There is one thing we can guarantee when you’re working with concrete:

 

No matter what, concrete will crack.

 

Almost 40 years and thousands of projects later, this rule still applies. Concrete cracks whether you want it to or not.

There are basically three types of everyday cracking. The first and most common, is shrinkage cracking. When concrete is placed, it is a liquid. You must keep it a liquid to get it to form into the shape you want. With the right water-to-cement ratio, you can get a liquid that flows. This makes it easier to push out of a truck or pump to an intended spot.

The amount of water added is tightly controlled. There is a constant battle between the supplier that has promised the concrete will get to a certain strength or hardness, and the placement contractor whose job it is to get the wet gray material out to the middle of a slab and get it flat. The supplier wants it drier because that helps achieve strength and the placement contractor wants that mud wetter because it makes it much easier to get it into place.

The key point to understand in relation to cracking is that water is a certain percentage of the concrete mix. Any material that contains water will shrink as it dries, and the water evaporates. Concrete is no different. A typical 4-inch slab will shrink at least ¼ inch for every 100 sq. ft. of surface space.

As the shrinkage begins, the concrete will crack where it is the weakest. Cracking typically starts within 12 hours of the finishing process. Weather conditions will slow or accelerate it. Shrinkage cracking is typically planned for and handled with control joints.

Control joints are designed cuts. These are intended to cause weakness so that the concrete cracks along the bottom of the control joint which releases the stress from the evaporating moisture. These joints are typically spaced evenly through a project.

It is standard to see control joint placement across the slab with the cuts forming squares that are 10-by-10 or 15-by-15-feet wide.  We strive to have control joints forming smaller areas than standard.

The second most common type of cracking is structural cracking. Although I’ve seen it in new projects, structural cracking doesn’t typically occur there. It tends to be more prevalent in older structures.  Here the one thing that makes concrete valuable, its strength, is its greatest weakness. Because the concrete cannot flex, it must crack. These cracks show up in a variety of widths and directions. When we see elevation variances where one side of the crack is higher than the other. Structural cracks can become tripping hazards when the height variance becomes too high or the crack spreads too wide.

When asked how to repair cracks, it depends on type of crack.  There are several ways to repair. If a crack is 1/4" wide, it is recommended to chase the crack with a wider blade and filling the crack with a high-strength repair material. This can attempt to keep the smaller cracks along the edge of the larger crack from spreading. This method works well, but often highlights the cracking.  Cracks smaller than 1/4" are not necessary to repair and will look worse than crack itself.

The third most common type of cracking, and the one that seems to frustrate customers the most, is called “craze cracking.” This crack is purely aesthetic. Some call it “alligator skin cracking” or map cracking.” It only affects the surface skin of the concrete without creating any structural issues.

 

This type of cracking typically covers a large area and has very fine veins in the surface. The concrete’s skin drying faster than the inside of the slab caused this. In turn the moisture trapped underneath results in its flex. This usually occurs when dry, hot or windy conditions occur while a slab is being placed.  Because it covers a larger area, and because it is so visible, customers tend to be most frustrated about craze cracking. Typically, the cracks will appear white against the darker gray of the troweled slab finish.

There’s no disappearing act

They are a natural part of concrete curing and getting hard. There are very good repair options to keep cracks from getting worse if, but no good method for making them go away.  Remember, concrete — just like any other stone — gets hard and cracks.

Based on an article on concretedecor.net and our experience