Advanced Structural Technologies Implementation in Designing and Constructing RC Elements with C-FRP Bars, Protected Through SHM Assessment

Abstract

The need to strengthen the existing reinforced concrete (RC) elements is becoming increasingly crucial for modern cities as they strive to develop resilient and sustainable structures and infrastructures. In recent years, various solutions have been proposed to limit the undesirable effects of corrosion in RC elements. While C-FRP has shown promise in corrosion-prone environments, its use in structural applications is limited by cost, bonding, and anchorage challenges with concrete. To address these, the present research investigates the structural performance of RC beams reinforced with C-FRP bars under static loading using Structural Health Monitoring (SHM) with an Electro-Mechanical Impedance (EMI) system employing Lead Zirconate Titanate (PZT) piezoelectric transducers which are applied to detect damage development and enhance the protection of RC elements and overall, RC structures. This study underscores the potential of C-FRP bars for durable tensile reinforcement in RC structures, particularly in hybrid designs that leverage steel for compression strength. The study focuses on critical factors such as stiffness, maximum load capacity, deflection at each loading stage, and the development of crack widths, all analyzed through voltage responses recorded by the PZT sensors. Particular emphasis is placed on the bond conditions and anchorage lengths of the tensile C-FRP bars, exploring how local confinement conditions along the anchorage length influence the overall behavior of the beams.

1. Introduction

The necessity of strengthening existing reinforced concrete (RC) elements is increasingly becoming an imperative for modern cities in the context of developing resilient and sustainable structures and infrastructures [1]. The aging of materials, inadequate initial design, contemporary design regulations, and the ever-increasing demand for the construction of high-demanding projects have led the scientific community in recent years to seek alternative ways of reinforcing and strengthening structural elements. This need becomes more acute when the stresses of strong earthquakes and the subsequent damage that can occur are added to the structural capacity of a structure.

In addition, the influence of environmental factors on a structure, such as carbonation, humidity, exposure to chlorides (from seawater), freeze–thaw cycles, and the use of salt to melt ice in countries with very low temperatures, is equally important. These are factors that accelerate the corrosion of steel reinforcement, with the direct consequence of reducing the life span of structures [2,3]. In the USA, the estimated cost of maintenance and rehabilitation of damages that occurred due to corrosion of steel reinforcement in existing highway bridges as a result of the exposure to seawater or freeze–thaw shells is estimated at USD 150 billion with annual costs of 8.3 billion dollars. In comparison, in Canada, the total cost for all structures is 74 billion dollars; in Europe, the annual cost is three billion dollars, respectively [4,5].

In recent years, various solutions have been proposed to limit the undesirable effects of the phenomenon of corrosion in RC constructions, such as using galvanized bars or bars with epoxy coating instead of conventional steel reinforcements. Batis et al. [6], Kassellouris et al. [7], and Sideris and Savva [8] proposed and studied the use of corrosion inhibitors as an additive during the concrete production process. This solution affects the strength of concrete.

RC structures have traditionally relied on steel bars for reinforcement. However, in recent decades, innovative materials composed of glass, carbon, aramid, or basalt fibers embedded in a polymer matrix have emerged as viable alternatives to traditional steel reinforcement. These materials, known as fiber-reinforced polymers (FRPs), have already seen widespread use in enhancing the performance of deficient RC structural members [9]. FRPs (sheets, strips, laminates, bars, and ropes) are commonly employed as additional external reinforcement to improve the flexural or shear capacity of these structures, utilized in the repair and strengthening of various RC components, including beam-column joints [10], columns [11], frames [12], beams [13,14,15], deep beams [16], corroded beams [17], and slabs [18,19,20].

FRP bars are highly recommended in scenarios where steel-reinforced concrete structures are at risk of corrosion, which can lead to serious safety hazards and substantial financial costs, particularly in harsh or aggressive environments [17]. FRP bars provide several key advantages over traditional steel reinforcement, such as exceptional resistance to corrosion, high tensile strength aligned with the fiber direction, and being lightweight and non-magnetic. These attributes make FRP bars an increasingly attractive choice for improving the longevity and performance of RC structures in demanding conditions, where conventional steel might fail or require costly maintenance [21]. Due to these advantageous properties, using FRP bars as internal reinforcement in RC structural members subjected to monotonic or seismic loading has gained significant attention in recent years [22].

Researchers have explored the potential of FRP bars in various structural applications. However, despite their benefits, FRPs exhibit certain limitations, particularly their brittle behavior and linear-elastic stress–strain response [23,24]. Unlike steel reinforcing bars, FRP bars do not yield, which means they lack the ductility that is essential for energy dissipation in structures subjected to seismic loads [25]. Additionally, FRP bars possess a low modulus of elasticity and low shear strength, which pose challenges from a structural engineering perspective [26]. As a result, concrete beams reinforced with FRP bars display linear-elastic behavior up to failure, with no yielding. The final failure of these beams is characterized as brittle, whether it occurs due to FRP rupture or concrete crushing, though the latter is generally more desirable for RC flexural elements [27].

Moreover, in recent years, considerable interest has been found in the application of FRP bars as longitudinal reinforcement for the construction of beams, slabs, or decks in reinforced concrete bridges, especially in cases where the effects of corrosion of the reinforcement are of utmost importance for durability and construction safety [3,28,29]. FRP rods are composite materials consisting of synthetic or organic fibers of high tensile strength impregnated in a matrix in combination with some chemical additive. The fibers determine the strength and stiffness of the composite materials along the longitudinal axis, while the matrix is the binding material of the fibers. Chemical additives (catalysts, fillers, polymerization accelerators or retarders, etc.) are added to the matrix to improve its physical and mechanical properties, resistance to environmental conditions, and reduce the cost of producing FRP rods [30,31].

These are anisotropic, non-homogeneous, and brittle materials. The anisotropy of the rods results from the fact that the shear strength and their properties along the transverse axis depend on the physical and mechanical properties of the resin and fibers while along the longitudinal axis on the corresponding characteristics of the fibers. The anisotropic nature of the material leads to different physical and mechanical properties along the two axes, which should be considered when designing and determining failure mechanisms. Their properties depend on many parameters, the most basic of which are the type and volumetric percentage shot composite material of the fibers and the matrix, the orientation of the fibers, the production process, and the quality control of all stages during their formation. Each combination of the above parameters gives FRP rods with different properties along the two axes [32,33].

The main characteristics of the fibers are their extremely high tensile strength, high specific stiffness, and entirely linear elastic behavior until they break. Specific stiffness is defined as the ratio of their modulus of elasticity to their density. Their diameter ranges from 5 µm to 25 µm, and their length-to-diameter ratio is enormous. They are placed straight and uninterruptedly along the FRP rods’ longitudinal axis, determining the composite reinforcement’s strength and stiffness in this direction [34].

It is commercially available in a wide variety and is characterized by high tensile strength along the longitudinal axis of the fibers, fully linear elastic behavior until failure, and high resistance to environmental conditions. The most common types of fibers still used in the field of applications are aramid and glass carbon fibers, while recently, the use of basalt fibers has also been investigated [35].

Considerable interest is found in the experimental and analytical investigation of the application of FRP bars as the main longitudinal reinforcement in RC structural elements with or without transverse shear reinforcement and, in particular, FRP bars with glass fibers (G-FRP) due to their low cost and relatively high tensile strength presented by the specific type of bars [36,37,38].

Extensive experimental research has been conducted on concrete beams reinforced with G-FRP bars, revealing that these beams exhibit higher deflections and larger crack widths than those reinforced with traditional steel bars. The latter is owed primarily to the low elastic modulus of G-FRP bars, as outlined in the ACI 440.1R-15 guidelines [30]. Consequently, beams reinforced with G-FRP bars also demonstrate lower post-cracking flexural stiffness than those with steel reinforcement. Despite significant analytical and experimental efforts to understand the behavior of G-FRP-reinforced beams under various loading conditions, challenges remain.

However, in concrete elements with FRP bars, the problem of shear stress is particularly important as, compared to steel reinforcements, they present a relatively lower modulus of elasticity and lower strength along the axial axis, resulting in the opening of large cracks in width and height and their reduced contribution as a bark action, respectively. Further, combined with the wide variety of bars with different mechanical characteristics and properties available in the composite materials field, it is necessary to investigate further the behavior of reinforced concrete structural elements with other types of bars [39].

In contrast, Carbon Fiber-Reinforced Polymer (C-FRP) bars, although possessing a lower modulus of elasticity than steel bars, have a modulus three to four times higher than G-FRP bars. C-FRP bars also offer substantially higher strength than G-FRP or Aramide Fiber-Reinforced Polymer (A-FRP) bars [40,41]. However, the behavior of concrete beams reinforced with C-FRP bars under loading remains an open question; a relatively limited number of works on RC beams with C-FRP bars can be found in the international literature [42,43,44,45,46,47].

The current research seeks to implement advanced and innovative structural technologies to design and construct RC elements reinforced with C-FRP bars while applying an emerging Structural Health Monitoring (SHM) scheme to protect them. Thus, this work experimentally investigates the structural integrity of slender concrete beams C-FRP bars reinforced and subjected to incremental static loading by applying an Electro-Mechanical Impedance (EMI) monitoring system, using Lead Zirconate Titanate Piezoelectric transducers (PZT).

1.1. C-FRP Bars Applications and Limitations

C-FRP bars are gaining ground in construction and civil engineering thanks to their remarkable properties, such as high tensile strength, low weight, and strong corrosion resistance. Unlike traditional steel reinforcement, which is susceptible to corrosion and degradation over time, C-FRP bars offer a durable alternative that can significantly extend the lifespan of concrete structures, particularly in harsh corrosive environments.

However, despite their benefits, the widespread adoption of C-FRP bars faces several challenges. One primary concern is the high material cost of C-FRP compared to steel, making it a financially restrictive choice for typical structural applications with limited budgets. However, the ongoing advancements in material science and the necessity for more resilient and sustainable structures are leading to the exploration and development of more affordable and high-performance alternatives. Further, hybrid reinforcement strategies, combining traditional steel reinforcement with C-FRP bars, can also be implemented [48,49]. Moreover, the reduced required cross-sections of C-FRP bars compared to the steel ones and the enhanced durability, which refers to reduced maintenance and repair costs in structures’ service life, supports the reduction in overall costs while still providing some of the benefits of advanced materials.

Furthermore, the bonding and anchorage with concrete constitute another concern about using C-FRP bars. Inadequate optimization of these factors might have a detrimental impact on the overall efficacy of the reinforcement. Recently, Nam et al. investigated the bond strength as dependent on the higher modulus of elasticity and the diameter of the bar [50]. Further, C-FRP bars with helical and sand-coated surfaces can achieve improved bonding conditions [51]. In addition, Yoo et al. proposed a lap-splice, epoxy-filled tube, coupler, and expanded ribs as mechanical solutions to improve the anchorage performance [52], and Karayannis et al. proposed a spiral mechanical one [25].

In addition to their use in new constructions, C-FRP bars are often applied to strengthen and repair existing structures where weight reduction and corrosion prevention are vital for longevity and safety [45]. However, integrating C-FRP bars into pre-existing constructions might be intricate and require expertise and equipment. Thus, to mitigate the abovementioned rising difficulties in the pragmatic implementation, the structural integrity of the pre-existing material must be assessed to ensure compatibility, and the bonding process must be executed with high precision to avoid issues like poor adhesion, which could undermine the effectiveness of the reinforcement.

Moreover, C-FRP bars could be an ideal and efficient solution in external retrofitting schemes such as RC jackets, in harsh, corroded environments. In these applications, careful surface preparation, precise placement, and specialized equipment should be used to bond the bars to the existing concrete or structure. Further, specific guidelines arising from international standards should be followed and applied to such applications [30].

Summarizing, C-FRP bars offer significant benefits for tensile reinforcement and corrosion resistance. For compressive performance, materials like steel remain more reliable due to their ductility and robustness under compression. However, C-FRP bars can complement steel in hybrid reinforcements where each material’s strengths are maximized in a balanced structural design [53,54].

1.2. Challenges of the Applied SHM Scheme on the Proposed Experimental Study

Traditional SHM methods tend to be costly and may be insufficient, particularly for hard-to-reach structural elements, underscoring the need for cutting-edge monitoring technologies. Among these advanced techniques, Electro-Mechanical Impedance (EMI) has emerged as a promising localized SHM approach, demonstrating substantial potential for detecting damage in RC components early. With its cost-effective nature, EMI has proven effective in identifying issues such as concrete cracking, reinforcement yielding, and quantifying applied forces. Recent studies highlight the integration of PZT patches as part of EMI systems to monitor structural integrity effectively [55]. These piezoelectric materials offer several benefits, including low energy consumption, ease of installation, portability, and the ability to function as both sensors and actuators. By leveraging the electromechanical coupling properties of PZTs, EMI detects irregularities in the structural integrity of RC structures, with changes in impedance signals providing early warning signs of damage.

Recent research has further shown that embedding a network of PZT patches in high-risk areas of an element significantly improves the accuracy and effectiveness of SHM methods, enhancing the assessment of damage levels. Using a network of PZT sensors rather than relying on a single sensor at a specific location can increase the reliability of the monitoring system. Multiple sensors can cross-check data to ensure accurate results even if one sensor fails or degrades over time [56].

This research employed an array network of externally bonded PZTs to investigate the sensor placement optimization. This task is crucial for real-life applications, with dual significance as, on the one hand, optimized monitoring efficiency leads to reduced installation costs (PZTs patches, wires, computational resources, etc.), and further to enhanced durability and reliability of PZT sensors by protecting them with coatings (epoxy resin, cement-based non-corrosive mortars) to shield from environmental agents, improve their robustness and ensure longer operational lifetimes.

Notably, advancements in PZT-based SHM data analysis have been made through clustering techniques, which assess multiple measurements from the PZT network to diagnose damage in RC elements strengthened with fiber-reinforced polymer (FRP) materials. Studies have also demonstrated the application of this technology in detecting debonding failures in FRP-strengthened beams, proving the technique’s viability for monitoring such issues. Moreover, artificial intelligence is increasingly attractive to researchers. In particular, machine learning techniques and neural networks have been widely investigated in the last decade in SHM applications. However, adopting and establishing the abovementioned techniques requires a vast pool of acquired contextually relevant datasets under different conditions to develop reliable diagnostic models. Given the scarcity of SHM-acquired data specific to this context, the application of Root Mean Square Deviation (RMSD) as a damage index in this study fills an essential gap, providing a standardized approach that is validated by initial experimental outcomes. To the authors’ knowledge, no similar experimental investigation or SHM-acquired data exists in the extant literature [57].

By integrating advanced monitoring technologies, the study seeks to provide critical insights into the long-term performance and durability of C-FRP reinforced structures. Through real-time monitoring with PZT transducers, any degradation of the concrete, slippage, or loss of cohesion of the bars can be detected promptly, allowing for targeted maintenance actions and preventing further structural damage. This approach enhances the safety and reliability of real-life structures, offering targeted indications of potential damage assisting the role of traditional inspection techniques and human assessments conducted at periodic intervals to verify and supplement the acquired data. In addition, it provides valuable information for stakeholders to make decisions and minimizes the risk of catastrophic failure.

1.3. Significance and Objectives of the Proposed Experimental Scheme

The current research seeks to implement advanced and innovative structural technologies to design and construct RC elements reinforced with C-FRP bars while applying an emerging Structural Health Monitoring (SHM) scheme to protect them. Thus, this work experimentally investigates the structural integrity of slender concrete beams reinforced with C-FRP bars under increasing static loading by applying an Electro-Mechanical Impedance (EMI) monitoring system, using PZTs.

The investigation of how C-FRP bars and PZT sensors function together in structural monitoring schemes is crucial. Moreover, the study aims to optimize sensor placement, bonding conditions, and reinforcement configurations to maximize the accuracy of the SHM system. Special attention is given to the bond conditions and anchorage lengths of the tensile C-FRP bars, with an examination of how local confinement conditions along the anchorage length affect the overall response of the beams. Key aspects such as stiffness, maximum loading capacity, deflection at each loading step, and crack width development in terms of Voltage responses acquired by the PZTs’ implementation are presented and analyzed.

Further, the research aims to improve structural safety by enabling early damage detection. PZT sensors, known for their high sensitivity to minor structural changes, can capture early-stage damage that might be missed by traditional methods. This enhanced detection capability is crucial for maintaining the structural integrity and longevity of critical infrastructure.

In this research, the monitoring scheme is constituted by a low-cost, portable, custom-made impedance analyzer device with advanced computational and connection properties as a data acquisition system, and concurrently is equipped with a signal unit that can amplify and filter the generated signals. In addition, the advanced system can perform measurements, and collect the data wirelessly and in real-time, demanding low energy consumption. This advancement aids in developing predictive maintenance models, helping infrastructure stakeholders make data-driven decisions for repairs or replacements. Moreover, the low energy demands, permit the integration of a battery system or renewable power source as a power supply.

Furthermore, light and small PZTs were applied that can streamline monitoring processes and reduce inspection frequency, potentially lowering maintenance costs over the structure’s lifecycle. Further, the proposed method offers a non-invasive way to monitor existing structures without extensive modification. By attaching or embedding PZT sensors, the system can be adapted to older structures, extending their operational life without disrupting functionality. By addressing practical considerations such as cost, durability, and environmental effects, the study aims to provide actionable insights that can lead to the large-scale application of PZT-enabled SHM systems in the construction industry. Thus, the proposed monitoring scheme establishes the EMI method as a feasible real-time and continuous SHM method and permits a cost-effective analysis for real-life applications, considering the extracted advantages in the service-life horizon.

In addition, the use of C-FRP materials in construction is linked to lower maintenance demands, corrosion resistance, and an extended lifespan compared to traditional steel reinforcements. This contributes to sustainability goals by reducing resource usage and waste associated with frequent repairs.

2. Experimental Work

In this research work, the structural integrity of slender concrete beams of ratio a/d > 2.5 reinforced with C-FRP bars and transverse steel shear reinforcement is experimentally investigated. The beams were tested under increasing static loading. Furthermore, the efficiency of the Structural Health Monitoring (SHM) system in evaluating proposed alternative anchorage techniques of the longitudinal bars through continuous spiral steel reinforcement of circular and rectangular shapes is investigated, as well as their influence on the response of the examined specimens.

2.1. Specimen’s Characteristics

Two beams were cast and subjected to experimental testing under four-point loading conditions. The RC beams reinforced with C-FRP bars were identified as “CFRP10-C” and “CFRP10-R”. The first notates the circular spiral steel reinforcement anchorage, while the other notates the rectangular one.

Figure 1 illustrates the geometry and the cross-sectional dimensions. In addition, the reinforcement details of all the specimens are also depicted. Each beam has a total length of 2700 mm, with a width-to-height ratio of b/h = 200/250 mm, an effective depth of d = 200 mm, and a shear span of a = 1000 mm, resulting in a shear span-to-depth ratio of a/d = 5, characteristic of typical slender beams. All beams share identical dimensions, a high compression steel reinforcement ratio to prevent premature failure of the concrete’s compression zone, and the same transverse reinforcement ratio for comparative purposes. The compression reinforcement comprises standard deformed steel bars with a diameter of 14 mm (4Ø14 top), corresponding to a top longitudinal compression steel reinforcement ratio of 1.54%.

Additionally, the transverse shear reinforcement was kept consistent across all beams, consisting of mild steel closed stirrups with a diameter of 6 mm, spaced at 200 mm intervals (Ø6/200 mm), as depicted in Figure 1. The geometrical ratio of the bottom longitudinal tension reinforcement for each beam is provided in

Table 1. Geometrical ratio of the bottom longitudinal tension reinforcement for each beam.

Beam Name	Type and Diameter	Tensional Bars			Experimental Results
		ρf (%)	ρf ffu (MPa)	ωf	Pexp (kN)	Vexp (kN)	δpeak (mm)
CFRP10-C	2HD10	0.39	7.07	0.24	84.4	42.2	79.9
CFRP10-R	2HD10	0.39	7.07	0.24	76.1	38.1	60.3

.

Further, the beams also feature an enhanced anchorage configuration for the tensile C-FRP bars. Each C-FRP bar includes a continuous configuration of mild steel spiral transverse reinforcement with a circular or rectangular shape positioned around and at both anchorage ends. This anchorage system is designed to enhance the bond properties between the C-FRP bar and the concrete in these areas by creating local confinement. Implementing special anchorage systems and innovative methods to prevent the premature debonding of FRPs is crucial [58]. The spirals are applied over a length of 500 mm at both beams. The geometric and reinforcement details of the spiral used in beams are also illustrated in Figure 1.

2.2. Materials Properties

Various raw materials are used to produce carbon fibers, the most basic of which are polyacrylonitrile fibers, cellulosic fibers (such as viscose or cotton), and petroleum fibers. They are mainly prepared through the heat treatment of polyacrylonitrile fibers, resulting in high-quality fibers with excellent characteristics.

The main characteristic of these fibers is their low density and low specific weight (about 1/4 of steel) combined with their high tensile strength and high specific stiffness. In particular, their tensile strength ranges from 2.1 to 6 GPa, their modulus of elasticity from 215 to 700, and their reduced fracture strain from 0.2 to 2.3%. In addition, they show stability in terms of their chemical composition in strongly corrosive and acidic environments and high resistance to fatigue and creep phenomena. Thanks to the strong orientation of the crystallites along the longitudinal axis, they present a very low coefficient of linear thermal expansion.

The C-FRP bars used in this study, identified as HD10, were manufactured through a pultrusion process involving carbon fibers, following specific guidelines provided by the manufacturer. The process begins with continuous carbon fibers, characterized by high tensile strength and a fiber volume fraction of 60%. These carbon fibers boast a nominal tensile strength exceeding 4 GPa and an elastic modulus of over 230 GPa. After preparing the carbon fibers, they are saturated with resin to produce the synthetic C-FRP bars. These bars feature a rough external surface achieved through a special surface treatment with quartz sand, enhancing their bond properties. The final C-FRP reinforcing bars used in this study have a nominal ultimate tensile strength of 1.8 GPa and an elastic modulus of 130 GPa.

Standard 150/300 mm cylinders were tested for axial compression and splitting tension on the day the beams were tested to assess the concrete’s properties. The average compressive strength of the concrete used for all beams was determined to be 29.1 MPa, while the splitting tensile strength was 2.42 MPa. The concrete mix included a maximum aggregate size of 16 mm. Additionally, the yield tensile strength of the deformed steel bars was experimentally determined: 555 MPa for the Ø10 tension bars (diameter of 10 mm) in beam S10, 545 MPa for the Ø12 (diameter of 12 mm) tension bars in beam S12, and 550 MPa for the Ø14 (diameter of 14 mm) compression bars. The yield tensile strength of the Ø6 (diameter of 6 mm) mild steel stirrups and spirals used in the beams was found to be 310 MPa.

2.3. Test Setup and Instrumentation

A four-point bending test setup was employed to apply monotonic loading to the RC beams, as shown in Figure 2. The beam specimens were simply supported on roller supports, with a span of 2.25 m between the supports, within a rigid laboratory frame.

A hydraulic actuator, controlled by a servo controller, applied a gradually incremental load. The load was subjected to the beams’ upper surface via two evenly spaced steel rollers which were positioned 250 mm apart at the midspan of the specimens, creating a four-point loading configuration. This consistent load application allowed for accurate observation of the beams’ behavior under stress. Two steel roller supports on each side supported the beam on its edges. A load cell, boasting high precision with an accuracy of 0.05 kN, measured the applied force with utmost accuracy. Each shear span of the beams was 1 m long, resulting in a typical slender beam configuration with a shear span-to-depth ratio of five.

The deflections of the beams were meticulously measured using six Linear Variable Differential Transducers (LVDTs). Four of these LVDTs had an accuracy of 0.01 mm, while the remaining two had an accuracy of 0.005 mm. To enhance measurement accuracy, two LVDTs were strategically placed at the midspan of the beams—one on the front side and one on the backside. Additional LVDTs were positioned at the midpoint of the left shear span, at the midpoint of the right shear span, and at the supports (refer to Figure 2 for details). This arrangement allowed for precise estimation of the net deflections at the midspan and at the midpoints of both the left and right spans. Load and corresponding deflection data were continuously recorded throughout the tests until the beams ultimately failed.

2.4. Electro-Mechanical Impedance (EMI) Technique

The EMI technique leverages the piezoelectric properties of PZT transducers [59]. These transducers exploit the piezoelectric effect, which generates an electrical charge in response to mechanical stress, and conversely, produces mechanical vibrations when subjected to an electric field. By utilizing this effect, the EMI method activates PZT transducers that are either bonded to or embedded within a host structure, inducing vibrations.

Changes in the mechanical impedance of the structure (or its inverse, admittance) are reflected in variations in the electrical signal measured from the PZT transducers, which can be observed through changes in voltage or frequency response. The interaction between the PZT transducers and the host structure is captured as an admittance signature, which includes both a real component (conductance) and an imaginary component (susceptance). This admittance signature reveals structural characteristics and can be described by the complex admittance of the PZT patch, as outlined in Equation (1).

$$\overline{\mathrm{Y}}={\displaystyle \frac{\overline{\mathrm{I}}}{\overline{\mathrm{V}}}}=\mathrm{G}+\mathrm{B}\mathrm{j}=4\mathsf{\omega }\mathrm{j}{\displaystyle \frac{{\mathrm{L}}^2}{\mathrm{h}}}\left[\overline{{\mathsf{\epsilon }}_{33}^{\mathrm{T}}}-{\displaystyle \frac{2{\mathrm{d}}_{31}^2\overline{{\mathrm{Y}}^{\mathrm{E}}}}{\left(1-\mathsf{\nu }\right)}}+{\displaystyle \frac{2{\mathrm{d}}_{31}^2\overline{{\mathrm{Y}}^{\mathrm{E}}}}{\left(1-\mathsf{\nu }\right)}}\left({\displaystyle \frac{{\mathrm{Z}}_{\mathrm{a},\mathrm{e}\mathrm{f}\mathrm{f}}}{{\mathrm{Z}}_{\mathrm{s},\mathrm{e}\mathrm{f}\mathrm{f}}+{\mathrm{Z}}_{\mathrm{a},\mathrm{e}\mathrm{f}\mathrm{f}}}}\right)\left({\displaystyle \frac{\tan \mathrm{k}\mathrm{L}}{\mathrm{k}\mathrm{L}}}\right)\right]$$

(1)

where $\overline{\mathrm{V}}$ represents the Harmonic alternating voltage supplied to the circuit. Further, $\overline{\mathrm{I}}$ represents the Current passing through the PZT, where G represents the Conductance (real part of the admittance). In addition, B represents the Susceptance (imaginary part of the admittance), and j the Imaginary unit, respectively. Moreover, v is the Angular frequency, L is Half-length, and h the Thickness of the PZT patch, respectively. Further, d₃₁ represents the Piezoelectric strain coefficient of the PZT, Z_a,eff is the Short-circuited effective mechanical impedance, and Z_s,eff is the Effective structural impedance. In addition, Poisson’s ratio is symbolized with n, the Wave number related to the angular frequency with k. Furthermore, the $\overline{{\mathrm{Y}}^{\mathrm{E}}}$ is the Complex Young’s modulus of elasticity under a constant electric field, where the ${\mathsf{\epsilon }}_{33}^{\mathrm{T}}$ represents the Complex electric permittivity of the PZT patch along the axis at constant stress, respectively.

Any changes in the RC beam, such as variations in its mass and stiffness properties, will inevitably lead to modifications in the structural parameters, which, in turn, affect the effective structural impedance. These changes will be reflected in alterations to the admittance, as defined by Equation (1), thereby indicating the beam’s structural integrity [57].

In this experimental study, the approach for detecting changes using PZT transducers excited with a sinusoidal harmonic voltage of 2.5 V across a frequency range from 10 to 250 kHz, with 1 kHz incremental step. Serving as both actuators and sensors, the PZT transducers generated the voltage signals and captured the EMI responses, by involving low-pass filters to remove high-frequency noise [59]. These signatures were then processed using a custom-designed wireless device known as WiAMS [15,51]. The WiAMS device, depicted in Figure 2, is equipped with advanced processing capabilities that facilitate rapid and extensive calculations and can be operated remotely.

Initially, EMI measurements were performed on the beam in its pristine, undamaged state to establish a baseline response indicative of its healthy condition. Following this, the measurements were repeated under various simulated damage scenarios to mimic potential conditions that could affect the structural integrity of the beam. The results were then meticulously compared to identify deviations, utilizing statistical metrics such as the Root Mean Square Deviation (RMSD). This comparative analysis was crucial for detecting and assessing structural changes, offering significant insights into the health and integrity of the element under multiple loading conditions. It is important to note that all voltage measurements were carried out under controlled laboratory conditions to minimize the effects of temperature and humidity fluctuations on the data.

2.5. PZT Patches’ Installation

Each beam was equipped with eight small and thin PZT transducers, each one measuring 10 × 10 × 2 mm. These PZT patches, known as PIC151 and manufactured by PI Ceramics, were strategically placed on the beam in various configurations, as shown in Figure 1.

Four PZTs were externally epoxy-bonded to critical locations on the concrete facade surface on the CFRP10-C beam, explicitly targeting areas susceptible to flexural and shear diagonal cracks. These PZTs were labeled as “A–D”. Four more PZTs were placed at the middle of the bottom surface of the beam, directly below the surficial PZTs’ positions, and labeled as “1–4”.

On the other hand, for beam CFRP10-R, eight PZTs were externally epoxy-bonded on the bottom surface, directly below the C-FRP bars, and at the same positions as the ones of CFRP10-C, apart from the load points (see Figure 1).

2.6. Data Analysis

Statistical analysis plays a crucial role in transforming variations in EMI signatures into meaningful index metrics. One effective method for quantifying these variations is the Root Mean Square Deviation (RMSD). By comparing the acquired voltage responses of the beam in its pristine condition to those in subsequent conditions, the RMSD provides a clear metric of the deviations between these states.

The RMSD is a statistical measure representing the square root of the squared differences between corresponding points in the pristine and subsequent condition signatures. This metric is sensitive to any changes in the EMI responses, thus making it a valuable tool for identifying and quantifying stress-related changes.

The RMSD index is expressed mathematically as shown in Equation (2).

$$\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{D}=\sqrt{{\displaystyle \frac{{\sum }_{\mathrm{r}=1}^{\mathrm{M}}{\left({\left|{\mathrm{V}}_{\mathrm{p}}\left({\mathrm{f}}_{\mathrm{r}}\right)\right|}_{\mathrm{D}}-{\left|{\mathrm{V}}_{\mathrm{p}}\left({\mathrm{f}}_{\mathrm{r}}\right)\right|}_0\right)}^2}{{\sum }_{\mathrm{r}=1}^{\mathrm{M}}{\left({\left|{\mathrm{V}}_{\mathrm{p}}\left({\mathrm{f}}_{\mathrm{r}}\right)\right|}_0\right)}^2}}},$$

(2)

where ${\left|{\mathrm{V}}_{\mathrm{p}}\left({\mathrm{f}}_{\mathrm{r}}\right)\right|}_0$ represents the initial voltage response at the pristine state where ${\left|{\mathrm{V}}_{\mathrm{p}}\left({\mathrm{f}}_{\mathrm{r}}\right)\right|}_{\mathrm{D}}$ is the voltage responses in any subsequent state.

This index provides a single numerical value that summarizes the overall deviation between the EMI signatures, effectively transforming the raw voltage response data into an index metric. This approach allows for easy comparison and monitoring of the condition of the structural member over time.

3. Results and Discussion

3.1. Mechanical Responses

Figure 3 illustrates the bending moment curves in terms of deflection at the middle of the span of the tested beams with C-FRP bars. Each curve presents the experimental mechanical response of the beam as it is acquired from the load-cell values and the corresponding deflections of the LVDTs till the final failure. As it is shown, the beam with the circular anchorage configuration exhibits both higher load capacity and greater deflection, indicating improved performance. Similar findings by Bank [60] and Teng et al. [61] support that circular anchorage configurations often provide enhanced load distribution and prevent premature debonding, leading to greater load-bearing capacity.

Circular anchorage may improve the bond between the C-FRP bars and concrete, allowing the load to be more evenly distributed, delaying failure initiation, and enabling higher deflection levels under the same loading conditions. This effect aligns with Karayannis et al. [25], and Alkhateeb and Hejazi [62], who demonstrated that such anchorage configurations help control crack width and improve flexural response.

Moreover, some key points that should be mentioned from the experimental responses of the beams could be gleaned from the fact that further analyses of the mechanical responses of the beams and the curves initially show a small pre-cracking segment with high flexural stiffness. The primary segment, however, represents the behavior after cracking, which demonstrates significantly reduced stiffness compared to the pre-cracked portion. Thus, the experimental curves reveal distinct pre-cracking and post-cracking phases in the beams’ responses. Initially, a small pre-cracking segment demonstrates high flexural stiffness, attributed to the intact concrete’s cross-section fully resisting the applied load. The uncracked concrete and C-FRP bars together create a high initial stiffness, as the entire cross-section is effective in load-bearing, a characteristic observed in similar studies on FRP-reinforced beams [37,45].

In the post-cracking phase, stiffness significantly decreases, as only the concrete in the compression zone effectively resists further loading. Meanwhile, the C-FRP reinforcement primarily carries the tensile load; however, due to its relatively low modulus of elasticity, it offers limited stiffness compared to traditional steel. This aligns with the findings by Zheng et al. [63], who observed similar reductions in stiffness in post-cracking phases of C-FRP-reinforced beams, highlighting that stiffness transitions depend mainly on the reinforcement’s properties after concrete cracking occurs.

Figure 3 shows that each time a crack forms, there is a simultaneous drop in load, reflecting a localized stiffness decrease at the cracked sections. This response is common in beams with C-FRP reinforcement, as documented by Vijayan et al. [64], who attributed such load drops to the brittle nature of carbon fibers and the resulting sudden loss of load capacity when cracks initiate in the concrete matrix. The cracking results in stress concentration around the crack, which reduces local stiffness and manifests as a load drop in the curve. The behavior is exacerbated by the C-FRP’s low modulus, which does not bridge the cracks as effectively as steel reinforcement might. The work of Zheng et al. [65], similarly demonstrated this load-drop phenomenon in C-FRP-reinforced beams, noting that it becomes more pronounced with each new flexural crack formation due to the limited redistribution capacity of C-FRP compared to steel.

Further, as observed in the load-deflection curves (Figure 4), there is a simultaneous and noticeable drop in load each time a crack is formed in the C-FRP reinforced beams. This drop can be attributed to the localized decrease in stiffness at the cracked section of the beam. The formation of long flexural cracks, combined with the low elastic modulus of the C-FRP bars, contributes to this phenomenon.

Before cracking, the stiffness of the beam is relatively high because the entire cross-sectional area of the concrete is effective in resisting loads. Once cracking occurs, however, only the portion of the cross-section under compression remains effective, while the contribution of the C-FRP tensile bars is minimal due to their low modulus of elasticity. Consequently, for the same level of beam deflection, a higher load is required to achieve that deflection in an uncracked cross-section compared to the load needed to produce the same deflection after cracking has occurred. Further and detailed analysis regarding the mechanical responses of the beams has been conducted by Karayannis et al. [25].

The findings of this study align with and expand upon the work by Konsta et al. [66], who conducted detailed analyses of C-FRP-reinforced beams. They observed similar bending and deflection behaviors, with notable stiffness reductions post-cracking and distinct load drops associated with crack formation. These results collectively underline the critical role of FRP material properties—such as modulus of elasticity and anchorage design—in influencing the bending performance and post-cracking behavior of reinforced beams.

3.2. EMI Responses and Qualitative Evaluation

The process of acquiring EMI responses involved the use of four WiAMS devices. Measurements were taken at various loading levels (stations) for each beam, with each station chosen based on its significance to the beam’s structural integrity. This significance was determined through visual inspections, crack meter readings, and the rate of load increment. Similar approaches in selecting monitoring points have been noted in Shah’s study [67], who emphasized the importance of targeting areas of structural vulnerability for accurate integrity assessment.

For the “CFRP10-C” beam, a total of twelve individual EMI measurements were performed across different loading and damage states. The number of measurements was adjusted to capture the nuanced changes in structural response, as suggested by Bhalla and Soh [68], who showed that more frequent EMI measurements could yield more accurate insights into progressive damage, especially under variable loading conditions. In contrast, for the “CFRP10-R” beam, nine individual EMI measurements were collected. These measurements aimed to capture and analyze the beam’s response under various conditions to assess changes in structural integrity and performance.

Both beams were fitted with eight PZT transducers, though in different configurations. This variation in arrangement aimed to assess how PZT placement influences the sensitivity and accuracy of EMI measurements in detecting stiffness changes due to cracking and constitutes a critical aspect of the investigation. Studies by Naoum et al. [69] and Parpe and Saravanan [70] support the critical role of PZT placement in optimizing sensitivity, showing that strategic placement near anticipated crack zones significantly enhances the accuracy of EMI-based damage detection.

The varied configuration of PZTs allowed each device to capture distinct local stiffness changes, especially near cracks. The importance of configuration is significant as the transducer’s arrangement directly affects signal strength and fidelity, which are critical for accurate EMI response interpretation [69,71].

In Figure 4 and Figure 5 the curves of Voltage responses for all the PZTs of beam CFRP10-C are illustrated. As shown in the close-ups at the resonant frequencies, a considerable variation in the peak of the curves can be observed. As stiffness decreases with increased cracking, resonant frequencies shift in EMI signals reliably indicating local stiffness reductions [50,56,72]. These changes are essential indicators and refer to the changes in the stiffness of the beam’s mass, which occurred due to the crack formations near each PZT position.

Figure 6 and Figure 7 show the voltage response curves for beam CFRP10-R, where similar peak variations at resonant frequencies were observed. These shifts also correspond to stiffness changes, likely due to crack formations near the PZT transducers, consistent with the patterns observed in CFRP10-C. Comparing the results between the two beams indicates that despite different loading and transducer configurations, both beams exhibited stiffness degradation as cracking progressed. These findings parallel the results of Karayannis et al. [73], who demonstrated that resonant frequency shifts are consistent across different structural configurations when stiffness changes due to cracking.

The consistent variation in peak responses across both beams reaffirms the sensitivity of EMI measurements in detecting stiffness changes, regardless of configuration differences. Several studies in the literature confirm that resonant frequency-based methods are robust indicators of structural degradation across various reinforcement layouts and loading conditions [74,75].

At this point, it is worth mentioning that an important observation from the close-ups of the Voltage responses is that the alterations of the curves seem to fit with the alterations in the beam’s integrity with the formation of new cracks or further widening of existing ones.

An essential observation is the correlation between the alterations in the EMI voltage curves and the integrity of the beams, particularly in relation to the formation and widening of cracks. This correlation suggests that the voltage response curves reflect progressive changes in beam stiffness as damage accumulates. This behavior aligns with the findings that EMI response alterations closely tracked damage progression in concrete beams, especially as cracks formed and propagated.

Moreover, the voltage curve alterations provide real-time insights into the structural degradation process, supporting the potential of EMI monitoring as an early-warning system for crack propagation and structural integrity loss. Further, the EMI responses extract that voltage and frequency shifts are direct indicators of crack-related stiffness changes, allowing for continuous monitoring of structural health.

The results of this study are consistent with findings in the literature, where EMI-based approaches effectively capture structural stiffness changes due to cracking in concrete beams reinforced with carbon-fiber materials. In addition, the study highlights that EMI monitoring with strategically placed PZTs and frequent measurement intervals can serve as a valuable tool for real-time damage detection in RC structures. Additionally, it supports the idea that resonant frequency analysis is a reliable indicator of structural health, even in varying reinforcement and load conditions. This has practical implications for developing monitoring systems in infrastructures prone to crack formation, as timely detection can prevent further structural compromise.

3.3. Evaluation of the Proposed SHM Technique for CFRP10-C

The schematic presentation of the codified names and positions for all the PZT transducers of beam CFRP10-C is displayed in Figure 1.

The cracking pattern at each loading stage is illustrated in Figure 8. Moreover, the RMSD volume ratios for all the epoxy-bonded PZT patches to the facade and bottom surface, respectively, are also depicted in Figure 9, Figure 10, Figure 11 and Figure 12.

At Dam1, the first flexural crack appeared in the mid-span, directly below the right loading point, while the second flexural crack formed at the third state in parallel to the first (counting from the loading point) stirrup of the left span. The third crack developed at the fourth damage state parallel with the second stirrup of the right span. Thereafter, the fourth flexural crack formed at the fifth state, below the left loading point, while till the sixth state, the fifth crack became apparent parallel with the first stirrup of the right span. The development of the first, second, third, and fifth flexural cracks was recorded until the end of the experimental process by implementing crack-meter transducers. Finally, the beam led to fatal failure due to the shear-flexural cracking development of the second crack.

In Figure 9, the RMSD values of the pair PZT A and PZT 1 are illustrated. The Voltage responses of this pair are primarily affected by the formation of the third crack at first and the fifth crack subsequently, as they are placed in between these two cracks. As extracted from both figures, the RMSD values of the PZTs follow an ascending pattern during the states of the formation of the two cracks. Especially, PZT A seems to identify the potential formation of the third crack at an earlier state. After that, RMSD values for both PZTs are affected by the widening of the fifth crack. The arrows in both figures indicate significant increments of the RMSD values.

As cracks widen, they exacerbate the localized stiffness loss in the beam. The widened cracks cause greater disruption to the mechanical impedance of the beam, which is captured by the PZT transducers. As the beam’s stiffness continues to degrade, the voltage responses from the PZTs will increasingly diverge from their baseline values, resulting in higher RMSD values. The formation of multiple cracks, especially in close proximity, has a compounding effect on stiffness loss. The fifth crack, being wider, contributes more significantly to the overall reduction in beam stiffness. The wider the crack, the more significant the effect on the beam’s resonant frequency, as the beam behaves more like a fractured element. This is why both PZTs show an increase in RMSD values, with the widening of the fifth crack playing a major role in the noticeable increment.

The ascending pattern of RMSD values across both PZTs during the formation of cracks reflects the progressive deterioration in the structural integrity of the beam. The continuous increase in RMSD values indicates that as the cracks form and propagate, the stiffness loss in the affected areas continues to grow, influencing the overall mechanical impedance of the beam.

Figure 10 depicts the RMSD values of the pair PZT B and PZT 2. The Voltage responses of this pair are primarily affected by the formation of the first crack at first and the fifth crack subsequently, as they are placed in between these two cracking formations. As shown in both figures, the RMSD values of the PZTs follow an ascending pattern at the states of the formation of the two cracks. In particular, both PZTs seem to identify the potential formation of the fifth crack at an earlier state. RMSD values for both PZTs are affected by the widening of the fifth crack. The widening of the fifth crack further contributes to the increase in RMSD values for both sensors, which suggests that the sensors are detecting the progressive weakening of the beam’s structure as the crack propagates. This is consistent with the fact that as cracks widen, the stiffness of the concrete decreases, which is easier to detect with piezoelectric sensors due to their sensitivity to mechanical changes.

In the final stages, the RMSD values for PZT B increase due to the widening of the first crack. This shows that as cracks propagate, different cracks influence the beam’s overall stiffness at various points in the structure. The widening of the first crack introduces further stiffness degradation, and this is detected by the sensors as a continuing rise in RMSD values. The arrows in both figures indicate significant increments of the RMSD values.

In Figure 11, the RMSD values of the pair PZT C and PZT 3 are presented. The Voltage responses of this pair are primarily affected by the formation of the second crack at first and the fourth crack afterward, as they are positioned between these two cracks. As depicted in both graphs, the RMSD values of the PZTs follow an ascending pattern at the states of the formation of the two cracks. This is consistent with the previous observations: as cracks propagate, they cause a more significant reduction in the beam’s overall stiffness, which is detected by the piezoelectric sensors. The widening cracks lead to an increase in the deviation from the baseline response, reflected in the RMSD values. PZT 3 seems to identify the potential formation of the fourth crack at an earlier state. RMSD values for both PZTs are affected by the widening of the second crack.

In Figure 12, the RMSD values of the pair PZT D and PZT 4 are illustrated. The Voltage responses of this pair are primarily affected by the formation of the second crack, as they are placed in the “neighborhood” of this cracking formation. As shown, the RMSD values of the PZT 4 follow an ascending pattern at the states of the formation and the widening of the second crack, while PZT D seems to exhibit lower sensitivity in detecting the widening alterations. However, at the final states (Dam 10, Dam 11, Dam 12), the RMSD values are significantly high, indicating the successful detection of the formation of a new offshoot of the second crack at the state Dam 10. Despite its initial lower sensitivity, PZT D picks up on the newly developed damage, which demonstrates the sensor’s ability to capture not only the primary crack’s development but also the formation of additional crack branches, further confirming the efficacy of piezoelectric sensors in crack monitoring.

In conclusion, the RMSD values presented in Figure 9, Figure 10, Figure 11 and Figure 12 reveal the sensitivity of PZTs in detecting the formation and progression of cracks in concrete beams. The results demonstrate that PZTs are effective in capturing early-stage cracking and the subsequent widening of cracks, with the RMSD values rising as the cracks propagate. The positioning of the sensors relative to the cracks plays a significant role in the sensors’ monitoring performance.

3.4. Evaluation of the Proposed SHM Technique for CFRP10-R

The schematic presentation of the codified names and positions for all the PZT transducers of beam CFRP10-R is displayed in Figure 1.

The cracking pattern at each loading stage is illustrated in Figure 13. The RMSD volume ratios for all the epoxy-bonded PZT patches to the bottom surface are also depicted in Figure 14, Figure 15, Figure 16 and Figure 17.

At Dam 1 state, no obvious crack occurred. Until the next measuring state, the first, and second flexural cracks have appeared in the mid-span, directly below the loading points, while in the third state, the third flexural crack has been formed parallel to the first (counting from the loading point) stirrup of the right span. The development of these three flexural cracks was recorded until the end of the experimental process by implementing crack-meter transducers. At the Dam 4 state, the fourth flexural crack has also been formed parallel to the left span’s first (counting from the loading point) stirrup. In the next state, the fifth, and sixth cracks occurred. Both had a slight diagonal formation, indicating a potential shear-to-shear-flexural performance. The fifth crack was formed on the second stirrup of the left span, while the sixth crack was formed on the third one of the right span. In the next state (Dam 6), the seventh crack developed during the third stirrup of the left span. The seventh crack would later be the reason for the beam’s fatal failure due to a shear critical fracture. Between the states Dam 8 and the final state Dam 9, the fibers of the C-FRP bars have been fractured.

In Figure 14, the RMSD values of pairs PZT A and PZT 1 are illustrated. The Voltage responses of this pair are primarily affected by the formation of the third crack at first and the sixth crack subsequently, as they are placed in between these two cracks. As extracted from both figures, the RMSD values of the PZTs follow an ascending pattern during the formation of the two cracks. PZT A seems to identify the potential formation of the third crack at an earlier state. Thereafter, RMSD values for both PZTs are affected by the widening of the cracks. Once the cracks start widening, the RMSD values for both sensors begin to increase, as shown in the graphs. The widening of cracks represents an increase in the local stiffness degradation, which is more easily detectable by the sensors. Widening cracks lead to more pronounced mechanical shifts in the beam, which results in larger changes in the voltage response, as detected by the piezoelectric sensors.

Figure 15 depicts the RMSD values of the pair PZT B and PZT 2. The Voltage responses of this pair are initially affected by the formation of the first crack first and the third crack subsequently, as they are placed in between these two cracks. As shown on both graphs, the RMSD values of the PZTs follow an ascending pattern during the states of the formation of the two cracks. This is consistent with the earlier observations that the piezoelectric sensors are able to detect even small changes in stiffness when cracks first begin to form. The increase in RMSD values corresponds to a detectable reduction in the beam’s stiffness, as cracks initiate and propagate through the structure.

Thereafter, RMSD values for both PZTs are affected by the widening of the cracks. This indicates that the cracks are having a more significant impact on the overall beam stiffness. As the cracks propagate, the decrease in stiffness becomes more noticeable to the sensors, leading to a continued increase in RMSD values.

In Figure 16, the RMSD values of the pair PZT C and PZT 3 are presented. The Voltage responses of this pair are primarily affected by the formation of the second crack at first and the fourth crack afterward, as they are positioned between these two cracks. As depicted in both graphs, the RMSD values of the PZTs follow an ascending pattern at the states of the formation of the two cracks. Thereafter, RMSD values for both PZTs are affected by the widening of the cracks. RMSD values increase as the cracks widen, confirming the sensors’ ability to detect changes in the mechanical integrity of the beam due to crack growth. This pattern suggests that piezoelectric sensors are sensitive to both the formation and widening of cracks, as these processes degrade the beam’s overall stiffness, which is reflected in the voltage response.

In the last two states, PZT C exhibits decreased RMSD index values, indicating that the cracking was probably developed differently from the front and back sections of the beam or a local debonding of the epoxy resin of the PZT could have occurred.

In Figure 17, the RMSD values of pairs PZT D and PZT 4 are depicted. The Voltage responses of this pair are primarily affected by the formation of the fourth crack at first, fifth, and seventh cracks subsequently, as they are placed among these cracking formations. These cracks likely affect the beam’s structural integrity significantly, as they are positioned along critical regions of the beam that may experience high stresses. As shown in both figures, the RMSD values of the PZTs follow the ascending pattern at the states of the formation of the two cracks. This increase in RMSD corresponds to the reduction in stiffness due to the formation of these cracks. The sensors detect the initial damage from the crack formations, leading to a rise in RMSD values, as expected.

After the initial formation, the widening of the fifth crack and the subsequent formation of the seventh shear-critical crack result in significant increases in RMSD values. The seventh crack, being classified as a fatal shear crack, likely represents a critical failure point for the beam. As the crack widens, the stiffness reduction is more pronounced, leading to a significant increase in the RMSD values. This suggests that the sensors are effective in detecting both the initiation and propagation of critical cracks, particularly those that can lead to catastrophic failure.

In conclusion, Figure 14, Figure 15, Figure 16 and Figure 17 illustrate the sensitivity and effectiveness of piezoelectric sensors in detecting the formation and progression of cracks in concrete beams. The RMSD values increase as cracks form and propagate, reflecting the corresponding decrease in beam stiffness. PZT placement relative to crack sites is crucial for early detection, with sensors closer to cracks being more sensitive to early-stage damage. The widening of cracks, particularly fatal shear cracks, leads to significant increases in RMSD values, further confirming the utility of these sensors for structural health monitoring.

4. Discussion

In this experimental project, an investigation was conducted into the effectiveness of an SHM scheme to protect and prevent the failure mechanism of innovatively designed, and constructed RC structural members (beams) with C-FRP bars as longitudinal reinforcement and continuous spiral steel reinforcement as anchorage configuration. This assessment was conducted by employing an EMI-based method enabled by PZT sensors. Moreover, the statistical scalar index metric RMSD was applied to determine the efficiency of the proposed SHM method in damage detection.

Furthermore, the study seeks to assess the effectiveness of the PZT-enabled SHM system in detecting changes in the structural integrity of an existing reinforced concrete (RC) building. This will be accomplished by strategically installing PZT transducers in various configurations throughout the structure. By analyzing a comprehensive set of measurements gathered from the multiple PZT transducers employed, the study aims to ensure the reliability and accuracy of the proposed monitoring method. The diversity in sensor placements and configurations is expected to provide a robust dataset, enhancing the fidelity of the system’s ability to identify and evaluate any structural alterations.

All the PZTs on both beams successfully detected the formation of cracks within their monitoring “neighborhoods” with some even predicting the imminent development of these cracks. The RMSD index values proved effective in serving as indicators of damage severity and progression, enabling a more accurate assessment of the structural integrity of the specimens.

Moreover, the voltage response curves obtained from the PZTs demonstrated significant peak shifts at their resonant frequencies. These peak shifts indicate a strong correlation between the voltage measurements and the structural integrity, suggesting that voltage signals could be a reliable indicator for early damage detection and monitoring.

5. Conclusions

The placement of PZTs significantly impacts their ability to detect crack formation. Sensors located closer to cracks are more sensitive and can identify cracks earlier. As cracks widen, the RMSD values increase, confirming that cracks lead to progressive stiffness degradation, which can be detected by piezoelectric sensors. The ascending pattern in RMSD values in response to crack formation and widening suggests that PZTs can act as an early warning system, alerting maintenance teams to the presence of structural defects. The early detection of cracks using PZTs can provide significant advantages for preventive maintenance. By identifying cracks at their incipient stages, structural issues can be addressed before they escalate into critical failures, reducing the likelihood of catastrophic structural collapse. This can extend the lifespan of structures and reduce overall repair and replacement costs.

PZTs are effective in tracking multiple cracks, demonstrating the ability to monitor complex damage evolution in real time. The sensors’ sensitivity to crack formation and widening varies depending on placement, but they all successfully detect major damage events, especially fatal cracks. PZTs, placed in strategic locations within the structure, could facilitate remote monitoring, reducing the need for manual inspections in difficult-to-access or hazardous areas, and ensuring more frequent and comprehensive assessment of structural integrity.

Fatal and shear-critical cracks result in significant RMSD increases, indicating the sensors’ capability to detect catastrophic failures. The sensors provide continuous data, reducing the chances of missing critical damage that might occur between scheduled inspections. The ability to track the progression of cracks over time allows for more accurate forecasting of when specific components might fail. This predictive capability can optimize maintenance schedules, ensuring that repairs are carried out, when necessary, rather than on a fixed timetable, leading to better resource allocation.

For instance, in large structures, knowing exactly where cracks form and their extent can guide targeted repairs in high-risk areas, minimizing disruptions to the structure’s normal operation.

The implementation of the proposed SHM method for real-time monitoring of RC elements reinforced with C-FRP bars, to detect damage that occurred due to C-FRP slippage constitutes a novel contribution of this experimental project in RC structural applications. These results emphasize the potential of piezoelectric sensors in structural health monitoring, offering an effective method to track damage progression and prevent structural failures. Continuous monitoring using piezoelectric sensors can enhance the safety of infrastructure. In environments where safety is paramount, these sensors can provide real-time feedback on structural health, allowing for immediate actions to mitigate potential risks.

In addition, the circular anchorage configuration outperformed the rectangular configuration in terms of maximum load-bearing capacity, achieving this with greater ductility. The failure modes observed in both beams highlight the necessity of enhancing the design of RC structural members reinforced with C-FRP bars and improving the material properties of the C-FRP bars themselves. This suggests optimizing the anchorage configuration and reinforcement materials could lead to more resilient and sustainable structural designs.

The study opens up several avenues for future research and development that could further enhance the applicability and effectiveness of PZT-enabled SHM systems. Some of these potential directions include the integration with other sensing technologies, in a multi-modal monitoring scheme. Further, the development of advanced signal processing techniques could help identify subtle changes in the voltage responses that may be overlooked by traditional methods, improving the sensitivity and accuracy of crack detection. In addition, integrating multi-sensor systems will further improve the effectiveness of the proposed scheme, ultimately contributing to the resilience and sustainability of critical structure/infrastructure.