Interfacial Transition Zone Research Articles

Influence of the graphene oxide-coated steel fiber on the microstructure optimization of UHPC

Benefiting from its superior mechanical properties, large specific surface area and abundant oxygen-containing functional groups, graphene oxide (GO) can generate nucleation and pore-infilling effects to optimize the microstructure of the cementitious composites, thus reinforcing the mechanical performance and durability of cement-based materials. However, due to the dispersion issues and relatively high cost, the widespread use of GO in the cement industry has not yet been realized. In the present study, we applied three different methods to coat industrial GO nanosheets on the steel fiber’s surface and reinforce the UHPC microstructure. The proposed coating method effectively disperses GO within the ultra-high-performance concrete (UHPC) matrix and targets its enhancement effects on the interfacial transition zone (ITZ), which has the weakest pore structure in UHPC. The result presents that the three-step dip method of GO coating on the steel fiber demonstrates higher effectiveness in reinforcing the microstructure of UHPC, the porosity of the matrix is 1.88 %, with a relative decrease of 37.1 ∼ 60.3 %. In addition, the average distance of ITZ after GO enhanced UHPC is about 3.7 µm, about 37.3 %-68.1 % shorter, and the ITZ porosity is 3.8 %, which is 15.6 %-65.5 % lower. With the assistance of the metal intrusion technique, backscattered electron characterization and deep learning-based analysis, the pore structure features of UHPC were extracted with a classification recognition accuracy of 88.5 %, and the pore intrusion volume inside the characteristic region of the UHPC matrix increased by 1.89–2.14 times. The extraction characteristics illustrated that the GO strengthening on steel fibers was more effective in ITZ, with an optimization efficiency of about 8.1 % higher than in other regions. We expect that the findings of this study could provide deep insights into GO modification for cementitious composite reinforcement applications and open new pathways for affordable alternatives to nano-enhanced composites.

Research on humidity field and freezing resistance of green recycled ceramic internal cured concrete at early age

In cold and drought environment, concrete shrinkage cracking caused by water loss is serious, and thus the freezing resistance of concrete is deteriorated. The waste ceramic particles were used as internal curing materials to prepare concrete, based on this background. Then, the influence of internal curing with ceramic particles on humidity field and temperature of concrete is analyzed by using temperature and humidity sensor. Based on the change mechanism of the internal humidity of concrete, the relationship between the curvature radius of liquid surface and the relative humidity is revealed. Taking water loss and water loss rate as the index, the water transfer law of concrete surface under ceramic particle internal curing was defined. Through the drying shrinkage test, the mechanism of improving the drying shrinkage of concrete by internal curing in ceramic particles was mastered. Finally, the effects of ceramic internal curing on cement hydration, interfacial transition zone (ITZ) and freezing resistance of concrete were studied. The results show that the relative humidity, water loss and water loss rate of concrete increase with the increase of ceramic particle content, and the dry shrinkage value decreases with the increase of ceramic particle content. Compared with ordinary concrete, when the ceramic particle content is 10%, 15%, 20%, 25% and 30%, and the test period is 10 days, the internal relative humidity of concrete increases by 0.52%, 0.67%, 1.55%, 2.55% and 3.05%, respectively. The drying shrinkage values decreased by 3.42%, 7.69%, 10.26%, 15.17% and 30.13%, respectively. When the test period was 1032 hours, the water loss increased by 1.08%, 5.15%, 7.90%, 12.69% and 14.49%, respectively. Furthermore, when the ceramic particle content is 20%, the initial normalized heat flow of concrete increases by 2.74 mW/g, and the normalized heat flow minimum increases by 0.324 mW/g. The minimum ITZ microhardness increased by 2.64 MPa, the ITZ width decreased by 5 μm, and the maximum crack width at the contact between ITZ and aggregate decreased by 0.89 μm. At the same time, when the ceramic particle content is less than 20%, the effect of internal curing on the freezing resistance of concrete increases with an increase in ceramic particle content.

Numerical analysis of failure mechanics of concrete under true dynamic triaxial loading using a four-phase meso-model

In the present paper, the failure mechanics of concrete under true triaxial dynamic loading is investigated. A four-phase concrete meso-model is developed. The meso-model consists of aggregate, mortar, interfacial transition zone (ITZ) and macropores. A finite element (FE) model of a split Hopkinson pressure bar is also created to apply true triaxial dynamic loading. The development of cracks in concrete is presented for the uniaxial, biaxial and triaxial loading cases. The presence of local discontinuity is modelled, and its effect on the failure of concrete is extensively investigated and discussed. In the last, the effect of aggregate distribution is also analysed. It is found that the internal structure of concrete changes the stress distribution from true triaxial to compression or extension triaxial, which is the prime cause of internal cracking.

Rice husk ash and recycled sand as synergistic substitutes in ultra-high performance concrete: Microstructural, mechanical performance, and eco-economic analysis

Quartz sand and silica fume (SF), commonly used in UHPC production, are costly and contribute to dust pollution and significant carbon emissions. In this work, we explored the preparation of an environmentally friendly ultra-high performance concrete (e-UHPC) by substituting SF and quartz sand with rice husk ash (RHA) in ratios of 10 %, 20 %, and 30 %, and recycled building sand (RS) in ratios of 25 %, 50 %, and 75 %. We assessed the impact of these substitutions on workability, mechanical strength, hydration products, and the micromechanical characteristics of the interfacial transition zone. In addition, an environmental and economic assessment index for e-UHPC was introduced. Results showed that while RHA and RS reduced the flowability of e-UHPC, RHA helped mitigate the compressive strength loss due to RS (3.9 %-9.7 %). RHA accelerated early hydration, and improved densification in the transition zone, enhancing late hydration and refining the pore structure of e-UHPC. Notably, a 10 % RHA replacement rate optimally improved early hydration efficiency and minimized CH content. The combination of 10 % RHA and 25 % RS proved most effective in optimizing the interfacial transition zone and improving the pore structure of e-UHPC. Importantly, e-UHPC reduces the carbon footprint at a lower cost and offers significant environmental and economic advantages over traditional UHPC. The results of this study will promote the incorporation of construction and agricultural wastes into concrete.

An innovative method for mesoscale modelling of moisture diffusion in concrete

Moisture diffusion influences the durability and long-term performance of concrete and whilst it predominantly occurs via the cement matrix and Interfacial Transition Zone, most existing models consider concrete to be homogeneous. This paper introduces a novel micro-meso model that employs random packing and Voronoi tessellation. Rayleigh-Ritz pore distribution and Brunauer-Skalny-Bodor models are combined to determine the radius and fraction of various pores. The results indicate that relative humidity diffuses faster with increasing temperature, decreasing ambient relative humidity and tortuosity. Ambient relative humidity has a greater influence on diffusion compared to temperature and tortuosity. Numerical and experimental comparisons demonstrate that the proposed methodology effectively captures relative humidity distribution across various scenarios. Furthermore, explicit pore network modelling incorporates key parameters for a more accurate analysis. Integrating the proposed methodology into a fully coupled hygro-mechanical framework can potentially yield more accurate predictions of mechanical behaviour; enhancing the reliability of long-term performance assessments and enabling more durable concrete design.

Tailoring magnesium potassium phosphate cement-based adhesive for the improved bonding performance of CFRP/concrete interface

Magnesium potassium phosphate cement (MKPC) with rapid hardening and early high strength can be used as inorganic adhesive for externally bonded carbon fiber reinforced polymer (CFRP). This study aims to tailor MKPC as inorganic adhesive with good infiltration and bonding strength for CFRP-reinforced concrete. The influences of water-to-binder (w/b) ratios and particle sizes of potassium dihydrogen phosphate (PDP) on the fresh properties, hydration kinetics, pore structure, and strength development of MKPC were studied. The bonding strength of CFRP/concrete with MKPC-based adhesive was evaluated by double-shear tests. Results showed that a lower w/b ratio and a finer PDP enhanced the strength of MKPC due to a refined microstructure. The bonding strength of CFRP/concrete was dependent on the rheological characteristics and strength of MKPC. The use of a coarse PDP in MKPC was beneficial to enhance the bonding strength of CFRP/concrete. MKPC with coarse PDP and a w/b ratio of 0.2 resulted in the greatest bonding strength, corresponding to a 20% enhancement compared to epoxy resin. The superior bonding performance of MKPC was ascribed to its higher strength, a better infiltration for CFRP, and a larger MPC/concrete interfacial transition zone (ITZ).

Limestone calcined clay-based engineered cementitious composites (LC3-ECC) with recycled sand: Macro performance and micro mechanism

Limestone calcined clay cement (LC3) contributes to the development of sustainable cement-based materials. In this study, LC3 was combined with fly ash (FA) and recycled sands (RS) to develop engineered cementitious composites (ECC). Three FA/LC3 ratios (1, 1.5, and 2) and three RS particle sizes (0.15–1.18 mm, 0.15–2.36 mm, and 0.15–4.75 mm) were studied. Compared to ordinary Portland cement (OPC)-based paste, the LC3 substitution resulted in higher hydration heat and hydration degree, forming more hydration products. Although LC3 substitution produced a slightly lower compressive strength, a more robust tensile strain-hardening behavior than OPC-based ECC was achieved. The increasing FA/LC3 ratios gradually reduced compressive strength, tensile strength, and crack width. The multiple cracking behavior with controlled widths of LC3-based ECC was found to be more stable, which could be traced to the superior properties of fiber/matrix interfacial transition zone. In addition, utilizing RS as fine aggregates with various particle sizes could achieve an ultra-high tensile strain capacity exceeding 8 % for LC3-based ECC, thereby enhancing its utilization efficiency. Furthermore, the utilization of LC3 resulted in 28.6–47.1 % and 6.6–22.0 % reductions in carbon emission and embodied energy, respectively. The combination of LC3 and RS holds a promising approach to developing sustainable ECC with desirable mechanical performance.

Enhancement of Recycled Aggregate Concrete Properties Through the Incorporation of Nanosilica and Natural Fibers

This study introduces an innovative approach to enhancing recycled aggregate concrete (RAC) by incorporating nanosilica (NS) and natural fibers (NF), specifically sisal fiber (SF) and palm fiber (PF). This novel combination aims to overcome the inherent limitations of RAC, such as reduced strength and durability, while promoting sustainability in construction. The research focuses on evaluating the mechanical properties of RAC, including compressive and flexural strengths, through the integration of NS and NF. Our findings reveal that NS significantly improves the microstructure of RAC by enhancing the interface transition zone (ITZ) and filling nanovoids, resulting in a denser and more durable concrete matrix. Specifically, the addition of 3% NS increased the compressive strength of RAC by up to 22.5% and the flexural strength by up to 25.6% at a 100% replacement ratio of recycled aggregate. The addition of NF, treated to withstand the alkaline environment of concrete, further strengthens the RAC by providing a bridging effect that enhances flexural strength by up to 46.7%. This work not only advances the performance of recycled concrete but also aligns with the broader goal of environmental sustainability by utilizing waste materials and reducing the carbon footprint of concrete production. The findings have the potential to influence future construction practices, encouraging the adoption of more durable and eco-friendly building materials.

An experimental and numerical study on chloride transport in concrete under single bending load and coastal soft soil environment

This paper presents an experimental and numerical simulation study on the corrosion of chloride ions in concrete components, using a single pre-applied bending load to represent an additional bending moment caused by an external one-off disturbance. Besides, a novel correlation testing method, the Pearson-Mutual Information (PMI) method, is proposed based on Pearson correlation coefficients and mutual information theory. The findings indicate that a significant effect on chloride ion transport is only observed when the ratio of the applied bending load to the ultimate bending load (λ) exceeds 0.3. There is a distinctly positive and nonlinear relationship between λ and free chloride concentration (CCl), the apparent chloride diffusion coefficient (Da), as well as apparent surface chloride concentration (Cs). The bivariate time-varying model developed in this study demonstrates high fitting accuracy, with simulation results that closely correspond to actual measured data, achieving a coefficient of determination (R²) of 0.98 or higher. Furthermore, a sensitivity analysis of the model input parameters indicates that Cs is more sensitive than Da, and λ is identified as the key parameter. The impacts of the interfacial transition zone (ITZ) and its diffusion coefficient (Ditz) were found to be minimal on the output of the model.

Impact of PVA-coated aggregate on concrete impermeability and creep characteristics under hydraulic pressure and development of a creep model

In marine engineering projects such as sea dikes, concrete must exhibit both exceptional impermeability and plastic deformation to prevent cracking and subsequent internal corrosion due to synergistic deformation with the foundation. This study enhances the interface transition zone (ITZ) and plastic deformation of concrete by applying a PVA film to the aggregate surface. We use nuclear magnetic resonance to investigate the internal pore distribution and conduct permeability tests to assess its impermeability. Utilizing a self-developed triaxial hydraulic creep apparatus, we tests the concrete's creep performance under hydraulic pressure. Compared with conventional waterproof concrete, the findings reveal that the PVA film on the aggregate surface significantly increases the water penetration pressure of the concrete and markedly enhances the momentary deformation and creep deformation under hydraulic pressure, significantly extending the time to creep. Furthermore, the study proposes a creep model incorporating a fractional derivative-based dashpot model, combined with the Burgers model. This research offers a novel approach and theoretical support for developing impermeable and plastic concrete suitable for marine engineering applications.

Synthesis of a novel one-part alkali-activated material from solid wastes salt sludge and soda residue

In this paper, salt sludge (SS), a solid waste from sodium hydroxide production, was activated at high temperature for the first time. It was combined with soda residue (SR), a solid waste from sodium carbonate production, as a solid activator to activate the solid aluminosilicate precursor, blast furnace slag (BFS). This process developed a novel one-part alkali-activated material, termed salt sludge and soda residue co-activated blast furnace slag cementitious material (SRB), which effectively utilizes solid wastes. The effects of ratios of calcined SS (CSS) to SR, fine aggregate types, and curing methods on the physical and mechanical properties of SRB were investigated. The composition of the hydration products was characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared spectroscopy (FTIR), while the hydration process during the setting stage was examined with 1H low-field nuclear magnetic resonance (1H LF NMR). The influence of mix ratio, fine aggregate type, and curing method on the microstructure of SRB was analyzed using scanning electron microscopy (SEM). The results showed that the synergistic activation of BFS by CSS and SR was significantly more effective than individual activation. At the optimal mix ratio (CSS:SR:BFS = 15:15:70), the 3/28 days compressive strength of SRB was 14.3/53.7 MPa. The primary hydration products were C-(A)-S-H gels, along with Friedel's salt, ettringite, and hydrotalcite crystals. Using manufactured sand (MS) as a fine aggregate reduced mortar fluidity compared to river sand (RS) but enhanced the interfacial transition zone (ITZ) with the hardened paste, resulting in a denser microstructure. Heat curing promoted hydration product generation, increasing early (3 days) strength by a factor of 2.5. However, the rapid accumulation and insufficient diffusion of hydration products adversely affected the later strength.

Long term properties of Nano-engineered temperature self-controlled concrete served for mass concrete structure

Due to the advantages of low-cost and superior electrical conductivity, Nano-carbon black (NCB) can endow hydraulic concrete with long-term and stable self-heating capacities at affordable costs, thus allowing active and intelligent temperature control for high-altitude and high-latitude dams. This study, for the first time, investigated the long-term mechanical behaviors, freeze-thaw resistance and impermeability of temperature self-controlled concrete (TSC). Moreover, the modification mechanisms of TSC were revealed with the help of mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). Finally, the long-term performance and cost assessment of TSC are comprehensively evaluated by the entropy weight method. The results indicate that the incorporation of 4.5wt% NCB improves the compressive and splitting tensile strengths of TSC at 180 days by 17.4% and 7.9%, respectively, and shows an excellent response against impermeability and a slightly lower response against frost resistance. Besides, NCB accelerates the hydration degree and repairs the interfacial transition zone of TSC. The presence of NCB densifies the microstructures of the matrix and reduces the content of harmful pores by nearly 50%. Finally, TSC with 4.5wt% NCB is the optimal mixture that shows excellent comprehensive performances.

Shear tests and meso-scale simulation of cold joint structures in rock-filled concrete: Considering the mutual contact effects between component materials

To realistically reflect the shear performance of the rock-filled concrete (RFC) between layers, this study relies on an RFC dam project in Guizhou Province. Test blocks, measuring 5 m × 4 m × 2 m with cold joints, were cast on site and subjected to shear tests. Additionally, a three-dimensional four-phase meso-model was developed, considering the mutual contact effects between component materials. The mechanical response and damage mechanism of the interlayer were analyzed using the cohesive zone model. The results confirm that the cohesive zone model effectively simulates interlayer cold joint structures. The damage modes of the RFC interlayer include cold joint debonding damage (Mode I) and plastic extrusion damage of self-compacting concrete (Mode II). As the exposed height of the rocks increases, the damage mode shifts from Mode I to Mode II, and higher normal stresses intensify Mode II damage. Meanwhile, the paths of crack extension depend on the distribution of shear-resistant rocks, with weak interfacial transition zones serving as “bridges’’ for crack propagation. The interlayer shear strength is positively correlated with the average exposed height of rocks and normal stresses, with the former having a more significant impact. Finally, the accuracy of the proposed shear strength prediction model for the RFC interlayer was validated by comparing it with results from other studies, providing useful references for meso-numerical modeling of RFC.

Effect of cellulose nanofiber addition on the microstructure characterization and nano-mechanical behavior of interfacial transition zones in recycled concrete

Effect of cellulose nanofiber addition on the microstructure characterization and nano-mechanical behavior of interfacial transition zones in recycled concrete

Enhancing mechanical properties and crack resistance of high-strength SHCC/ECC for durable transportation through ethylene-vinyl acetate polymer modification

Strain-hardening cementitious composites (SHCC) possess excellent ductility and toughness, making them suitable for reinforcement and repair projects of transit infrastructures, such as tunnel linings, bridges, and highways. Especially, polyethylene (PE) fiber is admixed to SHCC for fabricating high-strength PE-SHCC. However, the hydrophobic nature of PE fibers results in relatively weak interfacial frictional bond strength with cementitious matrix, ultimately leading to larger crack widths and wider gaps. This poses durability and safety concerns for the practical applications of PE-SHCC in transit infrastructure. In this study, ethylene-vinyl acetate (EVA) dispersible polymer is selected to address this problem. The effect of admixing EVA on mechanical and direct tensile properties of PE-SHCC is investigated. The compressive strength, first crack strength and matrix fracture toughness of PE-SHCC tended to decrease after EVA modification, however, the tensile strength and tensile strain increased significantly with doping. After that, the single fiber pullout test combined with flaw size/distribution analysis is performed to explore the improvement mechanism of PE-SHCC after EVA modification. It was found that the addition of EVA could significantly improve the interfacial transition zone (ITZ), increase the interfacial friction between PE fibers and matrix, and optimise the defect distribution within the matrix. The recommended dosage of EVA is 3–6 % comprehensive consideration of the tensile strength, strain energy, crack width, and mechanical strengths of the EVA-modified PE-SHCC. The enhancement of mechanical strengths and crack resistance in PE-SHCC contributes to the development of transit infrastructure towards greater reliability, durability, and resilience.

Frost resistance and improvement techniques of recycled concrete: a comprehensive review

In the pursuit of sustainable construction practices, the utilization of recycled concrete has emerged as a pivotal strategy, distinguished by its commitment to resource conservation and environmental stewardship. Nevertheless, the inherent micro-porosity and micro-cracking within the old mortar of recycled concrete may lead to weak bonding performance at the interfacial transition zone, culminating in diminished strength, reduced density, and elevated water absorption rates compared to conventional concrete, which critically impairs its performance in cold climates subjected to freeze-thaw cycles. Consequently, this paper provides a structured examination of the frost resistance properties of recycled concrete subjected to freeze-thaw cycling. Initially, the study delineates the mechanisms of frost-induced damage in recycled concrete by synthesizing the degradation pathways observed in both conventional and recycled concrete during freeze-thaw exposure. Subsequently, a detailed analysis is conducted to identify the pivotal factors affecting frost resistance, encompassing the proportion and moisture affinity of recycled aggregates, the addition of silica fume and fly ash, the water-to-cement ratio, and the degree of water saturation. In the final segment, the study compiles and reviews the strategies for bolstering the frost resistance of recycled concrete, including the incorporation of air-entraining admixtures, fiber reinforcement, and aggregate modification approaches. The objective of this research is to offer a thorough comprehension of recycled concrete, with a concentration on the mechanisms of frost damage, the critical determinants of frost resistance, and interventions to augment its resilience against freezing conditions. On this basis, the present paper, in conjunction with the characteristics and current research status of recycled concrete, proposes recommendations for the application of recycled concrete in cold regions. This review is anticipated to facilitate researchers in gaining a comprehensive understanding of the freeze-thaw characteristics of recycled concrete and the measures to enhance its frost resistance. Furthermore, it aims to assist engineering and technical personnel in selecting appropriate treatment methods to improve the frost resistance of recycled concrete in cold regions, thereby promoting the practical engineering application of recycled concrete in such areas.

Characterization of Interface Transition Zone in Asphalt Mixture Using Mechanical and Microscopic Methods.

An enormous surge in the pavement sector requires the evaluation of interface bonding in asphalt composite, since the assessment of bonding brings considerable cost savings. Microscopic and mechanical analyses were performed to study the status of the interface transition zone of four groups of asphalt mixtures, using thin-slice preparation to obtain asphalt mixture slices with a flat surface for microscopic analysis. The interface transition zones were characterized using good knowledge of blending or diffusion phenomena by conducting tests both at the micro and macro levels to determine mixture quality. Asphalt mixture components were observed using fluorescence microscopy imaging and evaluated by the gray value change law. Asphalt mixture groups, (virgin, recycled of 30% aged and 70% unaged, 6%, and 4% rejuvenator dosage mixtures) under the same process parameters, which are a mixing time of 270 s and a mixing temperature of 150 °C, been considered optimum for component fusion in a hot asphalt mixture were used. This study relied on the influence of morphology law, assessed through rutting tests for high temperature performance, semi-circular bending tests for low temperature performance, and pull-off tests for interface bonding strength. The relationship between interface transition zones and macro performance was studied. The self-developed pull-off method was a research innovation which can be used as an alternative to study interface transition zones in asphalt mixtures, and provides the necessary data needed with 3D surface failure mode calculations. The device measured the bonding strength of a single aggregate in distinct positions using the bitumen penetration test method. The main goals were to determine a correction factor, identify the appropriate alteration, and compute the actual fracture surface area. Using scanning electron microscopy for interface characterization and micro-morphologies of mortar transition zone, our analysis provides adequate knowledge about interface position and the components present. The applied approaches to characterize asphalt mixture interfaces proved workable and reliable, as all methods have similar trends with useful information to determine asphalt pavement quality.

Microstructural Analysis of the Effect of Using Nano-silica on the Mechanical Properties of Cement–Sand Mortar Under the Effect of Heat

The performance of cement-based materials depends on the characteristics of solid particles at the nano-scale or nanometer porosities in the interfacial transition zone between cement particles and aggregate. Heat significantly affects the properties of these particles and the connection between them. Accordingly, the present study seeks to investigate the effect of nano-silica on the strength parameters of sand–cement mortar at high temperatures. In this regard, the sand–cement mortar was prepared by replacing 5, 10, and 15 percent of cement with nano-silica. The specimens were subjected to temperatures of 25, 100, 200, 400, 600, and 800 °C after curing at the ages of 3, 28, and 90 days. The effect of high temperatures on the physical and mechanical properties of sand–cement mortar was analyzed using macro-structural tests of compressive strength, loss in weight, and water absorption, and microstructural tests of X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results revealed that the macro-structural behavior of sand–cement mortar highly depends on the microstructure and changes in cement nanostructures during heat treatment. Primary portlandite and C–S–H nanostructure were destroyed at 600 °C, and alite, belite, and β-wollastonite were formed at 800 °C. Adding nano-silica improved the strength properties of sand–cement mortar against heat, so the compressive strength of 28-day specimens containing 15% nano-silica increased from 13.9 to 19.2 MPa at a temperature of 800 °C.

Sustainable and mechanical properties of Engineered Cementitious Composites with biochar: Integrating micro- and macro-mechanical insight

Engineered Cementitious Composites (ECC) are ductile, strain-hardening cementitious composites with a tightly controlled crack opening profile. A significant concern in the development and application of ECC is its high carbon emissions associated with high cement content consumption. As an emerging green additive, biochar can significantly cut down on the carbon emission of concrete products. However, research that integrates micro- and macro-mechanical insights with biochar incorporation is limited. In this study, micro-mechanical tools indicated that a certain amount of substitution biochar could enhance the tensile properties of ECC. The study assessed various properties including compressive strength, porosity, density, tensile performance, crack pattern, sustainability and cost of ECC with different biochar proportions. The findings showed that incorporating 10 %–20 % biochar effectively increased the tensile strain capacity and decreased the crack opening width in ECC materials. This improvement can be attributed to the introduction of finer biochar particles (≤75 μm) at the fiber/matrix interfacial transition zone, which alters the fiber-bridging force by reducing the chemical and frictional bond strength between the fiber and matrix, as revealed by micro-mechanical tests and microstructural inspection. Moreover, the utilization of biochar contributes to reducing the carbon emission of ECC, enhancing sustainability while maintaining a ∼10 % minimal reduction in compressive strength. Overall, this study introduces a novel approach for reusing low-carbon biomass waste in the production of building materials, offering potential advantages in micro- and macro-mechanical performance, cost-effectiveness, and environmental sustainability.

Application of recycled concrete aggregates in continuous-graded cement stabilized macadam

This study investigates the potential of using recycled concrete aggregate (RCA) derived from construction and demolition waste as a replacement for natural aggregate in cement-stabilized macadam (CSM). Various substitution rates of RCA, ranging from 0 % to 100 %, were examined with a focus on mechanical properties and durability, including unconfined compressive strength (UCS), indirect tensile strength (ITS), compressive elastic modulus, drying shrinkage, frost resistance, scour resistance, and water damage resistance. Nanoindentation was used to analyze the interfacial transition zones (ITZs) within the CSM. The results show that with a fixed cement content, the UCS, ITS, and compressive elastic modulus decrease as the RCA content increases. Higher RCA content also leads to reduced drying shrinkage, frost resistance, scour resistance, and water damage resistance. Nanoindentation revealed that ITZ1 and ITZ2 have similar thicknesses (∼50 μm), while ITZ3 is about 80 μm thick, highlighting the intricate physical composition of ITZs, including pores, low-density C-S-H, high-density C-S-H, and Ca(OH)2. These findings provide critical insights for optimizing sustainable CSM design using RCA, thereby promoting environmental sustainability in road construction.