Fine cracks in a concrete slab are rarely a cause for concern. Although they can't be completely eliminated, they can be controlled and should disappear within a month. This is due to the phenomenon of plastic shrinkage, which is when the concrete dries, cures and strengthens, causing it to contract and form small cracks. These cracks are usually very narrow, about the width of a human hair, and shallow.
They don't affect the overall performance of the structure, but they can cause leakage problems. The main cause behind the generation of fine cracks in concrete is plastic shrinkage, which is the rapid depletion of moisture from fresh concrete into its plastic state. Other causes include variations in air temperature, concrete temperature, relative humidity and wind speed on the concrete surface. Additionally, improper site preparation, a poor quality mix and bad concrete finishing practices can contribute to the formation of these cracks.
Repairing a fine crack in a concrete structure involves filling the gap with a material that adheres well to the concrete, restores its original appearance, and prevents liquids from penetrating inside the structure and staining the concrete. After the concrete has fully cured, you can also consider using a concrete sealing compound to improve the appearance and reduce cracking. If your concrete is a little older, a concrete repair professional can help you implement a solution for cracks. ACI 116R-90, Cement and Concrete Terminology, defines fine cracks as cracks in an exposed concrete surface that has widths so small that they are barely noticeable.
Fine cracks in concrete can be difficult to repair but are an acceptable crack that is very difficult to prevent. It's important to note that these cracks are not visible or affect the overall performance of the structure.