Aggregate-mortar Interface Research Articles

Exploring the Influence of Heterogeneity on Determination of a RVE for Concrete Structures

The mechanical behavior of a heterogeneous material is strongly influenced by the response of its constituents (and their respective interactions) at lower scales. This fact accounts for the nonlinearities observed at the macroscale, and consequently, the challenges in formulating constitutive models capable of providing accurate predictions in a practical application context. Essentially, there are two primary ways to represent a heterogeneous material, either through Phenomenological Models at a Single Scale or through Multiscale Models. Multiscale models represent the most robust approach to depicting a heterogeneous material. In these models, the connection between macro and meso levels is typically made through the concept of a Representative Volume Element (RVE). The RVE is the smallest sample of the material that can statistically represent the mechanical behavior of the material. Typically, in multiscale model applications, it is assumed that an RVE exists and that its size is initially prescribed. However, the appropriate determination of an RVE for quasi-brittle materials remains a subject of study. This paper discusses the determination of RVEs for concrete using normalized graded aggregate curves. For this purpose, numerical simulations were performed using the Finite Element Method, employing constitutive models based on Phase Field theory. Throughout the study, issues related to the size of the RVE and the determination of ergodic properties are discussed. The influence of the aggregate gradation curve on the mechanical behavior of the RVE and the influence of the aggregate-mortar interface are also examined. Finally, the difficulties encountered in determining the RVE in a practical application context are presented.

Failure criterion and damage evolution of high-strength geopolymer concrete under compression-shear composite loading after high temperatures

Geopolymer concrete (GPC) holds significant potential for building fire safety design due to its green, low-carbon, and high-temperature resistant properties. In practical applications, concrete structures often face multiaxial composite loads. Therefore, investigating the composite stress performance of high-strength geopolymer concrete (HGPC) after exposure to high temperatures is crucial for assessing structural damage and enhancing fire prevention measures. This paper examines the compressive shear performance of HGPC at various temperatures T (20°C, 200°C, 400°C, 600°C) and stress ratios k (0.15, 0.3, 0.45, 0.6). The study analyzes failure patterns, shear load-displacement curves, shear strengths and their components, and peak shear deformation. Failure criteria are proposed to describe the damage patterns of HGPC under compressive shear loading after high temperatures. Additionally, the damage evolution process is evaluated using the energy method. The results show that HGPC damage surfaces penetrate the coarse aggregate at room temperature under compression-shear composite loading but shift to the aggregate-mortar interface at 600°C. Shear stiffness, peak shear strength, and residual strength increase with higher stress ratios but decrease with rising temperatures. Temperature affects peak shear strength by over 95 %, significantly more than the stress ratio. The shear strength decreases more slowly than compressive strength up to 400°C but more rapidly beyond 400°C. The cohesive strength of HGPC decreases with increasing stress ratio under different temperatures, while contact friction strength exceeds 50 % of peak shear strength. Shear dilation strength generally increases, then decreases, peaking at k = 0.45. The failure criterion based on the modified twin shear unified strength theory aligns best with experimental results. Furthermore, an increased stress ratio retarded the development of damage evolution, which was first accelerated and then slowed with rising temperature.

Tensile strength prediction of crumb rubber concrete based on mesoscale modelling

ABSTRACT This study presents an experimental study and a mesoscale model for evaluating the tensile performance of crumb rubber concrete (CRC). In the experimental study, indirect tensile strengths of CRC with 6%, 12% 18% rubber contents were tested. Then the tested CRC samples were modelled as a five-phase material containing rubber, coarse aggregate, mortar, aggregate-mortar interface layer, and rubber-mortar interface layer. Numerical results from the mesoscale model agreed well with experimental tests. Parameter analysis was carried out to investigate the content, size, shape of rubber particle, thickness of interface layer on the tensile strength of CRC. Numerical results also indicated that rubber content was the governing influencing factor on the tensile strength reduction. As comparison, the influence from rubber size and rubber shape was relatively low. For a specimen with constant rubber content and size, changes in the distribution of rubber particles resulted in slight changes in the tensile strength of CRC as well. The effect of rubber particles on the tensile properties of concrete was similar to that of pores. Finally, tensile strength prediction formula was developed based on the pores model proposed in this study.

Quantitative image analysis on the filling proportion of corrosion products in reinforced concrete using X-ray computed tomography

Corrosion-product filling inhibits corrosion-induced cracking in reinforced concrete (RC) structures, thereby affecting structural deterioration. However, the accurate quantification of corrosion-product filling has challenged the scientific and engineering communities. In this study, a novel method for quantifying the filling proportion of corrosion products βF at the macro- and meso-scales using X-ray computed tomography and image segmentation algorithms was proposed. This method was applied to RC specimens exposed to chloride-rich environments and compared with conventional methods. The results showed that conventional quantitative methods underestimated βF owing to a lack of understanding of the corrosion products filling into air voids and the aggregate–mortar interface, and those overflowing from the specimen. A positive correlation between βF and corrosion propagation as well as moisture content in the environment was observed, whereas βF was negatively associated with concrete cover thickness.

Biaxial compressive failure criteria and constitutive model of high-strength geopolymer concrete after high temperature

This paper studies the effect of high temperatures (20℃, 200℃, 400℃, and 600℃) and stress ratios (σ2/σ3) (0.15, 0.3, 0.45, and 0.60) on biaxial compressive strength and strain development patterns of high-strength geopolymer concrete (HGPC). Experimental observations indicate that HGPC exhibits laminar damage similar to ordinary Portland concrete (OPC) before and after exposure to high temperatures, under biaxial compression. Unlike OPC, damage in HGPC initially passes through coarse aggregates at room temperature but shifts predominantly to the aggregate-mortar interface at 600℃. The biaxial compressive strength envelope of HGPC expands with rising temperatures. The increased multiple of biaxial compressive strength (σ3/fm) for HGPC at room temperature rises and then falls with increasing stress ratios, peaking at a stress ratio of 0.45 and surpassing the values for OPC and high-strength concrete (HSC). Before high temperature, the modulus of elasticity of HGPC is higher than that of OPC and decreases with increasing temperature, decreasing by about 45–60% for every 200°C increase, and the reduction is greater than that of OPC. The existing failure criterion is not suitable for HGPC after high temperature, this paper develops a new biaxial compression failure criterion for HGPC before and after high temperature based on Kupfer's criterion, taking into account the effect of temperature. The proposed failure criterion calculation results are well in agreement with the experimental data. A damage constitutive model for HGPC considering the effect of temperature and lateral pressure is also proposed, combining the Weibull distribution model for the ascending branch and a modified empirical model for the descending branch. This combined model effectively predicts the stress-strain relationship of HGPC under various temperature conditions.

Size effect on tensile strength of interface between coarse aggregate and mortar under freeze-thaw cycle

Through the cleavage tensile test and freeze-thaw cycle experiment, the variation of tensile strength of mortar and interfacial bond-splitting tensile strength of coarse aggregate-mortar interface with different specimen sizes under freeze-thaw cycle was studied. The results indicate that the splitting tensile strength of the mortar specimen and the interfacial bond-splitting tensile strength of the coarse aggregate-interface specimen have a significant size effect, and the splitting tensile strength of the coarse aggregate-mortar interfacial specimen is significantly lower than that of the mortar specimen. At the same time, it is found that the statistical size effect model, the multifractal size effect model, and the fracture mechanics size effect model all can well describe the size effect of the split tensile strength of mortar specimens. Finally, considering the splitting tensile strength of standard mortar and the number of freeze-thaw cycles, a two-parameter size effect calculation model for the interfacial bond-splitting tensile strength of the coarse aggregate-mortar interface was established, basing on the Bažant-Xi statistical theory size effect correction formula, which was verified to be universal by experimental data.

Study on the dynamic fracturing characteristics of aggregate-mortar interface under various loading rates using DIC

The dynamics of fracture characteristics at the aggregate-mortar interface under varying loading rates present a significant area of investigation, pivotal for comprehending the response of concrete to dynamic load conditions. In concrete, the interface between the aggregate and the mortar forms a significant region, fundamentally dictating concrete structures' comprehensive strength and durability. This study aims to investigate the interface crack propagation process, including the crack propagation velocity and the propagation direction using Digital Image Correlation (DIC). Three-point bending beams with a single-size aggregate were fabricated. Beams with different initial crack tip-to-aggregate distances, d0 (aggregate position) of 20 mm, 40 mm, and 60 mm, were tested at five different displacement-loading rates (0.012 mm/min, 0.12 mm/min, 1.2 mm/min, 12 mm/min, and 120 mm/min) using an electro-hydraulic servo testing machine. The crack propagation process was documented utilizing a high-speed video camera. The study reveals a direct correlation between loading rates and concrete's mechanical attributes. The results indicate that as loading rates rise, the compressive strength, tensile strength, and modulus of elasticity of concrete notably increase, while the Poisson's ratio decreases. The initial fracture toughness (KIcini) also escalates with increasing the rates, its sensitivity notably influenced by d0. Concurrently, fracture energy increases with the increase of loading rate and the decrease of d0, a trend more evident at high loading rates. The crack initiation load (Pini) for all single-size aggregate concrete beams was higher than that of plain concrete, and Pini decreased as d0 increased. Observations were made on the trajectory of crack formation, further scrutinizing the effects of d0 and the variable loading rate. Notably, the presence of the interface was found to slow the crack propagation velocity, and this ability to inhibit crack propagation decreased as d0 increased. As the loading rate increased, the crack propagation velocity at the mortar matrix and interface also increased. This study also reports additional material properties, such as critical crack length, and interfacial fracture mode.

Low-temperature fracture behavior of railway asphalt concretes under semi-circular bending: Experimental and numerical investigation

Asphalt concrete waterproofing layer (ACWL) based on custom-designed railway asphalt concretes has been proposed to stabilize subgrade moisture content of high-speed railways. However, given the temperature-dependent viscoelasticity of railway asphalt concretes, the low-temperature environment poses a great challenge for ACWL. Therefore, this study was motivated to perform experimental and numerical investigations on the low-temperature fracture behavior of railway asphalt concretes, to enable the material optimization of ACWL in cold regions. First, semi-circular bending (SCB) tests were performed at a low temperature of −10 °C, based on asphalt concrete prepared from two types of asphalt binders (A and B) and two aggregate gradations (AC-10 and AC-13). Subsequently, both the homogeneous 3D and the heterogeneous 2D model were established based on the extended finite element method (XFEM) to analyze the fracture behavior and the mesoscale mechanisms, respectively. In particular, an improved polygonal random aggregate generation and packing algorithm was proposed for the 2D model based on the actual morphology, which achieved reasonable accuracy and efficiency in fracture simulation. The results showed that the fracture process of railway asphalt concrete under SCB could be divided into four stages according to crack trajectory and fracture energy. In the first two stages, the crack length remained approximately 0 and the input energy accumulated in the elastic deformation. In the last two stages, the crack propagated through the specimen, and a rapid fracture energy dissipation was observed. Besides, the cracks tended to propagate upwards into the aggregate-mortar interface zone and the mastic zone for AC-13 and AC-10, respectively. Finally, the B13 exhibited superiority over its counterparts, attributed to the low-temperature deformation adaptivity of binder B and the crack deflection effect of AC-13 with coarser aggregates.

Microstructure, tensile strength and shear strength of aggregate-mortar interface: Effect of aggregate mineralogy

This work aims to study the mechanical properties of the interfacial transition zone (ITZ) through both experimental tests and microstructural observations. The study considers a mortar with water-cement ratio of 0.58 and four types of aggregates: sandstone, granite, and two different types of limestone with varying water absorption coefficients. Chemical compositions, physical characteristics, and surface micromorphology of the four aggregates were evaluated using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and water absorption measurements as well as SEM observations. Experimental setups were developed to investigate the bonding behavior of the aggregate-mortar interface. The bonding strength between granite and mortar through direct tensile test varied from 51% to 69% of tensile strength of mortar from young age to mature, while 61% to 74% for sandstone, 48% to greater than 90% and 81% to greater than 102% averagely for black and white limestone respectively. The fact of failure in the bulk mortar indicates ITZ adhesion for limestones may equal or exceed the mortar tensile strength generally from 28-day age. This conclusion is consistent with the results obtained by slant shear test. The significant difference of adhesion behavior of ITZ between two limestones due to their water absorption capability. High-water absorption turns out to be beneficial in forming a strong chemical bond at early age, however, the mineralogy remains the pivot under same surface condition. This chemical bond is evidenced by the failure pattern and morphology of the aggregate surface after failure and the presence of a dense accumulation of portlandite and C-S-H under SEM.

Compressive Test Investigation and Numerical Simulation of Polyvinyl-Alcohol (PVA)-Fiber-Reinforced Rubber Concrete

To investigate the mechanical properties of polyvinyl-alcohol (PVA)-fiber-reinforced rubber concrete, 13 groups of PVA rubber/concrete specimens with PVA volume fractions of 0%, 0.5 vol%, 1.0 vol%, and 1.5 vol% and rubber particles with volume replacement sand ratios of 0%, 10%, 20%, and 30% were prepared, and the uniaxial compression full curve test was performed. The findings indicate that the bridging effect of PVA, as well as the synergistic effect of PVA and rubber particles, can improve the compressive properties of concrete, and the failure of the specimens demonstrates obvious ductile characteristics. Furthermore, PVA has a better impact on rubber concrete’s bearing capacity, crack propagation of the failure surface, and compressive strength in the latter stages. PVA-fiber-reinforced rubber concrete is thought to be a six-phase composite made up of the aggregate phase, mortar matrix, PVA fiber, rubber particles, aggregate–mortar interface, and rubber–mortar interface on the mesoscale. To simulate the entire process of concrete with varying PVA rubber/content from integrity to damage and cracking, a meso-numerical model of PVA rubber/concrete was constructed. The simulation results and test results are in good agreement, demonstrating the validity of the mesomodel and offering a theoretical foundation for the structural analysis and design of this type of concrete.

Mesoscale Analysis of the Effect of Interfacial Transition Zone on the Compressive Damage of Concrete Based on Discrete Element Method.

The coarse aggregate-mortar interface transition zone (ITZ) has a great influence on the mechanical properties of concrete, which cannot be easily studied using laboratory tests in the mesoscale. In this paper, a series of axial compression tests were conducted using the discrete element method (DEM) on concrete specimens for four phases: coarse aggregates, mortars, aggregate-mortar interface transition zones, and voids. The effects of ITZ strength on macroscopic stress and microscopic cracks under different strength reduction factors were investigated through axial compression testing. With the increase in interface transition strength, the compressive strength of the concrete becomes stronger; moreover, the number of cracks decreases, and the anisotropy of contact orientation becomes weaker. Meanwhile, the direction of crack development and the damage mode of compressed concrete specimens were also dependent on the coarse aggregate-mortar interface strength coefficient.

Simulation and evaluation of fatigue damage of cold recycled mixtures with bitumen emulsion

In order to explore the fatigue damage mechanism of CRME under dynamic load, DEM of CRME was established based on PFC2D. The micromechanical response characteristics of CRME were obtained by simulation splitting test, and verified by indoor indirect tensile fatigue test. It shows that the results of simulation test are basically consistent with those of indoor test. The numerical value and quantity of tensile force chain in DEM increase obviously with the increase of the stress ratio, the fatigue life decays rapidly, and the horizontal displacement at different width positions in the specimen increases significantly. The tensile force chain is generated, accumulated and concentrated in the mortar and the aggregate-mortar interface. Eventually, the micro structure and bonding force of CRME are damaged. Tensile force chain is the main factor of fatigue damage. The improvement of the bonding performance of bitumen emulsion cement mortar (BECM) and RAP-mortar interface is suggested to improve the fatigue performance of CRME.

Effects of the embedding of cohesive zone model on the mesoscopic fracture behavior of Concrete: A case study of uniaxial tension and compression tests

The application of the Cohesive Zone Model (CZM) in concrete meso-mechanics solves the nonlinear problem of material crack tip well. At present, the application methods of CZM in concrete mesoscopic models include the bond mechanics characterization of the aggregate-mortar interface transition zone (ITZ) and the characterization of fracture characteristics between material micro-elements. From this, two different numerical models were derived—the combined Finite Discrete Element (FDEM) model and the Cohesive Finite Element (CFEM) model. To evaluate the difference between the two numerical models and the traditional Finite Element (FEM) model in simulating concrete mechanical behavior and crack propagation, a 3D concrete mesostructure modeling algorithm was developed in this study, including innovations in gravitational self-compacting of polyhedral aggregates. After that, the FDEM model and the CFEM model were successfully constructed based on the FEM model. Through uniaxial compression and tensile numerical simulations, it was found that the CFEM model is more in line with the theoretical and experimental mechanisms in the characterization of the static mechanical behavior and fracture process of concrete. The fracture failure angle variable defined by the CFEM model revealed that the compressed concrete material exhibited better strength and toughness due to a higher damage degree and more energy-consuming cracking mechanism. The above findings to a certain extent promote the future development of CZM in the field of concrete meso-mechanics.

Influence of Silica Fume on Mechanical Properties and Microhardness of Interfacial Transition Zone of Different Recycled Aggregate Concretes

Abstract Several countries have started using recycled aggregate as a partial replacement to natural aggregate in concrete. Recycled aggregate contains adhered mortar, which distinguishes it from the natural aggregate. In the present study, natural coarse aggregates were entirely replaced by two kinds of recycled coarse aggregates. The recycled aggregates obtained from the jaw crushing method were named recycled coarse aggregate-1 and the aggregates that were further processed by the ball milling method were named recycled coarse aggregate-2. The performance of control concrete and two kinds of recycled coarse aggregate concretes were studied experimentally with respect to mechanical properties. Results indicate that the processing method to obtain recycled coarse aggregates plays an important role in developing the required mechanical properties. The ball mill processed aggregates performed better than the jaw crushed aggregates in concrete. The performance was also assessed with respect to the microhardness of the interfacial transition zone around the surface of the aggregates. The presence of adhered mortar in recycled aggregate weakens it because of the presence of an old interfacial transition zone that affects the strength of concrete. The interfacial transition zone hardness at the aggregate-mortar interface is 53.94, 34.21, and 44.08 % of bulk concrete for control concrete, recycled coarse aggregate-1 concrete, and recycled coarse aggregate-2 concrete, respectively. The addition of silica fume improved the average microhardness, and the same was reflected in the mechanical properties of both the recycled coarse aggregate concretes. It is recommended to use ball mill processed recycled coarse aggregates as a complete replacement to natural coarse aggregates along with a 5 % addition of silica fume for better performance.

Effects of coarse aggregate surface morphology on aggregate-mortar interface strength and mechanical properties of concrete

Coarse aggregate surface textures have a significant effect on mechanical properties of aggregate-mortar interface in normal strength concrete. The authors used a scanning microscope to scan the surface of aggregates in concrete to observe surface morphology. The results showed that the surface morphology components of coarse aggregates were varying at different scanning scales, which included waviness and roughness. Each component was quantitatively characterized. New methods to calculate the interfacial tensile strength and shear strength considering the influence of 3D surface morphology were proposed. The methods were introduced into a 3D meso-numerical model. Uniaxial compression and direct tension tests were conducted to examine the effects of coarse aggregate surface morphology (CASM) on the mechanical behaviors of concrete. Compared to the test results, the numerical model can well predict the failure procedure of concrete and the cracking characteristics of interface with different aggregate surface morphologies. Moreover, it was found that concrete tensile strength was more sensitive to CASM than compressive strength.

Numerical investigation on fracture evolution of asphalt mixture compared with acoustic emission

ABSTRACT To reveal the fracture damage of asphalt mixture, an image-based numerical approach is proposed in this study considering the distribution of aggregate and asphalt mortar. Both digital image processing technique and discrete element method are applied to simulate the inhomogeneity of specimen and its fracture evolution at low temperature. At the same time, the acoustic emission (AE) activities are measured in laboratory to be compared with the micromechanical responses of numerical models. Results indicate that the developed method is suitable for modelling the numerical specimen of asphalt mixture, taking into account the size gradation and morphology of aggregates in the two-dimensional image. The numerical results of asphalt mixture have a good agreement with that of laboratory tests in cracking distribution and statistical probability. Cracks first appear on the aggregate–mortar interface, located in the upper and middle parts of the specimen, and then the brittle shear failure gradually propagates along the surface. Cumulative probabilities of both AE ringing counts and microfracture number follow the Weibull distribution. The numerical model has a significant advantage in the fracture evolution of asphalt mixture because the eccentric loading is unavoidable in laboratory tests due to personal and mechanical errors.

Workability and mechanical properties of microwave heating for recovering high quality aggregate from concrete

Microwave assisted concrete recycling is highlighted by a large number of research groups all over the world due to its potential for significant process benefit. In this paper, systematic experiments are established to investigate the heating process of mortar-aggregate under microwave irradiation. Four different aggregate types are selected that widely used as aggregates in concrete to represent different kinds of aggregates. The temperature field of both mortar and aggregate under different heating time and microwave power input are obtained. The crack formation, propagation and material damage during the heating process are graphically presented. XRD tests and SEM analysis are carried out to clarify the chemical content and microstructure variation around the aggregate-mortar interface before and after microwave heating. The mechanical properties of recycled concrete are evaluated through the UCS tests. Energy consumptions are tested during the heating process. The applications of microwave recovering aggregates from standard concretes are conducted based on the results obtained in the simplified concrete. The research proves that microwave irradiation could weaken the bonding between mortar and aggregate effectively, which provide a contribution and reference for the further application of microwave-assisted aggregates recycling.

A Fully Coupled Electromagnetic Irradiation, Heat and Mass Transfer Model of Microwave Heating on Concrete

Due to the potential for significant process benefit, microwave assisted concrete recycling is highlighted by many scholars and engineers over the world recently. In this paper, a multi-field model is established to simulate the microwave heating process of concrete specimen with single-particle aggregate by using of COMSOL Multiphysics. The validation of this model is verified by comparison with temperature from experiment. Mechanical properties of aggregate-mortar interface during microwave heating process was experimentally investigated. Electric, temperature, stress field evolution and moisture transformation during microwave heating process are discussed in detail. Temperature and stress gradient at mortar-aggregate interface influenced by microwave heating power, frequency and duration are analyzed and graphically presented, which are the dominant factors to achieve the interface debonding. Based on the comparison between concrete absorbed energy and temperature variation, energy efficiency for microwave heating concrete is also briefly discussed. Results of this investigation could provide a multi-field understanding for the microwave treatment on concrete.

Micro-scale characterization of the heterogeneous properties of in-service cement-treated base material

Compared with ordinary cement concrete, cement-treated base (CTB) material consumes much less cement and water, and its mechanical properties are also significantly different. The macroscopic properties of CTB are determined by microstructure. In order to understand the mechanical performance of in-service CTB, it is of great importance to investigate its microscopic structure and properties. In the study, the microscopic mechanical, structural, and chemical characteristics of the mortar and interfacial transition zone (ITZ) of in-service CTB were investigated through micro-scale characterizations. The results indicated the aggregate-mortar interface of in-service CTB is much weaker than that of concrete. A large number of micro-cracks and micro-pores were distributed in the ITZs and mortar. The micro-scale modulus of ITZ was approximately 85.6% of the modulus of the mortar and showed high variance. More compacted cement paste structure is beneficial to improve the mechanical properties of CTB.

Investigation of the microstructural fracture behaviour of asphalt mixtures using the finite element method

To limit and control the degradation of asphalt pavements and to increase the durability of the asphalt layers, a thorough understanding of the mechanical pavement response is necessary. As a contribution, it is imperative to investigate the fracture behaviours of asphalt mixtures. This paper applies two-dimensional (2D) cohesive zone modeling (CZM) techniques to simulate the microscale crack initiation and propagation within the asphalt mixtures. X-ray computed tomography (X-ray CT) scanning and digital image processing (DIP) techniques were applied to reconstruct the microstructure of asphalt specimens. Finite element (FE) simulations were conducted to simulate the quasi-static indirect tensile test (IDT) with the implementation of a 2D bilinear cohesive zone model. The FE models were optimised and validated based on experimental results. The validated model was then used to investigate the effect of temperature and the air voids on the fracture behaviour of asphalt mixtures. The results show that more significant damage is observed in broader areas outside of the main crack trajectory as the temperature is increased in the range considered in this study (−10 °C, 0 °C, +10 °C). The aggregate-mortar interface exhibited more significant damage than the bulk fracture within the mortar. The deformation voids which are defined as those not crossed by the final crack trajectory have no effect on the maximum reaction force at temperatures above 0 °C. Also, deformation voids are found to extend the time to failure. The results contribute to the current research on the fracture behaviour of asphalt mixtures and future investigations are recommended to facilitate the establishment of an asphalt mixture design method and evaluation method based on microscale failure mechanisms.