What causes cracks in concrete structures and why are they important?

The inherent properties of concrete make it one of the most durable materials to use in construction. Its lack of flexibility, however, makes it prone to cracking – a consequence of dramatic volume changes when it shrinks or expands in certain conditions.  

At first glance, we assume that cracks are the outcome of an error in the pouring procedure, but that’s not necessarily the case. The American Concrete Institute addresses this issue in their manual, ACI 302, 1-40:

“Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrences do not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction.”

Concrete cracks are the result of excess stress being loaded onto the slab. Depending on the type of crack, it might require filing with an adequate material or other repair methods, or no action at all. It all depends on the primary cause of cracking, and whether it’s affecting the integrity and durability of the structure or not.

Why cracks appear

There are many different reasons why they appear on concrete slabs. These are a few:

• Plastic shrinkage: these cracks form as a natural part of the curing process after a new pour. It occurs when concrete is in its plastic form (before hardening), due to the water in the mixture eventually evaporating. They’re usually 1-2mm in width and do not pose a threat to the structure’s stability. An excessively moist concrete mixture contributes to this shrinkage. 
• Thermal stresses: other cracks may be caused by extremely hot or cold environments that cause concrete to expand or contract. When it expands, it pushes against other static structures and causes cracking. Humidity and moisture play an important part in these volume changes as well. 
• Inadequate subgrade support: Concrete heaving or settling may also cause cracking. The main causes for heaving are when large tree roots or ground-freezes lift the concrete slabs out of place. Concrete settling cracks occur when a void is created in the soil underneath the slab, for example, when a tree is removed or pipelines are set into place. 
• Corrosion of steel reinforcements: oxygen and moisture are the main causes of corrosion to metals. This combination produces iron oxides and hydroxides that increase the metal’s volume, which builds pressure within the slab and eventually leads to cracking.  
• Overloading concrete by exceeding its weight capacity: this strong material has its limits, too. Concrete mixes are specially formulated to withstand a certain amount of pressure from external loads (2000, 3000, 4000, or 5000+ PSI). Placing excessive amounts of weight on top of a concrete slab can cause cracking. It is likely to overload the ground below the slab too; especially after freezing or heavy rain when the ground below is soft and damp. 

Concrete crack filling and sealing is necessary maintenance for the care and preservation of concrete slabs because it prevents water or moisture from getting in between the cracks causing rapid degradation. Small cracks are typically repaired with a concrete caulk or liquid filler. Wider cracks are usually patched and sealed with a concrete patching compound. They may also need to be enlarged with a grinder to create a v-shaped groove and then filled with foam and sealed with a polyurethane sealant. ProTech professionals exercise proper crack filling and sealing techniques in Houston, Texas in order to maintain the integrity of the slab.

Check out our Concrete Services. We have over 35 years of experience in the field. Our experts will assess the damage and pick the best solution to ensure your structure’s durability and strength. Our 24/7 response team will be happy to help – just contact us.