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.     

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.     

Effects of particle size and distribution on the mechanical properties of SiC reinforced Al-Cu alloy composites

This paper studied the combined effects of particle size and distribution on the mechanical properties of the SiC particle reinforced Al-Cu alloy composites. It has been shown that small ratio between matrix/reinforcement particle sizes resulted in more uniform distribution of the SiC particles in the matrix. The SiC particles distributed more uniformly in the matrix with increasing in mixing time. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength and elongation. Yield strength and ultimate tensile strength of the composite reinforced by 4.7 μm sized SiC particles are higher than those of composite reinforced by 77 μm sized SiC particles, while the elongation shows opposite trend with yield strength and ultimate tensile strength. Fracture surface observations showed that the dominant fracture mechanism of the composites with small SiC particle size (4.7 μm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, while the dominant fracture mechanism of the composites with large SiC particle size (77 μm) is ductile fracture of the matrix, accompanied by the SiC particle fracture.     

Mechanical properties of photocatalytic white concrete subjected to high temperatures

The paper describes the consequences of progressive damage in architectural high performance concrete when exposed to different heating treatments. Specimens were tested for uniaxial compressive, direct, and indirect tensile strengths at ambient conditions approximately one day after the exposure to the high temperature. Modifications in the microstructure, porosity, and pore size distribution of the fire deteriorated specimens were identified using scanning electron microscopy and mercury intrusion porosimetry techniques. Test results revealed no significant variations in the mechanical strengths for specimens exposed to temperatures up to 250 °C. Per contra, significant damage was observed for higher temperature, 500 °C and 750 °C, treatments, similar to that of ordinary concrete made with similar aggregates. Based on X-ray diffraction analysis, photocatalytic properties of the concrete were lost at 750 °C.     

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.     

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.     

Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence

Engineered cementitious composites (ECC) is a class of ultra ductile fiber reinforced cementitious composites, characterized by high ductility and tight crack width control. The polyvinyl alcohol (PVA) fiber with a diameter of 39 μm and a length of 6-12 mm is often used. Unlike plain concrete and normal fiber reinforced concrete, ECC shows a strain-hardening behavior under tensile load. Apart from the mix design, the fiber distribution is another crucial factor for the mechanical properties of ECC, especially the ductility. In order to obtain a good fiber distribution, the plastic viscosity of the ECC mortar before adding fibers needs to be controlled, for example, by adjusting water-to-powder ratio or chemical admixtures. However, such adjustments have some limitations and may result in poor mechanical properties of ECC. This research explores an innovative approach to improve the fiber distribution by adjusting the mixing sequence. With the standard mixing sequence, fibers are added after all solid and liquid materials are mixed. The undesirable plastic viscosity before the fiber addition may cause poor fiber distribution and results in poor hardened properties. With the adjusted mixing sequence, the mix of solid materials with the liquid material is divided into two steps and the addition of fibers is between the two steps. In this paper, the influence of different water mixing sequences is investigated by comparing the experimental results of the uniaxial tensile test and the fiber distribution analysis. Compared with the standard mixing sequence, the adjusted mixing sequence increases the tensile strain capacity and ultimate tensile strength of ECC and improves the fiber distribution. This concept is further applied in the development of ECC with high volume of sand.     

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。     

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.     

高温热处理对C/C-SiC复合材料制备与力学性能的影响

以针刺整体炭毡为坯体,采用树脂浸渍和化学气相沉积混合法制备C/C多孔体,然后熔硅浸渗制得C/C-S iC复合材料;研究了C/C多孔体的高温热处理对C/C-S iC复合材料密度、孔隙度、力学性能及断裂方式的影响。结果表明:炭涂层进行高温热处理可改变复合材料的弯曲断裂方式,使其具有一定的“假塑性”,弯曲强度下降约16%,压缩强度提高约20%,硬度增加;C/C多孔体的最终高温热处理可打开孔隙,有利于液S i的渗入,制备出密度较高(>2.0 g.cm-3)、开孔率较小(<4%)的复合材料,但导致其力学性能下降,基本上不影响其断裂方式。     

The influences of soft and stiff inclusions on the mechanical properties of cementitious composites

The embedment of microencapsulated phase change materials (PCMs) is a promising means for improving the thermal inertia of concrete. However the addition of such soft microcapsules degrades the mechanical properties, i.e., the elastic moduli and compressive strength, of cement-based composites. This study experimentally quantifies the effects of stiff quartz inclusions and soft PCM microcapsules, individually, and when added together, on the mechanical properties of cementitious composites. In addition, a variety of effective medium approximations (EMAs) were evaluated for their ability to predict the experimentally measured composite effective moduli. The EMAs proposed by Hobbs and Garboczi and Berryman (G-B) reliably estimate experimental data. The experimental data and the EMAs were applied to develop a design rule for performance equivalence, such that the composite modulus of elasticity can be maintained equivalent to that of the cementitious paste matrix, in spite of the addition of soft PCM microcapsules.     

Flax and polyparaphenylene benzobisoxazole cementitious composites for the strengthening of masonry elements subjected to eccentric loading

To address the structural problems caused by eccentric loads in unreinforced masonry, three different types of masonry were prepared based on clay bricks bonded with a natural hydraulic lime mortar combined with a flax or polyparaphenylene benzobisoxazole (PBO) fabric-reinforced cementitious matrix (FRCM) composite. The mechanical behaviour when subjected to concentric and eccentric loads was studied by performing axial compression tests, with eccentric load tests only carried out in instances of large eccentricities. Analysis of the load–displacement and moment–curvature response revealed that both the flax- and PBO-based strengthening systems improve the strength and deformability of masonry. However, compared to the PBO fabric composite, the use of flax fabric provides a greater deformability that helps prevent the composite and substrate debonding.     

Recommendation of RILEM TC 200-HTC: Mechanical concrete properties at high temperatures—modelling and application

Mechanical concrete properties at high temperatures depend on many parameters. The main parameters are the specimen type and the test conditions. The report describes the test parameters and test procedures for relaxation tests in the range of 20 to 750°C. TC Membership: The draft of this document has been prepared by the following 16 Committee full members representing 12 countries. Chairman: U. Schneider, Austria; Secretary: R. Felicetti, Italy. Members: G. Debicki, France; U. Diederichs, Germany; J.-M. Franssen, Belgium; U.-M. Jumppanen, Finland; G.A. Khoury, UK; S. Leonivich, Republic Belarus; A. Millard, France; W. A. Morris, UK; L. T. Phan, USA; P. Pimienta, France; P. Rodrigues, Portugal; E. Schlangen, The Netherlands; P. Schwesinger, Germany; Y. Zaytsev, Russian Federation.     

Effect of strain rate on the dynamic compressive mechanical behaviors of rock material subjected to high temperatures

Strain rate is not only an important measure to characterize the deformation property, but also an important parameter to analyze the dynamic mechanical properties of rock materials. In this paper, by using the SHPB test system improved with high temperature device, the dynamic compressive tests of sandstone at seven temperatures in the range of room temperature to 1000 °C and five impact velocities in the range of 11.0–15.0 m/s were conducted. Investigations were carried out on the influences of strain rate on dynamic compressive mechanical behaviors of sandstone. The results of the study indicate that the enhancement effects of strain rates on dynamic compressive strength, peak strain, energy absorption ratio of sandstone under high temperatures still exist. However, the increase ratios of dynamic compressive strength, peak strain, and energy absorption ratio of rock under high temperature compared to room temperature have no obvious strain rate effects. The temperatures at which the strain rates affect dynamic compressive strength and peak strain most, are 800, and 1000 °C, respectively. The temperatures at which the strain rates affect dynamic compressive strength and peak strain weakest, are 1000 °C, and room temperature, respectively. At 200 and 800 °C, the strain rate effect on energy absorption ratio are most significant, while at 1000 °C, it is weakest. There are no obvious strain rate effects on elastic modulus and increase ratio of elastic modulus under high temperatures. According to test results, the relationship formula of strain rate with high temperature and impact load was derived by internalizing fitting parameters. Compared with the strain rate effect at room temperature condition, essential differences have occurred in the strain rate effect of rock material under the influence of high temperature.     

超声法纳米颗粒悬浮液粒径测量中温度影响的实验研究

针对医药、化工领域高浓度纳米悬浮液颗粒粒径超声检测中温度影响,采用超声衰减谱法(UAS)对体积浓度30%的纳米铟锡金属氧化物(ITO)水性悬浮液在循环流速800 r/min,温度298～358 K时颗粒粒径分布进行实验。结果表明:温度升高,超声幅值A减小,超声衰减系数增大,颗粒中位径D50增大,颗粒系分布曲线整体朝大颗粒方向偏移,但是分布宽度保持稳定的趋势。同时,将室温(298K)测量结果与CPS离心沉降颗粒测量仪对比,结果较吻合。通过线性回归的方法修正温度对测量结果的影响,超声衰减法能够应用于358K的高温下高浓度纳米颗粒检测。     

The role of carbon nanofibers on thermo-mechanical properties of polymer matrix composites and their effect on reduction of residual stresses

In this research, the effects of carbon nanofibers (CNFs) on thermo-elastic properties of carbon fiber (CF)/epoxy composite for the reduction of thermal residual stresses (TRS) using micromechanical relations were studied. In the first step, micromechanical models to calculate the coefficient of thermal expansion (CTE) and Young's modulus of CNF/epoxy and CNF/CF/epoxy nanocomposites were developed and compared with experimental results of the other researchers. The obtained results of the CTE and Young's modulus of modified Schapery and Halpin-Tsai theories have good agreement with the experimental results. In the second step, the classical lamination theory (CLT) was employed to determine the TRS for CNF/CF/epoxy laminated nanocomposites. Also, the theoretical results of the CLT were compared with experimental results. Finally, reduction of the TRS using the CLT for different lay-ups such as cross ply, angle ply, and quasi-isotropic laminates were obtained. The results demonstrated that the addition of 1% weight fraction of CNF can reduce the TRS that the most reduction occurred in the unsymmetric cross-ply laminate by up to 27%.     

Effects of nano-SiO2 particles on the mechanical and microstructural properties of ultra-high performance cementitious composites

In this research the effects of nano-SiO₂ particles on the mechanical performance, hydration process and microstructure evolution of ultra-high performance cementitious composites were investigated by different methods. The results showed that the compressive and flexural strength increased with the increase of the nano-SiO₂ content up to 3% and due to agglomeration of nano-SiO₂ particles, the mechanical properties decreased slightly when the nano-SiO₂ content was more than 3%. The hydration process was accelerated by the addition of nano-SiO₂. The porosity and the average pore diameter decreased with the increase of the nano-SiO₂ content and aging. The microstructure was more homogenous and dense for nano-SiO₂ specimens as compared to the control specimen. All of these improvements could be mainly attributed to the pozzolanic and filler effects of nano-SiO₂.     

Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures

This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 °C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO₂ for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)₂ lowered stress from rapid recrystallization after exposure to 600 °C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance.     

几何尺寸对铠装信号引线性能影响研究

铠装信号引线是一种用于信号传输的无机铠装电缆,本文主要研究了引线制造过程中的几何尺寸对其绝缘电阻、分布电容、击穿电压、完整性和机械寿命等重要性能的影响,结果表明,在产品外保护管外径D3、芯线直径D1及引线长度确定的情况下,几何尺寸对各项指标的影响主要有外保护管内径D2、圆心偏差距离d,其中d只能尽量通过工艺控制,在满足引线密封完整性和机械寿命的前题下应适当增大D2来提高绝缘电阻、分布电容、击穿电压等性能.     

Effect of particle size distribution on green properties during high velocity compaction

Iron powders with two different particle size distributions were compacted by high velocity compaction. The influences of particle size distribution and impact velocity on green properties, including green density, springback, tensile strength and bending strength etc., were studied with scanning electron microscopy (SEM) and a computer controlled universal testing machine. The results show that the particle size distribution and the impact velocity strongly affect its properties. Wider size distribution results in green compact with higher density and better strength. Furthermore, springback of compacts is lower produced by the powder with wider size distribution, especially for radial springback. As impact velocity increases, its green density and green strength gradually increases, but the increasing rate of density decreases gradually. No special relation is found between springback and impact velocity. In addition, the axial springback and the bending strength are higher than the radial springback and the tensile strength, respectively.