Migrating Inhibitors for Preventing Carbonation Induced Corrosion of Reinforced Concrete

Abstract

Since its inception in the mid 19th century, Reinforced Concrete (RC) has become the most widely used construction material in the world. However, deterioration of RC structures, due to exposure of rebar to aggressive environment, has become a major concern all around the world. De-icing salts and carbonation are the two major factors responsible for the deterioration of RC structures. During the 20th century, extension in industrial emissions have raised the CO2 levels in air drastically, which has further led to increase in the carbonation rate of concrete and carbonation related distress of RC structures. Rebar corrosion ultimately affects the service life of the RC structures and rehabilitation of such structures has become a great challenge both technically as well as financially.

The prevention of rebar corrosion can be handled using several techniques such as denser concrete, latex or polymer modified concrete overlays, waterproofing membrane with asphalt overlays and cathodic protection to the rebar etc. One amongst these methods involves the use of corrosion inhibitors, which provide a simple and cost effective solution for prevention of corrosion of RC structures. Corrosion inhibitors are chemicals that are able to reduce corrosion rate and are generally used in relatively small quantities.

Based upon the mode of application, corrosion inhibitors are generally classified into two categories: (a) admixed corrosion inhibitors which are mixed during casting of concrete and (b) migrating corrosion inhibitors, which are applied on the surface of concrete during rehabilitation procedures and they subsequently diffuse into the hardened concrete to reach the rebar level thereby developing their inhibiting action at the rebar surface. Migrating inhibitors have an advantage over admixed corrosion inhibitors in terms of their negligible effect on cement hydration and properties of the resultant concrete. Present study explores the effectiveness of the migrating type organic based corrosion inhibitors in controlling carbonation induced corrosion in reinforced concrete.

The initial choice of chemicals to be used as corrosion inhibitors was based on the preliminary investigations carried out on the simulated pore solution in the form of saturated solution of calcium hydroxide bubbled with pure CO2 to reach a pH value of 8.1. In the preliminary investigation, eight types of corrosion inhibitors were tested for their effectiveness in carbonated conditions. The effectiveness of the bare steel specimens immersed in different solutions was assessed through the Potentiodynamic Polarization technique at a sweep rate of 60 mV per minute over a potential range of ─250 mV/SCE to 1500 mV/SCE, with an offset from the rest potential. Each time the measurement was performed on an undisturbed sample because the adopted high potential range during testing was considered to disturb the actual corrosion activity of the specimens. The testing was carried out till the exposure duration of 240 hours. Recorded polarization curves indicated that out of all the tested inhibitors, generic inhibitor 2-Aminobenzoic acid and amino alcohols based commercially available inhibitor C.I4 performed most efficiently, with an inhibitor efficiency of 61% and 71%, respectively. Also, both these inhibitors were able to keep the inhibiting action throughout the exposure period of 240 h. These two inhibitors were further investigated for their effectiveness as migrating type inhibitors in carbonated reinforced concrete specimens.

The performance of commercially available inhibitor C.I4 (SFG) and generic inhibitor 2-Aminobenzoic acid (ABA) applied as migrating type inhibitors was assessed for concrete mixture made at two different w/c ratios. The investigation was carried out by using both OPC and PPC cements in concrete. The carbonation acceleration of the specimens was achieved in carbonation chamber maintained at a relative humidity of 60-70%, a temperature of 30±2ºC and having 5±0.5% CO2 concentration by volume. The progress in carbonation front was monitored by the carbonation depth measurements made on 100 mm cube specimens. The experimental results indicated that the carbonation depth increases with an increase in w/c ratio. On making a comparison based upon the cement type, higher carbonation depth was reported in concrete made with PPC as compared to OPC cement because of lower content of portlandite present in PPC cement. Further, the treatment of concrete surface with inhibitors namely, SFG and ABA decreased the carbonation rate coefficient as compared to corresponding control specimens. With the application of inhibitor SFG the decrease in carbonation coefficient was 18-36% while a 29-46% decrease was observed in ABA treated specimens. This reduction in carbonation coefficient is attributed to the pore blockage effect of corrosion inhibitors. The 150 mm cube specimens tested under compression did not show any detrimental effect of applied corrosion inhibitors on the compressive strength of resultant concrete.

The effect of inhibitor application on rebar behaviour under accelerated carbonation conditions was studied by using electrochemical techniques viz. Half Cell Potential measurement (HCP), Tafel Extrapolation and Electrochemical Impedance Spectroscopy (EIS). For electrochemical measurements, prism specimens of size 300 × 300 × 150 mm were prepared with three steel rebars embedded into it; two steel rebars were embedded at the bottom with cover of 25 mm and one rebar was placed at the top with clear cover of 5 mm. The carbonation acceleration exposure conditions were kept same for carbonation depth measurement. HCP and Tafel extrapolation measurement were performed on prism specimens made with two different types of cement (OPC and PPC), two w/c ratios (0.43 and 0.52) and two inhibitor applications (SFG and ABA), while EIS was performed on concrete specimens made with two different types of cement, two inhibitor applications for concrete mixture designed for a w/c ratio of 0.43. All electrochemical measurements were made with the help of Advanced Corrosion Monitoring (ACM) field machine provided with the guard ring arrangement.

HCP results demonstrate higher corrosion risk in PPC concrete as compared to OPC concrete. HCP values for mixes made with PPC lie in the range corresponding to 90% corrosion risk; while in OPC concrete intermediate corrosion risk was observed till the exposure duration. Similar with the HCP value, corrosion current density (icorr) values also indicate less resistance of PPC concrete as compared to OPC concrete towards the carbonation induced corrosion. The application of inhibitors proved to be effective in retarding corrosion risk, for all concretes. However, the surface applied inhibitors took some time to migrate through the concrete pores to reach the rebar level. Once the inhibitors reached the rebar surface, they were able to form a protective layer around the rebar. It was demonstrated by HCP values which shifted towards more negative potential side till 30 days of exposure, followed by shift towards nobler potential side. Observed lower icorr value with the inhibitor application shows that inhibitors had reached the rebar surface and formed a protective layer around the rebar surface.

The physical interpretation of the various processes involved during rebar corrosion in carbonated environment could be studied through EIS measurement. In the present study, EIS was used as a non destructive technique (NDT) for studying the carbonation behavior of the concrete specimens treated with migrating type organic corrosion inhibitors. During the EIS measurements, an AC signal was supplied with a 25 mV RMS for a frequency range of 100 kHz to 10 mHz with five points obtained per decade. The interpretation of the obtained impedance plots were done by using equivalent electrical circuit in ZMAN software. It was observed that the two capacitive arcs obtained at high frequency and low frequency during EIS measurements could be correlated to the bulk properties of concrete and the rebar, respectively. Increase in the first capacitive arc of the Nyquist plot obtained at high frequency represented decrease in porosity of concrete with progressive carbonation resulting in denser microstructure, while decrease in second capacitive arc obtained at low frequency corresponded to the breakdown of protective oxide layer on the rebar surface. The concrete surface treated with inhibitors exhibited behaviour different form the control specimens. Both of the tested migrating type inhibitors were able to form a passive layer around the rebar surface and their efficiencies improved with time. Inhibition efficiency of 72% and 80% respectively, was achieved in OPC and PPC concrete with inhibitor SFG at the end of exposure duration of 90 days. Similarly, the inhibitor efficiency of 78% and 91% was observed with inhibitor ABA in OPC and PPC concrete, respectively.

Impedance results indicate that carbonation depth can be predicted by analyzing the high frequency arc of Nyquist plot. The developed model shows a strong correlation between the impedance difference measured between 10 Hz to 10 kHz frequency range, fitted resistive parameter R1 and carbonation depth measured by phenolphthalein indicator, with a very high regression coefficient of 0.9. The variation in the predicted carbonation depths by using developed relations and measured carbonation depths is below 15% for most of the exposure durations. Thus, all these relationships can be used effectively for predicting the carbonation depth from EIS, which is a popular NDT used for in-situ health monitoring of RC structures.