Showing papers by "Radhakrishna G. Pillai published in 2021"

Long-term performance and life-cycle-cost benefits of cathodic protection of concrete structures using galvanic anodes

Abstract: This paper presents a market study indicating that Patch Repair without galvanic anodes (PR strategy) can lead to continued corrosion (due to the halo effect and residual chloride effect) and another major repair in about five years Repeated patch repairs can lead to continued corrosion and eventual replacement of structures and huge life cycle cost (LCC) On the other hand, the strategy of cathodic protection using galvanic anodes (CP strategy) can enhance the service life and reduce LCC The data on long-term depolarized potential of steel, output current from the anodes and/or visual observations indicated that the galvanic anodes were successful in controlling the chloride-induced corrosion for up to 14 and 10 years, in a jetty and industrial building, respectively It was also found that the additional cost of galvanic anodes is only about 4% of the repair cost for the jetty structure – breaking the myth of high capital cost of CP strategy Then, a framework to estimate the LCC of PR and CP repair strategies is developed and it is found that CP and cathodic prevention (CPrev) strategies are highly economical than the PR strategy Also, the LCC of 30 repair projects confirmed that the use of CP strategy can lead to LCC saving of up to about 90% in about 30 years after the first repair More importantly, the CP and CPrev strategies can enhance the service life to as long as needed by the replacement of anodes at regular intervals and at minimal cost Also, a way forward to promote CP strategy in concrete repair industry is provided

Carbonation model for concretes with fly ash, slag, and limestone calcined clay - using accelerated and five - year natural exposure data

Abstract: Supplementary cementitious materials (SCMs) can be used in concrete to enhance sustainability and reduce the concrete industry's carbon footprint. However, some negative perceptions about their long-term carbonation resistance are obstacles for large-scale implementation of such concretes. This study evaluated the carbonation resistance of 34 concretes (with Ordinary Portland Cement, fly ash, blast furnace slag, and limestone calcined clay) in natural tropical exposure conditions (Open and Sheltered) for 5 years and in accelerated exposure conditions (1 and 3% CO2) for 112 days. Using these data and the square root of time function, the carbonation coefficients (KCO2, natl and KCO2, accl) of these concretes were estimated and a good correlation between them could not be observed. Hence, a more generic model (named as “A-to-N model”) to estimate the KCO2, natl using the KCO2, accl, CO2 concentration, and mixture proportion of concrete was developed, for which the mean absolute percent error is about 12% (reasonable accuracy). Using the A-to-N model, the carbonation depth at 50 years was estimated for various concretes. SCM concretes with low water-binder ratio and optimal binder content showed high resistance against carbonation at later ages; such information along with the target cover depth must be used while selecting materials for concrete design. Based on the model developed, a relatively simple ‘service life design chart’ was developed. This chart can be used by engineers to set the target KCO2, natl or KCO2, accl, and select the cover depth and binder type to provide the target service life (i.e., corrosion initiation time). This paper clearly shows that SCMs can be used to design concretes with comparable long-term carbonation depth as OPC concretes.

Long-term performance of galvanic anodes for the protection of steel reinforced concrete structures

Abstract: Corrosion is one of the major deterioration mechanisms of reinforced concrete structures . The conventional patch repair without addressing the root cause of the corrosion can lead to repeated repairs. Therefore, a form of cathodic protection (CP) using galvanic anodes is a viable electrochemical technique to mitigate corrosion. However, practitioners hesitate to adopt CP for repair due to the lack of evidence and limited knowledge on the long-term performance of galvanic anodes in concrete systems. For this, two reinforced concrete panels with and without discrete galvanic anodes were cast with admixed chlorides and exposed to a natural environment for 12 years. Electrochemical measurements, such as depolarized corrosion potentials and corrosion rate of the rebars, and output protection current density of the galvanic anodes were measured. In addition, physico-chemical characteristics such as elemental composition , residual lithium content, pH, pore volume, and pore size distribution in the encapsulating mortar were determined on a 12-year in-service galvanic anode. This paper indicates that the alkali-activated galvanic anodes can protect the steel rebars from corrosion for at least 12 years. Analysis after 12 years showed that the pores in encapsulating mortar were partially filled with zinc corrosion products , resulting in substantial pore blockage surrounding the zinc metal. This led to a reduction in the pH buffer in the vicinity of the zinc metal. Also, characteristics of tie wire-zinc metal interface may affect the long-term performance of galvanic anodes. Based on this study, specifications are proposed to help manufacturers to design durable galvanic anode systems.

Initiation of Stress Corrosion Cracking in Cold-Drawn Prestressing Steel in Hardened Cement Mortar Exposed to Chlorides

Abstract: Cold-drawn, high-strength, prestressing (PS) steel strands are widely used in pretensioned concrete (PTC) structures. This paper discusses the stress corrosion cracking (SCC) of PS steel embedded in cement mortar and gradually exposed to chlorides. Various stages of the passive to active (P-to-A) transition, which marks the onset of SCC, were investigated using the electrochemical impedance spectroscopy technique. The key mechanisms were identified and confirmed using scanning electron microscopy/energy dispersive x-ray analysis, x-ray diffarction, and confocal Raman spectroscopy. It was found that the passive film on unstressed PS steel has better electrochemical characteristics than that on conventional steel rebars. However, the residual tensile stress at the surface of PS steels can assist passive film cracking after chloride attack—contrary to the pitting corrosion without cracking of passive film in conventional steels. Further, tests indicated that the concentration of chlorides required to crack the passive film in PS steels can reduce by about 50% when prestressed—as in field structures. Chemical composition, stress state, and microstructural features at the PS steel surface were identified as possible factors influencing the initiation of SCC in PTC structures.

Performance indicators and specifications for fusion-bonded-epoxy(FBE)-coated steel rebars in concrete exposed to chlorides

Abstract: Fusion-bonded-epoxy (FBE)-coated steel rebars have been used in many concrete structures in anticipation of better corrosion resistance. However, due to premature corrosion observed, FBE-coated reb...

Effect of Test Methods on Corrosion Phenomena of Steel in Highly Resistive Concrete Systems and Data Interpretations

Abstract: Fly ash and limestone calcined clay cement (LC3) are being used in concrete to enhance chloride resistance. In this study, 60 specimens (with steel in three separate binder systems, namely 100% ordinary Portland cement [OPC], 70% OPC + 30% fly ash, and LC3 with surface resistivity of ≈10 kΩ·cm, ≈25 kΩ·cm, and ≈200 kΩ·cm, respectively) were subjected to impressed corrosion, and the results were compared with 15 lollipop steel-mortar specimens subjected to natural corrosion under a wet-dry chloride environment. It was found that the traditional method of impressed corrosion tests can induce microstructural changes in highly resistive concrete cover and at the steel/concrete interface; hence, these methods are not suitable for evaluating corrosion resistance (such as corrosion rate and corrosion-induced cracking) in highly resistive concrete systems. Further, the Raman spectra from the corroded steel surfaces indicated that the impressed corrosion and natural corrosion tests led to different forms of corrosion (i.e., uniform and pitting, respectively) and different compositions of corrosion products (i.e., α-Fe2O3 and β-FeOOH phases). This led to different expansive stresses, making the lab-to-field correlations inappropriate in the case of highly resistive concrete systems. This paper recommends natural corrosion tests exposed to wet-dry conditions and not the impressed corrosion tests for assessing corrosion phenomena of steel in highly resistive concrete systems.