Properties of UHPC with totally recycled fine aggregates and its mixture design method

Abstract

Using recycled fine aggregates (RFA) in preparing ultra-high-performance concrete (UHPC) is a critical challenge for developing sustainable building materials. This study explores the optimal gradation and mixture design method for using totally RFA to prepare UHPC. The modified Andreasen and Anderson model (MAAM) was adopted to design the gradation of RFA, and the effects of three key parameters including maximum particle size (Φ_max), minimum particle size (Φ_min), and distribution modulus (n) in MAAM on the flow and mechanical properties of UHPC were analyzed. Based on the experimental results, an optimized mixture design method for UHPC with totally RFA was proposed. The results showed that the flow of UHPC with totally RFA maintains a linear relationship with the total water absorption capacity of the RFA gradation. The compressive and flexural strengths achieved maximum values of 133 MPa and 12.4 MPa, respectively. Based on the synthetic strength evaluation, the optimal values of n, Φ_max, and Φ_min in MAAM were identified as 0.32, 2.36 mm, and 0.038 mm, respectively. The total density of the RFA gradation has an obvious correlation with the compressive strength of UHPC. Additionally, under the same RFA gradation, the proposed mixture design method increased the compressive and flexural strengths of UHPC without steel fibers by 6.5 % and 12.1 %, respectively. Adding 3 % steel fibers further enhanced the compressive strength by 10.7 % and improved the flexural strength by 183.9 %. These results highlight the benefits of optimized gradation in enhancing the performance of UHPC and validate the effectiveness of the proposed mixture design method. This work is of significant importance for the development and widespread application of environmentally friendly UHPC.