含粗骨料超高性能混凝土力学性能研究及机理分析

为探索含粗骨料超高性能混凝土的各项力学性能,研究了粗骨料体积掺量(0kg/m~3、280kg/m~3、400kg/m~3、480kg/m~3、560kg/m~3)、纤维掺量(2%、2.5%)以及纤维形态(平直型、端钩型)对超高性能混凝土抗压强度、弹性模量以及四点弯曲强度的影响,并引入纤维取向系数和纤维有效长度,探索粗骨料掺量对弯曲强度影响的微观机理。结果表明,粗骨料体积掺量对含粗骨料超高性能混凝土抗压强度的影响不大(0.4%~4.5%);对弹性模量的提高效果显著,最高可提高7.8%;对抗弯强度具有不利影响,并且随着粗骨料掺量增大,纤维取向系数下降,纤维有效长度减小,负面影响扩大。当粗骨料体积掺量为560kg/m~3时,弯曲强度下降了21.2%。增加纤维掺量或者掺入端钩型纤维可提高弯曲强度,掺入端钩型钢纤维可显著增大纤维有效长度,从而大幅度提高弯曲强度。     

钢丝网或纤维网增强超高性能混凝土双向板弯曲性能

为研究钢丝网或纤维网对混杂纤维超高性能混凝土(Ultra-high performance concrete,UHPC)板弯曲性能的影响,进行了四边简支双向板弯曲试验。UHPC中短切纤维为：单掺钢纤维、钢纤维分别与聚乙烯醇纤维、玻璃纤维、玄武岩纤维混掺等。研究参数为：钢丝网与玻璃纤维网层数、孔径、混掺纤维比例等。结果表明,单掺体积分数为1.5vol%的钢纤维时,铺设3层和4层钢丝网的UHPC板的极限承载力和25 mm挠度处的能量吸收值较2层分别提升14.9%、32.3%和14.1%、25.2%;孔径较小的钢丝网对UHPC板承载力和韧性提升明显。当混杂纤维总体积分数为1.5vol%且钢丝网2层时,混掺1.0vol%钢纤维和0.5vol%聚乙烯醇纤维对UHPC板增强增韧效果更好,0.5vol%钢纤维与1.0vol%玻璃纤维或玄武岩纤维混掺较0.5vol%钢纤维与1.0vol%聚乙烯醇纤维混掺对改善板峰后持荷能力更有利,即钢纤维与较高弹性模量非金属纤维混掺有利于提高裂后承载力。与玻璃纤维网相比,铺设钢丝网的UHPC板在峰后延性更好。提出了以素UHPC板峰值荷载挠度作为初裂挠度的韧性指标评定方...     

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%。     

超高性能混凝土动态冲击拉伸性能研究

超高性能混凝土(Ultra-high performance concrete,UHPC)作为一种具有超高物理力学性能的新型建筑材料,能显著提高军事防护工程的抗爆炸冲击能力,对保障防护工程中人员的生命安全具有重要意义。为揭示爆炸冲击波在防护工程自由面引起的动态拉伸破坏行为,利用霍普金森压杆装置(Split Hopkinson pressure bar,SHPB)对UHPC进行动态冲击拉伸试验,系统研究了粗集料种类、钢纤维掺量以及应变率对UHPC动态冲击拉伸性能的影响规律。结果表明:粗集料种类对UHPC的动态冲击拉伸强度有较显著的影响,相比于花岗岩和铁矿石,玄武岩粗集料对动态冲击拉伸性能的提高更为明显;UHPC的动态冲击拉伸强度会随着钢纤维掺量的增加而显著提高,但钢纤维掺量对UHPC动态拉伸强度的贡献存在4%的临界值;此外,UHPC表现出明显的应变率效应,当应变率为7~50s-1时,其效应最为显著。     

超高性能混凝土流变特性及其对纤维分散性的影响

优异的分散性能是纤维充分发挥增强增韧作用的关键。为了明确高掺量钢纤维在超高性能混凝土(UHPC)中的分散特征并提高纤维的分散性,采用抗压强度、抗弯强度等力学性能试验、混凝土流变仪以及图像分析技术,分别研究了降粘掺合料、钢纤维掺量对UHPC力学性能、流变性能以及纤维分散性能的影响。结果表明:降粘掺合料对UHPC力学性能无明显提升作用,但可显著降低UHPC基体的屈服应力和塑性粘度,同时可降低钢纤维导致的屈服应力和塑性粘度增加幅度;随着纤维掺量的增加,纤维轴向取向系数和有效利用率降低,而降粘掺合料可提高纤维轴向取向系数和有效利用率;UHPC基体的流变性能、纤维分散性能以及力学性能三者密切相关,基体流变参数越小,纤维轴向取向系数越高、纤维有效利用率越高,则UHPC力学性能越好。     

钢纤维橡胶再生混凝土的抗冻性试验

为使废弃混凝土和再生橡胶在北方地区混凝土工程中得以应用,采用正交试验法研究再生粗骨料掺量、再生粗骨料强化方式、钢纤维掺量与橡胶掺量对钢纤维橡胶再生混凝土(C45)立方体抗压强度和抗冻性的影响规律。利用扫描电镜和螺旋CT扫描技术研究了钢纤维橡胶再生混凝土的宏观和细观结构及其对抗冻性能的影响机理。结果表明:橡胶颗粒掺量是影响再生混凝土含气量、抗压强度和相对动弹模量的重要因素,再生粗骨料掺量是影响相对动弹模量和强度损失率的次要因素,钢纤维掺量对混凝土抗压强度增强作用较小,粗骨料强化方式对混凝土性能影响不大;橡胶颗粒与砂浆界面的裂缝宽度在5~55μm之间,二者之间的相容性较差;当橡胶颗粒掺量(与砂的体积比)大于20%后,随橡胶颗粒掺量增大,混凝土内部孔洞数目增多,钢纤维橡胶再生混凝土抗压强度降低、抗冻性减弱。     

钢纤维高性能轻骨料混凝土多轴强度和变形特性研究

为了研究钢纤维掺量和三轴应力比对高性能轻骨料混凝土破坏准则和本构关系的影响规律,进行了钢纤维全轻混凝土和钢纤维次轻混凝土多轴强度和变形特性的试验研究,考虑到试验机加载能力和新拌高性能轻骨料混凝土的工作性能,选取的钢纤维体积掺量为0、0.5%、1.0%和1.5%,试验加载路径有单轴拉、压,双轴等压和真三轴压。结果发现在单轴应力和低应力比条件下,钢纤维能够明显地发挥增强阻裂作用,随着钢纤维掺量的增加,中间主应力对极限抗压强度和峰值应变的影响越来越大,且钢纤维体积掺量对两种轻骨料混凝土应力-应变曲线下降段有一定的影响;在高应力比条件下,钢纤维体积掺量对峰值强度、峰值应变和应力-应变曲线下降段无明显影响,但对高应力比下轻骨料混凝土应力-应变曲线上特有的应力平台区域有较大的影响。考虑钢纤维含量特征参数的影响,对普通骨料混凝土的Kotsovos破坏准则进行了相应的修正,得出了适合钢纤维增强轻骨料混凝土的破坏准则表达式。     

钢纤维-聚丙烯纤维混杂对再生混凝土抗冲击性能的影响

为研究钢纤维(SF)与聚丙烯纤维(PPF)混杂后对再生混凝土(RAC)抗冲击性能的影响,采用落锤弯曲冲击试验装置对素RAC、SF/RAC、PPF/RAC和SF-PPF/RAC进行抗冲击试验;分析了不同纤维掺量和掺入方式对RAC抗冲击性能的影响;采用数理统计模型对冲击试验结果进行拟合和失效概率预测,并对SF-PPF/RAC抗冲击性能的阻裂增强机制进行深入分析。结果表明:单掺或混杂纤维均可提高RAC的抗冲击性能;其中混合掺入体积分数为1.5vol%的SF和体积分数为0.9vol%的PPF时,RAC抗冲击耗能的提高幅度最大,RAC基体的延性和韧性最佳。SF-PPF/RAC的抗冲击次数很好地服从两参数Weibull分布。SF与PPF混杂对改善RAC的抗冲击性能呈现出优异的混杂增强效应。      

废弃纤维再生混凝土框架中柱节点抗震性能试验研究

完成了再生骨料取代率为50%,废弃纤维长度分别为12 mm、19 mm、30 mm以及废弃纤维体积掺量分别为0.08%、0.12%、0.16%的5个废弃纤维再生混凝土框架中柱节点在低周反复荷载作用下的抗震性能试验研究。主要对不同废弃纤维长度、不同废弃纤维体积掺量下梁柱节点的破坏形态、滞回特性、延性性能、耗能特性等问题进行了对比分析。结果表明:废弃纤维再生混凝土框架节点的破坏均经历了初裂、通裂、极限、破坏四个特征阶段,当纤维长度为19mm,废弃纤维体积掺量为0.12%时,废弃纤维再生混凝土梁柱节点构件的滞回性能、延性性能及耗能性能较好;废弃纤维的体积掺量比废弃纤维长度对梁柱节点抗震性能的影响大。提出了废弃纺织纤维再生混凝土的抗剪承载力计算公式,计算结果与试验结果吻合较好。     

钢纤维类型对超高性能混凝土高温爆裂性能的影响

为了探寻可以有效改善超高性能混凝土（Ultra-high-performance concrete,UHPC）抗火性能的钢纤维类型,本文试验测定了不同类型钢纤维（3种普通钢纤维和2种来自于废旧轮胎的再生钢纤维）增韧UHPC及空白组混凝土的从常温至800℃高温爆裂行为和断裂能。结果显示,未掺入任何钢纤维的空白组UHPC试件全都发生了严重高温爆裂,钢纤维可以显著减轻其高温爆裂但却不能避免爆裂的发生,而掺入端钩型普通工业钢纤维（长度为35 mm,直径为0.55 mm）的UHPC呈现出最优的抗高温爆裂性能,其次是掺入未附着橡胶颗粒（RSF）的再生钢纤维（RSFR）增韧UHPC。可见,钢纤维自身性能特征显著影响了钢纤维增韧UHPC的高温爆裂,相同掺量情况下混凝土单位体积内分布密度较大的钢纤维或者分布密度较小但可以显著增加混凝土断裂韧性（断裂能）的钢纤维比较适合应用于具有较高抗火要求的UHPC结构中。     

Effects of coarser fine aggregate on tensile properties of ultra high performance concrete

This experimental research investigates the mechanical properties and shrinkage of ultra high performance concrete (UHPC) incorporating coarser fine aggregates with maximum particle size of 5 mm. To adequately design UHPC mixtures using various sizes of solid constituents, particle packing theory was adopted. UHPC mixtures containing either dolomite or basalt, and four fiber volume fractions up to two volume percent were investigated. Uniaxial tension test was performed to evaluate the first cracking tensile strength, ultimate tensile strength, tensile strain capacity and cracking pattern. The UHPC mixtures with dolomite and steel fibers with more than one volume percent achieved more than 150 MPa of compressive strength at the age of 56 days, and showed strain hardening behavior and limited decrease in tensile strength compared to typical UHPC without coarser fine aggregates. The experimental results highlight the potential of dolomite used as coarser fine aggregate in UHPC.     

再生钢纤维增韧超高性能混凝土的力学性能

采用来自于废旧轮胎的两种再生钢纤维制备含粗骨料的超高性能混凝土,并测定其抗压强度、劈裂抗拉强度、断裂能和静弹性模量等力学性能,空白组及普通钢纤维增韧超高性能混凝土作对比性能试验。结果显示,未附着橡胶颗粒的再生钢纤维使超高性能混凝土的抗压强度略微下降,降低幅度为3.91%,其余各类型钢纤维均有利于提高超高性能混凝土的力学性能;而附着橡胶颗粒的再生钢纤维显著提高了超高性能混凝土的断裂能,约为普通钢纤维增韧超高性能混凝土的4倍。此外,再生钢纤维对超高性能混凝土的劈裂抗拉强度和静弹性模量的提高效果均优于普通钢纤维。再生钢纤维,尤其是附着橡胶颗粒的再生钢纤维,可以作为一种增韧材料替代普通钢纤维应用到超高性能混凝土工程结构中。      

Strain-hardening UHP-FRC with low fiber contents

This research work focuses on the optimization of strength and ductility of ultra high performance fiber reinforced concretes (UHP-FRC) under direct tensile loading. An ultra high performance concrete (UHPC) with a compressive strength of 200 MPa (29 ksi) providing high bond strength between fiber and matrix was developed. In addition to the high strength smooth steel fibers, currently used for typical UHP-FRC, high strength deformed steel fibers were used in this study to enhance the mechanical bond and ductility. The study first shows that, with appropriate high strength steel fibers, a fiber volume fraction of 1% is sufficient to trigger strain hardening behavior accompanied by multiple cracking, a characteristic essential to achieve high ductility. By improving both the matrix and fiber parameters, an UHP-FRC with only 1.5% deformed steel fibers by volume resulted in an average tensile strength of 13 MPa (1.9 ksi) and a maximum post-cracking strain of 0.6%.     

Synergistic response of blending fibers in ultra-high-performance concrete under high rate tensile loads

This study investigated the synergistic tensile response of blending 1% long and 0.5% short steel fibers in ultra-high-performance concrete (UHPC) at high strain rates of 16–37 s⁻¹. Three ultra-high-performance hybrid-fiber-reinforced concretes (UHP-HFRCs) containing twisted, hooked, or smooth long (30 mm) fibers blended with short (13 mm) smooth fibers, as well as one sample (LS10MS05) blending long and medium (19 mm) smooth fibers, were examined. The blending of long and shorter steel fibers in UHPC generated high synergy in the tensile responses of the UHP-HFRCs, especially at high strain rates. Synergies were significant for strain capacity and peak toughness, but not for post-cracking strength and softening fracture energy. Among the long fibers, the hooked fibers generated the highest synergy at high strain rates, but smooth fibers produced the highest rate sensitivity in UHPC. Consequently, the LS10MS05 sample demonstrated the highest tensile resistance at high strain rates.     

钢纤维增强超高性能混凝土抗压性能的细观数值模拟

利用LS-DYNA软件在细观层次上建立了三维钢纤维增强超高性能混凝土（Steel fiber reinforced ultra-high performance concrete,SF/UHPC）圆柱体试件有限元模型,对其轴心受压下的力学性能和裂缝发展进行了数值模拟。在验证细观数值模型的有效性和合理性的基础上进行参数分析,着重研究了钢纤维体积率、钢纤维长径比、形状效应和尺寸效应对超高性能钢纤维混凝土抗压强度、韧性和破坏形态的影响。最终,根据模拟结果拟合了超高性能钢纤维混凝土抗压强度计算公式。结果表明：三维超高性能钢纤维混凝土细观模型可以较好地模拟单轴受压应力条件下混凝土的静力性能和损伤破坏机制,所拟合的公式也能较好地预测超高性能钢纤维混凝土的抗压强度。     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing

Ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHP-FRC) were introduced in the mid 1990s. Special treatment, such as heat curing, pressure and/or extensive vibration, is often required in order to achieve compressive strengths in excess of 150 MPa (22 ksi). This study focuses on the development of UHP-FRCs without any special treatment and utilizing materials that are commercially available on the US market. Enhanced performance was accomplished by optimizing the packing density of the cementitious matrix, using very high strength steel fibers, tailoring the geometry of the fibers and optimizing the matrix-fiber interface properties. It is shown that addition of 1.5% deformed fibers by volume results in a direct tensile strength of 13 MPa, which is 60% higher than comparable UHP-FRC with smooth steel fibers, and a tensile strain at peak stress of 0.6%, which is about three times that for UHP-FRC with smooth fibers. Compressive strength up to 292 MPa (42 ksi), tensile strength up to 37 MPa (5.4 ksi) and strain at peak stress up to 1.1% were also attained 28 days after casting by using up to 8% volume fraction of high strength steel fibers and infiltrating them with the UHPC matrix.     

Short-term tensile creep and shrinkage of ultra-high performance concrete

The tensile creep and free shrinkage deformations of ultra-high performance concrete (UHPC) were examined through short-term testing to assess the influences of stress/strength ratio, steel fiber reinforcement, and thermal treatment. The use of fibers and the application of thermal treatment decreased 14-day drying shrinkage by more than 57% and by 82%, respectively. Increasing the stress-to-strength ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by volume decreased the tensile creep coefficient by 10% and the specific creep by 40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 90 °C, after initial curing, decreased its tensile creep coefficient by 73% and the specific creep by 77% at 7 days, as compared to ordinarily cured companion mixes. Comparison of tensile creep behavior to published reports on compressive creep in UHPC reveal that these phenomena differ fundamentally and that further evaluation is necessary to better understand the underlying mechanisms of tensile creep in UHPC. Results from this study also showed that the effects of both thermal treatment and fiber reinforcement were more pronounced in tensile creep behavior than tensile strength results of different UHPC mixes. This emphasizes the importance of conducting tensile creep testing to predict long-term tensile performance.