To investigate the axial load response of reinforced concrete (RC) columns confined by carbon textile-reinforced concrete (CTRC) under chloride attack, 24 CTRC-confined square columns and 12 unconfined columns were tested under axial load, while considering the influences of dry-wet cycles, textile ratios, stirrup ratios, and section sizes. The experimental results indicate that the corrosion resistance and compression performance of RC columns under chloride ion erosion were significantly improved by CTRC. The corrosion of the RC column with CTRC confinement was remarkably reduced with a maximum of 63.9% in the chloride salt environment. The maximum increments of bearing capacity and ductility of CTRC-confined columns were 30.9% and 87.7%, respectively, compared with unconfined columns. In addition, bearing capacity, ductility, and deformation energy were also affected by stirrup ratios and section sizes. Finally, the semi-empirical and semi-theoretical analytical models of the stress-strain relationship and axial-bearing capacity were proposed based on the experimental data and theoretical analysis. The compound confinement effects of CTRC and corroded stirrups on core concrete was considered in the proposed models. The models correlated well with the experimental data.
Super-high cementitious roller-compacted concrete (SHCRCC) that have unit cementitious materials content of 220 kg/m3 or higher can be recognized as “construction friendly RCC”. In this study, the proposition of how to reduce construction costs without sacrificing the workability was investigated. To solve the issue, mix proportions replacing the high volume of cementitious materials, cement, and fly ash with stone powder (SP) were surveyed. Based on exhaustive investigations, it was found that the mix proportion can be realized with sufficient tolerance of workability. In the proposed mix proportion, the cementitious materials replaced with SP up to about 100 kg/m3 provide a large paste volume of 240 to 260 L/m3. In addition, it was verified that the SP plays a sufficient role as an alternative to cementitious materials since the compressive and tensile strengths of the RCC, and the watertightness and bond strength at lift joints are the same as, if not better than high cementitious RCC (HCRCC). Reducing cementitious materials also helps to control the temperature rise of the RCC. In the case of a large-scale RCC dam of 150 m in height and 2 million m3 volume with 5 zones, it is found that a cost reduction is about 25 to 30% for cementitious materials and chemical admixture, and a placement speed is about 20% faster than that of medium cementitious RCC (MCRCC) thanks to a large workability margin.
This paper intends to examine the influence of temperatures on the early age of cement paste hydration, as well as the setting time and the compressive strength of cement paste with the addition of Triethanolamine (TEA) and Triisopropanolamine (TIPA); hence, the mechanisms of TEA, and TIPA at different curing temperatures were explored. The outcomes show that adding 0.05% and 0.1% TEA, and 0.05% and 0.1% TIPA retarded the setting time of cement pastes; however, this retardation depended on both the dosage and the curing temperature. At 23°C curing temperature, both TEA and TIPA could be used as a hardening accelerator at the dosage of 0.05% by cement weight. Adding 0.05% TEA into the cement paste strongly accelerated the hydration of C3A and increased the early compressive strength, which was more efficient than adding 0.05% and 0.1% TIPA, especially after 8 hours of curing. However, at 50°C curing temperature, as TIPA strongly accelerated the formation of AFt, AFm, CH, using TIPA was more effective than using TEA to enhance the cement paste's compressive strength. These results can offer theoretical direction for the application of cement-based materials adding TEA and TIPA under hot and standard curing temperatures.
The coefficient of thermal expansion (CTE) of cement paste is an essential parameter for estimating cracks of cement-based structures, including under normal operating conditions. The CTE of low-heat Portland cement pastes dried for a long term at various relative humidities were measured by applying trapezoidal temperature history. The measured CTE was a convex function when displayed versus relative humidity and was highest at the relative humidity of 58%. At the relative humidity of 11%, the CTE was similar to the one of the fully dried sample. Based on a drying shrinkage model in the literature that classifies pore water as free liquid water and adsorbed water, we computed pore pressure change and corresponding strain, from which the CTEs were estimated. The microstructural rearrangements of cement paste due to long-term drying were taken into account by obtaining pore size distributions from water vapor sorption isotherm. The CTEs predicted with the model agree well with the measured ones.
This study investigates the wet carbonation of concrete fines with CO2 and natural air gas bubbling in a carbonation system at low temperatures. After the air- and CO2-wet carbonations, the properties of a solution and hydrated cement paste powder are determined. In the air and CO2-wet carbonations, more Ca is extracted into the solution at a low temperature of 5°C. This high Ca concentration in the solution through air-wet carbonation primarily originates from the portlandite and unhydrated phases of the cement paste. Even in solutions with high pH values, the rehydration process and C–S–H decomposition occur simultaneously in air-wet carbonation. Moreover, CO2-wet carbonation indicates that the decalcification of C–S–H occurs rapidly, even in the presence of portlandite. Air-wet carbonation presents a potential method for the direct air capture of CO2 using concrete waste fines in a short period.
Sodium silicate activators derived from silica-rich wastes have attracted increasing attention owing to their promotion on the sustainable production and development of alkali-activated materials (AAM). This paper presents the research progress on the feasibility of using silica-rich wastes-derived sodium silicate activator as an alternative to commercial sodium silicate activator in AAM. The basic factors affecting the quality of silica-rich wastes-derived sodium silicate activator are reviewed. The structure features between commercial sodium silicate and derivative sodium silicate are addressed and compared. Influences of the two different sodium silicates on flowability, mechanical properties, microstructure and durability are summarized and discussed. Previous studies reveal that utilizations of amorphously rich SiO2 containing materials, together with proper preparation technology, enable to produce highly reactive sodium silicate activators and as a consequence to obtain materials with comparable or even better performances as compared to the commercially available sodium silicate-AAM. Recommendations for future investigation are provided eventually.