Most cracks in concrete occur within 2-3 days of being laid. After a month, these fine cracks should have disappeared. Shrinkage cracks can appear on the surface within hours of pouring the concrete, but it takes a full month for the concrete to settle completely. The freeze-thaw cycle of the ground can cause it to rise several centimeters before settling again, which is a major factor in concrete cracking.
Concrete provides structures with strength, rigidity, and resilience against deformation. However, these characteristics make it difficult for the concrete to move in response to environmental or volume changes. Cracking is often the first sign of distress in concrete, but deterioration may occur before any cracks appear. Cracks can form in both hardened and fresh concrete due to volume changes and repeated loading.
In the first or second year, it's not uncommon for the foundation of a new home to settle a bit. As this happens, the slab may develop some cracks due to the settlement, but these are usually not a major concern. A seven-day cure time is all that's needed for concrete to reach 50% more strength than uncured concrete. Reducing the pH of concrete by carbonation or the ingress of chlorides (salt) can cause the passive film of steel to degrade and lead to volume reduction in hardened concrete. If your concrete is a bit older, a professional can help you fix any cracks.
These are usually caused by drying of the surface, especially when exposed to low humidity, high air or concrete temperature, or hot sun during placement of the mix. While they may not affect the structural integrity of the concrete, they can lead to further deterioration. When you hear that a concrete mix has a strength of 2000, 3000, 4000, or more than 5000 PSI, it refers to the pounds per square inch needed to crush that slab. Cracking is caused by shrinkage of the surface layer and can be prevented by sawing control joints into cured concrete when it's hard enough (usually 6-12 hours after pouring). Certain cracks can be repaired by targeted injection of appropriate material adapted to the diagnosis of each individual crack, followed by an appropriate protective coating.
Expansion joints made from compressible materials like asphalt, rubber, or wood should act as shock absorbers and relieve stress on concrete to prevent cracking. If sublayers are not well compacted when concrete is poured onto them, the weight of the concrete can cause them to sag and lead to cracking. The American Concrete Institute has no standards or recommendations that give an affirmative or negative answer as to which cracks need repair based on width and other factors. This can lead to stresses greater than the tensile strength of concrete and early thermal cracks appearing due to excessive heat generation in thick walls or temperature gradients in thick slabs. Once fully cured, you can also consider using a sealing compound to improve appearance and reduce cracking. Wetting the concrete and covering it with plastic or a tarp can reduce evaporation but requires daily re-wetting which could damage the slab.