Understanding Concrete Cracks: Why they happen and What to do

Concrete: it's strong, durable, and the foundation of much of our built world. But have you ever noticed cracks in concrete surfaces? It's a common sight, and contrary to what you might think, it doesn't always mean there's a problem. In fact, cracking is often an expected part of the concrete aging process.

The Inevitable Shrinkage

The primary reason concrete cracks is due to shrinkage. Concrete is a mixture of cement, water, aggregates (like sand and gravel), and sometimes admixtures. When the concrete is first poured, it's in a plastic state. As it hardens (a process called hydration), the water evaporates, causing the concrete to shrink in volume. This shrinkage creates internal stresses within the concrete.

Since concrete is strong in compression (withstanding squeezing forces) but relatively weak in tension (withstanding pulling forces), these tensile stresses can easily exceed the concrete's tensile strength. When this happens, cracks form to relieve the stress. This is normal and almost unavoidable.

Other Contributing Factors

Besides shrinkage, several other factors can contribute to concrete cracking:

• Temperature Changes: Concrete expands when heated and contracts when cooled. These thermal stresses can lead to cracking, especially in climates with significant temperature fluctuations.

  
  

• Settlement: If the ground beneath the concrete settles unevenly, it can cause stress and cracking.

  
  

• Applied Loads: Heavy loads or excessive weight on the concrete surface can also contribute to cracking.

  
  

• Improper Mix: An incorrect mix of concrete materials from the plant can lead to weaker concrete that is more prone to cracking.

  
  

The Role of Control Joints

Now, here's where engineering comes in. To manage and control cracking, concrete structures often incorporate control joints. These are intentionally created grooves or cuts in the concrete surface that create a weakened plane. The purpose of control joints is to encourage cracks to form at these predetermined locations, rather than randomly across the surface.

By controlling where the cracks occur, we can minimize their visual impact and prevent them from becoming wider or more problematic. Control joints essentially direct the cracking, ensuring that it happens in a controlled manner that doesn't compromise the structural integrity or aesthetic appeal of the concrete.

Benefits of Control Joints:

• Control the location of cracks.

  
  

• Minimize unsightly random cracking.

  
  

• Reduce the risk of wider, more damaging cracks.

  
  

• Help maintain the structural integrity of the concrete.

  
  

Spacing and Depth:

Control joints are typically spaced at intervals equal to 24 to 36 times the concrete slab thickness. They should be cut to a depth of about one-quarter to one-third of the slab thickness to be effective.

What to do about hairline cracks

Lowes, Home Depot, and other local hardware stores sell inexpensive crack fillers if you prefer that aesthetic however most homeowners choose to leave the cracks alone as the fill will be noticeable.

When to Worry About Cracks

While hairline cracks are usually nothing to worry about, certain types of cracks can indicate a more of a problem, especially if in your home.

• Wide Cracks: Cracks wider than about 1/4 inch.

• Uneven Cracks: Cracks where one side is higher than the other (indicating settlement).

• Cracks in Structural Elements: Cracks in load-bearing walls, columns, or beams. Any cracks here need immediate professional inspection.

  
  

In summary, concrete cracking is a normal and expected phenomenon caused primarily by shrinkage. Control joints are essential for managing these cracks and ensuring the long-term durability and appearance of concrete structures. While most hairline cracks are harmless, be vigilant about larger or more concerning cracks that may require professional attention.