Radhakrishna G Pillai

Currently, highway/railway bridges are designed for the service life of more than 100 y. In such reinforced concrete structures, fusion-bonded epoxy (FBE) coated steel rebars are being used in anticipation of delayed initiation of reinforcement corrosion. However, the FBE steel rebars get exposed to sunlight/ultraviolet rays during prolonged storage and delayed/staged construction. This paper presents microanalytical and electrochemical data (Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersion x-ray diffraction, and electrochemical impedance spectroscopy) and shows the adverse effects of sunlight/UV exposure on the corrosion resistance of FBE-coated steel reinforcement in concrete construction. Based on tests on steel-mortar specimens, the mechanisms of UV-induced chemical changes, shrinkage, and cracking of FBE coating, and the resulting steel corrosion mechanisms are proposed. Also, the adverse effects of sunlight/UV exposure on chloride threshold and reduction in the service life of FBE-coated steel in cementitious systems are presented. The paper recommends to minimize the exposure of FBE-coated steel rebars to sunlight/UV rays to less than one month.

This paper investigates the corrosion and bond characteristics of steel reinforcement with Cement-Polymer-Composite (CPC) coating, which is widely used worldwide to prolong the initiation of corrosion. CPC coating is supposed to be applied on sandblasted or cleaned surface to exploit its full potential. However, CPC coating is generally applied on the rusted or uncleaned surface, which can lead to premature corrosion initiation and associated degradation of the bond between coated steel and concrete. For corrosion studies, 20 lollipop specimens with as-received and sandblasted steels, and with and without CPC coating were cast. These were exposed to chlorides and tested using a recently developed test method based on the linear polarization resistance technique. It was found that as-received, CPC coated steels had 50% less chloride threshold than sandblasted, CPC coated steel. For bond studies, 16 pull-out specimens with CPC coated steel rebars were cast. It was found that even negligible corrosion can lead to ≈50 to 70% reduction in bond strength. This indicates that the corrosion propagation period in the case of CPC coated rebar systems would be negligible. Based on the corrosion and bond results, a new service life model for RC systems with CPC coated steel rebars is proposed. The results highlight that if preventive maintenance is not employed, many structures with CPC coated rebars can experience premature corrosion initiation and significant bond reduction.

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.

Typically, the post-tensioned (PTd) concrete bridges have an anticipated design life or corrosion-free service life of 100+ years. This paper highlights some of the concerns associated with the grouts and grouting practices, which have resulted in a large inventory of inadequately grouted PTd systems. Then, the relevant material properties and associated performance tests to qualify post-tensioning (PT) grouts are discussed. An experimental program on the fluidity, bleed resistance, volume change, and compressive strength of three commercially available PT grouts and one in-house developed PT grout is presented. Then, a compilation of existing standard specifications for PT grouts is presented. Following this, a set of comprehensive, and stringent performance specifications for PT grouts–to enable complete filling of PT ducts–was proposed. Finally, good grouting practices to achieve century-long corrosion protection for PT systems are recommended.

Many concrete structures are built using plain mild (PM) and cold-twisted deformed (CTD) steel rebars. Also, quenched and self-tempered (QST) and prestressing (PS) steels are extensively used in today’s construction. For estimating the corrosion propagation period (tp) of concrete structures, the current practice is to assume that the corrosion rate (icorr) of different steels are equal to that of PM steel—leading to erroneous estimation of tp. This paper provides icorr data from a 33-month experiment on PM, CTD, QST, and PS steels embedded in chloride contaminated mortar. Linear polarization resistance test was adopted for icorr measurement. A total of 100 specimens were tested. It was found that the CTD and QST steels exhibit higher icorr than the PM and PS steels. The paper also provides statistical distributions for icorr of these four steels. For PM, CTD, QST, and PS steel embedded in chloride contaminated mortar, and exposed to wet-dry conditions, these are found to be ∼Weibull (2.5, 20.7, 0) μA/cm2, ∼Lognormal (0.45, 3.2, 0) μA/cm2, ∼Gamma (6.8, 3.5, 0) μA/cm2, and ∼Weibull 3P (1.3, 6.5, −0.02) μA/cm2, respectively. Similar distributions for dry condition are also presented. These statistical tools would help engineers in estimating residual service life and scheduling repair activities.

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.

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 (Cl th ) of such highly resistive steel cementitious (S-C) systems (a key parameter to estimate service life). Most Cl th tests involve repeated measurements of polarization resistance (R p ) 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 R p . This paper experimentally evaluates the suitability of LPR and EIS techniques for assessing R p 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 R p of steel embedded in highly resistive S-C systems are provided.

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.