Radhakrishna G Pillai

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.

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.

Nowadays, various concrete systems with fly ash, slag, limestone calcined clay, etc. exhibiting high ionic resistivity are used to enhance the resistance against chloride-induced corrosion. This study deals with the corrosion assessment of steel in three cementitious systems, namely (i) Ordinary Portland Cement (OPC), (ii) OPC + 30% fly ash, and (iii) limestone calcined clay cement (LC3) exhibiting ‘low to moderate’, ‘moderate to high’, and ‘very high’ resistivities, as per AASHTO T358 (2017). Results from the ASTM G109 and impressed current corrosion (ICC) tests were evaluated. It was found that LC3 systems have excellent resistivity against the ingress of chlorides and provide better corrosion resistance. It was also found that the corrosion products formed on steel in LC3 systems are different and less expansive than that found in the OPC systems.

In general, pretensioned concrete (PTC) structures are being designed with a target service life of about 100 years. However, cases of premature/catastrophic corrosion-induced strand failures have been reported. This necessitates a re-examination of the current practices for service life-based design (SLD) as well as corrosion assessment of new and existing PTC structures respectively, especially the choice of limit states and the parameters considered. The work presented in this paper is a component of the research carried out to develop tools to assist SLD and assessment of PTC structures, and focusses on the estimation of time required for Passive-to-Active (P-to-A) transition, which is the first step of chloride-induced corrosion of prestressed steel strands. For this, ordinary Portland cement-based reinforced mortar specimens (both unstressed and stressed categories) were prepared using prestressing (PS) steel king wires extracted from 7-wire strands. Totally, 10 specimens (5 + 5) were cast, cured for 28 days and then subjected to dry-wet (5-day drying followed by 2-day wetting) cycles in simulated concrete pore solution containing sodium chloride. The P-to-A transition was detected by continuous monitoring of the open circuit potential and impedance measurements at the end of every wet exposure period. The specimens were then autopsied and the concentration of chlorides in the mortar at the level of steel was determined and defined as Clth. It was found that the Clth of PS steel can reduce by half in OPC system when prestress is applied. A case study was performed to understand the implications of using overestimated values of Clth on the service life. An overestimation in service life of about 40% was observed when the Clth of unstressed PS steel was used - emphasizing the importance of determining the Clth of stressed steel and using it for service life estimation.

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 LC 3. 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.

In the light of the increasing demand for cement in construction and dwindling reserves of cement-grade limestone, the blend of ground limestone and lower grade calcined clay has emerged as a potential candidate for large volume cement replacement.Studies of such ternary blended systems in paste and concrete reveal very interesting physical and chemical effects on the structure development, strength and durability performance. This paper describes the results of durability studies conducted at IIT Madras on concretes prepared with limestone calcined clay Cement, in comparison with ordinary Portland cement and fly ash-based cement. The focus of the study was to delineate the chemical and physical effects caused by the binder composition on durability indicators for chloride-induced corrosion. The experimental strategy involved the assessment of the pore structure evolution and electrical properties on cementitious pastes, along with measurement of the durability parameters based on moisture absorption, chloride migration and diffusion. The results from the study reveal the complex interplay of the various factors that lead to improved performance of the blended cementitious systems. The synergistic interactions of the blend of calcined clay and limestone impact the physical structure positively at early ages as opposed to fly ash systems, which require prolonged curing to realize their potential.

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.

The use of supplementary cementitious materials (SCMs) in concrete is a sustainable solution. However, many practitioners and researchers perceive the lower resistance of SCM-based concretes against carbonation as a concern for large-scale implementation. It must be noted that most literature discusses the carbonation resistance of SCM-based concretes performed in accelerated carbonation. These accelerated carbonation results should be compared with natural carbonation results to predict the reliable performance of SCM-based concretes against carbonation in natural exposure conditions. Therefore, studies on the long-term natural carbonation of concrete with various SCMs were conducted in Chennai, India (hot-humid, tropical climate). This study has 21 concrete mixes with various SCMs (ordinary Portland cement, fly ash, and limestone calcined clay systems), water-to-binder ratios, and compressive strength grades. The specimens were exposed to natural atmospheric conditions for about 10 years in sheltered and open exposure. The sheltered specimens showed higher carbonation depths than the open-exposure specimens. It was observed that the type and replacement levels of SCMs, water-to-binder ratio, paste content, etc., could influence the carbonation resistance. The long-term natural carbonation study concluded that concretes with SCMs offer higher resistance to carbonation at a later stage of carbonation—hence, necessitating the decision-making based on models made using long-term natural carbonation.