Effect of Shrinkage Reducing Agent and Steel Fiber on the Fluidity and Cracking Performance of Ultra-High Performance Concrete

Due to the low water-cement ratio of ultra-high-performance concrete(UHPC),fluidity and shrinkage cracking are key aspects determining the performance and durability of this type of concrete.In this study,the effects of different types of cementitious materials,chemical shrinkage-reducing agents(SRA)and steel fiber(SF)were assessed.Compared with M2-UHPC and M3-UHPC,M1-UHPC was found to have better fluidity and shrinkage cracking performance.Moreover,different SRA incorporation methods,dosage and different SF types and aspect ratios were implemented.The incorporation of SRA and SF led to a decrease in the fluidity of UHPC.SRA internal content of 1%(NSRA-1%),SRA external content of 1%(WSRA-1%),STS-0.22 and STE-0.7 decreased the fluidity of UHPC by 3.3%,8.3%,9.2%and 25%,respectively.However,SRA and SF improved the UHPC shrinkage cracking performance.NSRA-1%and STE-0.7 reduced the shrinkage value of UHPC by 40%and 60%,respectively,and increased the crack resistance by 338%and 175%,respectively.In addition,the addition of SF was observed to make the microstructure of UHPC more compact,and the compressive strength and flexural strength of 28 d were increased by 26.9%and 19.9%,respectively.

Spalling and Mechanical Properties of Fiber Reinforced High-performance Concrete Subjected to Fire

Spalling and mechanical properties of FRHPC subjected to fire were tested on notched beams. The results confirm that the internal vapor pressure is the leading reason for spalling of high-performance concrete （HPC）. At the same time, the temperature-increasing velocity and constrained conditions of concrete element also play significant roles in spalling. Steel fibers cannot reduce the risk of spalling, although they have obvious beneficial effects on the mechanical properties of concrete before and after exposure to fire. Polypropylene （PP） fibers are very useful in preventing HPC from spalling, however, they have negative effects on the strengths. By using hybrid fibers （steel fibers＋PP fibers）, both good anti-spalling performance and improved mechanical properties come true, which may provide necessary safe guarantee for the rescue work and structure repair after fire disaster.

Dynamic Mechanical Behaviour of Ultra-high Performance Fiber Reinforced Concretes

Ultra-high performance fiber reinforced concretes （UHPFRC） were prepared by replacing 60% of cement with ultra-fine industrial waste powder. The dynamic mechanical behaviour of UHPFRC with different fiber volume fraction was researched on repeated compressive impact in four kinds of impact modes through split Hopkinson pressure bar （SHPB）. The experimental results show that the peak stress and elastic modulus decrease and the strain rate and peak strain increase gradually with the increasing of impact times. The initial material damage increases and the peak stress of the specimen decreases from the second impact with the increasing of the initial incident wave. Standard strength on repeated impact is defined to compare the ability of resistance against repeated impact among different materials. The rate of reduction of standard strength is decreased by fiber reinforcement under repeated impact. The material damage is reduced and the ability of repeated impact resistance of UHPFRC is improved with the increasing of fiber volume fraction.

Seismic Performance of Steel Reinforced Ultra High-strength Concrete Columns

The seismic performance of steel reinforced ultra-high-strength concrete columns(SRSHC) with various shear-span ratios(λ) were studied through a series of experiments.The concrete compressive cube strength value of experimental specimens ranged from 92.9 MPa to 108.1 MPa.The main experimental variables affecting seismic performance of specimens were axial load ratio and stirrup reinforcement ratio.The columns(λ=2.75) subjected to low cyclic reversed lateral loads failed mainly in the flexural-shear mode failure and columns(λ≤2.0) subjected to low cyclic reversed lateral loads failed mainly in the shear mode failure.Shear force-displacement hysteretic curves and skeleton curves were drawn.Coefficient of the specimen displacement ductility was calculated.Experimental results indicate that ductility decreases with axial pressure ratio increasing,and increases with stirrup reinforcement ratio increasing.Limit values of axial pressure ratio and minimum stirrup reinforcement ratio of columns are proposed to satisfy definite ductility requirement.The suggested values provide a reference for engineering application and for the amendment of the current Chinese design code of steel reinforced concrete composite structures.

Charactersitics of Stress-strain Curve of High Strength Steel Fiber Reinforced Concrete under Uniaxial Tension

A whole of 110 specimens divided into 22 groups were tested with varying the volume fraction of steel fibers and the matrix strength of these specimens. The stress-strain behaviors of four types of steel fiber reinforced concrete （SFRC） under uniaxial tension were studied experimentally. When the matrix strength and the fiber content increase, the tensile stress and tensile strain vary differently according to the fiber type. The mechanisms of reinforcing effect for different types of fiber were analyzed and the stress-strain curves of the specimens were plotted. Some experimental factors for stress or strain of SFRC were given. A tensile toughness modulus Re0.5 was introduced to evaluate the toughness characters of SFRC under uniaxial tension. Moreover, the formula of the tensile stress-strain curve of SFRC was regressed. The theoretical curve and the experimental ones fit well, which can be used for references in construction.

Structural Behavior of Continuous Prestressed Steel Fiber Reinforced High Strength Concrete Beam

The flexural behaviors of continuous fully and partially prestressed steel fiber reinforced high strength concrete beams are studied by experiment and nonlinear finite element analysis. Three levels of partial prestress ratio （PPR） are considered, and three pairs of two-span continuous beams with box sections varying in size are designed. The major parameters involved in the study include the PPR and the fiber location. It is concluded that the prestressed high strength concrete beam exhibits satisfactory ductility; the influences of steel fiber on the crack behaviors for partially prestressed beams are not as obvious as those for fully prestressed ones; steel fibers can improve the structural stiffness after cracking for fully prestressed high strength concrete beams; the moment redistribution from mid-span to intermediate support in the first stage should be mainly considered in practical design.

Properties of High Strength Steel Fiber Reinforced Concrete under Compression

This paper mainly discusses the properties of high strength steel fiber reinforced concrete under compression. Steel fibers with volume content of 1% do not display significant effect on the strain at peak stress and the area of the ascending portion of

Seismic Behavior of Steel Reinforced Ultra High Strength Concrete Column and Reinforced Concrete Beam Connection

To investigate the seismic behavior of connections composed of steel reinforced ultra high strength concrete (SRUHSC) column and reinforced concrete (RC) beam, six interior strong-column-weak-beam connection specimens were tested subjected to reversal cyclic load. Effects of applied axial load ratio and volumetric stirrup ratio on ductility, energy dissipation capacity, strength degradation and rigidity degradation were discussed. It was found that all connection specimens failed in bending in a ductile manner with a beam plastic hinge. The ductility and energy dissipation capacity increased with the decrease of applied axial load ratio or increase of volumetric stirrup ratio. The displacement ductility coefficient and equivalent damping coefficient lay between those of steel reinforced ordinary concrete connection and those of reinforced concrete connection. The applied axial load ratio and volumetric stirrup ratio had less influence on the strength degradation and more influence on the stiffness degradation. The stiffness degraded sharply with the decrease of volumetric stirrup ratio or increase of applied axial load ratio. The experimental results indicate that SRUHSC column and RC beam connection exhibited better seismic performance and can provide reference for engineering application.

RESEARCH ON THE HIGH PERFORMANCE COMPOSITE ROAD CONCRETE

Ordinary concrete presents short service life when used for building and repairing high-grade road with heavy traffic due to its large brittleness, poor bending flexibility and serious shrinkage on drying. In this paper, a new kind of high performance concrete has been designed by means of combination of organic, inorganic material as well as metal material. The research and application have shown that this new concrete can significantly counteract the deficiency of ordinary concrete and give excellent mechanical properties and pavement performances. The application of this new kind of concrete is of great social and economic significance.

A Study on the Flexural Performance of UHPC Precast Deck-Joint Interface by the Exposure of Steel Fiber

As a solution against the serviceability problem caused by the cracks occurring at the UHPC precast deck-joint interface, this study proposes a method exposing the steel fiber at the interface and evaluates the corresponding flexural performance of the lap spliced construction joint. After having slowed down the strength development of the concrete placed in the joint of the precast deck by means of a curing retardant, the concrete at the interface is crushed so as to expose the steel fibers and the change in the flexural performance is observed experimentally according to the exposure of the steel fibers. The results show that, even if the ultimate strength and stiffness of the UHPC precast deck including the joint are mostly determined by the arrangement details of the rebar lap splice, the exposure of the steel fibers can secure stable ductile behavior and reduce the width of the cracks generated at the precast deck-joint interface under service load.

Durability of Concrete Subjected to the Combined Actions of Flexural Stress,Freeze-thaw Cycles and Bittern Solutions

Freeze-thaw durabilities of three types of concretesnormal portland cement concrete （OPC）, high strength concrete （HSC） and steel fiber reinforced high strength concrete （SFRHSC） were systemically investigated under the attacks of chemical solution, and combination of external flexural stress and chemical solution. Four kinds of bitterns from salt lakes in Sinkiang, Qinghai, Inner Mongolia and Tibet provinces of China were used as chemical attack solutions. The relative dynamic modulus （RDM） was used as an index for evaluating the damage degree during the course of chemical attack and stress corrosion. The experimental results show that the freeze-thaw durability of concrete is visibly reduced in the present of the flexural stress, i e, stress accelerates the damage process. In order to quantify the stress accelerated effect, a stress accelerating coefficient was proposed. The stress accelerating coefficient is closely related with the types of bitterns and the numbers of freeze-thaw cycles is. The more numbers of freeze-thaw cycles is, the greater the stress accelerating coefficient for various concretes will be. In addition, there also exists a critical ratio of external stress to the maximum flexural stress. If the stress ratio exceeds the critical one, the freeze-thaw durability of various concretes will be greatly decreased compared to the responding concretes without applied stress. The critical stress ratio of OPC, HSC and SFRHSC is 0.30, 0.40 and 0.40, respectively, indicating that HSC and SFRHSC have advantages over OPC and are suitable to use in the bittern erosion regions.

Mechanical Properties of SFHPC after Exposure to High Temperatures

The objective of this study is to increase the insight into the mechanical behaviors of steel fiber reinforced high-performance concrete （SFHPC） after exposure to high temperatures. Three types of steel fibers were used and three-point bending tests on notched beams of SFHPC were carried out. The results showed that the flexural toughness and fracture energy of SFHPC increased evidently with the fiber content. The hooked steel fibers with higher strength possess higher flexural toughness and fracture energy than the marked steel fibers with lower strength. After exposure to high temperatures of 300, 600 and 900 ℃ in an electrical furnace, the compressive strength, flexural toughness and fracture energy of SFHPC decreased less than that of HPC without fibers, Both steel fiber content and type had strong influences on the flexural toughness and fracture energy. In addition, the failure patterns of SFHPC beams changed from pull-out of steel fibers at lower temperatures to tensile failure of steel fibers at 900 ℃.

高强型钢超高性能混凝土短柱轴压性能试验及其有限元分析

为研究高强型钢超高性能混凝土短柱的轴压性能,对试件进行了轴心受压试验,设计参数包括配钢率、箍筋间距和箍筋形式。观察了试件的轴压破坏过程,获得了破坏形态及轴向荷载-位移曲线,分析了承载能力、变形能力、刚度等轴压性能指标,以及超高性能混凝土、纵筋、箍筋和型钢的应变发展规律。在试验研究基础上,采用ABAQUS建立了高强型钢超高性能混凝土短柱轴压性能有限元分析模型,计算结果与试验结果吻合较好,进而进行了参数分析。结果表明:试件发生的是典型轴压破坏,中部表面出现“锯齿形”裂缝;轴向荷载-位移曲线会出现2次荷载峰值;箍筋间距减小时,试件的承载能力和变形能力均提高,但刚度退化速率没有显著变化;配钢率增大时,试件的承载能力提高,变形能力先增大后减小,刚度退化变缓;型钢强度增大时,试件的承载能力和变形能力均提高,提高速度先快后慢;超高性能混凝土抗压强度增大时,试件的承载能力提高,变形能力降低;超高性能混凝土受拉能够达到峰值应变;纵筋在极限点之前发生受压屈服;箍筋在极限点时应变不足屈服应变的1/3,但在破坏点前达到受拉屈服;型钢翼缘在极限点前后发生屈服,腹板屈服晚于翼缘。

钢纤维形状及掺量对超高性能混凝土力学性能的影响

对比研究了钢纤维形状及掺量对超高性能混凝土(UHPC)工作性能及力学性能的影响,并通过SEM分析研究其演化机理。结果表明:随钢纤维掺量的增加,UHPC的工作性能变差,且端勾型钢纤维对工作性能劣化最明显。当钢纤维掺量为1.5%~2.5%时,端勾型和长直线型钢纤维UHPC的抗压、抗折及劈裂抗拉强度较高;当钢纤维掺量超过1.0%后,UHPC的拉压比会出现大幅提高,折压比相对提高不明显;当钢纤维掺量为1.5%~2.0%时,短直线型钢纤维UHPC的力学性能和工作性能最佳。SEM分析发现,钢纤维在基体中错向连续搭接分布,形成三维网状结构,能够阻止宏观裂缝的产生和微裂纹的发展,使UHPC表现出良好的力学性能。

钢-PVA混杂纤维高性能混凝土抗渗性能试验研究

为探究钢纤维掺量、聚乙烯醇(PVA)纤维掺量和矿粉掺量对钢-PVA混杂纤维高性能混凝土(HFHPC)抗渗性能的影响,本文开展了钢-PVA HFHPC抗渗性能试验,并对试验结果进行了极差分析、方差分析和回归分析。结果表明:对HFHPC抗渗性能的影响程度从高到低依次为钢纤维、矿粉和PVA纤维。在试验水平范围内,得出混凝土试件抗渗性能最佳水平组合:钢纤维体积掺量1.0%、PVA纤维体积掺量0.7%、矿粉质量取代率20%。当钢纤维掺量超1.0%时,HFHPC抗渗性能略有下降,但仍高于基准素混凝土。最后采用多元回归分析,建立了HFHPC渗透系数与钢纤维、PVA纤维和矿粉的预测模型。

超高性能海水海砂混凝土单轴受压应力-应变关系

为研究超高性能海水海砂混凝土(Ultra-High Performance Seawater Sea-sand Concrete,UHPSSC)轴压应力-应变关系,探究钢纤维体积掺量对UHPSSC轴压力学性能和轴压应力-应变全曲线的影响,使用Instron 1346万能材料试验机测得UHPSSC应力-应变全曲线,借助Pycharm曲线拟合程序与已有混凝土应力-应变全曲线模型进行拟合与修正.结果表明:钢纤维掺量越高,轴压作用下UHPSSC的破坏程度越小,抗压强度和峰值应变也随之增加;已有模型拟合的UHPSSC应力-应变全曲线上升段与试验曲线比较相近,下降段后期差距较大;通过修正下降段形状参数,得到了更适用于描述UHPSSC应力-应变全曲线的修正模型,并验证该修正模型的适用范围.

BFRP筋与超高性能纤维混凝土黏结性能试验研究

为探究玄武岩纤维增强复合材料(BFRP)筋与超高性能纤维混凝土(UHPFRC)之间的黏结性能,通过中心拉拔试验,对BFRP-UHPFRC试件破坏模式、黏结-滑移曲线进行分析,且探讨了筋材直径、单一纤维及钢-PVA混杂纤维对黏结强度的影响。结果表明,试件破坏模式包括BFRP筋拔出破坏与拉断破坏。BFRP筋直径的增大将导致黏结强度降低。随着单一纤维掺量增加,黏结强度呈先增大后减小的趋势。而相较于单一纤维,钢-PVA混杂纤维显著提高了试件的黏结强度,当掺入1%钢纤维和0.5%PVA纤维时,黏结强度最高(25.35 MPa)。此外,通过灰狼算法优化的支持向量回归模型实现对黏结强度的预测,预测值与实际值拟合较好。

不同类型钢纤维UHPC无腹筋梁受剪性能试验

基于单点加载下4根超高性能混凝土无腹筋梁构件受剪破坏试验结果,研究其受剪性能,以钢纤维类型为主要研究参数,分析钢纤维类型对无腹筋梁的受剪承载力、裂缝发展形态、挠度和韧性等发展规律。结果表明:端钩型钢纤维对提高超高性能混凝土无腹筋梁的受剪承载力、阻滞裂缝发展及控制试件变形能力作用优于光滑平直型和波浪型钢纤维,而端钩型Ⅱ钢纤维优于端钩型Ⅰ钢纤维;端钩型Ⅰ和光滑平直型钢纤维增韧效果较好,但分担纵筋拉力较少;波浪型钢纤维分担纵筋拉力较多,提高超高性能混凝土无腹筋梁抗裂性的效果较好,但增韧效果较差。经对比分析,陈璇公式计算值精确度较高,离散程度小。

部分充填式窄幅钢箱-UHPC组合梁抗裂性能

为研究部分充填式窄幅钢箱-UHPC组合梁的抗裂性能,开展了4根不同钢纤维掺量的部分充填式窄幅钢箱-UHPC组合梁与1根部分充填式窄幅钢箱-NC组合梁静力加载试验。试验表明:在负弯矩作用下,部分充填式窄幅钢箱-UHPC组合梁裂缝呈现数量多、短小且密集的特点;同等配筋率的条件下,不同钢纤维掺量(0、1%、2%和3%)部分充填式窄幅钢箱-UHPC组合梁与部分充填式窄幅钢箱-NC组合梁相比,在弹性阶段其刚度明显提高,最大裂缝宽度随钢纤维掺量的增加而减少,开裂强度分别提高了36.0%、48.8%、82.0%和112.0%,UHPC显著提高了组合梁的抗裂性能;可以参考按照JTG 3362—2018中裂缝宽度式乘以0.8的修正系数来计算部分充填式窄幅钢箱-UHPC组合梁裂缝宽度。

UHPC抗冲击性能的试验研究

抗冲击性能是混凝土结构重要技术特征。针对超高性能混凝土(UHPC)抗冲击性能,采用不同掺量的钢纤维和橡胶颗粒,试验研究了钢纤维和橡胶掺量对UHPC的工作性能、强度和抗冲击性能的影响,并对钢纤维和橡胶颗粒掺量对UHPC的影响机理进行了分析。试验研究结果表明:随着钢纤维掺量的增加,UHPC的坍落度和扩展度均不断降低,含气量增大,强度和抗冲击性能明显增大,钢纤维可有效提高UHPC的抗冲击性能,掺量宜控制在3%之内;随着橡胶颗粒掺量的增加,UHPC的抗压强度不断降低,但坍落度、扩展度、抗折强度及抗冲击性能呈现先增大后降低趋势,掺入适量的橡胶颗粒可有效提高UHPC的抗折强度和抗冲击性能,掺量宜控制在10%之内。通过采用钢纤维和橡胶颗粒,可配制出抗冲击性能良好的UHPC。