Effect of aggregate size on the fracture and mechanical properties of a simple concrete

The influence of the aggregate size on the fracture energy, tensile strength and elasticity modulus in different types of concrete are analyzed. For this purpose, nine simple cement-based composites have been designed, manufactured and tested, with one objective to provide experimental results that can be used as a benchmark for checking numerical models of concrete fracture, as this simple composite (a matrix, spherical aggregates of the same radius, and two types of matrix-aggregate interface) is amenable to modelling. All in all, 44 specimens were tested. From notched beam tests, values of the fracture energy and modulus of elasticity were obtained. The tensile stress was deduced from indirect standard tensile test. Data for bilinear softening functions extracted from the experimental measurements are also provided. Comparison with available experimental data is also included and discussed.     

Cohesive crack modelling of a simple concrete: Experimental and numerical results

Fracture tests were performed on six types of simple concrete made with two types of mortar matrix w/c = 0.32 and w/c = 0.42, two types of spherical aggregates (strong aggregates that debonded during concrete fracture, and weak aggregates, able to break), and two kinds of matrix-aggregate interface (weak and strong).The tensile strength, fracture energy and elasticity modulus of the six types of concrete were measured. These results are intended to serve as an experimental benchmark for checking numerical models of concrete fracture and for providing certain hints to better understand the mechanical behaviour of concrete.A bilinear softening function was used to model the fracture of concrete. The influence of the type of matrix, aggregate, and interface strength on the parameters of the softening curve are discussed: particularly, the fracture energy, the cohesive strength and the critical crack opening.     

Debonding of a thin rubberised and fibre-reinforced cement-based repairs: Analytical and experimental study

An analytical approach for the prediction of debonding initiation between a rubberised cement-based overlay and old concrete substrate under monotonous mechanical loading was applied. Based on the linear elastic fracture mechanics, a model has been developed taking into account the interlocking between two crack surfaces in the overlay. Assuming that the debonding initiation just occurs after the crack cutting the overlay layer reaches the overlay–substrate interface, the stress intensity factor of the debonding tip can be calculated, allowing prediction of stress fields near the interface debonding tip. Then with a criterion of debonding initiation and propagation depending on the interface tensile strength, the load associated could be determined and might be interesting for the design of thin bonded cement-based overlays. The adequateness of this analytical approach was verified by both experimental data and finite element calculations. It has been used to show the relevance of a cement-based material with low modulus of elasticity combined with a high residual post crack strength to achieve sustainable repairs.     

Analytical and experimental investigations on the fracture behavior of hybrid fiber reinforced concrete

In the present study, Mode-I fracture tests of hybrid fiber reinforced concrete (HFRC) composite beams were conducted and the fracture properties and other post peak strength characteristics of the HFRC composites were evaluated and analyzed. The HFRC composite was produced using three types of fibers namely steel, Kevlar and polypropylene. A total of 27 HFRC composite beam specimens were cast and tested using the RILEM recommended three point bending test. The main variables were the fiber volume content and combinations of different fibers. The load versus crack mouth opening displacement (CMOD) curves of HFRC composite beams were obtained. Inverse analysis was carried out to determine the tensile strength and crack opening relationship. Analytical models based on comprehensive reinforcing index were developed for determining the influence of the fibers on fracture energy, flexural tensile strength, equivalent tensile strengths and residual tensile strengths of HFRC composites. Based on the experimental results and inverse analysis, a model for predicting the tensile softening diagram of HFRC composite mixes was also developed. The analytical models show conformity with the experimental results.     

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.     

Quantitative characterization of accelerated aging in cement composites using flexural inverse analysis

A constitutive model consisting of a tri-linear tensile stress-strain with residual strength was applied in characterization and prediction of long term flexural behavior of several cement-based composite materials. Flexural test results were back-calculated to obtain material parameters and establish their relationship with aging. The material behavior is described by tensile stress-strain parameters consisting of elastic modulus, first cracking strain, post cracking stiffness, ultimate strain, and a residual strength parameter. The relationships between the material parameters and age were established by studying the time dependent flexural performance of various composites with glass and natural fibers as reported by Litherland et al. (1981), Marikunte et al. (1997), Bartos et al. (1996), and natural fibers reported by Toledo-Filho et al. (2000). An analytical model for prediction of rate and extent of damage as a function of time and temperature is proposed for degradation of flexural behavior of strain softening and hardening fiber reinforced concrete subjected to aging. This model is applicable to long-term durability of different classes of materials subject to accelerated aging under different environmental conditions.     

Analytical study of tensile behaviors of UHMWPE/nano-epoxy bundle composites

Ultra-high molecular polyethylene (UHMWPE) fiber reinforced nano-epoxy and pure epoxy composites in bundle form were prepared and tested for tensile properties. UHMWPE fiber composites are well known for their superior tensile performance, and this work was conducted to assess the effect of adding nanoadditives to the resin and to evaluate possible enhancements or degradations to that attribute. The results showed that tensile tests on various types of UHMWPE fibers/nano-epoxy bundle composites resulted in an increase in modulus of elasticity due to the addition of small amounts of reactive nanofibers (r-GNFs) to epoxy matrix. It was observed that the modulus of elasticity of the composite bundles depended on both volume fractions of the matrix and the weight percent (wt%) of r-GNFs in the matrix. A non-linear relationship was established among them and an optimal modulus was determined by calculation. A three-dimensional surface plot considering these two parameters has been generated which gives an indication of change in modulus of elasticity with respect to volume fraction of matrix and wt% of r-GNFs in the matrix. A Weibull analysis of tensile strengths for the various bundle composites was performed and their Weibull moduli were compared. The results showed that presence of r-GNFs in the composites increased the strength effectively, and 0.3 wt% r-GNFs based composites showed the highest strength. An important ancillary finding is that optimum tensile values are a function not only of the above parameters, but also strongly influenced by the addition of diluents which control the viscosity of the blend.     

Tensile elastic modulus, strength and fracture of delta-Al2O3f/Al alloy composites

Based on the experimental and theoretical analysis, the tensile elastic modulus, strength and fracture characteristics of squeeze casting delta-Al2O3/Al alloy composites were studied. The fracture characteristics of composites were observed by SEM. The elastic modulus was predicted by the finite element method based on the energy equivalence principle, and the strength was predicted by the statistical integration average method using the maximum energy criterion of composite strength. In the prediction, the distribution density functions of the fiber's orientation and length were considered. These functions were gained by experimental measurement. It is shown that the predicted results are in agreement with the experimental values well and the microstructure feature of composites controls the fracture characteristics.     

Tensile Elastic Modulus, Strength and Fracture of δ-Al_2O_(3f)/Al Alloy Composites

1. IntroductionThe tensile elastic modulus and strength are twoimportal material parameters of composites and hadbeen paid more attention by mad researchers[1-4].However, in the random short fiber reinforced metalmatrix composite (RSFRMMC), the tensile elasticmodulus and strength are influenced by many factorsand have not been predicted precisely. At present,the predicting precision for elastic modulus of corn-posites had been improved by using the finite elementmethod (FEM)l']. However, t…     

单向纤维增强陶瓷基复合材料单轴拉伸行为

采用细观力学方法对单向纤维增强陶瓷基复合材料的单轴拉伸应力-应变行为进行了研究。采用Budiansky-Hutchinson-Evans（BHE）剪滞模型分析了复合材料出现损伤时的细观应力场,结合临界基体应变能准则、应变能释放率准则以及Curtin统计模型三种单一失效模型分别描述陶瓷基复合材料基体开裂、界面脱粘以及纤维失效三种损伤机制,确定了基体裂纹间隔、界面脱粘长度和纤维失效体积分数。将剪滞模型与3种单一失效模型相结合,对各个损伤阶段的应力-应变曲线进行模拟,建立了准确的复合材料强韧性预测模型,并讨论了界面参数和纤维韦布尔模量对复合材料损伤以及应力-应变曲线的影响。与室温下陶瓷基复合材料单轴拉伸试验数据进行了对比,各个损伤阶段的应力-应变、失效强度及应变与试验数据吻合较好。     

定向钢纤维水泥基复合材料断裂理论分析及试验

基于非线性铰模型研究了定向钢纤维水泥基复合材料的裂缝断裂全过程理论分析方法,结合不同尺寸试件的三点弯曲梁断裂试验对本文方法进行了验证。进而利用该方法预测了大尺寸三点弯曲梁试件的裂缝断裂全过程,并研究了试件尺寸对名义强度的影响。通过理论分析与试验结果对比,表明本文方法可较好地预测定向钢纤维水泥基复合材料的裂缝断裂全过程;此外,定向钢纤维水泥基复合材料的名义强度存在一定的尺寸效应,但尺寸效应表现不明显。     

Influence of the fracture softening behaviour of wood on load-COD curve and R-curve

A cohesive crack model is used to analyse failure of wood in mode I along the grain. Several configurations of the gradual fracture softening behaviour of an interface, meshed with joint-elements located on the potential crack path, are investigated. Different constitutive laws, obtained from a single normalized polynomial function, are tested in order to estimate the influence of parameters such as, the tensile strength, the fracture energy or the ultimate opening of the interface, on the macroscopic response of a fracture specimen. Numerical results are compared with experimental data obtained on DCB specimen. We argue that the fracture energy related to the constitutive law must correspond to the plateau value of the R-curve. Moreover, this study reveals that the peak load of a load-COD (Crack Opening Displacement) curve is strongly affected by the slope of the softening behaviour. Finally, we present a review of the influence of each parameter describing the softening function on: (1) the load-COD curve and (2) the corresponding R-curve.     

Tensile behavior of glass/epoxy laminates at varying strain rates and temperatures

In the present work tensile tests at different strain rates and temperatures were performed in glass fiber reinforced polymer (GFRP). It is observed that such kind of composite presents an elasto–viscoplastic behavior – the rate dependency only occurs for loading levels above a given elasticity limit. Strain rate strongly affects the ultimate tensile strength (σ_u) and the modulus of elasticity is almost insensitive to it while temperature only influences the modulus. Analytical expressions to predict the modulus of elasticity and (σ_u) as a function of the temperature and strain rate are proposed and compared with experimental data showing a reasonable agreement.     

Prediction of Fracture Parameters of High Strength and Ultra-High Strength Concrete Beams using Minimax Probability Machine Regression and Extreme Learning Machine

This paper deals with the development of models for prediction of facture parameters, namely, fracture energy and ultimate load of high strength and ultra high strength concrete based on Minimax Probability Machine Regression (MPMR) and Extreme Learning Machine (ELM). MPMR is developed based on Minimax Probability Machine Classification (MPMC). ELM is the modified version of Single Hidden Layer Feed Foreword Network (SLFN). MPMR and ELM has been used as regression techniques. Mathematical models have been developed in the form of relation between several input variables such as beam dimensions, water cement ratio, compressive strength, split tensile strength, notch depth, and modulus of elasticity and output is fracture energy and ultimate load A total of 87 data sets (input-output pairs) are used, 61 of which are used to train the model and 26 are used to test the models. The data-sets used in this study are derived from experimental results. A comparative study has been presented between the developed MPMR and ELM models. The results showed that the developed models give reasonable performance for prediction of fracture energy and ultimate load.     

Early age fracture properties of microstructurally-designed mortars

This paper compares the fracture properties as well as crack initiation and propagation of real and equivalent mortars. The development of the elastic modulus, tensile strength, and fracture energy at different hydration stages were determined by inverse analysis of load-displacement curves obtained by the compact tension test (CTT). Further, the impact of the moisture content on the aforementioned material properties was also tested on oven-dried equivalent mortars. Digital image correlation (DIC) was used to follow the crack initiation and propagation.The elastic modulus, tensile strength, and fracture energy support the validity of the equivalent mortars approach. The load-displacement curves obtained by the CTT were also compared to those simulated by finite element method showing excellent correlations. DIC revealed the formation of similar crack patterns at comparable load levels between the two mortars. At early age, the moisture content has a considerable influence on the tensile strength and the fracture energy.     

界面对纤维增强陶瓷基复合材料拉伸性能的影响

建立了桥联纤维细观力学模型, 研究了界面对纤维增强陶瓷基复合材料拉伸模量及强度的影响。分别引入纤维应力均匀系数和界面脱粘率作为界面完全脱粘和局部脱粘条件下界面性能的表征参数。研究表明, 应力均匀系数及界面脱粘率越大, 材料模量越低, 而断裂时纤维所承担的应力越高。基于混合率给出了拉伸强度表达式, 同时也分析了基体裂纹分布、界面脱粘和纤维拔出对强度的影响。计算结果表明, 本文强度模型给出的预测值与试验值吻合较好。      

Defined-performance design of ecological concrete

Energy consumption and CO₂-emission of concrete can be reduced when cement is replaced by secondary materials such as residual products from other industries. However, for the design of such environmentally friendly concretes, predicting its performance is very important. In this article a cyclic design method is presented, which can predict the strength of a concrete mixture based on particle packing technology. In the procedure, the amount of water is estimated from the required workability and calculated packing density. After that, the strength of that mixture is predicted from packing density calculations and the amount of water in the mixture via the cement spacing factor. This cycle is repeated until the mixture composition does not have to be adjusted anymore to comply with the desired performance or strength class. With the presented cyclic design procedure cement contents can be decreased without changing concrete properties in a negative way, thereby saving up to 57 % of Portland cement and reducing CO₂-emission with 25 %. This is shown by experimental results of ecological concrete mixtures tested on compressive strength, tensile strength, modulus of elasticity, shrinkage, creep and electrical resistance. The results confirmed that relationships between cube compressive strength, tensile splitting strength and modulus of elasticity correspond to those for normal concrete. The experimental program showed the possibility to use cube compressive strength as the governing design parameter in the cyclic design procedure for ecological concrete. Furthermore, it is shown how the cyclic design method can be used for defined-performance concrete design.     

Crack formation and fracture energy of normal and high strength concrete

The crack path through composite materials such as concrete depends on the mechanical interaction of inclusions with the cement-based matrix. Fracture energy depends on the deviations of a real crack from an idealized crack plane. FRACTURE energy and strain softening of normal, high strength, and self-compacting concrete have been determined by means of the wedge splitting test. In applying the numerical model called “numerical concrete” crack formation in normal and high strength concrete is simulated. Characteristic differences of the fracture process can be outlined. Finally results obtained are applied to predict shrinkage cracking under different boundary conditions. Crack formation of high strength concrete has to be seriously controlled in order to achieve the necessary durability of concrete structures.     

Model representation of filled polymers

We have investigated the mechanical properties and dielectric relaxation in polyamide-6 composites with fiber glass and mica. We propose a new model for filled polymers, assuming that the material consists of two interpenetrating continuous phases. The first phase is the polymer sorbed on the surface of the filler particles, mechanically stronger and having a higher modulus of elasticity. The second phase is the unsorbed polymer. The calculated mechanical characteristics of the composites (tensile strength and modulus of elasticity) agree well with experimental data. Translated from Izmeritel'naya Tekhnika, No. 8, pp. 53–56, August, 1998.     

Influence of aggregate deformation and contact behaviour on discrete particle modelling of fracture of concrete

The discrete element method, DEM, has been used in fracture studies of non-homogeneous continuous media adopting circular or spherical particles. A 2D circular rigid DEM formulation developed with the purpose of modelling concrete is described and evaluated in uniaxial tensile and compression tests. According to this model, the aggregate can be modelled either as a rigid macro-particle or as a deformable group of particles. The inter-particle contacts can either be assumed as brittle or follow a given bilinear softening curve. It is shown that aggregate deformability, together with the consideration of pure friction contacts working under compression, increases the fracture energy in compression, leading to a better agreement with concrete tests. The softening contact model, by adding a higher capability of load redistribution, is shown to give a better agreement than the brittle model under tensile loading. The recognized crack mechanisms of the brittle model (tensile splitting, branching, bridging) are also present with softening.