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What can you tell me about curing process of concrete over time?

Q:    I was always under the assumption concrete came to a full cure in about 28 or 29 days. Recently I heard a piece of trivia that said Hoover dam has portions in it that won't fully cure for 500 years. What can you tell me about concrete cure time?


A:     Technically speaking, the concrete in Hoover Dam should continue to gain strength forever. This is true for any concrete that has sufficient moisture for the cement hydration process to continue.
Practically speaking, most concretes gain well over 90% of their total strength in the first few months after placement. After that, the strength gain will continue if moisture is present, but the rate of gain is very slight.
The 28-day strength of concrete is important because that is the basis of acceptance for most projects. For example, if a project specifies 4000 psi concrete, the concrete does not have to reach that level of strength for 28 days. Typically, it is expected that a concrete mix will reach 50% of its design strength in 3 days, 75% in 7days, and 100% at 28 days, as determined by breaking concrete test cylinders that are made on the day of placement.
The 28-day requirement is really a holdover from many years ago, as some projects now specify that the design strength be reached much sooner. In Tennessee, ready mix producers have supplied concrete that reaches 3000 psi in only 24 hours, and sometimes in as little as 16 hours. Of course, this concrete will continue to gain strength as long as it properly cured by keeping moisture available, and will often reach ultimate strengths of 8000 psi or more.