What Causes Spalling?: A Case Study

Concrete has two layers: an attractive, smooth, outer surface, and a rough, rocky interior. Concrete spalling occurs when the attractive top layer chips away, revealing that ugly interior material.

In my research, I’ve come across a great many explanations of what causes spalling. Some claim that damage and wear are the primary causes of spalling. Others say that spalling is caused by rebar corrosion. I have also seen it claimed that concrete spalling is caused by “carbonation of the concrete where carbon dioxide reacts to chemicals within the concrete”, while even others say that poor concrete construction habits are the culprit for most spalling issues.

The Real Cause of Spalling

severe spalling damage on a concrete staircase. Paving stones are used between the concrete steps and show no damage

Concrete spalling can ruin the appearance of an otherwise decorative landscaping design. In this case, an ornate latticework fence and appealing stonework design is framed by a badly-spalling concrete staircase.

Concrete is a much more complicated material than it may appear at first. There are many different varieties of concrete, each containing different chemical compositions in different quantities.

Each type of concrete is made for a different purpose, and is used in different weather conditions. The concrete mixture is also affected by building practices, time given for curing, weather conditions, and by how much air is allowed to be mixed into the concrete mixture.

These different mixtures are used in different ways. You’re not likely to use the same type of concrete to build a bridge or dam that you use to design your patio or sidewalk.

Additionally, concrete surfaces are treated in many different ways. Polished surfaces, stamped surfaces, and normal concrete surfaces will all be susceptible to different things. And on top of that, indoor concrete experiences different problems than outdoor concrete.

In the end, spalling happens for a wide variety of reasons, and each type of concrete has its own challenges. While rebar corrosion may be a primary cause of concrete spalling in a building, it may not be the foremost reason for a sidewalk to begin to spall and deteriorate.

As a homeowner or landscaper, your primary concern is probably going to be learning how to prevent spalling of outdoor concrete — such as porches, patios, sidewalks, walkways, and steps. But what causes spalling in this kind of concrete?

How Spalling Occurs In Outdoor Concrete

a concrete railing covered in spalling damage from ice and snow.

Spalling is worst where moisture collects the most. While walkways are often cleared of snow and ice during the winter, it may not be cleared off other surfaces. This can lead to additional spalling on those areas.

When it comes to concrete sidewalks, patios, stairways, and similar outdoor concrete, spalling is most commonly caused by two factors:

1. Freeze/Thaw Cycles

Concrete is a porous, absorbent material filled with air cavities for water to collect within. During cold weather, this moisture will collect between the attractive, finished top surface of concrete and the rough material underneath.

Water expands as it freezes. As the water expands within the concrete, it pushes up against this smooth concrete surface. Over time, this will cause the surface to “pop” off, revealing the ugly concrete below.

2. Improper Use of Concrete De-Icer

De-icer and rock salt are commonly abused products that can often lead to significant damage to concrete.

De-icers and rock salts are meant to be spread lightly on icy concrete surfaces, where they loosen the concrete and make it easier to remove. It’s expected that the concrete de-icer will be removed along with the snow and ice it helps to melt.

When this doesn’t happen, the de-icer seeps into the concrete along with the moisture. There, it lowers the freezing point of water and can lead to additional freeze/thaw cycles when the temperature drops. Additionally, rock salt will remain as the water evaporates, crystallizing beneath the surface. As these rock salt crystals build up, they can push up on the concrete surface as well, contributing to spalling.

Other Causes Of Damage To Outdoor Concrete

a concrete wall showing significant damage from water and ice.

Concrete spalling has ruined the appearance of this otherwise beautiful patio area.  By sealing the concrete when it was built, the beauty of this patio could have been preserved.

In the picture to the right (click on the photo to maximize), you can see that the concrete has experienced multiple forms of damage.

Even in the summer, it’s obvious where the water has been allowed to freeze on this concrete. The concrete along the top of the wall is pitted and badly damaged, where snow and ice have collected and led to spalling damage.

As the concrete surface has become more and more damaged and porous, it has begun to absorb stains that the finished surface would have been resistant to.

On the right-hand side of the image, you can see that a metal mounting has also been placed in the concrete, where it has rusted and corroded badly, staining the concrete. The concrete steps on the left-hand side have also been stained by rust damage from an unidentified source that has since been removed.

Outdoor concrete can also be permanently stained by soil, berries from trees, moss and algae, and a wide variety of other types of damage. Removing these stains can be difficult, if not impossible, to do.

Click to learn how to clean outdoor concrete.

Protecting Outdoor Concrete From Spalling

a spalling staircase, with steps showing ice damage but little wear and tear damage

Saying that “wear and tear” is the most common cause of concrete spalling simply doesn’t make sense. If true, wouldn’t the outer edges of these steps show the most wear and tear — and therefore be the most damaged?

When concrete is placed without any protection from the elements, it will eventually become spalled, stained, and damaged.

Your best bet for protecting your concrete from stains and damage is to seal it with a sliane-based concrete sealer before the damage occurs.

Sliane-based sealers work their way deep into the pores of the concrete, where they react with the chemicals of the concrete to create a glasslike barrier within the material.

This glasslike barrier will protect your concrete from damage from water and de-icers by preventing them from being absorbed into the concrete.

As an added benefit, sealing the concrete will also eliminate some of the need for de-icer material, as snow and ice will not be able to seep into the pores and “grip” the concrete as it otherwise would.

Concrete sealers such as Concrete Treat: Concrete Sealer And Blanket will not change the appearance of the concrete. Concrete Treat can be applied easily with a roller, sprayer, or brush, to both newly cured and cured concrete. Once applied, it will dry in just 2-4 hours.

Concrete Treat is available to customers worldwide via our online store. We would also like to invite you to contact us to order by phone, to ask questions, or to inquire about distributor prices. We look forward to hearing from you!!

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