This article is intended to provide you with some basic reasons as to why concrete cracks and answer some of your questions. Most cracks occur as a result of shrinkage of concrete. Shrinkage is simply a reduction in the volume of concrete as it hardens. If this reduction in volume were unrestricted, then a crack would not occur. However, in reality, ground friction and a number of things such as structural connections inhibit free shrinkage and thus cause cracks.
Lightweight concrete shrinks more.
It is important to note that concrete does crack and that this is normal. What is not normal is an unsightly and excessive amount of cracks. Nonstructural Cracks Not every crack threatens the structural safety of a building. In fact, in many instances, cracks are merely cosmetic in nature. These cracks are typically seen in flat work such as walkways and curbs.