Maturity testing is a non destructive testing method that accurately estimates the IN PLACE strength of concrete. Traditional strength tests include casting cylinders in either 6"x12" or 4"x8" molds per ASTM 31. The problem with this method is the results only tell you if the concrete MIX has met project specifications regarding strength, not the strength of the concrete that is in place.
So if your project is large enough with time and money constraints...look into testing for strength the mature way. Click here for the ASTM Standard.
The Concrete Promotional Group of Kansas City just completed a Maturity Testing Workshop on March 9th, 2010. Click here to see their 2 page handout on Maturity Testing.
Special "shout out" to Christy Martin with the Concrete Promotional Group for allowing me to share their document.