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.

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.

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.

This paper discusses the improvement expected in the service life of reinforced concrete (RC) structural elements subjected to chloride-rich environments through the use of blended cement. Comparisons are made between concretes with ordinary portland cement (OPC) and fly ash-based portland pozzolana cement (PPC) at three water-to-cement ratios (w/c=0.57, 0.47, and 0.37). Through a comprehensive experimental program, the apparent chloride diffusion coefficient (Dc) and corrosion current density (icorr) were evaluated for these concretes. The study reveals that (1) although service life depends on both initiation and propagation periods, the propagation period is less significant when the severity of the environment is high; and (2) the service life of an RC structure can double if PPC is used instead of OPC when chloride-induced corrosion is critical.

Fly ash and limestone calcined clay cement (LC3) are being used in concrete to enhance chloride resistance. In this study, 60 specimens (with steel in three separate binder systems, namely 100% ordinary Portland cement [OPC], 70% OPC + 30% fly ash, and LC3 with surface resistivity of ≈10 kΩ·cm, ≈25 kΩ·cm, and ≈200 kΩ·cm, respectively) were subjected to impressed corrosion, and the results were compared with 15 lollipop steel-mortar specimens subjected to natural corrosion under a wet-dry chloride environment. It was found that the traditional method of impressed corrosion tests can induce microstructural changes in highly resistive concrete cover and at the steel/concrete interface; hence, these methods are not suitable for evaluating corrosion resistance (such as corrosion rate and corrosion-induced cracking) in highly resistive concrete systems. Further, the Raman spectra from the corroded steel surfaces indicated that the impressed corrosion and natural corrosion tests led to different forms of corrosion (i.e., uniform and pitting, respectively) and different compositions of corrosion products (i.e., α-Fe2O3 and β-FeOOH phases). This led to different expansive stresses, making the lab-to-field correlations inappropriate in the case of highly resistive concrete systems. This paper recommends natural corrosion tests exposed to wet-dry conditions and not the impressed corrosion tests for assessing corrosion phenomena of steel in highly resistive concrete systems.

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.