Some characteristics of high strength fiber reinforced lightweight aggregate concrete

The effect of polypropylene and steel fibers on high strength lightweight aggregate concrete is investigated. Sintered fly ash aggregates were used in the lightweight concrete; the fines were partially replaced by fly ash. The effects on compressive strength, indirect tensile strength, modulus of rupture, modulus of elasticity, stress–strain relationship and compression toughness are reported. Compared to plain sintered fly ash lightweight aggregate concrete, polypropylene fiber addition at 0.56% by volume of the concrete, caused a 90% increase in the indirect tensile strength and a 20% increase in the modulus of rupture. Polypropylene fiber addition did not significantly affect the other mechanical properties that were investigated. Steel fibers at 1.7% by volume of the concrete caused an increase in the indirect tensile strength by about 118% and an increase in the modulus of rupture by about 80%. Steel fiber reinforcement also caused a small decrease in the modulus of elasticity and changed the shape of the stress–strain relationship to become more curvilinear. A large increase in the compression toughness was recorded. This indicated a significant gain in ductility when steel fiber reinforcement is used.     

Effect of aggregate properties on the strength and stiffness of lightweight concrete

An experimental program was carried out to obtain the compressive strengths and elastic moduli of cold-bonded pelletized lightweight aggregate concretes. Three types of aggregates were made with different fly ash contents. Experimental data were analyzed statistically. Test results of multivariate analysis of variance (MANOVA) with 95% confidence level (α=0.05) show that the properties of lightweight aggregates and the water/binder ratio are two significant factors affecting the compressive strength and elastic modulus of concrete.     

Interdependence of microstructure and strength of structural lightweight aggregate concrete

The demand for lightweight concrete is steadily increasing because of economic and practical considerations. Hence, the inherent internal and external features of lightweight aggregates have been a subject of intense research in recent years. This study provides new insight into the micro-structural and chemical factors which influence the strength properties of structural lightweight aggregate concrete. These are described with respect to four expanded clay lightweight aggregates used in nine concrete compositions containing various types and proportions of dispersing agents such as water-reducing admixtures and superplasticizers, with silica fume and ground granulated blast-furnace slag as optional mineral admixtures. The microstructural characteristics of the paste-aggregate interface and the paste porosity of these concretes are discussed. The methods used include scanning electron microscopy-energy-dispersive X-ray analysis, X-ray diffraction analysis, optical microscopy and compressive strength testing.     

Effect of very fine aggregate on concrete strength

The effect of very fine aggregate on the water requirement and strength evolution of superplasticized concrete was studied. The maximum aggregate size was 6 mm, fine aggregate content 0–15% of the total aggregate, binder (OPC and silica) content 80–800 kg m⁻³, and specific surface area of the finest aggregate material 18–1000 m² kg⁻¹. The water requirement for constant workability fell sharply with increasing amount and fineness of the very fine aggregate due to improved particle packing. At constant, relatively small binder contents of 80–250 kg m⁻³, modifying the aggregate distribution increased the 91-day strength of 6 mm concrete as much as threefold, e.g. from 20 to 65 MPa at 200 kg m⁻³. This effect was less significant the greater the binder content. The correlation between strength and porosity was similar regardless of concrete composition. The stress-strain behaviour was more dependent on strength than on composition

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Resume On a étudié l'effet d'un granulat très fin sur le besion en eau et l'évolution de la résistance d'un béton traité aux superplastifiants. La taille maximale du granulat était de 6 mm, la teneur en granulat fin de 0 à 15% de la quantité de granulat totale, la teneur en liant (OPC et silice) de 80–800 kg m⁻³ et la surface spécifique du plus fin granulat de 18–1000 m² kg⁻¹. Le besoin en eau pour une ouvrabilité constante a nettement diminué lorsque la quantité et la finesse du granulat ont augmenté en raison de l'amélioration de la compacité des particules. Pour des teneurs en liant constantes, relativement faibles, de 80–250 kg m⁻³, en modifiant la distribution du granulat, la résistance à 91 jours du mélange de béton de 6 mm s'est accrue jusqu'à trois fois, soit de 20–65 MPa pour 200 kg m⁻³. Plus la teneur en liant était élevée, moins cet effect a été significatif. La corrélation entre la résistance et la porosité ne dépend pas de la composition du béton. L'état du béton sous contrainte/déformation dépend davantage de la résistance que de la composition.

     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

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.     

预湿轻集料对混凝土内部相对湿度特性的影响

为缓解混凝土的早期自干燥,减小自收缩,用预湿轻集料对其内部相对湿度进行调控.采用湿度测试仪和混凝土收缩测试仪,研究了水胶比、矿物掺合料对混凝土内部相对湿度与自收缩的影响规律,及预湿轻集料对自收缩的抑制效果.研究表明:混凝土内部相对湿度早期下降快,后期下降慢;降低水胶比、掺加硅灰会加快内部相对湿度的下降速度,而粉煤灰的加入可延缓低水胶比混凝土早期内部相对湿度的下降过程.预湿轻集料的引入可对混凝土内部相对湿度起到补偿作用,减小自收缩,作用效果随轻集料引入水量的增大而增强.     

Lightweight geopolymer concrete containing aggregate from recycle lightweight block

In this research, the properties of lightweight geopolymer concrete containing aggregate from recycle lightweight block were studied. The recycle block was crushed and classified as fine, medium and coarse aggregates. The compressive strength and density with various liquid alkaline/ash ratios, sodium silicate/NaOH ratios, NaOH concentrations, aggregate/ash ratios and curing temperatures were tested. In addition, porosity, water absorption, and modulus of elasticity were determined. Results showed that the lightweight geopolymer blocks with satisfactory strength and density could be made. The 28-day compressive strength of 1.0–16.0 MPa, density of 860–1400 kg/m³, water absorption of 10–31% and porosity of 12–34%, and modulus of elasticity of 2.9–9.9 GPa were obtained. It can be used as lightweight geopolymer concrete for wall and partition.     

Non-steady-state accelerated chloride penetration resistance of structural lightweight aggregate concrete

This study aims to characterise the chloride penetration resistance of structural lightweight aggregate concrete (LWAC) produced with different types, volumes and initial wetting conditions of lightweight aggregates (LWA), types of cement and contents of fly ash and silica fume, w/c ratios and curing conditions. A comprehensive experimental study was carried out involving three types of non-steady-state tests, which simulate different exposure conditions and penetration mechanisms. It is shown that the chloride penetration resistance is mainly affected by the cementitious paste and that high performance LWAC of 30–70 MPa can be produced. Regardless of the type of aggregate, we propose exponential relations to estimate the diffusion coefficient of chlorides. The volume and initial wetting condition of LWA had little influence on the chloride resistance. A long-term higher reduction of the diffusion coefficient was found in less dense LWAC. Reasonable correlations between the non-steady-state tests were obtained. Contrary to what is suggested in some European standards, the concrete strength cannot properly predict the durability behaviour of LWAC.     

自密实轻集料混凝土双轴受力力学性能

为探究自密实轻集料混凝土双轴力学性能,采用三轴试验机对其进行双轴压-压和双轴拉-压试验,得到不同工况下的应力-应变曲线及其破坏形态,通过提取应力-应变曲线峰值应力和峰值应变,并与相关文献普通混凝土与轻集料混凝土研究成果对比,分析自密实轻集料混凝土双轴力学性能。研究结果表明:双轴压-压工况下,当侧向压应力较低时,试件主要呈现剪切破坏形态;当侧向应力较高时,试件呈劈裂破坏形态。双轴拉-压工况下,试件主要呈劈拉破坏形态,与侧向应力无关。随着侧向压应力的提高,自密实轻集料混凝土主压应力相对比无侧向应力工况明显提高,峰值应力最大提高均值幅度为68.08%,主拉应力随侧向压应力的提高逐步降低,最大降低幅度为62.35%。应用Kupfer双轴受力破坏准则验证自密实轻集料混凝土受侧向应力影响变化规律较为保守,同时基于Kupfer提出自密实轻集料混凝土双轴力学性能破坏准则,所得到的破坏准则方程具有良好的适用性。      

Engineering properties of lightweight aggregate concrete made from dredged silt

Silt dredged from reservoirs can be hydrated and sintered into lightweight aggregate for producing lightweight aggregate concrete (LWAC). The densified mixture design algorithm (DMDA) was employed to manufacture LWAC using 150 kg/m³ of water at different water-to-binder ratios (w/b = 0.28, 0.32 and 0.4) using lightweight aggregates of different particle densities (800, 1100 and 1500 kg/m³). The engineering properties of the LWAC thus obtained were examined. Results show that the fresh concrete meets the design requirement of having slump of 250 ± 20 mm and slump flow of 600 ± 100 mm. With respect to hardened properties, the compressive strength, ultrasonic pulse velocity and thermal conductivity were found to decrease with increasing w/b ratio but increase with increasing aggregate density. Moreover, higher aggregate density also resulted in less shrinkage. The surface resistivity exceeding 20 kΩ-cm also matched the design objective. The experimental results prove that LWAC made from dredged silt can help enhance durability of concrete.     

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.     

Effect of moist curing and use of lightweight sand on characteristics of high-performance concrete

Factorial design approach was undertaken to determine the effect of initial moist-curing duration (0 and 6 days) on properties of high-performance concrete (HPC) made with different water-to-cementitious materials ratios (w/cm) of 0.30 and 0.40 and sand substitution rates with lightweight sand (LWS) of 0 and 30 %, by volume. Mechanical properties of HPC are shown to be mainly affected by w/cm and total shrinkage of concrete by the LWS replacement rate and initial moist-curing period. It should be noted that concrete made with 30 % LWS replacement that did not receive any moist-curing exhibited lower shrinkage than moist-cured concrete prepared without any LWS, regardless of the w/cm. Combined use of 30 % LWS and 7 days of moist curing can lead to greater increase in compressive strength and larger decrease in total shrinkage compared to the use of 30 % LWS without moist curing or 7 days of moist curing without LWS. The use of LWS, however, is more practical and efficient to reduce shrinkage of HPC for deep concrete elements.     

A simple mix design method for structural lightweight aggregate concrete

Current design methods for structural lightweight aggregate concrete (SLWAC) are usually only valid for a limited range of concrete compositions that have previously been subjected to trial tests. The SLWAC mix design is more complex than that of normal weight concrete as more parameters need to be determined. Taking this into account, a simplified design method is proposed for SLWAC made with natural sand. The major advantages of the proposed method are that it is easy to apply and it can be generalized to any type of lightweight aggregate (LWA). For this, three additional design parameters are needed: the strength of LWA in concrete; the limit strength; the SLWAC potential strength. At most, two experimental mixtures are needed to determine these parameters. A biphasic model to estimate the strength of SLWAC is evaluated and high correlations are obtained. The good performance of the suggested method is demonstrated by examples of practical application and by the comparison with experimental results reported by the authors and other investigators.     

纳米SiO2改性轻骨料混凝土性能

纳米SiO₂（NS）具有极强火山灰活性、晶核作用和填充效应,因此用NS改善水泥基材料性能成为众多学者研究的热点。本课题对不同掺量的NS对轻骨料混凝土强度及耐久性的改性效果进行了研究。通过测试轻骨料混凝土的力学性能（抗压和抗折）和氯离子渗透性能及利用SEM和EDS测试分析了NS对混凝土宏观和微观结构的影响。研究结果表明:在适当的掺量下,NS能够有效地提高轻骨料混凝土的力学性能,其中28 d的抗压强度和抗折强度比空白组混凝土分别提高了21.6%和46.2%。氯离子渗透的结果表明,轻骨料混凝土的抗氯离子渗透性能随着掺量的增加而呈线性增强。混凝土界面过渡区（ITZ）也发生了显著变化,其厚度减小,形貌也更加致密。ITZ的钙硅比随着NS掺量增加而减小,说明该区域内水化产物C-S-H凝胶增多,Ca（OH）₂被消耗,从而形成致密的过渡区,有利于强度提高。      

自密实轻骨料混凝土压-剪复合受力力学性能

为探究自密实轻骨料混凝土压-剪复合受力力学性能,应用液压伺服机和材料压-剪试验机,对自密实轻骨料混凝土进行单轴受压、单轴劈裂抗拉和压-剪复合受力试验研究,通过试验得到不同加载工况下自密实轻骨料混凝土破坏形态和力-变形曲线,引用文献对普通混凝土和轻骨料混凝土压-剪复合受力研究数据,对比分析自密实轻骨料混凝土压-剪复合受力性能。研究结果表明:自密实轻骨料混凝土压-剪复合受力破坏形态与普通混凝土和轻骨料混凝土相类似,随着轴压比的提高,剪切破坏断面摩擦痕迹逐步明显,混凝土碎渣也逐步提高,自密实轻骨料混凝土剪切破坏强度、残余荷载和剪切破坏位移也随之提高;剪切破坏强度提高幅度高于普通混凝土和轻骨料混凝土,残余荷载受轴压比影响提高幅度高于普通混凝土,但略低于轻骨料混凝土。基于主应力空间结合普通混凝土和轻骨料混凝土压-剪试验数据,提出混凝土压-剪复合受力统一破坏准则,同时基于八面体应力空间,提出自密实轻骨料混凝土破坏准则,所提出的破坏准则具有良好的适用性。