无表面修饰PVA纤维增强高韧应变硬化水泥基复合材料的性能研究

制备高韧应变硬化水泥基复合材料(SHCC)通常用经表面涂油处理的聚乙烯醇(PVA)纤维.本文通过利用无表面修饰的PVA纤维及高掺粉煤灰,制得高韧SHCC.通过控制粉煤灰掺量,利用减水剂调节水胶比,实现对基体强度的控制,得到有利于制备SHCC的基体.弯曲和直接拉伸试验结果表明,由无表面修饰PVA纤维增强的水泥基复合材料呈现多缝开裂和应变硬化特征,具有优良韧度和延展性.纤维增韧作用主要体现在挠度硬化阶段,但对于强度较低的SHCC而言,挠度软化阶段中也呈现较明显的纤维增韧作用.高掺粉煤灰时,无表面修饰PVA纤维增强的SHCC所呈现出的直接拉伸极限应变达3％以上,与经表面涂油PVA纤维增强的SHCC相当.     

粉煤灰对应变硬化水泥基复合材料轴拉性能的影响

应变硬化水泥基复合材料（SHCC）中掺入粉煤灰可以有效改善纤维与基体接触面的特性,减弱纤维与基体间的界面黏结力,进而在一定范围内增强SHCC的应变硬化特性。选择I级、II级粉煤灰两组对比材料,并对SHCC单轴拉伸性能的影响进行了测定。研究结果发现无论是从强度上还是从应变硬化性能上,I级粉煤灰都比II级粉煤灰要好很多。     

高强高模PVA纤维表面改性及耐碱性能

采用疏水二氧化硅涂层与纳米石墨涂层两种方案对高强高模聚乙烯醇(PVA)纤维表面进行改性,并使用接触角测量仪、原子力显微镜及Fourier红外光谱仪对处理后的纤维表面进行表征,研究了高强高模PVA纤维的耐碱性及单丝拔出时的微观力学界面参数。结果表明:经过处理后PVA纤维表面粗糙度增加,由亲水状态成功转变为疏水状态,接触角均大于130°;PVA纤维耐碱性良好,碱浸泡后拉伸强度保持率大于95%;经过表面修饰后PVA纤维的化学粘结力大幅降低。纳米石墨涂层可以很好地调控纤维与水泥基体的界面,使纤维从水泥基体中被完整拔出。     

应变硬化水泥基复合材料高温后弯曲韧性研究

传统水泥基材料强度低、韧性差,表现出明显的脆性破坏.掺加聚乙烯醇(PVA)纤维的应变硬化水泥基复合材料(SHCC)可以改善传统水泥基材料的脆性,常温下表现出明显的应变硬化特征,提高了基材的强度和韧性.通过四点弯曲试验研究在不同温度作用下SHCC的弯曲韧性,结果表明,随着温度的升高,SHCC的强度和韧性呈现下降趋势,100℃之前降幅较小;约在10％以内;200℃时开始大幅下降(降低约50％),300℃时丧失韧性,发生脆性破坏.试验还表明,内掺Protectosil MH50硅烷乳液防水剂时,高温作用下,其韧性降低规律与未掺加防水剂时大致相同,但同条件下会使SHCC的强度略微降低.     

PVA纤维类型对应变硬化地聚合物基复合材料力学性能的影响

本文对比分析了4种不同聚乙烯醇(PVA)纤维分别在不同配合比地聚合物基体中的增韧作用,为利用国产无表面涂油PVA纤维制备应变硬化地聚合物基复合材料(SHGC)提供实验数据。主要研究矿渣与粉煤灰的比例、碱溶液的浓度、纤维尺寸以及纤维表面特性等因素对地聚合物基复合材料抗压和直接拉伸性能的影响。结果表明,经过7 d室温养护,含矿渣的地聚合物基体和复合材料的抗压强度均高于30 MPa,而纯粉煤灰地聚合物基体和复合材料的抗压强度较低,为12~15 MPa。表面涂油PVA纤维SHGC的延展性普遍高于无表面涂油PVA纤维SHGC。然而,通过调节地聚合物基体配合比,可以提高无表面涂油PVA纤维的增韧效果。当粉煤灰质量分数为33%时,无表面涂油PVA纤维SHGC的极限拉伸应变达1.44%,与表面涂油PVA纤维SHGC相当。在纯粉煤灰的情况,4种PVA纤维复合材料均呈现出稳定的多缝开裂和应变硬化特征。     

SAP对高性能工程水泥基复合材料性能的影响

高收缩率是限制高性能工程水泥基复合材料(HP-ECC)大规模工程应用的瓶颈之一。本文通过引入超吸水性聚合物(SAP)来缓解HP-ECC的收缩，研究了不同掺量的SAP对HP-ECC抗压强度、抗折强度、拉伸性能、自收缩和干燥收缩性能的影响，并采用扫描电子显微镜(SEM)研究了SAP对HP-ECC拉伸后纤维表面形貌变化的影响。结果表明，HP-ECC中掺入SAP后的抗压强度和抗折强度降低，自收缩和干燥收缩得到缓解，且自收缩和干燥收缩随SAP掺量的增加而降低。此外，HP-ECC的拉伸强度降低，拉伸延伸率提高。SAP的引入降低了基体的断裂韧度，使基体更容易形成微裂缝，从而改善了HP-ECC的应变硬化行为和多缝开裂现象。随着SAP掺量的增加，纤维从基体中拔出时的表面形貌越来越光滑，纤维-基体界面黏结性能降低。     

基于数字图像方法的混掺纤维应变硬化水泥基复合材料力学性能及变形特征

为揭示混掺纤维对应变硬化水泥基复合材料力学性能和变形行为的影响规律,研究了玄武岩–聚乙烯醇(PVA)纤维应变硬化水泥基复合材料的抗拉、抗压性能及压应变演化特征.设计了纤维掺量为材料体积分数的2％,玄武岩纤维和PVA纤维掺量比分别为3:1、1:1和1:3,同时控制粉煤灰与水泥掺量的比值(FA/C)分别为1.2、1.5和2...     

纤维对再生砖混ECC力学性能及微观结构的影响

为促进不同粒径再生砖混骨料的多元化利用，本试验采用再生砖混细骨料完全代替石英砂，采用不同掺量聚丙烯纤维制备再生砖混工程水泥基复合材料(ECC),研究其受力破坏特征、强度影响机理及微观结构对力学性能的影响。结果表明：未掺纤维的再生砖混ECC的失效模式为脆性破坏，而掺纤维的再生砖混ECC受拉时具有明显的应变硬化特征，随着纤维掺量的增加，其抗折强度、极限抗拉强度和极限拉应变持续增大，抗压强度呈先增大后减小趋势，表现出良好的延性和韧性破坏特征；再生砖混ECC的孔隙率在11.28%～13.68%,通过SEM观察，发现纤维与再生砖混ECC黏结性能较好，纤维破坏模式主要为拔出和拉断破坏，开裂后应变硬化拉伸幅度和拉伸强度低于普通ECC混凝土；新旧浆体界面黏结性能相对薄弱，破坏时微裂缝容易在界面过渡区产生和发展。     

ECC的材料组成与性能关系分析

通过设计10组配合比研究了不同PVA纤维掺量、水胶比和粉煤灰掺量对工程水泥基复合材料(ECC)强度(压缩、拉伸和弯曲)和韧性性能的影响,并进行了材料组成与性能关系分析.其中使用四点弯曲薄板来研究ECC的弯曲韧性,使用ASTM-C1018和DBV中提出的韧性指标来量化ECC的韧性特征.结果表明:ECC的抗压强度主要取决于粉煤灰置换率和水胶比,而抗拉强度和弯曲强度则主要依赖于PVA纤维体积掺量,且纤维掺量控制ECC的应变硬化和软化行为.虽然PVA纤维掺量的提高可以略微提高ECC的弹性模量,但主要还是受粉煤灰掺量控制.     

纳米粒子和石英砂对PVA纤维水泥基复合材料单轴拉伸性能的影响

为研究纳米粒子种类和掺量以及石英砂粒径对聚乙烯醇纤维(PVA纤维)水泥基复合材料单轴拉伸性能的影响,通过单轴拉伸试验测得了试件的极限拉应变和极限拉应力,并得到了试件应力-应变关系曲线.PVA纤维的体积掺量为0.9％,选择纳米SiO2质量掺量和石英砂粒径各四种.结果 表明,纳米SiO2的掺加对PVA纤维水泥基复合材料抗拉伸性能有一定的提高,随着纳米SiO2掺量从0％增大到2.5％,试件极限拉应变和极限拉应力整体上呈逐渐增大趋势.相对于纳米CaCO3,纳米SiO2对PVA纤维水泥基复合材料抗拉伸性能的增强效果更明显.石英砂的粒径对PVA纤维水泥基复合材料抗拉性能影响较大,石英砂的粒径越小,PVA纤维水泥基复合材料的极限拉应变和极限拉应力越低.     

Effect of Plasma Treatment of Polyethylene Fibers on Interface and ementitious Composite Properties

It is well known that interfaces in composites play an important role in determining composite properties. In this paper, preliminary results of the improvement in tensile properties of a fiber-reinforced cementitious composite due to plasma treatment of the discontinuous polyethylene fibers are reported. Specific focus is placed on the pseudo strain-hardening composite properties induced by fiber reinforcements and associated load transfer from crackbridging fibers to matrix. Single fiber pullout tests support that the composite property improvement is indeed derived from interfacial property enhancement of the plasma treatment process.     

Effect of fiber properties and matrix composition on the tensile behavior of strain-hardening cement-based composites (SHCCs) subject to impact loading

The article at hand describes the behavior of high-strength and normal-strength strain-hardening cement-based composites (SHCCs) made of fine-grained matrix and high-density polyethylene fibers under quasi-static and impact tensile loading. The dynamic tension testing of unnotched and notched cylinders was performed using the Hopkinson bar at strain rates of around 150 s^− 1. The responses of the materials under dynamic and quasi-static tensile loading were compared to the corresponding results for normal-strength SHCC made of polyvinyl-alcohol fibers as obtained in previous investigations. To explain the pronounced differences in rate effects on the material performance of various SHCC compositions, cracking pattern and fracture surface conditions were studied. Additionally, strain rate dependent changes in the mechanical behavior of individual fibers and in the fiber–matrix interfacial properties were deduced from single-fiber tension tests and fiber pullout tests, respectively. Altogether, the results obtained provide clear indications as to the decisive parameters for a purposeful material design of impact resistant types of SHCC for use in structural elements or protective overlays.     

原状粉煤灰对超高性能混凝土性能的影响

为提高粉煤灰的综合利用率,降低原料成本,采用未经磨细和分选的原状粉煤灰等质量替代硅灰来制备超高性能混凝土(UHPC),并研究了不同掺量的原状粉煤灰对UHPC力学性能及微观结构的影响。结果表明:原状粉煤灰的掺入可使UHPC中胶凝材料的粒度呈梯度分布,形成良好的微级配;并且使新拌混凝土的流动度增大,影响了钢纤维在UHPC基体中的分布;当原状粉煤灰掺重不超过30%时,UHPC抗折强度随着原状粉煤灰掺量的增加呈现不同程度的增长,30%原状粉煤灰掺量的UHPC抗折强度与不掺粉煤灰的空白样相比提高了34%;由于原状粉煤灰水化缓慢,当原状粉煤灰掺量在0%~40%时,UHPC抗压强度随着原状粉煤灰掺量的增加有所下降。孔结构分析表明:UHPC的平均孔径以及总孔体积均随着原状粉煤灰的掺入而减小,基体更加密实;当原状粉煤灰掺量为30%时,SEM照片显示钢纤维与UHPC基体结合紧密,界面黏结增强。     

Development of gypsum-based composites with tensile strain-hardening characteristics

Brittle nature of gypsum restrains its wide application in construction industry. For improvement, a novel type of composite material, gypsum-based engineered cementitious composites (GS-ECC), was developed using specially chosen polyethylene (PE) fibers. This study investigated the rheological and mechanical properties of GS-ECC, that is, workability, uniaxial tensile and compressive behavior, flexural strength, etc The investigation showed that GS-ECC possessed excellent tensile strain-hardening behavior and saturated cracking characteristics with the average tensile strain capacity more than 5%. To explore the underlying mechanism, the microstructure of interface transition zone (ITZ) between gypsum crystals and PE fibers were investigated through the use of SEM. Single fiber pull-out test, bending-fracture test, and single crack tension test were conducted to investigate the mesoscopic properties from fiber/matrix interface to matrix toughness and fiber bridging capacity. This study demonstrates the feasibility of achieving strain-hardening gypsum-based composites by adding the PE fibers.     

高掺量粉煤灰高延性水泥基复合材料的制备和性能

高延性水泥基复合材料(hjgh ductility cementitious composites,HDCC)是一种具有应变硬化、多缝开裂和高延性等特性的新型纤维增强水泥基复合材料,其材料设计必须取得基体韧度、界面黏结和纤维特性三者的最优组合,因此,HDCC的制备必须优选原材料和优化配合比,以取得最优的材料制备技术.从配合比设计入手,研究了粉煤灰含量、胶砂比等对HDCC力学性能的影响,优化了特定材料下的材料制各技术.结果表明:粉煤灰含量、胶砂比和养护条件对HDCC的拉伸性能均具有较大的影响.随着粉煤灰掺量的增大,砂含量的降低,拉伸应变增大.当砂含量较高时,基体开裂韧度较高,基体的极限拉伸应力下对应的极限拉伸应变较小,然而随着应力的下降,复合材料仍然能维持相当大的应变·     

Low-energy impact behavior of ambient cured engineered geopolymer composites

Engineered geopolymer composite (EGC) is a new kind of fiber reinforced geopolymer composite with tensile strain-hardening behaviors. This paper was intended to investigate the low-energy impact behaviors of EGC. To further reduce the carbon footprint and material cost of EGC, the feasibility of developing ambient cured EGC with cheap local PVA fiber was discussed according to the micromechanics-based analytical models. The compressive, tensile and impact tests of EGC, engineered cementitious composites (ECC), pure geopolymeric matrix and cementitious matrix were conducted and compared. It was found that the EGC specimens have similar tensile behaviors with ECC and the ultimate tensile strain of EGC can be as high as 7.5%. Under impact load, it was found that the PVA fibers could effectively restrict the crushing and spalling of geopolymeric matrix. Also, the dissipated energy of pure geopolymeric matrix is 3.8 times higher than that of cementitious matrix, indicating that it is recommendable to develop impact-resistant material based on geopolymeric matrix. The influences of NaOH molarity on the impact behaviors of EGC and geopolymeric matrix were discussed. It was found that the impact-resistance of EGC improved with the increase of NaOH molarity, while the threshold of NaOH molarity for geopolymeric matrix was recommended as 12 mol/L. Even though the compressive strength of EGC is lower than ECC, it can be concluded that EGC could have comparable or even higher impact-resistance than ECC under different low-velocity impact conditions.     

Tensile and fiber dispersion performance of ECC (engineered cementitious composites) produced with ground granulated blast furnace slag

An engineered cementitious composite (ECC) produced with ground granulated blast furnace slag was developed for the purpose of achieving moderately high composite strength while maintaining high ductility, represented by strain-hardening behavior in uniaxial tension. In the material development, single fiber pullout tests and matrix fracture tests were performed, followed by micromechanical analyses to properly select the range of mixture proportion. Subsequent direct tensile tests were employed to assess the strain-hardening behavior of the composite, which exhibited high ductility and strength with the addition of slag. High ductility is most likely due to enhanced workability and fiber dispersion performance which is attributed to the oxidized grain surface of slag, as verified by fiber dispersion tests. These results suggest that, within the limited slag dosage employed in the present study, the contribution of slag to fiber dispersion outweighs the side-effect of decreased potential for saturated multiple cracking, including a slight increase in matrix fracture toughness and fiber/matrix bond strength.     

Nanoscale characterization of engineered cementitious composites (ECC)

Engineered cementitious composites (ECC) are ultra-ductile fiber-reinforced cementitious composites. The nanoscale chemical and mechanical properties of three ECC formulae (one standard formula, and two containing nanomaterial additives) were studied using nanoindentation, electron microscopy, and energy dispersive spectroscopy. Nanoindentation results highlight the difference in modulus between bulk matrix (~ 30 GPa) and matrix/fiber interfacial transition zones as well as between matrix and unreacted fly ash (~ 20 GPa). The addition of carbon black or carbon nanotubes produced little variation in moduli when compared to standard M45-ECC. The indents were observed by electron microscopy; no trace of the carbon black particles could be found, but nanotubes, including nanotubes bridging cracks, were easily located in ultrafine cracks near PVA fibers. Elemental analysis failed to show a correlation between modulus and chemical composition, implying that factors such as porosity have more of an effect on mechanical properties than elemental composition.     

Properties of concrete incorporating high volumes of class F fly ash and san fibers

The results of an experimental investigation to study the effects of replacement of cement (by mass) with three percentages of fly ash and the effects of addition of natural san fibers on the slump, Vebe time, compressive strength, splitting tensile strength, flexural strength and impact strength of fly ash concrete are presented. San fibers belong to the category of “natural bast fibers.” It is also known as “sunn hemp.” Its scientific (botanical) name is Crotalaria juncea. It is mostly grown in the Indian subcontinent, Brazil, eastern and southern Africa and some parts of the United States (Hawaii and Florida). A control mixture of proportions 1:1.4:2.19 with W/Cm of 0.47 and superplasticizer/cementitious ratio of 0.015 was designed. Cement was replaced with three percentages (35%, 45% and 55%) of class F fly ash. Three percentages of san fibers (0.25%, 0.50% and 0.75%) having 25-mm length were used.The test results indicated that the replacement of cement with fly ash increased the workability (slump and Vebe time), decreased compressive strength, splitting tensile strength and flexural strength and had no significant effect on the impact strength of plain (control) concrete. Addition of san fibers reduced the workability, did not significantly affect the compressive strength, increased the splitting tensile strength and flexural strength and tremendously enhanced the impact strength of fly ash concrete as the percentage of fibers increased.