STUDIES ON THE CORROSION OF STEEL REINFORCEMENT IN CONCRETE IN PRESENCE OF FLY ASH AND OTHER POZZOLANIC MATERIALS

Abstract

The steel reinforcement in concrete remains in the passive state because of the high alkaline environment provided by cement paste. Concrete of low water-cement ratio, well compacted and properly cured, disallows the ingress of chloride ions, carbondioxide and water due' to its low permeability. For corrosion to occur either the alkalinity of concrete in contact with steel must be reduced or chloride ions get penetrated into the concrete. The former may occur due to leaching of alkalis ur by the reaction of pozzolanic materials with the lime liberated from cement hydration, or by carbonation and possibly by presence of chlorides. The latter may occur through the use of some admixtures or exposure to de-icing salts or saline solution (sea water) . Chloride ion has the special property of destroying the passivity of steel surface in the alkaline concrete system thus causing corrosion. Introduction of Portland pozzolana cement in the market in recent years has posed certain problems to the users parti cularly in respect of corrosion of steel reinforcement in the concrete. There is a general apprehension that the incorpora tion of pozzolanic materials in cement concrete might enhance the risk of corrosion of steel in concrete. Hence there is a need to investigate the effect of pozzolanic materials on the corrosion of steel reinforcements. Therefore, a study of corrosion of steel reinforcements in pozzolana cement concrete was systematically undertaken in order to arrive at definite -2 - conclusions regarding the effect of various parameters on corrosion phenomenon in such structures. The results of these studies are incorporated in this thesis. The experimental approaches for this study consisted of: (i) Electro-chemical measurements involving steel/solution electrolyte or steel/concrete electrolyte system: The anodic polarisation studies on the extracts of pozzolanic materials/cements, time-potential measurements of steel in concretes under normal tap water and 3 per cent sodium chloride solution and specific resistance of concrete speci mens in normal and aggressive conditions were carried out. (ii) Accelerated weight loss measurements of steel reinforce ment in concrete: Weight loss measurements of steel reinforcement embedded in concrete specimens were taken under accelerated wetting and drying cycles (wetting in 3 per cent sodium chloride solutions and drying at 55°C) . (iii) Chloride ion fixation by different cement pastes: Chloride fixation by different cement pastes were determined by adding known quantities of NaCl in the pastes and then measuring the free chloride ion in the water extracts at different periods of humid curing. XRD and SEM studies on different cement pastes were also carried out to identify chloroaluminate formations. - 3 - (iv) Field exposure tests on reinforced concrete specimens at different exposure periods: Observations regarding incidence of corrosion of the steel reinforcement and depth of carbonation of the reinforced concrete blocks exposed at number of places of different climatic conditions in the country were carried out. The concrete extracts of the exposed specimens were also analysed for pH as well as chloride and sulphate contents. The studies were basically a comparison of the two types of cementing materials i.e. OPC (ordinary Portland cement) and PPC (Portland pozzolana cement) in regard to their behaviour towards corrosion of steel reinforcement in concrete. Fly ash and calcined clay were the main pozzolanic materials used in the investigation. The concretes were designed for equal 28 days compressive strength. The anodic polarisation studies revealed that the pozzo lana extracts were more corrosive than the cement extracts but there was no difference in the nature of different cement extracts. The potential measurements showed that the protective behaviour of the two types of concrete was more or less the same in normal curing conditions but a distinct superiority of the OPC concrete over PPC concrete was observed in 3 per cent NaCl solution. Accelerated weathering studies indicated higher weight loss in PPC than in OPC concretes. From the chloride fixation studies it was observed that more free chlorides were present in PPC pastes than in the OPC paste indicating lower chloroaluminate formation in the PPC paste. - 4 - This creates favourable conditions for more corrosion of steel reinforcement to occur in the concrete made with pozzo lana cement, Higher extent of corrosion, greater depth of carbonation, lower pH were found in PPC concretes than the corresponding QPC concrete in the field exposure studies. Between fly ash and calcined clay the latter pozzolana was found to induce more corrosion than the former. It appears that although under normal environment the pozzolanic materials may not induce any significant corrosion to steel reinforcement in the PPC concretes, they may cause appreciably higher corrosion under aggressive, i.e. marine and industrial environments mainly due to reduction in alkali nity (pH) and lower tolerance of chloride and other deleterious salts.