Radhakrishna G Pillai

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.

Supplementary cementitious materials (SCMs) can be used in concrete to enhance sustainability and reduce the concrete industry's carbon footprint. However, some negative perceptions about their long-term carbonation resistance are obstacles for large-scale implementation of such concretes. This study evaluated the carbonation resistance of 34 concretes (with Ordinary Portland Cement, fly ash, blast furnace slag, and limestone calcined clay) in natural tropical exposure conditions (Open and Sheltered) for 5 years and in accelerated exposure conditions (1 and 3% CO2) for 112 days. Using these data and the square root of time function, the carbonation coefficients (KCO2, natl and KCO2, accl) of these concretes were estimated and a good correlation between them could not be observed. Hence, a more generic model (named as “A-to-N model”) to estimate the KCO2, natl using the KCO2, accl, CO2 concentration, and mixture proportion of concrete was developed, for which the mean absolute percent error is about 12% (reasonable accuracy). Using the A-to-N model, the carbonation depth at 50 years was estimated for various concretes. SCM concretes with low water-binder ratio and optimal binder content showed high resistance against carbonation at later ages; such information along with the target cover depth must be used while selecting materials for concrete design. Based on the model developed, a relatively simple ‘service life design chart’ was developed. This chart can be used by engineers to set the target KCO2, natl or KCO2, accl, and select the cover depth and binder type to provide the target service life (i.e., corrosion initiation time). This paper clearly shows that SCMs can be used to design concretes with comparable long-term carbonation depth as OPC concretes.

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.

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.

Grouted, post-tensioned (PT) concrete structures are protected from tendon corrosion by filling the interstitial spaces with cementitious grouts. To achieve complete grouting, the cementitious grout must be sufficiently flowable and bleed resistant. Nowadays, many commercial pre-packaged grouts are available. However, simulated bleed measurements with prototype-scale tendon grouting tests have shown that pre-packaged grouts tend to form a highly porous layer of grout, which poses a severe threat to corrosion protection. This study focuses on evaluating the performance of a novel pre-blended grout produced on an industrial scale using the fluidity and fluidity retention tests, standard, wick-induced, pressure-induced, and inclined tube bleed tests. Also, the ability of the fresh grout to retain its properties against slight variations in the ambient temperature and water content was studied. The performance of the pre-blended grout on a real scale was evaluated and compared with a widely used site-batched grout composition using prototype tendon grouting tests. In addition, a set of stringent and comprehensive specifications were developed for applications in PT systems. It was also observed that the pre-blended grout considered in this study met all the proposed specifications, therefore, can be used for the corrosion protection of tendons in PT structures.

Curing plays a vital role in enhancing the impenetrability of cover concrete, which is essential to ensure the desired service life of a concrete structure. However, their performance in the field is most often evaluated by compressive strength instead of durability parameters. This paper presents an investigation of the suitability of durability index tests in evaluating the performance of various field curing methods. Five reinforced concrete slab specimens were cured in the field using the following curing methods: 7-day wet hessian, three types of curing compounds, and air curing. The field-cured concrete was tested at the age of 28 days using oxygen permeability index, water sorptivity index, rapid chloride migration test, and surface resistivity. In this study, durability index tests were able to differentiate between wet and air curing. The results of this study indicate that compressive strength as a standalone criterion is not adequate for assessing the performance of field curing methods.

The article ‘‘Sustainability-based decision support framework for choosing concrete mixture proportions’’, written by ‘‘Ravindra Gettu, Radhakrishna G. Pillai, Manu Santhanam, Anusha S. Basavaraj, Sundar Rathnarajan, B. S. Dhanya’’, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 3 December 2018 without open access. The copyright of the article changed in December 2019 to The Author(s) © 2019 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

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.

In grouted post-tensioned (PT) systems, cementitious grouts are supposed to completely fill the interstitial spaces between the strands and act as the ‘last line defence system’ against corrosion. However, use of poor quality grout materials and grouting practices result in voided grout systems, ultimately leading to premature failure of tendons in many bridges around the world. To ensure an intact system, the grout must have excellent fresh properties, in particular the flow properties. Such high-performance grouts are not available in many developing countries, where grouting for post-tensioned structures is still a nascent technology. In this research, a two-stage test program was carried out to evaluate the fresh and hardened properties of seven commercial grouts, which includes three Pre-Packaged Grout mixes (PPG); three Site-Batched Grout mixes (SBG) and one standard Ordinary Portland Cement grout mix (PCG). Further, one PPG mix and SBG mix were chosen and their properties were evaluated for three levels of mixing speed and two ambient temperature conditions. Fresh properties such as wet density, efflux time and its retention, standard bleed, wick-induced bleed and pressure bleed, as well as set/hardened properties such as setting time, compressive strength and volume change were evaluated. Three batches of grout were tested for each grout material, to ensure reliability of results. The influence of binder fineness on the performance of grouts was also evaluated. The study serves as a strong evidence in substantiating that the most commonly used grout materials for PT system in developing countries, fail to meet the standard requirements and even the manufacturer's own specifications. It is also found that the performance of the grout is influenced by mixing speed, ambient temperature, and fineness. The study emphasises that the evaluation of the grout behaviour under simulated field conditions is essential to ensure void free and durable PT systems.