Role of matrix structure and flaw size distribution modification on deflection hardening behavior of polyvinyl alcohol fiber reinforced engineered cementitious composites(PVA-ECC)

The multiple cracking and deflection hardening performance of polyvinyl alcohol fiber reinforced engineered cementitious composites(PVA-ECC)under four-point flexural loading have been investigated.Matrices with different binder combinations and W/B ratios(from 0.44 to 0.78)providing satisfactory PVA fiber dispersion were specially designed.Effect of pre-existing flaw size distribution modification on deflection hardening behavior was comparatively studied by adding 3 mm diameter polyethylene beads into the mixtures(6%by total volume).Natural flaw size distributions of composites without beads were determined by cross sectional analysis.The crack number and crack width distributions of specimens after flexural loading were characterized and the possible causes of changes in multiple cracking and deflection hardening behavior by flaw size distribution modification were discussed.Promising results from the view point of deflection hardening behavior were obtained from metakaolin incorporated and flaw size distribution modified PVA-ECCs prepared with W/B=0.53.The dual roles of W/B ratio and superplasticizer content on flaw size distribution,cracking potential and fiber-matrix bond behavior were evaluated.Flaw size distribution modification is found beneficial in terms of ductility improvement at an optimized W/B ratio.

Mechanical properties of polyvinyl alcohol-basalt hybrid fiber engineered cementitious composites with impact of elevated temperatures

In the present study,the mechanical properties of polyvinyl alcohol(PVA)-basalt hybrid fiber reinforced engineered cementitious composites(ECC)after exposure to elevated temperatures were experimentally investigated.Five temperatures of 20,50,100,200 and 400℃ were set to evaluate the residual compressive,tensile and flexural behaviors of hybrid and mono fiber ECC.It was shown that partial replacement of PVA fibers with basalt fibers endowed ECC with improved residual compressive toughness,compared to brittle failure of mono fiber ECC heated to 400℃.The tension tests indicated that the presence of basalt fibers benefited the tensile strength up to 200℃,and delayed the sharp reduction of strength to 400℃.Under flexural load,the peak deflections corresponding to flexural strengths of hybrid fiber ECC were found to be less vulnerable ranging from 20 to 100℃.Further,the scanning electron microscopy(SEM)results uncovered that the rupture of basalt fiber at moderate temperature and its pullout mechanism at high temperature was responsible for the mechanical evolution of hybrid fiber ECC.This work develops a better understanding of elevated temperature and basalt fiber impact on the residual mechanical properties and further provides guideline for tailoring ECC for improved fire resistance.

Electromagnetic Shielding and Absorption Properties of Fiber Reinforced Cementitious Composites

In order to investigate the electromagnetic shielding effectiveness （SE） and absorbing properties of fiber reinforced concrete, steel fiber, carbon fiber and synthetic polyvinyl alcohol （PVA） fiber reinforced concrete were researched. The results show that with the increase of fiber Volume fraction, the SE and trend of frequency change of corresponding fiber reinforced concrete are enhanced. When the volume content of steel fiber is 3%, the SE of concrete is above 50 dB and its frequency is above 1.8 GHz. Moreover, in the range of 8-18 GHz, steel fiber, carbon fiber and PVA fiber all can improve the microwave absorption properties of concrete. The concrete with 0.5% carbon fiber can achieve the best absorbing property, the minimum reflectivity is about -7 dB; while steel fiber optimal volume fraction is 2%. The reflectivity curve of PVA fiber reinforced concrete fluctuates with the frequency, and the minimum value of the reflectivity is below -10 dB. The results show that fiber reinforced concrete could be used as EMI（electromagnetic interference） prevention buildings by attenuating and reflecting electromagnetic wave energy.

Multiphysics processes in the interfacial transition zone of fiber-reinforced cementitious composites under induced curing pressure and implications for mine backfill materials: A critical review

The mesoscale fiber-matrix interfacial transition zone(FM-ITZ) under induced curing pressure plays a key role in the effectiveness of fiber reinforcement and the engineering application of fiber-reinforced cementitious composites(FRCCs). This critical review establishes the link among induced curing pressure(i.e., external loading condition), multiphysics processes(i.e., internal governing mechanism), and interface behavior(i.e., material behavior) for FRCC materials through analysis of the state-of-the-art research findings on the FM-ITZ of FRCC materials. The following results are obtained. For the mechanical process, the induced curing pressure changes the stress state and enhances multicracking behavior, which can strengthen the FM-ITZ. For the hydraulic process, the strengthened seepage of the FM-ITZ under induced curing pressure weakens the effective stress and exaggerates the deficiency in water retention capacity between the bulk matrix and the FMITZ. For the thermal process, the induced curing pressure causes a steep temperature gradient in the FM-ITZ and thus influences the temperature evolution and thermally-induced microcracks in the FM-ITZ. For the chemical process, the induced curing pressure enhances hydration kinetics and results in the formation of additional hydration products in the FM-ITZ. Moreover, recommendations are proposed on the basis of findings from this review to facilitate the implementation of fiber reinforcement in cemented paste backfill technology.

High Temperature Flexural Deformation Properties of Engineered Cementitious Composites (ECC) with Hybrid Fiber Reinforcement

Engineered Cementitious Composites(ECC)is a class of high-performance fiber reinforced composites with ultra-ductility designed based on micromechanics,and it has been developed for increasing application in the construction industry during recent decades.The properties of ECC at room temperature have been tested and studied in depth,however,few studies focus on its performance after high temperature that is one of the worst conditions to ECC.To investigate the change tendency and mechanism for the high temperature flexural properties of hybrid fiber reinforced ECC and the feasibility of calcium carbonate whisker to reduce the cost of ECC materials,polyvinyl alcohol fiber(PVA)reinforced strain hardening cementitious composites(PVA-ECC),steel fiber+PVA fiber reinforced ECC(defined as HyFRECC-A)and steel fiber+PVA fiber+CaCO3 whisker reinforced ECC(defined as HyFRECC-B)subject to room temperature and 200℃,400℃,600℃,800℃elevated temperature exposure were experimentally compared.The results indicate that equally replacing PVA fibers by steel fibers degraded the flexural hardening ability of PVA-ECC at room temperature,while the addition of appropriate amount of CaCO3 whisker improved the flexural strength,toughness and flexural hardening behavior.The elevated temperature posed a significant effect on the flexural strength and toughness of the three types of ECCs.Flexural deflection hardening behavior of the three types of ECCs was eliminated after high temperature exposure.Flexural strength and toughness of PVA-ECC presented an exponential decay along with the increase of temperature.The addition of steel fiber slowed down the decay rate.Although the use of CaCO3 whisker increased the post-temperature flexural strength and toughness of HyFRECC-B,the decay rate was not further decreased.

Flexural behavior of steel reinforced engineered cementitious composite beams

In order to enhance the durability of steel encased concrete beams, a new type of steel reinforced engineered cementitious composite(SRECC) beam composed of steel shapes, steel bars and ECC is proposed. The theoretical analyses of the SRECC beam including crack propagation and stress-strain distributions along the depth of the composite beam in different loading stages are conducted. A theoretical model and simplified design method are proposed to calculate the load carrying capacity. Based on the proposed theoretical model, the relationship between the moment and corresponding curvature is derived. The theoretical results are verified with the finite element analysis. Finally, an extensive parametric study is performed to study the effect of the matrix type, steel shape ratio, reinforced bar ratio, ECC compressive strength and ECC tensile ductility on the mechanical behavior of SRECC beams. The results show that substitution concrete with ECC can effectively improve the bearing capacity and ductility of composite beams. The steel shape and longitudinal reinforcement can enhance the loading carrying capacity, while the ductility decreases with the increase of steel shape ratio. ECC compressive strength has significant effects on both load carrying capacity and ductility, and changing the ultimate strain of ECC results in a very limited variation in the mechanical behavior of SRECC beams.

Tensile properties of strain-hardening cementitious composites containing polyvinyl-alcohol fibers hybridized with polypropylene fibers

Partially replacing polyvinyl-alcohol(PVA)fibers with polypropylene(PP)fibers in strain-hardening cementitious composites(fiber hybridization)modify certain mechanical properties of these materials.The hybridization based on the introduction of low-modulus hydrophobic polypropylene fibers improves the ductility and the strain-hardening behavior of the cementitious composites containing polyvinyl-alcohol fibers of different types(PVA-SHCC).Pull-out tests indicate that adding PP fibers increases the energy capacity of the hybrid composite with respect to the material containing only PVA fibers under tensile loading,and PP-fiber geometry(i.e.,section shape and length)is a key factor in enhancing the strain capacity.

Main properties of ultra-high performance fiber reinforced cement composites under couple effect of load and environment

This study aims to reveal the mechanism that how the content of steel fibers and strength grades affect the macro performance of the ultra-high performance fiber reinforced cementitious composite （UHPFRCC） and to study the UHPFRCC durability under the combined effect of loads and environments. Three types of high and ultra-high performance fiber reinforced cement composites with different strength grades （100, 150, 200 MPa） and different steel fiber volume fractions （0%, 1%, 2%, 3%） are prepared. The main properties of mechanical performance and short-term durability are studied. A preloading frame is designed to apply a four- point load external flexural stress with a stress selection ratio of 0.5 for UHPFRCC150 specimens. The results show that the growth in strength grade with a proper content of steel fiber greatly increases the strength and toughness of the HPFRCC and the UHPFRCC while decreasing the dry-shrinkage ratio. For the loaded specimens, the existence of steel fiber can reduce the negative influence of tensile stress on the Cl- penetration resistance of the UHPFRCC in addition to improving its ability to resist the freeze-thaw damage.

Influence of Thickeners on Cement Paste Structure and Performance of Engineered Cementitious Composites

Hydroxypropyl methylcellulose （HPMC） and amphoteric polyacrylamide （ACPAM） were respectively used to prepare engineered cementitious composite （ECC） which exhibits strain-hardening behavior under uniaxial tension. The connections between cement paste structure and the performance of the composite in fresh and hardened state were investigated, aiming at achieving the desirable workability at a given solids concentration. The experimental results of viscosity and miCrostructure of cement pastes show that the intimate connections between flocculation groups lead to the growing increase in viscosity. The results of deformability and fiber dispersion demonstrate that fiber dispersion coefficient is a comprehensive index which can reflect the performance of deformability as well as uniformity. And the desirable fresh mixture can be achieved by optimizing the viscosity of cement paste. At last, the ductile strain-hardening performance of the ECC prepared with HPMC or ACPAM was investigated through uniaxial tensile test.

配筋PVA-钢混杂纤维增强水泥基复合材料梁柱边节点抗震性能试验研究

改善工程材料韧性和耐久性,提高框架梁柱节点抵抗变形和破坏的能力,是提高框架结构抗震韧性的有效途径之一。采用PVA-钢混杂纤维增强水泥基复合材料代替普通混凝土应用到梁柱边节点,考虑轴压比和加密区体积配箍率的影响,设计6个配筋PVA-钢混杂纤维增强水泥基复合材料、1个配筋单掺PVA纤维增强水泥基复合材料和1个钢筋混凝土梁柱边节点试件进行拟静力试验,分析其破坏形态、滞回曲线、骨架曲线、延性、耗能能力、钢筋应变和梁端塑性铰区转角,探讨混杂纤维的加入对梁柱边节点抗震性能的影响。结果表明:与钢筋混凝土节点和单掺PVA纤维增强水泥基复合材料节点相比,PVA-钢混杂纤维增强水泥基复合材料节点的承载力高、变形能力大、延性好、耗能能力强,抗震性能显著提升。当试验轴压比从0.12增加到0.24,梁端塑性铰区产生一定的外移,塑性性能发挥更充分,同时试件的变形能力、延性、耗能能力增加。在加密区体积配箍率减小的情况下,试件仍表现出良好的抗震性能。

玄武岩织物-钢-PVA工程水泥基复合材料单轴受压力学性能研究

本文研究了玄武岩织物和混杂短切纤维(钢纤维及PVA纤维,短切纤维总体积掺量为2%)复合作用对玄武岩织物-钢-PVA工程水泥基复合材料(BTR-HyECC)单轴受压力学性能的影响,并通过圆柱体单轴受压试验测得各试件受压性能指标,并绘制出应力-应变曲线。结果表明:当钢纤维掺量为0%(体积分数,下同)时,随着玄武岩织物的加入,试件的抗压强度、峰值应变以及抗压韧性显著提高;当钢纤维掺量增加到1%时,试件抗压强度和弹性模量达到最大;当钢纤维掺量为1.5%时,试件变形能力和抗压韧性达到最佳。基于试验数据,建立了适用于BTR-HyECC的单轴受压本构方程,拟合曲线与试验曲线吻合较好。

RTP-PVA混杂纤维的工程水泥基复合材料干缩性能试验研究

为研究回收轮胎聚合物(recycled tyre polymer,RTP)和聚乙烯醇(polyvinyl alcohol,PVA)混杂纤维对工程水泥基复合材料(engineered cementitious composites,ECC)干缩性能的影响,对RTP-PVA混杂纤维总体积分数为2.0%的ECC进行流动性、直接拉伸和干缩试验,并分析混杂纤维作用机理和干缩计算模型.结果表明:RTP纤维替代率为12.5%~50%对ECC流动性影响不大,比单掺PVA纤维的ECC流动性降低1.09%~3.69%.ECC的抗拉强度随RTP纤维替代率的增加而降低,不同比例的混杂纤维比单掺PVA纤维的ECC抗拉强度降低了23.6%~56.6%.不同比例混杂纤维ECC干缩率曲线趋势相近,7 d时干缩率为28 d时的80.92%~82.77%.ECC的干缩率随RTP纤维替代率的增加而降低,28 d时不同RTP纤维替代率的混杂纤维比单掺PVA纤维的ECC干缩率降低了0.80%~2.09%.RTP-PVA混杂纤维可在ECC中协同发挥作用抑制基体干缩,结合试验结果得到适合RTP-PVA混杂纤维ECC的指数型干缩预测模型.

钢-PVA混杂纤维增强水泥基复合材料永久模板叠合RC单向板短期刚度计算方法

钢-PVA混杂纤维增强水泥基复合材料(HFRCC)具有良好的受拉韧性和耐久性能,是永久模板的理想材料。首先探究了PVA纤维掺量对HFRCC工作性能和抗折强度的影响,随着PVA纤维掺量增加,HFRCC的工作性能显著减弱,抗折强度先增大再减小,最终选用含有1.5%钢纤维和0.25%PVA纤维的HFRCC来制作永久模板。随后对六个HFRCC永久模板叠合RC单向板和一个RC单向板进行了四点受弯试验,探究了HFRCC-RC界面处理方式和配筋率对叠合板受弯性能的影响。试验结果表明:抹平、等距凹槽和钢钉拉毛三种界面处理方式对叠合板受弯性能的影响可以忽略;HFRCC模板能够有效限制裂缝发展,减小裂缝宽度和间距,叠合板的开裂弯矩相比于普通混凝土板提高了20.1%~31.7%。分别采用刚度解析法和有效惯性矩法建立了HFRCC-RC叠合板短期刚度计算方法,两种方法均有较高的计算精度且离散性较小。

GO-PVA注浆材料的制备及其强度特征研究

由于瓦斯抽采钻孔周围稳定性较差,受扰动影响易致钻孔周围煤岩体产生新的裂隙,进而降低瓦斯抽采效率,为填补现有材料颗粒大、胶结能力差的缺陷,引入纳米材料和高分子聚合物材料改善瓦斯抽采钻孔封孔材料的性能,并分析纳米材料氧化石墨烯(Graphene Oxide,GO)、高分子聚合物聚乙烯醇(Polyvinyl Alcohol,PVA)纤维复掺于水泥基注浆材料中的宏微观强度特征。采用单因素试验探究GO最佳质量分数及PVA纤维最佳长度和质量分数,确定最优值后再与普通水泥注浆材料进行复合对比试验,并通过单轴压缩测试不同养护龄期下的强度变化规律,结合蠕变特性、相似模拟和扫描电子显微镜(Scanning Electron Microscope,SEM)图像,探讨复合材料的增强机理。研究结果表明:两类添加剂均对水泥基注浆材料抗压强度展现出有效的提升作用,尤其是当氧化石墨的质量分数为0.04%,PVA质量分数和长度分别为0.2%和3 mm时,其抗压性能达到顶峰,密封性能提升,抵抗外界应力变化的能力得到增强。研究结果可为封孔注浆材料的研发提供基础。

PVA/钢混杂纤维增强水泥基材料(HFRCC)配合比优化及评价方法

将钢纤维、国产PVA纤维和日本PVA纤维按照适宜比例进行配制,不同纤维材料性能相互补充、取长补短,可以更好发挥出混杂纤维增强水泥基材料(HFRCC)的力学性能,有利于其成本控制,具有广泛应用前景。通过正交试验极差结果择优和信噪比S/N稳定性择优两种方法分析粉煤灰掺量、水胶比、砂胶比、钢纤维掺量、国产PVA掺量和日产PVA掺量对混杂纤维增强水泥基材料(HFRCC)抗拉强度和抗弯强度的影响规律,对比两种方法的结果,并建立各因素和响应量之间的回归关系,与和易性工程性能相结合,给出HFRCC的最优配合比。结果表明:信噪比S/N稳定性择优方案更加准确和全面。粉煤灰掺量、钢纤维掺量、国产PVA掺量和日产PVA掺量对HFRCC的响应量影响较大,水胶比和砂胶比影响较小;建立数学模型预测和优选配合比,HFRCC抗拉强度最大可以达到6.36 MPa,抗弯强度最大可以达到13.90 MPa,预测值和试验值之间的相对误差绝对值均接近3%,且和易性能较好。研究结果可以为混杂纤维增强水泥基材料(HFRCC)的制备提供依据。

PVA纤维长度对水泥基复合材料抗拉强度的影响

为了研究PVA纤维长度对水泥基复合材料抗拉强度的影响,采用6 mm、9 mm和18 mm的PVA纤维结合新疆本地沙漠砂与粉煤灰制备PVA-沙漠砂水泥基复合材料,于恒温条件养护28 d后测试其抗拉强度并与普通混凝土试件进行对比。由试验得出:添加PVA纤维的水泥基复合材料相比于普通混凝土试件其抗拉强度显著提高;随着纤维长度的增加,PVA-沙漠砂水泥基复合材料的流动度和抗拉强度逐渐降低。

Experimental investigation of engineered geopolymer composite for structural strengthening against blast loads

The recent increase in blast/bombing incidents all over the world has pushed the development of effective strengthening approaches to enhance the blast resistance of existing civil infrastructures.Engineered geopolymer composite(EGC)is a promising material featured by eco-friendly,fast-setting and strain-hardening characteristics for emergent strengthening and construction.However,the fiber optimization for preparing EGC and its protective effect on structural elements under blast scenarios are uncertain.In this study,laboratory tests were firstly conducted to evaluate the effects of fiber types on the properties of EGC in terms of workability,dry shrinkage,and mechanical properties in compression,tension and flexure.The experimental results showed that EGC containing PE fiber exhibited suitable workability,acceptable dry shrinkage and superior mechanical properties compared with other types of fibers.After that,a series of field tests were carried out to evaluate the effectiveness of EGC retrofitting layer on the enhancement of blast performance of typical elements.The tests include autoclaved aerated concrete(AAC)masonry walls subjected to vented gas explosion,reinforced AAC panels subjected to TNT explosion and plain concrete slabs subjected to contact explosion.It was found that EGC could effectively enhance the blast resistance of structural elements in different scenarios.For AAC masonry walls and panels,with the existence of EGC,the integrity of specimens could be maintained,and their deflections and damage were significantly reduced.For plain concrete slabs,the EGC overlay could reduce the diameter and depth of the crater and spallation of specimens.

PVA-钢混杂纤维增强水泥基复合材料抗压性能试验研究

为实现纤维增强水泥基复合材料(ECC)受荷开裂过程中裂缝的全过程控制,提高其压缩变形性能,在传统ECC中掺加钢纤维,制备PVA-钢混杂纤维增强水泥基复合材料(HyFRCCs)。以PVA纤维掺量、钢纤维掺量及钢纤维长径比为变量,设计制作22组不同配合比HyFRCCs试件。通过轴心受压试验研究HyFRCCs单轴受压力学特性,分析纤维掺量、种类及几何特征对HyFRCCs抗压性能的影响。结果表明:PVA纤维掺量、钢纤维掺量及钢纤维长径比对HyFRCCs轴心抗压强度影响较小,纤维混杂能够显著提高材料峰值应变和峰值后延性,HyFRCCs的裂缝控制能力和压缩变形能力得到显著改善。

定量分布PVA纤维增强水泥基复合材料连续梁抗弯性能数值模拟

纤维增强是有效改善水泥基复合材料抗拉性能的方法之一,掺加聚乙烯醇(Polyvinyl Alcohol Fiber,PVA)纤维可以提高水泥基复合材料的抗裂性能和韧性,根据构件拉应力大小配制PVA纤维体积掺量,即定量分布PVA纤维,能够充分发挥纤维的增强作用。为研究PVA纤维定量分布的增强机理,开展了定量分布PVA纤维增强水泥基复合材料(Adaptively Distributed Polyvinyl Alcohol Fiber Reinforced Cement-based Composites,AD-PFRC)连续梁的抗弯性能数值模拟研究,对比分析了不同PVA纤维分布方式连续梁的应力云图及梁底部拉应力较大区域的合力。数值模拟结果表明:平均PVA纤维体积掺量为1.3%时,AD-PFRC的拉应力值相比PFRC明显提高,且达到极限荷载时,AD-PFRC梁跨中截面底部拉应力较大区域的合力相比PFRC提高了12%,AD-PFRC的PVA纤维增强作用明显提高。

PVA纤维对超高韧性纤维增强水泥基复合材料力学性能的影响

通过对超高韧性纤维增强水泥基复合材料制作的立方体试件、棱柱体试件,进行了抗压、抗折和I型断裂试验,研究了不同配合比下各组试块的抗压强度等力学指标与纤维掺量及基体材料强度的关系,研究了荷载与变形的关系、及不同切口高度试件的抗裂性能等。试验结果表明:合理的配合比及最佳PVA掺量能提高纤维增强水泥基复合材料的抗压强度、抗折强度以及抗裂韧度;研究了国产PVA纤维与进口PVA纤维的合理掺量,在相同力学指标的前提下,利用国内PVA纤维造价低的优势,可实现部分代替超高性能水泥基复合材料中的进口PVA纤维,从而达到这种复合材料国产化、本地化的目的。