42907 Chap.3 Environmental loads and exposure classification
Environmental loads and exposure classification
Exposure classification is the hinge of the subject: it converts a description of where a surface sits into a minimum concrete quality and a minimum cover. The codes name seven of them: six graded rungs from A1 up to C2, plus one more for an environment the table never describes.
This chapter reads the table as a decision tree, sets out the dimensional definitions of the marine bands, and explains the result that looks backwards: a permanently submerged surface is B2, two classes milder than the tidal and splash zone, because submerged concrete is starved of oxygen.
What this chapter covers
- 01
Six graded rungs from mildest to harshest, and the seventh code that is a referral
- 02
The macro environment, a climatic zone for the whole structure, against the micro environment of one surface
- 03
Ground contact rows, and the magnesium content of 1 gram per litre that splits them
- 04
Interior rows: too dry for corrosion, against repeated wetting and drying
- 05
Above ground exterior rows by distance: over 50 km inland, 1 to 50 km near coastal, within 1 km coastal
- 06
Fresh water as B1 and soft or running water as U, and leaching of the paste
- 07
The three marine bands and their dimensional definitions: one metre above wave crest, one metre beyond each tide
- 08
Why the permanently submerged band is milder than the tidal band
- 09
Minimum characteristic strength and initial curing by class, and where curing steps from three days to seven
- 10
Required cover as a function of class and strength, and what a dash in that table means
- 11
What the bridge code adds: a minimum binder content and a maximum water to cement ratio
Classifying the four faces of one marine column
- +1Below the lowest tide less its one metre margin the surface is permanently submerged, so it is B2: unlimited chloride, but starved of oxygen so corrosion cannot propagate quickly.
- +1The tidal and splash band runs from one metre below the low tide to one metre above the high tide. With a low tide at 1 m and a high tide at 2 m that band is 0 m to 3 m, and it is C2, the most severe graded class in the table.
- +1Above 3 m and within one metre of the wave crest the surface is in the spray zone, C1. Higher still and well clear of the crest it leaves the marine rows and is classified as a coastal above ground surface, B2.
- +1You do not cast one column in three concretes over three metres. The member is specified for C2 throughout, because that is the class its harshest surface attracts.
Key terms
- Exposure classification
- A code assigned to a concrete surface from its environment, which then fixes a minimum characteristic strength, a minimum curing period and a required cover. Six graded rungs run from A1 up to C2, with a seventh code reserved for an unlisted environment.
- Classification U
- Not a severity grade but a referral. The table declines to classify the environment and the severity has to be judged case by case; a protective coating on the surface is allowed to count towards that judgement.
- Macro environment
- The climatic zone, tropical, temperate or arid, that applies to the structure as a whole. Temperature and humidity strongly affect chemical deterioration, so the same detail behaves differently in different parts of the country.
- Micro environment
- The conditions at one particular surface of one particular member, which is what the classification table actually reads. Two faces of the same member can sit in different bands and therefore attract different design cases.
- Tidal and splash zone
- The band reaching one metre under the lowest astronomical tide and one metre over the highest, together with the underside of any horizontal member spanning the water. It is the most severe graded class, because wetting and drying cycles deliver both salt and oxygen.
- Spray zone
- Everything from one metre above the crest of the waves upward, one step milder than the tidal band. It receives salt carried in the air rather than by direct immersion.
- Required cover
- The tabulated distance from the concrete surface to the nearest reinforcement, given as a function of exposure class and characteristic strength. A dash in that table is a prohibition of the combination, not a missing value.
- Leaching
- The dissolution of calcium hydroxide and calcium silicate hydrate out of the paste by soft or running water, which is undersaturated in calcium and hydroxide ions. It raises porosity and lowers strength, so cover cannot protect against it.
Environmental loads and exposure classification FAQ
Why is permanently submerged concrete classified milder than tidal concrete?
Because corrosion is an electrochemical process that needs oxygen at the cathode as well as chloride at the anode. A permanently submerged surface is saturated, so oxygen has to reach the steel by diffusion through water rather than through air, and the supply is starved. Corrosion is therefore slow even after the steel depassivates.
Severity peaks where wetting and drying cycle, because each cycle delivers salt by absorption on the wet half and oxygen through air filled pores on the dry half.
What do I do when a surface qualifies under two branches of the table?
Classify it under both and carry the harsher class. A footing under the sea in aggressive ground is a maritime surface and a ground contact surface at the same time, and it has to survive both loads simultaneously. The same rule applies within a single member whose faces sit in different bands: the specification follows the worst face, even where only a small part of the member is exposed that way.
Can I use a thicker cover instead of a stronger concrete?
Only within the table. Where the table carries a dash at a given class and strength the combination is prohibited outright, and no cover thickness makes it acceptable. The physical reason is that a weaker mix has a higher water to binder ratio and a more connected capillary network, so the transport rate through every millimetre of the thicker cover is higher. Cover buys time against a given rate; it cannot change the rate.
How does the bridge code differ from the building code here?
In two ways that matter to every calculation. It adds requirements the building code does not make at all, a minimum cement material content in kilograms per cubic metre and a maximum water to cement ratio, and its covers are thicker at every severe class.
At the top of the ladder the building code asks for 65 mm and the bridge code for 80 mm on comparable high strength concrete, which is in effect the price of doubling the design life prescriptively.
Is the climatic zone part of the classification?
It is, but only for above ground exterior surfaces that are genuinely inland, more than fifty kilometres from the coastline and non industrial. There the ladder runs arid, temperate, tropical as the moisture available for corrosion rises. Once a surface is industrial, or nearer than fifty kilometres to the coast, the climatic zone stops mattering and distance takes over.
Assessment move
Practise this chapter with the tables open rather than from memory, because the examinable skill is finding the right row and justifying it, not reciting the grid. Build the habit of writing the classification as a sentence with its evidence in it: name the branch, quote the condition that put the surface there, and give the distance or the tide level that decided it.
Learn the three marine definitions by their dimensions, since they are the ones most often misread, and drill the counter intuitive ordering until submerged being milder than tidal feels natural. Then practise moving from a class to the two tables it unlocks, remembering to apply the cast against ground and curing compound additions before any comparison with a first principles result.
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