Although exposure classes for concrete structures are typically standard, it's important to be aware of non-standard classes as they can significantly influence the cost. Discover how these classes are utilized, the main differences between them, and their effects on concrete construction and precast.
To begin with, what exactly are they? Concrete structures are built to meet a durability standard, which is typically defined by a design life. This design life represents the number of years a structure is expected to resist degradation until it becomes unserviceable. Exposure classifications help establish the design criteria for typical combinations of design life and environmental conditions.
In the case of steel-reinforced structures, exposure concerns the duration it takes for contaminants and moisture to penetrate the reinforcement, leading to corrosion and eventual failure. The exposure of the reinforcement can happen through cracks or the corrosion of the concrete itself.
Common conditions which are included in the an exposure class assessment will include:
1. Soil Types - highly aggressive salty, sulfate bearing and contaminated soils may call for a higher classification.
2. Climatic Zones - coastal areas require higher classifications.
3. Industrial Zones - structures nearby industries with atmospheric pollutants.
4. Marine - structures that are submerged or in tidal and / or spray zones will require higher classifications.
5. Abrasion - structures subject to significant abrasion from traffic, ie studded wheels, may require a special classification.
6. Freeze / Thaw - elements subject to freezing and thawing cycles will require additional design.
Exposure classes for bridge structures are defined in AS5100.5 and referred to in many other Australian Standards. Common classes are set out in the table below:
| Classification Code | Common Uses |
| A | For elements and structures without steel reinforcing and the inside of full enclosed elements ie. box girders. |
| B1 | Standard class for majority of locations and uses across Australia. |
| B2 | Coastal zones, full submerged marine structures. |
| C1 | Marine spray zones |
| C2 | Marine tidal / splash zones |
| U | Elements in highly aggressive environments that require a custom classification. |
Exposure classes can affect both the design and cost of concrete structures. The following aspects may be impacted:
Elevated exposure classes might necessitate improved concrete mix formulations. This could involve a higher cement content, the addition of particular supplementary cementitious materials, specific types of sands and aggregates, and unique admixtures. Concrete supplied by reputable manufacturers will be produced to meet a designated exposure class.
Proper curing of concrete is crucial for maintaining its durability. Adequate curing helps prevent cracking and maximizes concrete strength to guard against corrosion. Advanced curing techniques may be necessary for higher exposure classes.
The reinforcing cover refers to the clear space between the reinforcement and the outer surface of an element or the formwork. Increasing this cover enhances durability by slowing the penetration of contaminants. A greater cover than standard can often lead to an increase in the overall thickness and weight of precast elements.
The cover may also be adjusted based on the type of formwork used. Steel formwork is considered rigid, allowing for more reliable maintenance of the cover, which can result in a lower cover requirement. Conversely, timber formwork or on-ground casting is less reliable in holding the reinforcement, necessitating a higher cover.
The reinforcing cover is verified during pre-pour quality control checks using a ruler or tape measure. It can also be assessed with an infrared cover meter after casting.
Beyond the aforementioned points, in extremely aggressive environments and critical structures, further durability measures might be necessary. These could involve testing the electrical continuity of reinforcing, implementing cathodic protection, using stainless or galvanized reinforcing, and applying protective coatings.
The expert team at Modcast can assist in assessing the exposure classifications on your project and provide advice on potential impacts to design and cost.