Concrete is a desirable construction material due to its inherent strength and durability, which make it well-suited for many applications in residential and commercial construction, especially foundations, floor slabs, driveways, sidewalks, and roads, among other things. In these applications, concrete that is properly designed, mixed, placed, and maintained will perform exceptionally well for many years. However, poor design or mix, improper placement, finishing techniques, or inadequate maintenance may result in concrete that does not perform as intended. Aside from cracking, which is probably the most common form of distress to concrete, since virtually all concrete will crack as it cures, surface distress due to scaling, spalling, and popouts are the most common forms of damage.

How is Concrete Made?

Understanding how concrete is made is important to understanding mechanisms that can damage concrete. Basic concrete is made from a mixture of portland cement, aggregate, i.e. rock, sand, water, and, in some instances, chemicals known as admixtures. Water within a mixture has two functions: first, it allows the mixture, in its wet state, to be placed, shaped, and finished; second, the water acts to "hydrate" the cement, which is a chemical reaction that results in hardened cement paste that binds all of the ingredients together. After the concrete is placed, any water that is not used during the hydration process eventually migrates or "bleeds" out and evaporates. The loss of the excess moisture results in a loss of volume, which causes the concrete to shrink. Migration of the water out of the concrete also results in small voids within the concrete, referred to as capillaries. This results in a material that is hard, but porous.

Weathering of Concrete

The constituents of concrete and how it is installed and maintained are highly dependent upon where it is installed and its intended function. Concrete that is installed outside of a structure is exposed to the elements. Depending on the geographical location of the concrete, such elements can be detrimental to the service life of the concrete. The following figure taken from the 2012 International Residential Code indicates the general severity of ambient conditions and the potential weathering of concrete based upon geography.

As is shown, concrete constructed in the northern portion of the continental United States is exposed to more severe weathering conditions than other areas of the United States. This is predominantly due to the concrete's exposure to moisture and below freezing temperatures. Since concrete, in its hardened form, is porous, it is able to absorb and release moisture. Any water trapped in the concrete will freeze when the temperature of the concrete drops below 32 degrees Fahrenheit (°F). Water expands and occupies up to 10 percent (%) more volume when it freezes. If there is not room inside the concrete for the water to expand, substantial internal stresses will develop that can damage the concrete.

Air Entrainment

The American Concrete Institute (ACI) recognizes the potential for damage due to freezing and thawing of trapped moisture, and recommends that concrete that may be exposed to these conditions be air-entrained. Air-entrained concrete is created through the introduction of certain admixtures into the concrete mix, which results in the formation of millions, or even billions, of microscopic air bubbles within the concrete. These air bubbles provide room for trapped water to expand as it freezes and minimize or eliminate freeze-thaw damage from occurring. The amount of air-entrainment needed varies, and is dependent upon the geographic location of the concrete, the harshness of the conditions to which it will be exposed, what the concrete will be used for, and the design strength of the concrete.

Although air-entraining admixtures may be added to the concrete mix, factors in transportation, mixing, and placement of the concrete can result in localized air entrainment that is more or less than what was intended or required. Air entrainment can be lost in transit, particularly if incompatible admixtures are present in the mix. Over-finishing of the concrete may also result in a loss of air-entrainment as some of the air bubbles may be worked out of the mix. Adding water to the surface of the concrete to aid in finishing can reduce the concentration of air bubbles, while simultaneously increasing the water-to-cement ratio, which results in a weaker concrete surface. A loss of air-entrainment can result in a concrete that is more susceptible to freeze-thaw damage. Too much entrained air reduces the strength of the concrete which can lead to durability problems.

Types of Surface Distress - Scaling, Spalling, and Popouts

Scaling is probably the most common form of surface distress, since it can result from multiple different factors.

Scaling is generally identified by the loss of a thin layer of the surface paste as is shown in figure 2. Although scaling typically occurs in layers about 1/16 inch in thickness, repeated scaling over time can penetrate deep into concrete slabs. Scaling can result from freezing of trapped moisture, but may also be a product of overloading, or, possibly, from improper placement and finishing techniques that resulted in weakening of the concrete's surface.

Spalling is the loss of concrete from the surface of the slab and is similar to scaling, but is generally more severe. Whereas scaling is generally no deeper than 1/16 of an inch or so, single instances of spalling may penetrate much deeper, sometimes up to an inch, into concrete. In some instances, large sections of concrete are dislodged (Figure 3). Spalling is more commonly the result of freezing of trapped moisture, which dislodges larger sections of concrete than does scaling. Spalling is common where sections of large aggregate are close to the surface, but can occur anywhere. Spalling may also be due to the expansion of corroding reinforcing steel within a concrete member, although this is more common in structural concrete than concrete flatwork, which often does not contain reinforcing steel bars or welded wire mesh.

Popouts occur when small sections of the concrete surface are dislodged (Figure 4). This type of damage can occur deeper into the concrete's surface, and can resemble spalling in some instances; however, popouts are most often located directly above aggregate material. Popouts result when lower quality, or deleterious aggregates are in the concrete mix. Aggregates, such as shale, chert, and quartz, are undesirable in concrete mixes because they may undergo expansion sufficient to cause disruption of the mix, resulting in popouts, particularly when they are located close to the surface.

De-Icing Compounds

Research has concluded that some de-icing materials adversely affect the long-term performance of concrete. Specifically, "there is significant evidence that magnesium chloride and calcium chloride chemically interact with hardened portland cement paste in concrete resulting in expansive cracking, increased permeability, and a significant loss in compressive strength."[1] As a result of the expansive cracking and increased permeability, moisture is able to penetrate deeper within the concrete than it would have otherwise. This, combined with the decrease in compressive strength greatly increases the rate and severity at which damage to concrete due to freezing of moisture can occur.

In residential applications, homeowners should utilize de-icers sparingly. These chemicals are intended to loosen snow and ice to help facilitate removal. They should not be relied upon as the only solution to clearing away snow and ice, as this requires them to be utilized in a much higher concentration, which can lead to concrete damage.

Maintenance/Repair

Even properly designed and installed concrete can experience problems due to moisture intrusion. The regular application of concrete sealers can minimize moisture intrusion and resulting damage from freeze-thaw cycles. There are a variety of types of sealers on the market, which have different properties, and are intended for different types of concrete. Instructions for installing the sealer will vary from sealer to sealer and even manufacturer to manufacturer. Sealant must be applied proactively, before damage begins and before previous applications are completely worn away. Aside from maintenance of the concrete, additional measures can be taken to avoid possible damage to the concrete. Surface water runoff should be directed away from concrete slabs. Water that is allowed to accumulate around or beneath a slab may enter into the slab from below. Directing runoff away from slabs will also minimize or eliminate the potential for erosion of supporting soils and undermining of the slab.

Damage from scaling or spalling that is identified early, can oftentimes be corrected. Loose material can be removed from the surface of the concrete, making sure that only sound concrete remains, and then applying an overlay on the damage locations. The surface of the existing concrete must be roughened or treated with a bonding agent to ensure the new concrete adheres to the existing concrete. It is recommended that a qualified contractor perform the repairs.

Conclusion

When attempting to diagnose surface distress to concrete, it is important to correctly identify the type or types of damage present, as each has its own set of possible causes. Further investigation into the formulation and placement of the concrete will then be necessary. In some cases, it will be possible to obtain the concrete mix design and/or batch tickets. However, if such information is unavailable, laboratory analysis, called petrography, can be performed on cored samples to determine the physical characteristics of the concrete, including the water-cement ratio and the percentage of entrained air. Understanding the weather conditions when the concrete was placed could also provide key information regarding how the concrete may have been placed or finished. Knowledge of these facts, along with other known facts, can be vital when attempting to determine if the cause of damage to concrete is the result of a design issue, application error, or inadequate or improper maintenance..

[1] "The Deleterious Chemical Effects of Concentrated Deicing Solutions on Portland Cement Concrete", Michigan Tech Transportation Institute