This work investigates the effects of mixture parameters on the slump flow and compressive strength of a new type of alkali-activated binder-based ultra-high strength concrete (AAB-UHSC) before and after exposure to high temperatures. The innovative UHSC adopts a combination of sodium silicate (SS) and potassium carbonate (PC) as the activator. Compared with the alkali hydroxide that is currently used in existing AAB-UHSC, alkali carbonate salts are environmental- and user-friendly and cost-effective. Four mixture proportioning parameters, including water/precursor ratio, activator/precursor ratio, silica fume (SF) replacement percentage, and SS/PC ratio, are evaluated regarding their quantitative effects on fresh and hardened properties of AAB-UHSC using Taguchi method. The analysis of variance (ANOVA) reveals that the activator/precursor ratio is the decisive parameter controlling the slump flow diameter of fresh AAB-UHSC, whereas all four parameters have considerable influence on compressive strength of AAB-UHSC at room temperature. Although increasing the activator/precursor ratio increases compressive strength of AAB-UHSC at room temperature, it is adverse for residual strength of AAB-UHSC after exposure to high temperatures. Based on the test results and ANOVA, a mixture proportioning method of AAB-UHSC, which is lacking in existing literatures, is proposed and experimentally validated.
The Nam Ngiep 1 (NNP1) Hydropower Project in Lao PDR has constructed a 167 m-high roller compacted concrete (RCC) dam. Class C fly ash (FA) procured from the Mae Moh coal-fired power plant in Thailand has been selected as a supplemental cementitious material for the NNP1 RCC dam, to control hydration heat generation and improve workability. Though rarely used globally for RCC, it was found that the Class C FA was acceptable for the NNP1 RCC because it did not undergo a large temperature rise in its early age and because of the relatively high compressive strength of the concrete as the age in the medium-term and long-term as compared with general features of the concrete with Class C FA. To clarify the reaction mechanism for Class C FA, factors affecting the above features of Class C FA are analyzed and evaluated by observing FA particles and concrete core specimens of NNP1 RCC through a variety of devices, including Field Emission-Electron Probe Micro Analysis (FE-EPMA). This paper clarifies the reaction mechanism of the concrete with Class C FA and demonstrates its applicability for RCC dam and other structures.
This study presents a kinematic model for flexural analysis of RC beams and columns subjected to freeze-thaw action based on upper bound theorem. The developed model enables analytical derivation of the contribution of damaged concrete when actual deterioration profile is idealized as an assemblage of undamaged and damaged zones based on freeze-thaw depth obtained from concrete core specimens. The accuracy of the analysis is verified by comparing its predictions with available 21 RC columns and beams failing in flexure after freeze-thaw exposure. The predicted results show good agreement with the test results within error of 6% on average. Thereafter, the developed analysis predicts the ultimate moment capacity of a RC beam, which was taken from an existing bridge slab replaced because of the combined effect of frost damage and fatigue. Results demonstrate that the present analysis could support a rational decision-making regarding the need for repair or rehabilitation. This paper is the English translation from the authors’ previous work [Kanazawa, T., Nakamura, T., Sakaguchi, J. and Kawaguchi, K., (2021). “Flexural analysis combined with freeze–thaw depth for RC linear members.” Journal of Japan Society of Civil Engineers, Ser. E2 (Materials and Concrete Structures), 77(4), 177-186. (in Japanese)].