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.

The rate of ingress of external elements (that is, chlorides, carbon dioxide, oxygen, moisture, and so on) through the cover concrete is a key parameter influencing the durability of concrete structures. It can take several decades to estimate the realistic rates of ingress of these elements through concrete systems in the field - under natural exposure. Therefore, several accelerated test methods have been developed by various researchers to qualitatively estimate and compare the durability of various concrete mixtures in a reasonable period of time. Three such methods are (i) Wenner resistivity test, (ii) Rapid Chloride Penetration Test (RCPT) [ASTM C1202] and (iii) Accelerated Chloride Migration Test (ACMT) [NT Build 492]. This work presents a study on the correlation between the results from the Wenner resistivity tests and the RCPT and ACMT methods. The experimental design included 20 types of concrete (selected combinations using four independent variables). These independent variables and their levels were: (i) water-binder ratio (0.5, 0.55, 0.6 and 0.65); (ii) total binder content (280, 310, and 340 kg/m3); (iii) supplementary cementitious materials (SCMs) content (slag, class C fly ash and class F fly ash with 0%, 15% and 30% replacement); and (iv) curing period (28 and 90 days).

The mixture proportioning of concrete for sustainability should consider four aspects, without sacrificing affordability: the lowering of the carbon dioxide emissions; the minimization of raw materials required; reduction of energy demand during manufacturing and construction; and the longevity of the structure or other applications. Taking a set of concretes with different binders, including ordinary portland cement (OPC), fly ash (FA) and ground granulated blast furnace slag (GGBS), sustainability is assessed using different types of indicators including those that take into account the binder and clinker content, compressive strength, carbon footprint and energy demand. A new set of indicators called A-indices has been proposed for combining the influence of carbon dioxide emissions obtained from life cycle assessment (LCA) and durability parameter that relate to the service life of a structure. Here, this concept is illustrated by obtaining a parameter based on the chloride migration coefficient of the concrete. It is proposed that the decision-making process for sustainable concrete be made by minimizing both the A-index and the energy intensity, defined as the energy demand for a unit volume of concrete and unit performance parameter, such as 1 MPa of 1-year compressive strength. The best concretes considered here come out as those with ternary binders having 40% of the OPC replaced by a combination of GGBS and FA.

This paper discusses the influence of blended cement on the service life of reinforced concrete (RC) structural components subjected to chloride-rich environments. The service life is assumed as the sum of the corrosion initiation and propagation periods. A comprehensive experimental programme was performed to obtain the chloride diffusion coefficient and corrosion current density that are used in the estimation of the corrosion initiation and propagation periods. The estimated service lives of ordinary portland cement (OPC) and portland pozzolana cement (PPC) concretes having thermo-mechanically treated steel reinforcement, when exposed to chloride environments, are presented. The results suggest that, under certain circumstances, the service life of an RC structure can double when PPC is used instead of OPC.

Understanding the stress-strain behavior of corroded rebars is essential for understanding the structural behavior of corroding reinforced concrete structures. However, conventional methods of testing have several drawbacks, which have led to differences in opinion among researchers regarding the mechanical behavior of corroded rebars. This article proposes an improved method using Digital Image Correlation (DIC) to assess the stress-strain behavior of corroded rebars. The study is conducted in two phases: (a) development of the testing and evaluation method and (b) determination of the mechanical properties (i.e., yield strength, ultimate strength, and ultimate strain) of naturally corroded cold-twisted deformed (CTD) steel rebars using the developed method. The stress-strain behavior of one pristine plain mild (PM) steel and 13 naturally corroded steel rebars collected from a 15-year-old building are investigated in this study. The experimental results show that the proposed method can provide more accurate estimates of mechanical properties than the conventional methods. The developed method facilitates a better understanding of the stress-strain behavior at the fracture location (FL) and other local points of interest on corroded steel rebars. Because DIC measures local strains, the proposed method is able to measure local strain heterogeneity and therefore distinguish between the effects of geometrical variations from those due to true material degradation. This is in contrast to the conventional methods wherein deformation is measured over a finite gauge length, which results in a measurement of average deformation rather than true local deformation. This study emphasizes the need for measurement of full-field deformation to assess the stress-strain behavior of the corroded rebars with highly uneven cross-sectional area.

The 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 measured statistical parameters of the corrosion rate of CTD bars.

Chloride ingress through diffusion is a major deterioration mechanism, making it absolutely necessary to account for it in the Service Life Prediction (SLP) framework. However, in most SLP frameworks, there are limited provisions for the estimation of the chloride diffusion coefficient (Dchloride), a parameter that has a direct strong influence on the chloride induced corrosion initiation. Developing a probabilistic model for Dchloride based on raw material characteristics and mixture proportions can help in making decisions on the selection of cementitious materials during the planning and design phases of projects. This thesis presents the development of probabilistic models for the estimation of Dchloride based on 42 data sets collected from literature. First, the predictor variables such w/b, specific surface area of binder (SSAbinder), and SiO2 and CaO contents in the binder were identified. Then, a diagnostic study was conducted to investigate the nature and degree of influence of each independent variable on Dchloride. Then, probabilistic models that relate Dchloride to the significant independent variables were developed.

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.

This paper describes a unique international inter-laboratory study on the carbonation resistance of concrete prepared with different supplementary cementing materials. Concrete specimens – from 45 different concrete mixtures – prepared centrally in Lafarge Centre de Recherche (France)were shipped in a sealed condition to 4 other academia research laboratories (located in USA, Canada, India and China). The specimens were exposed to the ambient environments and atmospheric CO2 concentrations in the five locations, including Lafarge Centre de Recherche in France, in both sheltered and unsheltered condition for a period of 5 years. Measurements of carbonation depth were performed at periodic intervals, and the data was analyzed to assess the influence of climatic conditions on the resistance to carbonation. The results indicate that the general trend of carbonation is not much different irrespective of the macroclimate. Further, the number of rainy days seems to have a more significant influence on the progress of carbonation than the total rainfall in the region.