The anti-degradation consequences of water repellent-based inhibitors for controlling mild steel corrosion in concrete composite

. Synthetic water repellent (WRep) generally blocks the ingress of corrosive factors like moisture and various gases to the reinforced steel (RS) surface through the concrete pores. Mixing such WReps in the concrete mix did not affect the anti-corrosive response to the RS infrastructures. Considering such consequences, the present work explored the uses of two synthetic water repellents (e.g., WRep-A & WRep-B) and plant-derived green inhibitors to control the corrosion stability level of the RS in the concrete matrix. The broader question of the work is to know whether the water-repellent affects the anti-degradation response to the steel in concrete at the same patterns as commonly practiced in the concrete matrix or differently when the cast reinforced concretes exposed to water-repellent suspension for a certain curing time. The concrete slab exposed for one week to the suspension mixture of 1000 ppm WRep-B+ Mangifera indica (WRep-B+MILE) and 1000 ppm WRep-B+ Psidium guajava (WRep-B+PGLE) extracts exhibited maximum anti-corrosive response, which is justified by corrosion potential shifting to a more positive potential zone where the reinforced steel corrosion damage state rated as <10% probability and anticipated more effective corrosion inhibiting activities compared to other concentrations used in this work.


Introduction
Reinforced concrete infrastructures (RCIs) are generally porous materials prone to corrosion damages at their early service time, mostly due to the ingress of a water molecule and the various aggressive ions from the surrounding environments [1].These corrosionprone agents are controlled by factors like the composition of concrete, additives, waterproofing chemicals or water repellent (WRep) so on [2].Such water-related corrosion problems of the RCIs could be minimized by using the WRep.Therefore, most commercial and residential concrete projects specify using of the WRep.They are principally functional for long-lasting protection of the high-consequence RCIs like bridges, dams, flyovers, multistory reinforced concrete buildings so on in a highly corrosive environment [3].Cement-based coatings are also practiced in making the concrete impermeable to moisture or aggressive ions, and they plug the pores and capillaries of the RCIs [4].However, cement-based coatings have some demerits of poor adherence and low resistance to electrochemical corrosion attacks, which cause a limited service life, and stability of the RCIs [5].
On the other hand, cement-based WRep offers several advantages over cement-based coatings.It noticed that the ingress rate of the moisture and aggressive ions/gases from the surrounding atmosphere to the reinforced metal surface through the capillary porosity of the concrete structures is assumed to be proportional with the fourth power of the capillary radius [6], which affects the critical flow factor of aggressive gases/ions including water into the concrete structures.Therefore, using cement-based water-repelling admixtures to reduce the capillary radius is necessary to decrease the electrochemical degradation rate of the RCIs [7].Such water-repelling chemicals are enhanced to modify the microstructure features and durability improvement of the concrete mixes by forming pore-blocking structures/phases [8].
It reported that the addition of the WRep showed a significant effect on the water penetration depth and permeability characteristics of the RCIs, mostly due to the development of pore-hindering crystalline phases [9].Untreated concrete samples were uncovered with these pore-blocking phases [10].The water-repelling with fly ash or silica fume helps to compact the hydrated concrete mixes by enhancing the calcium silicate hydrated (C-S-H) gel formation.[11].Besides, the waterproofing agents upgrade the denseness and moisture impermeability due to the partial filling of the pores, voids, or cracks by insoluble C-S-H gels in hydrated concrete infrastructures [12].Besides, it reported that the volumes of the fine capillary pore (i.e., 0.01 μm diameter or smaller) increased with the addition of the water-repelling admixtures.They improved the resistance against moisture, chloride, CO 2 , and SO 2 diffusion into the RCIs by forming Friedel's salt [13].
Using WRep in concrete mix or on the surface of the infrastructures can prevent water or aggressive ion penetration into the inner layers of the RCIs.Therefore, the use of WRep admixture in concrete enhanced to escalate the shielding nature of the RCIs, even exposing them for long periods in reactive environments like moisture, CO 2 , SO 2 , sulfate, chloride so on [14].Moreover, using the cement-based WRep is practiced, aimed at decreasing the porous nature of the concrete matrix [15].It shows that using a waterproofing agent in concrete gifted to a marked increase in mechanical properties.According to the experimental data of the compressive and tensile tests, it had been reported that the concrete mixes with a small amount of the WRep enhanced the mechanical properties of the concrete structures [16].
There are different brands of commercial WRep in the market of the Kathmandu Valley, and each has slightly different physical and chemical properties.All the companies have WRep products with almost the same action of blocking the penetration or ingress of moisture and pollutant gases to the reinforced steel through the concrete pores.However, the basic properties of each WRep product have slightly different.The users are not aware of such truths.They have been executing all the WRep products in the same utilization method.As a result, all the commercially available WRep products do not accomplish the expected achievement [17].
In a normal manner, the different ways of using any WRep products in the concrete matrixes could be performed in different ways, such as by a homogeneous mixing of the WRep chemical in the matrix before casting the RCIs, by coating the WRep suspension on the surfaces of the RCIs [18], or by exposing the cast RCIs in the WRep suspension so on.As reported in the previous works, mixing sufficient concentrations of the commercially available two synthetic WReps in concrete matrixes did not potentially affect the anticorrosive response of the reinforced steel in concrete slabs after three months or more exposure periods in an ambient atmosphere [19].The additions of 500−4000 mg/L of both the waterproofing additives did not show an effect to shift the corrosion potential in the areas of less corrosion zone of steel bar in the reinforced concrete slab [20].
Considering the above studies, the present study aimed to compare the effectiveness and efficiency of two water repelling additives (WRep-A, WRep-B), commonly used in concrete works in the Kathmandu Valley, and to investigate the anti-degradation consequences of two water repellent-plant based mixed inhibitors for controlling the mild steel corrosion in concrete composite.The broader question of this investigation is to know whether the WRep affects the anti-corrosive response to the reinforced mild steel in concrete structures at the same patterns, as usual, applied the water-proofers in the matrix or differently when the cast RCIs exposed in the WRep suspension for fixed curing time.

Materials and Methods
The commercially used ordinary Portland cement (grade-53) powder, locally available fine sand, coarse gravel (2 cm diameter), and rust-free mild steel rod (with 1.2 cm diameter) were applied for the preparation of concrete slabs with rectangular shaped (40 cm × 24 cm × 4 cm), as recommended by the ASTM C1582/C1582M-11 standard [21].The mild steel rod [Fig.1(a)] used herein as a reinforced metal has the elements (in weight %); carbon (0.17 to 0.25), silicon (0.4), manganese (0.9), sulfur (0.05), phosphorus (0.05), and remained Fe [20].The cement, sand, and gravel were mixed in a ratio of 1:2:3 [Fig.1(b)], and then the concrete mixture was made with the help of distilled water (0.45 w/c) to prepare a total of eight sets of steel-reinforced concrete (SRC) slab using a wooden box [Fig.1(c)].Two mild steel rods with 8 cm spacing between them were embedded at equidistant in the concrete mixture to prepare the SRC slab, and one end of each embedded steel rod was left exposed for the half-cell potential measurements, as explained elsewhere [19].Two water-repellents named WRep-A and WRep-B (undisclosed the commercial name for the ethic issue), which commonly use in the construction sector in Kathmandu Valley, were selected to accomplish this study.The physicochemical properties of water-repellents, as concrete additives were reported, and the results are summarized in Table 1.Also, the methanolic leaf extracts of Mangifera indica (MILE) and Psidium guajava (PGLE) powder were separately used to prepare two hybrid inhibitors with the synthetic WRep-B (i.e., WRep-B+MILE, WRep-B+PGLE), which were further applied to study their corrosion inhibition effects to the RS in the concrete composite.The details of the preparation methods of the MILE and PGLE were described elsewhere [22].

Table 1. Physicochemical properties of the water-repellents Wrap-A and WRep-B.
The eight molded SRC slab specimens were removed from the molding box after 24 hours of their casting.Among these SRC slabs, five sets of the slabs were immersed for seven days in aqueous solutions of 1000 ppm, 2000 ppm & factory advised dose (FA-dose or i.e. ~3700 ppm) of WRep-A, and 1000 ppm and 2000 ppm WRep-B waterproofing agents, separately.Similarly, the other two concrete slabs were immersed in aqueous solutions containing 1000 ppm (WRep-B+MILE) and 1000 ppm (WRep-B+PGLE).These seven SRC slabs were immersed for one week for wet curing.After the wet cured of these seven SRC slabs, they were kept in an outdoor dry atmosphere for one week, and then their half-cell potential or corrosion potential (ϕ corr ) was started to monitor at different time intervals for up to four months or more.The remaining one more SRC slab without the addition of WRep (named "control SRC slab" hereafter) was used to record its ϕ corr after the 7 th day of the casting in the wooden box.
The ϕ corr of four points of each steel bar was recorded on a digital voltmeter (UNI-T Business, Hong Kong) using a reference saturated mercury(I) chloride electrode (SCE).The exposed portion of the RS in concrete was used as the working electrode, as described elsewhere [23].Eight points, marking point-1, point-2, point-3, point-4, point-5, point-6, point-7, and point-8 at equidistant from all sides of the slabs were marked on the upper surface of each concrete slab.The reference electrode and exposed part of the RS rod were connected to negative and positive terminals of the voltmeter, respectively, for the ϕ corr measurement of all eight points of every slab at various time intervals for four months or more.Based on the recorded ϕ corr , different corrosion conditions of the RS in the concrete slab were confirmed based on the ASTM C876-15 standard [24,25].The probability of the reinforced concrete condition can be established with the recorded ϕ corr , as summarized in Table 2.   Almost the same trend of the ϕ corr change was observed for the remaining three SRC slabs after immersion for one week in 1000 ppm, and 2000 ppm WRep-B, which are illustrated in Figs.3(a), and 3(b), respectively.Also, Fig. 3(c) showed the ϕ corr change trend of the SRC slab without the WRep addition (i.e., control slab) for comparative study.The recorded ϕ corr values of every point of all these seven SRC slabs, except the control slab (i.e., without WRep) are slightly shifted to the more corrosive direction of the HiC state, especially the exposure time between 1-3 weeks at the outdoor atmosphere of the department.Such shifting of the ϕ corr towards the less noble potential direction up to three weeks is probably due to the instability of the RS in the curing period of the concrete slab for the initiation of an insoluble calcium silicate hydrate (C-S-H) gel formation [12].However, after three weeks of exposure, the ϕ corr of all the concrete slabs, except the control slab is shifted toward the more novel direction (i.e., MiC state) by exposing all the concrete slabs for about 3-7 weeks.The process of shifting the ϕ corr towards the less than 10% corrosion possibility region (LoC state) is continued except for the ϕ corr of the control SRC slab [Fig.3(c)], and eventually attained almost a steady state ϕ corr up to 4 months or more.Consequently, it can be assumed that the time-dependent C-S-H is a main binding polymeric phase in the concrete slab to block the ingress of moisture and atmospheric gases through the concrete pores to the reinforced steel surface [26].Moreover, as reported in previous studies that the amounts of the C-S-H phase formation at the initial time of hydration were found lower than that at a long time [27].This indicates an increase in the C-S-H phase formation with curing time.Therefore, the time-dependent properties of the C-S-H gel are competent in provoking the curing effect of concrete mixes.They have significantly high corrosion-resistant properties leading to the SRC corrosion control by a continuous shifting of the ϕ corr towards the less than 10% corrosion possibility regions.
Besides, the diffusivity of CO 2 , O 2 so on gases are reported high at the initial curing periods of the carbonated concrete structures [28,29].Hence, it is expected the shifting of the ϕ corr to the more negative potential (i.e., less noble) area of the HiC state till the 3 rdweek measurement.Moreover, obvious corrosion can be anticipated on the surface of the RS with mill scale than without the mill scale [30].Herein, it is meaningful to mention the possible facts that the mild steel rods have a series of ridges and grooves with rust, which might not be eliminated using sandpaper (No. 200-1500) to make a shining surface.In another study, the diffusivity of different environmental gases in the concrete depends on the moisture/relative humidity.The diffusivity of atmospheric gases was constant when the relative humidity was < 60% [31].
Apart from the above results of the corrosion change, Figs.4(a) and 4(b) exhibited a comparable change in the mean ϕ corr with the standard error bar of six types of SRC slabs.The results revealed that the mean ϕ corr of the first three SRC slabs with 1000 ppm, 2000 ppm, and 3700 ppm (FA-dose) WRep-A, except the control slab, is continuously changed from >90% corrosion probability region (i.e., HiC state) to <10% corrosion probability area (i.e., LoC state) with exposure time till 17 weeks or more, as illustrated in Fig. 4 [32], which has high efficiency to prevent the corrosion damage of the SRC slabs [33].Also, the C−S−H makes it impenetrable by filling cracks and capillary pores of the concrete composites [18].Consequently, the SRC slabs curing for one week in solutions of 1000 and 2000 ppm water-repellent agents significantly improved their corrosion-resistant properties.It is worth noting that the standard deviation of most of the 97% recorded ϕ corr is comparatively small, i.e., <10 mV, which suggests that localized types of corrosion of the RS in concrete composite might not be expected.
In recent years, interest has been prospering to use of plant-based additives in the concrete mix for altering reinforced concrete properties, especially the corrosion-inhibitory nature [19,[34][35][36].Because, they are environmentally green, and could be produced in large amounts at a low price without using sophisticated techniques [37].Considering such rational explanation and fact, the inhibitory ability of two hybrid corrosion inhibitors (i.e., WRep-B+MILE, WRep-B+PGLE) was also investigated, which was explored from the change of the electrochemical ϕ corr data with exposure time, and the results are illustrated in Fig. 5(b).The results suggested that both the hybrid inhibitors facilitate the corrosionresistant properties of the concrete matrix.
The experimental data revealed that both the hybrid inhibitors composed of waterrepellent and green-based plant extract can gain higher corrosion inhibitory efficiency than the addition of a single water-repellent or plant extract, as elucidated in Fig. 5(c), probably due to the formation of C−S−H/phyto-molecule complex films on the surface of the RS.The protective film greatly blocked the ingress process of moisture, corrosive gases, and the active areas of corrosion reaction of the SRC.Previous studies also reported such a dramatically hindering of the active corrosion sites of the corroded metals/alloys by the hybrid inhibitors of the mixture of inorganic salt and organic compound [38,39].Also, due to the formation of the C−S−H/phyto-molecule complex protective film, the RS in concrete composite with WRep-B+MILE or WRep-B+PGLE displayed a notably high corrosion resistance from the shifting of the ϕ corr towards the region of the less corrosive degree.The high corrosion resistance effect of these two types of hybrid inhibitors to the SRC slab is attributed mostly due to the presence of high amounts of secondary metabolites (i.e., phenols, alkaloids, flavonoids, saponin, steroids, tannins, terpenoids, etc.) in the plant extracts [22,40], as noticed from the FTIR analysis data of these two types of plant-based extracts (Table 3).Such phyto-molecules of the plant extract are constituted mainly of O, N or S hetero-atoms together with aromatic, and π-electron-rich compounds in the methanol extracts of the plant-based materials [41], and they show generally high basicity including electron density properties [42].
The presence of such phyto-molecules in the plant extract can substantiate their binding onto the surface of the iron metal ions formed on the corroded reinforced steel rod [43], which enhances the formation of a protective passive film on the corroded reinforced steel rod by adsorption process and hence improve the anti-corrosive influence, as described elsewhere [44].Besides, the MILE extract-based additive performed better corrosion inhibiting action than the water repellents to control the SRC corrosion by enhancing the formation of C-S-H gel in concrete composites.The presence of these functional groups in the plant extract is evocative of the entanglement in the process of the phyto-molecules binding with iron ions to form a protective passive film [45,46].As illustrated above in Fig. 3(c), the corrosion potential of the RS in the control concrete mix shifted to a less noble ϕ corr direction to the MiC state after the 28 th -day exposure, and it attained a quasi-steady state till 4 months of exposure.The experimental fact is also supported by the optical images of the RS surfaces, as demonstrated in Fig. 6.The image of the RS after 120 days in the control concrete mix shows a significantly corroded morphology, as noticed in Fig. 6(b).However, the surface of the RS rod in the concrete with 1000 ppm MILE extract is smooth with shining surfaces [Fig.6(c)] like as the mild steel surface before being embedded in the concrete mix [Fig.6(a)].This is due to the inhibition effect of the MILE extract and the hybrid MILE-WRep inhibitor, which effectively defeated the corrosion reaction occurring on the mild steel surface and modification of the concrete composite.Also, the optical images corroborated a potent method of regular ϕ corr monitoring for ascertaining different corrosion probability conditions of the reinforced concrete.

Conclusions
The corrosive degradation of the RS restricts the tenacity and functionality of the concrete structures.Such an unavoidable degradation problem could be impeded using anti-corrosive substances like water-repellents and plant-based phyto-molecules.The anti-corrosive properties of two water-repellents and their hybrid corrosion inhibitors with two plantbased extracts (i.e., MILE and PGLE) were explored for the first time to control the corrosion problems of the reinforced concretes using the ϕ corr method according to ASTM procedures in present works.
The experimental results indicate that exposing the SRC slab for one week in both water-repellents and their hybrid inhibitors with plant-based extracts at various concentrations has a remarkable effect on shifting the ϕ corr towards the less corrosive regions (i.e., <10% probability of corrosion risk) of the reinforcing steel bars in concrete slabs.The plant-based new hybrid Wrap-B-MILE acted as an efficient inhibitor to control corrosion of the embedded mild steel in the SRC slab.The ϕ corr measurement method of the ASTM standard is based on probabilistic reasoning, which contributes only an approximate assignment of the probability of a corrosive state rather than the true corrosion rate of the concrete structures.
The consequence of this study would be a milestone for outstanding contribution in ameliorating and advancing the industrial production of novel plant-based hybrid waterrepellents as anti-corrosive concrete additives in the future after more detailed studies of the corrosion control mechanism and kinetics using more electro-chemical, phase analysis and mechanical properties.

Figure 1 .
Figure 1. Materials for reinforcing concrete slab preparation; (a) mild steel rods, (b) sand-gravelcement powder, (c) concrete mix, and (d) casting SRC slab in a rectangular box.

Figure 2 .
Figure 2. Changes of corrosion potential of the SRC slabs, which were immersed for one week in aqueous solution with (a) 1000 ppm, (b) 2000 ppm, and (c) RA-dose (~3700 ppm) WRep-A, as a function of exposure time.

Figure 3 .
Figure 3. Changes of corrosion potential of the SRC slabs, which were immersed for one week in aqueous solution with (a) 1000 ppm, (b) 2000 ppm WRep-B, and (c) at control condition without WRep-B, as a function of exposure time.
(a).Also, the identical behavior of the change of the mean ϕ corr of the remaining two SRC slabs with 1000 ppm, and 2000 ppm WRep-B is recorded at various exposure times, as demonstrated in Fig. 4(b).Moreover, the SRC slabs with 1000 ppm of both WRep-A and WRep-B have almost the same trend of change of the mean ϕ corr with each exposure day, which is shown in Fig. 4(c).

Figure 4 .
Figure 4. Changes in the mean corrosion potential (ϕ corr ) show the standard error bar of six SRC slabs with different concentrations of WRep-A and WRep-B, as a function of atmospheric exposure time.The same trend of the change of the mean ϕ corr of the slabs with 2000 ppm WRep-A and WRep-B is observed from the results shown in Fig. 5(a), although a slightly more noble mean ϕ corr is recorded for the slab with 2000 ppm WRep-B as compared with the 2000 ppm WRep-A slab, particularly after two months or more exposure times.Consequently, the use of 1000-2000 ppm of both the WRep-A and WRep-B in concrete composites enhances the formation of calcium silicate hydrate (C−S−H) gel as concrete pore sealer[32], which has high efficiency to prevent the corrosion damage of the SRC slabs[33].Also, the C−S−H makes it impenetrable by filling cracks and capillary pores of the concrete composites[18].Consequently, the SRC slabs curing for one week in solutions of 1000 and 2000 ppm

Figure 5 .
Figure 5. Changes in the mean corrosion potential with the standard error bar of different types of SRC slabs, as a function of atmospheric exposure time.

Figure 6 .
Figure 6.Optical images with 20 times resolution of (a) a fresh mild steel, and embedded (b) at control and (c) with 1000 ppm MILE concrete composites for about four months exposure in atmospheric condition.

Table 2 .
Mean ϕ corr and corrosion risk probability of the RS rod in concrete.
and MILE (WRep-B+MILE) or WRep-B and PGLE (WRep-B+PGLE) were investigated to know their potentiality for controlling the corrosion of the reinforced-steel in a concrete mix.For this purpose, regular monitoring of the variation in the ϕ corr of 8 surface points of each SRC slab was recorded at different exposure times, more than four months, and the results are illustrated in Fig.2.The corrosion potential (ϕ corr ) values of all the points of all the SRC slabs analyzed herein are located in LoC state, i.e., more noble potential regions

Table 3 .
FTIR peaks of the MILE and PGLE.