Carbonation depth assessment in shotcrete with various initial damage degrees and accelerator dosages: Experimental study

Abstract

Underground structures widely use shotcrete and operate in elevated CO2 concentration environments. Accelerators significantly affect the hydration process and structure of concrete, while continuous tunnel construction leads to initial damage (cracks) in the hardened concrete, both of which have a significant impact on the carbonation resistance of shotcrete. The effects of accelerator dosage and the initial damage degrees on the carbonation depth of shotcrete were analyzed. The results show that initial damage accelerated shotcrete’s carbonation process. The carbonation depth of shotcrete samples with initial damage degrees of 0.06, 0.12, 0.18, 0.24, and 0.30 at 56 days increased by 35.3, 60.7, 74.8, 164.2, and 246.5%, respectively, compared to the reference group. Sodium aluminate accelerator increases the carbonation depth of shotcrete, and this influence becomes more evident at higher dosages. Microscopic analysis indicates that the accelerator reduces the content of Ca(OH)2 in the system, accelerates the formation of AFt hydrates in early hydration, and the aluminosilicate ions in the accelerator will also further facilitate the transformation of AFt into plate-like AFm, deteriorating the gel structure. After considering the carbonation depth trend and the requirements for setting time, the optimal dosage of the rapid-setting agent is determined to be 5% and try to avoid disturbing the early shotcrete. By fitting the experimental results, a carbonation depth prediction model was derived for concrete with consideration of initial damage degrees under standard testing conditions, based on Fick's second law. These findings are instrumental in the durability evaluation and residual life prediction of corroded structures with various damage degrees.