Effect of corrosion inhibitors on bond strength of reinforced concrete under various exposure conditions

ABSTRACT

Corrosion inhibitors delay the incidence of reinforcement corrosion by decreasing the permeability of concrete. The impact of the commercially available inhibitors on the durability attributes of cover concrete needs more studies, especially the ones exposed to severe environmental conditions. This study mainly aims to investigate commercial inhibitors' efficacy in the bond characteristics of reinforced concrete for three different chloride-induced corrosion exposures. Eighteen specimens with dimensions of 150 mm × 150 mm × 150 mm, with optimum inhibitor dosage, were exposed to one of the three corrosion acceleration methods (induced current, sodium chloride saltwater immersion, and potable water immersion) after normal curing. The bond specimens peak slip, pull-out force, bond strength, and rebar mass loss were assessed after three, five, or seven cycles of wet-dry corrosion exposure. With the rise in immersion cycles, a general decline in bond strength was observed in all specimens. For instance, OW3 exhibited a bond strength of 11.78 N/mm², which diminished to 9.08 N/mm² by the time of OW7. In contrast, adding corrosion inhibitors to the concrete mix increased bond strength, measuring 13.06 N/mm² for OIW3 and 10.03 N/mm² for OIW7. Using corrosion inhibitors enhanced the bond properties and reduced the mass loss in steel under severe corrosion exposures.

Keywords:
Corrosion; Induced current; Bond strength; Rebar weight loss; Corrosion inhibitor

1. INTRODUCTION

Corrosion has a significant economic and environmental impact on almost every aspect of the world’s infrastructure, including bridges, highways, oil and gas pipelines, industrial buildings, monuments, water supply - sewage systems, and prestressed structures [¹, ²]. According to the NACE impact study, corrosion is projected to cost around 2.5 trillion US dollars, 3.4 per cent of worldwide GDP as of 2013, excluding repair and rehabilitation demands. Therefore, researchers worldwide are interested in implementing alternative technologies to slow down the corrosive process and enhance the service life of concrete members exposed to corrosive conditions.

When concrete is exposed to any corrosive environment, corrosion agents such as carbonates, chlorides, and sulfates penetrate the pore spaces of the concrete, affect the concrete’s alkaline nature, and leach the protective passivation layer at the steel-concrete interface [³, ⁴], causing corrosion. The metal oxides in the rust put expansive stress on the concrete cover, causing cracking and damage. The pressure also weakens the link between steel and concrete and subsequent concrete spalling while lowering the effective cross-sectional area of the steel reinforcement. Thus, corrosion lowers the capacity of concrete buildings to perform their intended tasks, jeopardizing their serviceability and durability [⁵].

The bond resistance at the reinforcement and concrete contact region offers efficient stress transmission between steel and concrete. The following reasons are essential in bond stress between steel and concrete: enhanced load transfer, crack control, improved structural behaviour, safety and reliability, and efficient use of materials. The bond resistance at the reinforcement and concrete contact region offers efficient stress transmission between steel and concrete. Chemical adhesion, friction, and mechanical interlocking between the reinforcement and the surrounding concrete generate the required bond resistance [⁶]. Corrosion causes a reduction of the bond strength, eventually affecting the service life of reinforced concrete structures [⁷, ⁸, ⁹, ¹⁰, ¹¹]. As a result, it’s critical to estimate the bond strength between steel and concrete to guarantee that the structure lasts up to its design life. Many studies have been conducted to determine bond strength as a function of cover depth, rebar diameter, bonded length, stirrups, and other variables [¹²,¹³,¹⁴,¹⁵,¹⁶]. Also, some studies [¹⁷, ¹⁸] presented the association between the width of cracks generated and the level of corrosion attained in another set of trials. A study [¹⁹] on concrete bond specimens gave an exhaustive analysis of bond strength investigations conducted to date, while another investigation [²⁰] summarised the research gap in bond strength parameters.

To avoid the high maintenance costs associated with corrosion in coastal areas, the permeability of the cover concrete must be enhanced by utilizing plasticizers and mineral admixtures such as fly ash, silica fume, metakaolin, and slag and also discussed the effect of adding SCMs to OPC binder concretes on chloride penetration resistance [²¹,²²,²³,²⁴]. Some chemicals, like calcium nitrite, zinc oxides, benzoates, amines, etc., have been investigated for their effectiveness in corrosion resistance in the last three decades [²⁵]. Corrosion inhibitors have helped to lower the incidence of corrosion by lengthening the commencement period and decreasing the permeability of concrete [²⁶]. Even though several manufacturers produce and market various inhibitors, there is little evidence that these commercially available inhibitors impact the durability of cover concrete [²⁷].

Though many studies have been completed on using SCMs to delay the onset of corrosion, additional research is needed on the effect of commercially available corrosion inhibitors and cement when exposed to various severe environments. This research aims to see how adding corrosion inhibitors combined with OPC cement affects the bond characteristics of reinforced concrete when exposed to the worst corrosive environments. The novelty of the present research lies in examining the bond strength of concrete with the incorporation of corrosion inhibitors using various environmental conditions. The present study graphical abstract and research methodology are depicted in Figures 1 and 2.

Figure 1
Graphical abstract of this study.

Figure 2
Methodology of this study.

2. EXPERIMENTAL STUDY

2.1. Materials

Ordinary Portland Cement (OPC) of grade 53 and specific gravity 3.14 was used as a binder, adhering to the minimum cement content requirements of IS 10262:2019. The concrete of M³⁰ grade was adopted to suit harsh exposure circumstances. The fine aggregate used was manufactured sand with a specific gravity of 2.69 and a fineness modulus of 3.25 that conforms to zone II. Quarry crusher granite angular aggregates less than 20 mm and greater than 12 mm in size, with a specific gravity of 2.71 and an aggregate crushing value of 9.5 per cent, were employed as coarse aggregate. The physical properties of the materials are reported in Table 1.

Table 1
Physical properties of the materials.

PHYSICAL PROPERTIES		OPC	FA	CA
Specific gravity		3.14	2.69	2.71
Fineness (%)		2.62	-	–
Fineness modulus (%)		–	3.25	6.78
Water absorption (%)		–	1.26	0.36
Crushing value		–	–	9.50
Setting time (min)	Initial	31	–	–
	Final	280	–	–

Additionally, the chemical properties of the OPC are presented in Table 2. Potable water with a neutral pH designed to 0.40 water-binder ratio was employed for mixing. Conplast is utilized as a superplasticizer at 0.8% by weight of the binder to improve the workability of the concrete [²⁸]. Reinforcing steel, whose yield strength is 500 N/mm² with a diameter of 20 mm and 2000 mm long, was used as the embedded rebar in concrete. The present study has examined the bond strength of the concrete with the addition of the Concare corrosion inhibitors. On a trial basis, an anodic corrosion inhibitor was admixed with concrete in four different dosages (0.5 kg/m³, 1 kg/m³, 1.5 kg/m³, 2 kg/m³ and 2.5 kg/m³) to identify the suitable dose with improved mechanical properties of concrete.

Table 2
Chemical properties of the OPC.

CEMENT	CHEMICAL COMPONENTS (%)
	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	LOI
OPC	21.34	4.89	4.06	63.08	0.58	2.87	1.92

2.2. Bond strength specimens

In the current investigation, 18 concrete bond specimens were investigated. Nine specimens were cast with 2 kg/m³ of corrosion inhibitor admixed concrete; the others were cast without any inhibitor. The cube moulds made of centring plywood, measuring 150 mm on all sides, were used to cast concrete bond specimens, with reinforcing steel inserted in the center, as shown in Figure 3. Thermo mechanically treated, high yield strength deformed bars of 20 mm diameter and 2000 mm long were used as inserts in the center. With the help of two PVC casings, each 50 mm long, a fixed bonded length of 100 mm was provided in all specimens. After inserting the rebar, the sides of the PVC tube were wrapped with plastic tape to ensure no bonding beyond the bonded length. The more significant length of the rebar was extruded from one face of the cube to allow easy installation of the Universal Testing Machine (UTM) capacity of 400 kN.

Figure 3
Bond strength specimen – mould dimensions.

After applying mould-releasing oil to the sides of the mould, a fresh concrete mix with a 75 mm slump was poured into three layers with sufficient compaction. All specimens were cast in controlled conditions, and a trowel was used to produce a flawless surface finish. After 24 hours of undisturbed setting, the specimens were gently demoulded without disturbing the alignment of rebars in bond specimens. The specimens were allowed to cure for 28 days in potable water at room temperature without any instance of pre-corrosion during this period.

2.3. Exposure condition

The selection of exposure conditions and cycles in bond strength testing of concrete is essential for accurately replicating real-world scenarios and comprehending the impact of various environmental factors on the bond between concrete and reinforcement materials. The bond specimens were exposed to accelerated corrosion after 28 days of standard water curing. To accurately reproduce the corrosion phenomenon in real-time marine conditions, the concrete bond specimens were subjected to an alternating wet-dry cycle. Concrete specimens were tested for bond behaviour after exposure to 3, 5, and 7 alternate wet-dry cycles (i.e., 18, 30, and 42 days). Each wet-dry cycle consisted of three days of immersion and three days of drying at ambient temperature for six days (per cycle). The three different corrosion acceleration settings are demonstrated in Figure 4.

Figure 4
Specimens subjected to (a) Water immersion, (b) NaCl immersion (c) Induced current exposure.

2.3.1. Induced current exposure

In the short period available for research, this strategy helps to induce chloride ion infiltration into concrete and achieve higher levels of corrosion. An electrochemical setup was created using rebar inserted in the bond specimen as an anode, a 0.8 mm thick stainless-steel plate suspended near the specimen as a cathode, and a 5% sodium chloride solution as an electrolyte. During the wet cycles, one set of bond specimens was immersed in the salt solution at a constant temperature of 22°C (±2°C). Their protruded rebars were connected to a digital power supply unit with a steady voltage 220V and a current density of 100 µA/cm² [²⁹].

2.3.2. Sodium chloride immersion

During the wet cycles, one set of specimens was immersed in a 5 per cent sodium chloride solution post-conventional curing. These specimens were surface-dried and stored at room temperature for the dry cycles. Many investigations have shown that immersing concrete specimens in salt solutions in alternate wet-dry cycles accelerates corrosion through the diffusion of chloride ions. The salt ions are absorbed at a pressure during the wetting cycle due to the pore pressure formed in concrete during the drying cycle.

2.3.3. Water immersion

After standard curing, another set of specimens was immersed in potable water in alternate wet-dry cycles. Although corrosion is not very pronounced in this case, it was performed to compare the bond behaviour of concrete specimens in the other two exposures.

2.4. Experimental study

2.4.1. Instrumentation setup

The Pull-out test was performed per IS 2770: 1967 – Part-1 (Reaffirmed 2007) [³⁰]. After 3, 5, and 7 cycles of alternate wet-dry exposures, the concrete bond specimens in Figure 5 were surface-dried and installed in the UTM machine to determine their bond performance. The extruded rebar was held tight with pneumatic grips as the concrete specimen was placed inverted from the top of the UTM. For a loading rate of 2250 kg/min [³¹], the slip of the reinforcement was observed and measured using three dial gauges for every 5 kN increase in load.

Figure 5
Wet-dry conditions of specimens.

The bond strength of embedding rods in concrete is thought to be one of the potential markers of the severity of corrosion. During pull-out testing, reinforced concrete elements can fail in three ways: splitting failure of concrete, yielding failure of steel, and pull-out failure due to a failure of the steel-concrete bond. The pull-out force was recorded as the tremendous load applied to the specimen at the time of failure, and the bond strength of the reinforced concrete was calculated using the equation (1).

(1) $$\tau =\frac{F}{\pi LD}$$

where:

Ʈ – Bond Strength (N/mm²)

F – Pull out force (N)

L – Length of rebar in bond with concrete (mm)

D – Diameter of the reinforcement (mm)

Before insertion into bond specimens, the initial mass (mi) of the embedded rebars was measured in each specimen. Following testing, the rebars were collected, cleaned with a metal brush for adhering concrete bits and corrosion products, wiped with a towel, and weighed to obtain the mass of rebar after corrosion exposures (mf), regardless of its failure mechanism. As in equation (2), the percentage mass loss in rebars is used to quantify the level of corrosion obtained.

(2) $$\text{Rebar Loss \% =}\frac{m_i-m_f}{m_i}\ast 100\%$$

3. RESULTS AND DISCUSSION

3.1. Mechanical properties

A total of twelve cubes (150 mm × 150 mm × 150 mm), 12 Cylinders (300 mm height × 150 mm diameter), and 12 Prisms (100 mm × 100 mm × 500 mm) were cast with corrosion inhibitors admixed in different dosages under controlled laboratory settings as per the IS 516:1959. The average compressive strength, split tensile strength, and flexural strength of the cubes, cylinders, and prisms were determined after 28 days of water curing per the Indian standards as listed in Table 3.

Table 3
Mechanical properties of the hardened concrete.

BINDER	INHIBITOR DOSAGE kg/m3	AVERAGE STRENGTH AT 28 DAYS (MPa)
		CS	STS	FS
OPC	0	41.90	3.78	6.72
	0.5	41.17	3.81	6.76
	1	41.39	3.83	6.79
	1.5	41.92	3.83	6.80
	2	42.56	3.85	6.81
	2.5	42.38	3.79	6.72

From this trial, a dose of 2 kg/m³ of inhibitor was chosen as the suitable dosage for the upcoming bond strength investigations, as the drop in mechanical properties of the concrete mix was less pronounced. The compressive strength decreased beyond the optimal mix due to inadequate bond strength between the aggregates and the cement paste [³²]. While corrosion inhibitors initially improve the compressive strength by protecting the steel reinforcement, the onset or acceleration of corrosion can lead to a sudden loss of compressive strength due to the damaging effects of rust formation, internal pressure, and deterioration of the concrete structure [³³]. Calcium nitrite inhibitors have the potential to interact with various elements in the concrete mixture, resulting in by-products that could compromise the concrete’s integrity. For instance, an overabundance of nitrite-based inhibitors might cause the generation of calcium nitrite, which may not enhance the concrete strength.

3.2. Bond strength

All concrete specimens bond-slip curves featured a linear ascending branch till collapse. For inhibitor admixed specimens, the bond stress increased progressively. Adding ductility-improving admixtures can demonstrate the non-linear behaviour of bond specimens [³⁴]. The bond parameters of all 18 specimens tested are listed in Table 4. All the specimens failed due to splitting failure (S), except OC7, which failed due to a combination of pull-out and splitting failure (S-PO). The rebar inside OC7 had undergone loss of ribs, weakening the interfacial bond. The rise in the number of cycles of corrosion exposure was found to disintegrate the friction bond that formed at earlier stages.

Table 4
Bond strength and rebar losses of concrete.

SPECIMEN ID	PEAK SLIP (mm)	PULL-OUT LOAD (kN)	BOND STRENGTH (N/mm2)	REBAR LOSS (%)
OW3	5.11	74	11.78	2.32
OIW3	6.23	82	13.06	1.76
OW5	4.28	65	10.35	2.93
OIW5	5.34	76	12.10	1.89
OW7	3.72	57	9.08	3.27
OIW7	4.23	63	10.03	2.13
ON3	4.92	65	10.35	3.46
OIN3	5.24	74	11.78	2.56
ON5	4.72	62	9.87	3.91
OIN5	4.96	68	10.83	2.87
ON7	3.87	56	8.92	4.78
OIN7	4.56	63	10.03	3.12
OC3	3.96	42	6.69	4.99
OIC3	4.68	56	8.92	3.52
OC5	3.52	35	5.57	5.43
OIC5	4.34	47	7.48	3.68
OC7	3.27	28	4.46	5.78
OIC7	3.86	34	5.41	3.92

• Specimen Id description: O – OPC, I – Inhibitor admixed mix, W – Water exposure, N – Sodium chloride exposure, C – Induced current exposure, 3/5/7 – Number of cycles of exposure

3.3. Effect of the addition of inhibitor

Considering specimens exposed to induced current, adding inhibitors has contributed to the increase in bond strength. The rebar in OC7 underwent severe corrosion, erasing the ribs and causing maximum loss of steel. When the inhibitor was added to the same specimen, the OIC7 rebar loss fell by 72%. The deformation capacity or slip values are also higher for the inhibitor admixed specimens than for the no-inhibitor specimens. The bond strength has increased with increasing exposure cycles for mixes with inhibitors. Calcium nitrite facilitates the development of a protective oxide layer on the surface of steel reinforcement. This layer is a barrier against the penetration of corrosive substances, including chloride ions and moisture. The mechanism of inhibition is based on the adsorption of nitrite ions onto the steel surface, which hinders the anodic dissolution of iron. As a result, the overall corrosion rate is diminished, contributing to preserving the steel reinforcement’s structural integrity.

3.4. Effect of method of exposure

All OPC concrete specimens that included corrosion inhibitors were subjected to failure testing, with the results detailed in Table 4. The specimens containing corrosion inhibitors demonstrated a gradual increase in bond stress. All specimens failed due to splitting and pull-out, with the embedded rebar losing its ribs, compromising the bond with the concrete. An increase in corrosion exposure cycles resulted in the deterioration of the initially established friction bond.

The bond strength of the concrete specimens showed considerable variation depending on the immersion conditions and the presence of corrosion inhibitors. Specifically, the specimens OIW3, OIW5, and OIW7, which contained corrosion inhibitors, exhibited greater bond strength than their counterparts without inhibitors (OW3, OW5, and OW7). As the immersion cycles increased, a general decline in bond strength was observed across all specimens, as shown in Figure 6. For instance, OW3 recorded a bond strength of 11.78 N/mm², which decreased to 9.08 N/mm² in OW7. Additionally, the percentage of rebar loss escalated with the number of immersion cycles. The specimens lacking corrosion inhibitors (OW3, OW5, and OW7) experienced more significant rebar loss than those with inhibitors (OIW3, OIW5, and OIW7). For example, OW3 exhibited a rebar loss of 2.32%, which rose to 3.27% in OW7, while OIW3 began with a rebar loss of 1.76% and increased to 2.13% in OW7.

Figure 6
Bond behaviour of all samples.

The mass loss observed in rebar for OIW3 is minimal, while OC7 experiences the highest loss due to the absence of an inhibitor and the most severe exposure conditions. Under accelerated corrosion exposure, the steel’s cross-sectional area reduction becomes significant. The rebar extracted from OC7 exhibited a substantial decrease in cross-section, as illustrated in Figure 7.

Figure 7
Rebar pulled out of specimen OC7.

3.5. Effect of duration of exposure

Specimens subjected to 3 cycles (18 days) of alternating wet-dry conditions demonstrate higher slip values across all corrosion inhibitor-admixed samples compared to those exposed to 5 cycles (30 days) and seven cycles (42 days). In contrast, specimens without corrosion inhibitors show a different pattern, with peak slip values gradually increasing for water immersion while progressively decreasing for those with NaCl admixture. The slip for ON3 decreases by 11.91% relative to OW3 after just two additional cycles of induced current exposure. Consequently, extending the duration of exposure correlates with a reduction in slip values. The pull-out and bond strength have significantly diminished for specimens exposed to induced current, with reductions reaching as high as 22.72% in OW3 and OW7. For water, NaCl, and appropriate induced exposure, OIW7, OIN7, and OIC7 reveal declines in bond properties of 23.16%, 14.86%, and 39.29%, respectively, when compared to OIW3, OIN3, and OIC3. Additionally, the weight loss in rebars due to corrosion has consistently increased with the number of exposure cycles.

The findings on bond strength in concrete have significant practical implications for marine and coastal structures, which are often exposed to harsh environmental conditions. These implications include enhanced durability, cost savings, safety and reliability, reduced environmental impact, and design improvements [³⁴]. GOODNEWS et al. [²⁹] examined thirty-six 150 mm concrete cubes with embedded steel bars divided into uncoated, corrosion inhibitor-coated, and control groups immersed in a 5% sodium chloride solution for 360 days. Corroded samples exhibited 31–26% lower bond strength and 82–87% higher maximum slip than controls, indicating significant corrosion damage. Inhibitor-coated samples displayed 24–36% higher bond strength and 42–43% lower maximum slip versus corroded samples, demonstrating the inhibitors’ effectiveness in protecting bond properties. However, they did not fully restore the original bond strength. The present study shows a slight reduction in pull-out load, bond slip, bond strength, and rebar losses compared to previous results. Corrosion products in reinforced concrete generally consist of various iron oxides, hydroxides, and iron chlorides. Notable examples include hematite (Fe²O³) and magnetite (Fe³O⁴). These compounds arise from the oxidation of iron when exposed to oxygen and moisture. Common iron hydroxides, such as Fe(OH)² and Fe(OH)³, typically form in aqueous environments and can further oxidize into iron oxides. In coastal or marine settings, the presence of chloride ions can lead to the formation of iron chloride compounds, which exacerbate the corrosion process. The accumulation of these corrosion products can severely compromise the integrity of the steel reinforcement and the surrounding concrete, ultimately diminishing structural strength and longevity [³⁵]. The corrosion products travelled to the outer face of the specimen and stained the exterior, as in Figure 8.

Figure 8
Corrosion products inside a split specimen.

4. CONCLUSIONS

The present has completed the bond behaviour studied with and without inhibitors in the concrete mix. According to the experimental study, the following conclusions are drawn:

• The different exposure methods helped us understand the variation in the severity of corrosion during the corrosion acceleration process. The induced current exposure, though unrealistic, presents a long-term behaviour of concrete specimens under extreme conditions.

• The concrete bond specimens exposed to three wet-dry cycles underwater exposure demonstrated an appreciable bond behaviour. This corresponds to the production of concrete under controlled settings. The NaCl immersion exposure shall be compared to concrete under submerged marine tidal zones in offshore structures.

• An increase in the duration of exposure from 18 to 42 days has exhibited an inevitable fall in all the bond parameters of concrete.

• Inhibitor addition has improved specimens’ ability to undergo deformation with higher peak slip values regardless of the type of exposure. The mass loss in rebar is also considerably less for inhibitor mixes than for no-inhibitor mixes, thus proving the efficacy of concrete usage.

• Inhibitor admixtures can be recommended for concrete construction in corrosion-prone constructions to ensure sufficient protection to reinforcement bars and increase the time for corrosion onset under severe exposure conditions.

• The present study examines the bond strength of concrete under various environmental conditions. Furthermore, the analysis can be analysed using different types of cement. Incorporating inhibitor admixtures is advisable for concrete structures susceptible to corrosion, as they provide essential protection for reinforcement bars and extend the duration before corrosion begins, particularly in harsh environmental conditions.

5. ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial grant approved for this research, during the 33rd meeting of the Board of Governors, conducted at the Government College of Technology in Coimbatore, Tamil Nadu (India) through the institution’s TEQIP III Cell.

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Publication Dates

• Publication in this collection
  02 June 2025
• Date of issue
  2025

History

• Received
  06 Dec 2024
• Accepted
  12 May 2025