Journal of Advanced Concrete Technology最新文献

The workability of high fluidity concrete (HFC) also depends on its segregation resistance besides fluidity, and gap-passing ability, etc. Currently, there is a lack of easy, quantitative method for evaluating segregation resistance. Efficient assessment is crucial for construction applications of HFC. This paper aims to propose a simple test method for the segregation resistance of HFC on basis of the J-ring test that has been generally used for evaluating the fluidity and passing ability of HFC. Experiment and numerical simulation of J-ring test were conducted for HFCs with different fluidity and segregation resistance. The fresh concretes were treated as two-phase granular fluids of matrix mortar and coarse aggregate in simulation by a newly developed particle meshless method, called DPMP-MPS. The flow and segregation behaviors of the HFCs during J-ring test under different lifting speeds of slump cone were investigated. The numerical results demonstrate a close correlation between the final flow value to slump value ratio (SF_-J/SL_-J ratio) and the segregation resistances of HFCs. Consequently, the J-ring test can assess the segregation resistance based on the SF_-J/SL_-J ratio. Notably, it emphasizes that, for precise evaluation of HFC workability using the J-ring test, the lifting speed of the slump cone should fall within the range of 10 to 15 cm/s.

This work is designed by coupling fly ash (FA) with dune sand (DS) for high-strength geopolymer activated in an alkaline environment under pressure-applied casting. Initially, the proportion of FA and DS is optimized with the least activator dosage to obtain higher than the compressive strength of 50 MPa. A uniaxial pressure is applied on a semi-dry mixture containing the least activators and immediately demolded, involving rapid production for the industrialization purpose of the paving blocks. The experimental study revealed that the FA-DS proportion of 1:1, with a liquid-to-solid ratio of 0.16, achieved a compressive strength of 54.4 MPa. Consequently, the coupling of DS provides an occupying effect and reduces the required activator quantity. The strength gain mechanism is discussed at the molecular level by analyzing Fourier-transform infrared. Finally, the technical performance of the strength and the density is evaluated on the real size 203 × 101 × 80 mm prism and compared with the commercially available conventional concrete blocks. Besides, the enviro-economic performance in terms of CO₂ emissions and the cost are analysed as well. It is concluded that the developed block is a more environmentally sustainable and economically viable alternative to conventional concrete blocks.

To explore the dynamic compressive mechanical properties of carbon fiber modified polymer reinforced concrete (CFMPRC), the 100 mm diameter SHPB (Split Hopkinson Pressure Bar) test system was used to carry out impact compression tests of CFMPRC with different carbon fiber volume content (0.1%, 0.2%, 0.3% and 0.4%). The dynamic stress-strain curves and fracture morphology of CFMPRC under different strain rates (37.7 s^-1 to 132.2 s^-1) were obtained, and the effects of strain rate and carbon fiber content on dynamic compressive strength, deformation and toughness of CFMPRC were analyzed. The results show that the dynamic compressive strength, deformation and toughness of CFMPRC have obvious strain rate strengthening effect and carbon fiber strengthening effect. The dynamic compressive strength, dynamic increase factor (DIF), dynamic peak strain and impact toughness of CFMPRC increase with strain rate gradually. The dynamic compressive mechanical properties of polymer reinforced concrete are improved by adding carbon fiber, and the optimal carbon fiber content is 0.2%. When carbon fiber content is 0.2%, the strain rate sensitivity of CFMPRC is the strongest, and the increase of strength is the maximum. Carbon fiber can bridge the internal cracks of concrete, and shows the co-modification effect with polymer. Polymer can enhance the carbon fiber/concrete matrix interface, which makes carbon fiber exert its effect more effectively.

The freeze-thaw resistance of concrete is significantly lower in salt water than in fresh water. Concrete deteriorates through repeated freezing and thawing, but in salt water, freezing alone leads to destruction. This paper investigated the effect of calcium hydroxide in concrete on the failure of concrete under such low temperatures. Calcium hydroxide precipitates at the transition zone between aggregate and cement paste due to the hydration of cement. The lower the temperature and the higher the concentration of salt water, the more calcium hydroxide dissolves. From concrete, more calcium hydroxide is eluted in salt water than in fresh water. This accelerates the deterioration of mortar and concrete due to freeze-thaw action. Mortar and concrete using ground granulated blast-furnace slag produces less calcium hydroxide. In mortar and concrete using blast-furnace slag sand, calcium hydroxide precipitated around the aggregate reacts with cement paste and blast-furnace slag sand to modify the transition zone. From these results, it was clarified that concrete using blast-furnace slag exhibits high freeze-thaw resistance even in salt water.

This paper is the English translation of the authors’ previous work [Ayano, T., Fujii, T. and Okazaki, K., (2023). “Freeze-thaw resistance of concrete using ground granulated blast-furnace and blast-furnace slag sand in salt water.” Japanese Journal of JSCE, 79(12), 23-00042. (in Japanese)].

Effects of fly ash (FA) content and environmental factors on the water permeability were studied, and the similarity relationship of time-dependent water permeability coefficient in site and laboratory environment was discussed. Meanwhile, the main microstructure parameters and their time-dependent characteristics were analyzed by the NMR method. Finally, the correlation between water permeability and porosity in two environments was analyzed. Results show that water permeability coefficient of FA concrete both decreased with exposure time in two environments. FA can effectively improve the water impermeability, and the improvement effect increased with FA content in the later exposure period. Laboratory environment accelerated the decrease of water permeability and porosity. However, in the later stage, the decrease degree was not as good as that in the site environment. Pores with size of 10 to 100 nm occupy the main part of pores in FA concrete and the proportion of harmful pores of diameter 100 nm or larger decreased with exposure time. The water permeability coefficient and porosity of concrete exposed for 520 days in laboratory are close to that exposed for 800 to 1000 days in site, showing a good time dependent correlation in both environments, and the correlation with exposure time is stronger than that considering FA content.

Electric Arc Furnace slag (EAF slag) reuse is currently limited by its inconsistent chemical composition and volume instability. However, the alkaline composition suggests the possibility to use this material for carbon capture and storage. This study investigated the CO₂ uptake of EAF slag using a direct aqueous carbonation technique. The process was implemented at room temperature and ambient pressure, with minimized energy consumption. The CO₂-reactive phases were identified through X-ray diffraction analysis. Different CO₂ quantification techniques were employed: thermogravimetric analysis, acid digestion and thermal decomposition. The replicability of experiments and quantification techniques was assessed through analysis of variance and pairwise comparisons. The average CO₂ uptake and coefficient of variation resulted respectively 7.9% and 9.0%, with a carbonation degree of about 34%, proving that this simple mineralization process can be promising even in mild conditions.

In previous studies on the bond behaviors of FRP sheets attached to concrete, specimens for bond tests that contained FRP sheets with relatively low stiffnesses were used. However, in actual strengthening design, high stiffnesses of FRP sheets are required because the scale of the structure is very large. Therefore, in this study, bond tests were conducted using specimens with many different sheet stiffnesses and with polyurea resin. As a result, the bond strength increased as the stiffness increased with multiple CFRP sheets. Nevertheless, existing bond strength models overestimated the bond strength when the stiffness exceeded 200 kN/mm. In addition, 3D scanning measurements of patterned and indented concrete thin layers behind CFRP sheets revealed that the interfacial fracture energy was strongly related to the surface area of the concrete thin layer, not to the CFRP sheet stiffness or the resin properties.

This paper is an English translation of the authors’ previous work [Ozaki, M., Sato, Y., Yoshida, E., Takeuchi, A., Yamada, Y. and Nagashima, F., (2023). “Assessment on bond strength of CFRP sheet bonded to concrete focused on sheet stiffness.” Journal of JSCE, 79(6), 22-00289. (in Japanese)].

Reinforced concrete (RC) structures in cold regions are susceptible to surface deterioration due to freeze-thaw cycles (FTC). For sustainable development goals (SDGs) and a decarbonized society, damaged structures should be repaired and reinforced. Post-installed anchors are commonly used for seismic retrofitting and equipment fixation. However, research on the bond characteristics of damaged concrete is limited. Therefore, in this study, the bonding performance of adhesive anchors in damaged concrete was investigated. Liquid nitrogen was employed to subject the concrete surface to FTC; subsequently, bond-slip tests were conducted with the degree of deterioration serving as a parameter. The results suggested, the bond strength decreased as the degree of damage increased. The reduction ratios of the post-installed anchor with epoxy and cement-based resins were almost identical. Furthermore, a bond strength equation was proposed by referring to the bond-slip model between the rebar and concrete (fib 1990). The test results were well predicted with a correlation coefficient of 0.94. This study is based on previous studies (Yano et al. 2022, 2023) but presents new findings.

To better utilize topology optimization theory to assist in designing reinforced concrete (RC) D-region members, a novel application mode, the Genetic Bi-directional Evolutionary Structural Optimization (GBESO) method based on Finite Element Analysis (FEA) with discrete models is proposed. Correspondingly a design method for reinforcement layout of RC D-region members is also derived. Non-linear FEA verification is conducted on numerical examples involving deep beams with openings. The results demonstrate that the GBESO algorithm exhibits better global optimization capacities compared to Evolutionary Structural Optimization-type (ESO-type) algorithms. It also provides rebar topologies that are more in line with the optimization objective, bringing lower steel consumption and higher rebar utilization rates. Moreover, by introducing inclined rebar to the members, their shear strength is enhanced to a level comparable to the flexural one, significantly improving ultimate load-bearing capacity, elastoplastic deformation capacity, and better ductility compared to empirical method.

Steam curing is a widely used technique for producing precast concrete components in practical engineering. Chloride attack is a main factor that leads to the corrosion of rebars in concrete structures, which significantly affects the durability and safety of concrete structures in marine environments. In this paper, XRD, LF-NMR, ²⁹Si NMR, and ²⁷Al NMR were used to investigate the microstructure evolution of steam-cured concrete and standard-cured concrete. Then, the steam-cured and standard-cured concrete were subjected to actual marine exposure tests to explore the microstructure evolution of concrete and the chloride erosion behavior in the tidal zone. The results showed that steam-curing can increase the average molecular chain length (MCL) and polymerization degree of C-(A)-S-H, promote the transformation of silicon-oxygen tetrahedral dimer to polymer, and increase the Q²/Q¹ value of steam-cured concrete to twice that of standard-cured concrete. Compared to standard-cured specimens, steam-cured concrete specimens had significantly more harmful pores and multi-harmful pores. With the increase of exposure time to the marine environment, the depth of the convection zone inside the concrete did not change significantly, however, the peak value of chloride concentration increased gradually. The addition of SCMs to steam-cured concrete reduced the content of free and total chloride, which was well explained by microscopic tests. The concentration of chloride on the surface and chloride diffusion coefficient of steam-cured concrete showed a quadratic function relationship with the increase of slag content, and an exponential decay relationship with the content of fly ash.