Risk of Corrosion

Performance Guidance: Risk of Corrosion and Material Selection

The long term cost effective performance of structures subject to an environment where corrosion is a risk factor is dependent upon designing structures with the right materials for the conditions. Conditions of the environment and the factors that can affect the design can be summed up as follows :

• Given the service exposure conditions, the risk of corrosion is deemed likely to cause major repair prior to the end of the service life

• The structure is classified as being sufficiently important from a usage, strategic, economic or logistical perspective that a repair during the service life can’t be tolerated

• The site conditions for construction and/or maintenance and repair are challenging – get in, get out and stay out!

• The avoidance of future repair and maintenance costs outweighs any additional initial cost of using more durable corrosion resistant materials; the impact is compounded by factoring in cover reduction and user costs

• There are numerous durability solutions though the performance longevity varies widely.

Given the vast range of the service and construction conditions, the structure importance defined by the usage rate and/or being a critical route, the elements of structures which could be under risk of corrosion, and the various design and material solutions that can be applied, there is a need to develop a set of guidelines to assess the conditions and drive to a design solution of the necessary durability measures.

Table 4 [ CLICK TO OPEN: Guidance Framework PDF ] presents a suggested decision framework to guide the owner/designer in selecting an appropriate type of stainless steel rebar for a structure subjected to specific service conditions.

The framework is a three step systematic process involving an assessment of :

1. the level of corrosion risk to which the structure is exposed, 

2. the elements of the structure requiring an extended durability provided by stainless steel rebar, and 

3. a recommendation of the appropriate type(s) of stainless steel rebar which will ensure the design service life is achieved.

The initial step assesses the extent of the risk of corrosion to the structure based on a number of factors such as :

• the general exposure conditions and expected chloride levels, 

• the level of importance and criticality of the structure,  

• the design service life without major repair,

• the likelihood of major repair without durability measures, 

• the ease of future repair access, and

• the extent of the impact or consequences of a major repair

Based on this assessment, a determination can be made as to whether stainless steel is required or not. The output at this point in the assessment process is the designation of a specific Risk Class pertaining to the structure from four options -- LOW, MODERATE, HIGH, VERY HIGH. Based on the Risk Class, the Owner/designer can make a rational fact based preliminary decision as to whether stainless steel rebar is required.  

The second step involves selecting what elements of the structure require this material based on the level of risk of exposure to corrosion.

A non-comprehensive list of the various elements of the different types of structures that are normally at risk of corrosion are as follows : 

Where to use SS

• Direct contact with chlorides or the splash zones

• The wet/dry tidal and the “misting” zones of marine structures

• Specific structure components

1. Bridges 

   a. Decks
   b. Barriers and medians
   c. Bridge beams
   d. Piers
   e. Towers adjacent the roadway
   f. Approach spans
   g. Abutment walls
   h. Roadway joints
   i.  Abutment-bearing seats

2. Underpasses

   a. As per bridges above
   b. Underside of decks in tire splash zone
   c. Columns and piers in tire splash zone

3. Tunnels

   a. In wall construction
   b. Barriers and medians
   c. Entry and exit portals

4. Parking garages

   a. Decks
   b. Columns

5. Marine structures

a. Elements in wet/dry splash zones

The final step involves the selection of the appropriate type(s) stainless steel rebar for the structure. The Owner/designer can choose to specify the recommended group of stainless steel rebar candidates that are deemed to provide sufficient durability to achieve the service life requirements in the Risk Class assessment. Alternatively, a specific type of stainless material from the group can be selected. Generally, given the fluctuations in pricing of the various types of stainless steel rebar due to market forces, it is suggested to specify the recommended group. Due to economies of scale and market forces, in recent years the spectrum of the cost of the various types of stainless steel reinforcement has narrowed accompanied by an overall decline. The Owner/designer should consult with a local rebar supplier for current values.  

Although there may be variations in the physical and corrosion resistant properties among the various types of SSR, the properties are all closely clustered and far superior to those of conventional and coated bars. 

As part of this final assessment it is reasonable for the Owner/designer to conduct a comparative life cycle cost analysis which would consider the cost saving benefits of reduced concrete cover, and the impacts of extensive traffic management, the user costs and the additional repair costs attributable to the option of using less durable materials.

TABLE 4: GUIDANCE FRAMEWORK PDF