Is hairline cracks normal for new concrete?

Yes, in about a month, fine cracks should disappear. While shrinkage cracks can appear on the surface within hours of pouring concrete, it takes a full month for new concrete to fully settle. Don't be surprised if the cracks that seemed visible at first are almost impossible to see after a month of settlement. A well-made concrete structure should not develop structural cracks quickly.

However, fine cracks can be expected. Don't stress; these cracks aren't a sign of poor quality and can be fixed without much effort. Concrete naturally cracks on its own, unless it is given a place to crack, such as a control joint or expansion joint. As the concrete cures, it heats up and expands.

In summer, I saw the slab crack before I could walk on it. Grooves in concrete are “control joints”, placed at regular intervals to control the location of shrinkage cracks, which are difficult to avoid. Crusting cracking generally occurs during the concrete stamping process, which is a way of adding texture or pattern to concrete surfaces. The concrete shrinks as it dries and causes a kind of tug-of-war that causes the slab to crack as the tremendous forces in the slab literally separate it and cause cracks.

It's so frustrating to go through all the trouble and work to pour a really good concrete slab only to have it break for no apparent reason, so what to do? Some shrinkage cracks are normal, but cracks throughout the slab are a sign that the concrete does not meet standards, may be weaker and less durable than necessary. This ground movement caused by the freeze-thaw cycle is a huge factor contributing to concrete cracking. Synthetic fiber additives can help reduce this type of cracking, but they do little once the concrete has cured. It may be tempting to add more water to concrete to make it easier to work with, but this is a mistake and cracks could form and greatly reduce the strength of the concrete.

If the cracks are stable and less than ¼ inch wide, you can repair them with a flexible sealant made for concrete, such as Sikaflex Concrete Fix. Since these cracks do not affect the overall structural integrity of the slab, concealing and sealing the space is often the primary purpose of such repair. The American Concrete Institute addresses the topic of concrete cracking in its American Concrete Institute manual, ACI 302. After heavy rain or snow melt when the ground below is soft and damp, excessive weight on the slab can press the concrete down and cause cracking. Sedimentation cracks often occur in situations where a vacuum is created in the ground below the concrete surface.

Typically made of a compressible material such as asphalt, rubber, or wood, expansion joints should act as shock absorbers to relieve the stress that expansion exerts on concrete and prevent cracking. Repairing a fine crack in a concrete structure involves filling the space 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.

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