Radhakrishna G. Pillai

Surface condition effects on critical chloride threshold of steel reinforcement

Deicing and anti-icing salts and seawater are the main sources of chloride ions that cause the corrosion of steel reinforcement embedded in reinforced concrete (RC) bridge and marine structures. This article reports on a study undertaken to evaluate the influence of the steel reinforcement surface condition on the corrosion performance. In the study, the critical chloride threshold values of five uncoated steel reinforcement types (ASTM A 706, ASTM A 615, microcomposite, stainless steel 304, and stainless steel [SS] 316LN) with as-received and polished surface conditions were quantitatively determined using the accelerated chloride threshold (ACT) test procedure. Micrographs of the surfaces (characterizing the mill scale and surface topography) for all steel reinforcement types were obtained using both optical and scanning electron microscopy (SEM). Results showed that the mean critical chloride threshold values increased with the complete removal of the as-received surface and with surface polishing for the ASTM A 706, microcomposite, and stainless steel 304 reinforcements. In addition the mean critical chloride threshold values decreased with the complete removal of the as-received surface and with surface polishing of the ASTM A 615 and SS316LN steels. The authors conclude that removal of the mill scale on the microcomposite steel reinforcement showed a significant improvement in the critical chloride threshold and removal of this mill scale may be economically justified.

Parameters Influencing Corrosion and Tension Capacity of Post-Tensioning Strands

A 12-month long strand corrosion test program with 298 specimens was conducted to identify and quantify parameters influencing corrosion and tension capacity of strands in post-tensioned bridges. The parameters investigated were grout class, moisture content, chloride concentration, void type, and stress level. The test specimens were 41 in. (1041 mm) long, in unstressed or stressed conditions, partially or completely embedded in cementitious grout, and exposed to various environmental conditions representing possible field conditions. After the exposure period, the grout material was removed and the strand surfaces were cleaned and visually evaluated for corrosion damage. The tension capacities of the strands were then determined. Results indicate that the corrosion was most severe at or near the grout-air-strand (GAS) interface. Corrosion evaluation and statistical analysis of the strand tension capacity results show that orthogonal, inclined, and bleedwater void conditions caused more corrosion and tension capacity loss than parallel and no-void conditions. The change in grout class did not result in statistically significant changes in the tension capacity of the strand samples evaluated. Statistically significant changes in tension capacity were observed with changes in the GAS interface, stress level, moisture content, and chloride concentration.

Probabilistic Capacity Models for Corroding Posttensioning Strands Calibrated Using Laboratory Results

The presence of air voids, moisture, and chlorides inside tendons or ducts was cited as a reason for the early age strand corrosion and failure in the Mid-bay, Sunshine Skyway, and Niles Channel posttensioned (PT) bridges in Florida, United States. Although rare, these incidents call for frequent inspection and structural reliability assessment of PT bridges exposed to moisture and chlorides. This paper develops and presents probabilistic strand capacity models that are needed to assess the structural reliability of such PT bridges and recommends a time frequency of inspection. A total of 384 strand test specimens were exposed to various void, moisture, and chloride concentration conditions for 12 and 21 months; the remaining tension capacities were then determined. Using this experimental data and a Bayesian approach, six probabilistic capacity models were developed based on the void type. The mean absolute percentage errors of these models are less than 4%, indicating that reasonably accurate prediction of the strand capacity is possible, when void, aggressive moisture, and chloride conditions are present.

Probabilistic Models for the Tensile Strength of Corroding Strands in Posttensioned Segmental Concrete Bridges

The presence of air voids, moisture, and chlorides inside tendon systems on segmental posttensioned (PT) bridges has been cited as a reason for the early age corrosion and failure of strands in these bridges. This paper develops probabilistic models to predict the time-variant tension capacity of PT strands exposed to wet-dry conditions. A total of 384 unstressed and 162 stressed strand test specimens were exposed to various void, moisture, and chloride conditions for 0, 12, 16, and 21 months; the residual tension capacities of the strands were then determined. Using these experimental data, a Bayesian approach is used to develop probabilistic capacity models for unstressed and stressed strands. The tension capacities of stressed strands under potential void, wet-dry, and chloride conditions in the field are predicted using the developed models. Probabilistic time-variant models are formulated in such a way that they can be updated by other researchers using additional information from the testing of unstressed strands only, avoiding expensive and cumbersome testing of stressed strands. The mean absolute percentage errors of these models are less than 3.2%, indicating good overall model accuracy.

Time-Variant Flexural Reliability of Posttensioned, Segmental Concrete Bridges Exposed to Corrosive Environments

Posttensioned (PT) bridges are used on major thoroughfares because they are economical structures for traversing long spans. Special inspections of these bridges have revealed corrosion of the strands at void locations in the tendons. The integrity of these strands has significant influence on the safety of these bridges. Inspections indicate that many ducts (parts of the tendon that is supposed to protect strands from exposure to aggressive environments) are cracked and many grout holes and vents are opened, allowing direct ingress of moisture and chlorides. This paper presents a framework for assessing the flexural reliability of PT bridges exposed to various environmental conditions. The moment capacity of a PT girder is formulated using probabilistic models for the tension capacity of corroding PT strands exposed to various void and environmental conditions. Using Monte Carlo simulations, the flexural reliability of an example PT bridge is assessed. The research indicates that the flexural reliability index reaches a value below the recommended value within a relatively short period of time when moisture and chlorides infiltrate the tendons. These findings emphasize the critical need for new inspection, assessment, and repair methods for these bridge types.

Evaluation of Corrosion Rates of Reinforcing Bars for Probabilistic Assessment of Existing Road Bridge Girders

AbstractThe rate of corrosion of the reinforcing bars is one of the important parameters required to estimate the residual service-life of a reinforced concrete (RC) bridge deck. In the present study, first, the linear polarization resistance technique was used to measure the corrosion rates of plain mild steel and cold twisted deformed (CTD) bar specimens, which were typically used in the older existing bridges. To consider the variability of a corrosion rate, the frequency distributions of the corrosion rates for the two types of bars were determined. Next, a probabilistic approach was adopted for assessing an existing RC girder-and-slab road bridge deck, subjected to corrosion of bars attributable to air-borne chlorides. A computational model was developed using the Monte Carlo simulation method, to assess the reduction in the flexural capacity of a typical girder. It was observed that the reduction in the mean capacity and the dispersion of the capacity with respect to time, were high with the measure...

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

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.

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

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.

Bond Performance of Pretensioned Concrete Systems

Prestressed concrete technology has revolutionized the infrastructure growth in many countries, especially that of the bridge sector. The bond between prestressed strand and concrete is very important for achieving good structural performance. However, some of the codal provisions have not given enough consideration to the bond strength of pretensioned concrete system in design. This paper presents the results from a preliminary experimental program on the bond strength of 7-wire strands embedded in M35 and M55 concretes. A pull-out test method was developed, and the same was used to determine the bond strength. The bond behavior and the mechanisms at the strand–concrete interface are also discussed. Bond strength of 7-wire strand in M55 concrete is found to be about two times more than that in M35 concrete.

Chloride-Induced Corrosion Rates of Steel Embedded in Mortar with Ordinary Portland and Limestone Calcined Clay Cements (OPC and LC3)

Chloride induced corrosion is a serious deterioration mechanism in concrete structures. Corrosion rate is an important parameter required to estimate the service life, especially propagation life, of concrete structures. The corrosion rate of the embedded steel significantly depends on the properties of the surrounding concrete and cementitious systems. The thermo-mechanically-treated (TMT) steel is widely used in Indian construction. However, literature provides very limited information on corrosion rates of TMT steel embedded in concrete with Ordinary Portland Cement (OPC) and Limestone Calcined Clay Cement (LC3). This makes it difficult to quantify and compare the service life of such systems. This paper presents experimental results on the corrosion rates of TMT steel embedded in mortar (w/c = 0.5) with OPC and LC3. Each test specimen (lollipop type) consisted of an 8 mm diameter steel rod embedded in a 100 mm long mortar cylinder with a 10 mm cover. To accelerate the corrosion studies, chlorides were premixed to the mixing water/mortar. Four levels of premixed chloride content (i.e., 0, 3, 6, and 9 % NaCl) were used. A total of 40 lollipop specimens with 5 replicas for each variable combination were prepared. Corrosion rates were measured using Linear Polarization Resistance (LPR) technique and were monitored for a period of 2 months. Comparison of the corrosion rates and propagation periods for the steel embedded in systems with OPC and LC3 are presented.