Embedded steel rebar corrosion in concrete is a time-dependent electromagnetic phenomenon and is one of the major causes of structural deterioration. It occurs due to the infiltration of carbon dioxide and chlorine through the protective concrete cover. Any loss of protective cover can lead to rebar corrosion and concrete deterioration, such as cracking, spalling and debonding. Consequently, structural capacity deteriorates and the service life could be compromised [1]. Many previous catastrophic collapses of concrete structures were due to a lack of knowledge on the extent of embedded rebar corrosion [2]. According to the National Association of Corrosion Engineers (NACE) report, approximately 1 to 5% of the USA Gross Domestic Product (GDP) is allocated to repairing and rehabilitating structures with corrosion damage [3]. Developing a reliable monitoring and condition assessment methodology for affected concrete structures is essential to make informed decision on the extent of rebar corrosion, any effect on structural safety and serviceability, and to effectively reduce the maintenance/rehabilitation budget.
Visual inspection is one of the conventional ways for condition assessment of structures. Rebar corrosion detection at an early stage through visual inspection is quite difficult and requires and may not be effective or accurate. In general, Non-Destructive Evaluation (NDE) is considered a more reliable method than visual inspection. Electrochemical methods such as Half-Cell Potential (HCP), Linear Polarization Resistance (LPR), and X-ray Computed Tomography (XCT) have been widely used for the condition assessment of corroded concrete structures. However, these methods have certain limitations. The HCP and LPR are semi-destructive methods requiring physical contact with embedded rebars, and they only detect instantaneous corrosion activity. The XCT is unsuitable for on-site measurements due to its large size and transportation challenges [1].
Apart from the methods mentioned above, Ground Penetrating Radar (GPR) is another NDE technique to determine reinforcement cover, concrete thickness, rebar location and spacing, localization of voids and cracks and concrete deterioration mapping [4, 5]. Past studies showed that GPR can detect rebar corrosion through differential amplitude reflection. Narayanan et al. [6], found that reflected GPR amplitude can indicate possible corrosion in steel rebars. Hubbard et al. [7] investigated the difference between GPR signals collected before and after accelerated rebar corrosion for 10 days. It was found that the GPR amplitude was affected, and the two-way travel time (TWTT) was influenced by the moisture introduced during the accelerated corrosion process. Zaki et al. [2] used GPR for corrosion detection in acceleratedly corroded rebars in a slab and deduced that the reflected wave amplitude was reduced due to corrosion. Lai et al. [8] also monitored the accelerated corrosion of concrete specimens with GPR and observed amplitude reduction as the corrosion level increased. This agreed with the finding from Hong et al. [9, 10]. In addition, Lai et al. [11] proposed a new corrosion evaluation approach using GPR, employing changes in lapsed travel time, amplitudes and peak frequencies. The maximum positive amplitude of the reflected wave was found to change at different phases of corrosion, and the reflected amplitude decreased with an increase in travel time. Zhan et al. [12] investigated the changes in GPR parameters during accelerated corrosion. A reduction in TWTT and an increase in the amplitude of the reflected wave with an increase in corrosion rate were found.
Hasan and Yazdani [4] corroded three steel rebars with accelerated corrosion, then placed them at different depths under known dielectric constant materials to simulate concrete embedment. The rebars were then scanned using a GPR, which showed an increase in TWTT and a decrease in amplitude, agreeing with the findings from other researchers [13–15, 5].
Actual rebar corrosion in real life is often a slow process, and waiting a long time to measure the corrosion extent is not practical. Hence, accelerated corrosion through impressed current techniques is a suitable solution to induce rebar corrosion in a relatively short time. In addition to saving time, it is cost-effective, and the corrosion rate can be controlled as needed [16]. Several previous accelerated corrosion studies were conducted using the impressed current technique [17–19]. Impressed current techniques are also known as galvanostatic, which uses a direct current (DC) from an external power source to corrode steel embedded in the concrete
Prior studies have shown the differences in GPR data collected before and after the rebar corrosion process. However, none of them investigated the effect of important parameters, such as rebar diameter and concrete cover, concrete strength, and corrosion duration on GPR data response. Understanding the effect of these parameters will allow more precise and targeted rebar corrosion determination than the general GPR-based methods from the past. The current study was conducted to bridge this significant knowledge-gap through an experimental investigation of these parameters under the accelerated corrosion environment. In addition, a reliable multivariate equation was developed to quantify the amount of embedded rebar corrosion in consideration of these parameters.