For those who wanted the answer to the title in these comments, paraphrased it was: with proper maintenance, indefinitely. The &quot;with proper maintenance&quot; phrase should be pointed out to management next time they think something we build should last forever; even these remarkable structures that appear formidably unchanging undergo continuous, hands-on, expert maintenance.<p>There is a tangentially-related Straight Dope thread [1] asking &quot;Is the Hoover Dam concrete still curing?&quot;, because if you take the tour (highly recommended for hackers with an affinity for industrial- and megastructure-scale engineering), you will be told &quot;the concrete is still curing!&quot; by the guides.<p>This is apparently not technically precise; someone from the concrete industry please correct me if I&#x27;m wrong here. I gathered from a civil engineering page that Hoover Dam&#x27;s concrete is considered cured, but it is still undergoing the hydration process as long as water is present in the concrete. [2] I think the Hoover Dam guides can be cut some slack for colloquially using the terminology &quot;curing&quot; in light of this technical distinction that most laypeople won&#x27;t understand.<p>[1] <a href="http:&#x2F;&#x2F;boards.straightdope.com&#x2F;sdmb&#x2F;showthread.php?t=258217" rel="nofollow">http:&#x2F;&#x2F;boards.straightdope.com&#x2F;sdmb&#x2F;showthread.php?t=258217</a><p>[2] <a href="http:&#x2F;&#x2F;www.engr.psu.edu&#x2F;ce&#x2F;courses&#x2F;ce584&#x2F;concrete&#x2F;library&#x2F;construction&#x2F;curing&#x2F;curing.html" rel="nofollow">http:&#x2F;&#x2F;www.engr.psu.edu&#x2F;ce&#x2F;courses&#x2F;ce584&#x2F;concrete&#x2F;library&#x2F;co...</a>