Steam curing is widely used for precast concrete, but mixtures containing polycarboxylate ether superplasticiser often develop surface bugholes and coarser pores near the mould-contact region. This study examined whether a water-based mould release agent (MRA) modified with a defoamer based on mineral oil (ODA) or a polyol-based defoamer (ADA) could improve the surface quality of steam-cured concrete. Concrete compressive strength, demoulded surface appearance and rebound-based near-surface uniformity were assessed. Steam-cured mortar was used to compare image-based pore characteristics in the mould-contact region and the interior. The image-based pore-structure response was more evident in the mould-contact near-surface region than in the interior. Compared with the blank group, ADA-6 (0.6% defoamer by mass of the MRA) increased the demoulded and 28-day compressive strengths by 21.84% and 10.13%, respectively, whereas ADA-9 (0.9% defoamer) gave the greatest near-surface pore refinement, reducing the mean pore diameter from 0.67 mm to 0.26 mm and the porosity from 6.58% to 1.63%. ODA was more dosage-sensitive, and complete replacement of the MRA reduced the 28-day strength by 15.70%. The results indicate that defoamer-modified water-based MRAs can improve demoulded surface quality, but strength and surface pore refinement should be optimised separately.
Basalt fibre (BF) is frequently used as a reinforcing material in concrete. In this study, to enhance mechanical interlocking at the fibre–matrix interface, BF was surface-modified using the silane coupling agent KH550 to produce modified basalt fibre (MBF). The physico-mechanical properties of concrete reinforced with unmodified basalt fibre (BFRC) and MBF-reinforced concrete (MBFRC) were evaluated. With an increase in the content of MBF, the P-wave velocity and mechanical properties of the MBFRC initially increased and then decreased. The mechanical strength was the highest with 0.05% volume fraction of BF. Compared with BFRC, after 28 days of curing, the uniaxial compressive strength, splitting tensile strength and flexural strength of the MBFRC increased by approximately 7.1%, 4.8% and 14.9%, respectively. Although the single-fibre tensile strength of the MBF was not increased, scanning electron microscopy and atomic force microscopy observations showed that modification with KH550 roughened the surface of the BF. These results suggest that the increased surface area of the MBF may facilitate additional nucleation sites for hydration products, thereby reducing interfacial pore defects and improving load transfer and crack-bridging efficiency. The study provides a new method for the strength improvement of BFRC.
Concrete-filled steel tubes (CFSTs) with alternative binders are a promising direction for sustainable construction. However, they are hindered by the current durability testing standards. Existing international standards were developed for concrete made with ordinary Portland cement (OPC) and therefore fail to address the thermo-mechanical properties, hydration mechanisms and pore structures of innovative binders such as geopolymers, alkali-activated materials. This critical review identifies specific gaps in current testing protocols by synthesising the fragmented literature on CFST mechanics, geopolymer durability and bond behaviour. Findings reveal that rapid chloride penetration tests (RCPTs), thermal cycling procedures and bond assessment methods calibrated for OPC concrete produce misleading results for alternative binders. Moreover, the confinement of CFSTs creates durability challenges, which are not addressed in existing standards, particularly regarding the tube’s corrosion and internal reinforcement protection. Modifications to current testing frameworks are proposed, including adapted RCPT protocols for geopolymer systems, standardised thermal cycling procedures for CFSTs and enhanced push-out testing specifications. This study offers practitioners and standards bodies helpful suggestions for validating CFSTs with alternative binders by identifying critical knowledge gaps and suggesting five test domains, thereby enabling more confident adoption of sustainable composite structures.
Fibre-reinforced mortar (FRM) has attracted much attention owing to its excellent mechanical properties. To further explore the influence of blended fibres on the mechanical properties and damage evolution of internal cracks in FRM, the effects of blended additions of basalt fibre, carbon fibre (CF) and polyester fibre (PETF) on the mechanical properties and microstructure of mortar were investigated. The results showed that the addition of 0.75% CF and 0.25% PETF had the best effect on enhancing the compressive strength, flexural strength and splitting tensile strength of the mortar, with increases of, respectively, 44.5%, 53.6% and 70.9% compared with mortar without fibres. Correlation analysis of rise time/amplitude (RA) and average frequency showed that the synergistic effects of blended fibres made the expansion path of cracks in mortar more complex. The RA percentage in the mortar with 0.75% CF and 0.25% PETF was the highest, increasing by 19.9% compared with the mortar without fibres.
Waste tires generate one of the largest sources of solid waste, and crumb rubber (CR) as aggregate in concrete is a viable option to valorise it. While the interaction of CR with water is complex, there are no standardised tests to measure this interaction and quantify non-reactive water in the cementitious matrix retained by CR (absorption). Conventional methods are not suitable for CR owing to its properties. Consequently, reported absorption values range widely from 0% to over 43%, and some authors even suggest treating CR as fully hydrophobic (negligible absorption). This way undermines the reliability of rubberised concrete (RC) mix design and hinders accurate estimations of its properties. This study proposes two complementary methodological approaches to estimate the water absorption of CR aggregates: (a) thermogravimetric analysis (TGA); and (b) a simplified mechanical method using California Bearing Ratio moulds. TGA results demonstrated that CR aggregates exhibit water absorption approximately 2 to 3 times greater than natural sand, challenging the widespread assumption of CR’s hydrophobic behaviour in RC. The mechanical method provided an indirect indicator associate to the water absorption of the CR within the cementitious matrix. The findings underscore the need to consider CR absorption and refine methods for measuring water absorption of CR.
India's extensive coastline poses significant durability challenges for marine infrastructure owing to corrosion of steel reinforcement, resulting in high maintenance and rehabilitation costs. Glass-fibre-reinforced polymer (GFRP) bars present a promising alternative. However, their durability in highly alkaline concrete environments remains a concern and requires further investigation. The influence of alkaline exposure on the tensile strength retention and bond performance of bars of different diameters (8, 10 and 12 mm) was examined in this study. Specimens were conditioned in simulated alkaline solutions for 28 days and 45 days and tested in accordance with D7205. The results showed a reduction in tensile strength exceeding 30% after 45 days of exposure, accompanied by microstructural degradation observed through scanning electron microscopy analysis. Pull-out tests conducted after 90 and 180 days revealed minimal bond degradation (3-4%), noticeable only after prolonged exposure. Regression analysis identified bar diameter and exposure duration as key parameters influencing durability performance. Predictive models were developed, which are applicable for bar diameters of 8-15 mm and exposure durations up to 250 days under accelerated conditions. The findings support durability-based design and highlight the potential of bars for sustainable coastal infrastructure, contributing to Sustainable Development Goals.
The recycling of low-grade waste clay into alkali-activated calcined clay (AACC) pastes represents a promising strategy for sustainable waste management. The synergistic effects of ground granulated blast-furnace slag (GGBS) on the properties of pastes synthesised from low-grade calcined clay were investigated in this work. The influence of slag content (0-60%) on setting time, density, ultrasonic pulse velocity and early mechanical properties was systematically evaluated to identify an optimal mix formulation. The results showed that the incorporation of significantly shortened the setting times and enhanced early-age strength. The pastes with 50% demonstrated optimal performance, combining rapid setting with high compressive strength. Long-term water immersion tests revealed excellent volume and strength stability; however, a notable reduction in deformation resistance was observed. Microstructural analysis using low-field nuclear magnetic resonance showed that prolonged water exposure increased overall porosity and promoted the conversion of gel and capillary pores into larger macropores, leading to a coarsened pore structure. These findings provide valuable insights into the durability and microstructural evolution of slag-enhanced pastes, supporting their potential application as sustainable construction materials.
Microcracks significantly compromise the long-term service life of concrete structures, prompting extensive research into mitigation strategies. Embedding healing agents to impart autonomous crack repair capabilities offers a promising solution to reduce maintenance costs and enhance sustainability in the construction sector. Cementitious capillary crystalline waterproofing materials (CCCW), as inorganic rigid waterproofing agents, have garnered increasing attention owing to their excellent waterproofing performance, ease of application, environmental compatibility and cost effectiveness. Recent studies have increasingly recognised the potential of CCCW to impart self-healing functionality to concrete. This review systematically examines recent advances in CCCW-based self-healing concrete, covering the fundamental composition, mix design principles, underlying self-healing mechanisms and the efficacy of crack healing evaluated through various performance metrics and assessment methodologies. Furthermore, future research directions are proposed, providing a theoretical foundation and technical reference to facilitate the industrial application of CCCW.
Engineered cementitious composites (ECC) still face the issue of limited self-healing ability, even with their superior crack control capability. In this work, in an attempt to improve the self-healing properties of ECC, superabsorbent polymer (SAP), light-burned magnesia and calcium sulfoaluminate (CSA) were added to ECC. The healing effect and mechanism were studied using various means, including crack closure tests, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. The results showed that the composite additive significantly enhanced the self-healing efficiency with a synergistic action. The SAP-CSA combination exhibited the most pronounced effect. Cracks with an initial width of 0.18 mm were completely sealed within 30 days and the compressive strength recovery rate reached up to 116%. continuously releases water, facilitating the rehydration of unhydrated cementitious materials and accelerating the formation of calcium silicate hydrate gel and calcium carbonate. CSA efficiently healed cracks by rapidly reacting with water to generate ettringite, which directly obstructed the cracks. This study offers a theoretical and practical foundation for enhancing the self-healing performance of ECC.
Low carbon dioxide concrete () has emerged as a critical strategy for reducing greenhouse gas emissions associated with the construction industry, which is responsible for approximately 8% of global carbon dioxide emissions as a result of the production of traditional Portland cement. This review paper examines the environmental implications of technologies, including supplementary cementitious materials, alkali-activated binders, approaches for carbon dioxide capture and utilisation, and novel aggregate alternatives. Life cycle assessment results from numerous studies were synthesised to evaluate reductions in embodied carbon dioxide, energy consumption and resource depletion relative to conventional concrete. The paper also explores challenges related to durability, materials availability, standardisation and performance variability, which can influence both environmental outcomes and large-scale adoption. In addition, the review highlights emerging trends such as bio-based binders, carbon-negative concrete formulations and digital optimisation techniques for mix design. Overall, this paper demonstrates that can significantly reduce environmental burdens, often achieving 20-70% carbon dioxide savings, but emphasises that holistic evaluation - including durability, transport and end-of-life considerations - is essential for accurate impact assessment. The review concludes with recommendations for research priorities and policy interventions needed to accelerate the transition towards a more sustainable and resilient concrete industry.
Alkali-activated concretes (AACs) have been extensively investigated in laboratories around the world. However, there are only a limited number of demonstration projects and applications of alkali-activated materials in building and civil infrastructure. The work described in this article involved the development of geopolymer concretes in the laboratory for a range of precast concrete products to be produced in the UK and Malaysia – load-bearing wall panels, reinforced concrete slabs, precast concrete stair units, voussoirs for a pedestrian concrete arch bridge (FlexiArch) and concrete building blocks. Upscale to factory production has shown that these relatively new concretes can be used in the construction industry without any problems other than the need to ‘optimise’ the laboratory mixes to the needs of the precast concrete factories. This included careful selection of materials (precursors and activators) to be used by the factory and modifications to the laboratory mixes for longer workability retention required at the factory. The use of an in-house produced sodium silicate activator from ground glass cullet reduced the cost of the AACs and made them comparable to the cost of Portland cement mixes. The embodied carbon dioxide of these mixes was also considerably reduced.
Cracks that appear during the initial setting and hardening stage of concrete can compromise the durability and service life of structure. The focus of this study was on the crack-resistance properties of recycled aggregate concrete () doped with basalt fibre () reinforcement to enhance its early-stage crack resistance. By investigating the relationship between recycled fine aggregate () content and volume fraction, intrinsic connections among mechanical properties, drying shrinkage and the crack resistance of were revealed. The results showed that incorporating at 0.2% by volume significantly enhanced the RAC's mechanical properties, regulated drying shrinkage and reduced early-stage cracking. The improvement became more pronounced with higher contents of RFAs. For the mix with 100% RFA, the addition of at 0.25% by volume reduced the total crack area per unit area of test specimens by 85.4% and decreased the maximum crack width by 60.1%. Combining grey correlation theory with the microhardness distribution at the RFA-mortar interface, the correlation ranking for crack resistance in was found to be drying shrinkage strain > flexural-compressive ratio > tensile-compressive ratio > splitting tensile strength. locally reinforces cement paste, leading to a relatively uniform stress distribution. During cracking, effectively dissipates energy, mitigating the cracking risk associated with high contents of RFAs.
Basalt fibre-reinforced polymer (BFRP) bars have attracted increasing attention as corrosion-resistant reinforcement for concrete structures; however, their structural performance under elevated temperatures remains insufficiently understood. This study experimentally investigates the tensile behaviour of BFRP bars and the flexural response of BFRP-reinforced concrete short slabs subjected to sustained elevated temperatures. Tensile tests on BFRP bars were conducted under steady-state heating conditions at room temperature, 210 degrees C, 300 degrees C, and 500 degrees C, representing critical thermal degradation stages of FRP composites, including polymer matrix softening, progressive fibre-matrix degradation and severe thermal decomposition. In addition, reinforced concrete short slabs were tested under flexural loading while exposed to room temperature, 200 degrees C and 600 degrees C to evaluate the structural response under moderate and severe thermal exposure conditions. The results showed that the tensile strength of BFRP bars decreased by approximately 34% at 210 degrees C, 22% at 300 degrees C and 93% at 500 degrees C compared to ambient conditions, while the elastic modulus remained relatively stable up to 300 degrees C. The flexural capacity of BFRP-reinforced concrete short slabs decreased by approximately 20% at 200 degrees C and 54% at 600 degrees C, accompanied by substantial stiffness degradation and increased deflection capacity. Although energy absorption increased at elevated temperatures, this behaviour was primarily associated with bond deterioration, stiffness reduction and progressive damage development rather than improved structural performance. The study also proposes preliminary temperature-dependent reduction models for BFRP tensile properties and evaluates the applicability of ACI 440 design provisions under elevated temperatures. The findings provide important experimental insight into the thermal performance and failure mechanisms of BFRP-reinforced concrete members and contribute toward the development of elevated temperature-resilient design approaches for FRP-reinforced structures.
The degradation behaviour of ultra-high-performance concrete () and conventional reinforced concrete () under accelerated stray-current corrosion was investigated. Cube and prism specimens, both plain and reinforced, were subjected to direct current to simulate the subway stray-current environment. The evolution of current, cumulative charge passed and mechanical properties was recorded, and the corrosion morphology of the embedded steels was analysed. The results showed that the (grade C50) exhibited severe electrochemical activity and rapid strength loss, with over 80% reduction in compressive strength and 95% in flexural strength after 14 days of current exposure. In contrast, the displayed only slight degradation owing to its dense microstructure and stable steel-matrix interface, which effectively limited ion migration and localised corrosion. The findings highlight UHPC's superior electrochemical stability and mechanical performance under stray-current environments, providing technical guidance for the performance design of structures in metro and tunnel systems.
The influences of concrete compressive strength (f c), bar diameter (d b), concrete cover (c) and water/cement ratio (w/c) on the bond behaviour between thermo-mechanically treated rebars and concrete were investigated through pull-out tests. Both pull-out and splitting failure modes were observed, primarily governed by the combined effects of d b and c. The reference specimens mostly exhibited ductile pull-out behaviour with gradual slip and a sustained post-peak response, whereas specimens with reduced cover or larger bars failed abruptly in splitting. Increasing f c from 21 MPa to 28 MPa enhanced the bond strength (tau max) by nearly 35% for 12 mm dia. rebars owing to improved interfacial adhesion and confinement, although the gain was less pronounced for larger bars. Increasing c from 20 mm to 75 mm improved both bond capacity and ductility, delaying splitting onset. Conversely, varying w/c in the range 0.41-0.48 had negligible influence when f c was constant. Despite the differences in tau max, all specimens exhibited similar tau-s curve shapes, indicating that f c, d b and c affect the magnitude rather than the fundamental bond mechanism. The proposed analytical method for estimating tau max correlated well with the test results, predicting bond stress-slip response within +/- 10% of measured values.
Panel concrete structures of concrete-faced rockfill dams (CFRDs) in China’s Three-North regions are frequently exposed to freeze–thaw (FT) cycles and carbonation, which pose significant durability challenges. In this study, the durability performance of panel concrete under the coupled effects of FT and accelerated carbonation (AC) was investigated. The mass loss, relative dynamic modulus of elasticity, splitting tensile strength and carbonation depth were evaluated under individual and combined environmental conditions. The degradation behaviour of the panel concrete was systematically analysed. The results showed that the loss of dynamic modulus under coupled action exceeded that observed under individual exposure, regardless of whether AC preceded FT cycles or whether FT preceded AC. The interactive deterioration mechanism between FT and AC was further explored through scanning electron microscopy and energy dispersive X-ray spectroscopy analyses. The results revealed that FT cycles led to the propagation of microcracks and accelerate the penetration of carbon dioxide and carbonation reactions. Although carbonation increased the compactness of the concrete, the volume shrinkage of reaction products caused internal stress and cracking, and the long-term coupling effect of FT and AC two will accelerate damage. A damage prediction model for panel concrete subjected to the coupled effects of FT and AC was subsequently established. The remaining life of panel concrete in different climatic zones of China was estimated, offering critical scientific support for the durability design of CFRD engineering.
One of the primary approaches for reducing global carbon dioxide emissions associated with concrete production is the partial replacement of Portland cement (PC) with supplementary cementitious materials. In this context, an experimental study on the mix design and performance of high-strength concrete (HSC) incorporating ternary and quaternary binder blends was developed. Silica fume, metakaolin, rice husk ash and limestone filler were combined with PC in predetermined proportions, replacing up to 30% of cement volume. Mechanical and durability properties were quantified and microstructural analysis was performed. The environmental performance of mixtures was evaluated based on their global warming potential (GWP). The results indicated a systematic loss of workability at the highest levels of cement replacement (quaternary blends). At 28days, only three of the ten mixtures showed notable decreases in mechanical strength relative to the reference concrete. At later ages (91 and 180days), most ternary and quaternary concrete mixtures showed significant improvements in mechanical properties and durability, and a dense and compact microstructure was observed. From the environmental perspective, higher cement replacement levels enabled the production of concretes with lower GWP. Overall, the findings demonstrate the technical feasibility of employing ternary and quaternary binder blends for producing eco-efficient HSCs.
The thermo-mechanical degradation of fibre-matrix bond and the flexural behaviour of steel-fibre-reinforced concrete () specimens subjected to elevated temperatures (200 degrees C, 400 degrees C and 600 degrees C) were investigated. Fibre pull-out and four-point bending tests were conducted independently to characterise the temperature-dependent degradation of interfacial bond and flexural response. The results showed that moderate heating (400 degrees C) enhanced the bond strength and pull-out energy to, respectively, 158% and 188% of their room-temperature (20 degrees C) values, owing to frictional strengthening and preserved matrix integrity. Severe deterioration occurred at 600 degrees C, and both indices decreased to approximately 63% of their reference values owing to microcracking and chemical bond loss. Flexural strength decreased progressively with temperature, retaining 42.8% for and 24.7% for plain concrete at 600 degrees C. Despite matrix damage, the exhibited stable ductility and higher fracture toughness, confirming the sustained efficiency of fibre bridging under thermal degradation. These findings provide insights into the temperature-dependent fibre-matrix bond degradation and corresponding flexural response of within the investigated temperature range of 200-600 degrees C.
A large amount of construction waste could be reused if recycled aggregate () is used to prepare pervious planting concrete (). This would reduce the consumption of natural resources and environmental burdens, in line with the concept of sustainable development. The effects of the water/binder ratio (w/b), polyvinyl alcohol powder () content and aggregate size on the compressive strength and water permeability of RA-PPC were studied in this work. Elymus dahuricus Turcz () was planted on the RA-PPC matrix to study its vegetative property and bacillus was used to improve growth. The results showed that the compressive strength and water permeability of the RA-PPC first increased and then decreased with an increase in w/b and content. The performance of RA-PPC made with larger aggregate (10-20 mm) was superior to that of RA-PPC made with smaller aggregate (5-10 mm). The pore structure characteristics and the paste thickness on the surface of the aggregates were the primary factors influencing the performance of the RA-PPC. The incorporation of bacillus promoted the growth of on the concrete matrix and enhanced the vegetative property of RA-PPC. The water permeability and compressive strength of the RA-PPC decreased slightly after planting.
The shear and torsional strengths of reinforced concrete (RC) members are affected by the diagonal crack angle. According to the space truss model used for torsional design in international codes (ACI 318-19, EN 1992-1-1:2004, CSA A23.3 and JSCE-17), the torsional crack angle is dependent upon the ratio of transverse to longitudinal reinforcement. However, owing to differences in the crack initiation mechanism, the actual crack angle often differs from the value predicted by these models, which may lead to over- or underestimation of the torsional strength. In this study, experimental tests were performed on ten RC beams under torsion to evaluate the effect of the torsional reinforcement ratio on the crack angle. Additionally, 207 test results were collected from the literature to examine the relationship between crack angle and torsional strength. The results showed that the difference between the predicted and measured crack angles became more pronounced as the reinforcement ratio deviated from 1.0. Unlike shear, the crack angles caused by torsion remained nearly constant throughout the loading process. A simplified equation incorporating the tensile strength of concrete was developed to improve crack angle prediction. The proposed equation provided better agreement with test results compared with current design equations and thus offers a more reliable approach for evaluating the torsional behaviour of RC members.
