Corrosion caused by chloride attack largely affects the long-term performance of structures involving steel

Concrete offers physical and chemical protection to the reinforcing steel. Steel reinforcement in concrete is protected against corrosion by passivation of the surface due to the high alkalinity of concrete. Chloride resistance depends on:

1. Porosity of the concrete, and the level of cover to the reinforcement: when a necessary amount of chlorides reach the steel reinforcement it penetrates the passivating layer and increases the risk of corrosion.
2. Cracking and crack width of concrete: corrosion of steel reinforcement is an electrochemical process, meaning the electrochemical properties of the concrete, such as resistivity, are critical to the rate of corrosion of the reinforcing steel.

To limit the level of chloride within the concrete and the reinforced or prestressed structure, NZS3101:2006 restricts acid soluble chloride of any reinforced concrete exposed to moisture or chloride to 0.8 kg/m3.

In most cases, excessive amounts of chloride in concrete originate from external sources. This means the transfer of chlorides into concrete occurs through various mechanisms that depend on exposure.

1. Diffusion
   A transfer of the mass of ions in the pore solution – resulting in a net flow from regions of higher concentration to regions of lower concentration of the diffusing substance. This operates in fully submerged concrete structures.
   For porous material like concrete, the diffusion coefficient is the material characteristic property describing the transfer of a given substance driven by concentration gradient.
2. Capillary absorption
   The transport of liquids due to surface tension acting in capillaries – meaning chloride can be transported in liquid solution. When concrete is not in permanent contact with a liquid, like in the tidal zone, the amount of liquid absorbed at the surface of the concrete is a function of time.
3. Porosity
   The connectivity of the pore system depends on the amount of original mixing water filled space, and the degree to which it has been filled. The water-cement ratio and binder type has been used to specify for durability. However, various types of cements can affect chloride resistance of concrete differently. The cement chemical composition affects both the porosity and chloride binding capacity of cement.

A Chloride diffusion coefficient is a measure of the resistance of concrete to chloride diffusion. The diffusion coefficient and concrete cover combine to measure the overall resistance of concrete to chloride ingress.

They also enable the prediction of service life, being the most direct means of specifying chloride penetration resistance. The most common test method utilised is NordTest Build 443.

Diffusion coefficients from a short-term exposure period (28 or 56 days) are a practical specification used for major infrastructure projects – especially when there is adequate lead time for the concrete mix design to be developed and tested.

Diffusion coefficients are good indicators of longer-term chloride resistance.

Durability design in chapter three of NZS3101:2006 is based on limiting the ingress of marine chlorides and the corrosion of steel reinforcement during the design life of the structure

The most aggressive exposure classifications C and B2 are based on chloride ingress in the exposure conditions:

• Permanently submerged
• Tidal
• Splash
• Spray

NZS 3101:2006 has used prescriptive requirements in the durability section, detailing Cement Binder Type, Minimum Binder Content, Maximum Water/Cement Ratio, Concrete Grade, Minimum Concrete Cover and the Minimum Curing Period.

Golden Bay products are durable solutions to combating marine environments.

1. Microsilica or EverPlus Class C Fly Ash comply to the binder requirements of NZS 3101:2006.
2. A ternary blend of EverSure Type GP, Microsilica SF and EverPlus Class C Fly Ash from our product solution range will offer the highest level of chloride resistance.