Sripriya Rengaraju

Bio: Sripriya Rengaraju is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Corrosion & Cementitious. The author has an hindex of 3, co-authored 7 publications receiving 83 citations.

Service life and life cycle assessment of reinforced concrete systems with limestone calcined clay cement (LC3)

Abstract: This paper presents data on the chloride diffusion coefficient (Dcl), ageing coefficient (m) and chloride threshold (Clth) related to seven concrete mixes (four M35 and three M50) with OPC, OPC + PFA (pulverised fuel ash) and limestone-calcined clay cement (LC3). Using these, the service lives of a typical bridge pier and girder with the PFA and LC3 concrete were found to be much higher than those with OPC concrete of similar strength. From life-cycle assessment, the CO2 footprint of PFA and LC3 concrete were found to be significantly lower than those of OPC concrete of similar strength. Further, the CO2 emissions per unit of concrete per year of estimated service life, as a combined indicator of service life and carbon footprint, are similar for concrete with PFA and LC3, which are much lower than that with OPC.

Investigation on the polarization resistance of steel embedded in highly resistive cementitious systems – An attempt and challenges

Abstract: Concretes with fly ash, slag, limestone calcined clay, etc. exhibiting high resistivity are being used to enhance the chloride resistance of structures – to achieve durability. Prior to use, the engineers need to determine the chloride threshold (Clth) of such highly resistive steel cementitious (S-C) systems (a key parameter to estimate service life). Most Clth tests involve repeated measurements of polarization resistance (Rp) and detection of corrosion initiation of steel embedded in hardened cementitious system (a sol-gel structure with partially filled pores). The high resistivity of such systems should be considered while interpreting the electrochemical response to determine Rp. This paper experimentally evaluates the suitability of LPR and EIS techniques for assessing Rp of steel embedded in highly resistive systems. Experiments were conducted with lollipop type specimens (steel reinforcement embedded in mortar cylinders). The following three types of mortar having various resistivities were prepared: (i) ordinary portland cement (OPC), (ii) OPC + fly ash, and (iii) limestone calcined clay cement. Experimental observations on how the following three factors affect the electrochemical response in highly resistive S-C systems are provided: (i) resistivity of concrete covering the embedded steel, (ii) electrode configuration, and (iii) electrochemical test parameters. It was found that electrochemical impedance spectroscopy (EIS) can detect corrosion initiation in highly resistive systems at earlier stages than the linear polarization resistance (LPR) technique. Also, the guidelines on how to use EIS technique to determine the Rp of steel embedded in highly resistive S-C systems are provided.

Macrocell corrosion mechanisms of prestressing strands in various concretes

Abstract: This paper investigates the mechanisms of corrosion of seven-wire strands and the reasons for failures in detecting corrosion at early stages. As per ASTM G109 and ACI 222 R-01, a 1-year study on c...

An accelerated chloride threshold test for uncoated steel in highly resistive cementitious systems (hr-ACT test)

Abstract: Currently, many supplementary cementitious materials are being used along with cement to reduce the CO2 emissions. These cementitious systems exhibit very high resistivity due to pozzolanic reactions and the traditional test methods are inadequate to assess their durability performance related to corrosion, especially chloride threshold (Clth). This paper presents development of a test method (hr-ACT) for determining the Clth of such highly resistive systems with guidelines for identifying the same. The test specimen consisted of a mortar cylinder (lollipop) (cement:sand = 1:2.75) with steel embedded at the centre. Three binders, namely, OPC (w/b = 0.5), PC-FA (70% OPC + 30% Class F Fly ash) (w/b = 0.5) and Limestone Calcined Clay Cement (LC3 − 50% OPC clinker, 31% calcined clay, 15% limestone and 4% gypsum) (w/b = 0.4), and Quenched and Self-tempered (QST) steel were used in this study. The specimens were subjected to chloride in a cyclic wet-dry regime (2 day wet and 5 day dry) and the electrochemical impedance spectroscopy (EIS) test was conducted at the end of each wet period. Statistical analysis was done on the repeated polarization measurements (Rp) to detect corrosion initiation. Once corrosion initiation was detected, the total chloride content (acid soluble chlorides) in the mortar at the Steel–Cementitious binder (S-B) interface was determined using SHRP 330 and reported as Clth. The time required to complete hr–ACT test for an S–B system is about 3–4 months. The Clth was in the order OPC > PC–FA > LC3. Also, the synergistic effects of Clth and other parameters on service life are discussed.

Large-scale laboratory trials of smart aggregates for self-healing in concrete under different curing regimes

Abstract: To develop concrete with self-healing properties, a promising approach, through the incorporation of smart aggregates within the concrete mix, was tested in large-scale laboratory trials. In this work, lightweight aggregates impregnated with sodium silicate and sodium nitrate, as potential self-healing agents, and encapsulated in polyvinyl alcohol coating were prepared and deployed as 30% replacement of the coarse aggregates by volume in 150 mm × 200 mm × 1000 mm beams. The beams were then cured in three different environments: outdoor exposure, distilled water and seawater. To evaluate the self-healing efficiency, multiple cracks, 100–700 μm in width, were created using four-point bending tests and confirmed through microscopy observations and ultrasonic wave measurements. The concrete beams containing the developed smart aggregates showed a considerable regain of stiffness and flexural strength after healing compared to the control specimens under all three curing conditions. The measured reduction in water ingress of the healed sections demonstrated significant recovery of water tightness (50% more) compared to the control specimens. The contribution of the sodium silicate and sodium nitrate cargo released from the aggregates to the proliferation of the healing products was evident but needs further investigations on the size effect to realise the full potential of the healing mechanism.

Limestone calcined clay cement and concrete: A state-of-the-art review

Abstract: This article reviews the rapidly developing state-of-the-art literature available on the subject of the recently developed limestone calcined clay cement (LC3). An introduction to the background leading to the development of LC3 is first discussed. The chemistry of LC3 hydration and its production are detailed. The influence of the properties of the raw materials and production conditions are discussed. The mixture design of concrete using LC3 and the mechanical and durability properties of LC3 cement and concrete are then compared with other cements. At the end the economic and environmental aspects of the production and use of LC3 are discussed. The paper ends with suggestions on subjects on which further research is required.

Reinforced concrete structures: A review of corrosion mechanisms and advances in electrical methods for corrosion monitoring

Abstract: Steel corrosion is the main cause of deterioration of reinforced concrete (RC) structures. We provide an up-to-date review on corrosion mechanisms and recent advances in electrical methods for corrosion monitoring. When assessing corrosion mechanism, the inherent heterogeneity of RC structures and the significant effect of environmental factors remain major issues in data interpretations. The steel surface condition and local inhomogeneities at the steel–concrete interface appear to have an important effect on corrosion initiation. Considering uniform corrosion in atmospherically exposed RC structures, the two main influencing factors of the corrosion process are the water content and the pore structure at the steel–concrete interface. However, irrespective of the depassivation mechanism, i.e. carbonation or chloride-induced corrosion, non-uniform corrosion is expected to be the main process for RC structures due to local variations in environmental exposure or the presence of interconnected rebars with different properties. Future studies may then be focused on their effect on macrocell corrosion to gain further insights in the corrosion mechanisms of RC structures. Concerning corrosion monitoring using electrical methods, the half-cell potential technique with potential mapping is accurate for locating areas with a high corrosion risk. Recent developments in the measurement of concrete resistivity have shown that the use of electrical resistivity tomography allows to consider appropriately the inherent heterogeneity of concrete and provides more insights on transport phenomena (e.g. water and salts ingress) in the material. Nevertheless, during the corrosion propagation stage, the polarization resistance remains the most important parameter to be determined as it provides quantitative information of the corrosion rate. If conventional three-electrode configuration methods can supply an accurate determination in the case of uniform corrosion, they often fail in the case of macrocell corrosion in field experiments. Recent advances have shown that a four-electrode configuration without any connection to the rebar can rather be used for the non-destructive testing and evaluation of corrosion. If studies are still required to quantify the corrosion rate, this method appears sensitive to localized corrosion and thus more suitable to field investigations. Finally, the coupling of numerical simulations with complementary electrical and other non-destructive testing methods is essential for consolidating the results to provide a better diagnosis of the service life of RC structures.

Towards ternary binders involving limestone additions — A review

Abstract: The review summarises literature to examine the transition from portland limestone cement system to composite ternary binder systems involving limestone. Interest in limestone addition as an ideal component in multicomponent binder systems has surged as evident from the large volume of literature published in the recent past. A ternary blended system, with co-substitution of limestone, has the potential to complement the reaction of the supplementary cementitious materials (SCMs). The direct addition of limestone powder helps to attain higher substitution levels of portland cement clinker, improve early age properties, and supplement SCM's reactivity. However, the dilution of hydrates could hamper the long-term benefits. In this review, the interaction of fine limestone is classified and elaborated under two broad umbrellas: physical and chemical interactions. The physical interactions can manifest in three ways, namely, filler action, shearing action and improved packing, which alters reaction rate and extent at early ages. The chemical interactions also modify the reaction kinetics and phase assemblage due to nucleation of C-S-H on calcite surfaces, preservation of the ettringite phase and formation of carboaluminates. Two different forms of carboaluminate hydrates — hemicarboaluminate and mono-carboaluminate can be present in the hydrate matrix depending on the balance between carbonate ions and aluminates in the pore solution. Several factors such as replacement level, particle size, choice of SCM, its reactivity and reactive aluminates content, sulphate levels, curing temperature, and duration of curing can control the carboaluminate formation, reaction degree of SCMs and chemical interaction from limestone additions. A combination of physical and chemical effects makes fine limestone a potential material for co-substitution with aluminosilicate based SCMs, mainly fly ash, slag, and calcined clay. In this review, the factors affecting limestone-SCM composites are summarised based on a detailed literature survey. The effects of SCM-limestone cement composites on hydration kinetics, reaction chemistry, the reactivity of SCMs, the stability of hydrated phases, and contribution to the physical structure development and macroscopic properties by evaluating hydration and mechanical properties are discussed. The importance of AFm (Al2O3–Fe2O3-mono) phases in various deterioration mechanisms in concrete and their influence on performance characteristics in different exposure environment is critically appraised.

The effect of the steel–concrete interface on chloride-induced corrosion initiation in concrete: a critical review by RILEM TC 262-SCI

Abstract: The steel–concrete interface (SCI) is known to influence corrosion of steel in concrete. However, due to the numerous factors affecting the SCI—including steel properties, concrete properties, execution, and exposure conditions—it remains unclear which factors have the most dominant impact on the susceptibility of reinforced concrete to corrosion. In this literature review, prepared by members of RILEM technical committee 262-SCI, an attempt is made to elucidate the effect of numerous SCI characteristics on chloride-induced corrosion initiation of steel in concrete. We use a method to quantify and normalize the effect of individual SCI characteristics based on different literature results, which allows comparing them in a comprehensive context. It is found that the different SCI characteristics have received highly unbalanced research attention. Parameters such as w/b ratio and cement type have been studied most extensively. Interestingly, however, literature consistently indicates that those parameters have merely a moderate effect on the corrosion susceptibility of steel in concrete. Considerably more pronounced effects were identified for (1) steel properties, including metallurgy, presence of mill scale or rust layers, and surface roughness, and (2) the moisture state. Unfortunately, however, these aspects have received comparatively little research attention. Due to their apparently strong influence, future corrosion studies as well as developments towards predicting corrosion initiation in concrete would benefit from considering those aspects. Particularly the working mechanisms related to the moisture conditions in microscopic and macroscopic voids at the SCI is complex and presents major opportunities for further research in corrosion of steel in concrete.

Investigation on the microstructure-related characteristics to elucidate performance of composite cement with limestone-calcined clay combination

Abstract: This paper discusses the role of physical structure alterations on three binder types: plain portland cement, fly ash-based binder and calcined clay-limestone binder. The kinetics of physical structure development and the relevance in transport properties were distinguished using an interlinked parameter in concrete and paste. A systematic experimental investigation was carried out on a range of critical parameters such as strength development, resistivity development, transport characteristics and the time-dependent change in transport parameter. The role of microstructure in terms of the chemical composition of C-A-S-H and its physical states in the different systems is identified as the critical factor governing the development of microstructure. Chloride resistance was assessed by chloride migration experiments for a period of 4 years. The durability behaviours of the concrete with various binder were generalised using pore network parameters such as formation factor and tortuosity. A sensitivity analysis was used to dissect the contribution of the pore solution dilution and pore connectivity to the change in the pore network parameter. Based on the rise in macroscopic physical characteristics (i.e., formation factor here), a two-fold structure development mechanism to conceptualise microstructural evolution in cement composites is presented. Initially, capillary pore space reduces to a critical size range (i.e., 10–30 nm), which is followed by the densification of the physical state of the microstructure. At the point of densification, the pores become largely disconnected which leads to a dramatic increase in the formation factor. The binding matrix in calcined clay concretes reaches the critical pore size at an early age which leads to early densification of capillary pore space region in comparison to fly ash concretes.