Search Results (57)

To address the limitations of traditional hardened concrete road surfaces in coal mine tunnels, which are prone to damage and entail high maintenance costs, this study proposes using modular concrete blocks composed of fly ash and coal gangue as an alternative to conventional materials. These blocks offer advantages including ease of construction and rapid, straightforward maintenance, while also facilitating the reuse of substantial quantities of solid waste, thereby mitigating resource wastage and environmental pollution. Initially, the mineral composition of the raw materials was analyzed, confirming that although the physical and chemical properties of Liangshui Well coal gangue are slightly inferior to those of natural crushed stone, they still meet the criteria for use as concrete aggregate. For concrete blocks incorporating 20% fly ash, the steam curing process was optimized with a recommended static curing period of 16–24 h, a temperature ramp-up rate of 20 °C/h, and a constant temperature of 50 °C maintained for 24 h to ensure optimal performance. Orthogonal experimental analysis revealed that fly ash content exerted the greatest influence on the compressive strength of concrete, followed by the additional water content, whereas the aggregate particle size had a comparatively minor effect. The optimal mix proportion was identified as 20% fly ash content, a maximum aggregate size of 20 mm, and an additional water content of 70%. Performance testing indicated that the fabricated blocks exhibited a compressive strength of 32.1 MPa and a tensile strength of 2.93 MPa, with strong resistance to hydrolysis and sulfate attack, rendering them suitable for deployment in weakly alkaline underground environments. Considering the site-specific conditions of the Liangshuijing coal mine, ANSYS 2020 was employed to simulate and analyze the mechanical behavior of the blocks under varying loads, thicknesses, and dynamic conditions. The findings suggest that hexagonal coal gangue blocks with a side length of 20 cm and a thickness of 16 cm meet the structural requirements of most underground mine tunnels, offering a reference model for cost-effective paving and efficient roadway maintenance in coal mines. Full article

The utilization of steam-cured steel slag fine aggregate concrete (SC) faces challenges in accurately predicting its stress–strain relationship. The mechanical properties of steam-cured SC and its stress–strain relationship have been systematically investigated through combined tests and machine learning (ML) approaches. The results showed that steam curing at 50 °C greatly increased the peak stress and ductility of SC. Specimens, the steel slag fine aggregate (SA) content of which was 40% by volume, and which were subjected to steam curing at 50 °C, exhibited superior mechanical and deformation properties. The prediction performance of three ML models—random forest (RF), backpropagation neural network (BPNN), and support vector regression (SVR)—was compared based on the test data. The analysis results revealed that the RF model achieved optimal performance (R² = 1.00), whereas the SVR model underperformed overall. Through the transfer validation method, it was found that the BPNN model, after parameter optimization, demonstrated a superior generalization ability in cross-mix-proportion predictions. It exhibited satisfactory prediction stability for steam-cured SC with an untrained mix proportion. In contrast, the RF model tended to overestimate peak stress. The theoretical reference for realizing the comprehensive utilization of steel slag in precast concrete components has been provided. Full article

To address the insufficient early strength development of steel-fiber-reinforced high-performance concrete (SF-HPC) under subzero temperatures, this study proposes an electric-induced heating curing method for SF-HPC fabrication at −20 °C. The effects of mix parameters, including steel fiber content, water-to-binder ratio, silica fume dosage, and fine aggregate gradation, on the curing temperature and mechanical properties of SF-HPC were systematically investigated. The optimal mix proportion was identified through the curing temperature and compressive strength development for the specimens. Results revealed that compressive strength initially increased and then decreased with higher silica fume content and fine aggregate replacement ratios, while increased water-to-binder ratios positively influenced curing efficiency and strength development. The optimal mix comprised 2.0 vol% steel fibers, a water-to-binder ratio of 0.22, 20% silica fume, and 60% fine aggregate replacement. Further, comparative analyses of electric-induced heating curing, room-temperature curing, and high-temperature steam curing revealed that electric-induced heating curing can promote the strength formation of SF-HPC in a negative-temperature environment. Microstructural characterization via BET analysis demonstrated that electric-induced heating curing refined the pore structure of SF-HPC. These findings highlight the benefits of electric-induced heating as an efficient strategy for fabricating SF-HPC in cold climates, providing theoretical and practical insights for winter construction. Full article

In order to improve early-age strength, steam curing is mostly used for railway prefabricated components, which consumes a lot of energy and affects the durability of concrete. Synthetic calcium silicate hydrate (C-S-H) has an excellent early-age strength effect, which can improve the early-age strength of concrete and help to reduce the energy consumption of steam curing, but C-S-H will increase the shrinkage of concrete and affect the durability of concrete. In this work, C-S-H/SRPCA was synthesized using a shrinkage-reducing polycarboxylate superplasticizer (SRPCA) in order to increase the early-age strength and decrease the shrinkage of concrete. The effects of 0.5%, 4.0%, and 8.0% C-S-H/SRPCA on the shrinkage and strength of concrete were studied. Meanwhile, the internal mechanism was also explored through cement hydration, the physical aggregation morphology of hydration products, pore structure and classification, and the chemical properties of pore solution. The results suggest that C-S-H/SRPCA can shorten the setting time and accelerate cement hydration. Specifically, when the dosage of C-S-H/SRPCA is 4.0%, the initial setting time of concrete is shortened by 2.5 h and the final setting time is shortened by 6.2 h compared with the control group. As a result, the 1-day compressive strength is effectively increased by 29.5%, and the plastic shrinkage is reduced. In the stage of plastic shrinkage, the plastic shrinkage time of the concrete with 4.0% C-S-H/SRPCA is 4.1 h, which is 6.1 h shorter than that of the control group. In addition, C-S-H/SRPCA decreases the porosity. When the dosage is 4.0%, the porosity of the hardened cement paste at 28 days is reduced by 15% compared with the control group. It lessens the content of the capillary pores at 10–50 nm. At 24 h, the content of 10–50 nm capillary pores in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. It also reduces the surface tension of the pore solution. The surface tension of the simulated pore solution with 4.0% C-S-H/SRPCA is 34 mN/m, which is 53% of that of the control group, and it inhibits the volatilization of the pore solution. At 28 days, the evaporation rate of the pore solution in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. Thus, the drying shrinkage of concrete is inhibited. Given the above, at the optimum content of 4.0%, C-S-H/SRPCA improves the 1-day compressive strength of concrete by 29.5%, reduces the 28-day total shrinkage by 21.7%, and restrains the development of microcracks. Full article

To enhance the microstructure and mechanical properties and to optimize the formulations of high-strength and high-flow ultra-high-performance concrete (UHPC), the effects of different quartz powder contents, fineness, and curing temperatures of UHPC were systematically studied. It was hypothesized that using the appropriate content and fineness of quartz powder can improve the microstructure of UHPC and, thus, improve its mechanical properties, especially at higher curing temperatures. To test this hypothesis, the flowability, compressive strength, flexural strength, and tensile strength of UHPC mixtures with different quartz powder dosages (0%, 15%, 30%) and fineness (4 µm, 8 µm), cured at 20 °C, 45 °C, and 90 °C, were investigated. The results indicate that as the dosage of quartz powder increases and the particle size decreases, the flowability of UHPC decreases. The compressive strength of UHPC first increases and then decreases with an increase in quartz powder dosage. Finer quartz powder usage considerably enhances packing density and pore structure. When the content of quartz powder is 15%, UHPC achieves optimal mechanical properties and pore structure, showing an improvement of 3.6% to 14.4% compared to UHPC with a coarse particle size. Additionally, an increase in curing temperature leads to the consistent growth of the compressive strength of UHPC. Under 90 °C steam curing, UHPC incorporating 15% fine quartz powder (4 μm) achieved a peak compressive strength of 182.1 MPa, increased by 19.8% compared to that under 20 °C, which is attributed to the enhanced pozzolanic activity of fine quartz powder. These findings provide valuable guidance for the mix design of UHPC in precast concrete components, long-span bridges, and marine structures, where high early strength and durability are critical. Full article

The utilization of aeolian sand (AS) as a substitute for river sand (RS) in ultra-high-performance concrete (UHPC) offers a sustainable solution to address natural sand resource shortages while enhancing AS utilization. This study systematically evaluates the influence of AS content (0–100% RS replacement by mass) on the workability, mechanical properties, and microstructure of UHPC under different curing regimes. All mixtures incorporate 0.65% by volume of straight steel fibers to ensure adequate fiber reinforcement. The results reveal that the spherical morphology, smooth surface nature, and fine particle size of AS enhance the matrix fluidity and reduce the early autogenous shrinkage of UHPC. By employing steam curing at 90 °C for 2 d followed by standard curing for 7 d (M3), UHPC samples with a 60% and 80% AS substitution achieve a compressive strength of 132.4 MPa and 130.8 MPa, respectively; a flexural strength exceeding 18 MPa; a porosity below 10%; and a gel pore content exceeding 60%. The steel fiber reinforcement contributes significantly to the flexural performance, with the fiber–matrix interface quality maintained even at high AS replacement levels. These findings highlight the feasibility of AS as an alternative fine aggregate in UHPC. Full article

Steam curing is a widely used method in the production of industrial precast concrete but it often leads to thermal damage that negatively impacts the material’s long-term durability and mechanical strength. The use of supplementary cementitious materials (SCMs) has shown considerable promise in improving pore structure and alleviating these adverse effects. This study employs high-resolution X-ray computed tomography (X-CT) to thoroughly assess how steam curing temperatures and various subsequent curing regimes influence the pore characteristics of mortars containing high volumes of mineral admixtures. The results shows that steam-cured specimens under water curing (ST8012-WA) achieved a compressive strength of 51.72 MPa and flexural strength of 5.85 MPa, representing improvements of 9% and 19.8%, respectively, compared to natural curing (ST8012-NA: 47.32 MPa and 4.88 MPa). The standard-cured specimen (SD) exhibited the highest compressive strength of 54.18 MPa, highlighting the detrimental effects of elevated steam curing temperatures. The findings reveal that higher steam curing temperatures result in increased porosity and decreased mechanical strength, challenges that can be effectively mitigated through appropriate postcuring techniques. Notably, water curing following steam curing proves especially effective in reducing pore size variability and improving the material’s durability. This research offers new insights into the intricate relationships among curing temperature, pore morphology, and mechanical performance, providing practical recommendations to optimize the quality and longevity of steam-cured precast concrete components. Full article

This study explores the use of volcanic glass powder (VG) derived from Shirasu volcanic deposits as a substitute for silica fume (SF) in producing high-strength precast concrete piles with a compressive strength of 123 MPa. Initially, mortar specimens with varying VG replacement ratios and curing temperatures were prepared to assess their compressive strength. After identifying the optimal mix ratios and curing conditions for high-strength mortars, concrete specimens incorporating VG were produced. Subsequent testing revealed that a VG replacement ratio of 20% by cement volume and a curing temperature of 70 °C were optimal for achieving the target compressive strength. Although the Young’s modulus of VG-incorporated concrete was slightly lower than that of pure cement and SF concrete, its performance remained satisfactory. These findings suggest that VG is a viable alternative to SF in high-strength concrete applications, providing a sustainable method to enhance concrete properties using locally available volcanic deposits. Full article

The construction waste of brick and concrete can be used to produce recycled powder and recycled sand, which can replace cement and natural sand, respectively, in concrete. This can reduce the cost of concrete, reutilize construction waste and decrease environmental pollution. The idea of producing UHPC incorporating both RP and RS by standard curing instead of steam curing is proposed in this study. The optimal mixture design of ultra-high-performance concrete (UHPC) with both recycled powder and recycled sand is investigated. Based on the revised Dinger–Funk model, the optimal mix proportions of green UHPC (GUHPC) with recycled powder and recycled sand were calculated on this basis, and the effects of the superplasticizer content, water–binder ratio, recycled powder and recycled sand replacement ratio on the workability and mechanical properties of GUHPC at different ages were investigated through the designed experimental program. The test results show that when the superplasticizer to cementitious material ratio was 0.8%, the flowability and the 28 d compressive strength were highest. When the water–binder ratio was 0.16, the flexural strength and compressive strength of the GUHPC at different ages were the largest. As the replacement ratio of the recycled powder increased, the workability of the GUHPC decreased. However, even when replacement ratio of recycled powder was 30%, the flowability was still higher than 180 mm. The flexural strength and the 28 d compressive strength increased first and then decreased. Compared with mixtures without RP, the 28 d compressive strength increased by 6.4% and reached the maximum value when the replacement ratio of the RP was 30%. The comprehensive contribution of recycled powder to the strength was analyzed. Recycled powder can enhance the contribution of cement to GUHPC strength, and the enhancement effect increases with increases in the recycled powder content and age. The optimal replacement ratio of recycled powder is 30%. As the replacement ratio of the recycled sand increased, the flowability of the GUHPC first increased and then decreased, and the flexural and compressive strength decreased. The toughness was analyzed by the flexural strength to compressive strength ratio (f:c). With increases in the recycled sand, the f:c at 3 d of age increased, the f:c at 7 d of age showed no significant change, and the f:c at 28 d of age first increased and then decreased. The f:c at 28 d of age reached a maximum value of 0.316 when the replacement ratio of recycled sand was 50%. Therefore, the replacement ratio of the recycled sand was selected to be 50%. The optimum mix proportions of GUHPC were obtained by considering the workability, mechanical properties and amount of recycled material. Full article

In order to improve the dispersion state of silica fume (SF) in cement concrete, accelerate the hydration rate of high-strength steam-cured cement concrete, and reduce production costs. In this paper, SF was made into a wet-ground silica fume (WSF) suspension solution through a wet grinding process and was applied to high-strength steam-cured concrete to replace the SF so as to improve the difficult dispersion of the inner SF and enhance the compressive strength of concrete. The physical and chemical properties of WSF were studied by XRD, SEM, and ZETA potential, and its effects on the mechanical properties, hydration development, and microstructure of cement concrete were studied using XRD, SEM, TG, BET, and NMR. The results show that SF mixed with water is transformed into a kind of suspension solution by wet grinding. After adding WSF, the compressive strength of concrete at 1 d increases when the substitution of WSF increases. Compared with SF-0, the 1 d compressive strength of SF-1 and SF-2 increased by 9.2% and 12.9%. When the WSF substitution was greater than 50%, the compressive strength of concrete did not improve significantly; the 1 d compressive strength of SF-3 and SF-4 is 14.3% and 15.4% higher than SF-0. With the increase in WSF substitution, the porosity of concrete at 1 d decreases, and the structure becomes denser. XRD, TG, and NMR analyses show that WSF can promote the hydration development of cement to form a C-S-H gel. As the amount of WSF substitution increases, its effect on the cement hydration reaction increases first and then tends to be flat. Full article

Solid wastes have been widely used as a cement substitute in precast concrete. On the one hand, solid waste can effectively ameliorate a series of problems caused by steam curing. On the other hand, the use of solid waste can reduce the amount of cement used in the construction industry and reduce carbon emissions. However, due to the complexity of the steam curing system, the performance of precast concrete prepared under different steam curing conditions varies greatly. Moreover, there are a wide variety of solid wastes, and the differences in the physicochemical properties of different solid wastes are significant. Therefore, it is necessary to systematically determine the mechanism of action of commonly used solid wastes. In this paper, the steam curing system is introduced in detail, and the mechanism of action of solid waste in precast concrete is systematically summarized. It was found that an appropriate increase in the temperature and duration of steam curing facilitates the strength development of precast concrete. In addition, there is a difference in the effect of the addition of solid wastes on the early and late strength of precast concrete, which usually leads to a decrease in the demolding strength of precast concrete, but increases the late strength of precast concrete. This study provides a reference for rationally regulating steam curing systems and realizing the comprehensive utilization of solid wastes in precast concrete. Full article

This research investigates the effects of various curing regimes, the incorporation of titanium slag, and the utilization of quartz sand on the strength properties and shrinkage behavior of ultra-high-performance concrete (UHPC). By using low-heat silicate cement to prepare UHPC, this study conducted standard curing and steam curing, and comprehensively analyzed the macro and micro performance of UHPC under different curing conditions. The findings indicate that the application of steam curing markedly enhances the mechanical attributes of UHPC while efficiently decreasing its drying shrinkage. In the comparative tests, we found that the compressive strength of concrete that had undergone 2 days of steam curing was 9.15% higher than that of concrete cured for 28 days under standard conditions. In addition, under the same curing conditions, titanium slag sand had higher mechanical properties than quartz sand. Under standard curing conditions, the 28-day compressive strength of UHPC using titaniferous slag aggregate was 12.64% higher than that of UHPC using standard sand. Through the data analysis of XRD, TG, and MIP, we found that the content of Ca(OH)₂ in the hydration products after steam curing was reduced compared to the standard curing conditions, and the pore structure had been optimized. The UHPC prepared with titanium slag sand has greater advantages in mechanical properties and drying shrinkage, and has a smaller pore structure than the UHPC prepared with quartz sand. Moreover, the use of titanium slag sand offers ecological and economic benefits, making it a more sustainable and cost-effective option for high-performance construction applications. Full article

The fabrication process of pretensioned prestressed concrete (PC) girders involves temperature changes, which affect the effective prestress and mechanical properties of the girders. Currently, there is a lack of a holistic understanding and accurate calculation methods for the prestress variation due to temperature change (PVTC), leading to technical challenges in calculating effective prestress in pretensioned PC girders. This study investigates the PVTC in three stages considering the time-varying interaction between concrete and tendons, proposes a new method to consider the effect of a deviator on the PVTC of a bent tendon, conducts an experimental study to validate the theoretical analysis, and develops measures for reducing the PVTC. The results show that the presented method provides reasonable predictions of PVTC, and the PVTC of the girder with steam curing is up to 80.3 MPa. Based on the presented method, measures for reducing the PVTC are proposed. This study provides new insights into computing the PVTC and improves the design and fabrication of pretensioned PC girders. Full article

To enhance the physical and mechanical characteristics of steam-cured concrete, an orthogonal experimental design was utilized to examine the effects of varying contents of fly ash (0 wt%, 10 wt%, 15 wt%, 20 wt%), silica fume (0 wt%, 5 wt%, 10 wt%, 15 wt%), basalt fiber (0 vol%, 0.05 vol%, 0.1 vol%, 0.2 vol%), and polypropylene fiber (0 vol%, 0.05 vol%, 0.1 vol%, 0.2 vol%) on its mechanical properties. Utilizing range and variance analyses, this study identified four preliminary optimized compositions of concrete incorporating fly ash, silica fume, basalt fiber, and polypropylene fiber. On this basis, in order to determine the optimal mix proportion, the mechanical performances, the pore characteristics, and the microstructure of four optimized mix proportions were analyzed. According to the results of macroscopic, fine, and microscopic multi-scale tests, the addition of 15 wt% fly ash, 10 wt% silica ash, 0.2 vol% basalt fiber, and 0.1 vol% polypropylene fiber to the steamed concrete is the best to improve the performance of the steamed concrete. Compared to ordinary concrete, the compressive strength increases by 28%, the tensile strength increases by 40%, and the porosity decreases by 47.2%. Full article

New building materials (manufactured sand and recycled coarse aggregates) can conserve raw materials and protect the environment. Prefabricated members can shorten the construction time of a structure. To use manufactured sand and recycled coarse aggregate in the preparation of precast member concrete, an economical and practical steam-curing scheme must be developed such that the compressive strength of precast manufactured sand recycled concrete (MRC) meets the requirements for hoisting. The effects of different steam-curing temperatures (standard curing, 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C) on the compressive strength of MRC with three water/binder ratios (W/B) (0.46, 0.42, and 0.38) were studied. In addition, the microstructure of MRC was examined using a scanning electron microscope. The equivalent age–compressive strength model was used to estimate the recycled concrete with manufactured sand. The results showed that the strength of MRC with a water–cement ratio of 0.46, 0.42, and 0.38 reached 33.9, 38.7, and 45.1 MPa, respectively, after 28 days of standard curing. The results also indicated that an increase in the steam-curing temperature had a positive effect on the early compressive strength of MRC and a negative effect on the 28 d compressive strength. This behavior was more obvious for MRC with a low W/B ratio. For MRC with a W/B of 0.46, 0.42, and 0.38, after steam-curing for 6 h, the compressive strength reached 32–65%, 36–70%, and 40–77% of the design strength, respectively. The optimum steam-curing temperatures for MRC with W/B of 0.46, 0.42, and 0.38 were 60 °C, 60 °C, and 50 °C. A decrease in W/B has a negative impact on the accuracy of MRC estimation using the equivalent age–compressive strength model. The maximum deviation of the prediction was within 10%, and the accuracy of the model was acceptable. This study provides a useful reference for the production of prefabricated MRC components in factories and subsequent construction. Full article