Heating process monitoring and evaluation of hot in-place recycling of asphalt pavement using infrared thermal imaging

The original asphalt pavement heating plays a key role in the entire construction procedure of hot in-place recycling (HIR). At present, the temperature of heating process in HIR is monitored by infrared temperature gun with single or multiple test points which cannot fully understand real heating situation. Moreover, there are no evaluation indices for quantifying heating quality. To objectively perceive and evaluate the heating quality, the infrared thermal imaging (ITT) system was used to monitor heating process of HIR and the combination of digital image processing (DIP) and artificial neural network (ANN) was applied to extract and correct the distorted thermal image to restore pavement surface temperature. Based on corrected thermal images, the heating quality of HIR was firstly quantitatively evaluated using three proposed indices: transverse uniformity T-u for evaluating the temperature uniformity, represent temperature RT for assessing the heating effect and proportion of temperature above 200 degrees C PTA200 for judging the overheating. Furthermore, on-site heating quality of three heating systems: respectively flame, infrared radiation and recycled hot-wind system, was analyzed and compared based on the above three indices. The results indicate that the superiority of the flame system and the infrared heating system is strong heating effect to realize the higher pavement surface temperature, but the temperature uniformity is poor, even some areas are overheated. On the contrary, the recycled hot-wind heating system provides the better temperature uniformity and avoids overheating, but the heating effect is poor. In addition, a comprehensive analysis reveals that high RT and poor T-u are mainly cause of overheating and the better T-u is required to match the increasing pavement surface temperature for high level of the heating quality. Finally, when using 200 degrees C as the high temperature boundary, the evaluation indices T-u and RT preferably satisfy the following relationship RT + RT x T-u < 190 degrees C to avoid occurrence of overheating in HIR.