Problems occur when the surface of concrete flatwork crusts over and looks like it's ready to finish while the underlying concrete is still soft, like jelly. If finishers get on the crusted surface with a power trowel, the soft underlying concrete may bulge beneath the trowel, creating a wavy or cracked surface. But if they wait for the underlying concrete to stiffen, the top surface is usually too hard to float and trowel.
The article describes the causes of surface crusting and the steps you can take to avoid it. The key is to minimize temperature and moisture differences between the top and bottom layers of concrete. These differences are most influenced by the temperature of the subgrade or supporting formwork and by the condition of the surrounding environment.
Even when you take steps to avoid it, you can still end up with crusted concrete. But all is not lost: This article also offers tips for finishing crusted surfaces.