Frost resistance and microstructure of Engineered Cementitious Composites: Influence of fly ash and micro poly-vinyl-alcohol fiber

One of the most damaging environmental conditions to concrete structure is cyclic freezing and thawing. This paper discusses the influence of the high volumes of fly ash (FA) and micro poly-vinyl-alcohol (PVA) fibers on the cyclic freeze-thaw resistance and microstructure of the Engineered Cementitious Composites (ECC). ECC mixtures with two different FA-cement (FA/C) ratios (1.2 and 2.2 by weight), and at constant water-cementitious materials (fly ash and cement) ratio of 0.27 are prepared. To compare the behavior of ECC with ECC matrix, all of the preceding properties are also investigated for ECC matrix mixtures of same composition without PVA fiber. For frost resistance, mixtures are exposed to the freeze and thaw cycles up to 300 cycles in accordance with ASTM C666, Procedure A. Experimental tests consist of measuring the residual mechanical properties (flexural strength, mid-span beam deflection and flexural stress - deflection curve), ultrasonic pulse velocity and mass loss. The roles of PVA fibers and FA are discussed through the analysis of microstructure and fiber-matrix interactions as function of frost exposure. The microstructural characterization by measuring pore size distributions is examined before and after exposure to frost deterioration by using mercury intrusion porosimetry (MIP). The air-void characteristics of mixtures are also studied using linear transverse method. Test results confirm that both ECC mixtures with high volumes of FA remain durable, and show a tensile strain capacity of more than 2% even after 300 freezing and thawing cycles. On the other hand, before completing 300 freezing and thawing cycles, matrix (ECC without fiber) specimens have severely deteriorated, requiring removal from the freeze-thaw machine. Therefore, results indicate that the addition of micro PVA fiber to the ECC matrix substantially improved the frost resistance. The results of freeze-thaw tests also indicated that the reduction of residual physical and mechanical properties with increasing number of freeze-thaw cycles is relatively more for ECC mixture with FA/C ratio of 2.2 than for ECC mixture with FA/C ratio of 1.2.     

Influence of micro-cracking on the permeability of engineered cementitious composites

Engineered Cementitious Composites (ECC) forms multiple micro-cracks under tension when loaded to beyond the elastic stage. Unlike normal concrete, such tight cracks help to maintain low water permeability even in the cracked stage. Therefore ECC shows great potential for application in hydraulic structures, such as dams and levees for which water seepage control is critical for their performance. In this paper, the permeability of ECC under constant tensile load was experimentally studied using a specially designed displacement-control loading device, providing permeability data for ECC under realistic loading conditions. In addition, an analytical model capable of predicting permeability property of ECC composite based on tensile strain and crack patterns has been proposed and experimentally verified on two different ECC mixtures. The findings of this research are expected to support future design and application of ECC for hydraulic structures.     

粉煤灰细度对超高韧性水泥基复合材料性能的影响

为了探究粉煤灰细度对超高韧性水泥基复合材料(ECC)性能的影响,设计对比三组由不同细度粉煤灰制作的ECC试件的抗拉及抗压试验性能,并进行了灰色关联度分析。结果表明:ECC的拉伸应变与粉煤灰细度之间不呈简单的线性关系;对于ECC的抗压强度,其主导因素并非是粉煤灰的细度,而是粉煤灰的活性。在特定条件下,通过改变粉煤灰的细度,可以在不影响基体强度的情况下改善ECC的延性。     

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.     

Water permeability of engineered cementitious composites

The water permeability of a unique class of high performance fiber reinforced cementitious composites (HPFRCC) called engineered cementitious composites (ECC) is investigated. These composites are deliberately tailored using microcmechanical design principles to exhibit pseudo-strain-hardening characteristics in uniaxial tension, up to greater than 4% strain. While undergoing tensile deformation, microcracks are designed to saturate the specimen rather than localize into large cracks. This tendency to form microcracks, which are experimentally shown to be approximately 60 μm in width, allows ECC material in the cracked state to maintain water permeability similar to that of uncracked concrete or mortar, and magnitudes lower than cracked reinforced mortar or concrete. It is also shown that the self-healing properties of cracks within ECC material significantly aids in reducing the coefficient of permeability of cracked ECC.     

Influence of curing condition and precracking time on the self-healing behavior of Engineered Cementitious Composites

This paper investigates the self-healing behavior of Engineered Cementitious Composites (ECC) with focus on the influence of curing condition and precracking time. Four-point bending tests were used to precrack ECC beams at different age, followed by different curing conditions, including air curing, 3% CO₂ concentration curing, cyclic wet/dry (dry under 3% CO₂ concentration) curing and water curing. For all curing conditions, deflection capacity after self-healing can recover or even exceed that from virgin samples with almost all precracking ages. After self-healing, flexural stiffness was also retained significantly compared with that from virgin samples, even though the level of retaining decreases with the increase of precracking time. The flexural strength increases for samples pre-cracked at the age of 14 days and 28 days, presumably due to continuous hydration of cementitious materials afterwards. Furthermore, it is promising to utilize nanoclay as distributed internal water reservoirs to promote self-healing behavior within ECC without relying on external water supply.     

用纳米压痕技术表征超高韧性水泥基复合材料(ECC)的裂缝自愈合特性

为了探究ECC裂缝自愈合体系中不同物相的微观力学性能,应用纳米压痕技术对经历10个干湿循环环境后裂缝自愈合ECC体系中不同物相的荷载-位移、接触刚度-位移、弹性模量及硬度进行了研究。结果表明:当荷载相同时,压入深度大小顺序为:纤维ITZSHP基体粉煤灰砂子;接触刚度与压入深度近似呈线性关系;粉煤灰和砂子的弹性模量及硬度是体系中最高的,远远高于其他相,其次是基体,接下来是SHP、ITZ,最差的是纤维。     

Influence of mineral additives and environmental conditions on the self-healing capabilities of cementitious materials

The effects of the addition of minerals of various compositions (i.e., silica-based materials, chemical expansive agents, swelling minerals and crystalline components) on the self-healing performances of cementitious materials were investigated. The self-healing capabilities were assessed by performing water permeability tests, quantifying the widths of the surface cracks and studying the water absorption of mortars that were pre-cracked by either splitting or compression. The results showed that the cracks that appeared early on healed more efficiently when they were cured in still rather than flowing water. High pHs and high temperatures accelerate crack healing. The healing efficiency can be further improved by utilizing a combination of minerals rather than a single mineral. A self-healing mechanism was discussed by combining these results with micro-observations. The precipitation of calcium carbonate, which is aided by higher pH values and higher calcium ion contents, was found to be the main contributor to the healing of surface cracks.     

Specimen size effect on the residual properties of engineered cementitious composites subjected to high temperatures

In this study, size effect on the residual properties of Engineered Cementitious Composites (ECC) was investigated on the specimens exposed to high temperatures up to 800 °C. Cylindrical specimens having different sizes were produced with a standard ECC mixture. Changes in pore structure, residual compressive strength and stress–strain curves due to high temperatures were determined after air cooling. Experimental results indicate that despite the increase of specimen size, no explosive spalling occurred in any of the specimens during the high temperature exposure. Increasing the specimen size and exposure temperature decreased the compressive strength and stiffness. Percent reduction in compressive strength and stiffness due to high temperature was similar for all specimen sizes.     

Phenomenological interpretation of the shear behavior of reinforced Engineered Cementitious Composite beams

This paper describes an experimental investigation of the shear behavior of beams consisting of steel Reinforced Engineered Cementitious Composites (R/ECC). This study investigates and quantifies the effect of ECC's strain hardening and multiple cracking behavior on the shear capacity of beams loaded in shear. The experimental program consists of R/ECC beams with short (8 mm) randomly distributed Polyvinyl Alcohol (PVA) fiber and conventional Reinforced Concrete (R/C) counterparts for comparison with varying shear reinforcement arrangements. Beams were loaded until failure while a Digital Image Correlation (DIC) measurement technique was used to measure surface displacements and crack formation. The shear crack mechanisms of R/ECC are described in detail based on findings of DIC measurements and can be characterized by an opening and sliding of the cracks. Multiple micro-cracks developed in a diagonal arrangement between the load and support points due to the strain-hardening response of ECC in tension. The strain-hardening response strongly influenced the shear response of the beam specimen.     

Effect of self-healing on strength and durability of zeolite-immobilized bacterial cementitious mortar composites

There is a compelling economic incentive to develop concrete materials that can repair its own damage, increase durability and prevent structural failure. This research investigated the potential of adding two different mineral producing bacteria into two types of cementitious mortar matrix to enhance self-healing ability for autonomous crack repair. In this study, zeolite was used as a carrier material to protect bacteria in high pH environment normally exists in concrete. The spore forming ability and ureolytic activity of zeolite-immobilized bacteria were investigated in order to examine potential for producing healing compounds. The self-healing ability of bacteria incorporated normal and fiber reinforced mortars was judged based on the development of compressive strength and permeation properties of cracked specimens with age as well as micro-structural characterization of crack healing compounds using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction.     

Influence of microcracking on water absorption and sorptivity of ECC

This paper presents the results of an experimental investigation on the water absorption and sorptivity properties of mechanically loaded Engineered Cementitious Composites (ECC). ECC is a newly developed high performance fiber reinforced cementitious composite with substantial benefit in both high ductility and improved durability due to tight crack width. By employing micromechanics-based material design, ductility in excess of 3% under uniaxial tensile loading can be attained with only 2% fiber content by volume, and the typical single crack brittle fracture behavior commonly observed in normal concrete or mortar is converted to multiple microcracking ductile response in ECC. In this study, water absorption (ASTM C642) and sorptivity tests (ASTM C1585) were conducted to determine absorption capacity and sorptivity of microcracked ECC. The experimental program described in this paper indicated that microcracks induced by mechanical loading increases the sorptivity value of ECC without water repellent admixture. However, the use of water soluble silicone based water repellent admixture in the production of ECC could easily inhibit the sorptivity even for the mechanically loaded ECC specimens. Moreover, the incorporation of the water repellent admixture reduced the absorption capacity of the resulting ECC mixture. Based on this study, the risk of water transport by capillary suction in ECC, cracked or uncracked, is found to be low compared with that in normal sound concrete. The incorporation of water repellent admixture further lowers this risk.     

Influence of matrix flowability,fiber mixing procedure,and curing conditions on the mechanical performance of HTPP-ECC

In this study, the influences of matrix flowability, fiber mixing procedure, and curing conditions on the mechanical properties of Engineered Cementitious Composites (ECC) made with High Tenacity Polypropylene (HTPP) fibers are investigated. While the HTPP-ECC examined in this study possesses moderate compressive strengths (30–70 MPa), their tensile ductility (1.91–3.91%) is similar to that of ECC with Polyvinyl Alcohol (PVA) fibers. For the purpose of controlling matrix flowability, different dosages of HRWR admixture were introduced to a matrix with fly ash/cement weight ratio of 2.8 and water/cementitious material weight ratio of 0.23. Dogbone-shaped and 50 mm cube specimens were used to investigate uniaxial tensile and compressive properties of HTPP-ECC, respectively. Test results showed that control of flowability in a certain range is required to achieve robust tensile ductility. A further improvement in tensile ductility and mechanical properties of HTPP-ECC was achieved through water-curing instead of air curing typically used for PVA-ECC. The basic mechanisms that enhance tensile ductility of HTPP-ECC through flowability control, mixing procedure modification, and water-curing are discussed from the view point of micromechanics underlying ECC design, with supporting evidence from fiber bridging stress–crack width (σ–δ) relations.     

高韧性纤维混凝土单轴受压性能及尺寸效应

高韧性纤维混凝土(ECC)具有优异的韧性、卓越的耗散能力及裂缝无害化分布的特点,能够明显改善结构的抗震性能与耐久性。通过对三种不同高厚比的立方体与棱柱体共60个试件进行单轴受压试验,探究高韧性纤维混凝土的受压性能、变形机制及尺寸效应对试件力学性能的影响,测得了不同高厚比试件受压的应力-应变全曲线。结果表明:高韧性纤维混凝土在裂缝发展及破坏模式上与普通混凝土存在明显的区别,由于纤维的桥接作用,在加载过程中材料表现出较强的压缩韧性,试件破坏以后仍保持相对完整,极限压应变约为普通混凝土的10倍;当高厚比大于1时,材料抗压强度对尺寸的敏感性降低;峰值应变与抗压韧性系数随着高厚比的增加逐步减少。结合电镜扫描结果,对高韧性纤维混凝土中纤维的分布、桥接情况及纤维增韧增强机制进行了分析与讨论。     

超高韧性水泥基复合材料自愈合研究进展

在总结了最近几年国内外相关研究进展的基础上,对超高韧性水泥基复合材料(ECC)及裂缝自愈合进行了综述。着重介绍了裂缝自愈合的最大允许宽度限值以及自愈合机制。提出利用ECC所独具的对裂缝宽度的可控性及紧密细小的微裂纹、较低的水胶比及矿物掺合料的二次水化效应可实现其良好的自愈合特性。最后指出该研究领域所面临的挑战及今后的研究方向,为ECC裂缝自愈合的研究提供有价值的理论参考。     

Compression after impact assessment of self-healing CFRP

The development of advanced fibre-reinforced polymer’s (FRP’s) to achieve performance improvements in engineering structures focuses on the exploitation of the excellent specific strength and stiffness that they offer. However, the planar nature of an FRP’s microstructure results in relatively poor performance under impact loading. Furthermore, significant degradation in material performance can be experienced with minimal visual indication of damage being present, a scenario termed Barely Visible Impact Damage (BVID). Current damage tolerant design philosophies incorporate large margins to account for reduction in structural performance due to impact events, resulting in overweight and inefficient structures. An alternative approach to mitigate impact damage sensitivity can be achieved by imparting the ability for these materials to undergo self-healing. Self-healing composites would allow lighter, more efficient structures and would also offer a potentially substantive increase in design allowables and reduction in maintenance and inspection schedules and their associated costs. This paper considers the development of autonomic self-healing within a carbon fibre-reinforced polymer (CFRP), and demonstrates the significant strength recovery (>90%) possible when a resin filled hollow glass fibre system is distributed at specific interfaces within a laminate, minimising the reduction in mechanical properties whilst maximising the efficiency of the healing event.     

Tensile fracture process of Strain Hardening Cementitious Composites by means of three-dimensional meso-scale analysis

There are a wide variety of short fiber reinforced cement composites. Among these materials are Strain Hardening Cementitious Composites (SHCC) that exhibit strain hardening and multiple cracking in tension. Quantitative material design methods considering the properties of matrix, fiber and their interface should be established. In addition, numerical models to simulate the fracture process including crack width and crack distribution for the material are needed.This paper introduces a numerical model for three-dimensional analysis of SHCC fracture, in which the salient features of the material meso-scale (i.e. matrix, fibers and their interface) are discretized. The fibers are randomly arranged within the specimen models. Load test simulations are conducted and compared with experimental results. It is seen that the proposed model can well simulate the tensile failure of Ultra High Performance-Strain Hardening Cementitious Composites (UHP-SHCC) including strain-hardening behavior and crack patterns. The effects of matrix strength, its probability distribution inside the specimen and fiber distribution on the tensile fracture are numerically investigated. Consideration of the probability distributions of material properties, such as matrix strength, appears to be essential for predicting the fracture process of SHCC.     

ECC材料力学性能与本构关系研究进展

混凝土在国内外应用广泛,但普通混凝土材料存在抗拉强度低、韧性差和脆性特征明显等缺点。自20世纪90年代采用性能驱动设计方法(PDDA)成功配制工程水泥基复合材料(ECC)后,仅在几年时间里,ECC材料受到了研究者的广泛关注。相比普通混凝土,采用PDDA得到的ECC的外掺纤维与基体界面有良好的粘结作用,这使得ECC材料具有应变硬化和多缝开展等重要特征。由于ECC优异的力学性能,使用其替代混凝土便成为解决混凝土脆性、裂缝开展等相关问题的一种有效的新途径。 然而, ECC的制备极不容易,由于基体胶凝材料产地不同或者纤维种类不同,某一地区配制成功的配合比大多无法适用于其他地区。因此,根据当地情况进行ECC材料的配合比设计仍然是各国学者青睐的课题。一方面,欲使用ECC代替混凝土用于建筑结构等,就必定要深入研究ECC材料层面的基本力学性能。建立ECC的本构模型对ECC构件甚至结构层面的研究都十分重要,但相关研究较少。另一方面,由于ECC材料良好的裂缝控制能力,国内外学者也致力于使用ECC材料进行结构加固修复的研究。 各国学者先后成功配制极限拉应变大于3%的ECC,这为进一步广泛开展ECC的研究和应用创造了很好的条件,用于配制ECC的纤维种类也更加丰富。在ECC拉伸、压缩、弯曲和剪切等大量的材料试验研究基础上,近几年,一些科研团队开始尝试用ECC部分甚至完全代替混凝土来浇筑梁、柱等构件,然后进行ECC构件层次的力学性能和耐久性等研究;另有部分研究人员也致力于建立ECC的本构模型,开展数值模拟分析。此外,由于ECC优异的力学性能,也有学者提出可以采用3D打印技术来建造无筋ECC建筑。 本文从ECC材料层面的单轴单调拉压力学性能、单轴循环拉压滞回性能、多轴力学性能及破坏准则、ECC与普通钢筋和纤维增强塑料(FRP)筋两种筋材的粘结性能方面进行综述,相应介绍了几种本构模型并简要对其进行评价,以期为用ECC代替混凝土进行建筑结构设计、选取本构模型进行数值模拟分析、编制规范和技术规程等提供参考。最后,对今后进一步开展ECC力学性能研究、建立本构模型提出展望。     

Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst embedded in various thermosetting systems

In self-healing polymers and composites, the activity of the embedded chemical catalyst within the thermosetting matrix is critical to healing efficiency. Rheological behavior of ring-opening metathesis polymerization (ROMP)-based healing agents, triggered by 1st or 2nd generation Grubbs catalyst suspended in various thermosetting resins, was investigated using an oscillatory parallel plate rheometer. Gel times for various healing agents were determined from the crossover of storage and loss moduli vs. time curves to indicate the activity of the ROMP reaction. Gelation of healing agents initiated by 1st generation Grubbs catalyst occurred faster than those triggered by 2nd generation catalyst. It is suggested that the dissolution rate of the catalyst by the healing agent is an important factor in determining the overall ROMP reaction rate in situ. Optical and scanning electron microscopic observations showed that the finer, rod-like solid particles of the 1st generation catalyst were distributed more homogeneously throughout the cured matrix. The effects of different healing agents and thermosetting matrix systems on the ROMP reaction are discussed in detail.     

Influence of handling effects on the permeability of preforms

Handling manipulations during the process chain of dry fibre preforms starting with the production of semi‐finished products, throughout the transport and storage until the build‐up, lead to mechanical stress on the textile structure. The knowledge of the relation between the stress in preforms caused by handling processes and their effect on the quality of the subsequent composite product could be of great importance for the development of automated solutions. The reduction of permissible requirements in the handling of textiles would lead to significantly simpler and thus more cost‐effective automation solutions. Using effects of defects methodology, impacts on preforms due to handling processes and the resulting permeability will be investigated. The analysis of the preforming process chain shows various visible effects as a result of handling and storage loads on textiles. One representative effect is a spherical shaped buckling within handled textile layers. Therefore the buckling will be used as a representative imperfection in this analysis. To create measurable imperfections textile samples will be impacted with a defined and reproducible load. Followed by the measurement of permeability during vacuum infusion of imperfect textile samples the relation between handling effects and the quality of preforms will be shown. In der Prozesskette zur Herstellung von Vorformen werden textile Halbzeuge durch zahlreiche Handhabungsprozesse belastet. Von zentraler Bedeutung ist hierbei die mechanische Beanspruchung der textilen Halbzeuge bei deren Handhabung und Lagerung während des Prozesses zur Herstellung der Vorform.