Causes of Concrete Cracks

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

Concrete cracks, corrosion carbonation

Figure 1: Process of carbonation [Image Credit: Robert Pirro]

 There are two major causes of corrosion in the reinforcing steel: carbonation and chloride penetration. Carbonation occurs when carbon dioxide and moisture infiltrate the concrete, reducing the pH level of the concrete (Pirro 2012, p. 29). This process can be seen illustrated in Figure 1. Corrosion of the reinforcing steel in concrete can be a major structural issue. Under normal conditions, the pH level of concrete is high (above 12.5). The high pH of concrete allows an inactive layer of ferric oxide to form around the reinforcement, preventing rust (Khan 2006, p. 14).

Concrete cracks, concrete corrosion chloride penetration

Figure 2: Process of chloride penetration [Image Credit: Robert Pirro]

Chloride penetration reduces the pH level of the concrete when oxygen, chlorides, and moisture all penetrate the concrete (Pirro 2012, p. 20). Chlorides can be found in potable water, which should never be used to mix concrete. There are also environmental factorsthat may add up over the lifespan of a structure. For instance, buildings exposed to salt water or de-icing salts may experience faster chloride build up from the salts (Emmons 1993, p. 12). The chloride penetration process can be viewed in Figure 2.

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

Concrete cracks, crazing

Figure 3: Crazing pattern [Image Credit: Victoria Interval]

Crazing is a web-like series of fine cracks, usually at the surface of the concrete. These can be caused by surface shrinkage, which can occur in low humidity, hot air or sun, and wind (PCA 2001, p. 3). Since these concrete cracks occur on the surface and do not penetrate deeper into the concrete, they do not indicate a deeper structural issue. A general pattern of crazing can be seen in Figure 3.

Disintegration

concrete disinigration

Figure 4: Concrete disintegration around column base [Photo Credit: Robert Pirro]

Concrete disintegration can be a result of freeze/thaw cycles on the surface. Moisture enters concrete pores and expands. The expansions can cause microcracking or they may force off a small amount of the surface. Figure 4 depicts the disintegration of a concrete surface around the base of a steel column and surrounding area. When tiny pieces of the surface come off, it is called disintigration (Pirro 2012, p. 38).

 Plastic Concrete Cracks

plastic concrete cracks

Figure 5: Plastic cracks [Photo Credit: Robert Pirro]

Plastic cracks occur before the concrete has hardened. They are caused by rapid loss of water during curing or settlement in the concrete itself (PCA 2001, p. 2). Hot, dry air and excessive water in the mix may both cause cracking. Hairline cracks may occur in as little as a few hours after a concrete pour, depending on the weather. The thin lines may be misleading; although they may be very thin, these hairline cracks may extend through the entire thickness of the slab (VandeWater 2012, p. 1). See Figure 5 below for an example of plastic cracking. This kind of cracking mostly affects slabs and other large flat surfaces, whose surface area is high relative to the volume of the concrete. This allows the water to evaporate quicker than it can bleed to the surface, causing the cracking (Khan 2006, p. 9). These kinds of cracks may initiate other cracking issues because the plastic cracks sometimes are initiation points for drying shrinkage (Emmons 1993, p. 68).

Hardened Concrete Cracks

hardened concrete cracks

Figure 6: Hardened cracks [Photo Credit: Robert Pirro]

Hardened cracks occur after the concrete has hardened, and are generally caused by drying shrinkage, settlement of the structure below grade, and thermal contraction effects. The cracks form because while the concrete is drying, its volume is being reduced. The condition of the concrete is restrained, so instead of just shortening the slab or member length, cracks form throughout to allow the reduction in volume (Emmons 1993, p. 30). This kind of crack is depicted below in Figure 6. Drying shrinkage is the shrinking (or reduction in volume) of the concrete due to loss of water (evaporation through the concrete surface) (Barth 2001, p. 2). These kinds of cracks may indicate improperly spaced joints (PCA 2001, p. 2).

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

concrete sidewalk disintetgration and delmination

Figure 7: Delamination of concrete caused by premature finishing [Photo Credit: Robert Pirro]

Delamination occurs when the surface of a slab is finished prematurely. When concrete cures, it is necessary for the excess water to escape to the surface (a process called “bleeding”). If a slab is finished before bleeding has occurred, it can trap the water below the surface. When the water does escape, it leaves hollow patches just below the surface. These patches may break open, resembling shattering, to expose the aggregate below as seen in Figure 7 (PCA 2001, p. 12). This type of defect occurs near the surface, and does not indicate a structural threat (unless over a cantilever, where the reinforcing steel is near top portion of the slab).

Overloading Cracks

diagram overloading concrete cracks

Figure 8: Diagram of locations and directions of overloading cracks [Diagram Credit: Robert Pirro]

Overloading a concrete member may cause several types of cracks. Depending on the direction and location of the crack (vertical, diagonal, top, bottom, etc), the type of loading stress can be identified. For example, vertical cracks at the bottom of a simply supported beam and in the center indicate positive flexural cracks. Negative flexural cracks show up over the supports on the top of the beam, also as vertical cracks (Pirro 2012, p. 47). It should be noted that flexural cracks may be related to longitudinal splitting cracks. This relationship is based on splitting cracks allowing moisture to reach the steel pieces in the concrete and corrode them, reducing their ability to resist flexure cracks. Reduction in resistance may cause additional flexural cracks (Giuriani 1998, p. 1). Shear cracks may appear as diagonal cracks at quarter points along the beam member (Pirro 2012, p. 47). See the diagram below in Figure 8 for better understanding of locations of cracking. These cracks can indicate a deeper structural issue if the crack width or lateral displacement exceeds 1/4″ (CFA 2005, p. 3).

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.

concete cracks and damage, spall area concrete reinforcement corrosion

Figure 9: Small spall area caused by corrosion of reinforcment [Photo Credit: Robert Pirro]

concete damage, spall corrosion concrete balcony

Figure 11: Large spall area on all balconies of building [Photo Credit: Robert Pirro]

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