Concrete is a highly consumed construction material used in the built environment and civil infrastructure. Therefore, condition monitoring using non-destructive techniques such as infrared thermography (IRT) should be optimised to extend the safe and continuous operation of existing structural asset and to reduce the requirement for new construction, which is more carbon intensive. The objective of this paper is to present an in-depth understanding of the heat flow mechanism in concrete slabs with hidden subsurface defects using IRT experiments. The experiments were performed on six concrete slabs with dimensions of 250 × 250 × 100 mm. Five of these slabs had 5 to 25 mm concrete cover over the simulated subsurface defect, and the sixth slab was plain concrete with no defect, used as control sample. Two sets of experiments using step heating thermography (SHT)technique were conducted using an IR heater as the excitation mechanism. By thermography of slabs, several sequences of IR images were recorded during both heating and cooling phases. Subsequently, the sequences of IR images were post-processed using a routine developed in MATLAB to achieve the evolution of temperature on surface of the samples. The temperature records from the tests during both heating and cooling phases were used to calculate thermal contrast, as well as first and second time derivatives of thermal contrast. By close examinationof the sequences of IR images as well as sign analysis of the thermal contrast and its time derivatives, the mechanism of heat transition in defective concretes was characterised and interpreted in three stages during both heating and cooling phases. The three stages of heating phase are named as simultaneous heat-up, lateral heat flow and maximum thermal contrast. The formation of the lateral heat flow that corresponds to the transition from stage one to two causes the thermal gradient on the surface observable by the thermal camera. Likewise, three stages of cooling phase are simultaneous cool down, lateral heat diffusion from void cover to sound concrete and inverse thermal contrast. The conclusions of this study increase the basic understanding about mechanism of heat transfer in defective concrete and put forward new directions toward optimised implementation of IRT for the detection of hidden defects in concrete bridges.