粗骨料与钢纤维对超高性能混凝土单轴拉伸性能的影响

为了提高含粗骨料超高性能混凝土(Ultra-high performance concrete,UHPC)的单轴拉伸性能,采用单轴拉伸试验和图像分析技术分别研究了粗骨料掺量、颗粒粒径对含粗骨料UHPC单轴拉伸性能和钢纤维在UHPC体系中分散性能的影响规律。结果表明,随着粗骨料掺量及颗粒粒径的增大,钢纤维在UHPC体系中的分散系数和取向系数显著降低,含粗骨料UHPC的单轴拉伸初裂强度、裂后强度和耗能也随之减小。根据粗骨料颗粒最大粒径与钢纤维体积分数、直径间的匹配关系式(Dmax=3df/(Vf)0.5),采用纤维混杂可以充分发挥多尺度纤维与具有不同粒径分布的骨料间的分级匹配关系;粗骨料体积分数和颗粒最大粒径分别为10%和10mm时,采用平直钢纤维(直径0.12mm、长度10mm、体积掺量1.2%)和端钩钢纤维(直径0.35 mm、长度20mm、体积掺量1.8%)混杂实现了含粗骨料UHPC的单轴拉伸性能的提升,其裂后强度和耗能分别为8.69 MPa和11.10J。     

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%。     

钢纤维埋深与类型影响钢纤维-UHPC基体界面粘结性能的试验研究

为研究钢纤维-超高性能混凝土(UHPC)基体界面粘结性能的影响因素，进一步阐明不同纤维类型、埋深下双根钢纤维、UHPC基体粘结性能及界面破坏形式，本工作通过双丝拉拔试验对不同埋深的高强钢纤维在UHPC基体中的拔出行为进行研究，以了解纤维的拉拔性能和UHPC的界面粘结性能。纤维拉拔试验以纤维类型及埋深为变量，对两根钢纤维在不同纤维埋深下的评价参数进行了表征和分析，并观察了纤维拔出后的微观形貌和UHPC基体的隧洞形貌。试验结果表明：端钩型纤维的拉拔性能优于直圆型纤维；利用SEM观察到拔出的直圆型纤维表面粘附有絮状或簇状的微小UHPC基体颗粒，并有不同程度的擦伤或长而宽的刮痕，在UHPC基体隧洞中发现了微裂纹，且在纤维拉拔过程中拔出口附近的基体会发生剥落。同时，钢纤维的拉拔性能与纤维埋深有关，但埋深对端钩型纤维影响更大，拉拔荷载峰值达到402.66 MPa,材料的强度利用率为94.9%;纤维的破坏模式也与纤维类型有关，端钩型纤维比直圆型纤维更易发生断裂。本研究可为进一步改善钢纤维增强超高性能混凝土的力学性能提供参考。     

浆体流变性能对超高延性水泥基材料性能的影响

浆体的流变性能是影响纤维在水泥基材料中分散性的关键因素，是采用聚乙烯（PE）纤维制备超高延性水泥基材料（Ultra-high ductility cementitious composites, UHDCC）的重要指标。本工作通过调整水胶比和外加剂的掺量调控浆体的流变性能，研究浆体的屈服应力和塑性黏度对UHDCC流动性、拉伸、抗压和断裂性能的影响。结果表明：调整水胶比和外加剂可以调控UHDCC浆体的流变特性，其流变行为符合假塑性流体。浆体塑性黏度在1.91～6.00 Pa·s范围内的UHDCC呈现不同程度的拉伸应变硬化行为；塑性黏度的最佳范围为3.06～4.60 Pa·s，此时纤维在基体中分散更加均匀，因此UHDCC具备更加优异的拉伸性能和断裂韧度，其拉伸应变可以超过10%。     

钢纤维对超高性能混凝土干燥收缩的影响

研究了在(20±2)℃、相对湿度为(50±5)%的环境中钢纤维体积掺量为0%、1%、2%和3%的超高性能混凝土(UHPC)的干燥收缩。结果表明:UHPC在前7d的干燥收缩发展速率较快,7d后发展速率逐渐减缓;但当钢纤维掺量超过2%后,钢纤维对干燥收缩的改善作用明显降低,相比钢纤维掺量为2%的UHPC,3%掺量UHPC的干燥收缩仅仅降低了1.5%。钢纤维高弹模及它与基体的界面粘结有效降低了混凝土的干燥收缩,但钢纤维掺量过多可导致多孔薄弱的界面区增加,从而使其对混凝土的收缩抑制作用减小。粉煤灰对超高性能混凝土干燥收缩的抑制作用大于矿粉。提出的新的数学拟合指数公式相比于文献中常用的ACI和王铁梦公式与实测结果吻合度更好。     

混杂纤维对大掺量矿物掺合料混凝土干燥收缩的影响

通过测定混凝土的长期干燥收缩值,研究了单掺聚丙烯(polypropylene,PP)纤维、聚酯纤维和钢纤维及其混杂技巧对大掺量矿物掺合料混凝土干燥收缩的影响.结果表明,大掺量矿物掺合料混凝土的干燥收缩比较大,纤维及混杂技巧有利于降低其干燥收缩,纤维的弹性模量越高,其抑制干燥收缩的效果越好;(0.05%PP纤维 0.05%聚酯纤维)的有机纤维混杂效果优于(0.1%PP纤维 0.7%钢纤维)和(0.1%聚酯纤维 0.7%钢纤维)的有机与无机纤维混杂;混杂纤维对混凝土干燥收缩的抑制效果与纤维之间的搭配有关.综合分析表明,(PP纤维 聚酯纤维)的二元混杂技术具有良好的工程应用价值.     

钢纤维聚合物混凝土抗压本构关系

钢纤维聚合物混凝土是由聚合物混凝土基体和钢纤维共同组成的纤维增强复合材料，它的力学行为不仅依赖于聚合物混凝土基体的行为，而且与钢纤维的掺量、分散特征以及钢纤维的几何尺寸有关，本文将基于损伤力学原理研究在不同纤维掺量下的钢纤维聚合物混凝土的全程压力-应变曲线及其本构模型。     

钢纤维形状与掺量对UHPC施工及力学特性的影响

选用四种平直及两种端钩钢纤维,研究钢纤维体积掺量、长径比、形状、同形及异形纤维混掺对超高性能混凝土(UHPC)施工及力学性能的影响.通过相关实验得到了UHPC扩展度、抗压强度、抗折强度、能量吸收、断裂能及弯曲应力-挠度曲线;基于弯曲应力-挠度曲线及改进后的规范方法计算了UHPC的弯曲韧性指标;最后,开展了最佳纤维混掺比例的研究.结果表明:纤维掺量每增加0.5％,UHPC扩展度平均降幅为2.72％,抗压强度平均增幅为5.79％.抗折强度、弯曲韧性指数和能量吸收则先增后减(临界掺量为3.5％),断裂能呈上下波动(在3％时达最低值).随着纤维长径比的增大,UHPC扩展度降低,抗压强度、抗折强度、弯曲韧性指数、能量吸收值和断裂能基本呈递增趋势.相同长径比时,端钩形纤维UHPC扩展度、弯曲韧性指数优于平直形纤维,抗压强度、抗折强度、能量吸收、断裂能低于平直形纤维.同形纤维混掺UHPC扩展度、抗压强度稍低于对应的单掺纤维,弯曲韧性、能量吸收、断裂能总体上优于单掺试件;异形纤维混掺UHPC扩展度、抗压强度稍低于单掺试件,抗折强度与单掺试件各有所长,弯曲韧性、能量吸收及断裂能绝大多数优于单掺纤维.UHPC抗折强度变异性高于其抗压强度.单掺和混掺纤维时,UHPC试件的抗压强度、抗折强度综合最优分别为173.53 MPa、44.9 MPa和160.9 MPa、55.72 MPa;纤维混掺最佳组合为18 mm平直形、16 mm端钩形,且两者混掺比例为1:1时,UHPC的综合力学性能较优.     

UHPC三轴受压力学性能研究

为了研究三轴受压下UHPC的受力性能,以围压大小和钢纤维掺量为试验参数,进行20组UHPC试件常规三轴试验,分析UHPC的破坏形态、应力-应变曲线、峰值应力和应变等力学性能。结果表明：围压和钢纤维掺量均为零的试件破坏时呈劈裂破坏,其他试件则呈剪切破坏;围压和钢纤维掺量对应力-应变曲线弹性模量和弹性段曲线形状影响较小;随着围压增大,峰值应力和应变呈不断增大趋势;随着钢纤维掺量增大,峰值应力和轴向峰值应变呈先增大后不变和先增大后减小趋势,环向峰值应变则呈增大趋势。通过对UHPC八面体正应力-体积应变和剪应力-剪应变关系进行分析,基于Drucker-Prager二参数准则,建立了UHPC八面体破坏准则计算方法。     

基于钢纤维掺量的混凝土的冲击性能研究

为了研究钢纤维掺量对高性能混凝土冲击性能的影响,本试验通过改变纤维掺量,依照ACl544．2R的纤维混凝土性能测试方法,对不同钢纤维掺量的试件进行研究。结果发现：在纤维掺量从0-25kg／m3变化时,钢纤维高性能混凝土的冲击性能随纤维掺量的增加而增强,但超过25kg／m3后其冲击性能降低。     

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。     

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。     

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。      

Laboratory investigation of rheological properties and scaling resistance of air entrained self-consolidating concrete

An experimental investigation was undertaken to analyze the influence of various admixtures on the rheological properties and scaling resistance of self-consolidating concrete. Such concrete is intended for use as a repair material for filling highly restricted areas, such as forms with closely spaced reinforcing steel bars. Several self-consolidating concrete mixtures having slump flow of 550+50 mm were prepared with water-to-cement ratios varying between 0.35 and 0.41. The mixtures were cast with 0 and 3 percent silica fume, with and without air-entraining admixture. All concretes incorporated superplasticizer and viscosity-modifying admixture to enhance deformability and stability. Rheological parameters (yield value and plastic viscosity) were measured using a concrete viscometer. The air content, unit weight, and consistency were also determined. The consistency was assessed using the slump flow and L-Flow methods. Tests performed on hardened concrete included compressive strength at 28 days (ASTM C 39), scaling resistance (ASTM C 672), durability to freezing and thawing (ASTM C 666) and measurement of the air-void parameters (ASTM C 457). Relationship between the simple slump flow and yield value and plastic viscosity measurements determined using a concrete viscometer are also discussed. In general, the laboratory test results indicate that it is possible to produce a frost durable, self-consolidating concrete with low yield value and high plastic viscosity (for such fluid concrete) which can be use as a repair material to fill highly restricted areas.     

Static and dynamic compressive properties of ultra-high performance concrete (UHPC) with hybrid steel fiber reinforcements

Ultra-high performance concrete (UHPC) is promising in construction of concrete structures that suffer impact and explosive loads. In order to make UHPC structures more ductile and cost-effective, hybrid fiber reinforcements are often incorporated. In this study, a reference UHPC mixture with no fiber reinforcement and five mixtures with a single type of fiber reinforcement or hybrid fiber reinforcements of 6 and 13 mm in length at a total dosage of 2%, by the volume of concrete, were prepared. Quasi-static compressive and flexural properties of those mixtures were investigated. Split Hopkinson press bar (SHPB) testing was adopted to evaluate their dynamic compressive properties under three impact velocities. Test results indicated that UHPC with 1.5% long fiber reinforcements and 0.5% short fiber reinforcements demonstrated the best static and dynamic mechanical properties. The static compressive and flexural strengths of UHPC with 2% long fiber reinforcements were greater than those with 2% short fiber reinforcements, whereas comparable dynamic compressive properties were observed. Strain rate effect was observed for the dynamic compressive properties, including peak stress, dynamic increase factor, and absorbed energy. The reinforcing mechanisms of hybrid fiber reinforcements in UHPC were eventually discussed.     

Plastic viscosity of fresh concrete – A critical review of predictions methods

Rheological properties of fresh concrete, namely plastic viscosity and yield stress, are critical for the concrete industry because they affect placement and workability. Moreover, these rheological properties influence the productivity and quality of concrete, including mechanical properties and durability. Therefore proper characterization of these properties is needed to control the quality of fresh concrete and ensure sustainability of concrete structures.Fundamental and phenomenological rheological models have been proposed in the literature for characterizing the behaviour of fresh concrete. Establishing a model for predicting the plastic viscosity of concrete based on its composition will be extremely valuable for the concrete industry. This paper provides a critical review of the most prevailing models in concrete technology as well as models proposed in the literature for predicting the plastic viscosity of dense suspensions to a total of eight models. Review has revealed that Mahmoodzadeh and Chidiac models based on the cell method provides a higher degree of correlation to the experimental data as well as a more consistent and reliable predictions in comparison to the models currently proposed in the literature for concrete and/or dense suspensions.     

PEN短纤维增强PTT复合材料的流变性能及力学性能

研究了聚对苯二甲酸丙二酯(PTT)/聚对萘二甲酸乙二酯(PEN)短纤维复合材料的流变行为和力学性能,讨论了复合材料的组成、剪切应力和剪切速率及温度对熔体流变行为、熔体黏度的影响,以及不同配比复合材料的力学性能。结果表明,PTT/PEN短纤维复合材料熔体为假塑性流体,熔体表观黏度随着温度升高而下降,且熔体黏度随着PEN短纤维含量增加而不断上升。随PEN短纤维加入量的增加,复合材料的拉伸强度、断裂强度、弹性模量均明显提高,无缺口冲击强度略有提高,说明PEN短纤维的加入对PTT起到了明显的增强作用而不降低材料的韧性。     

植物纤维基包装材料中纤维悬浮体流变行为研究

目的 采用可再生的植物纤维机械搅拌制备植物纤维悬浮体系,探究纤维悬浮液的流变性影响因素和体系中纤维的分散与取向,为后续工业化产品的加工和运输设计奠定基础。方法 利用旋转流变仪测试纤维质量分数、长径比（纤维长度）、温度和水溶性高分子等各个因素对体系流变性能的影响。结果 纤维悬浮体系为非牛顿流体,表现剪切变稀现象。溶液的黏度随温度的变化经线性拟合符合Arrhenius方程。纤维质量分数、长径比（纤维长度）的提高均会增加体系表观黏度,促进纤维的分散及网络结构形成,而温度的提高,黏度会呈指数形式减小。加入水溶性高分子PVA和聚丙烯酰胺分散剂均会提高体系黏度,有助于纤维的分散;PVA醇解度越高,亲水基团越多,易与纤维产生更多氢键作用,形成纤维网络结构,有助于后续发泡结构控制,醇解度过高则易凝胶化。阳离子型CPAM有助于改善纤维表面负电荷,破坏纤维静电吸附,使得纤维分散更均匀。结论 纤维质量分数、长径比（纤维长度）、体系温度等因素会影响纤维悬浮液流变特性,水溶性PVA和分散剂聚丙烯酰胺的加入有助于纤维的分散,对工业化纤维浆料配比加工具有参考意义,同时体系的黏流特性有利于生产线浆料输送系统的设备设计...     

Multi-scale investigation of microstructure,fiber pullout behavior,and mechanical properties of ultra-high performance concrete with nano-CaCO3 particles

The mechanical properties of a fiber-reinforced concrete are closely related to the properties of the matrix, fiber, and fiber-matrix interface. The fiber-matrix bond property is mainly governed by the adhesion between the fiber and surrounding cement materials, as well as the strength of materials at the interfacial transition zone. In this paper, the effect of nano-CaCO₃ content, varying between 0 and 6.4%, by mass of cementitious materials, on microstructure development, fiber-matrix interfacial bond properties, and mechanical properties of ultra-high performance concrete (UHPC) reinforced with 2% steel fibers were investigated. The bond properties, including bond strength and pullout energy, were evaluated. Mercury intrusion porosimetry (MIP), backscattered electron microscopy (BSEM), optical microscopy, and micro-hardness testing were used to characterize the microstructure of matrix and/or interfacial transition zone (ITZ) around an embedded steel fiber. Test results indicated that the incorporation of 3.2% nano-CaCO₃ significantly improved the fiber-matrix bond properties and the flexural properties of UHPC. This was attributed to densification and strength enhancement of ITZ as observed from micro-structural analyses. Beyond the nano-CaCO₃ content of 3.2%, the fiber bond and mechanical properties of UHPC decreased due to increased porosity associated with agglomeration of the nano-CaCO₃.