高韧性纤维增强水泥基复合材料的单轴拉伸力学性能

试验研究了3种纤维掺量聚乙烯醇纤维增强水泥基复合材料(PVA-ECC)的单轴拉伸力学性能及纤维掺量对ECC力学性能的影响规律,探讨了裂缝扩展宽度与轴拉应力的关系,分析了ECC拉伸断裂能与特征长度.结果表明:当纤维掺量由0.308%提高到0.462%时,PVA-ECC的断裂能提高128%,特征长度提高146%,最大拉应变提高128%.PVA-ECC的断裂能和最大拉应变比聚丙烯纤维混凝土分别提高209%和3 614%.表明PVA纤维具有良好的阻裂增韧效用,显著提高了ECC的抗裂性能和变形能力,ECC在单轴拉伸荷载下能实现应变硬化和多重裂纹开裂.     

耐碱玻璃纤维ECC复合材料受压应力–应变关系

为研究耐碱玻璃纤维工程用水泥基复合材料（耐碱玻璃纤维ECC）的抗压性能及应力-应变关系，对33组高性能水泥基材料试件进行了轴压性能试验，分析了纤维掺量、纤维长度及水灰比对耐碱玻璃纤维ECC的受压性能及应力-应变关系的影响，提出了耐碱玻璃纤维ECC受压应力-应变关系计算模型。结果表明，掺入耐碱玻璃纤维可以明显改善水泥基材料在单轴受压状态下的抗裂、受力和变形性能；耐碱玻璃纤维ECC试件抗压强度和变形能力的提升程度与纤维掺量、纤维长度及水灰比有关；随着纤维掺量和长度增加，耐碱玻璃纤维ECC试件的抗压强度和变形能力大致呈递增趋势，但掺量过多会因“团聚”现象明显导致试件抗压强度降低；水灰比主要影响试件的抗压强度，水灰比越大，抗压强度越小；当纤维质量掺量为6.5%、纤维长度为18mm及水灰比为0.32时，碱玻璃纤维ECC的综合力学性能相对较优，其抗压强度和变形能力分别可提升25.6%和88%；提出的应力-应变关系模型的计算值与试验值吻合较好，可用于描述耐碱玻璃纤维ECC的受压破坏全过程。     

大掺量粉煤灰超高性能钢纤维混凝土的静动态力学行为

研究了4种不同钢纤维掺量(体积掺量分别为0%,1.0%,1.5%,2.0%)的大掺量粉煤灰超高性能混凝土的单轴压缩强度、弹性模量、单轴抗拉强度、弯曲韧性、断裂韧性、断裂能等静态力学行为,以及高速冲击、压缩作用下的应力波传播规律、应力–应变曲线和破坏特征等动态力学行为.结果表明:掺加钢纤维的大掺量粉煤灰超高性能混凝土的轴心抗压强度、弹性模量和抗拉强度略有增大,韧性指数、残余强度、断裂韧度和断裂能成倍提高;未能增加冲击、压缩作用下的应变率效应程度,但却增大动态应力–应变曲线下的面积,提高试件破坏的应变率阈值,使混凝土存在裂而不散的破坏现象.     

粉煤灰掺量对PVA-ECC应变硬化性能的影响

粉煤灰与水泥反应产生的水化硅酸钙凝胶可以改善骨料与水泥间的界面结构,由此提升聚乙烯醇纤维增强工程水泥基复合材料(PVA-ECC)的韧性.为了确定PVA-ECC的最佳粉煤灰掺量范围并建立其获得高拉伸应变的评价指标,在保证不同配合比PVA-ECC良好流动性的前提下,应用拉伸试验研究粉煤灰掺量对PVA-ECC应变硬化性能的影响,并应用光学扫描研究试件断口处的破坏特征.提出"拔出型""断裂型""滑移硬化型"3种不同纤维破坏模式下PVA-ECC的拉伸应力指数建议值.研究结果表明:粉煤灰掺量在45%～65%时,PVA-ECC的抗拉性能随粉煤灰掺量增大而提高;粉煤灰掺量在60%～65%时,PVA-ECC的抗拉性能最好.     

再生纤维素纤维增强水泥砂浆的早期抗裂和自收缩行为

早期收缩开裂是导致混凝土劣化的重要因素之一,纤维的加入可以延缓甚至减少裂缝的延伸及扩展。使用具有吸水特性的再生纤维素纤维,与UF500纤维素纤维和无吸水的聚丙烯短纤维对比,研究再生纤维素纤维对水泥砂浆早期抗裂和自收缩行为的影响规律。将掺量为水泥质量1%或2%的各类纤维掺入水泥砂浆,密封养护,分析水泥砂浆强度、折压比及自收缩应变。研究结果表明：水灰比0.3时,添加再生纤维素纤维不能增强砂浆的抗裂性能和自收缩性能;水灰比大于0.35时,添加1%的再生纤维素纤维虽然降低了砂浆的抗折强度和抗压强度,但提高了该样品的折压比,增强了砂浆的抗裂性能和减缩效应。     

粉煤灰掺量及纤维体积率对ECC基本力学性能及断裂性能的影响

为研究粉煤灰掺量及纤维体积分数对ECC基本力学性能及断裂性能的影响，对不同粉煤灰掺入量和纤维体积分数的ECC进行了抗压强度和拉伸性能试验，并基于DIC进行了抗折和断裂性能试验。结果表明：大量粉煤灰的掺入提高了ECC的断裂韧性，但其各项强度及断裂能均有所下降；当掺入的纤维体积分数在1%～2%时，可以有效提高ECC的强度和断裂性能，增加拉伸应变的同时保证试件的完整性。当水泥与粉煤灰比例为1.2,纤维体积分数为2%时，在使ECC保持较高强度的同时还可以较大幅度改善一般混凝土所具有的“脆性”,使其具备较好的抗变形能力。     

上浆处理对PVA纤维增强水泥基复合材料力学性能的影响

为提高水泥基复合材料的韧性,通过在高强聚乙烯醇纤维表面上浆的方式,调控高强聚乙烯醇纤维与水泥之间的界面粘结状态,考察了上浆前后和上浆量对水泥基复合材料抗压和抗弯性能的影响。结果表明:高强聚乙烯醇纤维表面上浆后,2%体积掺量聚乙烯醇纤维增强水泥基复合材料的抗压强度稍有增加,抗弯强度有一定程度下降,但抗弯断裂应变和抗弯断裂能大幅度提高,抗弯时受拉侧呈现多裂纹开裂模式;上浆率过高或过低,水泥基复合材料抗弯韧性均有下降。     

混凝土特征拉应变随龄期发展规律研究

为了更好地预测混凝土抗开裂性能,研究了混凝土特征拉应变随时间的变化规律。通过弯拉试验方法,研究水泥强度等级与掺加纤维对混凝土1～180 d各龄期的初裂拉应变和极限拉应变的影响,分析初裂拉应变与弯拉强度和静力受压弹性模量比值的关系,建立了极限拉应变随龄期发展的关系式,提出最终极限拉应变与特征龄期的概念。试验结果表明,混凝土的初裂拉应变与混凝土弯拉强度和静力受压弹性模量的比值具有良好的线性关系,混凝土极限拉应变随时间成负指数增长,提高水泥强度等级与掺加纤维均可以提高混凝土最终极限拉应变,但提高幅度并不显著,相对而言,以特征龄期来评价混凝土抗裂性能更加重要。     

PVA纤维增强水泥基复合材料的性能试验研究

通过单轴抗压试验、直接拉伸试验、四点弯曲试验、抗折试验对不同配比和不同试件形状的PVA纤维增强水泥基复合材料进行力学性能试验研究,结果表明：加有PVA纤维的水泥基复合材料的抗压强度提高不大,大约是10%,而抗拉强度和抗弯强度则提高很多,且出现了许多细小裂缝,呈现出应变硬化特性,极限拉应变可达1.8%;利用直板型试件做直接拉伸试验比胫型试件做直接拉伸试验的试验结果准确,且易于试验操作.试验结果证实了PVA纤维增强水泥基复合材料具有很好的韧性.     

PVA纤维增强水泥基复合材料的抗拉性能

通过单轴拉伸试验,研究了粉煤灰种类、掺量、水胶比和砂胶比对聚乙烯醇(PVA)纤维应变硬化水泥基复合材料(SHCC)拉伸性能的影响.研究结果表明:粉煤灰替代部分水泥会降低SHCC的抗拉强度,随着粉煤灰掺量的增加,SHCC的应变硬化和多缝开裂特性能得到更好的发挥;在相同掺量下,Ⅰ级粉煤灰相较于Ⅱ级粉煤灰而言,能使SHCC具有更好的拉伸性能;水胶比的增加与砂胶比的减小,都会使SHCC的初裂强度和极限拉伸强度降低,但其极限拉伸应变会有所提升,较高的水胶比与较低的砂胶比更利于SHCC应变硬化和多缝开裂特性的发挥.     

Numerical study on flexural behaviors of steel reinforced engineered cementitious composite (ECC) and ECC/concrete composite beams

Engineered cementitious composite(ECC)is a class of high performance cementitious composites with pseudo strain-hardening behavior and excellent crack control capacity.Substitution of concrete with ECC can largely reduce the cracking and durability problems associated with brittleness of concrete.In this paper,a simplified constitutive model of the ECC material was applied to simulate the flexural behaviors of the steel reinforced ECC and ECC/concrete composite beams with finite element method.The simulation results are found to be in good agreement with test results,indicating that the finite element model is reasonably accurate in simulating the flexural behaviors of the steel reinforced ECC flexural members.The effects of the ECC modulus,ECC tensile ductility,ECC thickness and ECC position on flexural behaviors in terms of ultimate moment,deflection and the maximum crack width of the steel reinforced ECC or ECC/concrete composite beam are hence evaluated.     

高延性纤维增强水泥基复合材料研究应用新进展

首先综述了以ECC为代表的高延性纤维水泥基复合材料的发展过程及其基本力学性能,包括在单轴拉伸作用下优异的应变硬化以及多缝开裂的性能、抗压性能以及在弯曲作用下较高应变性能.其次较为详细地介绍了钢筋ECC结构构件,包括弯曲构件、受剪构件、梁柱连接构件以及预制填充墙等在往复周期荷载作用下的性能研究进展.最后简要介绍了目前ECC在工程中的应用情况,提出了今后应开展的主要研究内容.     

ECC材料的抗冻融性能试验研究

采用快冻法试验研究高韧性纤维增强水泥基复合材料(ECC)的抗冻融耐久性能,并与混凝土和砂浆抗冻融性能进行对比,分析国产与进口聚乙烯醇(PVA)纤维对ECC抗冻融性能的影响规律,探讨砂灰比对ECC抗冻性的影响.结果表明:300次冻融循环后,掺入国产和进口PVA纤维的ECC质量损失率分别控制在2.5%和1.5%以内;ECC的纵向相对动弹性模量比混凝土和砂浆提高1.62～1.87倍;ECC的横向相对动弹性模量比混凝土和砂浆提高1.61～1.79倍.随着砂灰比的增加,ECC质量损失率逐渐增加;ECC纵向与横向相对动弹性模量稍有增加.ECC抗冻等级高于F300,完全可用于寒冷地区混凝土结构的加固与维护.     

PVA纤维水泥基材料力学性能试验研究

为了制备超高韧性的水泥基复合材料(ultra high toughness cementitious composites,UHTCC),通过抗压、抗折以及直接拉伸试验,结合扫描电镜(SEM)测试,探讨粉煤灰掺量、石英砂掺量对UHTCC力学性能的影响;通过粉煤灰-石英砂复配,研究超高韧性水泥基材料的最优粉煤灰-石英砂掺量配比.结果表明:随着粉煤灰掺量的增加,抗压、抗折强度降低,拉伸变形增大,但是当粉煤灰质量/水泥质量(m(FA)/m(C))大于2.7后,拉伸变形提高缓慢;当石英砂质量/胶凝材料的质量(m(S)/m(B))为0.36时,拉伸变形性能最好;本文确定的最优粉煤灰-石英砂体系掺量为:m(FA)/m(C)-m(S)/m(B)=1.2-0.48,m(FA)/m(C)-m(S)/m(B)=2.2-0.36.     

Theoretical modeling of steel reinforced ECC column under eccentric compressive loading

Fiber reinforced cementitious composites(ECC) are a class of advanced composites with strain hardening and multiple cracking behaviors. Substitution of concrete with ECC can significantly improve the seismic resistance and durability of the infrastructures. In this paper, it is proposed to use ECC as the matrix of frame columns for improving its load carrying capacity, ductility, and avoiding the brittleness of concrete. Based on the assumption of plane remaining plane and constitutive models of materials, theoretical models for calculating the load-carrying capacity of the steel reinforced ECC columns under small and large eccentric compression are proposed. With the parameters of the constitutive models from the existing experimental data, the relationship between ultimate axial load and moment capacities is also derived with the proposed models. To verify the validity of the proposed theoretical models, finite element analysis with the software of ATENA is conducted to simulate the mechanical behavior of the steel reinforced ECC columns under eccentric compressive loading. The calculation results from the theoretical models show good consistency with the simulated results, indicating that the proposed models are feasible and reliable for design. Finally, based on the theoretical models, the effect of the ultimate tensile strain and compressive strength of ECC, longitudinal reinforcement ratio on the load carrying capacity of the steel reinforced ECC column are comprehensively studied.     

Methods of modifying the brittle behavior of cementitious composites

We put forward effective methods of increasing the tensile strain of cementitious composites with 2% PVA fiber and high fly ash content. The test results show that curing condition has a significantly effect on the tensile performance. It is approved that the specimens incorporated appropriate volume fraction rubber powder and lightweight aggregate greatly increase the tensile strain of composites at medium-term age, but indefinitely at long-term age. To a certain extent, EVA can limitedly enhance the tensile performance of comentitious composites owing to the formation of polymer membrane and the hindered hydration of cement.     

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.     

Methods of Modifying the Brittle Behavior of Cementitious Composites

We put forward effective methods of increasing the tensile strain of cementitious composites with 2% PVA fiber and high fly ash content. The test results show that curing condition has a significantly effect on the tensile performance. It is approved that the specimens incorporated appropriate volume fraction rubber powder and lightweight aggregate greatly increase the tensile strain of composites at medium-term age, but indefinitely at long-term age. To a certain extent, EVA can limitedly enhance the tensi...