Critical Chloride Content in Reinforced Concrete: A Review

The author details the reactions of various concretes on steel reinforcement. Although portland cements, slag cements and high alumina cements are all hydraulic binders, each possess special properties which are examined. The discussion of causes and methods of preventing the corrosion of steel reinforcement covers such aspects as galvanised steel reinforcement, effects of concrete composition, corrosion of steel reinforcments in concrete and prestressed reinforcement. It is concluded that the most significant influences on the corrosion of prestressing wire in concrete are: the presence of chloride; the presence of nitrates; the composition of the concrete; the degree of carbonation of the concrete; concrete compaction and, chlorides and sulphates should be used as far as possible when steel is embedded. (TRRL)

Corrosion of steel by concrete

Chloride threshold level for corrosion of steel in concrete

Probability, Reliability and Statistical Methods in Engineering Design

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This paper illustrates the effect of chloride-induced corrosion in the flexural capacity of a pretensioned concrete girder in an existing girder-and-slab deck bridge. The numerical study of the time-wise variation of the flexural capacity is based on a proposed model for the loss of cross-sectional area of the prestressing strands. It was observed that almost 46% of the total area of strands can get affected due to chloride-induced corrosion of the girder, by the end of its service life. The corresponding flexural capacity of the girder gets reduced by 50% of its initial capacity.

An Assessment of the Deterioration of Flexural Capacity of a Pretensioned Concrete Girder Due to Strand Corrosion

The occurrence of environmentally assisted cracking (EAC) is as dependent on the aggressiveness of the environment, as the susceptibility of the material and the presence of static tensile stresses. However, the influence of the environment has not been adequately considered in the past investigations on EAC mechanisms of prestressing strands. This study utilizes various characterization techniques to evaluate the surface/bulk deterioration of corroded pretensioned concrete (PTC) specimens after natural chloride exposure (by diffusion through cover concrete). Corroded strands in two PTC prism specimens (3000 × 150 × 200 mm) were characterized using EIS and other microanalytical techniques. The EIS and SEM images obtained after one and two years of exposure revealed a negligible residual protectiveness of the passivated surface (although the concretes possessed high resistivity). Raman spectra, SEM and X-Ray CT images of extracted corroded strands also provided unique insights on the pattern of corrosion propagation in PTC systems subjected to realistic chloride exposure. Microcracks in the bulk metal beneath flat bottomed corrosion pits revealed the possible EAC at low chloride levels expected in service (< 0.6 % by weight of binder). The findings serve as a basis to define chloride-induced passive-to-active transition as the end of risk-free service life of PTC structures, and for considering it as the limit state for both service life design and corrosion assessment to avoid the onset of EAC.

Electrochemical/microstructural studies on the corrosion of prestressed steel strand in concrete with naturally ingressing chlorides

NACE impact report (2016) states that nearly 50% of reinforced concrete (RC) structures experience major repair in about ten years. The existing approach of patch repair does not address the root cause and may not be durable – resulting in re-repair and huge economic loss. Galvanic anodes (GAs) are gaining widespread acceptance to achieve maintenance-free repair life for a few decades. However, a few GAs with inadequate characteristics are prematurely failing (within a few months). There are no short-term test methods to evaluate the longevity of GAs. Therefore, this work focuses on developing a short-term test method (Galvanic Anode Performance (GAP) test) to assess the longevity of GAs. For this, the GAP specimen was designed by simulating CP-protected RC structure as follows: (i) GA embedded in bedding mortar (i.e., anode), (ii) Nichrome mesh (i.e., cathode simulating rebars in RC structures), (iii) position of anode and cathode, (iv) application of potential difference (0.5, 1, 5, 10, 20, and 30 V) to accelerate the degradation of GAs, and (v) electrolyte to simulate conductivity of concrete. Applied potentials > 5 V could not capture the difference in characteristics of GAs. However, potential differences of 0.5, 1, and 5 V could show the true behavior of GAs in various exposure conditions. Then, an approach is proposed to evaluate the service life of GAs. Possible reasons for the premature failure of anodes were investigated by evaluating pH and pore volume of encapsulating mortar. The GAP test can help practicing engineers to estimate the longevity of GAs.

Development of galvanic anode performance (GAP) test for assessing the longevity of galvanic anodes for reinforced concrete structures

Reinforced concrete is, in general, a very durable system. However, as designers pursue more efficient structural designs and subject these structures to more aggressive environments, these systems become increasingly susceptible to corrosion. Corrosion of steel reinforcement is one of the more prevalent mechanisms of deterioration in reinforced concrete systems. As the world’s infrastructure ages, the cost of repair and replacement of these systems increase at rapid rates. As new models, designs, materials and construction methods become available, the service life of these systems should be extended. This Special Issue initially focuses on current practices used throughout the world to mitigate corrosion of the steel reinforcement embedded in concrete. Alexander et al., Li and Ueda, and Geiker et al. provide an overview for durability based design in South Africa, Asia and Europe. The authors note that both prescriptiveand performance-based methods are currently in use with the objective of ensuring durability. All authors note the use of models, especially models to predict the ingress of chlorides into concrete, should be used to better predict the service life. However, Alexander et al. critique exposure classifications and conclude that both rational service life designs and relevant environmental exposure classifications are sorely needed. The authors also recommend that exposure classifications account for the various factors that influence reinforcement corrosion and the resulting structural damage. Li and Ueda review the state-of-theart of durability design in Asia and highlight the strengths and weaknesses of the current practices. The authors ultimately recommend a ‘multi-barrier’ strategy to achieve long-term performance and corrosion resistance of reinforced concrete systems. Geiker at al. provide a European perspective on durability design and argue that designers must understand basic deterioration mechanisms and resulting damage to better design the infrastructure systems. The authors also note that service life models should include the time from corrosion initiation to the end of life (i.e., the propagation phase) to provide more resilient designs. In addition to the design for durability perspectives from the different regions, understanding how to better predict and quantify factors that influence the service life are critical for improving resilience. Ogunsanya et al. present how the use of different de-icing chemicals can influence the critical chloride threshold, a critical parameter for assessing service life. Boschmann Käthler et al. present a review of how the critical chloride threshold values are assessed and make recommendations on how to quantify these critical chloride values. Interestingly, such a critical parameter for assessing the service life of reinforced concrete system has no standardized testing protocol (although advances are underway in several locales). Ahmed and Vaddey present interesting work on chloride testing of various cementitious systems and recommend that water-soluble chloride testing be used to quantify chlorides in concrete. Standardizing testing requirements are essential for ensuring corrosionresistant structures and yet the pursuit is on-going. Shakouri and Dhandapani & Santhanam focus on buildup and transport rates of chlorides in concrete systems. Shakouri reported on the surface build-up rate of chlorides and assesshow these build-up rates influence service life. He concluded that a universal test is needed to assess surface chlorides; interestingly, this is another critical input parameter for assessing the service life and yet there is limited standardization. Shakouri also reported the need for long-term field data. Dhandapani and Santhanam compared various test methods currently used to quantify chloride transport rates under various exposure conditions and report good correlation between several testing methods. Although much of the literature on chloride transport and service life of reinforced concrete systems focus on uncracked concrete, cracking in concrete is common. Yet limited work has been performed to assess how cracks influence corrosion and resulting service life of reinforced concrete systems. O’Reilly et al. assessed the corrosion performance of reinforced concrete specimens containing narrow cracks and reported that these narrow cracks can promote corrosion and potentially

Special Issue on a vision for corrosion-resistant and resilient reinforced concrete systems: An introduction

The Rashtrapati Bhavan, the official residence of the President of India, is a Grade-I heritage structure. The reinforced concrete cantilever sunshades in this building are at a height of about 20 m above ground and span about two-kilometer along the perimeter of the building. In the late 1990s, some sunshades experienced corrosion and were repaired using polymer modified cementitious mortar. However, these repaired sunshades and others are now exhibiting severe corrosion and concrete spalling- posing a serious falling hazard for visitors and inhabitants. This paper presents a systematic evaluation of the concrete used in the sunshades and the assessment of corrosion and structural conditions of the sunshades. Concrete was found to be made of non-hydraulic lime and carbonated - indicating high probability of corrosion. Hence, about 200 sunshade locations from various parts of the building were visually and non-destructively assessed and distress-maps were developed. For this, an instrumented hammer was used on 15 test points per sunshade panel area (of about 1 × 2 m size). Based on the impulse waveform patterns, estimated strengths and visible damage, the panels were classified into distress levels of negligible, moderate and severe. About 58 to 86% of the sunshades were found to be severely damaged. Also, service level load test was conducted (upto a load of 75 kN) at a representative location, to assess the effect of corrosion on the load-deflection behaviour of the cantilever. The possible repair strategy and challenges associated with adopting conventional methods, are discussed in the paper.

Radhakrishna G. Pillai

Papers

An Assessment of the Deterioration of Flexural Capacity of a Pretensioned Concrete Girder Due to Strand Corrosion

Electrochemical/microstructural studies on the corrosion of prestressed steel strand in concrete with naturally ingressing chlorides

Development of galvanic anode performance (GAP) test for assessing the longevity of galvanic anodes for reinforced concrete structures

Special Issue on a vision for corrosion-resistant and resilient reinforced concrete systems: An introduction

Restoration of Reinforced Lime Concrete Sunshades of a Century Old Heritage Building in New Delhi, India