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Cheaper and Greener Stainless Steel Processing

Rebar Revelation?

MTO's Highway Infrastructure Innovation Funding Program (HIIFP) recently financed a significant project with the goal of finding less expensive and more environmentally sound alternatives to the current preparation process of stainless steel reinforcing bars. There are high financial and environmental costs associated with the chemical processing that prepares stainless steel bars for use in construction. For this reason, it is in the interest of MTO and other transportation agencies to find a better preparation alternative. In search of this alternative, University of Waterloo engineering professor C. Hansson used MTO's HIIFP funding to carry out preliminary research on stainless steel processing. The results of this research have brought new insights that are valuable to MTO as it builds new structures using stainless steel reinforcement.

The results of Professor Hansson's research may allow MTO to use stainless steel reinforcement without needing to remove the mill scale. Mill scale is an oxide residue that appears after the stainless steel bars are rolled and have cooled. MTO has the mill scale removed before bars are used because it is recommended by the stainless steel industry. The removal process begins by blasting a bar's surface with stainless steel abrasive, and is then completed with a chemical process called 'pickling' that removes remnant residue. This pickling process is carried out in a tank that contains a mixture of nitric and hydrofluoric acids. Clearly, handling and disposing of pickling acid poses a serious environmental problem.

Stainless steel is used because of the extremely slow rate at which it corrodes in a chloride-contaminated concrete environment. Policy introduced by the ministry in the fall of 2000 specified the use of stainless steel reinforcement for structural components with the greatest potential for corrosion and related damage. Transportation authorities are increasingly specifying stainless steel reinforcement for critical elements of structures or for entire structures exposed to severe conditions. Use of stainless steel extends the service life of a structure while minimizing requirements for regular maintenance. This option is preferred for bridges where accessibility for future repairs may be difficult. Ultimately, it reduces maintenance-related delays to highway users, and this is a vital benefit for MTO.

While it is more expensive to purchase than black or epoxy-coated steel, stainless steel reinforcement can be more cost-effective over the long term because there is less of the corrosion- induced concrete damage typically seen with other types of steel and this results in lower life cycle costs.

During Professor Hansson's research, the corrosion behavior of stainless steel reinforcement was evaluated in two environments; 1) a synthetic concrete pore solution with increasing additions of sodium chloride, and 2) ordinary Portland cement concrete prisms with admixed chlorides. The chlorides were added to simulate the behavior of concrete structures exposed to de-icing salts. Tests were performed exclusively on 316LN and 2205 grades of stainless steel, as these are the only ones that MTO specifies for construction use. Initial results indicate that when comparing the average corrosion rates of bars with mill scale to those without, both rates fall within the same scatter of values when undergoing the same surface treatment. There does not appear to be any significant advantage to treatment that attempts to remove mill scale.

These initial results are promising because they indicate that when testing 316LN and 2205 duplex stainless steel, surface condition does not appear to play a major role in determining corrosion resistance.

This finding may encourage MTO to consider eliminating the blasting and pickling steps of processing. Down the road, this study's initial results may produce environmental advantages and could lower MTO's stainless steel reinforcement costs. This project also shows how innovative university research can produce results that have the potential to improve construction techniques. The ministry is optimistic that the results of this research will play a role in the optimization of MTO's specifications for the surface treatment of stainless steel. The results are currently undergoing peer review and must be confirmed by further research and testing.