Abstract: The authors investigate the operational state of fire-retardant coatings in tunnels on 16 highways in Hunan Province, China. Numerous in-service tunnels are equipped with fire-retardant coatings, with thicknesses ranging from 12 to 20 mm. In 82% of the tunnels, the coatings have shed, posing a safety risk to tunnel traffic. However, assessment methods and treatment measures for evaluating the operational state of these in-service fire-retardant coatings remain limited. Based on the key performance indices outlined in relevant fire-retardant coating standards and the actual operational states observed in highway tunnels in Hunan Province, a coating operational state assessment model is developed. This model incorporates evaluation indices such as the shedding area rate, shedding frequency, bulging area rate, crack length and number, and average daily tunnel traffic flow. In addition, the types of coating defects, along with their occurrence and development mechanisms, are analyzed, and treatment measures are proposed for different operational state levels of existing fire-retardant coatings. A field assessment test conducted on the Paishaping tunnel yields an operational state value of 85.20—classified as a good level—which aligns with the assessment results provided by three independent testing and assessment units.

Key words: highway tunnels, fire-retardant coatings, state assessment, operational safety