Interfacial Transition Zone Research Articles

3D mesoscale discrete element modeling of hybrid fiber-reinforced concrete

To investigate the relationship of hybrid fiber volume friction on the mechanical properties and failure mechanisms of steel-polypropylene hybrid fiber reinforced concrete (HFRC), the study constructs a three-dimensional, five-phase mesoscale model using the Discrete Element Method (DEM), which consists of mortar, coarse aggregate, polypropylene fibers (PPF), steel fibers (SF), and the interfacial transition zone (ITZ). The axial compression test results of twelve cubic specimens with four different hybrid fiber combinations (0.5 % SF + 0.05 % PPF, 0.5 % SF + 0.1 % PPF, 1.0 % SF + 0.05 % PPF, 1.0 % SF + 0.1 % PPF) were used to validate the model’s accuracy, examining the influence of fiber content on the mechanical performance and cracking features of HFRC. Directed by experimental mix proportions, the model employed cluster particle modules with realistic aggregate geometries and randomly distributed SFs to simulate concrete microstructure. Utilizing the Flat-Joint Model (FJM) in DEM software (PFC3D), two contact models are developed: one FJM with bi-nonlinear softening segments to describe the ITZ between steel fibers and PPF-reinforced mortar (PFRM); another FJM considering the volume fraction effect of SF to describe the ITZ between coarse aggregate and FRM. Both models were experimentally validated, proving their accuracy. The results indicate that the established DEM model can accurately simulate the stress-strain relationship and failure modes of HFRC under compression. The hybrid fibers increased the compressive strength by 2.13–17.33 % and reduced the cracking strain by 5.25–9.33 %. Furthermore, the computational findings reveal that PPF primarily affects the crack initiation phase, while SF plays a significant role in the overall crack propagation stage. This discovery holds significant importance for understanding and optimizing the structural performance of HFRC.

Influence mechanism of further hydration on the compressive strength of ultra-high performance concrete with coarse aggregates

A further hydration test was conducted to study the effects of temperature and humidity conditions on the compressive strength and microstructure of ultra-high performance concrete with coarse aggregates (UHPC-CA), while analyzing the mechanisms influencing its compressive strength. The results indicated that the higher temperatures resulted in more significant strength and pore structure variations, with strength and pore structure changes being greater in liquid water environments compared to water vapor environments. The hardness of interfacial transition zone (ITZ) increased initially and then declined. The influence of further hydration on the strength of UHPC-CA was primarily divided into three stages: enhancement, damage and filling. These findings provide a theoretical foundation for the evaluation and design of UHPC materials for long-term service in aquatic environments.

Study on the damage mechanism of hydraulic concrete under the alternating action of freeze-thaw and abrasion

This study investigates the damage mechanism of hydraulic concrete under the alternating action of freeze–thaw (F) and abrasion (W) in cold regions. To this end, three groups of working conditions were established with a constant water-binder ratio. The mass loss and relative dynamic elastic modulus (RDEM) of each cycle were evaluated using the fast-freezing method and underwater method. The phase composition and relative content change of hydration products, pore characteristic parameters and pore size distribution, morphology and microstructure of interfacial transition zone (ITZ) were analyzed by X-ray diffraction (XRD), mercury intrusion porosimeter(MIP), scanning electron microscopyn (SEM) and image recognition technology, respectively. The results indicate that the alternating effect will accelerate the mass loss of concrete and the reduction of the RDEM compared with the single factor. Following four consecutive alternating cycles, the mass loss rates of working conditions W and F group were found to be 4.61 % and 97.66 % lower, respectively, than those of working condition F-W group. The RDEM damage coefficient (γ < 1) was also observed. At the micro level, the freeze-thaw effect will result in the formation of a considerable number of micropores and the expansion of microcracks, with the abrasion effect continuing to promote this process. In the process of alternating action, the dissolution of Ca(OH)2 by external water through microcracks and interconnected pores is accelerated. Concurrently, the pores and microcracks in the ITZ expand, the morphology is roughened, and the microstructure is complicated. The interfacial bonding ability and compactness of different material components decrease. Under the action of external disturbance, cement paste and aggregate fall off, resulting in mass loss and a decreased RDEM.

Suppressing of alkaline aggregate reaction (AAR) through waterproofing of concrete surface, aggregates, and mortars

Water is necessary for the alkali aggregate reaction to occur and this study investigates the impact of waterproofing on alkali-aggregate reaction (AAR) in concrete by separating water from alkali reactive aggregates through surface, aggregate, and matrix treatments. Accelerated mortar bar tests (AMBT) are conducted to analyze the expansion caused by alkali aggregates. Furthermore, the suppressive mechanism of waterproofing on AAR is explored using scanning electron microscopy (SEM), while the influence of waterproof concrete aggregate and matrix on pore characteristics and hydration products is assessed using nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). The results demonstrate that surface waterproofing with silane and polyvinyl alcohol (PVA) effectively suppress AAR. Moreover, PVA-coated aggregates significantly enhance the compactness of the interfacial transition zone (ITZ) in concrete. Based on these findings, an improved model considering waterproofness is proposed to quantify the degree of alkali aggregate reaction. These findings offer valuable guidance for controlling AAR.

PET particles modify strain hardening cementitious composites: An approach to introduce defects to enhance deformation capacity

Enhancing deformation capabilities remains a critical direction in the research of Strain-Hardening Cementitious Composites (SHCC). This investigation introduces Polyethylene Terephthalate (PET) particles as a novel method to induce micro-defects within SHCC, aiming to reduce the material's initial crack strength and improve its deformability. Through the exploration of various PET particle volumetric substitutions for traditional aggregates, the development of PET particle-modified SHCC (PMSHCC) was achieved. The paper examines the impact and mechanisms of different PET volumetric substitution rates (0 %, 5 %, 10 %, 15 %, 20 %, and 25 %) on the workability, axial compression strength and tensile properties of PMSHCC. By employing Digital Image Correlation techniques, the progression of crack width and density in PMSHCC under tensile load was documented, facilitating an analysis of the influence mechanisms of varying PET volumetric substitution rates on the tensile strain-hardening behavior of PMSHCC. The findings indicate that the increased Interfacial Transition Zone (ITZ) thickness between PET particles and the cement matrix, combined with the lower elastic modulus of PET particles compared to the cement matrix and quartz sand, collectively contribute to a reduction in tensile initial crack strength and an enhancement in ultimate tensile strain of PMSHCC. Within the examined range, an increase in the PET volumetric substitution rate significantly improves the ultimate tensile strain of PMSHCC by up to 76.0 %, with a minimal reduction in tensile strength of only 3.99 %. Moreover, the study proposes a tensile linear strengthening elasto-plastic model for PMSHCC incorporating the PET substitution rate, enabling accurate prediction of the material's tensile stress-strain behavior. These findings not only offer a novel pathway for the utilization of waste plastics but also provide significant insights for enhancing the deformability of SHCC.

Issues in the Use of Aggregates from Demolition Sites, in Construction, with a View to Protecting the Environment

Construction professionals are increasingly confronted with issues related to the preservation of the environment. For several years, they have been facing a double problem. In highly urbanized regions, the number of buildings is increasing, and with them the need for raw materials. Concrete is one of the most used materials in modern works, the need for aggregates, one of its components is very high. However, the quarries of aggregates are running out. Potentially usable spaces are becoming scarce, and construction professionals are facing supply difficulties. On the other hand, in the building sector, millions of tons of waste from demolitions are still too often deposited in storage facilities. It is becoming clear that the recovery of this waste is a major environmental issue, and that their reuse in the form of aggregates would be a beneficial alternative for the environment. It would allow pollution by landfill to be limited. In order to determine a relevant use of recycled aggregates in construction, we have done the tests for determining the density, los angeles, water absorption, proctor test on its aggregates. We got the following conclusions: recycled concrete aggregates are generally more absorbent and less dense than ordinary aggregates. The form of recycled aggregates is similar to that of crushed stone. The concretes made with these aggregates have acceptable workability. Due to their composition, these recycled aggregates have different characteristics from natural aggregates, and their mechanical behavior is distinct. Indeed, recycled aggregates are more porous, have a higher water absorption coefficient, contain hydrates, and have interfacial transition zones, which are weak zones for the mechanical strength of the concrete. Recycled aggregates are also suitable for road foundation work.

Durability properties of concrete containing copper heap leach residue as aggregates: experimental and analytical assessments

This paper investigates durability related properties of concretes containing copper heap leach residue (CHLR) as partial replacement of natural fine and coarse aggregates. The use of CHLR as aggregates in concretes promotes the reduction of the dependance on natural aggregates, upcycling of the waste as aggregates and the reduction of carbon footprint associated with natural aggregate productions. In this study, CHLR was washed, dried, and sieved to separate fine aggregate and coarse aggregate, and concretes were prepared with a cement content of 400 kg/m3 and water-cement ratio of 0.435 by replacing 25-75% natural fine and coarse aggregates. The concrete containing 50% CHLR as a partial replacement of natural CA and FA gained compressive strength of 52.9 and 54.0 MPa; drying shrinkage of 662 and 538 με; volume of permeable voids of 6.2 and 5.7%; 1485 and 2640 coulomb of charge passed in chloride permeability; and primary sorptivity coefficients of 4.0 × 10−3 and 4.3 × 10−3 mm/sec0.5 at 180 days, respectively. In contrast, these properties for the control specimens at the same age were 59.1 MPa, 394 με, 5.19%, 1280 coulomb, and 2.5 × 10−3 mm/sec0.5, respectively. The compressive strength and durability aspects declined in concretes using 75% CHLR coarse and fine aggregates. Existing analytical models for durability related properties of concrete containing natural aggregates are compared to that of the concretes using CHLR. Finally, backscattered electron images coupled with energy dispersive x-ray spectroscopy of the CHLR concretes were analyzed to understand the pore refinement, interfacial transition zones, microcracks, and hydration products influencing the durability aspects.

Experimental study on the effect of carbonate ions on ITZs of geopolymeric recycled concrete

In this study, carbonated recycled aggregates (CRA) were used in preparing geopolymeric recycled concrete (GRC). The influence of CRA with different carbonation durations on the compressive strength of GRC was studied, and the physical properties of CRA were tested. Bi-material Brazilian disc (BBD) was explored to simulate interfacial transition zones (ITZ). According to the test results, CRA was found to introduce quantities of carbonate ions, causing a similar adverse effect of efflorescence in GRC. Therefore, the compressive strength decreased in the first 24 h of carbonation. After 96 h carbonation, the reduction in compressive strength was insignificant. When using CRA to prepare GRC, it would be advisable to carbonate recycled aggregates for longer than 96 h. The results of the splitting tensile tests and nanoindentation test of BBD revealed that the carbonate ions introduced by carbonation treatment could slightly weaken the bonding properties of ITZ in GRC.

Impact of calcium formate and anhydrous sodium sulfate on the flowability, mechanical properties, and hydration characteristics of UHPC

Ultra-high performance concrete (UHPC) is not an ideal material for rapid repairs of bridge expansion joints due to its slow early strength development. To mitigate this early strength limitation, this study investigated the use of calcium formate (CF) and anhydrous sodium sulfate (SS) as early strength agents to enhance the early strength of UHPC. The findings indicate that a composite addition of 0.25 % SS and 0.25 % CF significantly improves both strength and workability, resulting in a 28-day compressive strength of 141.3 MPa, a 28-day flexural strength of 12.6 MPa, and a flowability of 227 mm. SS facilitates the formation of ettringite (AFt) and calcium hydroxide (C-H), while CF promotes the generation of smaller gismondine particles. This substantial increase in hydration products reduces the porosity of UHPC, thereby densifying the matrix and enhancing its mechanical strength. The synergistic effect of both additives leads to a higher concentration of calcium ions (Ca2+) around the smaller gismondine particles, which generates additional hydration products, optimizes the pore structure of UHPC, and narrows the width of the interfacial transition zone (ITZ). Consequently, this combination significantly improves the early strength of UHPC, achieving a 45.6 % enhancement in the early hydration rate with the use of 0.25 % CF and 0.25 % SS.

Experimental investigation and numerical simulation of chloride diffusion in rubber concrete under dry-wet cycles

Chloride erosion caused by the dry-wet cycles is one of the most significant factors leading to durability problems in concrete structures. In this paper, the diffusion behavior of chloride in rubber concrete with varying rubber content is studied by experimental investigation and numerical simulation based on setting up artificial dry-wet cycles exposure conditions. The determination of chloride concentration in rubber concrete is achieved through chloride concentration titration, and as a result, the surface chloride concentration and the apparent chloride diffusion coefficient are expressed as a function of time, thus providing a theoretical guide for Comsol Multiphysics to perform numerical simulations. The findings indicate that the chloride diffusion behavior in rubber concrete complies with Fick's second law and that rubber can effectively reduce the diffusion rate of chloride in rubber concrete. Furthermore, the surface chloride concentration and apparent chloride diffusion coefficient are fitted with high accuracy according to the experimental data, which ensures the precision of the numerical simulation. In addition, numerical simulation results indicate that the diffusion of chloride in rubber concrete is impeded by the aggregate, that is, chloride diffusion is faster in areas where the local aggregate volume fraction is small. Also, it is evident that the presence of interfacial transition zone (ITZ) negatively affects the resistance of rubber concrete to chloride diffusion.

A study of fine-scale low-temperature cracking in geopolymer grouted porous asphalt mixtures based on real aggregate profile modeling

At a micro-scale level, grouted porous asphalt mixtures are multiphase composites consisting of aggregates, asphalt mortar, filler, asphalt mortar-aggregate interfacial transition zone, and asphalt mortar-filler interfacial transition zone. It exhibits different mechanical behavior under different temperatures and loading conditions. Compared with aggregate, asphalt mortar, and filler, the interfacial transition zone (ITZ) is more prone to damage at low temperatures, with both the fine structure and the nature of the contact interface having a significant effect on the overall strength and crack resistance of the material. To evaluate the influence of ITZ and filler material properties on the cracking resistance of the material, a grouted porous asphalt mixture model based on the real aggregate shape was established using finite element modeling. A cohesive zone model (CZM) was employed to characterize the internal mechanical behavior of the material, and a mesoscale damage simulation was performed for the grouted asphalt mixture. The study revealed the influence of different interfacial and filler material properties on the low temperature cracking resistance and proposed a design approach for anti-cracking grouted asphalt mixtures. The findings indicated that interfacial transition zone (ITZ) and filler properties play a crucial role on determining the damage characteristics. Increasing the ITZ and filler properties significantly enhanced the low temperature crack resistance of the grouted asphalt mixture.

Electrical characterization of freeze-thaw damage in saturated concrete and its interfacial transition zones

Determining the freeze-thaw (F-T) damage of the interfacial transition zone (ITZ) is crucial to understanding the mechanism of F-T in concrete and improving its frost resistance. In this paper, the F-T damage of saturated concrete/mortar and ITZ is characterized using AC impedance spectroscopy, and the micro-mechanisms of F-T damage in cement-based materials (CBMs) are summarized. Firstly, the damage factor was defined by the effective resistivity of the CBMs, and a correlation between the damage factor and the compressive strength loss was established. The F-T damage process was divided into a damage development stage and a damage stabilization stage based on the damage factor and fractal dimension. Then, the effective resistivity separation formula was proposed to calculate the ITZ effective resistivity, and the ITZ damage factor was calculated for non-destructive quantitative characterization of the F-T damage in the ITZ. Finally, the microscopic mechanism of F-T damage in CBMs and their ITZs was summarized based on damage factor, fractal dimension, super depth of field microscopy, and nanoindentation. The results showed a linear relationship between the damage factor and compressive strength loss (Lc = 0.176Er+0.030); the ITZ had damage factors of 80 % and 70 %, which were 1.6 and 1.4 times higher than those of the matrix, respectively; F-T cycles produce micro-damage, part of which connects with the disconnected pores to form connected pores, the micro-damage extends and accumulates along the surface of the aggregates, leading to ITZ debonding from aggregate and deterioration of the mechanical properties.

Modeling the effect of material heterogeneity on the thermo-mechanical behavior of concrete using mesoscale and stochastic field approaches

The adverse impact of high temperatures on concrete is a well-recognized issue that can lead to significant mechanical deterioration and structural integrity loss. Factors such as aggregate type, cement composition, temperature, duration of exposure, and moisture content can substantially influence the fire resistance of concrete. To simulate and better understand the effects arising from the heterogeneity of concrete in a fire situation, a mesoscale approach is proposed, using the Mesh Fragmentation Technique (MFT) to assess the complex thermo-mechanical behavior of concrete. The MFT introduces high aspect ratio interface elements to model crack propagation and interfacial transition zones by means of an appropriated tensile damage constitutive law. In this extended framework, a fully-coupled thermo-mechanical model is proposed. The modeling approach includes considerations of both macroscopic and mesoscopic scales, in which the coarse aggregate, mortar matrix and interfacial transition zones are represented. Besides, a stochastic distribution is assumed for the material properties to account for the lower scale heterogeneity. The main novelty proposed in this study consists in the synergy of mesoscale and stochastic approaches that are herein combined to model the effect of heterogeneity on the concurrent macro-mesoscale thermo-mechanical behavior of concrete. To validate the numerical model’s capabilities in capturing thermally induced cracks, benchmark cases and a simulation of a bending beam exposed to elevated temperatures are presented. The results demonstrate the potential of the proposed approach in predicting the behavior of concrete subjected to thermal loading and the role played by heterogeneity in the thermally induced cracking.

Effects of colemanite, heavy aggregates and lead fibers on physical mechanics and radiation shielding properties of high-performance radiation shielding concrete

Radiation shielding concrete (RSC) is widely utilized as a construction material in specialized infrastructure of nuclear power plants and medical facilities. In this study, a high-performance-RSC (HPRSC) with excellent mechanical properties and radiation shielding properties was developed based on the modified Andreasen and Andersen model. HRPSC can effectively shield a broad spectrum of radiation rays due to the combination of neutron absorber (colemanite) and gamma scatterers (e.g., heavy aggregates of barite and magnetite, and lead fibers). The experiment results revealed that the compressive strength of HPRSC exceeds 70 MPa due to a scientific skeleton structure design, which contributes the formation of an exceptional interfacial transition zone between the heavy aggregate/lead fiber and the cementitious matrix. The type of heavy aggregates influenced the physical mechanics properties of HPRSC. The compressive strength of magnetite-based HPRSC was higher than that of barite-based counterparts under the same dosage of heavy aggregates. The incorporation of heavy aggregates and lead fibers enhanced the γ-ray shielding capacity of HPRSC. The μ index of designed HPRSC reached 0.1988 due to the synergistic complementary effects between the cementitious matrix and the radiation shielding additives. HPRSC shows a promising application prospect in high radiation environments.

Physical characteristics identification of the concrete interfacial transition zone via 3D image scanning

The interfacial transition zone (ITZ) is the weakest link in concrete materials. The physical characteristics of the ITZ, such as its microhardness and thickness, significantly influence the mechanical properties and durability of concrete. Typically, microhardness testing is employed to identify the physical features of the ITZ through line measurements with a certain degree of randomness. In order to accurately describe ITZ physical characteristics, this study proposes a novel physical characteristics identification method via three-dimensional (3D) image scanning. The fundamental principle of this method lies in the varying hardness of each concrete phase, resulting in different surface elevations after polishing. As the polishing abrasiveness changes from coarse (240 mesh) to fine (800 mesh), the ITZ can be clearly observed in the 3D point cloud images of the experimental concrete. The ITZ geometrical information (thickness and different surface heights) can be easily identified using 3D scanned images. It was also found that there was a considerable linear regression relationship between the height difference and the relative microhardness variation; thus, the ITZ microhardness could be indirectly recognized through surface measurements. The physical characteristics of ITZ can be statistically analysed, effectively mitigating the bias inherent in traditional test results and significantly improving identification efficiency and accuracy.

A low carbon treatment to enhance recycled aggregate concrete durability by continuous loading at the curing period

This study investigates the impact of continuous loading treatment during the curing period on the durability of recycled coarse aggregate concrete with a high replacement ratio. Evaluations were conducted on key parameters including compressive strength, and several durability indicators such as chloride ion permeability, freeze-thaw resistance, and carbonation resistance. Microscale examinations, involving air void structure, microhardness, hydration products, and microstructures, were also performed. The results demonstrate that continuous loading treatment significantly enhanced compressive strength by 4.13%–26.34%, reduced the chloride ion migration coefficient by 14.89%–38.30%, and improved freeze-thaw cycles by 20%. These performance improvements are primarily attributed to the optimized pore structure and strengthened interfacial transition zone. Additionally, this method presents a promising an eco-friendly approach for producing high-durability prefabricated concrete components.

Efficient utilization of coral waste for internal curing material to prepare eco-friendly marine geopolymer concrete

To address shortages in construction materials for island engineering, tackle the accumulation of solid waste, and inhibit the shrinkage of geopolymers, coral waste was utilized as the internal curing material to prepare high-performance marine geopolymer concrete (MGC) with seawater, sea-sand, and normal limestone aggregate (LsA). The coral coarse aggregate (CorA) used in this investigation has a total porosity ranging from 50% to 58.3% with internal pore diameters spanning 50–400 μm. The water desorption of CorA followed a two-stage pattern within a relative humidity (RH) range of 75%–85%, becoming nonlinear above 90% RH, which released about 85% of its moisture within 200 h at 97% RH, demonstrating potential for internal curing. Adding a small amount of CorA to MGC increased slump and setting time by providing internal curing water. However, as CorA content exceeded 30%, the slump significantly decreased due to reduced mixing water and elevated activator concentration, while the initial setting time slightly decreased. Furthermore, the inclusion of saturated CorA in MGC significantly reduced autogenous shrinkage, with higher CorA contents (exceeding 30%) leading to slight expansion in the early stages and nearly eliminating shrinkage at contents above 40%. The greater drying shrinkage in geopolymer systems compared to ordinary Portland cement is due to capillary pressure compressing the product framework, converting larger gel pores into smaller ones. Additionally, the layered calcium aluminosilicate hydrate (C-A-S-H) gel exhibits more pronounced creep characteristics under low internal humidity conditions. The higher CorA content in MGC promoted the formation of hybrid C, N-A-S-H gel and hydrotalcite-like phases, and reduced carbonation issues. The interfacial transition zone (ITZ) between CorA and the geopolymer matrix formed a robust mechanical interlock, enhancing tensile strength and minimizing shrinkage-induced cracks. Based on overall performance and marine material utilization, an optimal substitution rate of CorA between 40% and 50% is recommended.

Using waste to improve the weak: Recycled seashell as an ideal way to regulate the interfacial transition zone in biochar-cement composites

The strategy of partially substituting cement with carbon-negative biochar to fabricate biochar-cement composites can represent a promising approach to reducing the carbon emissions of the construction industry. However, the detrimental impact on strength development from excessive biochar incorporation remains a significant challenge. To address this issue, this study pioneers a strategy to modify wood waste-derived biochar using seashell waste for enhancing its binding with the cement matrix. Employing a mechanochemical process, the shell-modified biochar was synthesized using oyster shell (OS) and wood waste (WW). The compressive strength of cement composites incorporating 5 wt% modified biochar improved by 3.9–17% compared to the referenced biochar. Even with high biochar incorporation (30 wt%), the compressive strength increased by 58.2% upon modifying the biochar at the OS/WW ratio of 1:9 (10SBC). The modified biochar refined the pore structures, accelerated the hydration kinetics, improved the degree of hydration, and fostered the development of the C-S-H gel network with a longer silicate chain and a higher polymerization degree. Furthermore, the strength enhancement of the biochar-cement composites was attributed to the improved micro-mechanical properties at the interfacial transition zone (ITZ), which increased the volume fraction of high-density C-S-H by 64.4–112% and decreased the low-density C-S-H fraction by 41.8–52.6%. Notably, 10SBC demonstrated a superior improvement due to its cohesive bond with the C-S-H gel and the concurrent densification of C-S-H in the ITZ, which was facilitated by the growth of Ca-/Al-rich hydration products. Overall, this study synergistically upcycled wood and seashell waste into value-added cement additives, thereby achieving strength enhancement of the cement composites.

Estimation of cement paste stiffness and UHPC elastic modulus through measured phase-property upscaling

The elastic stiffness of bulk concrete materials results from the complex interaction of aggregates, voids, and hydrated cement (which can have multiple hardened phases at multiple length scales). Given the complexities associated with understanding the arrangement of these particles within bulk concrete volumes, estimations for elastic modulus often rely on empirical correlations with unit weight and compressive strength. Such estimations are inherently scale-dependent and fail to capture variability in mix designs, particularly the variability found in specialty concrete mixes. To develop a scale-independent method for estimating elastic modulus from mix-design volume fraction information, this study explores a novel bottom-up approach using cement paste phase stiffness values determined through micro-mechanical experimentation and randomized Monte-Carlo spring arrangement simulations. Statistical representations of cement paste phase stiffness distributions and bulk volume fraction data are combined to provide estimations for elastic stiffness in both the composite cement paste and bulk concrete containing fine aggregate and fibers. Resulting a priori estimations of UHPC cement paste stiffness from the micro-mechanical upscaling simulations were within 4% of measured values (based on mix-design and void volume fraction information alone) for a selected sample of mix proportions. When applied to the two UHPC mixes containing fibers and fine aggregate, upscaling simulations consistently overpredicted the measured elastic modulus, likely due to the aggregate-cement interfacial transition zone (ITZ) properties that were not captured in the micro-mechanical testing.

Study on the mesoscopic mechanical behavior and damage constitutive model of micro-steel fiber reinforced recycled aggregate concrete

This study employs in-situ 4D CT experimental techniques to investigate the mesostructured changes and mechanical behavior of micro steel fiber-reinforced recycled aggregate concrete (MSF-RAC) under uniaxial compression and cyclic loading conditions. The research aims to address the scientific problem of limited understanding of how micro steel fibers affect the mesoscopic mechanical behavior and structural damage characteristics of recycled aggregate concrete. High-resolution CT scanning technology was utilized for real-time observation of crack initiation and propagation, revealing the micro-mechanisms by which micro steel fibers enhance RAC. The study found that incorporating micro-steel fibers significantly improves the density of the interfacial transition zone (ITZ), inhibits crack development, and enhances the mechanical properties of the concrete. Based on these observations, an improved stiffness degradation damage constitutive model was proposed, which comprehensively considers the distribution orientation of micro steel fibers and ITZ characteristics. The model effectively predicts the stress-strain relationship of MSF-RAC under various conditions, demonstrating high accuracy and practicality. These findings provide important theoretical support and experimental evidence for the optimal design of RAC, showcasing significant engineering application value by promoting the sustainable use of recycled materials in construction.