Nice information Nick, but as I was reading, I notice information that might not be correct. I did not read all of the information afterwards.
Curing is usually required for 7 days for slab on grade and foundation walls are required to have the forms left on for at least three days, but most are removed after 24 hours and a curing compound applied.
Test cylinders are taken at the time of the pour (3) and sometimes (4).
Test cylinders are kept in a curing box and picked up the next day and brought to the lab and put in another curing tank.
At 7 days they break a cylinder and break another at 28 days. The 28 day break should achieve the designed strength. If not, another cylinder is kept for 56 days and broken again.
Read below for the rest of it.
ACI 318-02, “Building Code Requirements for Structural Concrete,” and ACI 301-99, “Specifications for Structural Concrete,” contain standards information related to this question.
Section 5.6.2.4 of ACI 318-02 requires a strength test to be the average of the strengths of two cylinders made from the same sample of concrete and tested at 28 days or at the test age designated for determination of the specified compressive strength.
ACI 318-02 doesn’t state a percentage of the specified compressive strength that must be reached at 28 days. In accordance with Section 5.6.3.3 of ACI 318-02, the strength level of an individual class of concrete is considered to be satisfactory if every arithmetic average of any three consecutive strength tests equals or exceeds the specified compressive strength and no individual strength test (average of two cylinders) falls below the specified compressive strength by more than 500 psi (3.45 MPa) when the specified compressive strength is 5000 psi (34.5 MPa) or less.
Results from the 7-day tests mentioned are usually not used for acceptance purposes, and thus there isn’t a percent of the specified compressive strength that the cylinder must meet in order to pass the compressive strength test. Section 1.6.4.2.e of ACI 301-99, for instance, requires molding and curing three cylinders from each concrete sample and testing one specimen at 7 days for information and two specimens at 28 days for acceptance, unless otherwise specified.
The 7-day test result is used to monitor early strength gain and is often estimated to be about 75% of the 28-day strength (Kosmatka, Kerkhoff, and Panarese, “Design and Control of Concrete Mixtures,” PCA, 2002). Neville (Properties of Concrete, 4th Ed.,” Prentice Hall, 1995), however, suggests that if the 28-day strength is to be estimated at 7 days, a relationship between the 28-day and 7-day strengths has to be established experimentally for the given concrete. For this reason, he states that various expressions for the ratio of the two strengths (expressions that were discussed in the previous edition of his book) are no longer thought to be reliable.
Hi, Marcel.
But can you be more specific? What is in error in the article? And, please try to identify the location of the article content in question. Thanks.
Hi Ben, this is where I saw a discrepancy.
Concrete
The concrete compressive strength used in residential construction is typically either 2,500 or 3,000 psi, although other values may be specified. For example, 3,500 psi concrete may be used for improved weathering resistance in particularly severe climates or unusual applications. The concrete compressive strength may be verified in accordance with ASTM C39 (ASTM, 1996). Given that concrete strength increases at a diminishing rate with time, the specified compressive strength is usually associated with the strength attained after 28 days of curing time. At that time, concrete generally attains about 85% of its fully cured compressive strength.
28 day strength should be achieved at that time.
7 day breaks usually indicate the 75% strength and wether or not the 28 day strength will make it.
Curing is usally 7 days for slabs and 3 days for form work unless specifications calls for otherwise.
Not to be confused with test cylinders that cure for 7 days and (2) or (4) that cure for 28 and sometimes 56 if necessary.
Hope that explains it. 
Yep. Engineers use the 28 day compressive strength as the standard. But 28 days is a long wait, so 3-day and 7-day strengths are useful in predicting the ultimate 28-day compressive strength of the concrete.
There is actually one last chemical bond that occurs after 30 years. I’ve core drilled thousands of slabs and old concrete is hard concrete.
So that is what my head is made from…“old concrete”