Take your standard mix design; blend in aggregates, sand, cement, and water. Then, cast in a cylinder and boil immediately for 2 to 3 hours. That's what Scandinavian testing laboratories are doing these days. They're rapidly adopting a boiling test that indicates the 28 day compressive strength of concrete after only 3 hours. Despite the highly unorthodox idea of a boiling test, it has been used to control the concrete on a number of Copenhagen construction sites for the past six years. Results have proved to be more exact than the normal 7 day compression test. The boiling test is less susceptible to errors, under site conditions, then other types of accelerated tests. The test uses standard 6 inch concrete cylinders cast in heavy, closed steel molds. Immediately after casting, the molds are lowered into a bath of boiling water. At the end of the 2 or 3 hours, the cylinders are removed for the boiling water and immersed for 15 minutes in water at room temperature. There should be ample water present during this cooling off period. The cylinders can then be demolded and subjected to normal compression testing. The strength value obtained is known as the "boiling strength."