What Are Concrete Shrinkage Cracks?

Even the most experienced contractor ends up with cracks in freshly poured concrete. But what exactly are concrete shrinkage cracks, and why are they so hard to avoid? Fortunately, while ugly, these naturally occurring cracks do not affect the structural integrity of the slab. That being said, it's good to know how to avoid concrete shrinkage cracks to keep your driveway looking its best.

What Are Concrete Shrinkage Cracks?

Concrete shrinkage cracks are non-structural cracks that form during the concrete drying process. Concrete is a mixture of water, cement, and various aggregates for strength and appearance. As concrete dries, the water in the top layer of the slab will naturally evaporate more quickly than the bottom half because it’s exposed to sun and air.

During this process, the concrete naturally shrinks as it loses moisture, but not at the same rate throughout the slab. The imbalance causes what is known as tensile stress—or the force on an object when something is stretched. The stress on the top of the slab is greater than the stress at the bottom, causing the concrete to crack.

Your local concrete company may take several precautions to keep shrinkage cracks from forming. They might delicately time out the installation with the weather, place wind blockers around your concrete on windy days, or add expansion joints to encourage cracks in one concentrated spot.

Identifying Shrinkage Cracks

It's important to note that shrinkage cracks only occur as the concrete dries. They are quite common and very tricky to avoid. Thankfully, they are not a threat to your concrete's strength. However, since there are other types of concrete cracks, it's important to know their differences.

Concrete shrinkage cracks are:

• Hairline in width

• Hard to see from afar

• Only sit on the surface of the concrete

Concrete shrinkage cracks are not:

• Thicker than 3mm

• Higher on one side of the crack

• Continuous from one side to the other

• Accompanied by breakage

Common Causes of Concrete Shrinkage Cracks

The major cause of concrete shrinkage cracks is the evaporation of water at uneven speeds throughout the mixture. While there isn't much you can do to prevent it, a few factors can exacerbate concrete shrinkage and lead to larger and more widespread cracking. Here are some ways to prevent concrete cracks from shrinkage.

Too Much Water

Learning how to mix concrete may be a simple DIY, but it's easy to throw off the balance between water and solids. Too much water in concrete will lead to higher evaporation and more tensile pressure as the mixture sets. While it's best not to cut down water too much—this can lead to consistency problems—be sure to create the ideal mixture for your project needs.

Direct Sun or Heat

Avoiding concrete shrinkage cracks all comes down to even evaporation. If you choose to pour the concrete on a hot day in an area with direct sun, you could end up with rapid evaporation and more cracks. The ideal temperature to pour concrete is between 50 and 60 degrees Fahrenheit, ideally when the sun is not at its peak.

High Winds

Remember that both heat and airflow can play a role in concrete shrinkage cracks. Whenever possible, avoid pouring concrete on days with high winds. The quick motion of the air will set the top layer of concrete even faster, leading to more tensile cracks.

How to Repair Shrinkage Cracks in Concrete

Shrinkage cracks are quite small and do not pose a threat to the structure of the slab. That being said, they're not the most attractive addition to a new floor or driveway. Repairing concrete cracks yourself, particularly small ones such as these, is an easy DIY.

The top concrete slab repair methods include:

• Covering the cracks with an epoxy or concrete crack filler and using a putty knife to smooth the surface.

• Resurfacing your concrete driveway with a DIY concrete repair kit and power resurfacer.

• Applying a finishing sealant to your concrete to hide the cracks.

You can also call a professional to repair concrete cracks if you're concerned about the final appearance of your slab. Not only can they enhance the look of your concrete, but they can also offer tips on how to prevent further damage from occurring in the future.