Crack growth in hybrid fibrous composites

A preliminary analysis is made of the energetics of transverse crack growth in a brittle elastic matrix bridged by elastic fibres fictionally bonded to the matrix. Studies made of the stability of a crack of finite length in a brittle polymeric material reinforced with steel wires are found to be in reasonable agreement with the predictions of the theory. It is proposed that the stability of transverse cracks in a very brittle matrix could be increased substantially by the inclusion of a second fibre component designed specifically to increase the work of fracture of the matrix. This has been shown to be possible using a very small volume fraction of glass fibres as a matrix toughening component and it has also been observed that stable transverse matrix crack growth can be achieved with composite systems of this type. This principle might have applications in the design of hybrid composites utilizing either a brittle polymeric or ceramic matrix.     

Interfacial degradation in fibre-reinforced cement-based composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Interfacial degradation in fibre-reinforced cement-based composites

Abstracts are not published in this journal     

Crack growth resistance of hybrid fiber reinforced cement composites

Crack propagation in cement-based matrices carrying hybrid fiber reinforcement was studied using contoured double cantilever beam (CDCB) specimens. Influence of fiber type and combination was quantified using crack growth resistance curves. It was demonstrated that a hybrid combination of steel and polypropylene fibers enhances the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing high performance hybrid fiber composites. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper emphasizes the desired durability characteristics of these composites and discusses their current and future applications.     

Application of rock wool waste in cement-based composites

This study investigates the properties of cement-based composites with addition of various rock wool wastes. The rock wool wastes are an insulating material. This study used rock wool waste with a cylindrical size distribution ranging from 17 to 250 μm, 30% of which is less than 150 μm. Rock wool waste can be used as a suitable substitute for coarse and fine aggregates, saving on the cost of natural aggregates and minimizing the environmental impact of solid waste disposal. In addition, because the composition of rock wool waste is similar to other pozzolan materials such as fly ash, ground granulated blast-furnace slag (GGBS), and silica fume, it can be considered as a supplementary cementitious material. Experimental results show that partially replacing natural aggregates with rock wool wastes improves the compressive strength, splitting tensile strength, abrasion resistance, absorption, resistance to potential alkali reactivity, resistivity, and chloride-ion penetration of cement-based composites. These improved properties are the result of the dense structure achieved by the filling effect of pozzolanic product. Pozzolanic strength activity index (PSAI) results and scanning electron microscope (SEM) observations confirm these findings. Therefore, rock wool wastes can act as either a cementitious material or inert filler in cement-based composites, depending on the particle size. The critical size appears to be 75 μm.     

Dynamic analysis of sandwich cement-based piezoelectric composites

Theoretical analysis of a sandwich cement-based piezoelectric composite is presented based on the theory of piezo-elasticity. The steady-state responses of two kinds of this composite under different loading cases are obtained by the use of displacement method. The effects of piezoelectric phases on the performance of this kind of devices are simulated and discussed. The solutions are compared with both the numerical and experimental results, and good agreements are found. Sandwich cement-based piezoelectric composites have great application potential in civil structure health monitoring. The results obtained in this paper are beneficial to the design of this kind of smart devices.     

Water permeability of cracked strain-hardening cement-based composites

Strain-hardening cement-based composites (SHCC) are characterized by the formation of numerous fine cracks under tensile loading. The water permeability of the cracked material and, consequently, the durability of respective structural members will strongly depend on these cracks. Experimental investigations into the water permeability of both SHCC specimens pre-cracked under uniaxial tension and SHCC overlays on cracked concrete substrate were performed. It is proposed to determine certain crack pattern parameters which may be linked to the theoretical water permeability of the cracked material. For this linking, a weighting function for the observed crack widths has been derived on the basis of the Hagen-Poiseuille equation.     

Crack propagation in polymeric composites

The velocities of rapidly moving cracks in polymethylmethacrylate, an epoxy resin, a rubber modified epoxy resin, coupled and decoupled glass bead filled epoxies and randomly oriented glass fiber reinforced epoxies were measured with a crack propagation gage that was electrolissecally plated on the surfaces of the materials.

When fracture was initiated from a natural crack it was found that the velocity conformed to Mott's equation , while fracture initiated from a blunted notch resulted in a velocity that conformed to Dulany and Brace's equation . A general energy balance was used to show how one could develop these two equations as bounds to the velocity of catastrophic crack propagation.

The terminal crack velocity in the unfilled materials and the glass bead filled materials was , where E/ρ was the modulus to density ratio of the matrix phase at the macroscopic strain rate of the fracture test. The proportionality constant of 0.28 was independent of matrix type, temperature and degree of adhesion. Cracks in the rubber reinforced epoxies always tended to become blunt, resulting in breaking loads that were higher than that expected for materials possessing a natural crack. In addition, the average terminal velocity was less than 0.28√E/ρ, indicating the retardation effects of the rubber particles. These facts were used to explain the higher fracture toughness of these composites.

Fracture surface roughness was primarily a function of crack extension and breaking stress and was less sensitive to crack velocity. An empirical modification of the Mott energy balance was used to qualitatively explain this behavior.     

Brazilian waste fibres as reinforcement for cement-based composites

Fibre reinforced cement-based composites were prepared using kraft pulps from sisal and banana waste and from Eucalyptus grandis pulp mill residues. The study adapted conventional chemical pulping conditions for the non-wood strands and a slurry vacuum de-watering method for composite preparation followed by air-curing. Plain cement paste and Pinus radiata kraft reinforced cement composites were used as reference materials. Mechanical testing showed that optimum performance of the various waste fibre reinforced composites was obtained at a fibre content of around 12% by mass, with flexural strength values of about 20 MPa and fracture toughness values in the range of 1.0–1.5 kJ m⁻². Experimental results showed that, of the waste fibres studied, E. grandis is the preferred reinforcement for low-cost fibre-cement.     

混杂纤维增强水泥基复合材料的力学性能

研究了化学改性聚丙烯（PP）纤维以及掺加聚丙烯纤维和芳纶纤维混杂比例和混杂效应对水泥基复合材料力学性能的影响,并构建了纤维增强水泥砂浆界面层的物理模型,描述了纤维对水泥砂浆的增强机制。实验表明,聚丙烯纤维经改性后使水泥砂浆前期抗折强度明显提高,聚丙烯纤维和芳纶纤维的混杂使水泥砂浆的后期抗折强度显著提高。改性聚丙烯纤维掺加体积分数为0.56%,芳纶纤维的体积分数为0.24%时,混杂纤维增强水泥砂浆试样较空白试样,3天、28天抗折强度分别提高了18.48%、31.17%,3天、28天抗压强度分别提高了7.16%、5.19%。     

Examples of the multicriteria optimization of cement-based composites

The materials considered in the paper are called concrete-like composites or materials with cement-based matrices. These are not only ordinary and high performance concretes, but also materials with different admixtures, dispersed fibre reinforcement, polymers, etc.

The selection of the most appropriate material composition and internal structure is proposed using the methods of the optimal design. In the paper basic methods of mathematical optimization are briefly discussed and applied to the formulation and solution of six different problems.

The main aim of the paper is to demonstrate how the optimization approach may be used in practical situations of the design of high performance concretes.     

碳纤维增强水泥基复合材料的吸波性能研究

利用弓形反射法(NRL)测试了碳纤维掺量分别为0.2%、0.4%、0.6%、0.8%和1.0%(质量分数)时碳纤维增强水泥基复合材料(CFRC)在低频段4～8GHz和高频段8～18GHz时的反射率,讨论了纤维掺量、频率、反射率之间的关系.结果发现,在纤维掺量相同条件下:低频段时,反射率＜-10dB,CFRC表现出吸波性;高频段且纤维掺量超过0.6%(质量分数)时,反射率＞-10dB,CFRC对电磁波表现出反射性.随着纤维掺量的增加,低频段时反射率先降低、后又有所回升,吸波性由弱变强、又变弱,纤维掺量为0.6%(质量分数)时出现最小反射率-15.0dB;高频段时反射率总体上呈上升趋势,材料对电磁波的反射性越来越强,纤维掺量为0.4%(质量分数)时出现最小反射率-19.4dB.     

Durability of strain-hardening cement-based composites (SHCC)

Strain-hardening cement-based composites were named after their ability to resist increased tensile force after crack formation, over a significant tensile deformation range. The increased resistance is achieved through effective crack bridging by fibres, across multiple cracks of widths in the micro-range. Whether these small crack widths are maintained under sustained, cyclic or other load paths, and whether the crack width limitation translates into durability through retardation of moisture, gas and other deleterious matter ingress, are scrutinised in this paper by evaluation of test results from several laboratories internationally. This contribution is a short version of the State-of-the-Art report by RILEM TC 208-HFC, Subcommittee 2: Durability, developed during the committee life 2005?C2009.     

掺纳米石墨烯片的水泥基复合材料的压敏性

研究了掺入少量纳米石墨烯片(GnPs)的水泥基复合材料的力学性能、电学性能和压敏性,结果表明,GnPs的掺入显著提高了水泥砂浆的早龄期抗压强度,但对后期抗压强度的影响不大。GnPs表面积较大,将阻塞离子移动通路,故少量GnPs(体积掺量低于0.15%)对改善水泥砂浆导电性的作用效果不明显。GnPs水泥基复合材料能感知应力、应变,GnPs的掺入提高了其灵敏度和重复性。     

放射性"自吸收"水泥基功能复合材料的研究

主要针对建材中粉煤灰的放射性过量问题,通过加入对氡和射线具有吸附和屏蔽作用的功能基元材料制备放射性“自吸收”水泥基功能复合材料,同时通过超细化技术,提高功能基元材料的活性,从而进一步增强材料对放射性的吸收和屏蔽作用。得到了氡释放率比普通粉煤灰水泥砂浆低64．8％、γ射线的发射率减小45％的生态型放射性“自吸收”水泥基功能复合材料,最后对放射性“自吸收”机理作了一定的探讨。     

Non-contacting strain measurement for cement-based composites in dynamic tensile testing

This paper presents the development of a test procedure and application of non-contacting strain measurement for cement-based composites under moderately high strain rate tensile tests. The strain time histories of test specimens measured by a laser extensometer in high speed mode were derived by a phase-shift technique based on zero-crossing method. The accuracy of the linear variable differential transformer (LVDT) of the actuator in a servo-hydraulic high rate testing machine was verified by image analysis using sisal fiber reinforced cement composite at a strain rate of 25 s⁻¹. The same procedure was then applied to Alkaline Resistant (AR) glass fabric reinforced cement composite tested at an average strain rate of 17 s⁻¹. Comparison between the strain values measured by the laser extensometer and the LVDT shows a good agreement between these two measurement techniques. The test results show that the Young’s modulus, tensile strength, maximum strain, and toughness of the AR-glass fabric-cement composite increase with increasing strain rate. However, under both static and dynamic loadings the composite has similar behavior: multi-crack development and one dominant crack leading to final failure. In order to ensure the accuracy of dynamic tensile test procedures, non-contacting devices and techniques should be used as an independent means of verification of test results. The accuracy required in quantifying relative improvements in mechanical properties necessitates the various methods of measuring the displacement and strain rate properties.     

Tension stiffening in reinforced high performance fiber reinforced cement-based composites

High Performance Fiber-Reinforced Cement-based Composite (HPFRCC) materials carry tension to strains greater than the yield strain of reinforcing steel and exhibit distributed compression damage with minimal spalling. Characterization of the interaction between the composite and steel reinforcement to large strains (i.e., >0.005) remains largely unknown. Three HPFRCC materials as well as concrete with a single reinforcing bar are tested in a prismatic specimen in uniaxial tension up to fracture of the reinforcement. Multiple cracking of the composite led to uniform bar yielding throughout the specimen and early hardening of the reinforcement at the location of dominant cracks. The reinforcement fractured within the HPFRCC at lower strain levels relative to the reinforced concrete. A modified approach based on planar analysis to estimate flexural strength of reinforced HPFRCC components using tension-stiffening data is proposed.     

水泥基复合材料中碳纤维的分散性研究

运用正交试验方法研究以羧甲基纤维素钠(CMC)和硅灰按不同比例配制的分散体系对碳纤维在水泥浆体中分散性的影响,采用新拌料浆法从多份新拌的水泥浆料中分离出碳纤维,并计算碳纤维质量的变动系数,由变动系数评价碳纤维的分散性和分散剂的作用效果.在各种CMC掺量下,硅灰均能显著改善碳纤维的分散性.随着CMC掺量的增加,碳纤维分散性提高.当CMC掺量为0.8%,硅灰掺量为15%时,CMC和硅灰的共同作用使变动系数最小,此时碳纤维在水泥基体中分散性最好,为最佳的分散剂配比.     

Effect of aggregates on drying shrinkage microcracking in cement-based composites

In this paper the effect of the aggregate content on drying shrinkage microcacking in cement-based composites is investigated. Experiments were performed on cement-based composites containing spherical glass particles as aggregates. It was found that the use of these model particles is an excellent tool to study the basic mechanisms of drying shrinkage microcracking in cement-based composites. The microcracks were made visible by means of an impregnation technique in combination with optical microscopy. Crack-patterns were quantified by using image analysis software. The effect of aggregate size and quantity on the defined crack-parameters is reported in this paper. Cracking was recorded on two stages of drying, which allowed to elucidate the long-term evolution of drying shrinkage microcracking.

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Résumé Dans ce papier, l'effet des granulats sur les microfissurations dues au retrait de dessiccation dans les matériaux en ciment est étudié. Les expériences sont réalisées sur des composites en ciment contenant des particules de verre sphériques comme granulats. Il a été trouvé que l'utilisation de ce modèle de particules est un excellent outil pour étudier les mécanismes basiques des microfissurations dues au retrait dans les composites en ciment. Ces microfissurations sont visibles au moyen d'une technique d'imprégnation en combinaison avec un microscope optique. Le comportement des fissures est quantifié en utilisant un logiciel d'analyse d'images. L'effet de la taille des granulats ainsi que la quantité sont retranscrits dans ce papier. Les fissures se réalisent en deux phases de séchage, ce qui permet d'élucider l'évolution à long terme du retrait de dessiccation.

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Editorial Note Prof. Jan G. M. van Mier is a RILEM Senior Member. He is Chairman of RILEM TC HFC ‘Hybrid Fibre Concrete’ and is also a member of the RILEM TAC (Technical Activities Committee). Jan van Mier and Jan Bisschop participate in the work of RILEM TC 181-EAS (‘Early age shrinkage induced stresses and cracking in cementitious systems’). Delft University of Technology is a RILEM Titular Member.     

Crack initiation in laminated metal-intermetallic composites

Several mechanisms have been proposed to describe crack initiation and propagation in ductile-brittle composites. This experimental study shows that the failure of metal intermetallic (metal-aluminides) composites was initiated by cracking initiation in the intermetallic layers. For metal layers that allowed shear deformation, crack initiation in adjacent intermetallic layers resulted from shear bands propagating from a crack tip in the intermetallic layer through the metal layer and producing stress concentration points at the interfaces of adjacent intermetallic layers. For metal layers that did not support shear deformation, crack initiation in the intermetallic layers resulted from the continued build up of stresses within the intermetallic layers, resulting in a relatively uniform distribution of cracks within the individual intermetallic layers. Prior to failure, lateral constraints produce lateral cracks in the intermetallic layers. The final fracture features of both failure mechanisms were similar for both metal-intermetallic systems.