In this study, for developing a silicate-based surface impregnation solution with high permeability to repair degraded concrete from the surface to the interior, experiments were conducted to investigate the effect of blending sodium hydroxide (NaOH) on the permeability of the lithium silicate solution (LS) that is one of generally used silicate surface impregnation materials. The high permeability of the modified LS solution, composed of LS, NaO, and water, was confirmed by solution immersion test using several grades of concrete and heated concrete with different strengths, and by ICP-AES analysis. Appropriate NaOH blend significantly increased the penetration of LS into concrete. The appropriate NaOH blends and the mechanisms of increasing permeability were investigated by the setting time test of fresh pastes of modified LS solution and unheated or 650°C-heated Portland cement paste powders, and the SEM-EDS, XRD, TG-DTA analyses of hardened pastes. As a result, when NaOH is blended in an amount such that the Na/Si molar ratio of NaOH-modified LS solutions is in the range of 0.5-1.25, they have high permeability. The addition of NaOH reduces the solubility of Ca(OH)₂, being a hydrate of Portland cement (PC), and therefore delays the calcium silicate formation between Ca(OH)₂ and LS. This delay prevents the penetration path of LS from being quickly blocked by calcium silicate, thus LS permeability is improved. However, the addition of excessive NaOH destroys other hydrates of PC to dissolve Ca ions, instead increasing the calcium silicate formation. In addition, the blend of NaOH would increase the alkalinity of neutralized concretes.

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.

Moisture transport is the key phenomenon indicating the deterioration of the durability and structural performance of concrete structures. Although various studies have attempted to evaluate moisture transport in concrete, an anomalous behavior, which does not follow the root-t law compared to other porous material, was not explicitly taken into account. To quantitatively evaluate anomalous moisture transport, this study developed a couple of numerical methods between the truss-network model (TNM) and the rigid-body-spring model (RBSM) for this purpose. The colloidal behavior of calcium-silicate-hydrate (C-S-H), which is the major phase of cement-based material, was introduced to consider the anomalous behavior and mechanical response regarding the microstructural change of cement paste as well as cracks that significantly accelerate the moisture transport in concrete. The numerical results indicated that both microstructural change of cement paste and rapid absorption through cracks cause anomalous behavior. In addition, the numerical results suggest that volumetric change of cement paste should rely on water content related to the colloidal behavior of C-S-H in order to reproduce the realistic expansion and the closure of cracks during a rewetting process that affects structural performance and durability of concrete.

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.

Compressive strength development of concrete is essential for cold weather concreting. It is well known that the strength development of concrete at temperatures below the freezing point is much delayed, however, the evaluation method for sub-zero temperatures has not been shown clearly yet. Therefore, when the average outside air temperature is under freezing point the planning method for cold weather concreting is not clear. This study focused on the effect of temperature on the strength development of concrete under sub-zero conditions by conducting various experiments. As a result, a function is proposed to calculate the maturity below the freezing point. The proposed function could explain the strength development of the concrete exposure in winter.

This paper is the English translation from the authors’ previous work [Taniguchi, M., Katsura, O. and Hama, Y., (2009). “A proposal of maturity function on the strength development of concrete under sub-zero temperature conditions.” Journal of Structural and Construction Engineering (Transactions of AIJ), 74(640), 995-1003. (in Japanese)].

One carbon neutralization measure applied in the concrete sector is the use of artificial carbonate in concrete for immobilization. This CO₂ reduction technology corresponds to the CO₂ emitted during concrete production. When considering the marketability of these technologies, especially for newly developed products in the carbon market, it is essential to quantify the amount of CO₂ fixed as inorganic carbonate. Additionally, as a representative test specimen for concrete containing aggregate, a φ100 × 200 mm cylinder specimen is conventionally used for physical property evaluation. To evaluate the amount of CO₂ fixed in one batch of concrete, a mass far from that of the conventional chemical analysis sample may need to be analyzed. Therefore, in this study, we investigated a pulverization process for concrete analytical materials. We also propose a new analytical apparatus that can be used to measure large cylinder specimens. Experimental results showed that the newly developed analyzer, equipped with a mass balance and CO₂ and H₂O gas analyzer for large cylinders, exhibited excellent analytical variability and measurement speed performance. It was also inferred that the homogenization process is necessary to grind the entire cylindrical concrete specimen into a fine powder and homogenize it to improve the representativeness of the concrete.