Corrosion Classification

[LearningAlways]

We are designing a precast concrete structure roughly 800 feet from the ocean. What corrosion classification do you feel comfortable assigning to foundations?

We are not exposed to salty humid air from oceans but our foundations are subject to fluctuations in seasonal high groundwater table. But... how do I know if water that is exposed to our foundation has high enough contents of salt that it would raise concern regarding corrosion? I read in ACI that foundations exposed to fluctuations in seawater are most at risk versus a foundation completely submerged due to little oxidation occurring when foundation are under water.

How can I be sure that seasonal high groundwater does or does not contain salt? Or should I play it conservative and assume C2, severe, exposed to an external source of chlorides from seawater even though the ocean is 800 feet away?

 

[Agent666]

Testing the water is how you would find out. Dig a test pit and get some samples tested and make an informed decision one way or the other.

If its simply a matter of additional concrete cover vs needing to add supplementary cementitious materials to the mix to improve the durability, then adding more cover is pretty easy to cover the more severe requirement unless adding the supplementary cementitious materials makes sense in terms of any added cost or you are otherwise constrained to a certain cover.

 

[SlideRuleEra]

Or should I play it conservative and assume C2, severe, exposed to an external source of chlorides from seawater even though the ocean is 800 (250 meters) feet away?

If the structure's location (elevation above sea level, NOT distance from shoreline) is subject to storm surge (as from a hurricane or similar event), assume "severe". Otherwise, judgement... as suggested by Agent666.

Concerning coastal atmospheric corrosion, at least 400 to 600 meters from the shoreline is considered at risk... further inland if, for example, there is salt marsh behind a barrier island. Here is a typical graph for a simple shoreline:

See the abstract of: Effect of Distance from Sea on Atmospheric Corrosion Rate

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[elreedyman]

If it is near to the sea so sure the ground water will be salt and its concentration like the sea/ocean salt concentration...but as mention it is flactuated it is called a splash zone which is 2 ft up and down the MSL. So you shall take a lot of precaution for this foundation to maintain its durability with time. The another point the soil test present there is a sulphate or not. In any way you should use a different types of protections.
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