Daksh Baweja

CHLORIDE-INDUCED STEEL CORROSION IN CONCRETE: PART 1--CORROSION RATES, CORROSION ACTIVITY, AND ATTACK AREAS

This paper presents results from a major long-term study on chloride-induced steel corrosion in concrete. The performance of a set of 50 reinforced concrete slabs made with a range of portland and blended cement binders was evaluated. Two portland cements, a high C3A and a low C3A, a blended fly ash cement, and a blended blast furnace slag cement were used. All reinforced concrete slabs were exposed to high chloride conditions by partial immersion in a 3% NaCl solution modeling seawater conditions. The reinforcement was cleaned and weighed prior to inclusion into the concrete slabs. Periodic nondestructive measurements of concrete performance included half cell potential monitoring, concrete resistivity, and electrochemical measurements of rates of corrosion of steel in concrete using potentiodynamic anodic procedures. Individual slabs were broken for reinforcement recovery at predetermined times during the study. Measurements were made of the area of corrosion and the weight loss of steel through corrosion. This information was related to the nondestructive data acquired on the slabs. In Part 1, estimated corrosion current (Ic) data and areas under the Ic versus time envelope for reinforcement within concrete slabs is presented. These data were obtained using potentiodynamic anodic polarization techniques. Measurements of the reinforcement corroded area in concrete are also presented and related to the electrochemical measurements taken. It was found that concrete water:binder ratio significantly influenced the corrosion rate of steel in concrete. Relationships were established between corrosion activity and the area of chloride-induced steel corrosion. Observed relationships were different for reinforcement within portland cement concretes and blended cement concretes considered.

CHLORIDE-INDUCED STEEL CORROSION IN CONCRETE: PART 2--GRAVIMETRIC AND ELECTROCHEMICAL COMPARISONS

This is the second of a two-part paper covering research into chloride-induced steel corrosion in concrete. The work focused on relationships between electrochemical data on chloride-induced reinforcement corrosion and gravimetric steel weight losses. Data were collected on a series of reinforced concrete slabs that were partially immersed in 3% NaCl solution for a period of 5 years. Slabs were made with a range of water-binder ratios (w/b) with portland cements having high and low C3A contents, slag-blended cement, or fly ash-blended cement. Data focused on measurements over time of concrete resistivity, corrosion rates, and gravimetric weight losses of steel taken at the end of the exposure period. Rates of corrosion of steel in concrete were measured using potentiodynamic anodic procedures. An analysis of estimated corrosion currents (I sub c) and the area under the I sub c versus time envelope for reinforcement within the concrete slabs is described. Measurements of weight loss of steel through corrosion in concrete are analyzed and related back to the electrochemical measurements taken. It was found that concrete w/b highly influenced the corrosion rate of steel in concrete. Quantitative links between steel weight loss, the electrochemical data, and concrete resistivity have been found. Under high-chloride conditions, the blended cement concretes having low w/b were found to perform better than other concretes investigated. The data suggest that such concretes had higher resistivity characteristics, had lower corrosion rate characteristics, and were likely to result in lower reinforcement weight losses when compared with equivalent portland cement concretes. Reinforced concrete performance under high-chloride conditions did not reflect concrete strength data from the materials considered. Results provide some guidelines for the design of durable concrete structures.

Corrosion characteristics of steel in solutions derived from cements and blended cements

Potentiodynamic Anodic Polarization techniques were used to investigate the corrosion characteristics of steel in solutions. The solutions were prepared by mixing different cements at a water-binder ratio of 2.0 and extraction after 3 and 7 days. Cements used in this investigation included an OPC with low C3A content, an OPC with high C3A content, a Slag Cement and a Fly Ash Cement. Results indicate that in the absence of chloride ions, the corrosion characteristics of steel in these solutions are very similar to those of steel in lime-saturated water. Despite differences in the rest potentials found in solutions made with the range of binder materials, steel passivation could be observed in all solutions. Passivation imparted to the steel by binder solutions was found to vary in the presence of increasing levels of chloride. A threshold chloride ion concentration for depassivation was established for each binder solution tested. Solutions made from the High C3A Cement and the Slag Cement displayed the best steel passivation characteristics, white the Low C3A Cement solution rated worst.

Relationships between anodic polarisation and corrosion of steel in concrete

Abstract Corrosion of reinforcement in concrete is a topic of concern mainly because of the high cost of repair and rehabilitation of concrete structural elements. There is as yet no method of assessment that would enable the rapid and accurate prediction of the extent of corrosion of reinforcing steel in concrete on site. Half cell potential techniques commonly used in situ give only probabilistic information on corrosion activity. Research effort is thus needed in both investigating and developing methods to assess more accurately the corrosion characteristics of steel in concrete with an ultimate view of site application. Long-term investigations on chloride induced corrosion of steel reinforcement have been conducted on a series of concrete slab specimens to establish relationships between electrochemical data and chloride induced corrosion of steel reinforcement. Potentiodynamic anodic polarisation procedures were used to monitor corrosion of steel reinforcement in concrete slab specimens over a period of four years. A statistically significant relationship between the area under the corrosion current and time relationship and the weight loss of steel reinforcement was established. Assessments of corrosion rates of steel in the concretes studied were thus verified. Reinforcement corrosion was found to be localised under the high chloride conditions occurring mainly in an area adjacent to the chloride source.

Improved Electrochemical Determinations of Chloride-Induced Steel Corrosion in Concrete

A significant amount of infrastructure is located in a marine environment or in environments where high levels of chloride are present. Current specification methods are generally lacking in regard to predicting service life of reinforced concrete (RC) structures in aggressive environments and qualitative judgments are typically used to make predictions concerning service life. Much research has focused on prediction of chloride diffusion coefficients where more quantitative links to service life can be established. The problem with such techniques is that assumptions of the chloride threshold level for steel depassivation must be made, and this introduces generally unacceptable levels of uncertainty for asset owners. One approach appears to lie in improving the understanding of the electrochemical behavior of steel in concrete such that more accurate predictions of service life can be derived in the future. Work presented in this paper relates to the analysis of a significant amount of data on electrochemical corrosion rates of steel in concrete.

Specifications of Concrete for Marine Environments: A Fresh Approach

There is significant debate around the world regarding the most appropriate test methods for assessing the performance of concretes in marine environments. Much research work has been conducted on chloride ion penetration into concrete and its relationships to embedded steel passivity and rate of corrosion. This paper summarizes work from a series of studies. The focus of this work was on the long-term performance of reinforced concrete under high-chloride conditions. Work included the monitoring of concrete performance using half-cell potential measurements, concrete resistivity, and the determination of gravimetric weight loss of steel through corrosion. Corrosion rates of steel in concrete were also measured using anodic polarization techniques. Concretes considered were made with a range of commercially available portland cement and supplementary cementitious materials. Concretes were classified on the basis of observed long-term performance based on resistivity and the time taken for embedded steel to reach a probable active corrosion state. The development of a method for designing and assessing concretes for critical marine structures based on the information is reported. Use of this method will lead to improved specifications for concrete when compared with existing design specifications.

Compliance Acceptance of Concrete Drying Shrinkage

Abstract In Australia, strength, and in particular compressive strength, has been used in AS 3600 and AS 1379 as a benchmark for structural performance, for durability and as a contractual basis for concrete supply. With tightening financial constraints and the ensuing need for increased construction efficiencies, there is a bigger emphasis on the provision of concrete quality. Contractually, quality provision is handled by measuring parameters such as compressive strength, flexural strength, chloride ion penetration by charge transfer (ASTM C1202-91), and concrete drying shrinkage. The correct assessment of such results, together with good concreting practices, will greatly increase the certainty of achieving desired quality in terms of design intent. Concrete drying shrinkage is recognised as important in the design process for maintaining serviceability requirements particularly on large horizontal structural elements, a common example being the limitation of cracking in large slabs. The compliance basis for compressive strength is well established in AS 3600 and AS 1379. Regarding concrete drying shrinkage, however, it is common to find in many design specifications average and absolute maximum values specified at 56 days. This can create immense constructional and contractual problems when individual samples fail to comply with the specified shrinkage limit.

MEASUREMENT OF CORROSION OF STEEL REINFORCEMENT UNDER HIGH CHLORIDE CONDITIONS

The background to a major study into the corrosion characteristics of steel reinforcement within portland and blended cement concretes is presented. The objectives of this work included an investigation into relationships between chloride ion concentration and the onset and rate of steel corrosion in concrete. Corrosion activity of steel reinforcement within concrete specimens was measured using procedures including half cell potential, resistivity and potentiodynamic anodic polarization. A total of 4 binder types which included a slag blended cement and a fly ash blended cement were used. The development of a procedure to electrochemically measure the corrosion rate of steel in concrete slab specimens partially immersed in chloride solutions is described.

The Acquisition and Analysis of Data on Factors Affecting the Field Performance of Plain and Blended Cement Concrete Structures

This paper describes how concrete was considered a material with a long-lasting and functional in situ life in the past. More recently, many durability and serviceability problems in structures have been reported. Codes and standards relating to both plain and blended cement concretes are currently being written in Australia, where new durability design requirements have recently been formulated. The long-term performance of both plain and blended cement concretes in structures has at this stage not been quantitatively defined. This paper briefly discusses the methods that have been developed to aid in determining the durability of in situ concrete structural elements. Consideration is given to correlations between reinforcement corrosion occurrence in building structures and coastal proximity. The effects of fly ash usage in concrete on other aspects of the durability of structures are also considered. Data relating to the durability of a series of inspected structures are presented. Attention is drawn to existing and potential durability and serviceability issues. Wherever possible, comparisons are made between the relative performances of plain and blended cement concretes.

Hydration Characteristics and Morphological Features of Concretes from Structures in Australia, England and the United States

Scanning electron microscope studies were carried out on concretes obtained from structures in Australia, England and the United States as part of an assessment of their durability performances. The ages of the structures ranged from nine to thirty years. All structures contained fly ash concretes in some sections, and, in a few cases, direct comparison between these concretes and ordinary portland cement (OPC) concretes was possible. Water: cement ratios of the concretes varied from 0.44 to 0.90. The sources of fly ash were identified in each case. The structures chosen had a range of use classifications encompassing building, hydraulic structures, bridge structures, towers and foundations. Deterioration effects of concretes in some structures were more common than in others. Based on observations, conclusions were drawn on the morphological features and hydration characteristics of the concretes.