1 Introduction

Corrosion of reinforcing bars stands as the predominant cause of deterioration in reinforced concrete (RC) structures. This corrosion process typically commences with the de-passivation of the reinforcing bar, commonly triggered by exposure to chlorides or carbon dioxide. Once de-passivated, the iron molecules on the reinforcing bar react with oxygen and hydrogen, generating corrosion products. Notably, these corrosion products occupy a larger volume than the original steel, thereby exerting radial pressure at the steel/concrete interface [1]. These Reinforced concrete structures are a crucial component of modern infrastructure, but they are susceptible to corrosion-induced deterioration. Corrosion of reinforcing bars is the primary cause of damage to these structures. Exposure to chlorides or carbon dioxide triggers de-passivation of the reinforcing bar, leading to a corrosive process that weakens the structure. The corrosion products generated occupy more space than the original steel, causing cracking and spalling of the concrete cover [2].

Moreover, the cross-sectional area of the reinforcing bar is reduced, compromising its strength and ductility. This impairment directly affects the structural safety of RC structures. To ensure the safety of existing infrastructure and optimize future designs, precise predictions of corrosion-induced damages are essential. Several models have been developed to predict the various stages of corrosion, from initiation to propagation, and evaluate their impact on the structural behaviour of RC structures. However, developing comprehensive predictive models requires a deep understanding of many parameters, some of which are difficult to measure and controversial [3]. Therefore, it is crucial to continue researching and refining these models to ensure the safety and longevity of our infrastructure. Pressure escalation ultimately leads to concrete cover cracking and spalling. Concurrently, corrosion induces a reduction in the cross-sectional area of the reinforcing bar, thereby compromising its strength and ductility. Consequently, the corrosion process impairs the structural behaviour and safety of RC structures. Accurate predictions of corrosion-induced damages are imperative for assessing the safety of existing infrastructure and refining future designs [4]. Recently, numerous models have emerged to delineate the various stages of corrosion, from initiation to propagation, and to evaluate their ramifications on the structural behaviour of RC structures. However, comprehensive predictive models necessitate a profound understanding of numerous parameters, some of which are challenging to measure and, in certain instances, contentious [5].

The bond between concrete and reinforcement is critical because it influences the behavior and performance of reinforced concrete structures. Steel is a common reinforcing material used in concrete providing additional strength [6]. Cracks or excessive deformation of reinforced concrete sections may result in the slippage between the reinforcement and concrete. As a result, the load-bearing capability of the components is reduced, resulting in structural deterioration. Hence, an elaborate study on the performance of the bond between concrete and reinforcement becomes a very important aspect of the study as far as the performance of concrete is concerned [7].

The bond performance between concrete and reinforcements is influenced by various factors, e.g. freeze-thaw cycles, erosion caused by chloride salts & sulfates including concrete carbonation. Prolonged exposure of reinforced concrete structures to severe environmental conditions can cause detrimental effects such as concrete cracking, corrosion of reinforcements, degradation of bond strength, decreased load-bearing capacity of structural elements, and ultimately, structural collapse [8]. Corroded steel reinforcements in reinforced concrete structures may result in poor bond performance and compromise structural integrity [9]. The corrosion process degrades the reinforcement, reducing its mechanical properties and cross-sectional area. The presence of corrosion-induced oxides on the reinforcement can cause tensile stresses in surrounding concrete, resulting in spalling and failure of the bond. The bond-slip behaviour and ultimate pull-out strength of reinforced concrete structures provide crucial information for their durability and are mainly caused by stress differentials between concrete and reinforcement, leading to cracks in the surrounding concrete [10].

Because of the negative impacts of the corrosion process on rebar, it is critical to examine the bond performance between concrete and corroded reinforcement. Through conceptual analyses and practical studies, researchers have thoroughly researched and analyzed the binding mechanism between concrete and corroded reinforcement [11]. Corrosion of steel reinforcement in concrete structures, particularly produced by chloride ions in marine environments and de-icing salts in freezing situations, poses a threat to the structural integrity of reinforced concrete structures. The degree of corrosion has a major impact on the bond performance between reinforcement and concrete. Bond strength is weakened by the loss of mechanical interlocking and the accumulation of corrosion products [12]. When the force of corrosion expansion exceeds the tensile strength of the surrounding concrete, it results in reduced friction and bond strength. Another research found that deformed bars without confinement undergo a significant decrease in bond strength with increasing corrosion levels. Conversely, in deformed bars with confinement, corrosion has no substantial impact on bond strength [13]. However, a lower corrosion percentage [in the range of 0.5–0.6%] leads to an increase in bond strength while a higher corrosion percentage [above 1.5%]reduces the bond strength. One study reported that the rise in bond strength was generally observed when the degree of corrosion of rebar was within 5%, a significant [14]. However, in cases where the degree of corrosion exceeds 5%, a significant decrease in adhesion between concrete and steel rebar is observed. Interestingly, mild corrosion of the embedded reinforcing steel increases the bond strength between concrete and reinforcement [15]. However, as corrosion progresses, there is a gradual degradation of the mechanical interlocking between the two materials have conducted a study on recycled aggregate concrete (RAC) reported that a low level of corrosion improves the bond performance of RAC compared to traditional concrete, while a high level of corrosion results in similar bondability. The use of geopolymer concrete in conjunction with steel fibres improves its chloride resistance along with bond tension and corrosion resistance [16]. This degradation is attributed to a material loss in the transverse rib and the growth of corrosion by-products at the concrete-rebar interface, leading to the formation of a smear layer that reduces adhesion. The direct relationship between the level of corrosion and the ultimate bond strength as long as the degree of corrosion did not exceed 4%. When the degree of corrosion in the reinforcement exceeds 4%, however, a significant decrease in the bond strength was observed, however beyond the corrosion level of 6%, no significant variations in bond strength were observed [17]. In another investigation, the influence of corrosion on bond strength is insignificant if the corrosion loss is below 2.4%. D. However, the presence of corrosion agents reduces the bond strength of RAC. In terms of concrete, up to a corrosion level of 2%, there was a modest improvement in bond strength, which thereafter decreases as the corrosion level goes up. This pattern was comparable to that seen with traditional concrete [18]. On the other hand, found that the bond strength was highest in the corrosion range of 1 to 1.5%, but gradually decreased as the corrosion level went beyond 1.5%. Further, the authors found that mild corrosion below 2.1% can improve bond behaviour, however, the inclusion of 15% fly ash in concrete can boost the ultimate bond strength and compensate for the decrease caused by severe rebar corrosion of rebars. The bond behaviour was relatively unaffected when the degree of steel corrosion was less than 0.05% or the exposure temperature was less than 200 degrees Celsius. However, at temperatures above 400 °C, a significant decrease in bond strength was observed up to 600°C or with a degree of corrosion more than 0.05 % [19].

To mitigate the negative impact of corrosion on bond performance, various preventive measures can be implemented. These include the use of corrosion-resistant reinforcements, such as stainless steel or fibre-reinforced polymer (FRP) rebar, as well as protective coatings and corrosion inhibitors [20]. Proper concrete cover thickness, adequate quality control during construction, and regular maintenance and inspection are essential for preserving the bond performance and overall durability of reinforced concrete structures. The research gap scanned through the above literature suggests a comprehensive study on corrosion development in reinforcing steel versus its impact on the bond between concrete and steel [21].

The study delves into different aspects of concrete performance under varying conditions and with different supplementary materials. The first part examines the effect of harsh environments on cement mortar with pumice powder (natural pozzolana, NP) as a partial substitute for ordinary Portland cement (OPC). Mortar mixtures containing 0%, 5%, 10%, and 20% NP were exposed to tap water and a harsh environment (seawater with 5% Na2SO4) for 360 days, undergoing 90 cycles of 48-hour drying and wetting [22, 23]. The second part focuses on high-strength concrete (HSC) using palm oil industry waste, specifically ground POFA (GPOFA) treated to create ultrafine POFA (UPOFA). This treatment enhanced fineness and pozzolanic content, resulting in high-strength concretes HSCg and HSCu, with the latter achieving a maximum strength of 116 MPa. The third part examines high-strength green concrete (HSGC) with up to 60% ultrafine POFA, treated to a 2 µm particle size, replacing 0%, 20%, 40%, and 60% of OPC [24, 25]. Finally, the study investigates the impact of manufacturing processes and rusting on the bond behaviour of deformed bars in ordinary-strength concrete exposed for up to 36 months in the Arabian Gulf. Sixty-three pullout tests on bars with three geometries and two manufacturing processes indicated that short-term exposure enhances bond performance, while extended exposure leads to a 10% decrease in bond strength after 36 months. The quenching process displayed superior mass loss resistance compared to the hot-rolled process, although both processes exhibited similar bond behaviour [26].

The present study presents a groundbreaking approach to understanding the impact of corrosion on the bond performance between different grades of concrete and reinforcement. By utilizing the innovative impressed current technique to induce controlled corrosion, the research provides a precise means to study the effects of corrosion on bond performance. The study examines three typical grades of concrete, M20, M25, and M30, and impresses corrosion levels ranging from 5 to 20% upon them. Through meticulous experimentation, the bond performance between reinforcement and concrete is comprehensively evaluated. This methodology allows for a thorough examination of how corrosion influences bond behavior across different concrete grades. The significance of this research lies in its ability to provide valuable insights into the structural deterioration mechanisms of reinforced concrete structures. By investigating bond performance under controlled corrosion conditions, the study offers a thorough understanding of how corrosion affects the bond behaviour across different grades of concrete. These insights will be useful in developing effective mitigation strategies for reinforced concrete structures.

Studies show that corrosion affects the bond between reinforcement and concrete. This paper takes a unique approach by combining advanced experimental techniques and detailed analytical methods to assess the degradation of the bond strength of concrete. The novelty of this work lies in its comprehensive examination of microstructural changes and their direct impact on bond strength, providing a more detailed understanding of the deterioration mechanisms. Additionally, this study introduces an innovative accelerated corrosion process that simulates long-term environmental exposure within a significantly reduced timeframe, offering valuable insights that bridge the gap between laboratory findings and real-world applications. This new methodology not only improves the accuracy of the results but also sets a new benchmark for future research in the field of structural durability and resilience.

In conclusion, this study is an important contribution to the field of structural engineering and construction. By utilizing the impressed current technique to induce controlled corrosion, the research provides a precise means to study the effects of corrosion on bond performance. The results of this study will be invaluable in developing effective mitigation strategies for reinforced concrete structures, ultimately leading to safer and more durable infrastructure.

2 Experimental program

The overall experimental program has been planned as presented through the block diagram in Fig. 1 below.

Fig. 1
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Flowchart of experimental program

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The study includes induction of 5 different corrosion levels (0%, 5%, 10%, 15%, & 20%) with the help of the Impressed Current Technique on three different usual grades of concrete specimens M20, M25 &M30 after 28 days of curing, followed by the pull-out test to observe the loss of bond strength. The detailed process for the experimentation is outlined below:

2.1 Materials

The study used ordinary Portland cement of 43 grade manufactured by Ultratech company following IS-8112-2013 standards [27]. The cement underwent laboratory testing to evaluate its properties, and the detailed results are provided in Table 1. The study also employed locally sourced fine aggregates that adhered to IS383:1970 specifications. Before usage, these aggregates were tested for essential physical properties required for mix design, as outlined in IS-2386-part-1-1963 [28], and the findings are presented in Table 2. Additionally, the investigation utilized locally available coarse aggregate with an average size of 20 mm following IS-383-1970 standards [29]. Tests were conducted to assess the significant physical properties of the coarse aggregate, and the results are outlined in Table 3. Regular tap water with a pH of 7.6 was used as a component in the experimental setup throughout the study. However, the yield strength and Ultimate Tensile strength of used rebars were 444.53 Mpa and 523.44 Mpa respectively.

Table 1 Physical characterisation of cement
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Table 2 Physical characterisation of fine aggregates
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Table 3 Physical characterisation of coarse aggregates
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Providing detailed information about the materials used is crucial for ensuring standardization and reproducibility of experimental setups. When other researchers can replicate a study using the same materials and methods, it enhances the reliability and credibility of the findings. The properties of materials, including aggregates and cement, can significantly impact experimental results. By specifying the source, type, and characteristics of materials, the study ensures that any observed effects can be attributed to the variables under investigation rather than variations in material properties. While the materials themselves may not be directly analyzed, they play a crucial role in influencing the structural behaviour of the system under study. For instance, the quality and properties of aggregates can affect concrete strength, durability, and bond performance, which, in turn, influence the structural response. Providing information about the materials used offers context and background for the study, helping readers understand the experimental setup and the rationale behind certain decisions. This information may also be useful for comparing the results with previous studies or industry standards. Including details of materials used is essential for ensuring completeness of reporting, transparency, and reproducibility in scientific research. It allows readers to assess the validity and robustness of the experimental methodology and draw meaningful conclusions from the results. While the materials themselves may not be directly analyzed in the results section, their inclusion is crucial for ensuring standardization, quality control, understanding the structural behaviour, providing context, and maintaining the completeness of reporting in the research study.

2.2 Preparation of specimens for pull-out test

The following section outlines the detailed procedure for preparing specimens for the pull-out test. The pull-out test is a critical component of the experimental setup, aimed at evaluating the bond strength between concrete and reinforcement. Proper specimen preparation is crucial to ensure accurate and reliable test results. First, suitable specimens are selected based on predetermined criteria, such as concrete grade, reinforcement type, and dimensions required for testing as depicted in Fig. 2. Then, concrete mixtures are carefully prepared with the help of the material discussed above in section 2.1 and materials quantities after mix design have been depicted in Table 4 to establish standards or project-specific requirements.

Fig. 2
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Geometric dimension of Sample

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Table 4 Design mix proportions quantities of different grades of concrete
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For the present study, 45 specimens (15 specimens for each grade of concrete under consideration) were prepared. Further, five corrosion levels were selected, with values ranging from 0 to 20% in increments of 5% (i.e., 0%, 5%, 10%, 15% and 20%) and a detailed description has been given in Table 5 below. The samples were assigned codes based on the type of concrete and the desired corrosion levels as shown in Table 6 below:

Table 5 Description of the grade of concrete, reinforcement and corrosion level
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Table 6 Description of concrete and corrosion level codes
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Further, formwork is constructed to mould the concrete into the size of each cylindrical sample was 100 mm in diameter and 225 mm in height for the pull-out test specimens as depicted in Fig. 3a below. The formwork should be sturdy, leak-proof, and capable of maintaining the integrity of the specimen during casting and curing. However, 12 mm diameter reinforcement bars along with 500 mm height are carefully positioned within the formwork according to the specified configuration and spacing. Proper alignment and placement of reinforcement ensure uniform distribution of forces during the test. The prepared concrete mixture is poured into the formwork, surrounding the reinforcement bars as depicted in Fig. 3a. Care is taken to prevent segregation and ensure proper compaction of the concrete to eliminate air voids. After casting, the specimens are allowed to cure under controlled conditions as depicted in Fig. 3b to achieve optimal strength and durability. Once the concrete has sufficiently cured, the formwork is carefully removed to reveal the hardened specimens. Care is taken to avoid damaging the specimens during de-moulding. However, Fig. 3c shows the curing procedure for the sample. Any irregularities or debris are removed, and the surfaces are cleaned to ensure proper bonding between the concrete and the testing apparatus.

Fig. 3
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Casting to curing process; a Specimens after casting; b Specimens after removal of Formwork; c Specimens during 28 days of curing

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2.3 Accelerated corrosion tests

The reinforced concrete samples underwent the impressive current method to initiate corrosion, which was a highly effective approach. The specimens were exposed to a direct electric current, via an embedded steel bar, which accelerated the corrosion process of the reinforcement. Before this step, the samples were immersed in a 3.5% NaCl solution for 3 days to further accelerate the process. During the electrochemical process, the anode was reinforced while the cathode was constructed from stainless steel, ensuring that the samples remained in good condition. The power sources used were adjustable for both voltage and current, ensuring that the process was highly efficient. Figure 4a, b, and c depicts the accelerated electrochemical corrosion phenomenon, which is an impressive sight. The amount of current applied was carefully calculated to achieve the desired mass loss. Overall, the impressive current method is a highly effective and efficient approach to initiate corrosion for reinforced concrete samples.

Fig. 4
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Accelerated electrochemical corrosion phenomenon; a schematic view of accelerated corrosion technique: b before Accelerated corrosion and; c after accelerated corrosion

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Faraday's law was used to estimate the expected amount of rebar corrosion versus rebar mass loss to achieve different levels of corrosion in the rebar. The mathematical expression of Faraday's law is as follows from equation (1) [2]:

$$i_{app} \, = \,\frac{{\rho W_{i} F}}{100 \pi dlwt}$$
(1)

where ‘iapp’ is applied current in Amp; ‘ρ’ is the degree of corrosion in percentage; ‘Wi’ is the initial weight of the reinforcement in gm; ‘F’ is Faraday's constant in Amp-Seconds; πdl is the surface area of reinforcement in mm2; l is the length of the reinforcement in mm; w is the equivalent weight of iron in gram; t is time in seconds; d is the diameter of the reinforcement in mm. Table 7 presents the time required to achieve the desired level of corrosion.

Table 7 Amount of time required to achieve corrosion levels
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The real corrosion loss was established by measuring the mass loss of the rebar to determine the average degree of corrosion along the bond length. The weights of the original flat and deformed bars per unit length were recorded before producing the drawable samples. Following the completion of the pull-out test, the corroded steel bars were removed from the specimens and any concrete residue on their surfaces was gently tapped off. After cleaning, the rusted reinforcement was weighed. The percentage loss of reinforcement mass was calculated using Eq. (2):

$$\eta_{s} \, = \,\frac{{m_{o} l - m_{c} l}}{{m_{o} l}}\, \times \,100$$
(2)

where ηs is the actual percentage loss in percentage; m0 is the original mass per unit length of the bar in gram; mc is the mass of corroded steel bar per unit length in gram; l is the actual length of the steel bar in mm.

2.4 Pullout assembly

The assembly consisted of two square plates of thickness 12 mm with dimensions of (270*270) mm. The bottom plate consisted of an extended rod of 25 mm diameter at the centre towards the down direction, while the upper portion had a hole of 16 mm diameter at the centre of the plate, which enabled the extra length of the reinforcement in the specimen to adjust within the assembly. This extra length of reinforcement was used for grip during the pullout. Both plates were connected using a 20 mm diameter Plain rod having 80 mm of threaded length that could be used to adjust the specimens within the assembly according to the sample size and could be locked with the help of nuts. The Plain portion of the rod was welded to the bottom plate while the threaded portion was used to tighten the upper plate using nuts. The bottom extended rod of the assembly was clipped with a wedge hydraulic grip while the pullout was done from the upward side during the Pullout Test. The prepared test assembly is shown in Fig. 5.

Fig. 5
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Schematic diagram of pullout assembly

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2.5 Pull-out test procedure

After the completion of the corrosion acceleration test, the specimens were loaded for pullout tests in a Universal Testing Machine (UTM) using two sets of pullout fixtures specifically designed to meet the required requirements, as shown in Fig. 6a, b. The elongated rod of the extractor was clamped by the upper jaw while the lower jaw clamped the rebar. The application of pulling force was facilitated by the mandibular extension bar. The relative slippage between concrete and reinforcement was measured on a digital monitor connected to the UTM. Specimens were loaded gradually at the rate of 2250 kN/min. Figure 6a, b depicts the pullout test setup.

Fig. 6
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Pull-out test Setup: a pullout test fixture; and b Specimens in universal testing machine during pull-out test

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3 Results and discussion

This experiment was carried out to gather valuable data on the performance of the specimens under different stress rates. The photographic summaries of typical post-failure pull-out test specimens provided crucial insights into the cracking and failure modes of the specimens. Figure 7a, b, c, and d clearly illustrates the effects of loads applied to the specimens until partial or full pull-out occurred. These results are summarized in Table 8, making it easy to compare and analyze the data. To ensure accuracy, three specimens were taken for each percentage set of corrosion shown as (1), (2) and (3) in Table 8. This experiment provides significant value to the field of research and will help inform future studies.

Fig. 7
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Pull-out Test; a Specimen in assembly; b Split Failure of the specimen; c Crack pattern on corroded specimen & d Tested Corroded specimens

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Table 8 Results of the Pull-out test
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3.1 Failure mode

This study presents a detailed analysis of the behaviour of specimens in the pullout test, which is crucial to ensure the safety and durability of concrete structures. The results show that the failure mode of each specimen was the cracking of the concrete cover, as illustrated in Fig. 7. The study also identified the pull-out mechanisms of the reinforcement in unbound and corroded concrete. At corrosion levels above 5%, the crack propagation along the initial corrosion-induced cracks was observed at peak load, resulting in loud noise and fragmentation of the concrete samples into 2–3 blocks along the initial cracks [30]. These findings highlight the importance of corrosion prevention measures to ensure the integrity and longevity of concrete structures. It is also worth noting that both the crack and the corroded rebar samples were covered with a red-brown corrosion layer, and the concrete surrounding the reinforcement showed a red-brown colour due to the diffusion of the corrosion [31]. This emphasizes the need for regular inspection and maintenance of concrete structures to prevent corrosion and ensure their safety [32].

3.2 Bond stress versus slip relationships

The data collected from Figs. 8, 9, and 10 reveals an interesting finding. It has been discovered that different levels of corrosion do not affect the effectiveness of bond strength across different concrete grades, including M20, M25, and M30. The bond stress-slip response for each of these concrete types is nonlinear, with a peak point followed by a reduction in bond stress and an increase in pull-out slip until the reinforcement rebar is completely pulled out or the samples fail. By examining the relationship between bond stress and slip for individual types of concrete, we can better understand the importance of proper construction techniques and the impact it has on the durability of structures.

Fig. 8
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Bond stress v/s slip relationship for M20 concrete

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Fig. 9
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Bond stress v/s slip relationship for M25 grade concrete

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Fig. 10
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Bond stress vs slip relationship for M30 grade concrete

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The bond stress versus slip relationship for the M20 grade of concrete with varying percentages of corrosion is represented by Fig. 8. The graph shows that corrosion has a detrimental effect on the bond stress and slip values. The bond stress and slip values were evaluated for different levels of corrosion for the M20 grade of concrete. The experimental results indicate that at a corrosion level of 0%, the bond stress was found to be 5.59 MPa, and the slip was 6.47 mm. However, as the corrosion level increased, the bond stress and slip values decreased significantly. At the maximum corrosion level of 20%, the bond stress was reduced to 2.34 MPa and the slip to 3.50 mm. Similarly, at a corrosion level of 5%, the bond stress was reduced to MPa, and the slip decreased to 5.59 mm. At a corrosion level of 10%, the bond stress was found to be MPa, and the slip was 4.69 mm. The experimental results show that bond stress is more susceptible to corrosion than slip. Corrosion significantly affects the bond stress and slip values for M20-grade concrete. Therefore, it is essential to take measures to prevent corrosion, as it can lead to a decrease in the strength of the concrete structure. The results indicate that the corrosion level should be kept to a minimum to ensure the longevity and durability of the concrete structure.

The correlation between bond and slip for M25 grade concrete at varying levels of corrosion is showcased in Fig. 9. The graph demonstrates that as corrosion levels increased from 0 to 20%, the bond stress decreased from 6.78 MPa to 3.41 MPa, and the slip decreased from 7.54 mm to 4.09 mm. This information highlights the importance of monitoring corrosion levels to prevent significant deterioration in structural integrity. With proper maintenance and regular inspections, it is possible to identify and address corrosion before it causes significant damage. By doing so, we can ensure safe and reliable infrastructure that meets quality standards and serves communities for years to come.

Figure 10 displays the relationship between bond and slip for M30 grade concrete, with varying levels of corrosion. At 0% corrosion, the bond stress and slip values were 8.06 N/mm2 and 8.08 mm, respectively. As the level of corrosion increased to 5%, the bond stress decreased to 7.09 N/mm2 and slipped to 7.10 mm. At a corrosion level of 10%, the bond stress decreased to 6.30 N/mm2, with a slip reduction of 6.03 mm. The bond stress further decreased to 5.42 N/mm2 and slipped to 5.53 mm at a corrosion level of 15%. Finally, at the highest level of corrosion (20%), bond stress reduced to 4.34 N/mm2 and slipped to 5.04 mm.

The results indicate that there is a significant reduction in bond stress and slip as the degree of corrosion increases, which is consistent across all concrete types [26]. This decrease is particularly pronounced for higher levels of corrosion, between 15 and 20%. The cause of this phenomenon is the deterioration of concrete material due to the corrosion process of the reinforcing steel, resulting in reduced adhesion between the concrete and the steel member. The reduction in bond stress and slip can be attributed to the increase in permeability and porosity of the concrete due to corrosion, leading to the creation of voids and cracks in the concrete, which ultimately weakens the bond between the concrete and steel [33]. Although the M20 grade concrete had higher slip values and lower bond stress values, suggesting it was more permeable than the higher-grade concrete samples M25 and M30, the trend of decreasing bond stress and slip with increasing degrees of corrosion remained constant across all concrete grades.

3.3 Effect of corrosion on pull-out load

Figures 11, 12, and 13 demonstrate the correlation between the pull-out load and the corrosion level of the tested specimens. Upon analyzing the test data, it was found that the pull-out load was significantly decreased in lower-grade concrete compared to higher grades. Additionally, there was a gradual reduction in the pull-out load as the level of corrosion increased. The impact of the pull-out load at different levels of corrosion has been discussed in detail in the following paragraphs:

Fig. 11
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Pull-out load vs corrosion level for M20 grade concrete

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Fig. 12
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Pull-out load vs corrosion level for M25 grade concrete

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Fig. 13
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Pull-out load vs corrosion level for M30 grade concrete

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The results presented in Fig. 11 are a clear indication of how corrosion can impact the pull-out load values for concrete grade M20. The pull-out load values decreased as the degree of corrosion increased. At 0% corrosion, the pull-out load was 47.39 KN, but this value was significantly reduced to 19.88 KN at 20% corrosion. The non-uniformity of the reduction in pull-out load values with increasing corrosion levels is worth noting. The steepest drop in pull-out load values was observed in the 0% to 5% corrosion range, with a 17% reduction from 47.39 KN to 39.31 KN. However, as the degree of corrosion increased further, the decrease in pull-out load values was less significant. For instance, between 5 and 10% corrosion levels, the reduction was only 10.51%, and between 10 and 15% corrosion levels, the decrease in pull-out load values was merely 8.24%. It is crucial to point out that at the highest corrosion level of 20%, the pull-out load was reduced by a whopping 58% compared to the pull-out load at 0% corrosion level. These findings indicate the importance of protecting concrete structures from corrosion to ensure their longevity and safety.

Figure 12 displays the pull-out load values for concrete grade M25 at various levels of corrosion. The results indicate that as corrosion levels rise, there is a downward trend in pull-out load values. At 0% corrosion, the pull-out load is measured at 57.53 KN, while at 20% corrosion, the pull-out load value falls to 28.91 KN. Furthermore, the rate of decrease in pull-out load values is not uniform as the degree of corrosion increases. For example, the pull-out load between 0 and 5% degree of corrosion only drops by 11.92% (from 57.53 to 50.67 KN). However, between 5 and 10% corrosion levels, there is a steeper decrease in pull-out load, with the pull-out load decreasing by 16.93% (from 50.67 to 42.09 KN).

Figure 13 illustrates the pull-out load values at different levels of corrosion. The data shows that the pull-out load values decrease as the level of corrosion increases. At 0% corrosion level, the pull-out load is 68.33 KN, while at 20% corrosion level, the pull-out load decreases to 36.88 KN. It is worth noting that the decrease in pull-out load values is not consistent across all corrosion grades. Between 0 and 5% corrosion, the pull-out load decreases by 12% from 68.33 to 60.11 KN. Between 5 and 10% corrosion, the decrease is less significant at 11.09%, with the pull-out load decreasing from 60.11 to 53.44 KN. Similarly, the pull-out load decreased between 10 and 15% corrosion was 14%, from 53.44 to 45.96 KN.

However, Fig. 14 the most significant decrease in pull-out load occurred between 15.27 and 20.98% corrosion, with a 20% reduction in pull-out load from 45.96 to 36.88 KN. Therefore, we can conclude that the relationship between the pull-out load reduction and the corrosion level increase is non-linear, and the rate of decrease varies with different corrosion levels. It has been found that the reduction in pull-out load increases as the degree of reinforcement corrosion increases. This is due to the volumetric expansion of rust, which fractures the interaction surface of reinforcement and concrete surface. As a result, it leads to a reduction in load-bearing capacity and loss of structural integrity .

Fig. 14
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Bond strength v/s corrosion level

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3.4 Effect of corrosion on bond strength

The impact of corrosion on bond strength was studied across five levels of corrosion, ranging from 0 to 20%. The results showed that the degree of reinforcement corrosion significantly affected bond strength for all concrete grades, including M20, M25, and M30. As the level of corrosion increased, bond strength decreased in all samples. Higher quality concrete demonstrated higher bond strength with the same degree of corrosion, due to lower permeability, good interaction between reinforcement and concrete materials, and an increase in cement paste and a decrease in water content. Minor cracks were observed in specimens with 15% and 20% corrosion levels in all concrete types, indicating that increased corrosion levels could lead to spalling and cracking, reducing the material's load-bearing capacity. The degree of corrosion could be attributed to factors such as changes in surface finish and loss of cross-sectional area. As the corrosion level increased, the mechanical force holding the reinforcement to the concrete was weakened. This reduction in bond strength was due to rust formation on the reinforcement surface, leading to surface roughness and uneven stress distribution within the concrete. This phenomenon contributes to the deterioration of concrete over a longer period [34]. Reducing the cross-sectional area of the rebar resulted in reduced ductility and reduced bond strength between concrete and rebar.

3.5 Bond strength degradation due to corrosion

This study aims to examine the weakening of the bond in pull-out specimens made of M20, M25, and M30 concrete, at different levels of corrosion. The corrosion levels tested were 0%, 5%, 10%, 15%, and 20%. The samples where no corrosion was initiated were classified under the 0% level and served as the control group. In the next section, the experimental result has been discussed:

Figure 15 demonstrates a notable reduction in the bond strength with an increase in the level of corrosion. It was concluded that there was a significant correlation between the two factors. The research revealed that the corrosion process resulted in a reduction in the bond strength of concrete with a grade of M20. The controlled sample that did not undergo corrosion was compared to samples with varying corrosion levels of 5%, 10%, 15%, and 20%, resulting in reductions of 17%, 26%, 41%, and 58%, respectively.

Fig. 15
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Bond strength reduction due to corrosion for M20 grade concrete

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Figure 16 shows the results of the experimental investigation on concrete samples of grade M25 exposed to different degrees of corrosion and shows a positive correlation between the degree of corrosion and the decrease in bond strength. As the degree of corrosion increased, so did the joint strength. According to the results of the study, a 12% decrease in bond strength was observed when the degree of corrosion reached 5%. In contrast, at corrosion levels of 10%, 15%, and 20%, a 27%, 42%, and 50% reduction in bond strength was observed, respectively, compared to the control sample that was not exposed to corrosion.

Fig. 16
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Bond strength reduction due to corrosion for M25 Grade Concrete

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Figure 17 demonstrates the results of M30 grade concrete showing a direct relationship between the bond strength and the degree of corrosion. According to the results of the study, it was observed that when the corrosion level reached 5%, the bond strength decreased by 13%. Corrosion levels of 10%, 15% and 20% resulted in an escalation of the earlier decrease to 22%, 37% and 47% respectively compared to the controlled sample which showed no corrosion initiation.

Fig. 17
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Bond strength reduction due to corrosion for M30 Grade Concrete

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Thus, the results obtained from the tests with concrete types M20, M25 and M30 showed that the reduction in bond strength caused by corrosion was more pronounced at higher corrosion levels for the above concrete types. The observed decrease in bond strength can be attributed to several factors, including metal depletion due to the onset of corrosion, roughness and porosity of the corroded steel surface, reduced cross-sectional area, and compromised structural integrity of the steel due to rusting and associated cracking. The above variables showed an upward trend with a corresponding escalation in the degree of corrosion, leading to a reduction in the cross-sectional dimensions of the steel and a consequent reduction in the load-bearing potential of the reinforced concrete building [30, 34].

3.6 Degree of corrosion and reduction of bar diameter

The pull-out test was performed on cylindrical specimens using 12 mm diameter bars. The corroded test bars were cleaned with sandpaper and weighed to determine the mass loss. The degree of corrosion was evaluated by measuring various degrees of corrosion ranging from 0 to 21.32 per cent. The assessment of the expected and actual levels of corrosion shows an error of up to 5%. It was found that the decrease in reinforcement diameter is related to the degree of corrosion. Specifically, it has been observed that the reduction range is 11.3–11.6 mm for a 5 per cent corrosion level, 10.7–10.9 mm for a 10 per cent corrosion level, 10.1–10.3 mm for a 15 per cent corrosion level and 9.4–9.7 mm for a 20 per cent corrosion levels. The reduction in the diameter of the reinforcement can be attributed to the metal depletion caused by the corrosion process triggered by the impressed current technique. Corrosion of rebar is a chemical reaction that leads to the formation of rust, a material that is both weaker and bulkier than the original steel. The oxidation process that leads to the formation of rust on the steel surface can lead to structural damage such as cracking and spalling, ultimately leading to a reduction in the cross-sectional area of the reinforcement. A reduction in the cross-sectional area leads to a reduction in the load-bearing capacity, thereby jeopardizing the strength and durability of the reinforced concrete structure [19, 26, 34].

4 Conclusions

The study examined how corrosion affects the bond between reinforcement and concrete. It looked at different concrete grades (M20, M25, and M30) and varying corrosion levels. The key findings are as follows:

  1. 1.

    Lower-grade concrete (M20 and M25) significantly decreased bond strength compared to higher-grade concrete (M30). The bond strength was highest in non-corroded specimens for all concrete mixes.

  2. 2.

    Lower-grade concrete displayed a noticeable decrease in pull-out load compared to higher-grade concrete. As corrosion levels increased, the reduction in pull-out load was gradual, indicating that higher-grade concrete (M30) had better bond strength and was more corrosion-resistant.

  3. 3.

    The reduction in pull-out load showed non-linear behaviour, with the decrease in bond strength becoming less pronounced as the corrosion level increased.

  4. 4.

    The bond stress-slip relationship for M20, M25, and M30 concrete displayed a non-linear response. Higher-grade concrete showed smaller variations in bond strength across different corrosion levels, indicating better resistance to corrosion effects.

  5. 5.

    Higher levels of corrosion led to a more significant reduction in bond strength, with factors such as metal loss, increased surface roughness, reduced cross-sectional area, rust formation, and cracking contributing to this reduction.

  6. 6.

    Corrosion reduced bar diameter, highlighting the impact of metal loss on the load-carrying capacity of reinforced concrete structures.

These findings emphasize the negative impact of corrosion on the bond between reinforcement and concrete, with higher-grade concrete demonstrating better corrosion resistance. The study underscores the importance of considering concrete grade and corrosion levels when assessing bond performance and provides valuable data for predicting bond degradation, planning rehabilitation, and retrofitting concrete structures. Further research on the effect of corrosion on structural components like beams and columns is recommended for a better understanding and practical application.