ASSESSMENT OF SUSCEPTIBILITY TO CORROSION OF REINFORCEMENT IN RCC STRUCTURES

Abstract

Reinforced cement concrete (RCC) is one of the most versatile and widely used construction materials throughout the world, but sometimes, such structures show distress. One of the main reasons of the distress is corrosion of the reinforcing steel which mainly depends on quality and quantity of construction materials, exposed environment, concrete cover thickness etc., besides design and construction factors, maintenance etc. The assessment of corrosion of reinforcing steel is rather difficult because the reinforcement is inside the concrete. Thus, destructive methods, wherever permissible, are generally employed to assess the corrosion. Apparently, there is still a need to have more experimental data using simple and non-destructive / partially destructive techniques, which will reflect on the corrosive state of the reinforcement and the life of the structures. The corrosion studies of reinforcement in the concrete made with pozzolana cement (fly ash admixed cement) have led to contradictory results. The work of a number of workers shows that mixing of fly ash in cement reduces the susceptibility to corrosion of reinforcement compared to that observed with Ordinary Portland Cement (OPC) while the work of others shows that susceptibility to corrosion of reinforcement is enhanced in the presence of fly ash. Similarly, the results of the effect of fly ash admixed cement on the depth of carbonation, change in pH, chloride content and permeability of concrete are also contradictory. Further, it has also been observed that inspite of concrete cover of proper thickness over reinforcement, there are occasions when the reinforcing steel has got corroded. Keeping these reports in view, investigations have been carried out to study the effect of (i) fly ash content in concrete, (ii) use of saline water in preparation of concrete and (iii) concrete cover thickness on corrosion of reinforcement under various environments through simple and non-destructive / partially destructive techniques. Thus, the corrosion of bare mild steel deformed bars was first investigated during their exposure to air, potable and saline water upto 1600 hours. Then cubical and cylindrical specimens with concrete cover thickness of 71 and 26mm respectively were cast by reinforcing the same mild steel deformed bars in the centre of the specimens made of three types of concretes; (i) concrete made with ordinary portland cement (OPC), aggregates and potable water (plain concrete), (ii) concrete made with OPC, aggregates and saline water (saline concrete), and (iii) concrete made with fly ash admixed cement (20% replacement of OPC by fly ash), aggregates and potable water (fly ash admixed concrete). Both the cubical and cylindrical specimens were then exposed to three environments - (i) air (normal atmosphere), (ii) potable water (tap water), and (iii) saline water for different time periods, maximum upto 40 months. During the exposure, it was observed that the reinforcements in the specimens did not undergo any significant and measurable corrosion and therefore, only assessment of susceptibility to corrosion of reinforcement at various intervals of exposure could be made. The assessment was made by determining the (i) half cell potential, (ii) pH of the concrete. in (iii) chloride content, (iv) depth of carbonation, (v) visual inspection and (vi) gravimetric tests. The results of these investigations have been discussed here and certain conclusions were drawn. The mild steel deformed bars, without concrete cover, on exposure to air, potable and saline water got corroded in the expected order, saline water > potable water > air. The same MS bars, when used as reinforcements in concrete, did not get corroded on exposure to the same environments during the exposure period from 18 months to 40 months. However, monitoring of half cell potential, pH, carbonation depth and chloride content indicated that the reinforcement, though not corroded during the period of exposure, has become more or less susceptible to corrosion depending upon the nature of the concrete, environment and concrete cover thickness. The half cell potential studies had indicated that it tends to shift towards more negative values in presence of moisture and salts while it shifts towards positive values in the presence of oxygen in dry atmospheric conditions which is in agreement with earlier reports. Therefore, due attention is to be paid to these factors while concluding about the state of corrosion on the basis of half cell potential data. It was also found that fly ash admixed concrete showed less free chloride content at initial stage of exposure (upto 8-10 months) to saline environment. However, on later stage of exposure, the free chloride content in fly ash admixed concrete increases as usual and reaches to more or less same magnitude as in plain concrete. IV Regarding the comparative effect of exposure to various environments, it has been concluded that susceptibility to corrosion of reinforcement is maximum in saline water and minimum in air. The specimens made with fly ash admixed concrete exhibited maximum negative values of half cell potential, maximum carbonation depth and maximum decrease in pH as compared to plain and saline concrete indicating that the reinforcement in fly ash admixed concrete has most favourable conditions for the susceptibility to corrosion. The values of half cell potential, pH, carbonation depth and chloride content studies of concrete specimens of different cover thickness exposed to different environments have led to the conclusion that increase in cover thickness causes decrease in susceptibility to corrosion of reinforcement. The visual inspection of reinforcement and gravimetric test indicated no measurable corrosion of the reinforcement during the period of exposure of concrete speciments in air (40 months), potable water (18 months) and saline water (18 months). These observations when combined with half cell potential, pH, depth of carbonation and chloride content data lead to the definite conclusion that the simple tests can enable us to know the conditions created in the concrete which may make the reinforcement more or less susceptible to corrosion. Thus, simple monitoring like half cell potential, pH, carbonation depth and chloride content may be useful to assess the possibility of corrosion of reinforcement in RCC structures.