Abstract
The application of Fiber Reinforced Polymer (FRP) materials in concrete structures has been rising due to their several advantages, including lightweight, high tensile strength, ease of installation, and corrosion resistance. They have been mostly implemented for strengthening and repairing existing structures in the form of an externally bonded system, i.e., sheet, jacket, near surface mounted. Furthermore, they have been recently utilized as internal reinforcement of concrete elements in the form of strands, bars, tendons, etc. Although higher durability and performance are associated with the FRP material in some aspects compared to steel, concerns remain regarding damages and defects in this material, many of which are related to their unique features. Importantly, debonding of FRP materials from a concrete surface or within a concrete element has always been an issue resulting in the premature failure of the structure. To this end, concrete elements strengthened or reinforced with FRP materials has to be inspected periodically to detect potential issues and hence prevent any premature failures. This study first determines all possible or potential damages and anomalies attributed to FRP reinforced/strengthened concrete (FRP-RSC) elements. It then investigates Non-Destructive Testing (NDT) methods that can be applicable to the inspection of FRP-RSC elements from a literature survey of past studies, applications, and research projects. Furthermore, this study evaluates the ability of two of the most commonly used NDT methods, Ground Penetrating Radar (GPR) and Phased Array Ultrasonic (PAU), in detecting FRP bars/strands embedded in concrete elements. GPR and PAU tests were performed on two slab specimens reinforced with GFRP (Glass-FRP) bars, the most commonly used FRP bar, with variations in their depth, size and configuration, and a slab specimen with different types of available FRP reinforcements. The results of this study propose the most applicable methods for detecting FRP and their damage/defects in FRP-RSC elements. This study further investigates the feasibility of two new methods for improving the detectability of embedded FRP bars. By providing the inspection community with more clarity in the application of NDT to FRP, this study offers means for verifying the performance and, therefore, help the proliferation of FRP materials in concrete structures.
Keywords
Introduction
In recent decades, Fiber-reinforced polymers (FRP) composites have emerged as a promising solution in the field of civil engineering, thanks to their exceptional mechanical properties and chemical resistance. These composites are being widely adopted as an alternative to mitigate corrosion, retrofit and strengthen deteriorated steel-RC structures. FRP composites are well-known for their superior corrosion resistance, high tensile strength, low weight, anisotropy, and elasticity up to rupture [1, 2].
Despite these advantages, the performance of FRP reinforced/strengthened concrete (FRP-RSC) elements may be influenced by mechanical and environmental factors. Exposure to harsh climatic conditions, such as water, alkaline or acidic solutions, saline solutions, high temperatures, and UV radiation, may lead to stiffness and strength reduction in FRP composites. Consequently, the strength, use, and durability of FRP-RSC elements may be compromised [3]. Debonding, delamination, loss of cross-sectional properties, and aging due to UV exposure are common damages that can significantly weaken FRP-RSC members, ultimately leading to structure failure if left undetected. Therefore, similar to any other structural system, regular inspection of FRP-RSC elements is essential to prevent exacerbation of potential damages and determine necessary repair actions. Periodic inspection helps identify damages and defects at an early stage, ensuring timely repairs and preventing the potential for more extensive and costly damages [4, 5].
Although inspection methods and damage codification have long been established for conventional steel and reinforced concrete elements [6], there is a lack of standardized methodology or guidelines for inspecting and detecting damages in FRP-reinforced or strengthened concrete elements [7]. This has significantly restrained the widespread use of FRP-RSC components, as there are no clear guidelines or effective methods for assessing their condition. Without proper inspection and maintenance, the construction industry cannot confidently use engineering materials whose conditions cannot be effectively assessed. Therefore, there is an urgent need for a unified inspection and condition assessment methodology specifically tailored to FRP-RSC elements. Such methods and guidelines will increase the application of FRP-RSC elements and ensure their safety and longevity. Furthermore, NDT methods that are common for detection of damages and defects in steel reinforced concrete elements may not be as effective for FRP reinforced elements. Detection of embedded FRP bars/strands is more challenging and there is only limited work performed in this area which leaves a knowledge gap that needs to be addressed. This mostly stems from the fact that FRP and concrete have similar electromagnetic and acoustic properties which makes it difficult for the NDT devices to detect FRP that rely on contrast of these properties between the target and sound concrete.
The objective of this study is to identify the most suitable methods for inspecting and detecting damage in FRP-RSC elements. To achieve this, the study explores and categorizes potential damage and defects of these elements, and then identifies the most appropriate non-destructive testing (NDT) methods for their detection. Several concrete slab specimens with various FRP materials and sizes were cast and tested with conventional Non-Destructive Testing methods such as GPR and PAU. GPR and PAU (Ultrasonic Testing) are common methods for detection of damages and defects in steel reinforced concrete elements, both for damages in rebars and in concrete. This is due to the fact that these NDT methods can penetrate the concrete and detect damage within element. The results of this study will be useful in developing a comprehensive guide for inspectors to select the most appropriate NDT methods on-site, which can significantly reduce costs and save time. Additionally, this study helps the development of future innovations in non-destructive testing and damage detection techniques for FRP-RSC structures.
Damages and defects in FRP-RSC elements
It is important to recognize the common/potential defects for the application of FRP-RSC because the selection of appropriate NDT tools depends on the understanding of the damage to be detected. The potential damages/defects in FRP-RSC elements based on their application and location of occurrence are discussed in the following.
FRP strengthened concrete elements (external application) can be broadly divided into three distinct materials: FRP, adhesive, and reinforced concrete, including three interfaces: FRP-adhesive interface, adhesive-concrete interface, and concrete-reinforcement interface, as shown in Fig. 1. Damages and defects can occur anywhere within these three materials and/or interfaces which can be broadly categorized as material defects and interface defects, respectively. In this paper, for the purpose of classification, the defects occurring in FRP are termed “Defects in FRP Composites”, the defects at interfaces and adhesive layer present anywhere between concrete substrate and FRP strengthening system are termed “Bond Defects”, and the defects in reinforced concrete substrate are termed “Defects in Concrete”.
Defects in externally applied FRP system.
The FRP reinforced concrete (internal application) includes FRP reinforcement, concrete, and concrete-FRP reinforcement interface as shown in Fig. 2. The defects associated with internal application of FRP can be located anywhere within the two materials and/or at the concrete-FRP interface. In this paper, the defects that occur in FRP reinforcing bars and strands are termed “Defects in FRP Composites”, defects at the concrete-FRP reinforcement interface are termed “Defects in the Interface”, and the defects in concrete are termed “Defects in Concrete”.
Defects in internal application of FRP.
All the identified defects associated with FRP-RSC elements can be summarized into a comprehensive classification diagram, as shown in Fig. 3 [7]. In addition to classifying the damages and defects based on where they occur, they can further be divided based on when the defects are initiated, such as manufacturing/installation/casting defects and in-service defects.
Damages and defects in FRP-RSC elements.
Analysis of the information about various NDT methods and their capabilities in relation with detection of damages in general and for FRP-RSC elements in specific can lead to identification of promising or potential methods for damage detection in FRP-RSC elements. In the following sections, the promising NDT methods for external application of FRP will be determined based on the statistical overview of the prior relevant investigations whereas the potential methods for internal application will be devised based on the experiences of the authors of this study.
It should be noted that to allow effective evaluation of NDT methods, the damages/defects in FRP-RSC elements are categorized into defect groups that have similar features as it concerns the application of NDT methods. As shown in Table 1 the NDT methods for FRP inspection are grouped for FRP and concrete in the internal and external applications.
Damage categories for statistical summary of applicable NDT for FRP inspection
Damage categories for statistical summary of applicable NDT for FRP inspection
The findings of a comprehensive literature survey of more than100 past studies [7, 8] on the application of NDT methods in detecting damages in external application of FRP (for damages in FRP) along with its statistical summary is shown in Fig. 4 [9]. In recent studies focusing on the non-destructive testing of FRP-strengthened concrete elements, there is a growing preference for techniques like GPR, PAU and Infrared Thermography (IR). Recent studies by Zatar et al. investigated the application of GPR and IR for evaluating RC slabs externally bonded with GFRP [10] and CFRP [11]. The studies concluded that the combined use of GPR and IR can accurately detect possible internal defects such as cracks in concrete substrate, delamination, debonding, and voids in concrete elements strengthened with FRP.
Statistical summary of NDT applicable for A. bond defects and B. defects within FRP composite layer. (AE = Acoustic Emission, TT = Tap Testing, IE = Impact Echo, MW = Microwave, GPR = Ground Penetrating Radar, UT = Ultrasonic Testing, LT = Laser Testing, IR = Infrared Thermography Testing, RT = Radiographic Testing).
Similarly, the statistical summary of the NDT methods applicable for detecting defects in concrete have been provided from a past study of the author [12] which are shown in Fig. 5.
Statistical summary of NDT applicable for A. cracks in concrete, B. voids in concrete, C. delamination in concrete and D. corrosion in steel reinforcement (TT = Tap Testing, IE = Impact Echo, GPR = Ground Penetrating Radar, UT = Ultrasonic Testing, IR = Infrared Thermography Testing, IRT = Impulse Response Testing, MFL = Magnetic Flux Leakage, RT = Radiographic Testing).
Due to the fact that literature on the application of NDT methods for internal application of FRP is limited and scarce, the potential NDT methods for internal application will be devised based on the limited available literature and experience of the authors. This represents a research gap that this research study attempts to address as much as possible. Hence this study can be used as an initial step for propelling the future studies on non-destructive testing for internal application of FRP.
In a study conducted at the University of British Columbia (UBC) by Ékes [13] it was demonstrated for the first time that GPR can detect both CFRP and GFRP bars embedded in concrete. However, this study does not give any information about the damage detectability of the GPR testing, let alone the level of its accuracy. Hence the authors of this study further propose to determine the complete capability of using GPR as a tool for damage detection in FRP-RSC elements.
Further, Acoustic Emission (AE) testing has been used in the past for detecting debonding between the embedded FRP rebars and surrounding concrete [14, 15]. Nevertheless, AE will not be considered in this study as an applicable NDT method for damage detection in FRP reinforced concrete element as this method cannot detect damage/defects that are already present in the elements and can only be implemented for detecting initiation of damages.
Based on the review of available literature and the experience of the authors, the following non-destructive devices are identified to be evaluated as potential methods for the detection of damages (defects in the interface and defects in FRP reinforcing bars and strands) in internal application of FRP: Ultrasonic Testing (Phased Array Ultrasonic), Ground Penetrating Radar (GPR), Infrared Thermography (IR), and Impact Echo (IE). Similarly, it should be noted that for the defects in concrete for the internal application of FRP, the findings shown in Fig. 5 A., B., and C. will still be applicable.
Order of priority for applicability of NDT methods
Based on the findings of the above sections, a flowchart showing the priority order of NDT methods suitable for each type of defect is prepared as shown in Fig. 6. This flowchart shows which NDT method is the most promising/applicable for the respective types of damage.
Order of priority for NDT methods suitable for each type of defect.
As mentioned earlier, the research on application of NDT methods for damage detection in FRP Reinforced Concrete is scarce. The ability to assess the condition of the relatively new and unique FRP reinforcements will increase the confidence of the construction industry in their use as a reliable substitute for steel reinforcements. To address the need, this research further investigates the ability of two of the most commonly used NDT methods, Ground Penetrating Radar (GPR) and Phased Array Ultrasonic (PAU), in detecting FRP bars/strands embedded in concrete elements. Accordingly, several slab specimens reinforced with GFRP (Glass-FRP) bars, the most commonly used FRP bar, with variations in their depth, size and configuration were fabricated and tested with PAU and GPR.
Experimental work
Slab specimens
The slab specimens were fabricated, targeting different parameters such as FRP bars/strands type (Glass FRP, Carbon FRP (CFRP), Basalt FRP (BFRP), bar diameter, bar direction and bar depths. Table 2 shows the identification of the slab specimens by group, highlighting their main parameters. Because GFRP bars are the most commonly used FRP reinforcement in concrete elements, the first two slabs constructed (Slabs C and J) were only reinforced with GFRP bars of different sizes, at different depths and different configuration (bars in one direction and bars in two orthogonal directions, i.e., mesh). The concrete cover specified by ACI CODE-440.11-22 [16] for the concrete members reinforced with GFPR bars ranges from 0.02 m (0.75 in.) to 0.08 m (3 in.), hence the depth variations in the slab specimens were included to represent the layers of FRP reinforcement, which could be anywhere within the concrete cover range specified. Further, the third slab (Slab L) has different types of internal reinforcement (bars and strands) embedded into it, including one steel bar whose detectability acts as a control for this research. Having GFRP, CFRP, BFRP and steel bars/strands on the same slab specimen allows comparison of the detectability of different FRP bar/stands with the steel bar under the same test conditions. More details can be found on [17].
Identification of small-scale concrete slab specimens
Identification of small-scale concrete slab specimens
All specimens were fabricated and labeled following the same layout as shown in Fig. 7. Every slab is identified with a letter (C, J and L), and every side has an identification number (from 1 to 4). The direction of the measurement will be determined by the number where the measurement is started to the end of the measurement.
Labeling of slab specimens.
Table 3 shows the details and dimensions of the specimens. For each specimen, the distance to edge, depth to surface, bar diameter, bar material and slab depth are presented based on the convention shown in Fig. 7. The formwork for each slab specimen constructed is shown in Fig. 8.
Reinforcement/Dimension details of detectability slab specimens
C-Std. (i.e., CFRP strands) and G-Std (i.e., GFRP strands) are labelled as Bar 5 and Bar 7 in Slab L.
FRP bars in different slabs: (a,b) Slab C, (c) Slab J, (d) Slab L.
Four different GPR systems with different center frequency ranges were used in this experiment to determine the effect of GPR frequency on the bar detectability, as shown in Table 4 and Fig. 9. The data acquired from grid scans were used to give the cross-section image of the test specimen through the plane parallel to the surface of the specimen along its depth. The cross-sectional image along the depth of the specimen is termed as a depth slice or time slice image.
Two different PAU systems were used in this experiment to determine FRP bar detection capability, as shown in Table 5 and Fig. 10. The MIRA 3D device had 64 ultrasonic transducers located in a 16×4 grid at 3 cm spacing (extended 16 rows of 4 transducers each) where the Pundit Live Array Pro had 24 ultrasonic transducers located in an 8×3 grid at 3 cm spacing (8 rows of 3 transducers each).
PAU systems used: (a) A1040 MIRA 3D (two devices were attached side by side to increase the number of channels to 16 rows of 4 transducers each) [27], (b) Pundit live array pro.
The depth slices results obtained for each slab specimen using each GPR device are illustrated in Fig. 11, where the bar detectability improves as the device central frequency increases.
Using depth slices results for Slab J with a top and bottom mesh of GFRP bars, all the GPR devices could clearly detect the top mesh of #6 GFRP bars at a shallower depth, while the bottom mesh was only visible for the higher frequency GPRs, as shown in Fig. 11d–f. In addition to the higher center frequency of the 2 GHz GPR device, its dual polarization feature further permits detection on both first and second levels of bars, whereas for the GPR device with maximum center frequency of 4 GHz and 6 GHz, the ability to detect the bottom mesh is solely due to the higher resolution, which it can afford because of its higher frequency.
Depth slices: (a) Slab C using Conquest 100 Enhanced, (b) Slab C using C-Thrue, (c) Slab C using Proceq GP8800, (d) Slab J using Conquest 100 Enhanced, (e) Slab J using C-Thrue, (f) Slab J using Proceq GP8800, (g) Slab L using Conquest 100 Enhanced, (h) Slab L using C-Thrue, (i) Slab L using Proceq GP8000.
The results of line scans and the area scans obtained from the PAU testing on each slab specimen using MIRA 3D and Pundit Live Array Pro devices are shown in Fig. 12. It can be seen that PAU could not detect bars in Slab C and Slab J. However, it was able to detect carbon strands, steel bars and GFRP strands in Slab L. As ultrasonic testing and PAU are very sensitive in detecting the presence of (air) voids, their ability for detecting CFRP and GFRP strands could be attributed to the presence of (air) voids within the twisted FRP cables of these strands.
The results indicate that NDT methods that rely on electromagnetic waves such as Ground Penetrating Radar (GPR) become less effective, if not obsolete for non-metallic/non-conductive embedded bars. However, with the increase in center frequency of the GPR device, the detectability of FRP bars can be improved. Similarly, other NDT methods such as Ultrasonic Testing (UT) or Phased Array Ultrasonic (PAU) that are based on stress waves have some capability for detecting steel reinforcement, but they perform poorly for the detection of the most commonly used FRP embedded bars (GFRP bars) but are good for detecting FRP strands.
PAU test results: (a) Slab C line scan using Pundit, (b) Slab C area scan using MIRA 3D, (c) Slab J line scan using Pundit, (d) Slab J area scan using MIRA 3D, (e) Slab L line scan and stripe scan using Pundit, (f) Slab L area scan using MIRA 3D.
To improve the detectability of FRP bars embedded in concrete, this study further investigated the feasibility of two innovative methods: 1) coating FRP bars with a suspension of metal particles in resin (Fig. 13) and 2) winding metal wire or strips over the FRP bars (Fig. 14). Metal wire can be wound on any type of FRP bar during or after manufacturing as they further increase the bonding of the FRP bars to concrete. However, in the case of metal particle coating, to provide both the bond strength and detectability, the particles can be mixed with sand or any other bond enhancing coating during the manufacturing process. It should also be noted that the corrosion of the metallic particles or wire would not be an issue as the introduced metallic presence would be covered with resin. Additionally, non-corrosive metals or other conductive particles may be considered.
Modification of FRP bar with metal particles coating; (a) FRP bar, (b) FRP bar with iron particle coating, (c) FRP bar with aluminum particle coating.
Modification of FRP bar with metal wire winding: a) FRP bar, b) FRP bar with iron wire winding, c) FRP bar with galvanized steel wire winding, d) FRP bar with copper wire winding, e) FRP bar with aluminum wire winding.
The concept and principle behind the proposed new methods is to provide metallic presence in FRP bars. Several customary NDT methods such as GPR, UT, Magnetic Flux Leakage (MFL) and others can be used to detect the proposed modified FRP bars embedded in concrete. The preliminary tests results obtained using GPR and Phase Array Ultrasonic (PAU) applications for detecting FRP bars with metallic coating and metallic wire are presented here to demonstrate the feasibility of the proposed methods. More tests are planned for the future.
In the investigation, two types of particles (iron and aluminum) for coating and a metallic wire for winding on the surface of the bar were employed in small concrete specimen as shown in Fig. 15. In this figure, the results from GPR and PAU show that the iron coated GFRP bar has the best detectability among others (red = best detectability, blue = least detectability). The results indicate that the addition of iron particles to the GFRP coating is most effective in improving the detectability of FRP bars. More importantly, the metallic particle coating makes PAU capable of detecting GFRP bars embedded in concrete which were previously undetectable by PAU.
A small specimen tested for the preliminary feasibility study for the new methods: a) Small slab formwork with aluminum coated, commercially available, iron wire wound, and iron particle coated GFRP bars (from left to right), b) Casted slab specimen, c) GPR test result d) PAU test result.
This study investigated non-destructive testing methods applicable for damage detection in FRP reinforced/strengthened concrete (FRP-RSC) elements. Damages and defects associated with the use of FRP in concrete elements were first investigated. Then, the most promising NDT methods for their detection in FRP-RSC elements were presented through a thorough literature survey of past works and studies. Additionally, the feasibility of using commercially available NDT methods such as GPR and PAU in detecting FRP bars embedded in concrete were evaluated. Several FRP reinforced concrete elements with different FRP materials and sizes were fabricated and tested with NDT devices with different frequencies. Finally, this study investigated the feasibility of using two new methods to improve the detectability of embedded FRP bars.
The following conclusions can be made from this study: Capability of non-destructive testing (NDT) techniques that utilize electromagnetic waves, like Ground Penetrating Radar (GPR) for detecting embedded bars made of non-metallic or non-conductive materials may not be as effective as detecting conventional carbon steel rebars. However, by increasing the center frequency of the GPR device, it is possible to significantly improve the detectability of embedded bars made of fiber-reinforced polymer (FRP). Other NDT methods such as Ultrasonic Testing (UT) or Phased Array Ultrasonic (PAU) that rely on stress waves can detect steel reinforcement to some extent, but they are not as effective in detecting the most commonly used FRP embedded bars (GFRP bars). However, they are still useful for detecting CFRP and GFRP strands. In FRP strengthened concrete, Infrared Thermography (IR), Ground Penetrating Radar (GPR), and Ultrasonic Testing (UT) can be considered the most applicable method for detecting bond defects. For detection of damages within FRP composites, UT, IR, and Tap Testing (TT) can be deemed the most trustworthy NDT methods. For all FRP surface anomalies, Visual Testing (VT) is proposed. GPR, Impact Echo (IE), Impulse Response Testing (IRT), and UT are the most reliable NDT methods for detecting damages in concrete. Also, GPR, UT, and IE are the most suitable NDT methods for detecting corrosion in FRP reinforced concrete elements. Metallic coating, specifically iron coating, on FRP bar noticeably improved their detectability by both GPR and PAU methods.
Footnotes
Acknowledgments
The authors greatly acknowledge the support by the Department of Civil and Environmental Engineering at Florida International University, the Department of Civil and Architectural Engineering at the University of Miami, especially Ana De Diego Castro, and Screening Eagle Technologies, Switzerland. The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented herein.