Volume Fraction Of Coarse Aggregate Research Articles

Mesoscopic Finite Element Method of the Effective Thermal Conductivity of Concrete with Arbitrary Gradation

The effective thermal conductivity (ETC) of concrete is the most important parameter in determining the temperature field and thermal stresses. A 2D random polygonal aggregate model and its modified model considering porosity were established in this paper in order to partially replace the experiment for parametric analysis on the ETC of concrete and to save the experiment cost. A mesoscopic finite element method for the ETC of concrete with arbitrary gradation was also proposed. In addition, the influence factors (thermal conductivity of coarse aggregate, cement mortar, and volume fraction of coarse aggregate) of the effective thermal conductivity of concrete were analyzed. The results show that the 2D gradation curve of coarse aggregates is proved to exist, and there is a corresponding relationship between the 2D and 3D gradation curves of coarse aggregates. The effective thermal conductivity of concrete has a positive exponential relationship with the volume fraction of coarse aggregates, a positive logarithm relationship with the thermal conductivity of coarse aggregates, and a positive linear correlation with the thermal conductivity of cement mortar. The most practical way to improve the effective thermal conductivity of concrete is to increase the ETC of the cement mortar, but the most effective way is to replace the aggregate with a material with a high thermal conductivity.

Comparisons of Tensile Fracturing Behaviors of Hydraulic Fully Graded and Wet‐Screened Concretes: A Mesoscale Study

The widely used wet‐screening method in the experimental testing of hydraulic fully graded concrete inevitably results in a gap between the real mechanical parameters of hydraulic fully graded concrete specimens and those of the corresponding wet‐screened specimens and therefore necessitates the comparative study on their mechanical behaviors. To this end, a two‐dimensional mesoscale modeling methodology is developed for simulating the tensile fracturing behaviors of hydraulic fully graded and wet‐screened concretes, and extensive Monte Carlo simulations are performed. The individual effects of specimen size variation, variation of gradation and volume fraction of coarse aggregates, and the weaker interfacial transition zones surrounding the large coarse aggregates to be removed by wet‐screening are detailed followed by the discussion on the combined effect of these three main factors. All the mean values of the macroscopic mechanical parameters related to tensile fracturing behaviors are found to show significant change in response to wet‐screening, and the underlying differentiation mechanism and governing factor(s) are identified. Furthermore, it is shown that the randomness of the investigated parameters can be roughly described by the Gaussian distribution, and the dispersion of each of the investigated parameters of hydraulic fully graded concrete is higher than that of the corresponding wet‐screened concrete.

Meso-fracture simulation of cracking process in concrete incorporating three-phase characteristics by peridynamic method

A mesoscopic peridynamic (PD) model is proposed for meso-fracture simulation of cracking process in concrete. In the novel procedure of implementing PD in the traditional framework of the finite element method, truss elements with appropriate stiffness properties are used to represent the PD bonds. Damage is involved by using a subroutine to reduce material parameters of the failed truss elements. The generate-and-place procedure is utilized simultaneously to generate the meso-structures consisting of cement matrix, aggregates and the interfacial transition zone, and consequently the typical three-phase characteristics of concrete are incorporated in the proposed PD model. Several benchmark numerical examples are performed to test both the accuracy and efficiency of the developed model in analysis of cracking process in concrete. These simulated results are consistent with the experimental data and observations. Additionally, these examples clearly demonstrate that the increasing number of grading segments tends to enlarge the tortuosity of crack propagation path in concrete, primarily because of its heterogeneity of the meso-structure. The results also show that the peak loads of concrete specimens with same dimension decrease with the increasing volume fraction of coarse aggregates.

Experimental Study on Functional Graded Concrete

The ultimate bearing capacity of thick walled cylinder can be improved significantly by using the concrete functional graded material. The functional graded characteristic of concrete is achieved by the certain variations of the compressive strength and elastic modulus. In order to make use of the functional graded characteristic of concrete, orthogonal experiment are designed in a L16(45) orthogonal array with four factors, namely, “different addition of air-entraining agent (AE)”, “volume fraction of coarse aggregate (VA)”, “maximum size of the coarse aggregate (MS)”, and “water/blinder material ratio (W/B)” and three experimental indexes, namely, compressive strength, elastic modulus and density. The experimental results are analyzed by using the intuitive analysis and analysis of variance (ANOVA). The results show that the addition of air-entraining agent may cause the decrease of the compressive strength, elastic modulus and apparent density of concrete, but it has no significant impacts on the three indexes. The elastic modulus and apparent density are affected significantly by the volume fraction of coarse aggregate and the two indexes increase with the increase of the volume fraction of coarse aggregate. The maximum size of the coarse aggregate has no significant effect on three indexes. The compressive strength, elastic modulus and apparent density of concrete decrease significantly as the water/ blinder ratio increase. The experimental results are used in the model test of thick-walled hollow cylinder. We find that the ultimate bearing capacity of functionally graded two-layered hollow cylinder is greater than that of the conventional thick-walled hollow cylinder.

Study on the Effect of Coarse Aggregate Volume Fraction on the Flow Behavior of Fresh Concrete via DEM

The coarse aggregate is the framework of the fresh concrete and thus plays an important role in its flow behavior. A discrete element method (DEM) model for the fresh concrete was established and validated by a concrete rheometer test with the contact parameters verified. Firstly, the model was used to study the effect of coarse aggregate volume fraction on the concrete yield stress. Then based on the DEM model, the pumping process of the fresh concrete was simulated in which the effect of the coarse aggregate volume fraction on the pumping pressure and wear on the wall were addressed. It is shown that both the pumping pressure and the wear degree increase with the coarse aggregate volume fraction.

Scattering-Filling Stone Concrete: Mechanism, Experiment and Utilization

The coarse aggregate volume fraction in conventional plastic concrete is controlled relatively low to ensure a required workability, but its low aggregate content play negative roll on the service properties of the concrete. In this paper, we revealed the mechanism of a coarse aggregate interlocking concrete prepared by Scattering-Filling Stone Concrete (SFSC), it can keep the fresh concrete its workability but increase its service performance by scattering coarse aggregate during the concrete casting, the experimental and engineering application of SFSC indicated that: the strength of concrete prepared with this method increases obviously whereas the shrinkage decreases significantly when 20% cement dosage is saved. The SFS process was utilized in a highway pavement, the core of the concrete is very dense and no obvious defect could be found when 8% pavement concrete was replaced by coarse aggregate. The SFSC is a type of low carbon concrete.

Mechanical Behavior Analysis of Asphalt Mixture in Meso-Scale Considering Locally Effective Material

Abstract Both conventional asphalt mix design and current pavement structure design method, known as macro research techniques, cannot take all micro–meso-parameters into consideration. The relationship between meso-structural information and macroscopic mechanical responses was established by using locally effective material method. First, elastic properties of asphalt mortar were obtained based on the resilient modulus test, and an appropriate moving window size was used to calculate the coarse aggregate volume fraction, which was treated as a locally homogenous distribution. Additionally, accounting for the generation of homogenous materials, a locally effective material method was implemented by a mechanical approach method for different asphalt mixtures, and then effective models for finite-element method (FEM) were established. Finally, effective models were calibrated within a small displacement domain based on the digital image correlation monitoring test, and then the FEM results illustrated the different mechanical responses, which show a more reasonable distribution than original models. In conclusion, although effective FEM model are locally equivalent materials, this approach is an effective method without lacking of the ability to figure out these meso-structural differences among various asphalt mixtures.

Experiment and Modeling of the Effect of Aggregate Shape on the Chloride Diffusivity of Concrete

The purpose of this paper is to evaluate the effect of aggregate shape on the chloride diffusivity of concrete through laboratory experiments and theoretical analysis. In the experiment, three types of crystal glass spheroids with aspect ratios of 1, 2, and 3 are used as coarse aggregate. The experimental results show that the chloride diffusivity of concrete decreases with an increase in aggregate aspect ratio. When the coarse aggregate volume fraction is equal to 0.1, 0.2, and 0.3, the chloride diffusivity of concrete at an aggregate aspect ratio of 3 is, on average, smaller than that at an aggregate aspect ratio of 1 by 5.29%, 11.4%, and 19.6%, respectively. An analytical approximate is derived for the chloride diffusivity of concrete with spheroidal aggregates and verified with the experimental results. The analytical approximate can predict the chloride diffusivity with an average relative error of less than 8%.

순환굵은골재, 황토, 고로슬래그 미분말 및 마섬유를 사용한 레인가든 구조물 콘크리트의 최적배합설계 및 역학적 특성

In this study, the optimum mix proportions of rain garden structure concrete were decided and the mechanical properties were evaluated. Experimental parameters were blast furnace slag, hwang-toh, recycled aggregates and natural jute fibers. The target compressive strength and chloride ion penetration were more than 24 MPa and less than 1000 coulombs, respectively. The response surface method was used for statistical optimization of experimental results. The optimal mixing ratios of the blast furnace slag, hwang-toh, recycled coarse aggregate and jute fiber volume fraction were determined 59.98 %, 8.74 %, 12.12 % and 0.2 %, respectively. The compressive strength, flexural strength and chloride ion penetration test results of optimum mix ratio showed that the 24.56 MPa, 3.88 MPa and 999.08 columbs, respectively.

Numerical Study of the Influence of Material Structure on Effective Thermal Conductivity of Concrete

A two-dimensional numerical model is presented to examine meso- and macroscopic structure effects on the effective thermal conductivity of concrete. The heterogeneity of concrete is considered at a mesoscopic level by Weibull distribution assumption. Simulations on several heterogeneous samples show that the effective thermal conductivity strongly depends on the degree of heterogeneity. Higher homogeneity indicates a greater effect on the effective thermal conductivity. Numerically simulated results also indicate that the size and shape of individual coarse aggregate appear to have negligible influence on the effective thermal conductivity of concrete. However, that greatly depends on the thermal conductivity and volume fraction of coarse aggregate. Modeling suggests that heat conductivity decreases when there is a drop in strength due to the damages creating a thermal barrier across the cracks and thus preventing heat flow through the matrix, that is, resulting in reduction of effective thermal conductivity. Moreover, the formation of cracks in the interfacial transition zone also leads to significant reduction of heat flow in coarse aggregates.

Effect of coarse aggregate characteristics on concrete rheology

In the present study, concrete was considered as a two-phase material, consisting of coarse aggregate (CA) and mortar. Coarse aggregate properties were characterized by fineness, uncompacted void and friction angle. The combined effects of CA characteristics and mix design on the rheological properties of the corresponding concrete were investigated using a portable IBB concrete rheometer. Experimental results indicated that a higher CA and fine aggregate content normally result in higher concrete rheological parameters (yield stress and viscosity). For a given type and amount of mortar, concrete yield stress and viscosity generally increase with the uncompacted void content and friction angle but decreased with the size (or fineness) of CA. Well graded CA, generally having low uncompacted void content, provides concrete with considerably reduced yield stress and viscosity when compared with single-sized CA. In addition, a multiple-parameter linear regression analysis was conducted to evaluate how different CA characteristics (fineness, uncompacted void and friction angle) and mix design parameters (mortar composition, and CA volume fraction) affect concrete rheological behavior.

Modeling Corrosion Initiation of Reinforcing Steel in Concrete: Effect of Non-diffusive Coarse Aggregate

The exposure time at which a threshold Cl— concentration, Ct, is initially achieved at the reinforcement depth in concrete, Ti, is an important service life determinant for reinforced concrete structures. However, there is no consensus in the literature of a value for Ct, as reported concentrations extend over one order of magnitude; and it has become increasingly apparent that Ct is distributed even for apparently identical specimens. In this research, models of randomly arranged coarse aggregate (CA) particles in concrete cover were generated for different aggregate types (particle size and size distribution), CA volume fraction, and cover; and, by finite element analysis, the effect of these factors on the time and location along the reinforcement at which Ct is initially achieved was investigated as a function of Ct. The results are discussed within the context of factors that contribute to the reported variability in Ct.

Investigation on the scattering-filling coarse aggregate self-consolidating concrete

To secure good flowability and workability of SCC, the volume fraction of coarse aggregate keep at an extremely low level. A new kind of SCC pouring method named scattering-filling coarse aggregate process was invented: it was method to scatter 20% (volume fraction to the finished concrete) of extra coarse aggregate into the fresh SCC mixture to replace the fresh concrete mixture while the concrete was pouring. A high strength (82 MPa) SCC just composing 360 kg/m3 cement and 120 kg/m3 class F fly ash was prepared with this process. With an increase of the extra coarse aggregate replacing ratio from 0 to 30%, the compressive strength of SCC increased steadily and reached a peak value when this ratio is 20%, then the strength dropped sharply. The drying shrinkage ratio and the chloride ion permeability decreased with the increase of that ratio. The scattering-filling coarse aggregate process can cast high strength SCC with lower cementitious materials content and produce concrete with better performance than the ordinary process.

Fresh and hardened properties of self‐consolidating concrete

AbstractSelf‐consolidating concrete, (SCC) is a new technology and the market share of its products is rapidly growing because of the economic opportunities and improvements of the quality of the concrete and the working environment. In view of the potential number of ingredients, SCC is a complex system that is usually proportioned with one or more mineral admixtures and one or more chemical admixtures. A key factor for a successful formulation is a clear understanding of the role of the various constituents in the mix and their effects on the fresh and hardened properties.The fundamental rheological properties of SCC are based on low yield stress, moderate viscosity and retention of the kinetic energy of the flowable mix by reducing the volume fraction of coarse aggregate. These measures are necessary for imparting the requested fluidity, segregation resistance and prevention of interparticle collision and blocking. A low yield stress is attained by incorporation of adequate contents of superplasticizer. The flow characteristics are further modified by changing the aggregate volume fraction, coarse‐to‐fine aggregate volume, and the composition of the other ingredients. The viscosity is controlled by the contents of the free water, superplasticizer, and the volume fraction of the solids in the mix. The two strategies for maintaining a moderate viscosity are based on either incorporation of high amounts of powder; i.e. total content of cementitious materials (referred to as powder type SCC) or addition of viscosity modifier agents (referred to as VMA type SCC).By analogy to the Powers and Brownyard gel–space ratio, the compressive strength can be approximated by calculating the binder‐space ratio. One of the main challenges is to impart the required rheological properties that allow desired flow and filling ability characteristics whilst retaining dynamic and static stability. With this respect, the density of the matrix and maximum size of aggregate cannot be overlooked as, within the range of practical attainable viscosities that allow a free‐flow, their effects on the resistance to segregation are as high or even greater than that of the effect of viscosity. Attention is also drawn to the higher shrinkage values of SCC, especially the inherent higher shrinkage associated with the powder type SCC.

Influence of aggregate content on the migration coefficient of concrete materials using electrochemical method

In order to investigate the effect of coarse aggregate content on the chloride ion migration coefficient of concrete, specimens with different coarse aggregate volume fractions and two water/cement (w/c) ratios of mortar matrix were used. The chloride ion migration coefficient of concrete is obtained using the electrochemical technique to accelerate chloride ion migration in cement-based material and the experimental results were plotted as a function of the fine aggregate volume fraction. The results are analyzed comparing experimental results and theoretical models that represent the concretes as three-phase composite materials. The three phases are the mortar matrix, the coarse aggregate, and the interfacial transition zone between the two. The chloride ion migration coefficient is used to assess the dilution, tortuosity, interfacial transition zone (ITZ) and, percolation effects of coarse aggregate in concrete. It appears that the dilution and tortuosity effects are a dominant factor affecting the chloride ion migration coefficient of concrete in the low volume fraction of coarse aggregate. As the volume fraction of coarse aggregate increases to 40 and 20% in concrete of w/c ratio 0.35 and 0.45, respectively, the ITZ with percolation effects are significantly.

Effect of aggregate volume fraction on the fracture parameters of concrete: a meso-mechanical approach

It is well known that fine and coarse aggregates play a vital role in the fracture of concrete. There is, however, little quantitative information available in the literature on the effect of the volume fraction of the aggregates on the fracture properties of concrete. The main objective of this work is to gain a better understanding of this effect using appropriate experiment and a meso-mechanical approach. For this, several control mixes ranging from hardened cement paste (HCP) to normal concrete were prepared in which the aggregate grading, w/c ratio, and the maximum particle size of aggregate were kept constant, but the volume fraction of aggregate was varied from 0·0 (HCP) to 0·60 in steps of 0·15. The critical stress intensity factor of the hardened cement paste, for which linear elastic fracture mechanics (LEFM) is valid, was determined from centrally notched disc specimens. The effective stress intensity factors of fine mortar, coarse mortar and concrete were then calculated from that of HCP using the appropriate toughening mechanism in meso-mechanical relations. The specific fracture energies, the macro-tensile strengths and the moduli of elasticity of fine mortar, coarse mortar and concrete were also calculated from meso-mechanical relationships. From these, the ductility of the mixes was calculated in terms of the characteristic length. It is shown how the meso-mechanical modelling approach gives a clear qualitative and quantitative picture of how a brittle matrix (i.e. HCP) progressively transforms into a tougher, stiffer and more ductile composite as the volume fraction of fine and coarse aggregates is increased without altering their grading.

Quantitative Description of Coarse Aggregate Volume Fraction Gradients

Abstract Within any cast cylinder of concrete, the coarse aggregate will tend to be inhomogeneously distributed. This variability may arise as a result of segregation caused by gravity or as a result of the wall effect that is caused by the inability of the aggregate to penetrate the walls of the mold. Using methods from image analysis, stereology, and statistics, local estimates of aggregate inhomogeniety are defined that quantify phenomena that have been qualitatively described in the past. These methods involve modification of the two-dimensional images to prepare them for analysis, as well as simple diagnostic statistics for determining the presence of a wall effect. While the techniques presented herein are developed specifically for cast cylinders, they can be generalized to other cast or cored concrete specimens.

Effect of coarse aggregate and [formula omitted] ratio on intrinsic permeability of concrete subject to drying

Intrinsic permeability of two types of concrete to nitrogen gas was tested; each had watercement ratios of 0.40 and 0.50 and a volume fraction of coarse aggregate of 0.41 ± 0.01. Two coarse aggregates of 10mm single size were used, one a crushed limestone of low porosity, the other a sintered fly ash of high porosity. Test specimens (100mm diameter by 50mm thick) were water cured, then dried, during which weight loss and permeability were measured. All concretes had low permeability but consistently higher weight loss and permeability were found in the lightweight aggregate concrete specimens, indicating that accessible pores became available fairly quickly, despite the good quality of mortar used. Such behaviour may result from too rapid drying of the lightweight concrete test specimens. For a more realistic indication of permeability it is recommended that testing should be done in a laboratory environment that would, ideally, simulate insitu conditions. For similar 28 day strength both types of concrete had comparable permeability.

圧裂引張試験におけるコンクリートの破壊機構に関連するパッドの影響

It is well known that the pads used in splitting tests have a great influence on the splitting tensile strength of concrete. This study was aimed at clarifying the effect of the padding materials on the splitting tensile strength of concrete in relation to the fracture mechanism of concrete by carrying out the experiments and the FEM analyses on both the actual and model concrete. The experimental results of the actual concrete showed that the influence of the padding materials on the splitting tensile strength depended on the types of fine and coarse aggregate, the volume fraction of coarse aggregate in concrete and the friction between pad and specimen. The FEM analyses showed that the stress distribution within the model concrete was not so much affected by the padding materials before cracking, while, after cracking, the padding materials influenced the initiation and development of cracks as well as the stress distribution of the model concrete. These facts suggest that the effects of the padding materials on the splitting tensile strength of concrete are closely related to the complicated structure of concrete.

セメント系複合材料の強度に関する確率論的研究 : 第 2 報 : モルタルおよびコンクリート強度の確率分布指標について

The results of the present study are summarized as follows : 1) The statistical distributions of the strength of cement-based composite materials can not be necessarily approximated by Weibull distribution, because the relation between ln (-ln(1-P)) and ln (F) can be generally expressed by a convex curve. Pareto distribution can hardly express the lower and the upper portions of statistical distribution curve. 2) The experimental results can be expressed more exactly by the proposed equation for the statistical distribution of strength than by previous equations, and the statistical properties of the strength of cement-based composite materials can be rationally explained by the proposed equations. 3) The value of β is affected by the type of loading, and the sizes of specimen and aggregate in macroscopic heterogeneous material, but is independent of the other factors. The value of β can be expressed by Eq. (11). 4) The value of parameter related to the quantity of defects in the mortar specimen (θ_m) increases with the increase of the sand-cement ratio (S/C), provided that the value of S/C is smaller than about 2, and can be expressed by Eq. (14). 5) The value of parameter related to the quantity of defects in the concrete specimen (θ_c) decreases with the increasing value of size of aggregate, the decreasing value of size of specimen and the increasing value of volume fraction of coarse aggregate, independently of the shape of specimen and the type of aggregate. These phenomena can be expressed by Eq. (16) and Eq. (17).