Combustion characteristics and mechanism of hydrothermally aged asphalt in underwater tunnels

Asphalt could suffer hydrothermal ageing in underwater tunnels, which varies its combustion characteristics significantly. The hydrothermally aged asphalt (HA) was comparatively studied with base asphalt (BA) and thermal-oxidatively aged asphalt (TA). Major findings are achieved: (1) The C＝O and S＝O bonds are more abundant in TA/HA than BA. HA contains more prominent –OH and H₂O but less substantial aliphatics. And it has lower S[dbnd]O than TA by about 20 %, attributing to the oxygen isolation of aqueous immersion during hydrothermal ageing. (2) Remarkably bimodal heat release rate (HRR) peaks are found for HA, while only unimodal HRR peak is observed for TA/BA. Importantly, the primarily main HRR peak of HA occurs at around 110 s, which exhibits to be earlier than those of TA/BA by over 120 s. Meanwhile, the fire growth rate index (FGRA) of HA is higher than those of TA/BA by more than 15 %, implying that hydrothermal ageing dramatically increases the asphalt's fire risk. (3) Herein, the combustion mechanism of bimodal HRR peaks found for HA is analyzed, attributing to thin-layer boilover effect. This could be supported by plentiful pores observed on the combusted HA via scanning electron microscopy technique.