Efficiency factor-based mix design of Class F fly ash concrete for rigid pavements: Calibration and experimental validation using Vietnamese materials

Abstract

This study systematically investigates the calibration of the efficiency factor (k) and the development of a mix design procedure for fly ash cement concrete using Class F fly ash from the Pha Lai thermal power plant, for use in road pavements in Vietnam. In light of the urgent need to reduce infrastructure's carbon footprint and valorise industrial waste, replacing a portion of cement with fly ash provides environmental benefits and significant technical enhancements. Using a modified Bolomey strength model, the research team established a quantitative relationship between the fly ash replacement ratio (f), the water-to-binder ratio (ω), and the strength ratio (R_s) in order to derive the experimental k values. The findings reveal that the k-factor varies with fly ash content: for a replacement range of 15% to 35%, k-values fluctuate between 0.70 and 0.40. An 8-step k-based mix design procedure was proposed and validated through comprehensive testing of workability, mechanical properties, and durability.  The experimental results showed that all of the fly ash mixtures achieved the target flexural strength requirement for the intended rigid pavement application. However, mixtures containing 15–25% fly ash provided the most balanced combination of workability, later-age strength, abrasion resistance and impermeability. The use of fly ash was also found to reduce the peak hydration temperature and delay the time to peak, which is beneficial in terms of the early-age thermal response of pavement concrete. Overall, this study provides a basis for the proportioning of Class F fly ash concrete for rigid pavements using Vietnamese materials that is supported by experimentation.