Are Surface Cracks in Concrete Normal?

Cracks in concrete are common and develop when stresses in concrete exceed its strength. Learn more about why surface cracks occur in concrete and how to prevent them.

Are Surface Cracks in Concrete Normal?

When you spot a crack in a concrete slab or wall, it's natural to assume something has gone wrong. But this isn't always the case. In reality, concrete cracks are very common and some are even inevitable. Cracks in concrete occur when the stresses in the material exceed its strength.

These cracks can range from being non-structural and unsightly to being detrimental to the structural integrity and safety of a building. When a homeowner notices a crack in their slab or wall, especially if the concrete is relatively new, they may assume something is wrong. Some types of cracks are unavoidable. The best thing a contractor can do is try to control cracking 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.

However, cracks can still occur despite these precautions. Concrete will crack naturally 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. The repair of mass concrete structures will depend on the width of the crack, the depth, whether it is idle or alive and on the service conditions of the structure.

Early heat shrinkage cracks are common in cantilever walls that are often used in reservoirs, dams, concrete tanks, retaining walls, bridge pillars, and basements. However, proper site preparation and good concrete finishing practices can greatly contribute to minimizing the occurrence of cracks and producing a more aesthetically pleasing project. Apart from appearance, cracking cracks do not greatly affect the strength or durability of concrete, provided that water intrusion does not occur, which can lead to further deterioration of concrete. If a house is on a hillside lot and there is a living room or open space area under the concrete floor of the garage, all cracks should be evaluated for safety reasons. The high pH (alkalinity) of concrete forms a passive film on the surface of embedded reinforcing steel rods and acts as a protective shield that prevents or minimizes corrosion. If the concrete does not meet industry standards or building code, then an extreme solution may be necessary - such as demolishing the slab and starting over with a new one - but this is better than building on a weak foundation.

Controlling the evaporation rate of the drying surface is key to preventing shrinkage cracking of plastic. Cracking is caused by the drying of the concrete surface, particularly when it has been exposed to low humidity, high air or concrete temperature or hot sun during placement of the concrete mix. While shrinkage cracks can appear on the surface within hours of pouring concrete, it takes a full month for new concrete to fully settle. Some aggregates containing vitreous silica (non-crystalline silicon dioxide) react with alkali hydroxides present in concrete. If you decide to repair a crack, keep in mind that the repair itself is likely to be visible unless you cover it with an overlay. The plug does not seal gaps throughout the wall or slab and its bond to the concrete often fails over time.

The best protection against structural cracking in residential structures is good compaction of soil and gravel under the slab.