When Is A Crack Just A Crack?
Concrete cracking is an inevitable phenomenon that poses challenges to the integrity and durability of structures. Structural cracks weaken concrete, endangering nearby individuals, while Non-Structural cracks can evolve into structural issues. Specific problems range from Alkali-Silica Reaction (ASR) to Thermal Contraction Cracking.
In this article, Joe Eixenberger, PE, PhD explores the old adage about concrete, which states that there are two types of concrete, cracked concrete and concrete that will crack.
There are two types of cracks: Structural Cracks and Non-Structural Cracks.
- Structural Cracks are cracks that have weakened the durability of the concrete and can endanger those near the concrete.
- Non-Structural Cracks are those that do not weaken the integrity of the concrete structure. In general, most cracks are non-structural, but if left untreated, non-structural cracks may lead to water penetration and other degradation methods of the concrete that can turn into a structural issue.
As there are a wide variety of cracks and causes, it is best to have these cracks evaluated by a professional. However, the earlier that a crack can be repaired, the better chance of extending the useful life of the concrete and its integrity.
Most Common Concrete Cracks: Causes and Generalized Repair Options.
Alkali-Silica Reaction (ASR)
Problem: Reaction of Silicon Dioxide
Alkali-Silica Reaction is caused by the reaction of silicon dioxide that Portland Cements reacts with the alkali hydroxides present in some aggregate. The reaction forms a gel that swells as it encounters moisture, and the pressure causes the concrete to crack. As moisture is required this type of cracking is found in damp locations such as piers. This usually takes five to twenty years to appear, but if the structure is in contact with moisture there are few repair methods to minimize further damage.
Solution: Corrective Steps During Mix Design
To minimize the chance of ASR, steps can be taken during the mix design such as using low-alkali Portland cement, selection of proper aggregate, or addition of cementitious materials such as fly ash. If ASR cracks are detected, the addition of waterproofing or improvement of drainage away from the structure can expand the useful life of the structure.
Corrosion of Reinforcement
Problem: Corrosion of Reinforcement in Concrete
Corrosion of reinforcement in concrete requires moisture, oxygen, and the passivation or protective alkaline layer of concrete must be removed due to chlorides or carbonation. Once the reinforcement starts to corrode it can expand up to four times its normal diameter. This expanding rebar adds pressure to the concrete causing it to crack.The cracking concrete and weakened reinforcement weakens the structure and can create a potential of falling concrete.
Solution: Removal of Unsound Concrete and Treatment
To repair this failure the unsound concrete is removed until sound concrete is found. The perimeter of this area is then saw cut and the concrete around the reinforcement is chiseled away. Depending on the level of corrosion and the structural requirements, the reinforcement can have the corrosion removed and a rust inhibitor and concrete primer applied.
If the corrosion is significant, the reinforcement may be removed and new reinforcement doweled into the structure. New concrete is then placed into the cavity to replace the old concrete. To minimize future corrosion a sacrificial anode and/or waterproofing system may be added.
Plastic Settlement Cracking
Problem: Plastic Settlement Cracking in Concrete
Plastic settlement cracking are fine cracks that are about 1 mm in width. This type of cracking usually occurs in deeper sections of concrete. Prior to the concrete hardening bleed water is rising to the surface and the aggregate and cement is settling to the bottom. This creates a weaker surface layer and cracks will generally appear in areas that are restrained such as at rebar. Sometimes these cracks are wider at the top and will penetrate to the reinforcement which will make the reinforcement susceptible to corrosion and further degradation.
Solution: Tevibrated or Trowelled
If these cracks are observed prior to hardening the concrete can be revibrated or trowelled again to close the cracks just after having been formed.
Plastic Shrinkage Cracking
Problem: Cracking is Caused by the Early Drying
Plastic shrinkage cracking is caused by the early drying of the surface of the concrete. This can be from things such as air or concrete temperature, wind speed, or relative humidity. These cracks are usually about 1 to 3 mm in width and can appear in a parallel pattern or random pattern.
While unsightly, these cracks are generally an aesthetic issue. However, the cracks should be watched as moisture and chlorides which can penetrate these cracks. This can lead to corrosion of the reinforcement and spall the concrete. In addition, the cracks can be widened or deepened by thermal movement or loading of the structure.
Solution: Concrete Can be Vibrated
If these cracks are observed prior to hardening the surface of the concrete can be vibrated to close the cracks and bring water to the surface to close the cracks.
Drying Shrinkage Cracks
Problem: Cracking During Shrinking
As concrete cures, it shrinks or contracts. In thin sections of concrete, if there are insufficient joints and excessive water loss the curing or drying of the concrete will cause these sections to crack. This is common near edges and corners, as there is more exposed surface area of the concrete.
Solution: Good Mix Designs
To effectively combat drying shrinkage cracks in concrete, a comprehensive strategy revolves around meticulous mix designs and thoughtful curing practices. Employing well-graded aggregates, shrinkage-reducing admixtures, and supplementary cementitious materials in mix designs can curtail shrinkage potential. Moist curing methods, gradual drying, extended curing periods, and protective measures for vulnerable edges and corners form the foundation of successful curing practices. This combined approach ensures the minimization of drying shrinkage cracks, bolstering the durability and aesthetics of concrete structures.
Thermal Contraction Cracking
Problem: Cracking because of Thermal Contraction
Thermal contraction cracking during the curing process due to differences in temperature in the concrete structure, usually in thicker sections. This is caused due to the exothermic reaction of concrete that causes the internal temperature of the concrete to increase or cool down slowly, while in an environment that is significantly cooler lowers the exposed sections of the concrete quicker. This causes the exposed sections to contract or shrink more than the interior sections and increases the stress in these areas leading to the concrete cracking.
Solution: Take Preventative Measures
To mitigate thermal contraction cracking, several preventive measures can be employed during the concrete curing process. Firstly, utilizing temperature-controlled curing environments can ensure uniform temperature distribution, minimizing rapid cooling and contraction disparities. Secondly, employing curing compounds or blankets that slow down the cooling rate of the concrete’s surface can help maintain even temperature conditions. Thirdly, utilizing concrete mixes with lower heat of hydration and incorporating fibers or other reinforcement materials can enhance the concrete’s ability to resist cracking due to thermal stresses. Lastly, strategic joint placement and proper construction sequencing can provide controlled points of relief for potential stress, preventing extensive cracking.
Crazing
Problem: Random Cracks on the Surface of the Concrete
Crazing is the formation of fine random cracks on the surface of concrete that occurs as the surface layer shrinks. This is common to concrete that is placed during hot temperatures in direct sunlight, low humidity, or excess wind. These are mostly noticeable in troweled finishes especially after the concrete gets wet and starts to dry.
Solution: Application of Protective Coating
To address the issue of crazing, which involves the development of fine random cracks on the concrete surface due to shrinkage, a strategic solution can be employed. Applying a protective coating over the affected areas offers a practical remedy. This solution not only serves to enhance the appearance of the concrete but also helps prevent further deterioration caused by environmental factors. By utilizing protective coatings, the structural integrity and longevity of the concrete can be effectively maintained.
Concrete Repairs
Once concrete has already hardened, the goal of repairing cracks in concrete is to extend its useful life. If the crack is a structural crack, this might require the replacement of the structure, or section of the structure.
While the crack is non-structural the goal is preventing further deterioration of the concrete. This is usually done by limiting the intrusion of exterior materials into the concrete. To do this, it is best to cut the concrete along the crack in a v shape and fill the crack with a suitable material. The suitable material will be dependent on the size of the crack and the environmental condition. The addition of a protective coating also helps mitigate further deterioration.
Conclusion
So, when is a crack just a crack? All cracks are a possible weakness to the durability or the integrity of the concrete structure. Whether it is something that needs to be repaired or looked at immediately will depend on the current and future use of the structure, the type of cracking, and the size of the crack. However, understanding what has caused the crack and how repairs are conducted will enable an individual to decide on its importance.
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