## 1 Introduction  
Durability and serviceability are important requirements for concrete structures over the whole of their service life. In reality, subsurface defects appear inside concrete structures, and are not noticed until they expose significant damage and failure. In developed countries, the cost of repair work ensuring acceptable operation conditions is even higher than the expense used to construct a new one [1; 2]. Hence, the prediction and detection of defects is essential, and should be carefully studied, in order to determine the structural conditions, and propose suitable repair methods for potentially damaged structure.  
Defects, such as crack, delamination, spalling, and air void, are primarily caused by the corrosion of embedded steel bars, successive freeze-thaw cycles, thermal effect, shrinkage, traffic load,or overloading, such as earthquake or heavy vehicles [3; 4]. Delaminations formed mainly from invisible hidden cracks in structures are usually at the region above the upper reinforcement layer in the protective concrete layer, or even at the place between reinforcement layers, as shown in Figure 1[5; 6].  
To detect and characterize delaminations in the concrete structures, various nondestructive techniques (NDTs) have been developed and applied. These NDTs include simple methods, such as chain dragging and hammer sounding, and other advanced methods, like ground penetrating radar (GPR), ultrasonic, and impact-echo [7]. All of them are available and suitable for the identification of delamination depths deeper than 5 to 10 cm [8].  
It needs to be noted that the types of structure, loading conditions, environmental conditions, and diameter of steel bars have a strong effect on the design of the thickness of concrete cover [9; 10; 11]. According to AASHTO LRFD (2014) [9], the minimum required thickness of the concrete cover of bridge structures is about 50 mm in most cases, and approximately 75 mm in the case of coastal and cast-against-earth structures. As required in ACI 318-05 (2014) [10] for building structures, the concrete cover thickness is at least 20 mm for structure not exposed to the weather or in contact with the ground, and 76 mm for concrete permanently exposed to the earth.  
Currently, among many NDTs, the active infrared thermography technique has been proven to be an effective tool to determine the location, and also the depth of delamination in concrete structures [6; 11; 12; 13]. This technique gives reliable results in the evaluation of the near-surface region with the depth of defects less than 10 cm [8; 14]. In several researches, when the ratio between the size and the depth of delamination (so-called width-to-depth ratio or WTDR) is smaller than 2.0, the delamination was considered undetectable [15; 16], while other authors stated that delamination with a WTDR smaller than 2.0 could be detected using advanced image processing techniques, as discussed in Section 2[3; 6; 11; 17; 18; 19]. In this study, the Long Pulsed Thermography (LPT) technique, one of the most common methods of active infrared thermography, is used to clarify the effects of depth, heating time, and steel bars on the detectability capacity of delamination inside a concrete specimen.  
In the present work, a series of experimental tests were carried out on a concrete specimen containing artificial delaminations of different sizes and depths, which create different WTDRs. Reinforcements are located above several delaminations having depth ranging from 5 to 8 cm, as mentioned in Section 4.1. The signal-to-noise ratio (SNR) criterion is applied to assess the detectability of delamination using active infrared thermography. A delamination can be considered as \"detectable\" when the SNR has a value higher than zero [20; 21; 22]. In addition, the absolute contrast of each delamination is monitored to determine the observation time, propose the nondimensional prefactor \(k\), and predict the depth of delamination.  
Figure 1: Location of delamination in concrete structures.