Aggregate Volume Fraction Research Articles

Hydraulic transport properties of unsaturated cementitious composites with spheroidal aggregates

Understanding of the coupled effects of aggregates and interfacial transition zones (ITZs) on hydraulic transport properties of unsaturated cementitious composites (CC) is of great importance for the design of CC durability. In this work, a comprehensive micromechanical theoretical framework is devised for the accurate predictions of the effective hydraulic conductivity, diffusivity and sorptivity of saturated and unsaturated CC, in which CC are regarded as a three-phase structure consisting of spheroidal aggregates with a certain gradation, their surrounding permeable ITZs with a specific thickness, and homogeneous cement paste at a mesoscopic level. Comparisons against experimental, numerical and analytical data suggest that the proposed framework is a reliable means to predict these effective hydraulic transport properties. The micromechanical framework can be regarded as a generic model that suits for predicting other hydraulic-like transport properties of multiphase composites not limited to cementitious composites involving spheroidal inhomogeneities and their surrounding interphase. This framework sheds light on the coupled effects of properties of aggregates (shape, gradation, mean size, and volume fraction) and ITZ (volume fraction, thickness, and hydraulic properties) on the effective hydraulic transport properties of CC. The results elucidate the complex relationship of components-structures-transport properties, which in turn can provide insights for understanding degradation of concrete in practical applications.

Mesomechanical creep model of fly ash-contained cement mortar considering the interfacial transition zone and its influential factors

A mesomechanical model was proposed to predict the creep behavior of cement mortar containing fly ash, with the Laplace transform principle applied. In this model, the mortar was regarded as a composite material, which is composed of the elastic fine aggregate phase, the viscoelastic cementitious paste phase, as well as the viscoelastic Interface transition zone (ITZ) phase between them. The generalized self-consistent Mori-Tanaka model in the Laplace transform domain was used as a base to establish the prediction creep model of mortar. The results showed that the proposed creep model of mortar considering the ITZ properly simulated the mortar creep behavior, with the coefficient of variation between the modeled and measured values locating in the range between 3% and 5%. Parameter analysis based on the grey relational analysis theory was conducted to clarify the key factors in this proposed model. Regarding the sensitivity of mortar creep to the factors, the decreasing order was as follows: ITZ viscoelastic parameter > ITZ thickness > fine aggregate volume fraction > the fineness of aggregate.

Estimation of constituent properties of concrete materials with an artificial neural network based method

Multi-scale models are developed for heterogeneous concrete materials to estimate their macroscopic mechanical properties in terms of micro-structural data. One crucial challenge of those models is the identification of local properties of constituent phases. In this paper, we present an efficient method based on Artificial Neural Networks (ANN). Typical concrete materials are taken as example. A macroscopic analytical strength criterion is established from three steps of nonlinear homogenization procedure. The macroscopic strength of materials is determined as a function of the frictional coefficient and cohesion of solid cement particles at nanometer scale, intra-particle pores, inter-particle pores and aggregates (inclusions). The objective is to identify the nanoscopic frictional coefficient and cohesion of cement particle from measured macroscopic values of uniaxial compression and tensile strengths. For this purpose, a numerical method based on the ANN is developed. With the analytical macroscopic strength criterion, sensitivity studies are first realized to identify the most important micro-structural parameters influencing the macroscopic strength of concrete. A simplified analytical macroscopic strength criterion is then proposed. A large dataset is further constructed through the inversion of the analytical strength criterion by using the aggregates volume fraction, porosity, macroscopic uniaxial tensile and compressive strengths as input variables and the frictional coefficient and cohesion of cement particles as output unknowns. An ANN model containing four hidden layers and 100 neurons in each layer is constructed and trained by using this dataset. Various types of validation of the ANN model are performed. It is found that the proposed ANN based model can effectively predict the frictional coefficient and cohesion of porous cement paste at the microscopic scale with a very good accuracy.

Phase field modeling scheme with mesostructure for crack propagation in concrete composite

Tracking the crack propagation in concrete at the mesoscopic level is of great importance for revealing the crack pattern and failure mechanism, due to the huge threat of cracking to structure safety. This paper develops a phase field modeling scheme with mesostructure of concrete composite to investigate its crack propagation behavior. With the classical phase field model and the staggered algorithm implementation by three-dimensional (3D) finite element method (FEM), crack propagation within concrete’s mesostructure composed of coarse aggregate, mortar matrix, interfacial transition zone (ITZ) and initial defects including micropores and microcracks is modeled. The proposed modeling scheme is tested on single-aggregate samples and complex mesostructured concrete samples subjected to uniaxial loading, showing reasonable accordance with the experimental observation and common failure modes of concrete material. The results indicate that mesostructural configurations are important factors affecting the crack pattern of concrete. The presence of coarse aggregates is prone to initiate and induce interfacial cracking in the ITZ effect, but also exerts the blocking effect on the crack growth. And the initial defects (micropores and microcracks) are also the important influential factor on crack initiation, propagation and the formation of the major crack. It is found that the higher the volume fractions of aggregates and initial defects, the cracking of concrete initiates earlier, propagates faster and penetrates more easily when subjected to the same loading condition.

Mesoscopic finite element analysis on dynamic direct tensile failure of lightweight aggregate concrete and corresponding size effect

A comprehensive finite element analysis at the mesoscopic level has been conducted into the complex topic of size effect coupling dynamic strain-rates. Taking the lightweight aggregate concrete (LWAC) dumbbell-shaped samples as the object of numerical investigation, the influence of strain-rate (with the range of 10−5/s ∼ 100/s) on direct-tensile failure of LWAC (including different lightweight aggregate volume fractions [Formula: see text] = 40%, 30% and 20%) was discussed. Subsequently, the structure size of LWAC samples was further expanded (width W = 100, 200 and 300 mm) and the dynamic size effect on direct-tensile strength was investigated. Numerical results show that both the direct-tensile strength and its corresponding size effect of LWAC exhibit a strain-rate dependent behaviour. The increasing strain-rate can gradually weaken the size effect of LWAC and direct-tensile strength would be independent to the structure size as the strain-rate reaches the critical strain-rate. The increasing lightweight aggregate volume fraction can reduce direct-tensile strength. Furthermore, a dynamic size effect model establishing the direct link between the strain-rate effect and size effect was proposed, which can quantitatively predict the dynamic direct-tensile strength of LWAC.

Numerical Investigation on the Effect of the Oxymethylene Ether-3 (OME3) Blending Ratio in Premixed Sooting Ethylene Flames

Synthetic fuels, especially oxygenated fuels, which can be used as blending components, make it possible to modify the emission properties of conventional fossil fuels. Among oxygenated fuels, one promising candidate is oxymethylene ether-3 (OME3). In this work, the sooting propensity of ethylene (C2H4) blended with OME3 is numerically investigated on a series of laminar burner-stabilized premixed flames with increasing amounts of OME3, from pure ethylene to pure OME3. The numerical analysis is performed using the Conditional Quadrature Method of Moments combined with a detailed physico-chemical soot model. Two different equivalence ratios corresponding to a lightly and a highly sooting flame condition have been investigated. The study examines how different blending ratios of the two fuels affect soot particle formation and a correlation between OME3 blending ratio and corresponding soot reduction is established. The soot precursor species in the gas-phase are analyzed along with the soot volume fraction of small nanoparticles and large aggregates. Furthermore, the influence of the OME3 blending on the particle size distribution is studied applying the entropy maximization concept. The effect of increasing amounts of OME3 is found to be different for soot nanoparticles and larger aggregates. While OME3 blending significantly reduces the amount of larger aggregates, only large amounts of OME3, close to pure OME3, lead to a considerable suppression of nanoparticles formed throughout the flame. A linear correlation is identified between the OME3 content in the fuel and the reduction in the soot volume fraction of larger aggregates, while smaller blending ratios may lead to an increased number of nanoparticles for some positions in the flame for the richer flame condition.

Shrinkage behavior of concrete containing bottom ash granules as partial replacement of natural sands

This study aims to provide databank and comprehensible design equations for the shrinkage strain curve of concrete using bottom ash granules to replace conventional natural sands. We prepared 27 concrete mixtures with various parameters, including the substitution level of natural sand for fine bottom ash aggregates, unit water content, and fine aggregate-to-total aggregate volume ratios. To derive reliable design models to predict the drying shrinkage of concrete using fine bottom ash aggregates, the time function specified in fib 2010 was modified by introducing the parameter of equivalent porosity of aggregates. Moreover, the shrinkage strain values at an age of 91 days were formulated by conducting a regression analysis using test data as a specified reference to determine the ultimate shrinkage strain. The test results showed that the shrinkage strain of concrete is marginally affected by the bottom ash aggregate content but significantly affected by the volume fraction of aggregates and their equivalent porosity. The proposed models are promising for estimating the shrinkage strain response of concrete using fine bottom ash aggregates by considering the restraining action of aggregates.

Stochastically homogenized peridynamic model for dynamic fracture analysis of concrete

Dynamic compact tension tests in concrete are investigated using an intermediately-homogenized peridynamic (IH-PD) model, and the results are compared with a fully-homogenized peridynamic (FH-PD) model as well as with experimental data from the literature. In the IH-PD model, concrete is considered as a two-phase material composed of aggregate and mortar, partially homogenized via a stochastic procedure. Both the FH and IH-PD models capture well the experimentally measured reaction-force time-evolution and crack paths for a wide range of dynamic loading rates, without the need for an explicit representation of microstructure geometry of the concrete material or of rate-dependent parameters. The IH-PD model is more accurate than the FH-PD in reproducing the peak loads and produces crack path tortuosity and randomness, similar to what is observed experimentally. These benefits come from the preservation of some micro-scale heterogeneity, stochastically generated to match the concrete’s aggregate volume fraction. We also find that it matters where one measures the reaction force when trying to connect it to fracture and damage progression in the sample: only when measured on the dynamically loaded side of the pre-notch are features in the reaction-force profile matching critical moments like crack initiation, crack branching, and sample final failure. These results provide guidance for future experiments seeking best placement of damage detection sensors and using elastic wave signal analysis in structural health-monitoring of concrete structures.

The meso-failure mechanism of lightweight concrete simulated by the phase field method

PurposeA mesoscopic phase field (PF) model is proposed to simulate the meso-failure process of lightweight concrete.Design/methodology/approachThe PF damage model is applied to the meso-failure process of lightweight concrete through the ABAQUS subroutine user-defined element (UEL). And the improved staggered iteration scheme with a one-pass procedure is used to alternately solve the coupling equations.FindingsThese examples clearly show that the crack initiation of the lightweight concrete specimens mainly occurs in the ceramsite aggregates with weak strength, especially in the larger aggregates. The crack propagation paths of the specimens with the same volume fraction of light aggregates are completely different, but the crack propagation paths all pass through the ceramsite aggregates near the cracks. The results also showed that with the increase in the volume fractions of the aggregates, the slope and the peak loads of the force-deflection (F-d) curves gradually decrease, the load-bearing capacity of the lightweight concrete specimens decreases, and crack branching and coalescence are less likely during crack propagation.Originality/valueThe mesostructures with a mortar matrix, aggregates and an interfacial transition zone (ITZ) are generated by an automatic generation and placement program, thus incorporating the typical three-phase characteristics of lightweight concrete into the PF model.

3D mesoscopic analysis on the compressive behavior of coral aggregate concrete accounting for coarse aggregate volume and maximum aggregate size

For better understanding the compressive behavior of coral aggregate concrete (CAC) with different volume fractions and maximum aggregate sizes (MASs) for coarse aggregate, an all-sided mesoscopic investigation was performed using a novel 3D mesoscale model assuming concrete to be a three-phase composite material composed of coarse aggregate, mortar matrix and interfacial transition zone. The random characteristics of coarse aggregate in shape and spatial distribution were considered adequately in the mesoscale model. Fourteen types of cubic mesoscale models (sized by 100 × 100 × 100 mm3) with various aggregate volume fractions (0–67.4%) and MASs (10–30 mm), were established to study the quasi-static compressive behaviors of CAC, such as the deformation processes, failure patterns and stress–strain relations. By comparing the simulation results and the available test results, it has proven to be that the present 3D mesoscale modelling approach was capable of simulating and investigating the compressive behavior of CAC. Besides, it was indicated from the simulation results that the compressive strength of CAC generally decreases with the increase of MAS, but when the MAS exceeds the critical value, the varying trend of compressive strength tends to slow down. Additionally, the compressive strength tends to decrease before reaching a certain aggregate volume, but when it exceeds a certain volume, that is, the optimum aggregate volume, the compressive strength exhibits a rebounding trend. And the optimum aggregate volume of CAC with the MAS of 20 mm was determined from 58.0% to 61.1%. Furthermore, a regressive relation between concrete strength and aggregate volume was preliminarily derived from simulation and available test results, which can provide some enlightenments for the mix design and performance prediction of CAC.

Heat transfer mechanism of glazed hollow bead insulation concrete

In this study, an improved random aggregate adding method was used to develop a microscopic model of glazed hollow bead insulation concrete (GHBIC), and the model's effectiveness was verified through a test. In addition, the heat transfer performance of GHBIC was also studied using the model. The effect of shape, volume fraction, and gradation of coarse aggregate as well as the volume fraction and gradation of glazed hollow bead on the effective thermal conductivity of this concrete was studied. The results show that the glazed hollow bead effectively blocked the heat flow channel of GHBIC. The smaller the particle size and the higher the volume, the blocking effect is more obvious. Finally, considering the fineness of glazed hollow bead, a mathematical model for the thermal conductivity of GHBIC was established.

Modeling of the tension stiffening behavior and the water permeability change of steel bar reinforcing concrete using mesoscopic and macroscopic hydro-mechanical lattice model

Mesoscopic and macroscopic hydromechanical lattice simulations are performed for studying the tension stiffening behavior, the change in permeability, and their linking of a tensioned reinforced normal strength concrete (NSC) member. In the lattice framework, the hydromechanical coupling is carried out by dual Delaunay triangles and Voronoi polygons, where transport elements are placed along the edges of the Voronoi polygons while mechanical elements are edges of Delaunay triangles. At the mesoscale, concrete is considered as a constitution of three phases: aggregate, cement paste and interfacial transition zones (ITZ). Two mesoscopic components cement paste and ITZ, as well as the concrete at the macroscale, are modeled by a damage model including softening strain feature. The crack opening is linked to the damage variable. Fluid flow obeys Darcy’s law in the intact conduit element and Poiseuille’s law in the crack. Validation against experimental results available in the open literature for NSC tie specimens shows that both current macroscopic and mesoscopic lattice hydromechanical models are appropriated to describe the tensile stiffening and damage induced permeability of NSC reinforced by a steel bar. Mesoscopic modeling helps to get insights of the aggregate volume fraction effect.

Mesoscale damage modelling of concrete by using image-based scaled boundary finite element method

As a thin layer surrounding aggregates, the interfacial transition zone (ITZ) has a significant influence on the mechanical performance of concrete, a large number of small elements is required to discretize the ITZ and its vicinity owing to its small thickness. In this contribution, the mesostructure of concrete is explicitly built by a novel method named improved random walking algorithm (IRWA), with which the grading, content, shape of aggregates and the thickness of ITZs, can be conveniently controlled by users, and high volume fraction of aggregate and remarkable efficiency can be achieved. In the framework of the scaled boundary finite element method (SBFEM), the image-based quadtree decomposition is employed to automatically generate the multi-level mesh for the concrete composites, with no trouble in dealing with hanging nodes or the dramatically changing element size in the vicinity of the material interface. The nonlocal damage model in integral format is extended to simulate progressive damage of concrete at mesoscale with good mesh independence. Four benchmarks are modelled to demonstrate the performance of the proposed approach, and the effects of model parameters on the structural responses are also discussed.

Mix design of self-levelling mortar prepared by curshed sand with high flowability and early strengthening

High flowability and early strengthening properties of self-levelling mortar (SLM) could greatly boost construction efficiency and save lots of labor force and materials costs. In this paper, absolute volume method was applied to design the mix proportions of SLM prepared by curshed sand. Further, the relationships between the flowability, strength of mortar and the calculated coating paste thickness MAPT under several paste to aggregate volume fractions (Vp/Va) were analyzed. Subsequently, ultrafine cement, silica fume and sodium sulfate were added to improve the early-age strength of SLM, and their effects on the flowability and rheological properties were also discussed. Finally, the XRD, TG-DTG analysis and SEM images were applied to study the microstructure of hardened matrix. Results showed that a linear equation could be employed to express the relationship between fluidity and the MAPT and a quadratic polynomial relationship for strength. Rheological parameters were increased by the ultrafine cement and silica fume, while sodium sulfate could enhance the rheological properties. It was found that combination admixture of silica fume and sodium sulfate could improve the mechanical strength greatly. And more C-S-H gels and higher chemical combined water (CCW) content were generated in the sample modified by the combination admixture.

Multi-scale analysis of asphalt pavement on curved concrete bridge deck considering effect of mesoscopic structure characteristics

• A two-dimensional (2D) random aggregate generation method was adopted to develop mesoscale models with different mesostructure characteristics. • A three-dimensional (3D) macroscopic model of the bridge deck pavement including two structural layers was established. • A multiscale coupling algorithm was used to establish the relationship between models at different scales. • Damage density is used to evaluate the damage possibility of asphalt mixture. • The influence factors of mesostructure damage are graded. Mesostructure characteristics of asphalt mixture, as essential factors, affect the mechanical response and mesoscopic damage of bridge deck pavement. This study explored the effect of different mesostructure characteristics on the mechanical properties and damage behavior of asphalt pavement on a curved concrete deck using a mesostructure-based multiscale method. A two-dimensional (2D) random aggregate generation method was adopted to develop mesoscale models with different aggregate volume fractions (30%, 40% and 50%), aggregate size ranges (4.75mm-2.36mm, 9.5mm-2.36mm, 13mm-2.36mm and 16mm-2.36mm) and aggregate shapes (quadrangle, hexagon, octagon and random). A three-dimensional (3D) macroscopic model of the bridge deck pavement including two structural layers was established. A multiscale algorithm was used to establish the relationship between models at different scales. In addition, the angle index (AI) was used as an evaluation index of coarse aggregates in irregular shapes, and the damage density was used to evaluate the damage possibility of mesostructure. The results showed that the damage density of asphalt mixture mesostructure had a linear relationship with AI, and the damage density became bigger as the AI decreased. Besides, increasing the range of aggregate size and the volume fraction of aggregate could reduce the damage density of asphalt mixture mesostructure, and ultimately improve the comprehensive performance of bridge deck pavement. For the damage density of bridge deck pavement, the angle index of aggregate was the most important factor, followed by the volume fraction and size range of aggregate.

Impact mechanisms of aggregate on rigid projectile normal penetration into concrete target

<p indent=0mm>Concrete failure mechanisms under projectile penetration at a mesoscale level have recently attracted researchers’ attention. Based on the three-dimensional (3D) mesoscale concrete model, the influences of aggregate strength, size, and volume fraction on deflection angle and penetration resistance of projectile were analyzed during the process of rigid projectile penetration into the concrete. Four stages of projectile deflected during the process of depth penetration were described. The measurement of the deflection angle of the projectile during the penetration process revealed the factors affecting ballistic stability. These factors were investigated quantitatively, and their critical values affecting the deflection angle of projectile were obtained. The effects of the key variables of coarse aggregate on penetration resistance were quantitatively studied. Then, the applicable conditions to satisfy the homogeneous assumption and the upper bounds of aggregate size and volume fraction were identified. The latter influenced the penetration resistance and were dependent on the aggregate configurations. Finally, the influencing mechanisms of coarse aggregate on the rigid projectile normal penetration into mesoscale concrete were described. The results reported here provide a theoretical and practical background for design optimization and safety assessment for protective engineering structures.

Analysis of the Aggregate Effect on the Compressive Strength of Concrete Using Dune Sand

In this study, the compressive strengths of concrete were investigated based on water content and aggregate volume fractions, comprising dune sand (DS), crushed sand (CS), and coarse aggregate (CA), for different ages. Experimental data were used to analyze the effects of the volume fraction changes of aggregates on the compressive strength. The compressive strength of concrete increases until the volumetric DS to fine aggregate (FA) ratio (DS/FA ratio) reaches 20%, after which it decreases. The relationship between changes in compressive strength and aggregate volume fractions was analyzed considering the effect factor of each aggregate on the compressive strength and at 2 conditions: (1) 0 &lt; DS &lt; CS &lt; CA and (2) 0 &lt; CA &lt; CS &lt; DS. For condition (1), when the effect factor of CA = 1, those of DS and CS were within 0.04–0.83 and 0.72–0.92, respectively, for all mixtures. For condition (2), when the effect factor of DS = 1, those of CS and CA were within 0.68–0.80 and 0.02–0.79, respectively.

Design of Concrete Mix Proportion Based on Particle Packing Voidage and Test Research on Compressive Strength and Elastic Modulus of Concrete

According to the basic principle of dense packing of particles, and considering the interaction between particles, a dense packing model of granular materials in concrete was proposed. During the establishment of this model, binary particle packing tests of crushed stone and sand were carried out. The fitting analysis of the test results determines the relationship between the particle size ratio and the remaining volume fraction of the particle packing, and then the actual void fraction of the particle packing was obtained, based on which the water–binder ratio was combined to determine the amount of various materials in the concrete. The proposed concrete mix design method was used to prepare concrete, and its compressive strength and elastic modulus were tested experimentally. The test results show that the aggregate volume fraction of the prepared concrete increased, and the workability of the concrete mixture with the appropriate amount of water reducing agent meets the design requirements. When the water–binder ratio was 0.42, 0.47, or 0.52, the compressive strength of the concrete increased compared with the control concrete, and the degree of improvement in compressive strength increased with the decrease in water–binder ratio; when the water-binder ratio was 0.42, 0.47, or 0.52, the static elastic modulus of the concrete increased compared with the control concrete, and the degree of improvement in elastic modulus also increased with the decrease in water–binder ratio. The elastic modulus and compressive strength of the prepared concrete have a positive correlation. Findings show that the concrete mix design method proposed by this research is feasible and advanced in a sense.

Model for Predicting the Thermal Conductivity of Concrete

Thermal conductivity of concrete greatly influences the heat transfer of buildings and affected by many factors. This paper presents a prediction model for thermal conductivity of concrete by adopting the theory of Wiener bounds and considering concrete to have four components (water, air, aggregate, and cement mortar). The proposed model considers the combined effects of porosity, water saturation, and the volume fraction of aggregate on the thermal conductivity of concrete by weighting parameters of $${\eta }_{1}$$ , $${\eta }_{2}$$ , $${\eta }_{3}$$ , respectively. By adjusting the weighting parameters of each component, the model can consider the influence of various factors on the thermal conductivity of concrete more comprehensively. Thermal conductivity of each component and expression of weighting parameters are determined by literature and experiments. The proposed model has been verified by the measured thermal conductivity of concrete under different porosities, water content, and volume fractions of aggregate with the prediction accuracy of $$\pm 12\mathrm{\%}$$ . Finally, the regularity of the change in the thermal conductivity of concrete with porosity, water saturation, the volume fraction of aggregate, and temperature is analyzed.

Thermal conductivity modelling of structural lightweight aggregate concrete

Various thermal conductivity models for multi-phase materials have been suggested in the literature. However, none have been specifically developed or adapted for structural lightweight aggregate concrete (SLWAC). In this context, the main purpose of this study was to develop a new simple model to predict the thermal conductivity of SLWAC, taking into account the volume fraction and thermal characteristics of each phase and an empirical factor that considers the uneven heat flux distribution through them. The study was based on an extensive experimental campaign carried out for different water to cement ratios and types and volume fractions of aggregate. Moreover, given that there are no standards for determining the thermal conductivity of lightweight aggregates (LWAs), a new technique was devised. The proposed model is easily implemented in practice, contributing to expeditious estimation of the thermal conductivity of the most common SLWAC regardless of the type of LWA and concrete composition.