In general, cracks in the floor are not something to be worried about. Unless the cracks start to change vertically, which could indicate settlement, tight cracks are usually just a result of the concrete curing. Many homeowners get anxious when they see cracks in their concrete and wonder if it's a sign of a structural problem. The truth is that 95% of the time, these cracks are harmless and nothing to be concerned about.
When concrete hardens, shrinkage is inevitable and cracks are unavoidable. The strength of concrete is measured in pounds per square inch (PSI). When non-crystalline silicon dioxide reacts with alkali hydroxide or other alkalis present in the environment, an alkali silicate gel forms and swells as it absorbs moisture from the cement pore solution. It's important to understand that most concrete cracks are harmless and there's no need to worry.
During the initial setting of concrete, plastic settling cracks form while the concrete is still plastic. Depending on the diagnosis of the individual crack, certain cracks can be repaired by targeted injection of the appropriate material followed by a suitable concrete protective coating. If there is steel reinforcement in the concrete, pressure can cause it to form cracks near the steel which can lead to more extensive cracking as rust builds up until the concrete begins to chip off and exposes the corroded reinforcing steel rods. Pouring concrete when conditions are too high (above 77°F) increases the chances of shrinkage cracking and creating a weak slab.
Stress will cause the concrete to crack from any corners since it cannot shrink around them. If the concrete cover protecting the reinforcing steel is damaged and the joint between the concrete and steel breaks, corrosion of the steel will begin. Old and new concrete do not mix so a “cold joint” forms which can lead to water ingress and deterioration of the concrete. For narrow cracks like this, you can use a self-leveling concrete crack filler to seal it before painting or finishing the surface.
Expansion joints are also used in concrete to allow for expansion and contraction with daily temperature variations. The alkali-aggregate reaction refers to a destructive expansion reaction within concrete that occurs over a long period of time (more than 5 years).