Types of Cracks in Concrete

Written By Mary Nguyen

Concrete's strength, rigidity, and resistance to deformation make it a reliable material for constructing structures. For example, highways that are made up of concrete pavement can last for up to 40 years (FHWA). However, just like other building materials, concrete is still susceptible to cracking. The primary causes of cracking include changes in temperature that result in the concrete expanding and contracting, the material composition of the concrete mix, the ground it is poured on, weather conditions such as sun exposure, erosion, and chemical reactions occurring within or on the surface of the concrete. Load and support circumstances also play a role in concrete deflection and cracking. If these factors are not considered during construction, cracks are more likely to appear. Proper site preparation, concrete mix, and finishing techniques can work to reduce these cracks and improve the concrete’s appearance.

Let’s have a look at the different types of cracks commonly found in concrete.

1) Plastic shrinkage cracks

Plastic shrinkage cracks appear within the first few hours of placing the concrete and prior to the concrete setting. These cracks occur when the top layer of freshly laid concrete loses water faster than it can bleed. As the water evaporates, empty spaces form between the solid particles, weakening the concrete and causing it to crack. This typically happens in concrete slabs, where the surface dries quickly, causing it to become very stiff and unable to handle the tensile stress.

Image of plastic shrinkage cracks

Plastic shrinking in concrete

2) Plastic settlement cracks

In reinforced structures, plastic settlement cracks can appear before the concrete sets. These cracks occur when there is severe bleeding, and when the settling of solids is obstructed by obstacles like reinforcement bars or reinforcing mesh (Figure 1).

Formation of plastic settlement crack (initial and final)

Figure 1 - Formation of plastic settlement crack (initial and after a few hours)

Diagram Credit: concrete.org.uk

Fresh concrete tends to settle or recede in deep formwork like a wall or column. If this settlement is hindered by obstructions such as steel bars or large aggregates, this can result in parallel fractures (Figure 2). Additionally, there could be shorter cracks perpendicular to the bars, extending in the opposite direction. As a result of settlement, noticeable undulations may appear on the concrete surface (Figure 3), with the highest points aligning with the top reinforcing bars.

General Plan View of Plastic Cracks

Figure 2 - General Plan View of Plastic Cracks

Diagram Credit: The Constructor

Section showing undulations

Figure 3 - Undulations on Concrete Surface

Diagram Credit: The Constructor

3) Expansion cracks

Concrete slabs expand when exposed to high temperatures in hot climatic conditions. If there isn't enough room for expansion, the concrete will crack. This is where expansion joints play a crucial role in helping to absorb stress and prevent cracking. Expansion joints are made up of compressible materials and are inserted between the slabs to prevent cracks from occurring.

4) Heaving cracks

Heaving cracks in concrete refer to cracks that result from the upward movement or displacement of concrete, leading to a visible separation or opening. These cracks typically form when there is a significant increase in volume within the concrete mass, exerting pressure and causing it to heave or rise.

Heaving cracks are caused by the freeze/thaw cycle. The freeze-thaw cycle occurs when water fills the voids of a rigid, porous material where it expands as it freezes and contracts as it thaws. Frozen ground can shift due to the freeze-thaw cycle caused by groundwater freezing and thawing during extremely cold weather. Plant roots, particularly those of trees growing near concrete slabs can also cause movement and cracks in the concrete. These two factors contribute to the damage in concrete.

Since concrete structures are rigid and are unable to accommodate movement, relief cuts are included in concrete slabs to allow the slab to move slightly without causing cracks in the concrete.

5) Cracks due to overloading

Putting excessive loads on a concrete slab can cause several types of cracks to occur, or even cause the concrete to fail. By observing the direction and position of a crack (such as vertical, diagonal, top, bottom, etc.), it is possible to determine the type of loading stress it indicates. For instance, positive flexural cracks can be identified by the presence of vertical cracks located at the bottom and centre of a simply supported beam (Figure 4). On the other hand, negative flexural cracks appear as vertical cracks over the supports on the top of the beam.

Diagram of locations and directions of overloading cracks

Figure 4 - Diagram of locations and directions of overloading cracks

Diagram Credit: Robert Pirro

6) Cracks from premature drying

When a concrete slab loses moisture too quickly, two types of cracks can occur as a result of premature drying. Crazing cracks resemble a spider's web and occur on the top layer, while crusting cracks appear during pattern embedding. Although these cracks may look unpleasant, they usually do not affect the structural integrity of the slab.

7) Chemical reaction cracks

Chemical reaction cracks are harmful chemical reactions that can cause concrete to crack. This is otherwise known as an alkali-aggregate reaction (AAR) which can occur due to the materials used in construction or from substances that come into contact with the concrete after it hardens. Over time, interactions between active silica aggregate and alkalis can lead to cracking. These alkalis can originate from various sources like cement hydration, admixtures, or external factors such as curing water, groundwater, and alkaline solutions that are retained or used in the structure. AAR can include alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).

  • Alkali-silica reactivity (ASR): Occurs when the high pH of the concrete's pore solution caused by alkalis in cement, causes certain reactive silica minerals in aggregates to dissolve. The dissolved silica combines with alkalis to form a gel-like material that expands when exposed to water. This expansion leads to the fracturing of aggregates and paste, often resulting in pattern cracking on the surface of the pavement.

  • Alkali-carbonate reaction (ACR): Certain dolomitic rocks have been found to cause significant expansion in concrete. ACR is relatively uncommon compared to alkali-silica reactions since the aggregates prone to this issue are less prevalent. The addition of supplementary cementing materials does not prevent the harmful expansion caused by ACR. Therefore, to prevent potential damage, the use of aggregates susceptible to ACR in concrete should be avoided.

8) Reinforcement corrosion cracks

When steel reinforcement corrodes, it expands and creates radial stresses, causing cracks.

Steel Corrosion Cycle

Figure 5 - Steel Corrosion Cycle

Diagram Credit: The Constructor

The two key causes of corrosion in the reinforcing steel are carbonisation and chloride penetration.

  • Carbonation is when carbon dioxide and moisture permeate the concrete, decreasing the concrete's pH level.

  • Chloride penetration is when the pH level of the concrete is diminished due to the infiltration of chlorides, oxygen and moisture.

These cracks can spread and lead to concrete spalling, i.e. the deterioration of steel reinforced concrete. Delamination can also occur when a broad crack forms at a plane of bars parallel to the concrete surface.

Our Recommendation

To obtain in-depth data about surface cracks on rigid pavement, we recommend using the following equipment to assess the condition of your concrete pavement:

  1. The ARAN LRMS – The ARAN LRMS (Automatic Road Analyser) system can measure roughness, rutting and surface texture using high speed lasers and a 1096-point laser line system for rutting over a 4.0m width. Click here for the technical brochure.

  2. The ARAN LCMS – The Automated Road Analyser (ARAN) is a 3D laser road profiler capable of undertaking automated measurement of road surface conditions and geometry. The LCMS sets itself apart from other crack detection methods by using a 3D imaging system to accurately identify cracks on the road. Click here for the technical brochure.

  3. The HWD - The Heavy Weight Deflectometer (HWD) is used for rigid pavements, airports and ports. It can be set up for assessing concrete slabs and interlocking blocks. Click here for the technical brochure.

Conclusion

Understanding the different types of cracks in concrete can help in preventing and repairing them effectively, resulting in more durable and aesthetically pleasing concrete projects. Our pavement engineers can direct you to the most suitable equipment to investigate your pavement’s condition. Click here to contact us.

Infographic about cracks in concrete

Infographic describing cracks in concrete

References

 

Mary Nguyen
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