>"Concrete can take 4 weeks to get to 90% strength [1], so if you didn't do the prefab route you would have to spend more than 90 days just waiting for the lower levels to cure so that you could build the upper levels."
Concrete made with Type III portland cement achieves most of its strength within seven days (concrete made with normal (Type I) cement reaches its design strength in 28 days).
When one needs significant strength more quickly, the mix can be adjusted to provide a higher ultimate strength than is required for the intended loads during the normal life of the building and accelerants added . This results in adequate capacity for construction to continue in as little as a few hours.
The general rule of thumb is 75% of 28-day strength in 7 days. Most concrete mixtures reach their design strength well before 28 days due to the fact that any particular batch of concrete will be mixed such that its 28-day strength is higher than the design strength by a few 10s of %s. This is due to the fact that failing a compressive strength test for any one test or any 3 averaged consecutive tests (guidelines are set forth for failure criteria in ACI) requires remedial action in general.
Concrete made with Type III portland cement achieves most of its strength within seven days (concrete made with normal (Type I) cement reaches its design strength in 28 days).
When one needs significant strength more quickly, the mix can be adjusted to provide a higher ultimate strength than is required for the intended loads during the normal life of the building and accelerants added . This results in adequate capacity for construction to continue in as little as a few hours.