"Concrete Cracks." Is this a title of an article or a statement of fact? Well, believe it or not, with most concrete it is a statement of fact. Most standard concrete does, indeed, crack. Understanding why and where these cracks occur is an important key to successful concrete design, maintenance, and repair. In this article, we will discuss why some of the most common cracks in concrete occur. Further, we will discuss how to control cracking, how to prevent problem cracks, and the basics of how to repair and treat cracks when they do occur.

While it may be surprising to hear, most cracks in concrete are expected and are not indicative of any latent problems. Conversely, some cracks in concrete may be indicators of significant issues or deficiencies, the extent of which should be investigated. Knowing which are which and how to treat problem cracks can be extremely important in prolonging the life and sustainability of concrete structures. Taking this into account, a basic understanding of concrete's strengths and deficiencies is required to properly anticipate and address concrete cracking.

WHY DOES CONCRETE CRACK?

Concrete is one of the most versatile building materials known to man. For this and many other reasons, twice as much concrete is used in construction as all other building materials combined.¹ One primary benefit of concrete is that it is very strong in compressive strength. This means that it is capable of supporting very heavy loads pushing against it. However, concrete is also relatively weak in tensile strength, with tensile strengths sometimes reaching only 10 percent of the concrete's compressive strength.² This means that concrete is much more susceptible to damage and cracking when tensile (pulling) forces are present.

Some of these tensile forces (such as shrinkage forces) are inherent in most concrete. Other forces (such as expansive forces that can develop within the concrete and flexural forces encountered during loading and use) can create tensile strains that exceed the concrete's relatively low tensile strength. Any time tensile forces exceed the concrete's tensile strength, cracking occurs. Knowing this makes it easier to control cracks, prevent problem cracks, determine their causes once cracks have occurred, and to provide proper treatment and repair of cracks.
Cracks in concrete fall into two categories: Those that occur in concrete during its plastic state and those that occur in hardened concrete.³

PLASTIC CONCRETE CRACKS

Plastic concrete cracks are those that occur at the surface of the concrete before the concrete has hardened. These cracks typically occur shortly after placing the concrete, during the interval when it is still possible to remold it. Most plastic cracks in concrete are relatively shallow in nature and do not run through the full depth of the concrete. However, it is important to understand that while plastic cracks typically begin as shallow surface cracks, some can develop into full-depth cracks later in the concrete's life.

Plastic Shrinkage Cracks:

Plastic shrinkage cracks are the most common cracks that occur in plastic concrete. They typically occur during high evaporative conditions when water evaporates from the concrete surface faster than it can be replaced by bleed water leaving the concrete. As the water evaporates from the surface, the drying surface shrinks and wants to pull inward on itself (tensile forces). Because the concrete below the surface is still wet and not shrinking at the same rate, tensile stresses develop in the weak, still stiffening, plastic surface. This can result in shallow cracks at the surface that may form a polygonal pattern (map cracking) or be longer in nature and essentially parallel to one another.
Cracks in concrete fall into two categories: Those that occur in concrete during its plastic state and those that occur in hardened concrete.

Concrete installers often try to combat high evaporative conditions by performing finishing early, while bleed water is still present on the surface, or by adding water to the surface. This is not recommended practice. Both actions weaken the surface of the concrete and exacerbate plastic shrinkage cracking. More successful measures can be taken during the placement of concrete to reduce surface evaporation and plastic shrinkage cracks (See ACI 305R-20).⁴ These measures often include the application of liquid evaporation retarders to the surface, fogging the air above the concrete, plastic sheeting to cover the concrete between finishing operations, wind breaks, sunshades, and even placing at night. The addition of microfibers into the concrete mix has proven to be very successful in reducing plastic shrinkage cracking. Recently developed evaporation-reducing admixtures have proven to be very successful as well.

HARDENED CONCRETE CRACKS

Hardened concrete cracking occurs after the concrete has appreciably hardened. Common causes include restrained drying shrinkage and expansive forces within the concrete that introduce tensile strains.

Restrained Drying Shrinkage Cracks:

Restrained drying shrinkage is the most common cause of concrete cracking. It is the reason why most in the concrete business agree with the statement "Concrete cracks." Typical concrete shrinks roughly 0.05 to 0.06 percent at 28 days.⁵ This equates to about 5/8 inch in 100 feet (16mm in 30.5m), or 1/16 inch in 10 feet (4mm in 3m).

As the concrete shrinks, it is pulling inward on itself. If it were able to pull and move unrestrained, the concrete would not crack. However, in the real world, concrete encounters a great degree of restraint while it is drying and shrinking. Anything that restrains movement of the concrete during drying and shrinkage can result in tensile strains and cracking, i.e., columns and bollards that penetrate through the concrete can limit the concrete's ability to move during shrinking, resulting in significant restraint. Reentrant corners in concrete are areas where shrinkage forces and restraint are concentrated. In the case of slabs on grade, the subgrade itself introduces friction that wants to restrain the shrinkage movement of the concrete. Once all this restraint produces tensile strains that exceed the relatively low tensile strength of the concrete, especially at an early age, cracks occur. Knowing this, measures can be taken in design and placement to anticipate and "control" restrained drying shrinkage cracks.

Control joints are saw cuts or tooled joints that are placed in the concrete surface in areas where restrained drying shrinkage cracking is anticipated. This is done to reduce the occurrence of random cracking. These joints create a thinner section of concrete that is weaker than the thicker concrete around them. When tensile forces from restrained shrinkage occur, the thinner section cracks first. In essence, the control joint is telling the concrete where to crack. So, control or contraction joints, as some refer to them, do not actually prevent cracking. Rather, the cracks simply occur in a straight line beneath the joints.

Steps can be taken to reduce restrained drying shrinkage cracks. By working closely with the ready-mix concrete supplier, the overall shrinkage of the concrete can be reduced or sometimes even eliminated. This is often done by using aggregates of proper quality, size and gradation, and admixtures such as water reducing, shrinkage, reducing, and shrinkage compensating admixture in the mix. Proper curing is also essential in controlling concrete shrinkage (See ACI PRC 308).⁶ Post-crack strength, crack widths, and the opening of control joints due to shrinkage can also be controlled by introducing reinforcing steel or macro fibers (See ACI 544.4R).⁷

Expansive Freeze/Thaw Stresses:

When water freezes within concrete, it expands-resulting in tensile strains that can fracture and crack the concrete. "D-Cracking" (Fig. 5) is a common type of cracking that occurs when soft or deleterious aggregates within the concrete absorb water from a poorly-drained subgrade and expand during freezing. This type of cracking starts unseen at the base of the slab near the joints. Over time the cracking progresses upward, to the surface of the slab.⁸ This results in a closely spaced cracking pattern radiating outward from joints and intersections of joints in concrete pavements.

Concrete in freeze/thaw environments should be air entrained for durability. "D-Cracking" can be prevented by using concrete with durable aggregates meeting ASTM C33, providing proper subgrade drainage and sealing pavement joints.⁹ Unfortunately, while "D-Cracking" can often be arrested by reducing moisture intrusion, repair of more serious damage is often impossible, requiring removal and replacement of the concrete.

Expansive Chemical Reactions:

Expansive chemical reactions within the concrete itself can result in tensile strains and cracking. These reactions can be due to materials that are used to make the concrete or materials that come into contact with the concrete after it has hardened. Two of the most common expansive chemical reactions that cause cracking in concrete are sulfate attack and alkali silica reaction (ASR).

Sulfate attack occurs when sulfates from soils, ground water, sea water, or other sources come into contact with hydrated calcium aluminate in the concrete's hydrated cement paste. The reaction is expansive, resulting in closely spaced cracking and quite often a whiteish "bloom" on the surface. Sulfate attack can best be prevented by utilizing sulfate resistant cements with low calcium aluminate contents in areas where sulfate exposure is anticipated.

ASR takes place when reactive aggregates, sufficient alkalis, and moisture are present. The alkalis (typically in concrete containing Portland cement) and moisture react with the aggregate to form an expansive gel around the aggregate. This expansion creates tensile strains which result in a distinctive Y-shaped crack pattern at the surface.

ASR can be controlled and prevented. Nonreactive aggregates can be used where available. Alkali content of the concrete can be reduced by using low alkali cement and supplementary cementitious materials, or reducing exposure to water.
While measures can often be taken to arrest and slow damage occurring due to sulfate attack and ASR, repair of more serious damage is often impossible, requiring removal and replacement of the affected concrete.

Corrosion of Reinforcing Steel:

Corrosion of embedded reinforcing steel in concrete is expansive (2 to 3.5 times in volume¹⁰) and can create some of the most serious damage we see in concrete structures. Cracks due to corrosion are typically near or over embedded reinforcing steel and usually exhibit rust staining.

Corrosion of embedded reinforcing steel can be reduced by providing low permeability concrete and adequate concrete cover in accordance with ACI Code 318.¹¹ Corrosion inhibiting admixtures can significantly delay onset of corrosion while sealers and coatings can protect against moisture penetration. Embedded galvanic anodes can also be used to help prevent corrosion of reinforcing steel at the perimeter of concrete repairs. This is sometimes referred to as "halo effect" corrosion.

Cracks and delamination of concrete due to steel corrosion should always be addressed as soon as possible. These types of cracks typically lead to further accelerated corrosion of the steel, eventually resulting in loss of load bearing capacity. ICRI 310.1R provides excellent guidance for preparing concrete for replacement in areas damaged by corrosion of reinforcing steel.⁹

Structural Cracks:

The most common structural cracks occur when in service loads or loads during construction exceed the design strength of concrete. These cracks are often present in structural members and may or may not have rust staining present. Cracks in structural members and other suspected structural cracks should always be evaluated by a licensed engineer to determine the cause, severity, risk of danger, and proper treatment and repair.

REPAIR AND TREATMENT OF CRACKS

ACI PRC 224.1R is an excellent resource for information regarding "Causes, Evaluation and Repair of Cracks in Concrete Structures".⁵ It tells us, "Cracks in concrete need to be repaired if they reduce the strength, stiffness, or durability of the structure to an unacceptable level, or if the function of the structure is seriously impaired." This would include cracks and joints that can allow migration of water to embedded reinforcing steel or subgrade.
When considering the repair of cracks in concrete, one should first determine the causes, locations, and severity of the cracks. As stated above, cracks can be a symptom of an underlying problem. Such underlying problems should be addressed prior to making crack repairs. If the crack is repaired without addressing the root cause, that same underlying cause will often result in failure of the repair or appearance of new cracks in the same general area.
Once the cause of the cracking has been determined, determine if the crack repair is being made in order to reinstate structural capacity. If so, structural repairs such as epoxy injection (ACI RAP Bulletin 1), or fiber reinforced polymer (FRP) (ACI PRC 440.2) may be required.¹³‚¹⁴ In some cases, additional steel can also be added to increase the strength across the crack.

If the repair is not structural in nature, determine whether future movement in the crack is anticipated. Often, non-structural, non-moving cracks can be repaired with cementitious materials or semi-rigid polymers. However, such rigid materials should not be placed so as to bridge moving cracks and joints. If movement is expected, repairs should be made with more flexible materials such as elastomeric sealants.

REFERENCES

1. Gagg, Colin R., 2014, "Cement and concrete as engineering material: An historic appraisal and case study analysis," Engineering Failure Analysis, Volume 40, pages 114-140.
2. International Code Council, 2015 Concrete Manual, Chapter 3. ICC Publications, Country Club Hills, Illinois, 2015.
3. Price, W.H., 1982, "Control of Cracking During Construction," Concrete International, V. 4, No. 1, Jan., pp. 40-43.
4. ACI Committee 305, Guide to Hot Weather Concreting {PRC 305R-20), American Concrete Institute, Farmington Hills, Ml, 2020.
5. ACI Committee 224, Causes, Evaluation and Repair of Cracks in Concrete Structures {AC/ PRC 224.1R - 01), American Concrete Institute, Farmington Hills, Ml,2007.
6. ACI Committee 308, Guide to External Curing of Concrete {AC! PRC 308-16), American Concrete Institute, Farmington Hills, Ml, 2016.
7. ACI Committee 544, Guide to Design with Fiber Reinforced Concrete {AC/ PRC 544.4-18), American Concrete Institute, Farmington Hills, Ml, 2018.
8. O'Doherty, John, May 1987, D-Cracking of Concrete Pavements, Material and Technology Division of the Michigan Department of Transportation, Issue No. 7
9. ASTM C33/C33M - 23, Standard Specification for Concrete Aggregates, ASTM International, West Conshohocken, PA, 2023, 11 pp.
10. ACI Committee 222, Guide to Protection of Reinforcing Steel in Concrete Against Corrosion {AC/ PRC 222-19), American Concrete Institute, Farmington Hills Ml, 2019
11. ACI Committee 318, Building Code Requirements for Structural Concrete {AC/ Code 318-19), American Concrete Institute, Farmington Hills, Ml, 2019.
12. ICRI Committee 310, Guideline for Surface Preparation for Repair of Deteriorated Concrete Resulting from Reinforcing Steel Corrosion ICRI 310.1R
13. ACI Committee E706, ACI RAP Bulletin 1, "Structural Repair by Epoxy Injection," American Concrete Institute, Farmington Hills, Ml, 2009. 7 pp.
14. ACI Committee 440, Design and Construction of Externally Bonded Fiber Reinforced Polymer {FRP) {AC/ PRC 440.2 - 23), American Concrete Institute, Farmington Hills, Ml, 2023.
15. Federal Highway Administration, March 2007, The Use of Lithium to Prevent or Mitigate Alkali-Silica Reaction in Concrete Pavement and Structures, Publication No. FHWA-HRT-06-133.

About the Author

Matthew Hansen is a graduate of the University of Toledo, Ohio. He has more than 40 years of successful experience in design, sales, marketing, and technical services within the construction and concrete industry. He has been employed by the Euclid Chemical Company for over 27 years, and currently holds the position of National Business Development Manager. Matthew is a member of the American Concrete Institute committees. ACI 362 Parking Structures, ACI 515 Protective Systems for Concrete, ACI 546 Repair of Concrete, ACI 563 Specifications for Repair of Structural Concrete in Buildings, and the International Concrete Repair Institute committees 110 Guide Specifications, 310 Surface Preparation, 320 Concrete Repair Materials and Methods and 510 Corrosion.