## 4 Tip  
TIP is a relatively new NDT method for PIT over the past decade. The technique is based on the fundamental relationship between thermal properties and structural integrity. An intact concrete pile structure is characterized by internal homogeneity, stable thermal conductivity, and uniform heat transfer. In the presence of defects such as cracks, necking, broken sections, segregation, honeycomb, among others, the thermal conductivity within the defective areas decreases substantially (with air-filled voids exhibiting a thermal conductivity of approximately 0.026 W/(m\(\cdot\)K)), leading to impeded heat transfer and localized temperature anomalies. TIP is applicable to both cast-in-place and precast piles, differing primarily in their heat sources. For cast-in-place piles, the thermal energy originates from cement hydration exotherm, while precast piles require controlled thermal excitation via either embedded heating elements or external heat sources. Early heat generation and dissipation are contin  
Figure 9: Received signals from intact piles and piles with different defects [41]. gent upon the concrete mix design, soil conditions, and pile geometry. Temperature sensors positioned at various depths within the pile are employed to track temperature variations at different locations within the concrete. By comparing the temperature measurements with a theoretical model, one can pinpoint the location of defects and evaluate their severity [89]. When this method is applied to circular concrete piles, a localized reduction in temperature suggests an insufficiency of concrete and can be interpreted as a reduction in pile diameter. Conversely, a localized increase in temperature indicates an expansion in concrete volume, implying a bulge in the pile [90]. In addition, TIP can be utilized to evaluate the concentricity of the reinforcement cage within the pile, as eccentricity can result in temperature discrepancies between the two sides of the cage. However, the precision and accuracy of TIP have been a matter of concern in laboratories and engineering. Defect detection by direct analysis of temperature profiles provides indicative or suggestive evidence. Validating the assessment's accuracy is challenging due to the lack of actual pile testing. Moreover, various defects can exhibit similar anomalous temperature profile characteristics, making it often difficult to distinguish and often relying on limited experience. [PERSON] et al. [91] developed a numerical model to simulate cement hydration in concrete piles and heat transfer at the pile-soil interface, enabling identification of both defect locations and sizes. The new assessment method could detect defects and locations with sizes smaller than 10% of the cross-sectional area, enhancing the precision in pile integrity assessment. However, the method is insensitive to defects in the center of the cross-section. The numerical modeling is based on the assumption of a uniform shaft, which does not correspond to reality. It is difficult to consider the complexity of the pile shape in 2D modeling. Therefore, more TIP data are needed to improve the evaluation method. In conventional TIP testing, an embedded hot wire is typically employed, comprising a series of discrete temperature sensors spaced at 300-mm intervals [7]. This configuration leads to potential data blind spots, which are influenced by the defect's size and location, along with the sensor's resolution and detection range. Consequently, this diminishes the reliability of both the data and the analyzed results. To address this challenge, enhancing sensor density represents an effective approach. Unlike conventional discrete-point sensors, distributed optical fiber sensing systems employ continuous sensing media installed along the entire pile length, enabling comprehensive data acquisition with complete spatial coverage and high resolution [92].