低成本PVA纤维对超高韧性水泥基复合材料力学性能的影响

超高韧性纤维增强水泥基复合材料（ECC）因其出色的高韧性及多缝开裂特性备受关注,然而一直以来因配比中进口PVA纤维的使用导致高昂的价格限制了其在工程中的大规模应用。为了进一步降低成本及实现原材料的本土化,研究低成本国产PVA纤维对ECC力学性能的影响十分必要。通过单轴拉伸、压缩、三点抗弯及单裂缝拉伸等宏观、细观试验研究两种国产低成本PVA-ECC的力学性能,并借助纤维分散性试验及SEM,探讨纤维的分散等微观特征。结果表明,低成本国产纤维在基体中具有良好的分散性,尽管其纤维桥接余能、最大桥接应力及PSH指数低于进口纤维,但均能满足能量与强度准则,即便相对较差的纤维A试件的3 d、7 d及28 d的极限拉伸应变也可达到2.52%、3.34%及3.08%,可实现良好的应力硬化行为及饱和多缝开裂特性,满足ECC的使用要求。     

高轴压比配筋PVA纤维增强混凝土柱抗震性能试验研究

为提高钢筋混凝土柱在高轴压比下的抗震性能,采用聚乙烯醇（PVA）纤维增强混凝土代替普通混凝土是可选择的措施之一。设计6个剪跨比为4的钢筋混凝土柱试件,其中4个试件采用PVA纤维增强混凝土,另外2个对比试件采用普通混凝土,进行拟静力试验以研究高轴压比下的抗震性能。通过改变轴压比和纤维掺量,在水平循环往复荷载作用下,观测试件裂缝开展及破坏过程,研究滞回性能、骨架曲线、延性性能及耗能能力。试验结果表明:高轴压比下,PVA纤维增强钢筋混凝土柱破坏时,裂缝开展缓慢,纤维的桥接作用有效地抑制了裂缝的开展;纤维增强混凝土柱主要表现为延性破坏模式,极限位移角约为普通混凝土柱的1.47倍~1.53倍,表明其具有良好的塑性变形能力和损伤容限;PVA纤维增强钢筋混凝土柱的耗能比约为普通混凝土柱的1.82倍~1.95倍,表明其耗能能力显著提高,抗震性能优越。     

聚乙烯纤维增强高延性碱矿渣复合材料的拉压性能及裂缝分布

在碱激发作用下,以矿粉为主要原材料,粉煤灰为辅助材料,共同制备聚乙烯(PE)纤维增强高延性碱矿渣复合材料。通过轴向拉、压实验,研究不同养护龄期(1天、3天、7天、28天、56天、120天)下材料的拉压性能,并借助数字图像技术(DIC)对裂缝进行了表征。结果表明:高延性碱矿渣表现出较好的延性,具有早强特征。7天强度值可达极限强度的84%以上(极限拉压强度分别为5.05 MPa、91.24 MPa),拉伸应变可达5.74%,多缝开裂基本饱和;28天后拉压性能趋于稳定(拉压强度、拉伸应变分别保持在6 MPa、100 MPa、6%);DIC数字分析云图直观地描述了裂缝的形成及发展过程,可从一定程度上对开裂破坏方向及位置进行可靠预判。      

超高性能混凝土的自愈合及抗冻性能

为研究带裂服役超高性能混凝土(UHPC)的自愈合及抗冻性能，对混杂钢纤维UHPC试件预加0.05%和0.1%两种应变损伤，置于水中养护28天自愈合后进行300次冻融循环试验。通过单轴拉伸性能、裂缝特征、质量损失及超声波脉冲速率(UPV)指标综合评价UHPC的自愈合及抗冻性能，并利用扫描电子显微镜和能谱仪(SEM-EDS)分析微观结构和愈合产物。结果表明：28天水养后，预损伤0.05%试件表现出较好的自愈合性能，抗拉强度、拉伸应变和应变能均高于参照试件，表面所有裂缝全部愈合；预损伤0.1%试件的拉伸性能低于参照试件，表面最大裂缝(宽度为69μm)并未完全愈合。300次冻融循环后，两种预损伤试件的初裂强度和抗拉强度均进一步增加，而拉伸应变和应变能均有所减小。相对质量与UPV的变化趋势能够很好地反映两种预损伤试件的再水化效应。SEM-EDS结果显示：距裂缝较近部位的纤维-基体粘结更牢固；裂缝表面的愈合产物主要为Ca(OH)2和CaCO3，内侧主要为水化硅酸钙(C-S-H)凝胶。     

低轴压比聚乙烯醇纤维增强水泥基复合材料中长柱抗震性能试验

已有对聚乙烯醇纤维增强水泥基复合材料(PVA/C)柱抗震性能的研究大多针对短柱,且PVA/C一般只在节点及其邻近部位局部设置。基于此,本文对低轴压比且沿柱全高设置的PVA/C中长柱进行低周反复荷载试验,变化参数为纤维体积分数ρ_f和体积配箍率ρ_v。通过试验,得出以下结论:所有试件均发生弯曲破坏;当ρ_f和ρ_v分别在试验设计范围内增大时,试件的裂缝控制能力、延性、截面转动能力及耗能能力均提高,刚度退化及承载力衰减速度减小;ρ_f的增大可较大程度提高试件开裂荷载,而对峰值荷载影响较小;ρ_f由0 vol%提高到2 vol%,位移延性系数、耗能比及开裂荷载分别提高52.9%、112.3%和51.1%;掺加适量纤维后,即使降低配箍率,试件也可保持良好的抗震性能和裂缝形态。根据本文试验数据并收集其他相关文献试验数据,拟合得出位移延性系数与ρ_f和ρ_v之间的关系式。最后总结了各类PVA/C柱抗震性能差异。      

绿色延性水泥基复合材料裂缝自愈合性能

绿色延性水泥基复合材料(green toughness cementitious composities,GTCC),是借鉴高延性水泥基复合材料(engineered cementitious composites,ECC)技术开发的一款聚乙烯醇((polyvinly alcohol,PVA)纤维增强尾矿砂水泥基复合材料,该复合材料以大比例尾矿砂替代天然细骨料,具有经济、环保和延性特性。目前对于该复合材料的研究主要局限于产品制备、力学性能和耐久性等方面,有关GTCC裂缝自愈合性能的研究至今尚未见报道。为了探究该复合材料的自愈合性能,设计了尾矿砂替代天然砂比率为50%的3组不同水胶比的立方体试件,采用抗压强度恢复率法对GTCC的自愈合性能进行了研究,研究了损伤龄期、养护龄期、养护环境及干湿循环周期等因素对该新型材料自愈合效果的影响,揭示了绿色延性水泥基复合材料在不同条件下的自愈合规律。结果表明:该复合材料的损伤龄期越早,其自愈合效果越好;其自愈合效果随着自愈合养护龄期的增加而增加,但后期增长率较早期增长率越来越弱;干湿循环养护环境相对其它环境,更有利于其自愈合发生;自愈合作用主要发生在21个干湿循环之前。     

超高韧性水泥基复合材料多缝开裂特性及其自生愈合

应变硬化水泥基复合材料(Strain hardening cement-based composites,SHCC)是超高性能水泥基材料的一种。通过三点弯曲加载试验,分别对普通砂浆和SHCC试件诱导开裂,对两种材料的开裂特性及其裂缝分布规律进行了研究。结果表明,SHCC材料试件与普通水泥砂浆试件相比具有更高的承载力和应变能力,初始开裂荷载约为普通砂浆试件的2.5倍。SHCC材料中的PVA纤维的桥接作用有效延迟了裂缝的产生与扩展,将普通砂浆试件中数量少而大的裂缝转化为多而细的微小裂缝,呈现多缝开裂特性,产生的裂缝宽度符合正态分布规律,且90%的微小裂缝小于30μm。这些微小裂缝在潮湿环境中产生一定程度的自生愈合,在水中的愈合程度和速度均高于在潮湿空气中。     

PVA-ECC加固桥墩抗震性能试验研究

为增强现役桥墩的抗震能力,研究基于聚乙烯醇纤维增强水泥基复合材料(PVA-ECC)的桥墩抗震加固性能,完成了5个剪跨比为4.2的圆形桥墩试件拟静力试验。通过试件的破坏形态与抗震性能指标,分析了PVA-ECC加固桥墩抗震性能,讨论了轴压比与PVA纤维体积掺量对桥墩抗震性能的影响。试验结果表明:采用PVA-ECC材料可以明显改善桥墩的破坏形态,控制裂缝的宽度和发展,提高桥墩的损伤容限与承载能力,表现出良好的延性和耗能能力;随轴压比增大,PVA-ECC加固桥墩的承载力提高,位移延性系数与耗能能力明显降低;随PVA纤维体积掺量增大,PVA-ECC加固桥墩的滞回曲线更加饱满,有效提升了桥墩的延性与耗能能力。     

不同纤维掺量下聚乙烯醇纤维/水泥复合材料徐变性能试验

通过对5组试件进行试验,研究了不同纤维掺量下聚乙烯醇纤维/水泥复合材料（PVA/ECC）的受压徐变性能。结果表明:PVA纤维的加入,降低了基材的密实度及弹性模量,使试件的徐变增加;在掺杂PVA纤维（体积分数为0~2vol%）的PVA/ECC试件中,PVA纤维掺量较高和掺量较低的试件徐变均较大,PVA纤维掺量适中的试件徐变较小;各组试件的徐变速率均具有前期快后期慢的特性,7天内发生的徐变可达总徐变的40%左右,60天内约为总徐变的80%,持荷约60天后,徐变逐渐趋于收敛。最后通过对试验值进行回归分析,提出了适用于PVA/ECC材料的徐变度数学计算模型及徐变系数预测模型。      

碳酸钙晶须对混杂纤维增强高延性水泥基复合材料力学性能的影响

为了探究碳酸钙晶须对钢纤维/PVA混杂纤维增强高延性水泥基复合材料(HyFRHDCC)力学性能的影响,利用2%体积掺量的廉价碳酸钙晶须替代部分纤维,研究了不同纤维掺量HyFRHDCC的压缩性能和拉伸性能,利用扫描电子显微镜观察了HyFRHDCC的微观结构。研究结果表明,引入碳酸钙晶须能够提高HyFRHDCC的初裂拉伸应变和峰前压缩韧性;在1.5%PVA+0.25%钢纤维HyFRHDCC中掺入2%碳酸钙晶须可以改善材料的拉伸性能;当PVA纤维减少至1%时,HyFRHDCC出现了明显的应变软化行为。微观形貌分析发现,碳酸钙晶须能够通过裂纹偏转、晶须拔出以及裂缝桥联等微观机制改善HyFRHDCC的应变硬化行为。     

Self-healing performance of aged cementitious composites

This study investigates the autogenous self-healing capability of one-year-old engineered cementitious composites (ECC) with different mineral admixtures to understand whether self-healing performance in late ages is similar to that of early ages. Sound and severely pre-cracked specimens were subjected to different environmental conditions including water, air, “CO₂-water,” and “CO₂-air” for one year plus 90 days of initial curing. Self-healing performance of ECC mixtures was assessed in terms of crack characteristics, electrical impedance testing, rapid chloride permeability testing and microstructural analysis. Laboratory findings showed that the presence of water is crucial for enhanced autogenous self-healing effectiveness, regardless of mixture composition. “CO₂-water” curing resulted in the best self-healing performance of all curing conditions, which was confirmed with results from different performance tests throughout the experimental study. By further curing specimens under “CO₂-water” (depending on the ECC mixture composition), cracks as wide as half a millimeter (458 μm) were easily closed by autogenous self-healing within only 30 days of further curing, and all cracks closed completely after 90 days. Because high levels of CO₂ emission are a global problem, the effectiveness of “CO₂-water” curing in closing microcracks of aged cementitious composites specimens through autogenous self-healing can help reduce the increasing pace of CO₂ release. The results of this study clearly suggest that late-age autogenous self-healing rates of ECC specimens can be significantly enhanced with proper further environmental conditioning and mixture design.     

Research on production,performance and fibre dispersion of PVA engineering cementitious composites

Abstract

Polyvinyl alcohol (PVA) fibre is considered as one of the most suitable polymeric fibres to be used as the reinforcement of engineered cementitious composites (ECCs). Research and application have shown that PVA–ECC can significantly counteract the deficiency of ordinary concrete. In the present paper, micromechanics based design theory and fracture mechanics formulation leading to energy and strength criteria to achieve strain hardening and multiple cracking are described. Engineered cementitious composites showing pseudo strain hardening behaviour with over 6% of strain capacity under tension is produced. Uniaxial tensile tests of PVA–ECC are conducted and the results support the validity of the proposed theory. Also viscosity modifying agent plays an important role in the dispersion of the fibres in the matrix. It is shown that a uniform distribution of fibres throughout the bulk of the composite material is crucial to its excellent workability, tight crack width and reduction in the autogenous and drying shrinkage strains.     

Influence of microcrack self-healing behavior on the permeability of Engineered Cementitious Composites

Engineered Cementitious Composite (ECC) is a family of high performance fiber reinforced cementitious composites featuring strain-hardening behavior and high tensile ductility (with tensile strain capacity of 3–5%). ECC achieves high ductility by forming multiple microcracks with crack width less than 60 μm under tension. The tight crack width of ECC naturally lends itself to low permeability even in the cracked stage. Such properties are of particular interest to hydraulic structure applications. In addition to the tight crack width, self-healing of microcracks further lowers the permeability of cracked ECC, enhancing the durability and safety of hydraulic structures. In this paper, the permeability of ECC composites under the influence of self-healing was experimentally studied. Single crack permeability tests were also conducted to directly correlate the permeability and self-healing behavior of a single crack with a given initial crack width. Additionally, an analytical model capable of predicting the permeability of ECC composites that undergo self-healing process is proposed and verified with experimental data. The research findings in the present paper can be used to accurately estimate the permeability of ECC and are expected to provide support for future design and application of ECC for hydraulic structures.     

循环荷载作用下超高性能混凝土的轴拉力学性能及本构关系模型

对具有不同拉伸应变特性(应变强化和应变软化)的超高性能混凝土(Ultra high performance concrete, UHPC)进行了单调和循环荷载作用下的直接拉伸试验。试验结果表明:应变强化UHPC基体开裂后进入多点微裂纹分布的应变强化段,达到极限抗拉强度后进入单缝开裂的应变软化段;应变软化UHPC基体开裂后直接进入单缝开裂的应变软化段;循环荷载下两种类型UHPC的轴拉应力-应变曲线包络线与单调荷载下的应力-应变曲线基本一致;基于刚度退化过程建立了两种类型UHPC的轴拉损伤演化方程,根据实测应力-应变曲线和试件的裂缝分布形态建立了两种类型UHPC的轴拉本构关系模型,与试验结果基本吻合;采用能量法研究了应变强化UHPC两阶段轴拉本构关系在数值计算时的等效方法。最后,通过无筋应变强化UHPC抗弯试验梁的数值模拟对本文建立的应变强化UHPC轴拉本构关系模型和损伤演化方程及相关假定进行了验证,结果表明本文建立的应变强化UHPC轴拉本构模型能较好地预测UHPC弯拉构件的极限承载力,轴拉损伤变量能在宏观层面上较好地反应试件的裂缝分布状态。      

Influence of treated palm oil fuel ash on compressive properties and chloride resistance of engineered cementitious composites

The influence of palm oil fuel ash (POFA) inclusion on the compressive properties and chloride resistance of engineered cementitious composites (ECC) were experimentally investigated. In the material development, pozzolanic reactivity of POFA, direct tensile test and matrix fracture test were performed for evaluating the performance of ECC with POFA. Different ECC mixes with varying POFA content and water–binder ratios were used. The results show that the use of POFA should be helpful for achieving strain-hardening behavior by enhancing the fracture toughness and interfacial bond between matrix and PVA fiber. Moreover, at 28 and 90 days, increasing the POFA/cement ratio up to 0.2 led to an increase in the compressive strength of the ECC. The ECC mix with 1.2 POFA–cement ratio achieved a compressive strength of 30 MPa at 28 days, which is within the normal range of concrete strength for many applications. In addition, the test results show that mechanically pre-loaded POFA–ECC specimens exposed to chloride solution remain durable. The results also indicated strong evidence of self-healing of micro-cracked POFA–ECC specimens, which can still carry considerable flexural load. The rapid chloride permeability test reveal that the total charge passed was gradually reduced with the inclusion of higher amount of POFA. The results presented in this study provide a preliminary database for the durability of cracked and uncracked POFA–ECCs under chloride environment or/and combined mechanical loading.     

Effect of sustained tensile loading on SHCC crack widths

Strain Hardening Cementitious Composites (SHCCs) exhibit multiple cracking when subjected to tensile loading and can achieve a strain capacity of more than 4% while resisting the full tensile load. Sustained tensile loading (creep) tests were performed to determine the time-dependant cracking behaviour of SHCC. The sustained load levels ranged from 40% to 80% of the ultimate tensile resistance. In this paper the crack widths during sustained tensile loading of SHCC were quantified. It was found that the average crack widths are not significantly dependant on the level of the creep load. The average crack widths were however found to increase significantly from around 70 μm to more than 200 μm after 7 days of sustained loading, even at a load of less than 50% of the ultimate tensile strength.     

Strain hardening behavior of lightweight hybrid polyvinyl alcohol (PVA) fiber reinforced cement composites

Experimental results on the strain hardening and multiple cracking behaviors of polyvinyl alcohol (PVA) fiber reinforced cementitious composites under bending are reported in this paper. Different hybrid combinations of PVA fibers with different lengths and volume fractions are considered to reinforce the mortar matrix. Among different hybrid combinations, the composite containing 2% thicker PVA fibers of 12 mm length and 1% thinner PVA fibers of 6 mm length and the composite containing 2% thicker PVA fibers of 24 mm length and 1% thinner PVA fibers of 6 mm length showed the best performance in terms of highest ultimate load, largest CMOD (crack mouth opening displacement) at peak load and multiple cracking behavior. The effects of four types of light weight sands on the strain hardening and multiple cracking behavior of hybrid fiber composites are also evaluated in this study. It has been observed that the ultimate load and CMOD at peak load for all light weight hybrid fiber composites are almost the same irrespective of volume fractions of light weight sand. The composites containing finer light weight sands exhibited higher ultimate load than those containing coarser light weight sands. It is also observed that the hybrid fiber composite containing normal silica sand exhibited higher ultimate load than the composites with light weight sands.