Radhakrishna G Pillai

Quenched and Self-Tempered (QST) steel reinforcing bar (rebar herein) is widely used owing to its high strength and ductility over conventional hot-rolled or cold deformed bars. A typical QST rebar constitutes a composite microstructure of a hard ‘tempered martensite’ (TM) peripheral ring and a ductile ‘ferrite-pearlite’ (FP) core. TM and FP are predominantly responsible for the strength and ductility, respectively exhibited by a QST rebar. A good quality QST rebar cross-section shows a uniform, continuous, and concentric TM-ring around the FP core. However, recent studies reported that poor quality QST rebars with inadequate/defective cross-sections could influence mechanical and durability properties. This study evaluates the effects of inadequate cross-sectional phase distribution (CSPD) on the mechanical properties of QST rebars. In stage 1, tensile test on TM and FP extracted from a QST rebar clearly showed that FP is ductile with low strength, while TM had a brittle behaviour with high yield and ultimate strengths. In stage 2, the composite response of QST rebars collected from different manufactures were evaluated using tensile and bend tests to correlate the effect of CSPD. Tensile test results show a significant variability (up to 15%) in the stress–strain behavior and mechanical properties exhibited by rebars with inadequate CSPD while the failure pattern shows a composite response. Poor-quality rebar specimens in bend tests showed cracks which could induce crevice corrosion. Hence, CSPD has a significant influence on the mechanical and corrosion properties of a QST rebar. Acceptance criteria based on TM-ring percentage area and distribution in the CSPD is proposed to check the quality of the QST rebars.

This paper illustrates the effect of chloride-induced corrosion in the flexural capacity of a pretensioned concrete girder in an existing girder-and-slab deck bridge. The numerical study of the time-wise variation of the flexural capacity is based on a proposed model for the loss of cross-sectional area of the prestressing strands. It was observed that almost 46% of the total area of strands can get affected due to chloride-induced corrosion of the girder, by the end of its service life. The corresponding flexural capacity of the girder gets reduced by 50% of its initial capacity.

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.

Fusion-bonded-epoxy (FBE)-coated steel rebars have been used in many concrete structures in anticipation of better corrosion resistance. However, due to premature corrosion observed, FBE-coated rebars are banned in many parts of the world. On the other hand, such rebars with damaged coating are still used widely in some other parts of the world. This paper discusses the thickness, continuity, flexibility, and chemical composition of coating. Also, the performance indicators such as electrical resistance, UV-resistance, moisture resistance, and chloride diffusion coefficient of coating, and the chloride threshold of FBE-coated rebars are discussed. Laboratory tests adopted techniques (EIS, LA-ICP-MS, and EDX) on samples of coating peeled-off from coated rebars and specimens of coated steel rebars embedded in cement mortar, indicate that more comprehensive and stringent specifications are required to promote the use of quality epoxy materials, FBE-coated steel rebars, and construction practices are recommended.

Prestressed concrete technology has revolutionized the infrastructure growth in many countries, especially that of the bridge sector. The bond between prestressed strand and concrete is very important for achieving good structural performance. However, some of the codal provisions have not given enough consideration to the bond strength of pretensioned concrete system in design. This paper presents the results from a preliminary experimental program on the bond strength of 7-wire strands embedded in M35 and M55 concretes. A pull-out test method was developed, and the same was used to determine the bond strength. The bond behavior and the mechanisms at the strand–concrete interface are also discussed. Bond strength of 7-wire strand in M55 concrete is found to be about two times more than that in M35 concrete.

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 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.

Nowadays, Cement-Polymer-Composites are widely used to coat steel rebars to delay the initiation of corrosion in reinforced concrete (RC) structures. However, Cement-Polymer-Composite (CPC) coating is sometimes inadequately applied on rusted steel and can lead to premature under film/crevice corrosion. This paper investigates the effect of such inadequate applications of CPC coating and premature corrosion on the service life of RC structures. For this, maximum surface chloride concentrations, diffusion coefficients, and chloride thresholds were determined by a one-year-long laboratory study on the specimens obtained from a 6-year-old bridge and prepared in the laboratory. Studies found that the chloride threshold of inadequately coated steel rebar (i.e., coating on ‘as received’ surface) is significantly lower than that of the adequately coated steel rebars (i.e., coating on ‘sandblasted’ surface). Also, the corrosion initiation time for systems with inadequately coated steel rebar was about 50% less than that of the systems with adequately coated steel rebars. The corrosion mechanisms were justified with micrographs. It is recommended not to use CPC coated steel rebars if adequate surface preparation (say, cleaning/sandblasting) is not implemented.

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.