Marine

Corrosion of steel reinforcement in concrete is the most frequent problem affecting the durability of reinforced concrete structures. Corrosion caused through chloride attack is one of the main mechanisms of deterioration affecting the long-term performance and serviceability of such structures. 

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

When a necessary amount of chlorides reach the steel reinforcement it penetrates the passivating layer and increases the risk of corrosion. The chloride resistance depends on the the porosity of the concrete (mix constituents, mix proportions, the degree of compaction and curing given to the fresh and hardened concrete) and the level of cover to the reinforcement.

The performance of the concrete cover under service load, in terms of cracking and crack width, also affects the resistance to chloride penetration. Corrosion of steel reinforcement is an electrochemical process. Therefore electrochemical properties of the concrete, such as resistivity, are critical properties affecting the rate of corrosion of the reinforcing steel. 

To limit the level of chloride within the concrete and then within the reinforced or prestressed structure NZS3101:2006 restricts the acid soluble chloride of reinforced concrete exposed to moisture or chloride in service to 0.8 kg/m

In most cases excessive amounts of chloride in concrete originate from external sources. The transfer of chlorides into the concrete occurs through various mechanisms depending on the exposure conditions.

The primary mechanisms involved with chloride ingress are:

Diffusion is 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 form of transport 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.

Capillary absorption is the transport of liquids due to surface tension acting in capillaries. Chloride can therefore be transported in the liquid solution.  When concrete is not in permanent contact with a liquid such as in the tidal zone, the amount of liquid absorbed at the surface of the concrete, as well as the amount of liquid transported at any distance from the surface, is a function of time.

Porosity of the concrete is highly sensitive to water-cement ratio and binder type. 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 with hydration products.  Water-cement ratio and binder type has therefore been used to specify for durability. However, various types of cements have been found to affect chloride resistance of concrete differently. The cement chemical composition affects both the porosity and chloride binding capacity of cement.

Significant amounts of New Zealand’s infrastructure is located close to the coastline and for exposed reinforced concrete structures, durability design in chapter three of NZS3101:2006 is based on limiting the ingress of marine chlorides and consequential corrosion of the reinforcement during the design life of the structure.

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

  • Permanently submerged
  • Tidal
  • Splash
  • Spray

NZS 3101:2006 has used prescriptive requirements in the durability section. This details Cement Binder Type, Minimum Binder Content, Maximum Water/Cement Ratio, Concrete Grade, Minimum Concrete Cover and the Minimum Curing Period.  These are shown in the tables below of use in exposure classification ‘C’ – Tidal/Splash/Spray:

 

Exposure Class

Cement Binder Type

Minimum Binder Content (Kg/m3)

Maximum Water/Cement Ratio

Concrete Grade
(MPa)

Minimum Concrete Cover – 50/100 year design life (mm)

Minimum Curing period
(Days)

C

EverSure and 8% Microsilica 600

350

0.45

45

60/na

7

C

EverSure and 8% Microsilica 600

350

0.45

50

50/50

7

C

EverSure and 8% Microsilica 600

350

0.45

60-100

50/50

7

 

 

Exposure Class

Cement Binder Type

Minimum Binder Content (Kg/m3)

Maximum Water/Cement Ratio

Concrete Grade
(MPa)

Minimum Concrete Cover – 50/100 year design life (mm)

Minimum Curing period
(Days)

C

EverSure and 30% EverPlus

350

0.45

40

60/na

7

C

EverSure and 30% EverPlus

350

0.45

45

60/70

7

C

EverSure and 30% EverPlus

350

0.45

50

60/60

7

C

EverSure and 30% EverPlus

350

0.45

60-100

55/60

7

Life prediction models can be used as an alternative for zones B2 and C however they are outside the scope of the NZ standard. The information in the tables will generally provide solutions which are more conservative than those derived from the use of a model.

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

They also enable the prediction of service life and hence are 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 the more practical specification used for major infrastructure projects when there is sufficient lead time for the appropriate concrete mix design to be developed and tested prior to commencement of construction. Such diffusion coefficients are good indicators of longer-term chloride resistance.

The inclusion of Microsilica 600 or EverPlus Class C Fly Ash at the specified replacement levels deems compliance to the binder requirements of NZS 3101:2006.  Alternatively the use of a ternary blend of Microsilica 600 and EverPlus Class C Fly Ash from our product solution range will offer the highest level of chloride resistance.

As part of our service offering we can provide specialist durability planning advice for major projects and undertake any necessary performance based durability testing.