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.

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.

The overall service life of concrete structures can be divided into corrosion initiation and corrosion propagation phases. The durations of these two phases depend on the chloride threshold (Clth) and corrosion rate (icorr), respectively, of the embedded steel reinforcement. Quantitative information on Clth and icorr of conventional steel reinforcement are available in literature. Now-a-days, the use of prestressed concrete elements are rapidly increasing and there is a dire need for estimating their service life. Quantitative information on the Cl th and icorr of prestressing steel are required for this estimation. However, very limited quantitative information is available on these parameters. As such, the current practice is to assume that both conventional and prestressing steels have similar Clth and icorr-which might result in unrealistic estimations. This paper provides data on i corr obtained from a 9-month long experimental program. The i corr data was obtained using linear polarization resistance (LPR) tests on prestressing wire embedded in mortar. 10 specimens (made using the center king-wires obtained from 7-wire strands) were cast, cured, and subjected to a cyclic wet-dry exposure using 3.5% sodium chloride solution. It is observed that the average value of icorr of prestressing steel exposed to chloride-contaminated mortar is around 5.8 μA/cm2. The paper also provides the probabilistic estimations on corrosion propagation period, t p (defined as the time to crack after corrosion initiation) by substituting the measured icorr data into two models from literature [i.e., Morinaga (1990) and Wang and Zhao (1993)]. It is found that the estimated average tp for prestressed concrete systems are 5.4 and 9.7 years with large scatter. This paper also provides probabilistic estimations on t p for the prestressed concrete systems with 50, 65, and 80 mm cover depths. © 2014 4th International Conference on the Durability of Concrete Structures.

The overall service life of concrete structures can be divided into corrosion initiation and corrosion propagation phases. Although the corrosion propagation period (tp) is usually found to be smaller than the initiation period (ti) it is important to estimate tp for planning and budgeting for the repair activities. The tp depends on the corrosion rate (icorr) of the steel reinforcement. India has many old concrete structures built using the Cold Twisted Deformed (CTD) steel bars. Now-a-days, CTD steel (being highly vulnerable to corrosion) is rarely used and the Thermo-Mechanically Treated (TMT) or Quenched and Self-Tempered (QST) steel bars are extensively used. Quantitative information on icorr of the CTD and TMT/QST bars are required for estimating tp. However, very limited quantitative information is available on icorr to estimate tp. Therefore, the current practice is to assume that the icorr of both CTD and TMT/QST steels are equal to that of plain mild steel, which might result in unrealistic estimations. This paper provides icorr data obtained from 20-month long experimental program. The icorr data were obtained using linear polarization resistance (LPR) tests on CTD and TMT/QST steel bars embedded in mortar. Twenty-five specimens were cast, cured, and subjected to a cyclic wet-dry exposure using 3.5 % sodium chloride solution. It is observed that the icorr of CTD and TMT/QST steel bars can be represented as ∼3PLN(σ, μ, γ); with ∼3PLN(0.3, 3.9, -24) μA/cm2 and ∼3PLN(0.2, 3.6, -20) μA/cm2, respectively. This corresponds to an average icorr of 26.6 and 16.7 μA/cm2, respectively, for CTD and TMT/QST steels. It is also found that, in general, icorr of TMT/QST steel exhibits less scatter than CTD steel. This paper also provides the probabilistic estimations on tp using the measured icorr data and the tp model developed by Wang and Zhao (1993). Based on the estimations, it can be concluded that the median time-to-crack for a system with CTD steel can be approximately 1.8 times less than that of a system with TMT/QST steel - indicating that early notification is required for engineers to prepare an optimized repair strategy for deteriorating 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.

This paper evaluates the suitability of various techniques such as half-cell potential, macrocell corrosion, linear polarization resistance, and electrochemical impedance spectroscopy (EIS) to detect corrosion initiation of fusion-bonded-epoxy (FBE) coated steel rebars in concrete. It was found that EIS is the best technique for this purpose. Then, a new test method (named as “cs-ACT” test) using EIS is developed to detect the initiation of corrosion and determine chloride threshold at the coating-steel interface, which was not a practice in the literature. Also, the reduction in the resistance of the FBE coating was monitored and a a 4-stage degradation process and corrosion initiation process are identified and discussed using SEM, EDAX, and statistical analysis of the change in the polarization resistance of steel (from repeated EIS tests - Nyquist/Bode plots). Then, a new method that uses the properties of epoxy coating, steel-coating interface, and concrete cover to estimate the service life of reinforced concrete systems with FBE coated rebars is demonstrated. Modifications to the existing specifications to achieve target service life are also proposed.

This paper presents data on the chloride diffusion coefficient (D cl ), ageing coefficient (m) and chloride threshold (Cl th ) 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 CO 2 footprint of PFA and LC3 concrete were found to be significantly lower than those of OPC concrete of similar strength. Further, the CO 2 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.

Environmental impact due to the emission of carbon dioxide during concrete production can be taken care by reducing the clinker content in the cement. The clinker content can be reduced by replacing it with fly ash and limestone calcined clay. Such systems can have a potential to exhibit enhanced durability/service life when exposed to chloride and carbon dioxide. However, estimating probabilistic service life of concretes with such alternative binder systems is difficult due to the lack of quantitative estimates of the input parameters such as chloride diffusion coefficient (DCl), ageing coefficient (m), carbonation coefficient (KCO2), and chloride threshold (Clth). This paper presents the experimentally observed estimates of these parameters for the following systems: (I) 100% OPC, (II) 70% OPC + 30% fly ash, and (iii) limestone calcined clay cement (LC3)-known as OPC, PFA, and LC3 concretes, respectively, herein. A total of three concrete mixes were designed. Also, based on these input parameters, the probabilistic service life estimates of a bridge pier and a girder made of these three concretes and exposed to chlorides and carbon dioxide are presented. For chloride ingress study, Fick's 2nd Law of diffusion and Clth have been used. For carbonation study, a recently developed model for estimating carbonation depth (using mixture proportion) have been used. Then, the life-cycle assessment (LCA) of these three concrete systems in terms of the CO2 emissions per unit of concrete per year of estimated service life is presented-for both chloride and carbonation induced corrosion. In chloride laden environments, the service life of the bridge pier and girder systems could be enhanced by about 10 times by using fly ash or LC3 concretes-for similar strength grade concretes. Also, the average annual CO2 emissions (during the expected service life) of PFA and LC3 concretes could be about 3 and 7 times, respectively, lower than that of OPC concretes of similar strength grade. In case of carbonation-induced corrosion, the limited experimental data indicate that the PFA and LC3 concretes could exhibit a lower service life and higher average annual CO2 emissions (during the expected service life) than OPC concretes.