In order to reduce the likelihood of cracks developing in structural concrete elements, various design and construction measures can be implemented. These measures are intended to minimize or eliminate the underlying causes of crack formation.
There are several common types of cracks that can occur in concrete, including plastic settlement cracks, plastic shrinkage cracks, thermal contraction cracks, long-term drying shrinkage cracks, cracks caused by reinforcement corrosion and alkali-aggregate reactions, and crazing.
To prevent shrinkage and settlement cracks in plastic concrete, the use of shrinkage-compensating admixture, adequate concrete cover, and controlled evaporation rates can be effective. Additionally, proper distribution of steel bars and provision of expansion joints can help control cracks due to thermal contractions and long-term drying shrinkage.
The quality of concrete constituent materials, as well as the concrete placement, compaction, and curing processes, all play critical roles in minimizing the likelihood of crack development.
Table-1: Types of Cracks in Concrete and their Preventive Measures
| Crack Types | Preventive Measures |
| Plastic settlement cracks | • Use a high-quality concrete mix containing shrinkage compensating admixture. • Compact poured concrete adequately. • Use sufficient concrete cover thickness to avoid the development of surface cracks. • Use adequately rigid forms for slabs. • Start concrete placement from deep sections to decrease surface crack development. • Wet soil before concreting of footings to prevent water loss from the base of concrete. |
| Plastic shrinkage cracks | • Wet substrate and formwork and remove excess water before pouring concrete. • Use synthetic steel fibers in concrete to offset the influence of plastic shrinkage. • Employ chilled water or ice to lower fresh concrete temperature in hot weather conditions. • Provide wind barriers. • Apply aliphatic alcohol over concrete surface after screening to control the evaporation of bleed water. |
| Early thermal contraction cracks | • Reduce heat of hydration by cooling concrete before placement. • Provide expansion joints early on, if applicable, to allow concrete expansion. • Insulate concrete to decrease thermal gradients. Place reinforcements at suitable spacing to control crack width. |
| Long term drying shrinkage cracks | • Provide suitable spacing between steel bars to control crack width. • Decrease water content to enhance concrete curing. • Consider low workability for concrete. • Use water-reducing admixtures. • Prevent the use of admixture containing calcium chloride because it increases drying shrinkage. • Utilize shrinkage compensating admixtures to decline the rate of drying shrinkage cracks. • Use rigid aggregate in concrete to lower cement content to reduce shrinkage because hard aggregates create restraints. • Provide expansion joints to eliminate external restraints. |
| Crazing | • Begin concrete curing as soon as possible as it prevents the appearance of craze cracks later on. • Keep concrete surface wet for at least three days. • Excessive segregation and bleeding create a low-strength concrete surface layer, which encourages crazing. So, consider low slump concrete or entrained concrete to prevent excessive segregation and bleeding. • Prevent large moisture differences between the concrete surface and its interiors. • Avoid any finishing work while bleed water is on the concrete surface. |
| Cracks due reinforcement corrosion | • Adequately compact concrete around steel bars. • Make sure bar spacings are according to applicable codes such as ACI 318-19 to permit both, easy flow of fresh concrete and adequate compaction. • Use clean steel bars to create a good bond at the concrete-steel interface. • A good bond ensures the transfer of tensile stresses to steel bars and hence reduces crack width. Reduce concrete permeability to make it withstand the penetration of harmful agents. • A low w/c ratio, adequate compaction, and curing would yield low permeable concrete. • Apply a protective coating system to the concrete surface to improve the resistance of concrete against the ingression of harmful substances. |
| Cracks due to alkali-aggregate reaction | • Use low-alkali Portland cement to produce concrete. • Prevent the use of aggregate that is susceptible to alkali-carbonate reaction. • Consider pozzolans as cement replacement material because they decrease the alkalinity in concrete. • Utilize protective coating and joint sealing to protect concrete structures. |
FAQs
What are the common types of cracks in concrete structures?
Concrete structures are prone to different types of cracks, which can be classified into several categories. One type of crack is the plastic settlement crack, which occurs during the early stages of the concrete’s curing process when the settlement of the solid particles causes the surface to crack. Another type is the plastic shrinkage crack, which happens when the surface dries too quickly, causing the concrete to shrink and crack. Thermal contraction cracks occur due to temperature changes that cause the concrete to expand and contract.
Long-term drying shrinkage cracks, on the other hand, take time to develop and occur due to the drying of the concrete over an extended period. Cracks due to reinforcement corrosion happen when the steel reinforcement inside the concrete corrodes and causes the concrete to crack. Additionally, alkali-aggregate reactions can cause the formation of cracks when there is a chemical reaction between the aggregates and the alkali in the cement paste. Lastly, crazing is a network of fine surface cracks that occur when the surface of the concrete hardens too quickly.
What are the causes of cracks in buildings?
- The permeability of concrete refers to its ability to allow liquids, gases, and other substances to pass through its pores and cracks. This can be a concern in structures where moisture or chemicals can cause damage or deterioration, such as in parking garages or wastewater treatment plants. The degree of permeability can depend on factors such as the quality of the concrete mix, curing conditions, and the presence of cracks or defects.
- Thermal movement refers to the expansion and contraction of concrete caused by changes in temperature. This can occur in structures such as bridges, where temperature fluctuations can cause the concrete to shift or crack. Proper design and construction techniques can help to accommodate for thermal movement and minimize the potential for damage.
- Creep movement is a type of deformation that can occur in concrete over time. This can be caused by the weight of the structure itself or by external loads, and can result in a gradual shift or settling of the concrete. Creep movement can be a concern in structures such as high-rise buildings or bridges, where long-term stability is critical.
- Corrosion of reinforcement occurs when the reinforcing steel within the concrete begins to rust or corrode. This can weaken the structure and compromise its ability to bear weight. Corrosion can be caused by exposure to moisture, chemicals, or other environmental factors, and can be accelerated by poor construction practices or lack of maintenance.
- Moisture movement refers to the ability of concrete to absorb or release moisture over time. This can be a concern in structures such as basements or water treatment facilities, where excessive moisture can cause damage or deterioration. Proper construction techniques and moisture barriers can help to minimize the potential for moisture movement.
- Poor construction practices, such as inadequate curing or improper placement of reinforcing steel, can lead to structural defects or weaknesses in concrete. These defects can compromise the strength and stability of the structure, and may require costly repairs or replacement.
- Improper structural design and specifications can also contribute to concrete failures. Inadequate consideration of factors such as loads, environmental conditions, and potential stresses can result in structures that are not able to withstand the forces placed upon them. Proper design and specification can help to ensure the long-term stability and safety of concrete structures.
- Poor maintenance can also lead to concrete failure over time. Lack of regular inspections, repairs, and upkeep can result in deterioration or damage that can compromise the structural integrity of the concrete. Regular maintenance and upkeep can help to identify and address potential issues before they become major problems.
- Movement due to chemical reactions can occur in certain types of concrete, such as those containing reactive aggregates or cement. This can cause the concrete to expand, contract, or crack over time. Proper selection of materials and monitoring of chemical reactions can help to minimize the potential for movement due to chemical reactions.
What are the different methods of concrete crack repair?
Concrete crack repair involves various methods such as epoxy injection, routing and sealing, grouting, stitching, drilling, and plugging, as well as gravity filling. These methods are used to repair cracks that can compromise the structural integrity of concrete structures.
Epoxy injection is a popular method of repairing cracks as it involves injecting epoxy into the cracks to fill them and prevent further cracking. Routing and sealing is another technique that involves widening the cracks and filling them with a sealant material.
Grouting involves filling the cracks with a mixture of cement, sand, and water, while stitching involves drilling holes on either side of the crack and inserting metal stitching devices. Drilling and plugging, on the other hand, involves drilling holes into the concrete and inserting steel dowels to reinforce the concrete.
Gravity filling is a simpler method that involves pouring a liquid material into the crack, allowing it to flow and fill the crack. Each of these methods has its advantages and disadvantages, and the choice of method depends on the severity of the crack, the size of the structure, and other factors.
How to prevent plastic settlement cracks in concrete structures?
To ensure the durability and longevity of concrete structures, it is essential to follow certain best practices during the construction process. One such practice is to use a high-quality concrete that meets the required specifications. This can help to prevent issues such as cracking and deterioration over time. Additionally, it is important to compact the concrete adequately to ensure that there are no voids or air pockets that could weaken the structure.
Another important factor to consider is the concrete cover. Sufficient concrete cover must be provided to protect the reinforcing steel from corrosion, which can significantly reduce the lifespan of the structure. Using rigid forms can also help to ensure that the concrete is properly shaped and does not slump or deform during the pouring process.
Starting concreting from the deep sections of the concrete element is also crucial. This can help to reduce the risk of segregation and ensure that the concrete is evenly distributed throughout the structure. Finally, it is recommended to wet the soil before pouring concrete for the footing. This can help to prevent the formation of voids and ensure that the concrete adheres properly to the ground surface. By following these best practices, builders can help to ensure that their concrete structures are strong, durable, and built to last.