investigated geopolymer mortar with fly ash and metakaolin as the primary binders, assessing the impact of different fiber types and volume fractions on the mortar’s flexural and compressive strength. The results indicated that optimal mechanical properties could be achieved with a fly ash-to-metakaolin ratio of 35:65. The mechanical performance is the best, with a compressive strength of 54 MPa, a flexural strength of 3.4 MPa, and a split tensile strength of 1.9 MPa at 28 days. Different fibers influenced the splitting tensile strength to varying degrees; with a 1.5% volume fraction of steel fibers, geopolymer mortar exhibited the best reinforcement effect, showing a 70% increase in flexural strength and a 142% increase in tensile strength. Mechanistic analysis revealed that the reinforcement from refined various fibers could refine the structure and further enhance the strength. Of steel geopolymer fibers’ The reinforcing effect of steel fibers is the best among them, and the internal structure is the most compact. The geopolymer mortar hydration products of geopolymer mortar reinforced with PP fibers, PVA fibers, steel fibers, and carbon fibers were amorphous network-structured zeolites (Na2[Al2Si3O10]·2H2O). The limitations of geopolymers can be effectively addressed through the aforementioned research, which can effectively reduce the use of cement and achieve the goal of sustainable development.

Experimental investigations on mechanical performance, synergy assessment, and microstructure of pozzolanic and non‐pozzolanic hybrid steel fiber reinforced concrete

AbstractThis paper investigates the effects of pozzolanic substitutions for ordinary Portland cement (OPC) with silica fume (SF) and metakaolin (MK) on the mechanical and toughness performances of steel fiber reinforced concrete (SFRC). Initially, a reference concrete mix with a water‐to‐binder ratio of 0.4 is blended with different volume fractions of steel fibers with varying geometry: crimped steel (CS) and straight steel (SS), both individually and in combination, to examine their mechanical properties. In the subsequent phase, the study investigates the impact of combining macro‐ and microsteel fibers on flexural toughness, to determine potential synergy for suitable combinations. Also, the possible influence of pozzolans in the variation of flexural toughness of hybrid steel fiber reinforced concrete (Hy‐SFRC) was evaluated. Hybridization of steel fibers was found effective in improving the workability of the concrete mix up to 11%. SFRC mixes containing pozzolans exhibited a significant enhancement in compressive strength, modulus of rupture, and modulus of elasticity compared to non‐pozzolanic SFRC. The hybrid combination of CS 1.5% and SS 0.5% was considered the best in terms of mechanical properties. Additionally, the results of synergy assessment showed that hybridization of steel fibers in the pozzolanic concrete mix was particularly effective in the post‐cracking stages with a positive 14% compared to a negative 8% in the pre‐cracking stage. The pozzolanic addition improved the flexural toughness of Hy‐SFRC to about 10%–20%. Blending of SF and MK in Hy‐SFRC was found effective in enhancing the toughness mechanism of concrete compared to Hy‐SFRC mixes containing binary SF and MK, indicating a stronger bond between the fibers and the matrix resulting from the pore refinement and hydration products developed at the interface. Hy‐SFRC containing a ternary pozzolanic mix of SF 10% and MK 10% gave the best results in flexural toughness, and the corresponding synergy values were found to be the maximum. The results were consistent with the morphology analysis, which revealed an increase in hydration products at the interface between the aggregate and concrete matrix, as well as between the steel fiber and concrete matrix, due to the ternary blending of SF and MK.

Compressive Bearing Capacity and Ductility of Slurry-Infiltrated Fiber Concrete Blocks with Two-Dimensional Distributed Steel Fibers

Slurry-infiltrated fiber concrete (SIFCON) has excellent potential for application as a new material with good crack resistance, impact resistance, and seismic performance. In this paper, SIFCON blocks were cast in a single pour using the custom-made two-dimensional directional steel fiber placement device, followed by compressive testing. Based on the test results, the effects of fly ash substitution rate, steel fiber aspect ratio, and steel fiber volume fraction on the compressive bearing capacity and ductility of SIFCON blocks were investigated. The results indicate that the custom-made device in this paper effectively achieves the directional placement of steel fibers, offering a cost-effective solution that optimizes the construction process. Regarding the test results, it was observed that the compressive bearing capacity of SIFCON blocks decreases linearly with increasing fly ash substitution rate, while the addition of fly ash improves the ductility of the blocks. Notably, a 31% decrease in bearing capacity was noted when the fly ash substitution rate increases from 0% to 40%, whereas the ductility ratio increased by 99% for the same substitution rate range. Furthermore, the results revealed a positive correlation between the bearing capacity and ductility of SIFCON blocks with higher steel fiber aspect ratios and volume fractions. Specifically, the bearing capacity increased by 19% and 33% with steel fiber aspect ratio increments from 33 to 70 and volume fraction increases from 10% to 14%, respectively. Additionally, the ductility ratio of the test blocks increased by 104% when the aspect ratio of the steel fibers increased from 33 to 70, and by 37% when the volume fraction of steel fibers rose from 10% to 14%.

Study on the performance of coal gangue ceramsite steel fiber high strength concrete

Our study aims to use coal gangue ceramsite (CGC) as coarse aggregate and add an appropriate volume fraction of steel fiber to prepare high strength concrete with lighter weight and meeting pumping requirements. The effects of (w/b) ratio, CGC content, the volume fraction of steel fiber, and water reducing agent content on the properties of coal gangue ceramsite steel fiber concrete (CGCSFC) were analyzed by orthogonal test, range analysis, and variance analysis. The optimal mix proportion of CGCSFC was obtained by using the efficacy coefficient method combined with the AHP-CRITIC method. The results showed that the crushing value of CGC was 37.26 % lower than that of coal gangue (CG). The reason for the increase in the strength of CGC was that there were few micro-cracks in CGC and the increase in quartz content. The (w/b) ratio had the most significant effect on the compressive strength of CGCSFC. CGCSFCs with compressive strength of 60–90 MPa and slump of 140–220 mm were prepared. The dry apparent density of CGCSFC with the same strength grade was 18.2–19.4 % lower than that of normal concrete (NC), and the specific strength was increased by 19.7–36.3 %. The average calculation error of Bowromi modified formula considering CGC content and the volume fraction of steel fiber is 3.82 %.

Distributions of coarse aggregate and steel fiber in ultra-high performance concrete: Migration behavior and correlation with compressive strength

The vertical distribution of coarse aggregates and steel fibers in ultra-high performance concrete (UHPC) both at fresh and hardened states was investigated to unveil their migration behavior and the correlation with compressive strength. To fulfill this purpose, customized design of specimens and tailored test method were developed. Specimens with two sizes (3–5 mm and 5–10 mm) and varying content (0–50 %) of coarse aggregate and different volume fraction of steel fiber (0–2 %) were proportioned. It was found that higher amount of coarse aggregates was located in the lower part whereas more steel fibers and voids were observed in the upper region of the specimens. Increasing the content of coarse aggregates resulted in worsened uniformness of its distribution, while incorporating steel fibers up to 2 % by volume imposed negligible effects on the heterogeneity. Migration of different phases primarily occurred during the fresh state. Compressive strength of the specimens at the middle layers was up to 30 % higher than those at the top and bottom layers. The inferior compressive strength was primarily ascribed to the worsened distribution of CAs and higher voids in the top layer.

Electrical resistivity of eco-friendly hybrid fiber-reinforced SCC: Effect of ground granulated blast furnace slag and copper slag content as well as hooked-end fiber length

Over the last 20 years, the development of electrically conductive composites for removing snow and ice from transportation infrastructure has received exceptional traction. However, these composites need to exhibit stable electrical conductivity and high mechanical properties to be sustainable and cost-effective. Towards this goal, the article investigates the roles of ground granulated blast furnace slag (BFS) and copper slag (CS) content, in addition to hooked-end steel fiber length, on the electrical properties of eco-friendly ultra-high performance hybrid fiber-reinforced self-compacting concrete (HFR-SCC) for the first time in the literature. For this purpose, sixteen eco-friendly electrically conductive ultra-high-performance HFR-SCC were designed based on the variable parameters of four different BFS/total binder ratios (20, 40, 60, and 80 %), a CS/total fine aggregate ratio of 50 %, and two different hooked-end fiber lengths (30 and 60 mm), while all mixes used 1.75 % by volume fraction of steel fibers. After determining the workability properties (slump-flow and T500 values) of all mixes, compressive strength and electrical resistivity/conductivity tests of 90-day specimens were conducted. Additionally, environmental and economic evaluations of all mixes in terms of sustainability were performed in order to clarify the effects of the variable parameters. Taking into account the experimental results obtained, it was observed that all electrically conductive ultra-high performance HFR-SCC mixes demonstrated satisfactory workability properties, while the compressive strength values reached to impressive values of 127 MPa. The optimum BFS/total binder ratio was identified to be 40 % for higher compressive strength and conductivity of ultra-high performance HFR-SCC specimens. On the other hand, the addition of CS to the mixes resulted in an increase of almost 9 % in compressive strength compared to one without CS, while at the same time, a significant increase of approximately 363 % was observed in the electrical conductivity values of the specimens. As for the influence of different lengths of hooked steel fibers, the use of 30 mm length hooked-end steel fibers in HFR-SCC mixes performed better in terms of compressive strength, whereas 60 mm fibers performed better regarding electrical conductivity. In conclusion, this experimental work has evidenced that it is possible to develop an eco-friendly and sustainable electrically conductive ultra-high performance cementitious composite (the optimal mix compressive strength and electrical resistivity values were 127 MPa and 2242 Ω.cm, respectively) by using waste from different industries such as iron and copper. Thus, it will provide important insights for the design and application of future electrically conductive concretes, which can be an important alternative in efficient active deicing and snow-melting applications.

Experimental investigation on bond-slip performance and damage evolution mechanism of deformed reinforcing bar embedded in steel polyvinyl-alcohol hybrid fiber high performance concrete after high temperature

Hybrid Fiber High Performance Concrete (HFHPC) utilizes the synergistic effects of macro and micro fibers at different stages, which can effectively improve its mechanical properties, fracture toughness, ductility, and impact resistance after high temperature. The bonding between steel bars and hybrid concrete is a fundamental prerequisite for ensuring that these two materials with completely different mechanical properties work together. In order to study on bond-slip performance and damage evolution mechanism of deformed reinforcing bar embedded in steel-polyvinyl-alcohol (S-PVA) hybrid fiber high performance concrete (HFHPC) after high temperature, a total of 75 specimens were designed and manufactured by selecting volume fraction of steel fiber and PVA fiber and dosing of slag powder as the orthogonal test factors, and the central pull-out test were completed after exposure to room temperature, 200℃, 400℃, 600℃, and 800℃. The experimental results indicate that the bonding failure mode of HFHPC specimens all showed steel bar pull-out failure after high temperature, and the bond stress-slip curve can be divided into 4 stages: elastic rise stage, nonlinear rise stage, plastic degradation stage, and residual stage. As the temperature increases, the bonding strength of HFHPC specimens gradually decreases and the peak slip gradually increases. The addition of hybrid fibers can significantly improve the bonding ductility and energy dissipation capacity of deformed reinforcing bar embedded in S-PVA HFHPC after high temperature. When the temperature is below 200℃, the bonding strength damage rate of S-PVA HFHPC specimens is relatively slow; when the temperature exceeds 200℃, the bonding strength damage rate of S-PVA HFHPC specimens increases.

Research on Layered Steel Fiber Reinforced Concrete Mix Ratio Design Based on Orthogonal Test

The aim of this study was to investigate the effect of different mix ratios on the mechanical properties of steel fiber-reinforced concrete (LSFRC) and to determine an optimum mix ratio. The effects of four factors, namely, fly ash content, volume fraction of steel fibers, water–cement ratio, and sand rate, on the mechanical properties of LSFRC were investigated through orthogonal experiments. The microstructure of LSFRC at different mix ratios was analyzed using scanning electron microscopy (SEM), and an optimal mix ratio was derived. The results showed that the water–cement ratio and the volume fraction of steel fibers were the main factors affecting the mechanical properties of LSFRC. When the water–cement ratio was 0.38 and 0.42, the combined mechanical properties of concrete were superior. Steel fiber content between 0.6% and 1% had a significant effect on the splitting tensile strength of concrete. The effect of sand rate on compressive and splitting tensile strengths was consistent, with a significant effect on both at a sand rate of 40%. In terms of microstructure, 20% fly ash content promotes the hydration of concrete. The optimum LSFRC mix ratio determined was 0.42 water–cement ratio, 0.6% steel fiber content, 40% sand rate, and 20% fly ash content. Experimental verification using this mix ratio showed that the compressive, flexural, and split tensile strengths were increased by 3%, 19%, and 33%, respectively, when compared to ordinary concrete.

Enhancing UHPC Tensile Performance Using Polystyrene Beads: Significant Improvements and Mechanisms.

This study investigates utilizing spherical polystyrene (PS) beads as artificial flaws to improve ultrahigh-performance concrete (UHPC) tensile performance using a uniaxial tensile test and explains the corresponding mechanisms by analyzing the internal material structure of UHPC specimens with X-ray CT scanning. With a hooked steel fiber volume fraction of 2%, three PS bead dosages were employed to study tensile behavior changes in dog-bone UHPC specimens. A 33.4% increase in ultimate tensile strength and 174.8% increase in ultimate tensile strain were recorded after adding PS beads with a volume fraction of 2%. To explain this improvement, X-ray CT scanning was utilized to investigate the post-test internal material structures of the dog-bone specimens. AVIZO software was used to analyze the CT information. The CT results revealed that PS beads could not only serve as the artificial flaws to increase the cracking behavior of the matrix of UHPC but also significantly optimize the fiber orientation. The PS beads could serve as stirrers during the mixing process to distribute fiber more uniformly. The test results indicate a relationship between fiber orientation and UHPC tensile strength.

Reinforcement Effects on Tensile Behavior of Ultra-High-Performance Concrete (UHPC) with Low Steel Fiber Volume Fractions.

Ultra-high-performance concrete (UHPC) with a low steel fiber volume fraction offers lower material costs than UHPC with typical steel fiber volume fractions, and has the potential to mitigate the ductility degradation of rebar-reinforced UHPC (R-UHPC). This study explores the reinforcement effect on the tensile behavior of UHPC with a low fiber volume fraction with the aim of facilitating more cost-efficient UHPC applications. The axial tensile behavior of 30 UHPC specimens with low fiber volume fractions at different reinforcement ratios was tested through direct tensile tests. The findings indicate that adopting UHPC with a low fiber volume fraction can significantly mitigate the ductility deterioration of rebar-reinforced UHPC (R-UHPC), and both increasing the reinforcement ratio and decreasing the fiber volume fraction contribute to the improvement in ductility. The failure modes of R-UHPC are determined by the ratio of reinforcement ratio and fiber volume fraction, rather than a single parameter, which also means that R-UHPC with different parameters may correspond to different methods to predict tensile load-bearing capacity. For UHPC with a fiber volume fraction low to 0.5%, incorporating steel rebars gives superior multi-crack cracking behavior and excellent capacity to restrict the maximum crack width. Increasing the fiber volume fraction from 0.5% to 1.0% at the same reinforcement ratio will yield little benefit other than an increase in tensile load-bearing capacity.

Experimental Study of Recycled Aggregate, Fly Ash and Steel Fiber on Properties of Concrete

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. The utilization of recycled aggregates is a good solution to the problem of an excess of waste material, provided that the desired final product quality is reached.The use of recycled aggregate in concrete can be useful for environmental protection and economical terms recycled aggregates are the materials for the future.The work focuses on the possibilities of the recycled aggregate concrete as structural material with replacement level of 25%,50% and 75% byweight of concrete. Fly ash is also used as a partial replacement of Portland cement with10, 20 & 30 % by weight of cement in order to improve the properties of RAC. The use of fly ash replacement in Recycled Aggregate concrete (RAC) could improve compressive strength to be higher than that of concrete without fly ash at the same W/B ratio.The fly ash is an organic, non combustible residue of coal after burning in power plant. The steel fiber are available imperforated, corrugated or with wide end for better bending.The volume fraction of the hook end steel fiber used in this study varied from 0.25%,0.5%a and 0.75 %. Key words- Fly Ash, Recycled Aggregate, Recycled Concrete Aggregate, Steel Fiber, Water Cement Ratio.

Properties of a steel fiber reinforced cementitious composite stool with digitally distributed steel fibers

This study aims to maximize the reinforcement of steel fibers in cementitious composite structural members. A 3D-printing stool was chosen as a case study. The orientation of the steel fibers is designed to be parallel to the direction of the tensile stress, and the volume fraction of steel fibers everywhere meets the tensile requirements, which is defined as digitally distributed steel fiber reinforced cementitious composite (DD-SFRC) stool. The DD-SFRC stool was prepared using magnetic field aligning and 3D-printing, to control the volume fraction and orientation of steel fibers in the stool. The results show that digitally distributed steel fibers reinforce the mechanical properties of the stool more effectively, in terms of ultimate load, energy absorption capacity, joint ductility, and the width of the crack, compared with that of the stool with random and aligned steel fiber distribution. The methodology of design and approach of preparation of DD-stool are presented, which are valid and applicable to other structures in either laboratory or practice.

Drying Shrinkage Properties of Arched Steel Fiber Reinforced Concrete by Volume Fraction of Steel Fiber

Drying Shrinkage Properties of Arched Steel Fiber Reinforced Concrete by Volume Fraction of Steel Fiber

Study on the biaxial mechanical behavior of steel fiber reinforced recycled aggregate concrete

Steel fiber reinforced recycled aggregate concrete (SFRAC) was proposed considering the enhancement of mechanical properties by incorporating steel fibers and the reduction of environmental impacts and cost due to the addition of recycled aggregates. To better understand the mechanical properties of SFRAC under biaxial compression and its failure criteria, 150 SFRAC specimens were prepared with different replacement ratios of recycled coarse aggregate (50%, 75%, 100%) and volume fractions of steel fibers (0%, 0.6%, 1.2%, 1.8%). In addition, various stress ratios were considered in the biaxial compression test (σ1:σ2=0:1.0, 0.2:1.0, 0.5:1.0, 0.7:1.0, 1.0:1.0). The results indicated that the failure of SFRAC was observed as columnar and splitting modes depending on the biaxial stress ratio. The impacts of the considered variables on the biaxial peak strength were studied. The increasing volume fraction of steel fibers tended to improve the biaxial peak strength of SFRAC within a threshold beyond which the impact was negative. Similarly, the biaxial strength of most SFRAC samples showed a pessimum effect when the biaxial loading stress ratio was higher than 0.7, though they were still higher than the uniaxial compressive strength. A failure criterion for SFRAC under biaxial compression loading is proposed, in which four parameters are included. The calculations are in good agreement with the experimental results.

Dynamic mechanical behavior of steel fiber reinforced-recycled aggregate concrete subjected to uniaxial loading

The objective of this study was to investigate the dynamic mechanical performance of steel fiber reinforced-recycled aggregate concrete (SFRAC) under uniaxial loading. A total of 22 groups of cylindrical samples, each measuring 100 mm × 200 mm, were subjected to uniaxial compression. The investigation placed particular emphasis on examining the influence of variables such as the strain rate, volume fraction of steel fiber (Vsf), volume fraction of polyvinyl-alcohol (PVA) fiber (Vpf) and substitution ratio of recycled coarse aggregate (SRCA) on the failure modes, stress-strain curves and deformation ability. The results revealed that the incorporation of steel fibers and PVA fibers enhanced the overall integrity of fractured samples, and induced changes in the failure modes. In particular, the maximum increase in uniaxial compressive strength attributed to strain rate, Vsf and Vpf reached up to 15.0%, 23.57% and 8.57%, respectively. Although the inclusion of recycled aggregates had no significant influence on the failure mode, it decreased the compressive strength by up to 16.34%. Finally, a dynamic stress-strain theoretical constitutive model was established in this paper. The predictions derived from this model closely aligned with the experimental stress-strain curves obtained during the conducted tests. This study provides valuable insights for applications of SFRAC in structural engineering.

Study on fracture characteristics of steel fiber reinforced manufactured sand concrete using DIC technique

Incorporation of steel fibers to improve the defects of manufactured sand concrete (MSC) has received a great deal of attention, while the fracture mechanism of the MSC by volume fraction of steel fibers (Vf) has been less studied. To investigate the effects of steel fibers on the fracture characteristics of the MSC, three-point bending tests were conducted on the MSC beams with different Vf (0, 0.5 %, 1.0 % and 1.5 %) using digital image correlation technique. The effects of Vf on the fracture parameters and crack propagation paths of steel fiber reinforced manufactured sand concrete (SFRMSC) were analyzed. In addition, fracture process zone (FPZ) of the SFRMSC was analyzed via the length of the FPZ (lFPZ) and area of the FPZ (AFPZ). The results indicated that the residual strength, initiation fracture toughness, unstable fracture toughness, fracture energy and characteristic length of the SFRMSC increased with the increase of Vf, whereas the crack propagation paths were not sensitive to the changes of Vf. The addition of steel fibers increased the interaction between the aggregate and the matrix, which led to an increase in the number of microcracks. Before approaching the Pc, both the lFPZ and AFPZ showed essentially stable changes. As Vf increased, the lFPZ and AFPZ developed faster to their corresponding maximum values. The AFPZ reached a maximum and increased with the Vf. At the Pc, the lFPZ and AFPZ changed in a similar trend when Vf ≤ 1.0 %. When Vf > 1.0 %, the lFPZ changed slightly while the AFPZ increases sharply.

A multiscale experimental study of CFRP-UHPFRC interfacial bond behaviour: Failure mechanisms and a fibre volume dependent bond-slip law

Carbon fiber reinforced polymer (CFRP) materials are being increasingly used with ultra high performance fiber reinforced concrete (UHPFRC) to construct composite structural members making use of their unique properties. As the interfacial bond behaviour between CFRP and UHPFRC plays a key role in the design of such members, it was investigated in detail by multiscale experiments of single shear pull-off tests of 26 CFRP-UHPFRC bonded joints with different bond lengths and widths of CFRP laminates and 0.5–4% volume fraction (Vsf) of straight high-strength steel fibers (length = 13 mm, diameter = 0.2 mm). 12 CFRP-normal strength concrete (NSC) bonded joints were also tested for comparison. The surface strain evolution and the interfacial debonding process were captured and analysed by the digital image correlation (DIC). The complex microscale failure mechanisms, such as the mortar cracking, the CFRP-UHPFRC interfacial debonding, and the steel fibers’ slipping and pulling out of the mortar, were visualised and analysed by the 3D micro X-ray Computed Tomography (μXCT) scanning. It was found that the debonded layer of mortar under the laminates was much thinner than that in the CFRP-NSC bonded joints, due to the lack of coarse aggregates in UHPFRC. The peak pulling forces for the specimens with Vsf = 0.5%, 1%, 2%, and 4% were 9%, 21%, 28%, and 29% higher than those without steel fibers, respectively. An empirical bond strength model and an interfacial bond-slip model were proposed based on the test results, accounting for Vsf, the material properties, and the laminate to UHPFRC width ratio. Finally, the proposed bond-slip model was applied in finite element modelling to simulate the flexural behaviour of an FRP-UHPFRC composite deck in the literature, with the interfacial debonding failure and the ultimate loads successfully predicted.

Effect of steel fiber orientation on punching shear resistance of steel fiber reinforced cementitious composites round slabs

The slab-column structure is commonly used in civil engineering. At the slab-column joints, the load from the slab is transmitted to the column. In the extreme state, punching shear failure may occur at the joints, and the failure is brittle without warning. When the punching shear failure occurs at the joint of round slab and column, usually the cracking is a conical shape because the direction of tensile stress caused by punching shear is generally radial. By incorporating steel fibers into cementitious composites round slabs, the steel fibers can bridge the conical surface, and therefore enhance the punching shear resistance of round slabs. If the steel fibers in the round slab are dispersed in the direction conducive to the punching shear resistance, the steel fibers can take full effect of strengthening and toughening, thus improving the punching shear resistance of the slab. So, the purpose of this investigation is to improve the punching shear resistance of steel fiber reinforced cementitious composite slabs by maximizing the reinforcement of steel fibers on the composites. Firstly, a model of punching shear resistance of steel fiber reinforced cementitious composite round slabs was derived based on the Modified Compression Field Theory with consideration of the orientation of steel fibers. According to the model, the slab with radial distribution of steel fibers around the punching shear area shows the highest punching shear resistance. Secondly, the approach of radially aligning steel fibers in the slab around the punching area by magnetic driving is proposed using a self-developed magnetic device. And, radially dispersed steel fiber reinforced cementitious composites (JD-SFRC) round slabs were prepared. Thirdly, the punching shear resistance of JD-SFRC round slabs, as well as unidirectionally aligned (1D), two-dimensionally aligned (2D), and randomly distributed (RD) steel fiber reinforced cementitious composites round slabs were tested. At last, the effect of the orientation of steel fibers on the punching shear resistance and failure characteristics of the round slabs were analyzed by comparing the results from tests and the model. Meanwhile, the effect of the volume fraction of steel fibers on the punching shear resistance of the slab was examined. The results show that, When the volume fraction of steel fiber is 1.0 %, compared with RD-SFRC round slabs, the ultimate shear resistance of 1D-SFRC, 2D-SFRC and JD-SFRC round slabs is increased by 14 %, 15 %, and 24 %, respectively.

Uniaxial behavior of UHPC under cyclic compression: Experimental investigation and constitutive model

The compressive behavior of ultra-high performance concrete (UHPC) is experimentally investigated in this paper for three different micro steel fiber volume fractions (Vf= 1.0%, 1.5% and 2.0%) under uniaxial monotonic and cyclic compression. The cube specimens are loaded and fully unloaded twice per displacement increment to track the evolution of damage. Cycles are included till the peak load and after attaining the peak load, all the way till the load carrying capacity becomes marginal. A one dimensional (1D) elastoplastic damage constitutive model is proposed to simulate the damage of UHPC under uniaxial cyclic compression, incorporating the damage parameters within a damage function deduced from the tests. Damage is incorporated as a nonlinear function and thus the hysteresis behavior (distinguishable unloading and reloading paths) is adequately captured in the model. The experimental results from this study and the accompanying model demonstrate that the strength and stiffness degradation in UHPC due to low cycle fatigue stabilize at higher fiber dosage levels.