Concrete Cracks…That is a fact
Concrete cracks. In fact, it is designed to crack to be able to fully engage the reinforcing steel. Concerns with concrete cracking come up when owners and maintenance workers are unsure of what to look for or are unaware of the implications of certain cracks. Some types of cracking indicate a structural issue, when others do not indicate any type of issue other than normal weathering.
There are many different causes of concrete cracks, which can lead to different types of cracking patterns. Each type of cracking pattern can be associated with a likely cause. If this cause is recognized, it can be identified as structurally vital or non-vital. Determining the type of crack and how it relates to structural issues can minimize economic damage, future deterioration, and in severe cases the loss of human life. In addition, concrete is so variable that properly constructing a concrete structure can be difficult. Issues related to workmanship can include over/under consolidated aggregates, improper location of rebar, over watering for workability, finishing surface before bleeding occurs. Each of these may be unimportant or may contribute to a structural failure. Determining the type of crack is vital so the concrete can be repaired or replaced correctly.
Concrete Repair Specialists offers concrete inspections to determine the cause of concrete damage. For an overview of the causes or cracking and damage, continue reading. We are located just north of Pittsburgh and serve the Southwestern Pennsylvania area.
Overview of the Causes of Concrete Cracks
Overloading
The cross section of concrete is designed with both calculated and estimated loads, determined from building codes. Design factors include the strength of the concrete; the number, sizing, and placement of reinforcing bars; and size and shape of the concrete cross section. When a structure is overloaded to the extent not covered in safety factors, concrete may be damaged or even fail. Overloading may be in shear, flexure, or tension. It may also be a result of fatigue or cyclic loading. Each of these has a different cracking pattern to look for (see Overloading Cracks below).
Corrosion
Both causes of corrosion end similarly. The pH level is the concrete’s last barrier against corrosion, so the reinforcement begins to rust (Khan 2006, p. 14). Rust expands the steel to 10 times the volume, which can cause major problems in the structure (see Spalling below).
Freeze/Thaw
Freezing and thawing cycles can be very detrimental to concrete over time. Unless a protective coating is applied to the concrete, each cycle allows more moisture to penetrate into the concrete. The stress of the moisture freezing inside the concrete causes larger defects with each cycle. Air-entrained concrete can be used to help alleviate some of the expansive stresses of harsh temperature changes. However, not all freeze/thaw effects can be solved in this way and many structures may succumb to cracking either caused or worsened by these cycles. Manufacturers of crack repair kits suggest that cracks less than 1/16″ in thickness can be repaired without professional contractors (“Types” 2012). However, tolerable crack widths may be significantly less than this (0.016″ and less depending on the environment) because cracks may allow deteriorating chemicals to damage the concrete in other ways (Emmons 1993, p. 13).
Alkali-Aggregate Reaction (AAR)
AAR refers to chemical reactions taking place within the concrete mix. Certain aggregates inside the concrete may react with alkalis, causing concrete expansion. The alkalis may be also be from within the concrete mix, or may be from outside sources like sea or ground water, or deicing salts. Depending on the type of aggregate, AAR also goes by other names. In siliceous aggregates, the reactions are called “alklali silica reactivity” (ASR). In dolomitic carbonate rocks, the reactions are called “alkali-carbonate reactivity” (ACR) (Khan 2006, p.15).
When these types of reactions occur, they create a gel-like substance that swells when moisture reaches it. The stresses from the swelling create internal tensile forces, which may crack the concrete from within (Khan 2006, p.15).
Shrinkage
Concrete shrinkage may occur throughout a structure’s life cycle for different reasons with the majority occurring within the first few months or years after casting. There are two primary categories of shrinkage:
- plastic (before hardening)
- drying (after hardening)
Immediately after concrete is poured, there can be settlement shrinkage, construction movement (e.g. formwork movement or removal), and drying shrinkage. After the concrete has fully hardened, a structure will undergo temperature, volume and chemical changes throughout the years (Winterbottom, p. 2). Each of these may also cause concrete shrinkage. Shrinkage is an expected phenomenon in a concrete structure, and can often be controlled with stress-relieving joints and properly placed reinforcing steel.
Types of Cracks
Concrete cracking and defect patterns can often indicate its cause or causes and can help to define whether the crack is
- architectural (affecting aesthetics only)
- structural (may affect the load carrying capacity)
Some of the main types of cracking are described below.
Crazing
Disintegration
Plastic Concrete Cracks
Hardened Concrete Cracks
Scaling
Scaling appears as small divets in the concrete surface in which aggregate may be exposed. Scaling is often caused by freeze/thaw cycles (PCA 2001, p. 10). Because scaling is a surface defect, it does not generally indicate a more serious structural issue.
Delamination
Overloading Cracks
Spalling
Spalling is primarily a result from the corrosion of the reinforcing steel and/or embedded objects such as clips, chairs, anchors, etc. When the steel corrodes, the rust expands to 10 times the original volume, creating internal tension forces in the concrete. Concrete is unable to handle the tension forces, and the pieces between the corroded steel and the nearest surface will break off, called “spalling” (PCA 2001, p. 12).
Even just a small spall can indicate a much larger issue for two main reasons.
- First, a small spall can expose the steel, leaving it ultra-vulnerable to more corrosive elements. This can been seen in Figure 9. If the steel corrodes more, there will be more spalling, as seen in Figure 10.
- Second, a spall in one area may be the first piece of a larger issue beneath the surface. It is likely that other rebar in the immediate area has also been affected by the corrosive effects and will begin to spall soon. Small spalls are relatively simple and inexpensive to fix, and repairing these early on can help to avoid large spalling areas.
A large spall area in a slab may indicate immediate danger to a structure. If enough concrete has spalled off of the bottom, exposing the reinforcing grid, then the concrete and steel are no longer working together to handle the compressive and tension forces. Essentially, when the concrete reaches its tensile limit, it will fail. The steel is not engaged by the concrete to take the excess tensile forces, and is only acting as a cage to hold up the concrete. At this stage, repairs may be enormously expensive. Figure 11 shows a whole building spalling failure.
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Contact Us Today
Concrete Repair Specialists is located just north of Pittsburgh and we serve Western Pennsylvania and Ohio. To learn more about how we can repair and improve the look of concrete in your business or home contact us.
- Online form
- Phone: 724-773-8001