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

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.

Mechanism of improving ductility of high strength concrete T-section beam confined by CFRP sheet subjected to flexural loading

For the purpose of inventing a new seismic retrofitting method for the reinforced high strength concrete (HSC) T-section beam using carbon fiber reinforced polymer (CFRP) sheet, three series, a total of twelve T-section beams with nine specimens confined by CFRP sheet in the plastic zone and three control beams were conducted up to failure under four-point bending test. The effectiveness of confining CFRP sheet on improving the flexural ductility of unstrengthened T-section beams was studied. The parameters such as the width and the thickness of CFRP sheet and the type of T-section were analyzed. The experimental results show that ductility and rotation capacity of plastic hinge can be improved by the confinement of CFRP sheet, and the ductility indices increase with the increment of width and thickness of CFRP sheet. A plastic rotation model considering the width of CFRP sheet and the effect of flange of T-section beam is proposed on the basis of the model of BAKER, and the test results show a good agreement with the perdicted results. The relevant construction suggestions for seismic retrofitting design of beam-slabs system in cast-in-place framework structure are presented.

Punching and Local Damages of Fiber and FRP Reinforced Concrete under Low-Velocity Impact Load

In recent years, the development and application of high performance fiber reinforced concrete or cementitious composites are increasing due to their high ductility and energy absorption characteristics. However, it is difficult to obtain the required properties of the FRCC by simply adding fiber to the concrete matrix. Many researchers are paying attention to fiber reinforced polymers (FRP) for the reinforcement of construction structures because of their significant advantages over high strain rates. However, the actual FRP products are skill-dependent, and the quality may not be uniform. Therefore, in this study, two-way punching tests were carried out to evaluate the performances of FRP strengthened and steel and polyvinyl alcohol (PVA) fiber reinforced concrete specimens for impact and static loads. The FRP reinforced normal concrete (NC), steel fiber reinforced concrete (SFRC), and PVA FRCC specimens showed twice the amount of enhanced dissipated energy (total energy) under impact loadings than the non-retrofitted specimens. In the low-velocity impact test of the two-way NC specimens strengthened by FRPs, the total dissipated energy increased by 4 to 5 times greater than the plain NC series. For the two-way specimens, the total energy increased by 217% between the non-retrofitted SFRC and NC specimens. The total dissipated energy of the CFRP retrofitted SFRC was twice greater than that of the plain SFRC series. The PVA FRCC specimens showed 4 times greater dissipated energy than for the energy of the plain NC specimens. For the penetration of two-way specimens with fibers, the Hughes formula considering the tensile strength of concrete was a better predictor than other empirical formulae.

Effects of fibers on mechanical properties of high-performance concrete subjected to elevated temperatures

The compressive strength and flexural toughness as well as fracture energy of fiber reinforced high-performance concrete (FRHPC) subjected to different high temperatures were studied. The results showed that after exposure at 300, 600 and 900 ℃, the concrete mixes retained 88.1%, 41.3% and 10.2% of the original compressive strength on average, respectively. Steel fiber and polypropylene (PP) fiber were both effective in minimizing the damage effect of high temperatures on the compressive strength. The HPC reinforced with steel fibers showed higher flexural toughness and fracture energy before and after the high-temperature exposures. In comparison, PP fibers had minor beneficial effects on the flexural toughness and fracture energy. The mechanical properties of HPC reinforced with hybrid fibers (steel fiber+PP fiber) were equivalent to or better than those of HPC reinforced with steel fibers alone. In addition, the failure pattern of FRHPC beams changed from pull-out of steel fibers at lower temperatures (20, 300 and 600 ℃) to tensile failure of steel fibers at higher temperature (900 ℃).

Mechanical Properties of Polypropylene Fiber Reinforced Concrete under Elevated Temperature

Apart from many advantages,High Strength Concrete(HSC)has disadvantages in terms of brittleness and poor resistance to fire.Various studies suggest that when polypropylene(PP)fibers are uniformly distributed within concrete,they play an active role in improving spalling resistance of concrete when exposed to elevated temperature while having no adverse effect on its mechanical properties.Therefore,there is a necessity to quantify the effect of the addition of polypropylene fibers in terms of the fiber dosage,the strength of the concrete,and the residual mechanical properties of fiber-reinforced concrete under exposure to high temperature from fire.The study was carried out on three water/cement(w/c)ratios(0.47,0.36&0.20)using granite aggregate for determining short term mechanical properties of Polypropylene fiber reinforced concrete in comparison to control mix.The experimental program includes 100×200 mm&150 x 300 mm cylinders with fiber volume of 0.5%,that were subjected to temperatures exposures of 400°C and 600°C for durations of 1 hour.From the results,it was observed that no significant enhancement in mechanical properties such as modulus of elasticity,Poisson’s ratio,split tensile strength,flexural strength,and compressive strength was observed at room temperature and at elevated temperatures.

玻璃纤维增强聚合物管约束高强混凝土柱轴压试验研究

为研究玻璃纤维增强聚合物管约束高强混凝土柱的轴压性能,设计了6个不同长细比的试件并对其进行了轴心受压试验,分析了其破坏过程及破坏形态、承载力、位移、荷载-位移曲线、骨架曲线、荷载-应变曲线。研究表明:玻璃纤维增强聚合物管约束混凝土柱轴压承载性能优异,随着长细比的减小,试件的轴压承载力提升显著;加载后期,该组合结构的破坏模式为玻璃纤维增强聚合物管发生环向破坏,内部混凝土被压溃;玻璃纤维增强聚合物空管轴压承载力相对较低,破坏时脆性特征明显;随着长细比的减小,玻璃纤维增强聚合物管约束高强混凝土柱的承载力和刚度逐步增加,长细比对结构的承载能力影响显著。研究结果对玻璃纤维增强聚合物管约束高强混凝土柱的理论研究和工程应用具有一定的指导意义。

聚乙烯醇纤维再生混凝土劈裂抗拉强度尺寸效应研究

为研究聚乙烯醇(PVA)纤维再生混凝土劈裂抗拉强度的尺寸效应,以5种再生骨料取代率、5种PVA纤维掺量、3种试件尺寸为变量,对81个立方体试件进行劈裂抗拉试验。结果表明:随着骨料取代率的提高,再生混凝土劈裂抗拉强度先增加后降低,25%再生骨料取代率下再生混凝土劈裂抗拉强度及尺寸效应度表现最佳;PVA纤维可降低再生混凝土劈裂抗拉强度尺寸效应,随着PVA纤维掺量的提高,其尺寸效应先减小后增大,0.1%PVA纤维掺量最优;基于试验数据,给出了试件劈裂抗拉强度的效应律计算式,可进行各种尺寸规格试块的劈裂抗拉强度计算。

矿物掺合料对FHPC性能影响及其尺寸效应研究

为提高纤维高强混凝土(FHPC)的性能,探究了硅灰、I级粉煤灰和S95矿渣粉在不同掺量和掺加方式下对FHPC力学性能及尺寸效应的影响。结果表明单掺矿物掺料中,S95矿渣粉对提高FHPC抗压强度和抗折强度效果最好,硅灰次之,I级粉煤灰效果最弱。当I级粉煤灰与S95矿渣粉以1:2复掺时,FHPC的抗压强度显著提高;硅灰与S95矿渣粉以1:1复掺时,FHPC的抗折强度提升效果最好。同时加入3种掺合料,保持硅灰掺量不变的情况下,I级粉煤灰与S95矿渣粉以1:1复掺,可以显著提高FHPC的力学性能。此外,低水胶比下FHPC的抗压抗折强度尺寸效应更为显著,但随着水胶比的增加,FHPC的抗压抗折强度尺寸效应逐渐减弱。

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 ℃.

高韧性纤维混凝土特性及工程应用

高韧性纤维混凝土具有高强、高韧、高耐久性、良好的环保性能及裂缝无害化分布等特点,能显著改善传统混凝土强而不韧、脆性易裂的根本缺陷。针对影响纤维混凝土性能的主要因素,主要从抗压强度、抗折强度等方面分析其力学性能,并从微观角度探索纤维混凝土中纤维发挥增韧作用的机理及工程应用状况,最后指出了高韧性纤维混凝土研究存在的问题并展望其研究与发展前景。

聚丙烯纤维混凝土纤维增强作用机理研究

为分析混凝土添加聚丙烯纤维后的应用效果,以水泥、粉煤灰等为材料,掺杂0.05%、0.10%、0.15%添加量的网状聚丙烯纤维,制备聚丙烯纤维混凝土试件。试验结果表明,添加聚丙烯纤维后混凝土的抗压、抗折、抗剪强度均得到不同程度的提升;添加0.10%纤维含量的试件在收缩时的裂缝面积、最大缝宽以及失水速率均低于普通混凝土试件;当试件经过拉伸试验后,添加聚丙烯纤维含量为0.10%的混凝土试件拉伸应力最高;在不同高温环境下,聚丙烯纤维混凝土试件的强度、质量损失率均低于普通混凝土试件,0.10%含量混凝土试件抗渗性最高。综合试验各方面分析可知,0.10%聚丙烯纤维含量的混凝土性能最佳。

压剪作用下PVA纤维再生混凝土力学性能试验研究

为了研究压剪作用下外掺聚乙烯醇(PVA)纤维对再生混凝土力学性能的影响,以再生粗骨料取代率、压应力比和PVA纤维体积掺量为变化参数,设计并制作144个立方体试件进行压剪试验。观察了试件的破坏形态,得到了荷载-位移曲线及垂直变形-剪切位移曲线,分析了不同变化参数对抗剪强度、峰值位移的影响规律。结果表明:0.1%体积掺量的PVA纤维对再生混凝土的抗剪强度提高了10%;随着压应力比的增大,试件的抗剪强度呈增大趋势,平均增大了2.03倍;与普通混凝土相比,再生混凝土试件的抗剪强度平均降低了13%;当再生粗骨料取代率为100%时,PVA纤维最佳掺量为0.1%;提出了纤维掺量为0.1%时压剪作用下PVA纤维再生混凝土抗剪强度计算公式。

墩粗钢筋在高强钢纤维混凝土中的黏结锚固性能

为探究墩粗钢筋与高强钢纤维混凝土之间的黏结锚固性能,以黏结长度、保护层厚度和钢筋类型为试验变量,开展拔出试验.通过黏结-滑移曲线、破坏模式、黏结强度及峰值滑移等分析墩粗钢筋-高强钢纤维混凝土的黏结-滑移性能.结果表明:混凝土的黏结强度随着黏结长度的增加而降低,且对墩粗钢筋试件的影响更明显;提高保护层厚度可以提升黏结性能,但作用有限;钢筋进行墩粗处理可以有效提高钢筋与混凝土之间的黏结强度,显著降低峰值滑移,改变破坏模式.基于试验数据,建立了混凝土的黏结-滑移关系.

型钢-钢纤维高强混凝土柱抗震性能试验研究

为改善高强混凝土的脆性,提升型钢高强混凝土柱的抗震性能,采用钢纤维混凝土制备型钢高强混凝土柱试件,开展型钢-钢纤维高强混凝土柱的低周往复加载试验,观察试件的破坏过程和形态,基于荷载-位移滞回曲线、骨架曲线及强度衰减分析钢纤维掺量对试件变形能力、承载力、延性等抗震性能的影响。结果表明:钢纤维体积分数在0~2.0%范围内,不同钢纤维掺量的试件均发生弯曲破坏,但钢纤维的掺入可延缓裂缝发展,有效防止混凝土大面积脱落;加入钢纤维,试件的滞回曲线更饱满,耗能能力显著提升,承载力提高了13%~21%,钢纤维体积分数为1.0%时提高最明显;随钢纤维掺量的增加,试件的延性和变形能力得到改善,钢纤维体积分数为2.0%时延性系数达5.34,强度衰减缓慢,控制位移增大,在变形较大的情况下,试件的强度衰减从28%降为16%。钢纤维的增韧效果能显著提升型钢混凝土组合结构的抗震性能。

方钢管超高强钢纤维混凝土柱轴压性能研究

为研究方钢管超高强钢纤维混凝土柱的受压性能,开展了12个方钢管超高强钢纤维混凝土柱的轴压试验,试验参数包括钢管屈服强度与厚度、混凝土基体强度和钢纤维掺量.结果表明:掺入钢纤维基本不改变试件的破坏形态;对于混凝土棱柱体抗压强度介于80~150 MPa之间、套箍指标介于0.33~1.43之间的方钢管钢纤维混凝土柱,其管内混凝土的抗压强度与对应混凝土棱柱体抗压强度接近;钢纤维对延性的提升作用随着混凝土强度的提高和套箍指标的增大而降低;当混凝土棱柱体抗压强度为130 MPa、套箍指标为0.90时,掺入1.5%的钢纤维对提升方钢管混凝土柱的延性不起作用.

冻融循环后PVA-ECC拉伸性能衰减规律研究

为研究聚乙烯醇纤维水泥基复合材料(PVA-ECC)冻融后的拉伸性能,分别对PVA-ECC试件冻融0、25、50、75、100、125、150次后进行拉伸试验,通过试件表面特征和拉伸特征参数综合评价PVA-ECC冻融后的拉伸性能,并采用向量自回归滑动平均(VARMA)模型探索冻融循环后拉伸特征参数之间的规律。结果表明,冻融循环试验后,试件均出现不同程度的损伤,损伤程度随冻融循环次数增加逐渐增大。初裂强度与抗拉强度随冻融循环次数的增加逐渐降低,拉伸应变与应变能随冻融循环次数的增加呈先升高后降低的趋势。基于试验数据建立了拉伸特征参数的关系式,进一步揭示了冻融循环后拉伸特征参数的衰减规律。