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Galvanised Reinforcement Rods in RCC Structures - technical & economical advantages |
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L Pugazhenthy |
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Corrosion of steel reinforcement in concrete is an electrochemical process which requires access of an electrolyte and oxygen to steel. Protective measures against corrosion rely on minimizing or preventing the corrosive electrochemical process. Four types of protective measures can be identified:
Galvanising - a protector Hot dip galvanizing is a viable means of protecting reinforcement, particularly where the durability of concrete cannot be guaranteed. Its use should be considered for aggressive exposure conditions, precast construction and prestige facades where long life, freedom from rust staining and low maintenance are important criteria. Rust-stained surfaces and cracking and spalling of concrete in recently completed structures demonstrate the wide need to protect steel reinforcement. Galvanized coatings provide important advantages for the protection of reinforcement. Research and practical experience since the 1950s have shown the corrosion resistance of galvanized steel reinforcement to be superior to uncoated steel, while the bond strengths of galvanized and black steel bars to concrete are not significantly different. The corrosion protection of the galvanized coating ensures that the design strength of concrete is maintained and the possibility of surface rust staining and eventual corrosion of reinforcement and spalling of concrete is removed. Steel accessories for use in reinforced concrete structures, particularly fittings and inserts, which may be partially exposed, are susceptible to the effects of corrosion and should be galvanized. Corrosion of reinforcement Corrosion of steel reinforcing bars inevitably weakens concrete members, reducing load bearing capacity and safety factors. In extreme cases failure of reinforced concrete members can occur, partly because of loss of strength due to corrosion of the reinforcement itself, and partly because of the breaking up of the concrete surrounding the reinforcement. When steel reinforcements corrodes, the corrosion product occupies more than three times the volume of the original steel, exerting great disruptive tensile stress on the surrounding conrete, leading to further cracking, more weather access and further corrosion. In mild cases rust staining occurs, in more serious cases, severe spalling of concrete may occur and ultimately concrete members may fail completely. In normal circumstances uncoated steel reinforcing bars give satisfactory service provided the following requirements are maintained:
The benefits of galvanizing reinforcement include:
Factors determining the durability of reinforcement Environment The external environment of the concrete provides the agents, which commonly cause corrosion in reinforcement: water, oxygen, carbon dioxide and chloride ions. Marine structures and structures close to coastal waters are particularly at risk from corrosion of reinforcement due to the ingress of chloride ions from sea spray and salt-laden air. Away from the seacoast most corrosion of reinforcement in concrete is due to the process of carbonation, which reduces the alkalinity of the surrounding concrete. This process can occur at any geographic location. The rate of carbonation is at a maximum when the relative humidity is about 50 per cent, and increases with increasing temperature. Surveys have shown that the corrosion problem in relatively new buildings is worst in coastal areas. Carbonation resistance. Galvanized reinforcement is better able to resist the effects of carbonation because of the much wider range of pH (to about 8) over which the zinc coating remains passivated. Since black steel typically depassivates when the pH of concrete drops below about 11.5, it is apparent that as the carbonation 'front' moves past a galvanized rebar, little or no effect will occur until the concrete adjacent to the reinforcement is almost completely neutralized. Chloride tolerance. Though zinc can be depassivated and attacked in the presence of chloride ions, the tolerance of galvanized reinforcement to chloride depassivation is substantially higher than that of black steel. In a survey of a number of long-serving marine structures [Tonini, DE and Cook, AR (1978) `The performance of galvanized reinforcement in high chloride environments ? field study reports.' International Corrosion Forum, NACE, Houston], galvanized bars were shown to have been exposed to chloride contents as high as 2.2 per cent (by approximately weight of cement) over periods of 10-20 years, with less than 10 per cent loss of original coating thickness and no record of failure. This should be compared to chloride levels in the range of 0.2-0.3 per cent by weight of cement leading to severe corrosion of black steel in similar circumstances. Quality of concrete In preventing corrosion of reinforcement, the most critical property of concrete is permeability. The degree of permeability determines the extent and rate of the diffusion of chloride ions and carbon dioxide through the concrete. Permeability is a function of mix design, compaction and curing. Mix design. To achieve low permeability, concrete must be dense, with a good bond between aggregate and cement paste. These desirable characteristics can be obtained by using good quality materials, with an adequate portland cement content, a low water/cement ratio, and small sized, well graded aggregates. Compaction: Proper compaction of concrete is of vital importance in minimizing permeability. Problems are likely to arise when placing and vibrating techniques are incorrect, reinforcement is congested, or form shapes that are not conducive to the necessary flow of concrete during placement. Curing. Proper curing of concrete is essential to achieve low permeability, as the continued hydration of the cement increases the volume of the gel and hence decreases pore spaces and blocks capillaries. Proper field curing must be provided for. Depth of Cover Lack of concrete cover for reinforcement has been identified as a major problem associated with `failures' in high rise buildings. In survey of 95 Sydney buildings ranging in height from 5 to 36 storeys and aged between 2 and 17 years, the average depth of concrete cover at sites where spalling occurred was 5.45 mm. The maximum depth of cover at any failure point was 18 mm compared with recommended covers in the range 25-30 mm, depending on the type of member. Cracks in concrete The type and size of cracks have an important influence on durability of concrete. Cracks caused by shrinkage or thermal stresses may contribute significantly to reinforcement corrosion, particularly when they run parallel to reinforcing bars and are close to the concrete surface. Crack widths of less than 0.1 mm are generally regarded as not causing significant corrosion risk, provided cover is adequate and the structure is not exposed to highly corrosive environments. Flexural cracks are not generally a problem as they decrease in width from a maximum at the surface and become narrower at the level of the reinforcing steel. Surface treatment of concrete In the production of architectural finishes the concrete surface is sometimes washed or treated to expose the aggregate. These practices are not recommended if there is any possibility of aggressive chemicals such as acids or salts being left behind to permeate the concrete. Etching, washing and mechanical concrete surface finishing may also result in loss of the valuable cement-rich paste which forms the surface layer of the concrete, reducing carbonation resistance and depth of cover. Reaction between galvanized coatings and concrete During initial contact of galvanized reinforcement with wet concrete, the outer zinc layers of the galvanized coating react to form stable insoluble zinc salts. Attack ceases as the concrete hardens and the galvanized coating remains intact. Corrosion protection provided by galvanizing In areas where the reinforcement may be exposed accidentally due to thin or porous concrete, cracking, or damage to the concrete, the galvanized coating provides extended protection. Since the corrosion product of zinc occupies a smaller volume than the corrosion products of iron, any small degree of corrosion, which may occur to the galvanized coating, causes little or no disruption to the surrounding concrete mass. Studies were made at the Structural Engineering Materials Laboratory, University of California, Berkeley California, of the effects of corrosion on reinforced concrete test prisms. Prisms 300 x 100 x 100 mm were axially-reinforced with 19 mm diameter galvanized or black steel bars. A 12.5 mm deep notch was cut at the mid section of each prism to enforce formation of a crack at the notch should corrosion products exert sufficient disruptive stresses. Prisms were placed in loading frames and the steel reinforcing bars stressed to 140 MPa. Prisms were then subjected to alternate immersion/drying cycles in a four per cent NaCI solution. Cracks occurred in test prisms reinforced with uncoated steel bars in less than ten months exposure. Large crack areas had developed by about 18 months and were still increasing at 24 months. No cracks were observed in prisms reinforced with galvanized bars until almost 16 months exposure. These crack areas were very small compared to those in prisms reinforced with uncoated steel bars and crack development ceased after a further 2 1/2 months exposure. Economics of galvanized reinforcement in concrete When the costs and consequences of corrosion damage to a reinforced concrete building are analysed, the extra cost of galvanizing is small. It can be regarded as an `insurance premium', but a premium, which is low and need be paid once only. Currently the cost of galvanizing of rebar is approximately Rs.80000 per tonne of steel and this depends on the quantity of steel to be galvanized, bar diameter etc. While the cost of galvanizing is an important factor, the cost of galvanized reinforcement as a percentage of total building cost is much lower than generally realised. It is as low as 0.5 - 1.0 per cent in many cases; and this is the correct life cycle costing approach to be adopted in such instances. For most structures, even in the most aggressive environments, the use of galvanized reinforcement can be confined to the exposed surfaces and critical structural elements such as:
When related to total project costs the added cost of galvanizing becomes very small indeed. Such costs represent a very small proportion of the cost of repairs, should unprotected reinforcement corrode. Frequently such repairs eliminate only the visible damage and cannot be relied upon as a long-term solution. Accordingly, whenever there is concern that premature corrosion of reinforcement might occur, reinforcement should be galvanized. The use of galvanizing however should not be considered an alternative to the provision of an adequate cover of dense, impermeable concrete. Bond strength of concrete to galvanized reinforcing bars The results of extensive programmes of pull-out testing by a number of researchers reveal no significant difference in the bond strengths of black and galvanized steel deformed (i.e ribbed) reinforcing bars in concrete. The bond strength to concrete has also been studied in tests conducted by University of California in accordance with American Concrete Institute (ACI) standard 208-58. Both corroded and uncorroded rebar were used. Tests were done on concrete beams with plain or; deformed bars cast inverted in the top of the beam. Galvanized rebars showed equal or better bond strength than ungalvanized rebars in all conditions in both plain and, deformed types. Passivation and additives The research into bond strengths also shows that there is little or no need for the current practice of chromate passivation of galvanized reinforcement by the galvanizes (other than to minimize the possibility of wet storage staining), or the alternative addition of chromium trioxide to the concrete mix. The addition of chromates to the concrete mix in the ratio of 35-150 ppm by weight of cement increases the bond strength of galvanized plain bars significantly. Application in India Convinced of the economic and technical benefits of galvanized rebars, the Indian building and construction sector has started using these in a number of projects. The following are some of the known applications for galvanized rebars in India.
Specifying galvanized reinforcement Reinforcing steel should be specified to comply with Indian Standard IS:12594-1988 "Hot DiP Zinc Coating on Structural Steel bars for Concrete Reinforcement-Specification". India Lead Zinc Development Association, has a vast repository of worldwide case histories using hot dip galvanized steel reinforcement in RCC structures. ILZDA welcomes enquiries from the civil engineering community on this emerging application. |
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