纳米SiO2对钢纤维/混凝土高温后力学性能及微观结构的影响

研究了掺纳米SiO2的钢纤维混凝土(NSFC)、钢纤维混凝土(SFRC)和普通混凝土(NC)三种材料在不同加热温度后的抗压、劈裂和抗折强度等力学性能,对不同温度热处理后的微观结构进行了SEM分析,对钢纤维与过渡区界面的相结构进行了XRD分析.结果表明:在测试温度范围内,NSFC的抗压、劈裂和抗折强度均高于SFRC和NC的强度,且在400℃时达到最大值.在常温下,NSFC的抗压、劈裂和抗折强度较NC分别提高27.01％、63.28％和54.12％,400℃高温热处理后比NC分别高35.09％、84.62％和87.23％; SEM分析表明,在钢纤维与过渡区的界面处,致密度提高,显微硬度提高.由于固相反应,使界面区结构发生变化,在钢纤维表层形成扩散渗透层(白亮层),即化合物层,呈锯齿状,XRD分析证明,白亮层主要由FeSi2和复杂的水化硅酸钙组成,从而增强了钢纤维与基体的粘结力,提高了混凝土的高温力学性能.     

纳米SiO2改性轻骨料混凝土性能

纳米SiO₂（NS）具有极强火山灰活性、晶核作用和填充效应,因此用NS改善水泥基材料性能成为众多学者研究的热点。本课题对不同掺量的NS对轻骨料混凝土强度及耐久性的改性效果进行了研究。通过测试轻骨料混凝土的力学性能（抗压和抗折）和氯离子渗透性能及利用SEM和EDS测试分析了NS对混凝土宏观和微观结构的影响。研究结果表明:在适当的掺量下,NS能够有效地提高轻骨料混凝土的力学性能,其中28 d的抗压强度和抗折强度比空白组混凝土分别提高了21.6%和46.2%。氯离子渗透的结果表明,轻骨料混凝土的抗氯离子渗透性能随着掺量的增加而呈线性增强。混凝土界面过渡区（ITZ）也发生了显著变化,其厚度减小,形貌也更加致密。ITZ的钙硅比随着NS掺量增加而减小,说明该区域内水化产物C-S-H凝胶增多,Ca（OH）₂被消耗,从而形成致密的过渡区,有利于强度提高。      

聚丙烯-钢纤维增强高强混凝土高温性能

通过对聚丙烯-钢纤维增强高强混凝土(混杂纤维/高强混凝土)试块的高温试验, 研究不同目标温度后混凝土表观特征、高温爆裂、质量损失及力学性能。结果表明: 高强混凝土在600 ℃时发生爆裂, 混杂纤维/高强混凝土直至800 ℃未出现爆裂, 混杂纤维有效抑制了高强混凝土的高温爆裂。混杂纤维/高强混凝土的质量损失随所受温度的升高而增大, 其抗压强度、抗折强度随温度的升高而降低, 并且400 ℃以后显著降低。相同温度下, 混杂纤维的加入提高了高强混凝土高温后强度。通过对试验结果的统计分析, 分别建立了混杂纤维混凝土质量损失、抗压强度和抗折强度随温度变化的关系式。     

聚丙烯-钢纤维/混凝土柱大偏心受压承载力计算

对270个聚丙烯纤维掺量(体积分数)分别为0vol%、0.1vol%、0.2vol%、0.3vol%、0.4vol%、0.5vol%、钢纤维掺量(体积分数)分别为0vol%、0.5vol%、1vol%、1.5vol%、2vol%的聚丙烯-钢纤维/混凝土试块进行立方体抗压试验、轴心抗压试验和劈裂抗拉试验,基于复合材料力学理论,考虑纤维的取向系数、长度有效系数和界面黏结系数,对其建立强度预测模型并进行机制分析,同时选取掺量分别为0vol%、0.1vol%、0.3vol%的聚丙烯纤维、掺量分别为0vol%、1.5vol%的钢纤维制作6根聚丙烯-钢纤维/混凝土柱,对其进行大偏心受压试验,在强度预测模型的基础上进行承载力计算,提出聚丙烯-钢纤维/混凝土承载力计算方法。结果表明:钢纤维对聚丙烯-钢纤维/混凝土立方体抗压强度、轴心抗压强度和劈裂抗拉强度均有提高;聚丙烯纤维可提高聚丙烯-钢纤维/混凝土的劈裂抗拉强度,但不能提高聚丙烯-钢纤维/混凝土的抗压强度;聚丙烯-钢混杂纤维加入混凝土柱可有效提高其极限承载力。      

再生砖骨料混凝土力学性能及破坏机理研究

以8种设计配合比的再生砖骨料次轻混凝土为研究对象,以水灰比、最大骨料尺寸和粗骨料种类为试验参数,开展了混凝土干密度、立方体抗压强度和劈裂抗拉强度试验研究,重点分析了骨料界面特征与再生砖骨料混凝土的破坏机理.结果表明:使用再生粘土砖骨料制备的次轻混凝土28 d抗压强度可达36 MPa,干密度不大于2200 kg/m3,能够减轻自重8％～16％;随着水灰比和取代率的降低,混凝土抗压和劈裂抗拉强度均呈显著增加的趋势;最大骨料尺寸对混凝土抗压和劈裂抗拉强度的影响较小;拟合所得混凝土干密度与力学性能换算公式的相关性较好;砖骨料-砂浆界面过渡区微观结构较为密实,混凝土界面性能得到增强;界面过渡区性能和砖骨料强度是影响再生砖骨料混凝土力学性能的两个关键因素,且砖骨料强度是其中最主要的因素.综合考虑,使用再生粘土砖骨料制备次轻混凝土具有良好的经济性和广阔的工程应用前景.     

纤维增强活性粉末混凝土高温力学性能的实验研究

研究了钢纤维、聚丙烯纤维和PVA纤维的不同掺量以及纤维复掺在90℃、200℃和400℃高温养护时活性粉末混凝土的力学性能和微观结构.结果表明,200℃高温养护,单掺钢纤维时,RPC的抗折强度与90℃的相近,但抗压强度提高,迭210.2MPa;单掺聚丙烯纤雏时,由于其高温熔解,形成三维网络结构,与RPC融为一体,抗压强度显著提高,当掺量为1.5%(体积分数)时,强度达242.6MPa.纤维复掺时抗折强度与钢纤维相近,但抗压强度有所提高,200℃养护时达265 MPa.400℃养护时,随水胶比降低,强度进一步增大,当水胶比为0.12时,抗压强度达333.4MPa.     

微生物矿化沉积对再生骨料界面过渡区的影响

骨料-水泥浆体的界面过渡区疏松多孔是导致再生骨料与天然骨料性能差异的重要原因。通过试验研究微生物矿化沉积技术对再生骨料界面过渡区性能的影响。使用水泥净浆包裹不同菌种和不同方式处理的再生骨料,养护后破碎获得骨料-水泥界面过渡区,通过SEM观察界面过渡区的微观形貌变化,利用纳米压痕试验测量界面过渡区的弹性模量和硬度,并结合再生砂浆块的抗压、抗折强度试验结果,分析微生物矿化沉积对再生骨料界面过渡区的改善效果。结果表明:再生骨料经假坚强芽孢杆菌和嗜碱芽孢杆菌处理后所制备的再生砂浆块的抗压、抗折强度及再生骨料界面过渡区的弹性模量和硬度均有明显提升,两菌种矿化生成碳酸钙的能力不同造成了性能提升效果的差异。     

不同胶凝材料的精细混凝土高温后力学性能

为了改善用于纤维编织网增强混凝土基体材料的精细混凝土的耐高温性能,该文进行了120个40mm×40mm×160mm棱柱体的高温后抗折强度试验和240个40mm×40mm×40mm立方体的高温后抗压强度试验。考察了不同胶凝材料对精细混凝土试件高温后力学性能的影响,包括外掺纳米材料以及以高铝水泥作为主要胶凝材料的影响。结果表明:体积掺量为1.5%和3.0%纳米Si O2气凝胶粉末未能改善精细混凝土的耐高温性能,质量掺量为5.0%纳米陶瓷粉在目标温度TR=800℃时使基体混凝土的抗压和抗折强度分别提高84.2%和120.9%。当TR=800℃时,采用高铝水泥作为主要胶凝材料的试件力学性能均比普通精细混凝土试件大幅提高;各组掺入活性粉末的高铝水泥混凝土试件在TR=800℃时,相对抗压和抗折强度均比未掺活性粉末时有所提高。     

钢纤维对超高性能混凝土抗弯力学性能的影响

为研究长、短钢纤维对超高性能混凝土（UHPC）受弯力学性能的影响,设计并制作了13组标准养护条件下的UHPC试件,其中3组为掺单一型短钢纤维,其他组均为掺混杂型钢纤维,对其进行立方体抗压及四点抗折试验。结果表明：对于掺加单一型短钢纤维的钢纤维/UHPC,钢纤维体积掺量为5vol%时,抗折强度最大,为19.98 MPa,继续增加钢纤维掺量,抗折强度反而降低;掺混杂型钢纤维的UHPC比单一型的抗折强度高,并且当长、短钢纤维体积掺量分别为2vol%和1vol%时,抗折强度达到最大,为23.55 MPa;钢纤维/UHPC的抗弯力学性能主要受长纤维的影响,短纤维影响较小;长纤维掺量对钢纤维/UHPC的抗折强度、延性以及抗弯韧性有一定影响,但是主要取决于长、短纤维的搭配,长、短纤维体积掺量最优搭配为2vol%和1vol%。     

玻化微珠保温混凝土高温后性能劣化及微观结构

配制普通混凝土（NC）和玻化微珠保温混凝土（GHB/NC）,研究从常温升温至1 000℃高温后表观现象变化、质量、抗压强度损失等性能的劣化过程,同时讨论超声波无损检测法评判混凝土高温后性能的普适性,对比分析相对波速、损伤度与受热温度、抗压强度损失率的关系,利用SEM观察不同高温后试件微观结构变化。结果表明:采用相对波速、损伤度评价混凝土高温后性能具有良好相关性,回归公式拟合度较高;随温度升高,NC和GHB/NC混凝土内部损伤逐步加剧,水泥胶凝受热分解、水分散失等在试件表面和内部产生空隙、裂纹并相互贯通,玻化微珠、粗骨料与水泥石界面黏结力逐步减弱甚至丧失,造成宏观力学性能劣化,抗压强度损失率增大。升温至800℃后NC强度损失72.3%,GHB/NC强度损失74.6%,1 000℃时基本丧失承载能力。      

纤维对超高性能混凝土残余强度及高温爆裂性能的影响

采用普通原材料制备56 d龄期抗压强度为140~160 MPa的空白组超高性能混凝土、钢纤维超高性能混凝土及混杂纤维超高性能混凝土,测定其遭受高温作用后的残余抗压强度和劈裂抗拉强度,并对100%含湿量的混凝土试块进行高温爆裂试验。此外,测定大小2种加热速率对超高性能混凝土高温爆裂行为的影响。结果表明:所配制混凝土的残余抗压强度均随着目标温度的升高呈现先增大再降低的趋势,800℃高温后的残余抗压强度约为常温强度的30%。钢纤维与混杂纤维混凝土的残余劈裂抗拉强度亦呈现先升高再降低的趋势,800℃高温后的残余劈裂抗拉强度分别为常温强度的15.1%和35.4%。空白组混凝土的残余劈裂抗拉强度随着目标温度的升高而单调下降,800℃高温后的强度值约为常温强度的20.3%。7.5℃/min加热速率下,100%含湿量的3种混凝土试块均发生了严重高温爆裂,单掺钢纤维可以改善超高性能混凝土的高温爆裂,但不能避免爆裂的发生,而混杂纤维对超高性能混凝土高温爆裂的改善效果并未显著优于钢纤维。2.5℃/min加热速率下,混杂纤维可避免部分超高性能混凝土试块发生爆裂。      

An experimental study on the tensile strength of steel fiber reinforced concrete

This paper deals with steel fiber reinforced concrete mechanical static behaviour and with its classification with respect to fibers content and mix-design variations. A number of experimental tests were conducted to investigate uniaxial compressive strength and tensile strength. Different mixtures were prepared varying both mix-design and fiber length. Fibers content in volume was of 1% and 2%. Mechanical characterization was performed by means of uniaxial compression tests with the aim of deriving the ultimate compressive strength of fiber concrete. Four-point bending tests on notched specimens were carried out to derive the first crack strength and the ductility indexes. The tensile strength of steel fiber reinforced concrete (SFRC) was obtained both from an experimental procedure and by using an analytical modelling. The experimental tests showed the different behaviour of SFRC with respect of the different fiber content and length. Based on the experimental results, an analytical model, reported in literature and used for the theoretical determination of direct tensile strength, was applied with the aim of making a comparison with experimental results. The comparison showed good overall agreement.     

混凝土劈裂抗拉强度和弯曲抗压强度尺寸效应的细观数值分析

混凝土作为非均质材料,其材料性能存在随试件几何尺寸变化的尺寸效应。该文在细观层次上将混凝土看作由粗骨料、砂浆和二者界面过渡区组成的三相复合材料,采用刚体弹簧元数值方法模拟了混凝土的劈裂抗拉强度和弯曲抗压强度的尺寸效应,并与已有的试验结果进行了对比验证。结果表明:劈裂加载的试件破坏形态和劈裂抗拉强度与试验结果均具有良好的一致性,并且小尺寸试件所表现出的尺寸效应要明显于大尺寸试件;对不同尺寸四点弯曲钢筋混凝土梁开展细观数值分析得到跨中截面混凝土的弯曲抗压强度,随着梁有效高度的增加,名义弯曲抗压强度整体上呈现降低的趋势,但当梁有效高度大于240mm时趋于稳定。     

钢纤维及陶粒掺量对轻质混凝土基本力学性能的影响

制备了3种强度等级共13组配合比的钢纤维增强轻质混凝土（Steel Fibre Reinforced Lightweight Aggregate Concrete,SFRLAC）,测量了立方体抗压强度、劈裂抗拉强度和轴心抗压强度,得到了SFRLAC轴心受压应力-应变曲线。试验结果表明,钢纤维能小幅度提高轻质混凝土（Lightweight Concrete,LC）的抗压强度,随陶粒比率（Haydite Ratio,V_h）的增大,抗压强度降低,且强度等级越低,降幅越大。钢纤维能显著提高LC的劈裂抗拉强度,钢纤维对低强度等级LC劈裂抗拉强度的贡献优于对高强度等级LC的贡献。低强度等级SFRLAC （LC30和LC40）的劈裂抗拉强度受V_h的影响较大,而高强度等级SFRLAC （LC50）与之相反。当V_h达到80%时,V_h不再是影响SFRLAC劈裂抗拉强度的主要因素,而钢纤维的增强效应显著。试块的破坏形态表明钢纤维能改善LC的塑性。V_h对抗拉强度的降低效应远大于对抗压强度的降低效应。建立了SFRLAC轴心抗压强度与立方体抗压强度的关系式。SFRLAC应力-应变曲线综合体现了钢纤维的增强效应和陶粒的削减作用,陶粒降低LC的峰值应力和韧性,钢纤维主要提高LC的韧性。     

Investigation of microstructure and flexural behavior of steel-fiber reinforced concrete

This paper presents microstructure and flexural behavior of steel-fiber reinforced concrete produced with different steel fibers volume fraction and aspect ratio. Prismatic concrete specimens of 100 × 100 × 350 mm were prepared with and without steel fiber. Two different steel fiber types (both is hooked-end) were used by ratio of 0% (control), 0.2, 0.4, 0.6 and 0.8% by volume. Specimens were de-molded after 24 h and cured in water until 7, 28, 56, 180 and 360 days. On the prisms, flexural strength has been defined for every age. The crack widths have also been measured after maximum bearing loads. Microstructure of SFRC was studied by scanning electron microscopy and optical microscopy for 180 aged specimens. The results showed that the polarized microcopy images may be used for observing the bond characteristic of SFRC as alternatively to SEM. A good bond was observed between steel fiber and concrete matrix interface zone by using polarizing microscopy, too. Flexural strength of SFRC increased with the concrete age and fiber volume fraction. Besides, the first crack development significantly decreased by increasing of fiber volume fraction in the all concrete ages.     

混合配筋钢纤维增强混凝土梁受弯承载力试验及理论计算

为研究玻璃纤维增强聚合物复合材料(GFRP)筋与普通钢筋混合配筋钢纤维增强混凝土(SF/混凝土)梁的受弯性能及其受弯承载力计算方法,在考虑受拉区混凝土抗拉强度的基础上,给出混合配筋SF/混凝土梁的界限配筋率及受弯承载力计算公式;在此基础上设计制作了三种配筋方式的SF/混凝土梁,重点探讨了混合配筋率及筋材面积比(A_f/A_s)对试验梁失效模式和受弯承载力的影响;同时,借助已有相关试验结果,对比分析了混凝土强度对混合配筋SF/混凝土梁受弯性能的影响。试验和对比分析结果表明:混合配筋SF/混凝土梁正截面应变仍符合平截面假定;相同配筋形式下,混合配筋SF/混凝土梁的受弯承载力和跨中挠度随筋材面积比A_f/A_s的增加而增大;单层配筋梁的受弯承载力比双层配筋梁大;合理提高混凝土强度可在充分发挥GFRP筋抗拉作用的同时进一步提高混合配筋SF/混凝土梁的受弯承载力;采用本文给出的界限配筋率公式能有效预测混合配筋SF/混凝土梁的失效模式;梁受弯承载力建议公式的预测值与试验值吻合较好,具有良好的适用性。      

Response of steel fiber reinforced high strength concrete beams: Experiments and code predictions

The shear-flexure response of steel fiber reinforced concrete (SFRC) beams was investigated.Thirty-six reinforced concrete beams with and without conventional shear reinforcement (stirrups) were tested under a four-point bending configuration to study the effectiveness of steel fibers on shear and flexural strengths, failure mechanisms, crack control, and ductility.The major factors considered were compressive strength (normal strength and high strength concrete up to 100 MPa), shear span-effective depth ratio (a/d = 1.5, 2.5, 3.5), and web reinforcement (none, stirrups and/or steel fibers).The response of RC beams was evaluated based on the results of crack patterns, load at first cracking, ultimate shear capacity, and failure modes.The experimental evidence showed that the addition of steel fibers improves the mechanical response, i.e., flexural and shear strengths and the ductility of the flexural members.Finally, the most recent code-based shear resistance predictions for SFRC beams were considered to discuss their reliability with respect to the experimental findings. The crack pattern predictions are also reviewed based on the major factors that affect the results.     

Effect of steel fiber on the mechanical properties of oil palm shell lightweight concrete

This paper reports the results of a study conducted to investigate the effect of low volume content of steel fiber on the slump, density, compressive strength under different curing conditions, splitting tensile strength, flexural strength and modulus of elasticity of a grade 35 oil palm shell (OPS) lightweight concrete mixture. The results indicate that an increase in steel fiber decreased the workability and increased the density. All the mechanical properties except the modulus of elasticity (E) improved significantly. The 28 day compressive strength of steel fiber OPS lightweight concrete in continuously moist curing was in the range of 41–45 MPa. The splitting tensile/compressive and the flexural/compressive strength ratio for plain OPS concrete are comparable with artificial lightweight aggregate. The (E) value measured in this study was about 15.5 GPa on average for all mixes, which is higher than previous studies and is in the range of normal weight concrete. Steel fiber can be used as an alternative material to reduce the sensitivity of OPS concrete in poor curing environments.     

Residual flexural behavior of fiber reinforced concrete after heating

In this study, the effects of fire on the flexural performance and residual strength of plain and fiber reinforced concrete are investigated. Three types of concrete are tested: plain, polypropylene (PFRC) and steel fiber reinforced concrete (SFRC). Prior to the flexural test, the specimens were exposed to fire for 15, 30, 45, and 60 min on a furnace. The burnt specimens were then tested under flexural load to measure their toughness and residual strength. Results indicate the reduction of flexural strength for both plain and FRC after being subjected to fire. For FRC, the effect of fire on the flexural response depends mainly on the fiber type and fire exposure duration. For PFRC, the flexural strength is found to drop significantly for every exposure duration, while toughness is found to increase at short exposure duration and then, drop quickly after long exposure duration due to the fiber evaporation effect. For SFRC, the flexural strength and toughness are found to drop gradually for every exposure duration due to the deterioration of cement paste and reduction in bond strength. SFRC exhibits a more consistent ability to maintain load carrying capacity after long exposure to fire than PFRC.