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Carbonation-induced microstructural evolution regulates multiscale fracture in UHPC incorporating recycled coarse aggregates
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DOI：10.1016/j.tafmec.2026.105826.png)
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Abstract
En 中文
Carbonation treatment refines RCA pore structure by means of Ca(OH)₂ consumption and partial C-S-H decalcification, leading to contradictory mechanical effects: improved compressive strength and durability as opposed to impaired tensile and fracture behavior. However, the influence of such carbonation on the multi-scale fracture behavior of ultra-high performance concrete incorporating carbonated RCA (UHPC-CRCA) and the underlying regulation mechanism remain poorly understood. This study investigates the cracking behavior of UHPC-CRCA by linking carbonation-induced phase transformation, pore-structure modification, microcracking evolution and strain localization, and macroscopic fracture response. The results show that carbonation promotes the formation of CaCO₃-rich products and markedly reduces the volume of capillary pores within the 10–1000 nm range. However, this densification does not translate into enhanced cracking resistance. Compared with UHPC containing natural coarse aggregates, the inherent defects of RCA promote earlier microcrack initiation, more low-amplitude microcracking events, and premature fracture process-zone development. After carbonation, although the RCA surface becomes visibly denser, an earlier abrupt decrease in b-value, stronger b-value fluctuations, earlier strain localization, and enlarged fracture process-zone evolution indicate more dispersed small-scale microcracking in UHPC-CRCA. At the mesoscale, the proportion of cracks penetrating aggregates increases from 19.45% in UHPC-RCA to 25.82% in UHPC-CRCA, resulting in more tortuous crack paths. Consequently, at the macroscale, the peak load and fracture energy decrease from 5.31 to 4.80 kN and from 1.09 to 0.88 J, respectively. This study clarifies the strength–fracture resistance trade-off in UHPC-CRCA and provides mechanistic guidance for the design of UHPC incorporating recycled aggregates.
Keywords:
Ultra-high performance concrete
Recycled coarse aggregate
Carbonation treatment
Microstructural evolution
Cracking evolution
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