Cracks in concrete are a common occurrence and develop when the stresses in the material exceed its strength. This is usually caused by the natural shrinkage of concrete as it hardens and dries. To prevent this, concrete can be given a place to crack, such as a control joint or expansion joint. As concrete cures, it heats up and expands, which can cause cracks to form.
In summer, I saw the slab crack before I could walk on it. Cracks in concrete are often misunderstood, and when a homeowner sees a crack in their slab or wall, especially if the concrete is relatively new, they may assume something is wrong. However, some types of cracks are inevitable and the best thing a contractor can do is try to control them. This is done by properly preparing the subbase, making sure that the concrete is not too wet, using reinforcement where necessary, and correctly placing and spacing the crack control joints and expansion joints.
Fine cracks are commonly observed in newly laid concrete and are due to plastic shrinkage. These cracks are very small, about 0.003 inches (0.08 mm) wide and can be very shallow. Cracks that are wider than a credit card and go through the depth of concrete are structural in nature and could be a sign of more serious problems (see Concrete Crack Repair Assessment). The American Concrete Institute addresses the issue of concrete cracking in its manual from the American Concrete Institute, ACI 302. If you're a decorative concrete contractor, synthetic fiber additives can help reduce this type of cracking, but do little once the concrete has cured.
Inclined cracks sometimes appear when coarse particles of aggregate near the surface form a skeleton in which the cement paste can settle and separate. Again, vibration and revibration will reduce or prevent cracking. Shrinkage cracks cannot always be prevented, but can be controlled by making planes of weakness to establish the direction of cracking when shrinkage occurs. The fine cracks of this floor were brushed with air with brown tint to achieve a beautiful crackle pattern. The following information is useful for diagnosing cracked concrete, but if you want expert opinion, contact a nearby concrete contractor for an in-person assessment. Plastic shrinkage cracks occur when wind speed, low relative humidity, high ambient temperature, or a combination of the three causes water to evaporate from a concrete surface faster than can be replaced by bleeding to the surface.
It is interesting to note that it often cracks in the same pattern that should have been articulated (see figure). Also, if you plan to install ceramic tiles over the slab in this area, make sure your contractor uses a “crack insulation membrane” over the slab since concrete cannot shrink around a corner and stress will cause it to crack from that point. In most cases when cracks appear in concrete, they can be identified and their cause established. However, even the American Concrete Institute does not have standards or recommendations that give a yes or no answer as to which cracks need repair based on width and other factors.