Cracks in concrete are a common occurrence, and they develop when the stresses in the material exceed its strength. This is usually due to the normal shrinkage of concrete as it hardens and dries. You can expect to see shrinkage and cracks at the base of the slab, which is very common and does not compromise the structural integrity of the home. Moisture, temperature changes, and other factors can cause hardwood floors, moldings, and wooden frames to shrink and acclimatize to reduce indoor humidity.
Similarly, expansion and contraction can occur daily and seasonally due to temperature fluctuations. Narrow cracks are a normal part of concrete slabs. Generally speaking, if the crack is stable and does not leak water, it does not indicate a structural problem. In most cases, these are shrinkage cracks that formed when the concrete cured.
Although they may seem alarming at first, they are actually quite common in almost every home with a concrete floor. If someone tells you that your concrete floor should not have any cracks, be wary - they don't know what they're talking about! It's important for both designers and contractors to inform owners that it's normal to expect some cracks and undulations on each project, and that this does not necessarily reflect negatively on the floor design or construction quality. The addition of water to ready-mixed concrete increases the likelihood of segregation and excessive bleeding, which will make the concrete surface porous, weak, and prone to cracking. This allows the patch to retain its moisture (and therefore be fixed correctly) rather than being absorbed by the surrounding concrete. If it loses its moisture too quickly, the patch may crack and not be able to bond properly with concrete. In most cases, cracks in foundations are beyond your control and can be attributed almost entirely to sudden and extreme changes in climate.
The pieces on the surface crack and are ejected, causing small, shallow blisters that bring the concrete surface closer. Depending on the climate and environment, there may also be a layer of crushed gravel under the slab to drain excess water, which if not properly cared for can cause cracks. A mesh is often used around the perimeter of a slab to help control cracking due to shrinkage. This will allow cracks to occur as the concrete is shrinking around this entire mesh and the mesh may not shrink with it. In addition, you need to consider how many holes you need to drill (for lifting slabs) or how many pillars you will need for the job (depending on the size of the building).
Other types of cracks in concrete, such as those due to impact or load, are discussed in other articles on this website. Water would always be a concern but even so, cracks would do little to prevent water from penetrating because concrete is not waterproof. For fine or non-structural cracks in concrete, it's less clear what is acceptable. In some cases, steel poles are inserted into unstable ground for reinforcement in pillars while hydraulic jacks are used to stabilize concrete slabs if subfloor movement has occurred. Keep in mind that if you decide to repair a crack, the repair itself is likely to be visible unless you cover it with an overlay.