Concrete Shrinkage: What Is It, and How Can It Be Prevented?

If installed properly, concrete is extremely durable and low maintenance. It absorbs and retains heat, it’s wind and water-resistant, and it’s non-combustible. Cracked concrete, on the other hand, is a different story.

One of the main culprits of cracked concrete is known as dry-shrinkage cracking, which can be a real pain during the curing process and will most likely lead to even bigger issues in the future.

Why Does Concrete Shrink?

In order for concrete to hydrate and gain strength over time, the minimum amount of water that is needed is 26 gallons per yard. All water over 26 gallons is only used for pumpability and workability of the concrete mixture. 

During the mixing stages, when more water is added than the design requires for a measured slump, it helps to place and work with the concrete. However, the extra water is not used for the hydration process and bleeds out of the concrete. As the water leaves the concrete, it creates a volume change, known as drying shrinkage. If the concrete is not strong enough during the curing process to withstand the tensile forces of this volume change, the concrete will crack.

Why Does Concrete Shrinkage Cause Cracks?

There are many reasons that may cause concrete to crack:

  1. On a very hot and windy day, the surface of the concrete may lose moisture faster than the concrete can tolerate and cause surface shrinkage, known as plastic shrinkage cracking. This can occur throughout the initial setting to final setting phases of the concrete-curing process. The cracking is only on the surface and is not full depth. 
  2. As the concrete goes past the final setting time (up to 7 days) the volume change of the concrete occurs as the excess water not used in hydration leaves the concrete. This type of cracking is typically full depth and can be controlled by placing joints within 8 foot to 12-foot square layouts to force this cracking to occur with aesthetics instead of random placement.
  3. When the top of the concrete is drying faster than the bottom of the concrete, this can cause what is known as thermal differential cracking. This type of cracking is very distinctive as the cracking appears to tear with wide gaps and has a very jagged edge in appearance.

How Is Concrete Shrinkage Prevented?

Maintaining the appropriate mix design water-cement ratios is critical to reducing the potential for concrete shrinkage cracking.  If an increase in workability is desired, look into admixtures to increase your slump and not water. Understand that ambient conditions will affect the concrete such as high heat and low humidity. Make sure the subgrade is moistened and not dry, as this can cause the water to leave quickly as it gets absorbed in dry soil. If you are using rebar, cool down the rebar prior to placing. All of this can be helped by immediately curing the surface of the concrete with a moisture evaporation inhibitor or curing compound.  With G3 Quality’s expertise in concrete placement, we can assist in creating a checklist to help reduce the chances of cracking on your project.

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