PVC coming handy in seawall construction

Can somebody shed more light on the wisdom of using concrete and steel in seawalls construction in place of PVC (vinyl)? Any chemist who can explain to me how saltwater reacts with steel or/and concrete?
I understand construction companies are shifting to PVC (Vinyl), which is not susceptible to saltwater corrosion. Check the latest press release from Vinyl News Service, available at,http:// www.vinylnewsservice.net/NewsAlerts/NewsAlertsFebruary282007/tabid/128/Default.aspx.
In my view, I think it makes sense, and I guess any material engineer will agree with me, to resort to more durable and cost effective building materials like vinyl.
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I guess you "could" use PVC but it has 2% of steel's modulus & 10% of concrete's.
Plus it's rather expensive compared to concrete.
Plastic pilings are making some progess in harbor applications but again.....plastic isn't very stiff.
cheers Bob
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