Compressive Behavior and Constitutive Modeling of CFRP-Confined Geopolymer Concrete Containing Hybrid Recycled Concrete and Brick Aggregates

The incorporation of hybrid recycled concrete and brick aggregates (HRAs) into geopolymer concrete presents a promising sustainable solution. This approach significantly reduces carbon emissions by utilizing industrial by-products and repurposing construction waste, thereby conserving natural resources. It demonstrates broad application prospects for advancing green construction and a circular economy. Furthermore, external confinement with carbon fiber–reinforced polymer (CFRP) can mitigate the inherent limitations of geopolymer HRA concrete (GHRAC), leading to a substantial improvement in its mechanical performance. However, the compressive behavior and constitutive model of CFRP-confined GHRAC remain poorly understood. This paper presents an experimental investigation on the compressive behavior of GHRAC under both unconfined and CFRP-confined conditions. A total of 45 cylindrical specimens with a diameter of 100 mm and a height of 200 mm were tested. The main variables included the replacement ratio of recycled aggregates and the number of CFRP layers. The fully recycled coarse aggregates, comprising blends of recycled concrete aggregates and recycled brick aggregates (RBAs), were tested with RBA replacement levels from 0% to 100%. The results demonstrated a clear trend: the compressive strength of the CFRP-confined geopolymer concrete (GHRAC) progressively decreased with higher RBA content. Specifically, strength reductions of 25.4%−32.0% and 36.1%−47.6% were recorded at the 50% and 100% RBA replacement levels, respectively, highlighting the significant influence of RBA incorporation. The compressive strength of GHRAC specimens confined by one to three layers of CFRP reached 2.4–5.8 times that of the unconfined specimens, while the ultimate strain reached 4.4–11.7 times that of the unconfined specimens. The secondary stiffness of the axial stress−strain curves increased with decreasing RBA replacement ratio or increasing CFRP layers. Based on the existing fiber-reinforced polymer confinement models, the new models for compressive strength, ultimate strain, and stress−strain response of CFRP-confined GHRAC were proposed.