Why Does Concrete Crack?

A crack in concrete, whether freshly poured or long-existing, does not necessarily mean that the project was unsuccessful, nor does it mean that there was anything wrong with the workmanship or installation at all. In fact, there are many different types of concrete cracking that occur and all of them are extremely common for a …

why does concrete crack

A crack in concrete, whether freshly poured or long-existing, does not necessarily mean that the project was unsuccessful, nor does it mean that there was anything wrong with the workmanship or installation at all. In fact, there are many different types of concrete cracking that occur and all of them are extremely common for a variety of reasons. Many are even considered inevitable, but there are ways to prevent them by understanding why they occur. 

What Type of Concrete Cracks Occur Most Commonly?

Concrete cracks typically belong to one of the following six categories:

Expansion Cracks

The term “expansion joint” is commonly used in relation to concrete, tile, brickwork, and stonework. This is because the materials used in these projects have a tendency to expand and contract under certain conditions, especially large swings in ambient temperature.

For concrete, hotter temperatures cause the material to expand and cooler temperatures cause the material to contract. Concrete can expand as much as ¾ of an inch for every 100 feet of concrete during the largest temperature swings. 

If the concrete is not allowed to expand as much as it needs to and a large concrete slab is either poured without expansion joints or is poured directly against an immovable surface like an exterior block wall, either the concrete, itself, will eventually crack or the abutting immoveable surface will crack, whichever is least flexible under the compression-like forces caused by the expansion.

These cracks are usually substantial in size and very visible. However, they are at least generally avoidable, which we will discuss in greater detail in a later section.  

Shrinkage Cracks

This type of crack will require a bit of a chemistry lesson to understand its causes fully: 

Prior to curing, concrete mix is generally made up of the components displayed in the above chart. As shown, water can account for up to a fifth of the total concrete mass prior to curing, and as you could likely tell by looking at a dry slab of concrete, this large volume of water doesn’t stay in your concrete shape forever (at least not as the water it once was). And yet, your final concrete shape does not change. How is that possible? If you are reducing 20% of something, wouldn’t the overall size decrease with it? This is where chemistry comes into play. 

Most people think that curing concrete is equal to drying concrete and that all water eventually leaves the mixture via evaporation. However, this isn’t the case at all. In actuality, water changes to become part of the structure itself by reacting with Portland cement to form a hydrated product, crystals. These strong, spindly formations connect the various aggregates to one another and retain the overall shape of the concrete. 

However, chemical reactions generally require very specific amounts of reactants to form an equally specific amount of product. This is a concept known as the conservation of mass, and in short, if not enough water is added to your mix, all of the crystals that should have been formed will not do so because there is not enough water for the amount of concrete you have. When this happens, your crystals will not be able to grab onto all of the aggregates consistently throughout the concrete, resulting in a weak structure that may not even “set”. 

On the other hand, if you add too much water, you have yet another unbalanced reaction. There will be too much water to react with the amount of concrete you have, thus resulting in “extra” water that will end up in your final product that still has to go somewhere. In this case, the excess water droplets settle in between the aggregates and the crystals that were able to form. As this water evaporates, it leaves voids throughout the structure and the concrete experiences increased shrinkage, a higher porosity, and greatly reduced compressive strength. The shrinkage of the material is what causes cracks. 

The hairline cracks that form due to shrinkage are not generally very obvious or unsightly. However, if this kind of crack is a concern for you at the surface level, it’s likely also a concern at the subsurface level in regards to the overall strength of your concrete. 

Settling Cracks

This type of concrete crack is much more straight-forward and generally occurs when a void is formed beneath the concrete structure. Concrete is a very heavy material, and gravity is a powerful mechanism. The weight of the concrete, over time, without the support of an underlying surface that has either changed shape, compacted, or has been otherwise removed can all quickly result in settling cracks in concrete. This is a common occurrence with large slab foundations that may have been poured on soil that either gets washed out, shifts, or compacts over time. These cracks are usually substantial in size and appearance. 

Lifting or Heaving Cracks

Imagine filling up a plastic water bottle to the brim with water, placing the cap back on, and tossing the water bottle into the freezer. When you return after the water has frozen, what do you find? A misshapen water bottle that is now larger than it was when you placed it in the freezer. As you likely know from real-life examples such as this, water expands when it freezes. 

This expansion occurs in frozen soil, too. Freezing and thawing cycles in the ground can cause lifting and settling cycles in overlying concrete. As the ground freezes, the ground can swell underneath sidewalks and other slabs, lifting the concrete up unevenly and then dropping it back down again once the ground thaws. This amount of motion can cause lifting or heaving cracks because the concrete is typically inconsistently supported throughout each motion. Tree roots lifting up a concrete sidewalk is another common example. 

Lifting or heaving cracks generally create multiple cracks in criss cross patterns and are very visible. 

Stress Cracks

Concrete generally has a specified design pressure rating, usually in terms of pounds per square inch (PSI). This rating is the amount of weight the concrete can withstand per square inch of a poured structure. If exceeded, the concrete can crack. 

For this reason, certain concrete mixes are designed for certain uses and each use has an allowed PSI rating for the concrete to be used. For example, driveways and sidewalks generally require a rating of up to 2,500 PSI while foundation slabs require a rating of up to 4,000 PSI. In other words, if you park a tractor trailer full of dairy cows on a sidewalk, it will likely result in a stress crack.  

Stress cracks are typically substantial and can span across a concrete slab or structure, following an uneven path. 

Crazing Cracks

When concrete cures too quickly and moisture is not retained long enough on the surface of the concrete, the surface of the concrete can dry too quickly and result in very fine web-like cracks on the surface of the concrete called crazing cracks, 

Slowing down the drying of the surface of a slab is especially a concern when stamping concrete. If the surface of the concrete dries too quickly or unevenly, the act of stamping patterns into concrete can cause a pushing motion on sections with greater moisture content while causing a pulling motion to sections that have begun to dry and cure. This can create a somewhat crinkled appearance on the concrete. These types of cracks are known as crusting cracks. 

Both crazing and crusting cracks are only superficial in nature and do not affect the integrity of the concrete whatsoever. 

How Can I Minimize Concrete Cracks?

Avoid Adding Excessive Amounts of Water to Your Mix

As we’ve discussed, excess water can cause excess shrinkage and result in cracking. You can avoid this issue by watching your water content or utilizing chemical admixtures that reduce the water content. 

Prevent Shifting Soils

By adequately compacting the subgrade beneath your pour, you can prevent settling cracks in your concrete. 

Ensure Proper Water Drainage

It is important to remember that the expansion and contraction of water-logged soil during large temperature swings can cause a variety of cracking types when it occurs underneath your soil, including heaving cracks, settling cracks, and possibly even shrinkage cracks. By ensuring that your subgrade drains properly, you can help extend the life of your concrete and prevent these types of cracks. 

Use Expansion Joints

The use of expansion joints on walls and slabs will allow your concrete to flex and contract, thereby helping to prevent the formation of expansion cracks. 

Provide the Longest Curing Period Possible

Ensuring your concrete does not cure too quickly and that it does not cure unevenly will help prevent excessive shrinkage and cracking while also increasing the compressive strength of your concrete. Curing compounds are available to provide more consistent curing times throughout your concrete. 

Consider Using Additives such as Synthetic Microfibers

Adding synthetic microfibers to concrete can help provide bridges for the crystals to grab onto your aggregates, adding strength and plasticity to the overall concrete structure. This will greatly reduce the occurrence of stress cracks over time. 

Julie Cochran

Julie Cochran

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