Evaluation of shear strength improvement of recycled concrete aggregate as a high-quality pavement material utilizing CO2 carbonation treatment

This study investigates the performance enhancement of recycled concrete aggregate (RCA) through CO₂ carbonation. RCA samples were treated with varying CO₂ concentrations in the range of 20–60 % and curing durations (T) in the range of 24–72 h. A series of large-scale direct shear tests, combined with SEM and EDS analyses, were conducted to evaluate the mechanical and microstructural changes. The optimal carbonation condition was identified as 20 % CO₂ concentration with 72-hour curing, resulting in a 51.7 % increase in friction angle and a 13.0 % increase in cohesion. In contrast, higher CO₂ concentrations (40 % and 60 %) yielded reduced strength improvements, with 60 % CO₂ treatment exhibiting diminished effectiveness. Strength parameters increased rapidly within the first 24 h before plateauing with extended curing durations. SEM analysis revealed that the 20 % CO₂-treated RCA developed a dense and well-compacted microstructure, with CaCO₃ effectively filling pores and bridging microcracks, whereas the 60 % CO₂-treated sample exhibited excessive surface carbonation that restricted internal modification. EDS results supported these observations, showing minimal elemental intensities in untreated RCA, increased Ca and C levels in the 20 % CO₂-treated sample, and surface accumulation in the 60 % CO₂-treated sample with stable Si content. The 20 % CO₂-treated RCA achieved greater strength with lower dilatancy than the untreated RCA; however, both exhibited comparable residual strength, governed primarily by particle friction. A predictive polynomial model was developed to estimate the evolution of friction angle and cohesion across varying CO₂ concentrations and curing durations, and its accuracy was confirmed through validation using the 10 % CO₂-treated RCA sample, demonstrating its reliability for practical applications.