Micromechanical properties of cementitious composites

Micromechanical properties of cement matrices in cementitious composites were investigated by means of the microindentation method. The research focused on the correlation between micromechanical properties such as modulus of elasticity and creep, and distance of the indentation from the aggregate-matrix interface. Composites based on ordinary Portland cement (OPC) matrix were examined after exposure to two types of ageing procedures. The sub-micron accuracy of the positioning system of the microindentation apparatus provided means for a meaningful investigation of cement matrix in close vicinity to the aggregate-matrix interface. For the purpose of statistical analyses, the data were divided into two groups with respect to the distance of the indent from the aggregatematrix interface. While the tests performed within a 30 μm distance from the interface were classified as indents within the interfacial transition zone (ITZ), indents outside this distance were considered to describe properties of the ‘bulk matrix’. The results provide quantitative comparison of the microstructural properties of the interfacial transition zone (ITZ) with those of the bulk cement matrix assessed by well understood characteristics such as elastic modulus and creep. It was shown that, for unaged specimens, the elastic modulus measured within the interfacial transition zone was about 25% lower than that of the bulk matrix. Such results have significant consequences for improvement in modelling of cementitious composites.

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Résumé La méthode de la micro-dureté a été utilisée pour déterminer les propriétés micro-mécaniques des matrices de composites à base de ciment. Cette recherche a consisté à déterminer la corrélation entre les propriétés micro-mécaniques telles que le module élastique et le fluage ainsi que la distance de l'échancrure de l'interface granulat-matrice. Les composites à base de ciment Portland ont été examinés après l'exposition à 2 types de vieillissement. La micro-précision du système de positionnement de l'appareil de micro-dureté a permis une évaluation précise de la matrice de ciment au voisinage de l'interface granulat-matrice. Pour l'analyse statistique, les données ont été divisées en deux groupes, vis-à-vis de la distance entre l'entaille et l'interface granulat-matrice. Les résultats obtenus pour une épaisseur de 30 μm de l'interface ont été classifiés comme la zone de transition de l'interface, au-delà de cette distance, ils ont été considérés pour la caractérisation des propriétés de la matrice principale. Ces résultats permettent une comparaison quantitative des propriétés microstructurales de la zone de transition de l'interface avec la matrice de ciment, afin de permettre l'évaluation d'autres caractéristiques comme le module élastique et le fluage. Il a été montré que pour certains échantillons qui n'ont pas subi un vieillissement accéléré, le module élastique mesuré dans la zone de transition de l'interface est 25% plus petit que celui de la matrice. Ces résultats ont des conséquences significatives pour modéliser les composites à base de ciment.

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Editorial Note Prof. P. J. M. Bartos is a Senior Member and the Chaiman of the RILEM 145-WSM: Workability of special concrete mixes. He is also a member of RILEM TC 162-TDF: Test and design methods for steel fibre reinforced concrete and RILEM TC 174-SCC: Self-compacting concrete.     

SHCC的配制与拉伸性能研究

采用聚乙烯醇纤维(PVA)纤维作为增强材料,选定不同的粉煤灰掺量、石英砂级配、纤维掺量和养护工艺配制应变硬化水泥基复合材料(SHCC),研究上述因素对SHCC力学性能的影响。研究表明,随着粉煤灰掺量的增加,SHCC极限拉伸强度有少许削弱,但极限拉伸应变不断增加,均高于3%。随着养护龄期增加,SHCC极限拉伸应变呈现先增加后减小的趋势,但拉伸强度随龄期增加而增大。自然养护有利于维持SHCC的高极限拉伸应变;蒸汽养护能提高SHCC早期的极限拉伸强度,但蒸汽养护使SHCC的极限拉伸应变随着龄期增加而明显降低。当m(FA)/m(C)=1.6,2.0和2.4,Vf=2.0%时,采用较细的石英砂和自然养护,28d龄期的SHCC极限拉伸强度在4 MPa以上,极限拉伸应变在3%以上。     

Correlation of tensile and flexural responses of strain softening and strain hardening cement composites

Closed form equations for generating moment–curvature response of a rectangular beam of fiber reinforced concrete are presented. These equations can be used in conjunction with crack localization rules to predict flexural response of a beam under four point bending test. Parametric studies simulated the behavior of two classes of fiber reinforced concrete: strain softening and strain hardening materials. The simulation revealed that the direct use of uniaxial tension and compression responses under-predicted the flexural response for strain softening material while a good prediction for strain hardening material was obtained. The importance of strain softening range on the flexural response is discussed using non-dimensional post-peak parameters. Results imply that the brittleness and size effect are more pronounced in the flexural response of brittle materials, while more accurate predictions are obtained with ductile materials. It is also demonstrated that correlations of tensile and flexural results can be established using normalized uniaxial tension and compression models with a single scaling factor.     

Self-healing behavior of strain hardening cementitious composites incorporating local waste materials

The self-healing behavior of a series of pre-cracked fiber reinforced strain hardening cementitious composites incorporating blast furnace slag (BFS) and limestone powder (LP) with relatively high water/binder ratio is investigated in this paper, focusing on the recovery of its deflection capacity. Four-point bending tests are used to precrack the beam at 28 days. For specimens submerged in water the deflection capacity can recover about 65–105% from virgin specimens, which is significantly higher compared with specimens cured in air. Similar conclusion applies to the stiffness recovery in water cured specimens. The observations under ESEM and XEDS confirmed that the microcracks in the specimens submerged in water were healed with significant amount of calcium carbonate, very likely due to the continuous hydration of cementitious materials. The self-healing cementitious composites developed in this research can potentially reduce or even eliminate the maintenance needs of civil infrastructure, especially when repeatable high deformation capacity is desirable, e.g. bridge deck link slabs and jointless pavements.     

Strength and ductility of RC beams strengthened with steel-reinforced strain hardening cementitious composites

Strengthening of Reinforced Concrete (RC) beams using strain hardening cementitious composites (SHCCs) layer cast to their soffit has recently been investigated. That work confirmed that strain localization occurs in the SHCC-strengthening layer, which severely limits the ductility of the strengthened beam. This paper reports the ductility enhancement achieved in tests on reinforced concrete beams that were strengthened with lightly steel-reinforced SHCC layer (0.3% and 0.6% steel reinforcement ratio). It has been found that the combination of the SHCC and a small amount of steel reinforcement helps develop higher strain in the SHCC strengthening layer at ultimate load and eliminates the observed early strain localization. The recorded averaged strain at ultimate load of SHCC-strengthening layer provided with 0.3% and 0.6% steel reinforcement was 2.10 and 3.76 times that of an unreinforced SHCC layer. Also, use of a 0.6% reinforcement ratio changed the mode of failure of the SHCC-strengthened beams from brittle to more ductile. Moreover, the SHCC-strengthening layer with 0.6% reinforcement ratio was able to develop uniformly distributed visible cracks, which were the only indication that failure was imminent. It needs to be emphasized that strengthening of RC structures using an unreinforced SHCC layer may lead to a brittle failure.     

超高性能水泥基复合材料弯拉作用下虚拟应变硬化机制分析

根据 Tjiptobroto 的工作 , 假设初始裂纹为最终失效裂纹 , 引入非初始裂纹纤维基体间的部分剥离能耗散项和单位失效面上随机纤维有效根数 , 依据初始裂纹总耗散能与非初始裂纹耗散能的平衡准则 , 研究了水泥基复合材料的多裂纹扩展失效机制。根据超高性能水泥基复合材料特性 , 修正和简化了各能量耗散项 , 建立了基于能量平衡准则的超高性能水泥基复合材料多裂纹开裂失效机理的理论模型 , 用以预报该材料的裂纹扩展规律。数值预报了 Tjiptobroto 实验模型的多裂纹扩展数目和能量耗散项 , 并与其实验结果进行了对比 , 吻合较好。表明对具有高弹模钢纤维的超高性能水泥基复合材料引入部分剥离能项是必要的。本文中的理论模型也可作为超高性能水泥基复合材料初裂承载能力和极限承载能力预报的理论参考。      

A review on durability properties of strain hardening fibre reinforced cementitious composites (SHFRCC)

This paper reviews and presents various durability properties of strain hardening fibre reinforced cementitious composites (SHFRCC). Published research results show that, due to its tight crack width properties compared to ordinary concrete and ordinary fibre reinforced concrete, SHFRCC significantly resists the migration of aggressive substances in to the concrete and improves the durability of reinforced concrete (RC). It is also reported that, due to the strain hardening and multiple cracking behaviours, SHFRCC meets the tight crack width limits for durability of RC structures proposed by different design codes. Based on the reviewed durability properties it is argued that SHFRCC materials can be used in selected locations of RC structural members to improve their overall durability performances.     

Multi-response optimization of post-fire performance of strain hardening cementitious composite

This paper presents results of an experimental program conducted to optimize the post-fire performance of Strain Hardening Cementititous Composites (SHCC) using Taguchi approach with utility concept. The experiments were first undertaken by determining nine SHCC mixes using a standard L9 (3⁴) orthogonal array of four parameters and each parameter with three levels. The four parameters of SHCC mixes included fly-ash/binder ratio, sand/binder ratio, water/binder ratio and fiber proportions. The responses of SHCC to be optimized were tensile strain capacity, compressive strength and post-fire compressive strength after subjected to 200 °C, 400 °C, 600 °C and 800 °C of isothermal heating. Utility concept was introduced to simplify the multi-response problem into mono-response question together with Taguchi method. The role of different parameters on the composite responses of SHCC was examined. Furthermore, an optimal SHCC mix to maximize multi-responses was determined based on statistical analysis and validated by additional confirmation tests.     

Shear banding and strain softening in plane strain extension: physical modelling

Localization of deformation in systems of shear bands or normal faults, respectively, as a consequence of extensional loading can be observed on a wide range of spatial scales in soil and rock formations. A series of plane strain model experiments was achieved in natural (1 g) and increased (ng) gravity field (geotechnical centrifuge) with dry and moist sand as well as with dry and moist sand-clay mixtures. In these experiments, the geometry of the shear bands (inclination, width, spacing, sharpness) was detected by means of the digital image correlation (DIC) technique. Comparison with existing analytical approaches for the determination of the spacing of shear bands is presented briefly. The stress-strain behaviour of the materials was determined in a new biaxial test device, which allows for the performance of biaxial compression and extension tests. The evaluation focuses on the strain softening gradient, which is seen as a key parameter in the explanation of shear band spacing.     

Micromechanical modelling of the transverse damage behaviour in fibre reinforced composites

A micromechanics damage model is presented which examines the effect of fibre-matrix debonding and thermal residual stress on the transverse damage behaviour of a unidirectional carbon fibre reinforced epoxy composite. It is found that for a weak fibre-matrix interface, the presence of thermal residual stress can induce damage prior to mechanical loading. However, for a strong fibre-matrix interface the presence of thermal residual stress is effective in suppressing fibre-matrix debonding and improving overall transverse strength by approximately 7%. The micromechanical model is subjected to a multiple loading cycle (i.e. tension-compression-tension), where it is shown to provide novel insight into the microscopic damage accumulation that forms prior to ultimate failure, clearly highlighting the different roles that fibre-matrix debonding and matrix plasticity play in forming the macroscopic response of the composite. Such information is vital to the development of accurate continuum damage models, which often smear these effects using non-physical material parameters.     

Special features of strain hardening of heat-resistant materials in tension

Calculations are carried out of linear and exponential parameters of heat-resistant metallic materials within the temperature range 20°C to 0.8 T_me. As initial data true tensile diagrams were used. Evaluation of the stability of the strain process is carried out and of the possibility of using the hypothesis of the single curve.Translated from Problemy Prochnosti, No. 9, pp. 21–25, September, 1990.     

Coupled temperature and moisture effects on the tensile behavior of strain hardening cementitious composites (SHCC) reinforced with PVA fibers

This paper reports the experimental findings on the tensile behavior of strain-hardening cement-based composites (SHCC). The composites were subjected to the combined effects of elevated temperatures and internal moisture condition. Uniaxial tensile tests on dumbbell-shaped SHCC specimens with in situ temperature control were performed at 22, 60 and 100 °C. In addition, the effect of the internal humidity of SHCC (95, 50, 20 and 0%) coupled to the elevated temperatures was investigated. It was shown that the tensile strength decreases and the strain capacity increases with an increase in temperature. The influence of the internal moisture conditions was more significant in high temperatures. The strain capacity reduced significantly with a decrease in the humidity level. The crack pattern of the SHCC specimens was determined. Furthermore, single fiber pullout tests were performed under the considered high temperatures condition. Finally, the results are discussed based on the thermogravimetry analysis of the PVA fiber, alterations on its microstructure and surface coating.     

Die drawn wood polymer composites. II. Micromechanical modelling of tensile modulus

The Cox–Krenchel micromechanical model was applied to give predictions for the tensile moduli of isotropic and oriented wood polymer composites (WPC). The oriented WPC were produced by the Leeds die-drawing process using polypropylene filled with softwood and hardwood powders. The wood particles were extracted from the composites to determine their density and aspect ratio by dissolving in hot decalin. To measure particle shape and size, image analysis was employed. These experimental parameters were then introduced to the Cox–Krenchel model which was found to give prediction of tensile modulus in very good agreement with the experimental values.     

Solid-solution hardening and softening in binary iron alloys

Six dilute (0.2, 0.5 and 1 at %) binary iron-base alloys with Co, Cr, Al, Si, Mn and Ni were prepared after scavenging inherent carbon with Ti. From tensile and stress relaxation tests in the temperature range of 77 to 450 K, stress-strain behaviours and thermal activation parameters were analysed as functions of solute content and temperature. In the four alloys containing Ni, Mn, Al and Si, solid-solution softening occurs below 250 K while solid-solution hardening occurs above 250 K. In the alloys containing Co or Cr, neither softening nor hardening due to solute additions occurs at any temperature. Detailed analysis of thermal activation parameters leads one to conclude that the solid-solution softening in the above mentioned four alloys is due to a decrease in kink energy with increasing solute content, while in the latter two alloys no change in kink energy occurs. On the other hand, there exists a strong solute concentration dependence of the athermal component, suggesting that the solid-solution hardening is due to the interaction of dislocations with groups of substitutional solute atoms that create lattice and modulus misfits.     

Fire resistance of ultra-high performance strain hardening cementitious composite: Residual mechanical properties and spalling resistance

Ultra high performance strain hardening cementitious composites (UHP-SHCC) is a special type of cement-based composite material with outstanding mechanical and protective performance at room temperature. But its fire performance is unknown and there is a lack of research in this aspect. This study presents an experimental program to study fire resistance of UHP-SHCC under two aspects, viz. high-temperature explosive spalling resistance and residual mechanical performance after a fire. Both compressive strength and tensile strength of UHP-SHCC were found to deteriorate with increasing exposure temperature. Tensile strain-hardening feature of UHP-SHCC would be lost at 200 °C and above. It was found that PE fibers are found not effective in mitigating explosive spalling, although they start to melt at 144 °C. FE-SEM (Field Emission Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray) techniques were used to study the state of fiber, fiber/matrix interaction, and microcracks development. Microscopic study found that melted PE fibers were still present in the cementitious matrix, and the melting did not introduce more microcracks. Furthermore, it was difficult for melted PE fibers to diffuse through the matrix, thus providing the reason that PE fibers did not mitigate explosive spalling in UHP-SHCC.     

Micromechanical model of nacre tested in tension

A modified shear lag theory is used to model the tensile behavior of Pinctada nacre. A two-dimensional model is used to analyze the stress transfer between the aragonite platelets of nacre assuming that the ends of the platelet are not bonded with the organic matrix. Elastic-perfectly plastic behavior of the organic matrix is assumed. A model for stress transfer between the platelets when the matrix between the platelets starts behaving plastically is developed. It is assumed that nacre fails when the matrix breaks after the ultimate shear strain in the matrix is exceeded. This theory can be used to model the stress transfer in platelet reinforced composites at high volume fractions.