Micromechanical investigation on size effect of tensile strength for recycled aggregate concrete using BFEM

In this paper, the validity and performance of base force element method (BFEM) based on potential energy principle was studied by some numerical examples. And the BFEM on damage mechanics is used to analyze the size effect on tensile strength for recycled aggregate concrete (RAC) at meso-level. The recycled aggregate concrete is taken as five-phase composites consisting of natural coarse aggregate, new mortar, new interfacial transition zone (ITZ), old mortar and old ITZ on meso-level. The random aggregate model is used to simulate the meso-structure of recycled aggregate concrete. The size effects of mechanical properties of RAC under uniaxial tensile loading are simulated using the BFEM on damage mechanics. The simulation results agree with the test results. This analysis method is the new way for investigating fracture mechanism and numerical simulation of mechanical properties for RAC.     

Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation

The properties of new Interfacial Transition Zone (ITZ) and old ITZ in Recycled Aggregate Concrete (RAC) were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and nanoindentation. From the SEM images, obvious voids and high concentration of calcium hydroxide can be found in both old ITZ and new ITZ in RAC. Based on the nanoindentation study, it is indicated that the thicknesses of old and new ITZs are in the range 40–50 μm and in the range 55–65 μm, respectively. It is also found that the average indentation modulus of old ITZ is 70–80% of that of old paste matrix, while the average indentation modulus of new ITZ is 80–90% of that of new paste matrix. Additionally, the influences of mix proportion, aggregate types and hydration age on the properties of ITZs in RAC are discussed in this study.     

Effect of carbonation of modeled recycled coarse aggregate on the mechanical properties of modeled recycled aggregate concrete

The modeled recycled aggregate concrete (MRAC) which is an idealized model for the real recycled aggregate concrete (RAC) was used in this study. The MRCAs prepared with two types of old mortars were modified by an accelerated carbonation process. The effects of carbonation of MRCA on the micro-hardness of MRCA and the mechanical properties of MRAC were investigated. The results indicated that the micro-hardness of the old interfacial transition zone (ITZ) and the old mortar in the carbonated MRCAs was higher than that in the uncarbonated MRCAs, and the enhancement of the old ITZ was more significant than that of the old mortar. The compressive strength and modulus of MRACs increased when the carbonated MRCAs were utilized, and the improvement was more significant for MRAC prepared with a higher w/c. In addition, a numerical study was carried out and it showed that the improvement in strength by carbonation treatment was less obvious when the difference between the new and old mortar was larger.     

Variation in mechanical properties of natural and recycled aggregate concrete as related to the strength of their binding mortar

Experimental work was performed to study the effect of binding mortar strength on the mechanical properties of recycled natural aggregate concrete mixes as well as reference corresponding natural aggregate concrete mixes. The moduli of elasticity of both NAC and RAC were found to be higher than that of corresponding mortar by about 40% and 10% respectively, for all compressive strengths investigated. It was possible to reach compressive strength for RAC of 53.5 MPa. The ratios of compressive strength of NAC or RAC to that of mortar varied between (1.05–1.56) and (1.02–1.26) respectively, these ratios decreased with the increase in compressive strength. Also from the results of compressive strength, it was found that the ratios cylinder/cube compressive strengths of RAC and mortar were smaller than those of NAC. The ranges of values obtained were (0.71–0.84) and (0.69–0.75) for RAC and mortar respectively, while for NAC this ratio ranged between (0.81–0.92), these values were obtained for compressive strengths ranging between 15 to 55 MPa. It was found that it is better to relate the cylinder/cube strength ratio to the modulus of elasticity of the concrete or mortar rather than to its compressive strength. The flexural strength showed an opposite trend, the ratios of NAC and RAC to that of mortar ranged between (0.72–0.95)% and (0.61–0.80)% respectively. These ratios increased with the decrease in compressive strength of mortars. On the other hand, the splitting tensile strength of NAC was higher than that of RAC and mortar for all strength levels investigated. The ratio of NAC to mortar splitting tensile strength ranged between (1.13–1.69), while this ratio for RAC ranged between (0.87–1.36). Finally, several regressions were developed that can relate the mechanical properties of the three materials investigated.     

加载速率对混凝土拉伸破坏行为影响观数值分析

探讨了加载速率及细观结构非均质性对混凝土破坏模式及宏观力学性能的影响。考虑到混凝土细观结构非均质性的影响,将混凝土看作由骨料和砂浆基质组成的两相复合材料。考虑材料的应变率效应,采用塑性损伤模型来描述砂浆基质的动态力学行为;由于骨料具有较高强度,假定不会产生断裂,设定为弹性体。对单边缺口的混凝土试件及L形试件在不同加载速率下的动态拉伸破坏模式进行了细观数值研究。数值结果表明:1) 混凝土动态破坏模式及裂纹扩展方向具有明显的加载速率相关性;2) 随着加载速率的提高,混凝土破坏模式从I-型模式到混合型模式转变;3) 混凝土细观结构越复杂,组分间相互作用越复杂,裂纹扩展路径越复杂,裂纹分支现象越为明显;4) 随着加载速率的提高,混凝土破坏时产生更多的裂纹扩展路径(分支裂纹),且损伤区域宽度增大,导致混凝土在高应变率作用下消耗更多的能量,可认为是混凝土材料动态强度提高的主要原因。     

考虑孔隙及微裂纹影响的混凝土宏观力学特性研究

混凝土是一种典型的多孔介质材料,孔隙分布错综复杂,孔径尺寸跨越微观尺度和宏观尺度,对混凝土弹性模量及强度等力学参数产生巨大影响.认为混凝土是由骨料、孔隙及砂浆基质组成的三相复合材料,采用Monte Carlo 法将孔隙、微裂纹及微缺陷与骨料颗粒随机投放在砂浆基质中.根据三相球模型及中空圆柱形杆件模型得到含孔材料的有效力学性质,并推导得到含孔材料的等效本构模型.建立含孔隙混凝土试件的细观单元等效化力学模型,对二级配含孔隙混凝土试件在单轴拉伸及压缩条件下的反应进行了非线性分析.结果表明:孔隙、微裂纹的存在对混凝土宏观弹性模量、强度及残余强度等力学性质都有很大影响,在对混凝土宏观力学特性分析及研究混凝土损伤断裂时不应忽略其影响.     

Base force element method based on the complementary energy principle for the damage analysis of recycled aggregate concrete

The finite element method (FEM) is an effective approach for exploring the failure mechanism of heterogeneous materials. According to the complementary energy principle, the use of FEM might suffer from several difficulties in terms of keeping the elements and their boundaries balanced, as well as finding interpolation functions. In this study, we introduced an efficient approach to researching the failure mechanism of the material, named base force element method (BFEM), according to complementary energy principle. Specifically, the element compliance matrix of an arbitrary quadrilateral element with four mid-edge nodes was expressed based on the complementary energy principle. Then, the node displacement was obtained by the governing equation using the Lagrange multiplier method. In addition, both the compliance matrix and the node displacement were represented as explicit expressions without the use of Gaussian integration. A numerical model of the recycled aggregate concrete (RAC) was established according to the Monte Carlo method. A comparative sample of the digital image model was also established using digital image technology. The influences of substituting recycled aggregate and the relative mechanical properties of adhered mortar to those of new mortar on the failure mechanism of RAC were studied. The simulation results indicated that the BFEM is an effective approach to researching the damage mechanism of heterogeneous materials.     

Optimization of normal and high strength recycled aggregate concrete mixtures by using packing model

This paper analyzes the possibility of applying the Compressible Packing Model (CPM) for the proportion of concrete mixtures produced with Recycled Concrete Aggregates (RCAs). As a matter of fact, the RCAs are composed of natural aggregates and attached mortar and, as a consequence, they generally present a higher porosity in comparison with ordinary natural aggregates. The higher porosity of RCAs can affect the resulting Recycled Aggregate Concretes (RACs) properties and, for this reason, the mix design procedure available in literature for ordinary concrete mixture cannot be applied as such in the case of RACs. In this context, the present work first presents a preliminary study in which the optimal mixing procedure for RACs is investigated and then, a possible extension of the CPM in the case of RACs is analyzed. Several structural RAC mixtures were designed for three strength classes (25, 45 and 65 MPa) by considering the variation of the aggregate replacement from 0 to 100%. Finally, the proposed procedure is experimentally validated by performing mechanical and durability tests on selected mixtures for the three strength classes with a RCAs content up to 60%. The results reported herein demonstrate the applicability of the CPM for recycled concrete mixtures and highlight as the rational use of RCAs lead to produce structural RAC without affecting its mechanical and the durability performance.     

Influence of industrially produced recycled aggregates on flow properties of concrete

This research aims at evaluating the main risks for the durability of concrete made of industrially produced recycled aggregates called Recycled Aggregate Concrete (RAC). A characterisation of recycled aggregates is performed and their peculiarities are highlighted. A comparison between the behaviour of RAC and that of ordinary natural aggregate concrete is carried out. The influence of both the composition and the curing conditions is discussed. The durability study is focused on the assessment of parameters representing the porous structure and concrete characteristics. Because of the high total water/cement ratio of RAC, their flow properties control their durability. It is established that RAC are characterised by significantly higher water absorption and air permeability. The diffusion of the carbon dioxide is faster, too. That leads to a weaker resistance of RAC to environmental attacks. Since the main durability problems are caused by the fine recycled fraction, its use needs to be restricted. Another way to increase RAC durability seems to be the extended curing in wet environment.     

Meso-scale numerical investigation on cracking of cover concrete induced by corrosion of reinforcing steel

Concrete cover cracking induced by corrosion of steel reinforcement is a major influencing factor for durability and serviceability of reinforced concrete structures. Here in this study, the influence of concrete meso-structure on the failure pattern of concrete cover is accounted for. The concrete is assumed to be a three-phase composite composed of aggregate, mortar matrix and the interfacial transition zone (ITZ). And a concrete random aggregate structure is established for the study on the mechanical behavior of radial corrosion expansion. In the present simulations, the plasticity damaged model is used to describe the mechanical behavior of the mortar matrix and the ITZ, and it is assumed that the corrosion of steel reinforcement is uniform. The cracking of concrete cover due to steel reinforcement corrosion is numerically simulated. The simulation results have a good agreement with the available test data and they are between the two analytical results. The failure patterns obtained from the macro-scale homogeneous model and the meso-scale heterogeneous model are compared. Furthermore, the influences of ratio of cover thickness and reinforcement diameter (i.e. c/d), the location of the steel reinforcement (i.e., placed at the middle and corner zones) and the concrete tensile strength on the steel corrosion rate when the concrete cover cracks are investigated. Finally, some useful conclusions are drawn.     

Failure Mechanism of Concrete: Combined Effects of Coarse Aggregates and Strength Level

Concrete is a composite, and its properties depend on the properties of the component phases and the interaction between them. It is known that the interfaces are the weakest link in concrete, playing a very important role in the process of failure. This process is strongly related with the characteristics of the aggregates (especially coarse aggregates) and with the relative differences in strength between matrix and inclusions. This paper analyzes the mechanical behavior of high strength and conventional concretes prepared with coarse aggregates having significant differences in strength, shape and surface texture, porosity and absorption, and interface bond strength. Two different gravels and two different crushed stones were used. Concrete mixtures with water/cement ratios of 0.30 and 0.50 were designed. The effects of aggregate type and strength level on concrete failure mechanism, including tensile and compressive strength, stiffness, energy of fracture, and crack pattern, are discussed.     

Load effects on fracture of concrete

Failure of concrete under any type of loading is associated with the development of visible cracks and very large inelastic deformations. These cracks are the precipitation of internal microcracks under high strains. The concrete system consists of microcracks even before the application of load. These microcracks propagate under different types of loading to form failure planes with a resulting loss in structural strength leading to failure. This paper discusses the formation of microcracks in concrete under short-term and sustained loadings. Concrete specimens were subjected to constant and incremental sustained loading and the deformations observed are related to the intensity of microcracking. The results show that at equal strains, the degree of internal microcracking is considerably reduced if the sustained load is applied incrementally over a period of time rather than applied as a step. It is concluded that the development of combined bond and mortar cracks is essential to cause failure under sustained loading. The longer the time to failure the higher the intensity of internal cracking at failure.     

Assessment of durability of recycled aggregate concrete produced by two-stage mixing approach

As more than 50% construction and demolition (C&D) wastes are composed of concrete debris in Hong Kong, recycling this debris into Recycled Aggregate (RA) for production of Recycled Aggregate Concrete (RAC) is an efficient way to alleviate the burden on landfill areas. Since RA is generated from concrete debris which has undergone years of services, the resulting RAC bears the weaknesses of lower density, higher water absorption, and higher porosity that limit them to lower-grade applications. Pinpointing to these weaknesses, Tam et al. [2005, Cement Concrete Res 35(6):1195–1203] developed the Two-Stage Mixing Approach (TSMA) for improving the strength of RAC, leading to the possibility in applying RAC for higher-grade applications. While the improvement in strength by TSMA has been proven in Tam et al.’s work [2005, Cement Concrete Res 35(6):1195–1203], the durability, in terms of deformation (shrinkage and creep) and permeability (water, air and chloride permeability), remains to be verified. In this paper, 0%, 20% and 100% of RA substitutions have been experimented to compare the durability performance of the Normal Mixing Approach (NMA) and the TSMA. Experiment results highlight that: (i) the higher the substitutions of RA, the weaker the performance of RAC; and (ii) the deformation and permeability of RAC can be enhanced when adopting TSMA. Therefore, it demonstrates that TSMA can help to improve the durability of RAC, on top of the previously verified strength improvement, and thus opening up wider applications of RAC.     

X-ray tomographic observations of microcracking patterns in fibre-reinforced mortar during tension stiffening tests

For the last decades, new reparation or fabrication processes have been studied to replace traditional rebar by roving of different mineral or organic fibres to avoid corrosion issues. Such materials refer to the family of cementitious composite. Their tensile strength would directly depend on the proportion of reinforcement and strongly on the interfacial mechanical properties between fibres and cementitious matrix. From now, evaluation of interfacial properties was mostly limited to the use of force–displacement curves obtained from mechanical experiments. This work presents a new methodology using micromechanical tension stiffening tests combined with X-ray computed tomography (XRCT) observations, performed at the Anatomix beamline at Synchrotron SOLEIL, and specific image processing procedures. Multi-XRCT acquisitions with suitable scanning strategy are used to image the whole fibre-matrix interface along centimetric samples at four to five different levels of loading magnitude. Intensive image processing is then performed on tomographic images including digital volume correlation (DVC), image subtraction and Hessian-based filtering. This experiment allows to study damage mechanisms at small scale. The proposed methodology shows great potential to provide both qualitative and quantitative elements on interfacial mechanical behaviour such as crack growth and crack orientation. The interface between mortar and sufficiently small multi-fibre yarn used in this paper is shown to behave in certain condition as traditional rebar interface producing conical cracks in the surrounding matrix rather than debonding in mode 2, permitting a much higher energy dissipation during debonding. According to this study, conical cracks repartition and geometry are mostly influenced by the cementitious matrix. The spacing between cracks goes from 50 to 100 μm, and the angle between crack normal vector and yarn orientation goes from 35° to 50°.     

骨料级配对二维模型混凝土单轴抗拉强度影响的理论研究

混凝土尺寸效应及其宏观力学非线性根源于其材料细观组成的非均质性。结合混凝土细观结构形式,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的复合材料。采用双线性弹性损伤模型来描述砂浆基质及界面过渡区的力学行为,假定骨料颗粒为弹性体而不发生破坏,进而推导并获得了单轴拉伸条件下不同骨料颗粒级配混凝土断裂裂缝扩展路径长度及其抗拉强度的理论解。最后,对比了建立的理论公式结果与细观尺度数值模拟结果,验证了构建的关于裂缝长度及抗拉强度理论解的准确性和合理性。     

A meso-mechanical model for concrete under dynamic tensile and compressive loading

We present a computational model, which combines interface debonding and frictional contact, in order to investigate the response of concrete specimens subjected to dynamic tensile and compressive loading. Concrete is modeled using a meso-mechanical approach in which aggregates and mortar are represented explicitly, thus allowing all material parameters to be physically identified. The material phases are considered to behave elastically, while initiation, coalescence and propagation of cracks are modeled by dynamically inserted cohesive elements. The impenetrability condition is enforced by a contact algorithm that resorts to the classical law of Coulomb friction. We show that the proposed model is able to capture the general increase in strength with increasing rate of loading and the tension/compression asymmetry. Moreover, we simulate compression with lateral confinement showing that the model reproduces the increase in peak strength with increasing confinement level. We also quantify the increase in the ratio between dissipated frictional energy and dissipated fracture energy as the confining pressure is augmented. Our results demonstrate the fundamental importance of capturing frictional mechanisms, which appear to dissipate a similar amount of energy when compared to cracking under compressive loading.     

考虑过渡区界面影响的混凝土宏观力学性质研究

混凝土材料的宏观力学特性及破坏机理由其细观组分来决定,界面过渡区是影响混凝土断裂破坏路径及宏观力学特性的重要因素。认为界面过渡区是区别于远处砂浆基质的一层含较高孔隙率的近场砂浆材料,采用“两步等效法”得到了混凝土细观单元的等效本构关系模型。最后基于细观单元等效化方法分析了在单轴拉伸、单轴压缩及弯拉载荷条件下混凝土试件的破坏过程及宏观力学性质,探讨了界面过渡区对混凝土力学特性的影响,并与随机骨料模型分析结果进行了对比。结果表明:界面相的存在对混凝土的弹性模量、强度及残余强度等力学性质有很大影响,在对混凝土宏观力学特性及细观断裂破坏过程进行研究时不可忽略其影响。     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

2D-SiC/SiC复合材料拉伸加卸载行为

为了研究国产2D-SiC/SiC复合材料的拉伸损伤行为以及低周循环载荷作用下的力学性能,通过试验和建立加卸载细观力学模型,对其拉伸加卸载行为进行了探讨。建立了单向连续纤维增强陶瓷基复合材料加卸载细观力学模型,得到了初始加载、卸载和重新加载时的应力-应变关系;利用断裂统计方法得到了基体裂纹数随应力变化的关系和复合材料失效判断条件。经过应力转化,将该模型应用于国产二维编织SiC/SiC复合材料。对单向加载试件,采用正交试验方法和最小二乘法得到基体Weibull模量和界面剪切阻力,通过控制材料失效强度与试验结果一致,得到纤维Weibull模量。由上述参数确定的2D-SiC/SiC复合材料拉伸循环加卸载应力-应变曲线与实测曲线吻合很好。通过Matlab编程得到2D-SiC/SiC复合材料单向加载时基体开裂过程图。结果表明,2D-SiC/SiC复合材料失效时,基体裂纹分布相对比较均匀;基体裂纹数随应力单调增加,未出现持平段,表明材料失效时,基体裂纹还没有达到饱和。     

再生粗骨料硅烷浸渍处理对混凝土介质传输性能的影响

由于残余砂浆的存在,再生粗骨料的物理力学指标远不及天然骨料,致使再生混凝土力学和耐久性能较差;此外,水分及有害离子侵入混凝土内部是引起混凝土材料性能劣化的主要原因。本试验用质量分数为8wt%的硅烷乳液浸渍强化再生粗骨料,通过抗压强度、毛细吸水和抗氯离子侵蚀试验对硅烷浸渍前后不同骨料质量取代率(0%、30%、50%)的再生混凝土介质传输性能进行了研究,最后利用SEM对再生混凝土内部的微观结构进行分析。试验结果表明,硅烷浸渍处理再生粗骨料的吸水率显著降低,由其制备的混凝土强度稍有所下降;再生混凝土毛细累积吸水量明显减少,且抗氯盐侵蚀性能显著提高,其中骨料质量取代率为50%的再生混凝土浸渍处理后氯离子扩散系数降低了37.5%。研究表明,硅烷浸渍处理再生粗骨料是提高再生混凝土耐久性的有效途径。