Section 2 Durability

2.1 Zones of exposure

2.1.1 For durability, three zones of exposure are to be considered for concrete structures:

Submerged zone: that part of the structure below the splash zone defined in item (b).
Splash zone: all areas subject to wave action or sea spray, and is to be considered to extend 3,0 m below lightship draught and up to upper deck level, see also Pt 9, Ch 3, 4.3 Analysis of sections for SLS.
Atmospheric zone: that part of the structure above the splash zone.

2.2 Cement content

2.2.1 A minimum content of 400 kg/m³ is to be used for the splash zone. In the submerged and atmospheric zones the minimum cement content is to be 320 kg/m³ where the maximum size of aggregate is 40 mm, or 360 kg/m³ where the maximum size of aggregate is 20 mm.

2.2.2 Cement contents in excess of 500 kg/m³ should generally not be used.

2.3 Water/cement ratio

2.3.1 The water/cement ratio is to be below 0,45 in the submerged zone and below 0,4 for the splash zone (defined in Pt 9, Ch 3, 4.3 Analysis of sections for SLS) and in the boundaries of oil storage tanks.

2.4 Minimum concrete strength

2.4.1 The minimum acceptable concrete strengths are indicated in Table 4.2.1 Minimum acceptable concrete strength.

2.4.2 Concrete tensile strength is also to be measured where required by the design Codes or Standards. For high performance concrete, direct tensile tests should be adopted.

2.5 Temperature

2.5.1 Consideration is to be given to the heat of hydration and shrinkage that may cause cracking.

2.5.2 In cold weather, precautions should be taken to prevent frost damage to the concrete.

2.5.3 Procedures are to be developed and agreed for hot weather concreting (ambient temperature >30°C) and cold weather concreting (ambient temperature <5°C) where applicable.

Table 4.2.1 Minimum acceptable concrete strength

Zone	Exposure conditions	Concrete strength N/mm²
Submerged	Directly exposed to salt water	40
Submerged	Directly exposed to crude oil or subject to severe abrasion	50
Splash	Directly exposed to salt water or salt-water spray	40
Atmospheric	Directly exposed to marine atmosphere	40
Atmospheric	Protected from direct exposure to marine atmosphere	30
NOTES 1. Concrete strength refers to the characteristic concrete strength obtained from testing standard 150 mm cubes of concrete at an age of 28 days. 2. The use of age factors is to be justified by testing.

2.6 Freezing and thawing

2.6.1 Parts of the structure that are subjected to freezing and thawing are to have adequate frost resistance. For severe situations, air entrainment is to be used, and reference is to be made to relevant standards for details of quality of air and spacing factors.

2.6.2 Freeze/thaw cycles may require special consideration for the storage of LPG and LNG in bulk depending upon tank arrangements and/or heating systems.

2.7 Concrete cover reinforcement

2.7.1 The nominal cover is to be not less than that shown in Pt 9, Ch 4, 2.7 Concrete cover reinforcement 2.7.3 or in accordance with the following, whichever is the greater:

1,5 times the nominal maximum size of aggregate.
1,5 times the maximum diameter of reinforcement or pre-stressing tendons.
For bundled bars, the greater of either 1,5 times the diameter of the largest bar in the bundle or the diameter of the equivalent bar, but not more than 100 mm. The equivalent bar is a single bar having the same cross-sectional area as the bundle of bars.

2.7.2 For the concrete given in this Section, the permeability is to be less than m/sec.

2.7.3 For certain types of structural configuration additional cover may be required to prevent deterioration due to acidic water or hydrogen sulphide gas.

Table 4.2.2 Nominal concrete cover in relation to zones of exposure

Zone	Nominal cover, mm, see Note
Zone	Reinforcement	Pre-stress
Submerged	40	85
Splash	50	95
Atmospheric (subjected to spray)	50	95
Atmospheric (general)	40	85
NOTE Nominal cover is defined as the cover to the shear reinforcement.

2.8 Concrete protection against chemical attack

2.8.1 For oil storage tanks, the possible attack by hydrogen sulphide, organic acids, etc. is to be considered.

2.8.2 Where flue gases are used as the inerting medium in tanks, consideration is to be given to the concrete being attacked by and/or in hot, high humidity conditions. This will need to be addressed on a case-by-case basis.

2.8.3 Where sufficiently high concentrations of chemicals may occur which could result in chemical attack, consideration is to be given to providing a suitable chemical resistant liner or partial liner.