Why Concrete Shrinks and How to Handle It

Hey there! If you’re working on a concrete project or simply looking to understand why concrete does some of the things it does, you're in the right place. One of the most common questions I get about concrete is, “Why does it shrink?” It’s one of those things that can sneak up on you and cause a lot of headaches if you’re not prepared for it. Whether you're pouring a slab for your driveway, building a foundation, or even creating a small patio, understanding the causes of concrete shrinkage and how to handle it can save you a lot of trouble down the road.

Concrete is tough, reliable, and durable—but it's also a living, breathing material that reacts to its environment. When you pour concrete, you’re not just dealing with a solid, immovable substance; you’re working with something that undergoes chemical changes and physical stresses as it cures and hardens. Shrinkage is one of those natural behaviors, and while it’s entirely normal, it can sometimes lead to cracks, gaps, and imperfections if not managed properly.

So, let’s take a deeper dive into why concrete shrinks in the first place, how you can minimize it, and what you can do if it happens on your project.

Why Does Concrete Shrink?

Concrete shrinkage is essentially the reduction in volume that happens as concrete cures. When concrete is mixed and poured, it starts out as a wet slurry of cement, water, and aggregates (like sand and gravel). As this mixture hardens, it goes through a process called hydration, where the cement particles chemically bond with the water molecules. This is how concrete gains strength.

However, as concrete continues to cure and harden over time, the water used in the mix begins to evaporate, and the material naturally contracts. Essentially, the concrete becomes denser as the water escapes, and this shrinkage leads to small gaps or cracks in the surface.

But wait, that’s not all. Concrete shrinkage isn’t just about water evaporating. It’s also influenced by a variety of factors, some of which are completely out of your control. Let’s break it down.

Types of Concrete Shrinkage

• Plastic Shrinkage: This occurs when the concrete is still in its "plastic" or liquid state, before it has fully hardened. Plastic shrinkage is typically caused by rapid evaporation of water from the surface, often due to hot weather, low humidity, or high winds. If this happens too quickly, it can result in surface cracking that may look like web-like patterns across the fresh slab.
• Drying Shrinkage: This happens after the concrete has started setting and begins to cure. As the water inside the concrete slowly evaporates, the mixture contracts. This is the most common form of shrinkage and can occur over a period of several weeks or even months. Drying shrinkage is often the culprit behind cracks that show up in concrete slabs, sidewalks, or driveways as they age.
• Autogenous Shrinkage: This form of shrinkage is more complex and happens within the concrete as it sets. It's caused by the hydration process itself, which draws water from the mix into the chemical bonds that are forming within the concrete. This leads to contraction even if the concrete is kept moist, and while it's less common than drying shrinkage, it can still contribute to some minor cracking in certain mixes.

The Real Problem with Concrete Shrinkage

While shrinkage is a natural part of concrete’s curing process, it doesn’t mean it’s harmless. Over time, especially in larger slabs or structures, the shrinkage can lead to issues that range from cosmetic to structural. Here's why it matters:

• Cracking: One of the most noticeable effects of shrinkage is cracking. Concrete may develop hairline cracks or larger, more visible fissures as it shrinks. These cracks not only look unsightly but can also weaken the concrete, leaving it vulnerable to further damage, such as water infiltration or even structural failure in more extreme cases.
• Loss of Surface Integrity: Shrinkage can also affect the appearance and usability of a concrete surface. For example, in patios or floors, shrinkage cracks can compromise the smoothness of the surface, making it uncomfortable to walk on or even dangerous in some cases.
• Foundation Issues: When shrinkage occurs in larger structures, like house foundations, it can lead to uneven settling or subsidence, which might cause additional structural problems over time. This could affect everything from doors and windows not closing properly to more severe foundation shifting that may require costly repairs.

How to Minimize Concrete Shrinkage

The good news is that shrinkage can be managed—and even minimized—through careful planning, proper techniques, and a little patience. Here are a few steps you can take to handle shrinkage effectively:

• Use the Right Concrete Mix: The mix ratio plays a crucial role in how concrete behaves. Too much water in the mix will increase the potential for shrinkage as it evaporates. Aim for a concrete mix that balances the water content, aggregates, and cement for optimal performance. Modern concrete formulas may also include shrinkage-reducing admixtures, which can help mitigate the effects of shrinkage. These products work by altering the chemical properties of the concrete to reduce the amount of water that evaporates.
• Control the Curing Process: Curing is one of the most important steps in handling concrete shrinkage. As the concrete hardens, it needs to stay moist for the first few days or even weeks. The longer you keep the concrete hydrated, the less likely it is to shrink and crack. Consider covering the concrete with plastic sheeting or wet burlap to retain moisture, especially in hot or dry weather conditions. Curing compounds can also be sprayed onto the surface to help retain moisture, especially for large slabs like driveways or foundations.
• Avoid Pouring in Extreme Conditions: While it may not always be possible, pouring concrete in extreme weather conditions can increase the risk of shrinkage. Hot, dry weather, in particular, encourages rapid evaporation, which leads to plastic shrinkage cracking. Try to schedule pours for cooler times of the day, or use cooling techniques (like wetting the subbase or using ice in the mix) to keep the concrete from drying out too quickly.
• Add Reinforcements: If you’re working on larger slabs or areas prone to significant shrinkage, consider adding reinforcing steel bars (rebar) or wire mesh to the concrete. These reinforcements help hold the concrete together and prevent cracks from spreading as the material shrinks.
• Properly Space Control Joints: Control joints are pre-planned cracks that allow the concrete to shrink in a controlled way without causing random cracking. These joints are cut into the surface after the concrete has started setting but before it fully cures. They help manage where the cracks will appear, keeping them straight and less noticeable.
• Monitor Moisture Levels: If you’re working in an area where moisture levels fluctuate, consider using moisture barriers beneath the concrete slab to control how quickly moisture escapes. This is especially useful for foundation slabs where ground moisture can play a big role in shrinkage behavior.

What to Do If Shrinkage Leads to Cracking

Let’s say shrinkage does occur, and cracks appear. It’s a disappointing reality, but it's not the end of the world. Cracks in concrete can usually be repaired, depending on their severity.

For hairline cracks, you can fill them with a concrete crack filler or epoxy resin. These are readily available at most hardware stores and are simple to apply. Just make sure to clean the crack thoroughly before applying the filler.

For larger cracks or more significant issues, like structural cracks in a foundation, it may be best to call in a professional contractor to assess the damage and recommend the proper repair method. Sometimes, cracks may indicate deeper issues that need addressing before they cause further harm.

Read More: How to Level Concrete Floors Like a Pro: A Homeowner’s Guide

Wrapping Up

Concrete shrinkage is one of those things that can throw you off guard if you don’t know what to expect. But once you understand why it happens and how to minimize it, you can approach your project with more confidence and better results.

Remember, shrinkage is a normal part of the curing process, and with the right techniques, you can manage it effectively. Whether you're working on a small DIY project or a larger structural job, keeping moisture in the mix, controlling the curing process, and using the right materials can go a long way in preventing shrinkage problems from turning into serious issues.

If you do end up with some cracks, don’t panic—there are ways to fix them. By following the steps in this article, you’ll be able to handle concrete shrinkage like a pro, ensuring your projects last for years to come without unnecessary stress or repairs.