Behavior Of Ultra-high Performance Concrete Research Articles

Hybrid effects of carbon nanotubes and steel fiber on dynamic mechanical properties of ultra-high performance concrete

This study aims to investigate the hybrid effects of carbon nanotubes (CNTs) and steel fiber (SF) on dynamic mechanical behavior of ultra-high performance concrete (UHPC) under the high-speed dynamic loading effects. The functionalized CNTs content varied from 0.0% to 0.20%, and SF volume content was added from 0.5% to 2.0%. The UHPC specimens were tested at strain rates ranging from about 60 s−1 to 240 s−1 using the split Hopkinson pressure bar. The dynamic behavior of specimens was evaluated in terms of stress–strain relationships, energy absorption capacity and failure patterns. Test results show that dynamic compressive properties of UHPC are strongly sensitive to strain rates. The individual addition of CNTs obviously improves the dynamic performance of plain UHPC, and there exists an optimal CNTs content of about 0.10% to achieve better improvement effects. The dynamic increase factor (DIF) is almost unrelated to CNTs content but closely correlated to strain rate variations, which can be well described by a modified empirical formula. Furthermore, it is found that the hybrid CNTs and SF display more significant reinforcing effects on dynamic properties of UHPC, as the UHPC specimen with 0.10% CNTs and 2.0% SF exhibits the highest dynamic compressive strength and toughness. Meanwhile, UHPC reinforced by hybrid fibers is less sensitive to high strain rates compared to plain UHPC. From microscopic observations, it is reasonably believed CNTs and SF exert synergistic reinforcing effects on UHPC matrix in view of their respective characteristics, which are more significant in improving the interfacial transition zone (ITZ) structure between SF and the surrounding matrix. This work promotes a fundamental understanding for the reinforcing effects of CNTs on UHPC, and provides an effective strategy to reinforce UHPC from multiscale perspectives.

Numerical Modelling of the Compressive and Tensile Behavior of Ultra-High Performance Concrete in Beams

INTRODUCTION Ultra-high performance concrete (UHPC) differs in its structural behavior from conventional concrete upon loading due to its high compressive and tensile strength, stiffness, toughness, and durability. Therefore, UHPC needs an appropriate constitutive model to simulate its mechanical properties in finite element analysis (FEA). METHOD In this study, numerical models were developed to trace the structural behavior of UHPC beams upon loading since beam behavior depends on the constituents' response to compression and tension. New numerical models were formulated to display the stress-strain relationships of UHPC in compression and tension by adopting a new methodology that depended on actual results. The compressive stress-strain relationship consisted of two portions; the ascending one for elastic and strain hardening up to compressive strength and a descending curve for the strain-softening until reaching a strain of 0.0062. RESULT A linear tensile stress-strain relation was applied for the elastic tensile behavior up to tensile strength. Then, a tri-linear relationship was applied for the stiffness degradation and crack propagation upon debonding fibers from the concrete matrix until fracture. These numerical models were used in Abaqus software to simulate the UHPC beam behavior. CONCLUSION The developed models were verified and approved for beams' behavior upon loading in flexure and shear. The results indicated that the models could predict the UHPC beams' response throughout the entire loading range from the beginning until failure. The verification included bear capacity, deflection, crack pattern, and stress distribution.

Effect of concrete composition on drying shrinkage behavior of ultra-high performance concrete

The concrete compositions have great effects on the drying shrinkage behavior of ultra-high performance concrete (UHPC). Five factors including water-to-binder ratio (w/b) and contents of steel fiber, superplasticizer, silica fume and fly ash were considered to evaluate their influences on the drying shrinkage behaviors of UHPC in this investigation. Based on the drying shrinkage test results, the effect degrees of these factors were analyzed by the gray relation entropy analysis. Then the microstructure and pore distributions of UHPC samples were measured by scanning electron microscope and mercury intrusion porosimetry for mechanism analyses. The results reveal that a lower w/b or higher content of fly ash is conducive to achieving a reduction in drying shrinkage of UHPC, while the additions of superplasticizer and silica fume have an adverse effect on drying shrinkage, resulting in increased drying shrinkage, and the effect of steel fiber on drying shrinkage is not monotonic, its optimum dosage is 50 kg/m3 in this work and a lower or higher amount will cause an increased DSS of UHPC compared with the one with an optimum dosage. The five factors can affect the drying shrinkage behavior of UHPC by principally changing the microtopography and internal pore structure of the concrete matrix, according to the gray relation entropy analysis, the effect degrees of studied factors on drying shrinkage of UHPC are as follows: w/b > superplasticizer > silica fume > fly ash > steel fiber.

The behavior of UHPC containing recycled glass waste in place of cementitious materials: A comprehensive review

An innovative leader in cutting-edge sustainable concrete technology is ultra-high-performance glass concrete (UHPGC). Through the utilization of post-consumer glass, UHPGC technology can improve the environment. Due to the fact that these materials are neither biodegradable nor ecologically benign, any economic benefit from a reduction in the amount of waste dumped in landfills is undesirable. This endeavor reviews the previous research on the behavior of Ultra-High-Performance Concrete (UHPC) incorporating waste glass powder as an eco-friendly cementitious material. A summarization was performed as well for previous works which used milled waste glass to replace cement, Quartz Powder (QP), Silica Fume (SF) and Sand or Quartz Sand (QS) in UHPC mixtures. Thus, various applications of Waste Glass Powder (WGP) in UHPC and its fresh and hardened characteristics were examined. The paper also pursues updating the database for more experimental and numerical work on UHPC incorporating waste glass from different sources.

Compressive behaviour of UHPC-filled square high-strength steel tube stub columns under eccentric loading

This paper presents experimental and numerical studies on the compressive behaviour of ultra-high performance concrete (UHPC) filled rectangular high-strength steel tube (UFHST) stub columns under eccentric loading. A total of 16 compressive specimens with a maximum concrete strength of 150 MPa and a maximum steel strength of 846 MPa were designed and tested. The test results showed that the use of high-strength steel tube and UHPC without steel fibres largely enhanced the load-carrying capacity of concrete-filled steel tube columns. The use of fibre-free UHPC adversely influenced the ductility of the columns. The incorporation of steel fibres into UHPC when casting the columns, however, improved the ductility of the composite columns and at the same time gained additional load-carrying capacity. A finite element (FE) model was then developed using ABAQUS and verified with test results. The numerical parametric study using the verified FE model showed that the confinement effect from the high-strength steel tube onto the UHPC was obscure before peak stress of the column was achieved, but was gradually activated/increased in the post-peak stress stage. Therefore, subsequent to the peak value, the axial stress of the confined UHPC experienced a short decrease first and then a continued growth. The findings from the present study set an experimental and numerical basis for future investigations to expose optimistic design of the UFHST stub columns.

Dynamic compression behavior of ultra-high performance concrete with hybrid polyoxymethylene fiber and steel fiber

Synthetic fibers are adopted to mix with conventional steel fibers (SF) for ultra-high performance concrete (UHPC) to improve chemical resistance and reduce self-weight. Polyoxymethylene fiber (POMF), as a type of lightweight, high-strength, corrosion-resistant synthetic fiber, is expected to replace SF generally used in UHPC. Due to the obvious difference of mechanical properties between POMF and SF, different POMF/SF hybrid ratios may affect the dynamic compression behavior of UHPC. To investigate the dynamic mechanical behavior of hybrid POMF/SF reinforced UHPC (HUHPC), axial compression and split Hopkinson pressure bar (SHPB) tests with strain rate from 40 to 170 s−1 of HUHPC were conducted. The fiber volume fraction was fixed at 3% and four different POMF/SF hybrid ratios (0:3, 1:2, 2:1, 3:0) were adopted. The effects of hybrid POMF/SF ratios on the dynamic compressive properties of HUHPC, including dynamic compressive strength, dynamic elastic modulus, dynamic increase factor (DIF), and impact toughness were discussed. The results show that HUHPC exhibits obvious strain rate sensitivity under impact loading. With the increase of strain rate, the dynamic compressive strength, dynamic elastic modulus, DIF and impact toughness of HUHPC increased. Based on the linear assumption of fiber volume fraction and dynamic compressive strength enhancement of UHPC, the calculated dynamic compressive strength of HUHPC is lower than the experimental values. As the strain rate increases, the propagation path of microcrack transforms from interface of fiber-matrix to penetrating the fiber, resulting in plastic deformation of SF and fracture of POMF, consuming a large amount of impact energy and improving the impact toughness resistance of HUHPC.

Torsional behavior of ultra-high performance concrete (UHPC) rectangular beams without steel reinforcement: Experimental investigation and theoretical analysis

Steel fiber is the main reinforcing fiber of ultra-high performance concrete (UHPC) and could greatly improve its tensile strength and toughness. As a result, steel fiber has a significant influence on the structural behavior of UHPC beams under pure torsion. However, related research in this regard is severely insufficient. To fill in this research gap, this paper presents an extensive investigation to explore the effect of steel fiber property on the torsional performance of UHPC beams without steel reinforcement. A total of 8 UHPC rectangular specimens (including one control beam without steel fibers) were tested under pure torsion until failure. The variables in the specimens included the volume fraction, type, dimension and hybrid effect of steel fibers. The failure modes, torque versus angle of twist curves, torque versus strain curves and distribution of cracks for all specimens were acquired based on the torsion tests. Results show that the addition of steel fibers significantly increased the cracking and ultimate torques of UHPC beams, with maximum increases of 79 % and 159 %, respectively. The volume content, shape and dimension of steel fibers exhibited significant influence on the torsional performance of UHPC beams, including torsional capacity, ductility and cracking pattern. The shape and dimension of steel fibers both affected the post-cracking load-carrying capacity of UHPC beams. The cracking and ultimate torques of the two UHPC beams with hybrid steel fibers were higher than those containing single type of steel fiber, both exhibiting positive hybrid effects. Based on the experimental results of this study, the hybrid use of 1 % end-hooked and 1 % long straight steel fibers is recommended in UHPC beams by comprehensively considering the torsional performance and the cost. Finally, by fully considering the excellent tensile behavior of UHPC, theoretical formulas were proposed for calculating the cracking and ultimate torques of UHPC beams without steel reinforcement based on the Bredt thin-tube theory. Comparison between the experimental and theoretical results on 36 UHPC torsional beams demonstrated the feasibility of the proposed formulas.

Impact of micromechanics on dynamic compressive behavior of ultra-high performance concrete containing limestone powder

To improve sustainability and reduce costs, interest in using limestone powder (LP) in ultra-high performance concrete (UHPC) has been increasing in recent years. Incorporating LP changes packing, characteristics of C–S–H and pore structure of interfacial transition zone (ITZ) and matrix by physical and chemical effects. However, the influences of LP replacement on the mechanical properties of UHPC, from micromechanics to macroscopic impact behaviors, have not been well understood. Herein, mercury intrusion porosimetry (MIP), thermal gravimetric (TG), nanoindentation test and Split Hopkinson pressure bar (SHPB) test are performed to reveal the relationships between the micromechanical properties and the dynamic performances of UHPC. Results show that appropriate amount of LP incorporation optimizes the packing, leading to a narrower space and more densely compacted C–S–H that improves the percentages of high-density C–S–H and ultra-high density C–S–H in ITZ. The higher density of C–S–H results in a higher steel fiber-matrix bond strength, which benefits a higher dynamic compressive strength and energy absorption while a lower dynamic increase factor. More low-density C–S–H are generated due to the dilution effect of excessive LP, leading to a weak steel fiber-matrix bond strength that deteriorates the impact resistance. Additionally, the mechanism of LP enhancing the quality of C–S–H in ITZ is proposed, which favors understanding the relation between macro dynamic performance and interfacial behavior at the micro and nanoscale.

Effects of Silica Fume Purity on Behavior of Ultra-High Performance Concrete

Abstract The use of silica fume in high-strength and ultra-high performance concretes has been studied from many perspectives including optimum replacement rate, implications on late-age strengths, and rheological properties. However, the effects of silica fume purity (e.g., silicon dioxide [SiO2] content) are not nearly as well documented, especially when shorter duration curing regimes at higher temperatures are utilized. In this paper, four silica fumes with varying SiO2 contents were used to quantify the effects of SiO2 purity in silica fume on the behavior of ultra-high performance concrete. Several variables were evaluated to determine if changes in SiO2 content significantly influenced properties of interest. Overall, silica fume purity had the most impact on thermal set time, maximum temperature generated during hydration, and compressive strength. Thermal set time values differed by over 7 h, and maximum temperatures differed by 7°C. After one day of room-temperature curing, compressive strength varied by 50 MPa, and elastic modulus varied by 14 GPa. When specimens were cured at elevated temperatures for short durations of time (e.g., one to six days), fc values ranged from 35 to 47 MPa.

Shear behaviour of ultra-high performance concrete beams with openings

An experimental investigation of the shear behaviour of Ultra-high performance concrete (UHPC) beams with web openings was conducted. Eight UHPC beams were experimentally cast and subjected to a four-point bending test. One beam, which was considered the control specimen, had no web openings, whereas the seven other beams contained openings. The studied parameters were steel fibre content, opening position, opening shape and reinforcement schemes around the web openings. Shear behaviour was evaluated on the basis of cracking patterns, load-deflection behaviour, ductility, initial stiffness, toughness and strain. Experimental results showed that increasing the volume fraction of steel fibres enhanced the shear strength, stiffness, toughness, ductility and strain in steel reinforcements. The presence of web openings in the shear region led to a decrease in the shear strength by 33.7%. The ultimate load of UHPC beams with openings has been evidently improved using reinforcement around the openings. An analytical verification was proposed to evaluate the experimental test results of UHPC beam by using design codes, different prediction equations and the empirical equation proposed herein. The proposed empirical equation showed an excellent agreement with available experimental data.

Tensile Behaviour of Ultra-High Performance Concrete with Different Steel Bars

Tensile Behaviour of Ultra-High Performance Concrete with Different Steel Bars

Dynamic Compression Behavior of Ultra-High Performance Concrete with Hybrid Polyoxymethylene Fiber and Steel Fiber

Dynamic Compression Behavior of Ultra-High Performance Concrete with Hybrid Polyoxymethylene Fiber and Steel Fiber

Compression behavior of ultra-high performance concrete (UHPC) confined with high-strength rectilinear ties

Compression behavior of ultra-high performance concrete (UHPC) confined with high-strength rectilinear ties

Long Term Behavior of Ultra High-Performance Concrete

Ultra-High-Performance Concrete (UHPC) is one of the important types of concrete technology breakthroughs in the 21st century. It achieved high results of mechanical properties, durability (resistance fire) and bonding strength. The aim of paper is to evaluate the long-time behavior of UHPC. The main variables were finesse modulus of sand, crushed quartz powder, fly ash and metakaolin and methods of curing (water& hot).The sand with different fineness modulus(3.2, 2.36 and 1.9) were used, Crushed quartz powder with ratio (10%, 20% and 30%) as a replacement of sand was used. Fly ash and metakaolin, with of (10%, 20%, 30 and 40%) and (5%, 10% and 15%) as a replacement of cement; respectively. The effect of these variables on the mechanical properties (compressive, tensile, flexural strength) at different ages. Also, the drying shrinkage strain was evaluated. The results showed that using and with fineness modulus (1.9), 20% ratio of crushed quartz powder to fine sand (CQ/S), 20% of fly ash to cement (FA/C) and 5% of metakaolin to cement (MK/C) give the best proportions of UHPC. The compressive strength for this mix was 900 kg/ cm2 .

A Numerical Model for Tracing Structural Response of Ultra-High Performance Concrete Beams

This paper presents a finite element-based numerical model for tracing the behavior of ultra-high performance concrete (UHPC) beams. The model developed in ABAQUS can account for stress–strain response of UHPC and reinforcing bar in both tension and compression, bond between concrete and reinforcing steel, and strain hardening effects in bars and UHPC and can trace the detailed response of UHPC beams in the entire range of loading. This model is validated by comparing predicted response parameters including load-strain, load-deflection, and crack propagation against experimental data governed from tests on UHPC beams with different reinforcement ratios, fiber volume fractions, and loading configurations (shear and flexural loading). The validated model is applied to quantify the contribution of stirrups and concrete to shear strength of beams so as to explore the feasibility of removing shear reinforcement in UHPC beams.

Experimental research on the spalling behaviour of ultra-high performance concrete under fire conditions

Explosive spalling is a severe risk for ultra-high performance concrete (UHPC) structures exposed to fire, fundamentally obstructing the comprehensive study of fire safety and the extensive engineering application of UHPC. In contrast to previous studies on the optimisation of components and mixture properties, this paper proposes a new curing regime, two-stage hot air curing (THAC), to inhibit the explosive spalling of UHPC in a high stress state and under an extreme heating rate. Two hundred and twenty-eight UHPC cubes with variations in parameters such as curing regime, fibre type, and water-binder ratio were tested under the ISO 834 fire standard. The influence of THAC on the mechanical properties and explosive spalling of the samples was investigated, and scanning electron microscopy (SEM) was utilised to observe the microstructures of the tested cubes. On this basis, twelve full-scale UHPC beams with various curing regimes and load ratios were tested under thermal–mechanical coupling conditions. The explosive spalling behaviour of the beams was observed, and the influence of spalling on the fire resistance was analysed. The test results indicated that UHPC formed a dense microstructure during the first stage of 50 °C curing. With hot air curing at 150 °C, further hydration and pozzolanic reaction of residual cement and mineral admixtures could be activated, generating massive calcium-silica-hydrate (C-S-H) gels. Moreover, almost all the free water could be consumed and evaporated after THAC curing, dramatically improving the explosive spalling resistance of UHPC. THAC could completely prevent the fire-induced explosive spalling of UHPC of different sizes and with varying stress states and improve the mechanical properties. This occurred despite conventional curing such as standard curing, water curing, and steam curing potentially leading to more serious spalling and a significant reduction in fire resistance, even possibly affecting the failure mode of UHPC beams.

Compressive behavior of ultra-high performance concrete confined with FRP

This paper presents an experimental study on the compressive behavior of ultra-high performance concrete (UHPC) confined with fiber-reinforced polymer (FRP) composites. A total of 18 cylindrical specimens were prepared from UHPC containing 2.5% of micro steel fibers by volume, among which 6 were unconfined and 12 were confined with carbon FRP (CFRP). They were tested to investigate the experimental compressive behavior of unconfined or CFRP-confined UHPC. The external confinement was provided by two to six plies of CFRP wraps with the confinement ratio ranging from 0.11 to 0.54. The test results obtained from this study were presented and compared with those from existing studies on unconfined and FRP-confined UHPC specimens in the form of concrete-filled FRP tubes or FRP-wrapped cylinders. Predictive equations were proposed for the compressive strength and ultimate strain of FRP-confined UHPC, based on the interpretation of test results from the present study and from the existing literature. An existing stress–strain model for FRP-confined conventional concrete has been recalibrated for FRP-confined UHPC. While the recalibrated model predicted the test results of FRP-confined UHPC reasonably well, the inadequacy of a parabolic curve for describing the stress–strain relationship of unconfined UHPC is noticed, as a result of the higher strength and more brittle behaviour of UHPC in compression.

Influence of steel fiber size, shape, and strength on the quasi-static properties of ultra-high performance concrete: Experimental investigation and numerical modeling

This investigation focused on identifying the impact of various types of steel fibers on the quasi-static mechanical behavior of ultra-high performance concrete (UHPC) through different laboratory experiments as well as their numerical modeling. UHPC specimens were fabricated with four steel fiber types: ZP305, Nycon type V, OL 10mm, and OL 6mm. Fiber shape and size had little impact on quasi-static properties of UHPC in compression, while they showed significant impact on flexural and tensile properties. The main benefits offered by the smaller fibers occurred prior to reaching the ultimate load carrying capacity. Once the ultimate strength was reached, larger fibers were more effective in bridging larger cracks. Numerical modeling of the presented experiments were performed using the Lattice Discrete Particle Model (LDPM) enriched with fibers effect. LDPM is a meso-scale model simulating concrete at the scale of coarse aggregate pieces, and it has been extensively used for the simulation of concrete mechanical behavior under various loading and environmental conditions. The extension of this model, the so-called LDPM-F, developed for the simulation of fiber-reinforced concrete, is employed in this research to verify and better understand the experimental observations.

Tensile and Flexural Behavior of Ultra-High Performance Concrete (UHPC) under Impact Loading

The rate-sensitive properties of ultra-high performance concrete (UHPC) subjected to flexural and direct tensile loads were investigated. Dynamic flexural tests were performed by means of a drop hammer impact machine, whereas a servo-hydraulic system was utilized to conduct direct tension tests. The absorbed energy of the UHPC specimens under flexure increased from 0.74 to 4.49 J as the input energy varied from 6.7 to 67 J. The load-bearing capacity and maximum flexural deformation exhibited identical trends as the input energy increased. The high-speed tensile properties, including strength, ductility, and energy dissipation capacity, improved upon increasing the strain rate up to 100 s-1. A nonuniform distribution of the deformation of UHPC specimens under high-speed tension was observed by identifying the zones of strain localization (near crack), uniform strain (far field), and shear lag (transition zone in between) using digital image correlation. The full-field data of displacement, strain, and strain rates were obtained and quantitatively evaluated. The results indicated that multiple mechanisms were involved in the rate-dependent behavior of the UHPC specimens.

Numerical study on the mechanical behavior of ultrahigh performance concrete using a three‐phase discrete element model

AbstractThis paper deals with numerical analysis on the mechanical behavior of ultrahigh performance concrete (UHPC) under uniaxial compression. For the modeling of the mechanical behavior of UHPC, the mesh‐free discrete element method was applied. To calibrate the model parameters and validate the numerical simulation results, a set of experimental investigations including mechanical tests, microscopy and tomography analysis, were performed. The scanning electron microscopy and polarized light microscopy were used to examine cross sections of UHPC as well as to characterize interfacial transition zone and aggregate in detail. X‐ray microtomography analysis was used to obtain information about the nonspherical shape of aggregate and to generate a realistic structural model. Simulation results have shown that the developed model predict stiffness reliably, strength, and breakage pattern of UHPC and shows good agreement with experimental results. Finally, the model has been applied to analyze the main crack initiation in static failure and to investigate the influence of different parameters such as aggregate content as well as aggregate and binder stiffness on the mechanical behavior of UHPC.