Search Results (531)

The transition to lower-clinker binders in heavy precast concrete is constrained by the need to combine rapid production cycles, high early-age strength, self-compacting performance, durability and measurable environmental benefits. The scientific gap addressed in this study is the limited integrated evidence comparing these criteria under conditions that are representative of industrial heavy precast production, particularly for CEM II/A-S 52.5 R and CEM II/B-V 42.5 R. Four concretes were assessed: CEM I 52.5 R, CEM II/A-S 52.5 R, CEM II/B-V 42.5 R, and a 50:50 CEM I/CEM II/B-V binder. The experimental programme included slump-flow, a plant-specific 600 mm flow-time indicator, compressive strength development, hardened density, water absorption, water penetration under pressure and freeze–thaw resistance. Environmental performance was evaluated using the manufacturers Environmental Product Declarations (EPDs) for the cement component within a cradle-to-gate boundary. All mixtures corresponded to at least strength class C50/60 at 28 days, while CEM I, CEM II/A-S and the 50:50 blend corresponded to C55/67. CEM II/A-S reduced water absorption from 5.49% to 4.29% and water penetration from approximately 61 to 23 mm relative to CEM I, but its freeze–thaw strength loss was 26.80% compared with 4.50% for CEM I. CEM II/B-V provided the lowest cement-related GWP, approximately 154 kg CO₂-eq/m³, about 25% below CEM I, whereas the 50:50 blend reduced this indicator by approximately 12% while achieving the highest 28-day compressive strength (approximately 78 MPa). The results show that cement selection for heavy precast concrete cannot be based on clinker content or strength alone. CEM II/A-S offered the most balanced technological and transport-property performance, whereas CEM II/B-V offered the greatest GWP reduction but requires consideration of its lower strength class and slower early-age development. Long-term durability and full life-cycle impacts remain to be verified. Full article

Fiber-reinforced high-water-resistance concrete backfill (FHWCB) is a rapid-setting cementitious backfill system used for underground support and backfilling, but its stability is strongly affected by mixture water content and fiber dispersion. This study investigated the fresh-state behavior, mechanical performance, acoustic emission (AE) fractal characteristics, b-value response, and energy evolution of FHWCB. Mixtures with water-to-cement ratios (w/c) of 1.0–1.8 and fiber volume fractions (Vf) of 0–0.5% were prepared and tested using fresh property measurements, unconfined compression, thermogravimetry, AE monitoring, correlation dimension analysis, b-value analysis, and strain energy partitioning. Increasing w/c improved flowability and delayed setting, but weakened the hydration skeleton and reduced early-age compressive strength by approximately 56–61%. Fiber reinforcement showed a non-monotonic effect: Vf = 0.3% increased compressive strength by approximately 16–26%, whereas excessive fiber addition reduced strength because of fiber clustering and weak local zones. AE amplitude sequences exhibited measurable fractal characteristics. A higher correlation dimension indicated distributed microdamage, while decreasing correlation dimension and b-value reflected the transition toward localized macrocrack growth. Energy analysis showed that the peak elastic strain energy density decreased from approximately 0.60 to 0.39 MJ/m³ as w/c increased. The proposed AE fractal–b-value–energy framework provides a quantitative basis for tracking damage progression and optimizing FHWCB for underground engineering. Full article

Classic elastic–plastic models for concrete do not fully account for either the material properties of shotcrete or the influence of environmental factors, resulting in significant limitations in the research and application of shotcrete in tunnels. This paper comprehensively considers the early-age strength of tunnel shotcrete and the physical and chemical attack effects of a sulfate environment, to investigate the mechanical degradation mechanisms of tunnel shotcrete under sulfate conditions and establish a corresponding nonlinear elastoplastic model. This study carried out sulfate attack tests on tunnel shotcrete and systematically revealed the stress–strain evolution characteristics of shotcrete under sulfate attack. Based on experimental data on both chemical and physical attack, this paper improves the classical elastoplastic constitutive model and constructs an elastoplastic constitutive model applicable to both the early and hardening stages of shotcrete. Overall, the improved model can describe the stress–strain response of shotcrete reasonably well; however, due to the high inherent discreteness of the shotcrete material itself, some fitting deviations still exist near points of sudden change in strain or stress. Under sulfate chemical attack conditions, the cracking stress of shotcrete exhibits a nonlinear trend of first increasing and then decreasing as corrosion time progresses. Under physical attack conditions, the cracking stress shows a clear linear decrease. Furthermore, in high-concentration sulfate environments, the influence of sulfate concentration on cracking stress is moderately reduced. The results of this study provide theoretical support for the durability assessment and constitutive modeling of tunnel shotcrete in sulfate-corrosive environments. Full article

The incorporation of supplementary cementitious materials derived from agro-industrial residues has emerged as a promising strategy to reduce the environmental impact associated with Portland cement production while promoting circular economy principles. This study investigates the influence of palm oil fuel ash (POFA) as a partial cement replacement on the mechanical and microstructural properties of concrete. Concrete mixtures containing 0%, 10%, 15%, and 20% POFA by mass of cement were produced using a constant water-to-binder ratio of 0.47. Compressive strength was evaluated at 7, 14, 28, and 56 days, while microstructural characterization was performed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) to assess hydration products and the interfacial transition zone (ITZ). The incorporation of POFA resulted in lower compressive strength at early curing ages, with reductions of approximately 16–21% compared with the control mixture, reflecting the slower kinetics of pozzolanic reactions. However, prolonged curing promoted significant strength development. At 56 days, concretes containing 10% and 15% POFA exhibited compressive strengths 3.7% and 9.2% higher, respectively, than the control concrete, whereas the 20% replacement level resulted in a 14.6% reduction. SEM observations revealed a denser cementitious matrix, improved aggregate–paste bonding, and a more refined ITZ in mixtures containing 10–15% POFA. EDS analysis showed Ca/Si ratios of 1.22 and 1.08 for the 10% and 15% POFA mixtures, respectively, indicating the formation of silica-rich C–S–H gel associated with effective pozzolanic activity. The results demonstrate that POFA can be successfully utilized as a supplementary cementitious material in concrete. While the 15% replacement level produced the highest 56-day compressive strength, both the 10% and 15% mixtures exhibited favorable microstructural characteristics and effective pozzolanic activity, indicating that both replacement levels are suitable for sustainable concrete production. Full article

The present study was conducted to systematically investigate the effect of adding Polypropylene (PP) fibre at various dosages (0–1.0 kg/m³) on the compressive strength and workability of Blended Portland Cement concrete. All mix variables were kept constant except for fibre content (w/c = 0.45), thereby allowing the dosage effect to be isolated. Standard-sized 150 × 150 × 150 mm cubes were prepared and tested for slump (ASTM C143), fresh density and early-age compressive strength BS 1881-116:1983 at 7 days and 14 days. Slump decreased monotonically from 200 mm to 160 mm as the dosage increased and remained within the EN 206 S4 (High) workability class throughout. Fresh density was essentially unaffected. The early-age compressive strength increased continuously for both test ages, reaching 51.7 MPa at 1.0 kg/m³ at 14 days. One-way analysis of variance confirmed a statistically significant dosage effect at both ages (7-day p < 0.001; 14-day p = 0.003). Based on the strength-workability trade-off, it was found that a pragmatic recommended dosage range is 0.6–0.8 kg/m³, which gives an appreciable strength improvement with a moderate decrease in workability without the need for the addition of extra admixtures. These results offer early age mix design guidance for practitioners and engineers in a wide range of construction applications and can be used to calculate a potential circular economy impact for the mix design: a 100 m³ pour at 0.8 kg/m³ could potentially divert 80 kg of PP waste from landfill or incineration if recycled PP fibres were used instead of the virgin fibres used in the study. Full article

The preservation of twentieth-century reinforced concrete buildings increasingly requires the integration of structural engineering assessment with heritage conservation principles. Although the deterioration mechanisms of reinforced concrete in marine environments have been extensively investigated, relatively few studies have examined how advanced material degradation affects the technical feasibility of preserving modern reinforced concrete heritage structures. This study addresses this gap through the structural assessment of the Patras Port Authority Building (OLPA), a reinforced concrete building constructed in the early 1970s and exposed for more than five decades to an aggressive coastal environment, providing the engineering basis for determining whether a complete code-based structural assessment is justified in accordance with KAN.EPE. and EN ISO 13822. A comprehensive inspection and testing program was carried out, including visual inspection, crack mapping, concrete core testing, carbonation-depth measurements, pH determination, chloride-content analysis, half-cell potential measurements, electrical resistivity measurements, and selective exposure of reinforcement. The engineering assessment revealed extensive deterioration of the structural system, including low concrete strength (approximately C8/10), carbonation exceeding the concrete cover, pH values between 7 and 8, chloride concentrations ranging from 0.0377% to 0.8975% by cement mass, and severe reinforcement corrosion. The measured average cross-sectional loss reached 34.5% for longitudinal reinforcement and 65.6% for transverse reinforcement (stirrups), accompanied by significant reductions in mechanical properties and ductility. It should be noted that concrete samples for chloride determination were collected at depths well beyond the reinforcement level. Additional deficiencies associated with inadequate confinement reinforcement, outdated seismic detailing, previous earthquake damage, cracking in columns and shear walls, and uncertainty regarding the geometry and condition of the foundation system further increase structural vulnerability. The engineering assessment indicates that the combined effects of long-term environmental exposure, corrosion-induced deterioration, obsolete design provisions, and existing structural deficiencies substantially reduce the reliability and seismic performance of the load-bearing system. Within this context, the study examines the implications of advanced deterioration for the preservation of reinforced concrete heritage buildings and proposes an integrated assessment framework that combines structural safety, durability, material integrity, intervention feasibility, and heritage significance. The proposed approach contributes to a more comprehensive engineering-based methodology for evaluating preservation strategies for aging reinforced concrete buildings exposed to aggressive marine environments. These findings also raise important concerns regarding the technical feasibility of preserving ageing reinforced concrete buildings located in highly seismic regions, where ensuring structural safety may require the introduction of new load-bearing elements together with the replacement of a substantial portion of the already deteriorated original material. Full article

Concrete shrinkage is significantly influenced by the restraint level, and cracking often occurs under specific restraint conditions, thereby adversely affecting structural performance. Investigating the effect of restraint on shrinkage cracking is of great significance for enhancing early-age durability and ensuring structural safety. In this study, four distinct restraint levels (0%, 35%, 55%, and 75%) were established by varying the thickness of the inner steel ring. The influence of varying degrees of restraint on the shrinkage behavior was investigated, with digital image correlation (DIC) and internal strain gauge measurement employed to observe the strain and predict the risk of cracking. As the degree of restraint increases, the inner steel ring inhibits the free radial shrinkage of concrete more significantly, thereby inducing greater tensile strains at both the outer circumferential surface and the interior. The surface strain accumulation far exceeds the interior response due to the drying gradient. During the first 60 h, the shrinkage strain measured by both methods exhibited the most rapid evolution, indicating a critical high-risk period for cracking. These results advance the understanding of restraint effects in concrete and comprehensively clarify the relationship between the degree of restraint and the shrinkage, which accurately captures the evolution of shrinkage, facilitates the transition from empirical to quantitative design for crack-resistant materials and supports their customized optimization under practical engineering loading conditions. Full article

Tunnel lining concrete is prone to early-age cracking and leakage during service, which provides pathways for water and harmful ions and consequently threatens the safety and durability of tunnel structures. To improve the crack resistance and water penetration resistance of lining concrete, this study investigates the effects of steel fibers, a CaO-MgO composite expansive agent, and fly ash–slag replacement on workability, mechanical properties, ultrasonic pulse velocity, early-age cracking, and water penetration resistance. The results show that steel fibers had the most pronounced effect on crack-width control; at 1.0% steel fiber content, the total cracking area and water penetration height decreased by 71.9% and 68.2%, respectively. The composite expansive agent showed an optimum dosage of 10%, at which cracking resistance and water penetration resistance were both improved. The largest reduction in water penetration height was observed when slag was fully replaced by fly ash, with an 87.5% decrease. These findings provide a material-design reference for improving the crack resistance and durability of tunnel lining concrete under restrained early-age conditions. Full article

To accurately predict temperature rise due to the concrete hydration heat released from the #0 segment of a continuous concrete girder bridge at an early construction stage for structural design purposes, researchers proposed an approach incorporating empirical predictive formulae with a preliminary numerical analysis. However, due to the uniqueness of the structural geometry and material in each engineering case and the limited data shared by the whole engineering community, no universal predictive empirical model for temperature rise due to hydration heat has yet been identified for practical use that can be applied to a variety of different projects. Moreover, the preliminary numerical analyses are usually based on questionable assumptions and simplifications of the physical truth, the accuracy of which also requires further validation. To this end, the present research measured the time-varying temperature samples of early age massive concrete in the #0 segment of Huangsha Harbor Bridge (a twin-deck three-span continuous concrete box girder bridge located in Jiangsu Province, China) and examined the accuracy of the predictive empirical models formulated by other researchers and the usability of a numerical modal established on a commercial finite element (FE) platform by comparing the corresponding results with the data from the present field measurements. The results suggest that the empirical formulae proposed can generally effectively describe the actual temperature distribution patterns related to the thermal issue, but they are characterized by inferior usability in some cases. In addition, the present comparison also indicates that the actual maximum temperature rise can be correctly predicted by the preliminary FE analysis in most cases. Full article

High-Early-Strength Concrete (HESC) is increasingly required in accelerated construction, yet most existing studies focus on single nano-additives rather than hybrid waste-derived systems. This study investigates the individual and combined effects of nanoclay (NC), nanosilica (NS), and cellulose nanofibers (NCel)—each produced from industrial or agricultural waste—on the mechanical and microstructural properties of HESC. A Box–Behnken response surface methodology (RSM) design was employed to optimize nanomaterial dosages with respect to early-age compressive strength, while microstructural evaluation (SEM, EDS, elemental mapping) clarified the mechanisms of enhancement. The results demonstrate that NC, NS, and NCel play complementary roles in hydration acceleration, particle packing, pore refinement, and crack-bridging. The optimized hybrid system (1.64% NC, 0.115% NS, 0.027% NCel) achieved a 3-day compressive strength of 59.7 MPa, 7-day strength of 71.2 MPa, and 28-day strength of 94.6 MPa, representing increases of 42.14%, 36.92%, and 21.59%, respectively, over the control mixture. Microstructural observations confirmed matrix densification, reduced Ca/Si ratio (from 2.05 to 1.68), refined pore structure (<0.4 μm vs. 0.9–1.2 μm in control), and enhanced ITZ in the optimized mixtures. Statistical analysis yielded robust predictive models (R² = 0.977–0.996) with significant interaction terms confirming synergistic effects among the three nanomaterials. This work demonstrates that waste-derived hybrid nano-systems offer a sustainable and effective strategy for producing high-performance HESC, with the RSM-derived optimum providing balanced early- and later-age strength while maintaining practical feasibility for field implementation. Full article

Against the backdrop of global climate change and the urgent need to decarbonize the construction sector, cement production remains a major contributor to anthropogenic CO₂ emissions, accounting for over 7% of the global total. Carbonation mixing has emerged as a promising green technology that integrates CO₂ sequestration with performance enhancement of cement-based materials. However, the combined effects of carbonation mixing and binary nano-oxide modification on the multi-performance attributes of cement mortars have not been systematically explored. This study aims to address this gap by investigating the synergistic modification of cement-based materials with nano-zinc oxide (nano-ZnO) and nano-zirconia (nano-ZrO₂) under carbonation mixing conditions. The results indicate that the quadratic polynomial models exhibit good-to-excellent goodness-of-fit, with the 28-day saturated water absorption model achieving an R² of 0.9650 and an adequate precision of 16.47, confirming reliable predictive capability. Compressive strength ranged from 62.29 to 116.45 MPa, representing a 90.05% increase in the lower limit and a 22.37% increase in the upper limit relative to the reference group. Nano-ZnO generally reduced early-age strength due to its retarding effect, while nano-ZrO₂ exhibited a continuous strengthening effect across the 0–10% dosage range. The optimal synergistic range for early-age strength was identified as 0–0.2% nano-ZnO and 0–2% nano-ZrO₂, whereas nano-ZrO₂ dominated long-term performance enhancement. Saturated water absorption decreased significantly from 3 to 28 days, reflecting progressive pore refinement through continued hydration and carbonation product filling. Autogenous shrinkage showed a non-monotonic trend with nano-ZnO content—initially increasing then decreasing—while drying shrinkage increased predominantly with nano-ZrO₂ dosage, attributed to increased capillary tension resulting from pore structure refinement. The carbonation environment accelerates CO₂ diffusion and reaction, generating stable calcium carbonate that partially fills microcracks and further refines the pore structure, which provides a viable technical pathway for developing low-carbon, high-performance cement-based composites, with the RSM-based optimization framework offering an optimization tool for tailored mix design in applications such as high-strength concrete, repair mortars, and prefabricated elements where early strength and dimensional stability are critical. Full article

High-volume fly ash binder and aeolian sand have been widely used in the production of low-carbon ultra-high performance concrete (UHPC) owing to their environmental and economic benefits. However, such low-carbon UHPC still faces the volume stability issues, including autogenous shrinkage, drying shrinkage and early-age cracking. In this study, a superabsorbent polymer (SAP) was incorporated into low-carbon UHPC containing high-volume fly ash and aeolian sand to improve its volume stability through internal curing. The effects of SAP particle size and dosage on the mechanical properties and volume stability of low-carbon UHPC were investigated. Results showed that the addition of 0.3% SAP with a particle size of 180–600 μm significantly reduced shrinkage and early-age cracking while maintaining the compressive strength. Furthermore, the microstructure of low-carbon UHPC was characterized using advanced techniques to shed light on the underlying mechanisms. Full article

This research systematically evaluated the durability performance of low-carbon cement concrete prepared with industrial solid wastes under harsh service conditions. Measurements included mechanical properties and drying shrinkage. Microstructural characterization was carried out using SEM-EDS, MIP, and TG-DTG, revealing a synergistic relationship between microstructural changes and the resulting mechanical and durability behavior of the concrete. The experimental results indicated that adding 25% fly ash (FA) lowered the compressive strength of the CF-25 and BF-25 concrete by 11.30% and 11.39%, respectively, while reducing drying shrinkage by roughly 9.2–9.5%. In comparison, incorporating 5% silica fume (SF) had contrasting effects. It significantly improved the compressive strength of the CS-5 and BS-5 concrete by 18.92% and 9.94%, respectively, but at the cost of increasing drying shrinkage by 6.30% and 18.68%, respectively. Fractal dimension analysis based on thermodynamic relationships showed that the pore structure fractal dimension (Ds) ranged from 2.88 to 2.93. Group C exhibited a higher Ds (2.93) than Group B (2.90), indicating a more intricate pore network associated with greater C-S-H gel formation. With FA addition, Ds decreased to 2.91601 for CF-25 but rose to 2.93244 for BF-25. With SF addition, Ds fell to 2.91182 for CS-5 and 2.88102 for BS-5. Micro-mechanistic analysis revealed that the limited pozzolanic activity of FA at early ages resulted in insufficient hydration products and increased porosity. In contrast, SF contributed to a dense, highly polymerized gel structure and an optimized pore size distribution through its strong pozzolanic reactivity and nano-filling action. The distinct chemical properties of high-calcium and low-calcium cementitious systems further accentuated the differential influences of these supplementary cementitious materials. Full article

This study investigates the mechanical properties and uniaxial compression behavior of nano-TiO₂-modified coral concrete (NTCC). Twelve groups of specimens with different nano-TiO₂ contents were prepared and cured in freshwater, seawater, and oxalic acid environments. Cube compressive strength, splitting tensile strength, and uniaxial compression tests were conducted according to relevant standards. The results indicate that nano-TiO₂ significantly enhances the mechanical performance of coral concrete. The compressive and tensile strengths initially increased and then decreased with increasing nano-TiO₂ content, with the maximum strength improvement reaching approximately 22%. Furthermore, increasing the nano-TiO₂ dosage reduced the brittle failure characteristics of NTCC under compression. The curing environment had a significant influence on the performance of NTCC. Specimens cured in seawater exhibited superior early-age strength, whereas those cured in freshwater achieved the highest later-age strength. The stress–strain response of NTCC under uniaxial compression can be divided into three stages: the elastic stage, elastoplastic stage, and descending stage. Based on the experimental results, an empirical constitutive model was proposed for NTCC. The predicted stress–strain curves showed good agreement with the experimental results, demonstrating the applicability of the proposed model for describing the compressive behavior of NTCC. Full article

Replacing Ordinary Portland Cement (OPC) with high volumes of fly ash (FA) offers a practical approach to reducing the environmental impacts associated with cement manufacturing and landfill disposal. However, high FA replacement levels, particularly up to 60%, often lead to lower early-age strength. This study developed a green cement mortar containing 60% FA and waste bottle glass nanoparticles (WBGNPs). The WBGNPs were incorporated at replacement levels of 2%, 4%, 6%, 8%, and 10% by volume of the OPC–FA binder. The findings showed that the addition of 4–6% WBGNPs significantly promoted the formation of dense reaction gels and enhanced compressive strength by 12.5–39.1%. Similar performance trends were observed in both the engineering and microstructural properties. The combined incorporation of FA and WBGNPs also improved drying shrinkage performance by reducing capillary stresses during water evaporation and minimizing crack development within the cement matrix. Additionally, a proposed shrinkage prediction model was validated using experimental data and demonstrated good agreement, with an average prediction error of approximately 8%. Overall, the incorporation of WBGNPs provides an effective method for producing high-volume FA cement mortars with satisfactory engineering properties suitable for concrete applications in tropical environments. This approach further supports sustainability by reducing waste generation, lowering landfill demand, and minimizing environmental pollution. Full article