Concrete Shrinkage

Concrete shrinkage is the loss of concrete volume during the curing process due to changes in moisture content. While you need water for concrete workability, losing moisture leads to shrinkage, which is the primary cause of cracks and deformations. Some shrinkage is a normal response for drying concrete, but it becomes problematic when it leads to shrinkage cracks.

Learn about the causes of concrete shrinkage cracks and how to control them.

What Causes Concrete Shrinkage Cracks?

Here are the various causes of concrete shrinkage cracks:

• Loss of moisture: The primary cause of shrinkage cracks in concrete is the loss of moisture from the mix, usually due to environmental conditions. When evaporation occurs, the concrete naturally shrinks, resulting in tensile stresses that cause cracks. Remember that some cracks are a normal aspect of the drying process and are not concerning unless the moisture loss is heavy.
• Restraint: External restraints are structures or connections attached to the concrete to increase strength and stability. Internal factors are reinforcements within the mix and aggregates. When external or internal factors restrict the concrete’s natural shrinkage, tensile stresses build up, leading to cracks.
• Mix design: The contents of a concrete mixture and their ratio can influence shrinkage and the severity of cracks. For instance, the type and amount of cement can impact shrinkage. The same goes for the ratio of water and cement inside the mix.

Types of Concrete Shrinkage

There are various ways in which shrinkage in concrete occurs. Here are the main types of concrete shrinkage:

Drying Shrinkage

The normal loss of capillary water over time causes concrete to contract. It’s a process that occurs over months or years when the concrete is dry and hardened. This type of shrinkage increases tensile stressors, which may result in cracking, internal warping and exterior deflection if the stresses exceed the concrete’s tensile strength.

Drying shrinkage cracks can appear anywhere in the concrete, from the surface to the internal structure, especially when there are restraints that prevent the volume reduction. While concrete is strong, drying shrinkage can cause significantly wide and deep cracks that may weaken the structure.

Plastic Shrinkage

Plastic shrinkage is the rapid evaporation of water from the concrete surface before it sets. It occurs due to the effect of climate conditions, such as high temperatures and wind velocity, that cause rapid loss of moisture, leading to surface drying and shrinking. Since the underlying concrete is still wet when the surface layer dries, shallow and irregular cracks begin to form on the surface.

Autogenous Shrinkage

Autogenous shrinkage occurs due to the chemical reactions in the concrete, also known as self-desication, and has nothing to do with climate conditions or the exchange of water with the external environment. In mixes that lack enough water, the chemical consumption of water leads to a high drop in internal humidity and the formation of cracks.

What Are the Consequences of Concrete Shrinkage?

If you’ve ever seen a crack in the sidewalk, you have seen the effects of concrete shrinkage. As the concrete shrinks, the areas where it shrinks experience an increase in tensile stress. The result is often cracking or warping, which disrupts the normal, smooth appearance of the concrete and weakens the concrete structurally.

How to Control Concrete Shrinkage

To reduce shrinkage in concrete and control the formation of cracks, try some of these measures:

Control the Amount of Water

While excess water in the concrete mix leads to rapid evaporation and plastic shrinkage, insufficient water can trigger autogenous shrinkage. You want to control the amount of water, ensuring it’s enough to minimize rapid evaporation and also support the cement hydration process.

 

Use Additives

There are various types of additives you can put in a concrete mix to reduce water without hurting the fluidity of your concrete mix. You can also strengthen the concrete’s curing process by adding curing compounds. If you expect shrinkage, using anti-shrinkage admixtures can help control its occurrence.

Add Control Joints

Another method that contractors commonly employ to deal with concrete drying shrinkage damage is adding control joints. These are joints that the contractor intentionally cuts into the slab of concrete. The idea behind control joints is that, when the concrete does crack due to shrinkage, it cracks along the control joints, which are chosen to minimize the effect the cracking will have on the concrete. In this way, you can maintain the integrity of the concrete even if it cracks from shrinkage.

Choose Dynamic Concrete Pumping for Your Concrete Pumping Needs

Your best bet in reducing the amount of drying shrinkage when using concrete in a construction project is to have a trained professional pour the concrete for you. They will know the right mix of cement and water to use, how to cure the cement to minimize the amount of shrinkage, whether or not control joints are necessary, and where to place them if they are. While some shrinkage is inevitable, it does not have to be a major setback for your project.

For more on the best ways to pour concrete, contact Dynamic Concrete Pumping today.