Determination of thermal irradiance on infrared thermographic detection of grouting defects in prestressed ducts

Infrared thermography (IRT) faces challenges in detecting grouting defects of prestressed ducts due to the shielding effect of the thicker concrete cover. For bridge service inspections, the thermal irradiance applied when using a heat excitation source to warm the target structure constitutes a critical factor. This study combines experimental and finite element simulation methods, selecting five test parameters closely related to the diurnal variation of solar irradiance—namely, 500 W/m², 600 W/m², 700 W/m², 800 W/m², and 1000 W/m², to conduct an in-depth comparative analysis of the performance of IRT, and finally systematically explores suitable thermal irradiance for detecting defects in prestressed duct grouting by IRT.The results indicate that when the thermal irradiance reaches 500 W/m², and with a heating duration of 120 minutes, the IRT can detect fully hollow defects with a depth of up to 110mm. When the thermal irradiance increased from 500 W/m² to 1000 W/m², both plastic and metal pipe-embedded concrete specimens exhibited an increasing trend in maximum absolute contrast (MaxΔT) at their defect area surfaces, and plastic ducts exhibit 1.4 times higher absolute contrast (ΔT) sensitivity to radiation increases than metal ducts. Thereby, this work will facilitate accurate and efficient defect identification in real-world engineering scenarios.