Concrete is prone to cracking during the first week after it is poured, which is known as early-age cracking. However, these cracks can also become visible in reinforced concrete slabs after more than a week. Any cracks that appear within the first 60 days after pouring are classified as early-age cracks.
Although early-age cracks do not typically cause structural failure in concrete, they can lead to issues such as premature corrosion of reinforcing steel bars and spalling of concrete cover. These problems can increase maintenance costs and potentially reduce the lifespan of the structure.
Early-age cracking can occur due to various factors, including the composition of the concrete mix, the environment in which the concrete is exposed, the rate at which the concrete hydrates, and the curing conditions. It is important to have a thorough understanding of the causes of early-age cracking to take appropriate measures to prevent it from occurring in concrete.
What are the Causes of Early-age Cracking in Concrete?
1. Internal Concrete Temperature
Cement hydration is a process that produces heat, known as the heat of hydration. This heat can cause cracks to form during the early stages of concrete development. If the temperature inside the concrete reaches approximately 50 degrees Celsius, the ettringite, a component of the concrete, may become unstable and dissolve. When the temperature subsequently decreases, the ettringite can expand, leading to both internal and external cracking in the concrete.
2. Temperature Gradient
Concrete structural elements can experience temperature gradients when exposed to high temperatures that cannot be dissipated. These gradients create thermal stresses between the internal and external surfaces of the concrete. When the concrete member is restrained, the early age thermal stress can exceed its tensile strength, causing cracks to develop.
There are various factors that can cause restraints in concrete, such as changes in section depth or shape. For instance, a T-beam or coffer slab can experience restraints at the junction between its web and flange. Similarly, top reinforcements in beams or slabs, stirrups in columns, formwork ties, and coarse aggregates bridging narrow sections and formwork can also cause restraints in concrete.
These restraints can exacerbate the thermal stresses generated by temperature gradients, leading to cracks in the concrete structural elements. Therefore, it is crucial to carefully consider potential restraints during the design and construction phases of concrete structures to avoid such issues.
3. Autogenous Shrinkage
Autogenous shrinkage is a phenomenon in which concrete experiences large tensile stress during the early stages of curing, especially in high-early-strength concrete. This stress is generated due to chemical shrinkage, which occurs when the volume of hydration product formed during the reaction between cement and water becomes smaller than the original volume of the constituents. In a typical Portland cement paste, the ultimate chemical shrinkage can be as high as 10% by volume.
When there is no additional water available for curing after the cement has set, the chemical shrinkage leads to the formation of empty capillary pores within the hydrating cement paste, known as self-desiccation. The partially filled pores create menisci, which in turn produce autogenous stresses and cause autogenous shrinkage. This shrinkage can result in the development of cracks in the concrete during the early stages of curing.
These cracks usually form around internal restraints such as steel reinforcements and aggregates, or they can occur through the depth of the concrete member when there is sufficient external restraint. Autogenous shrinkage can be a significant concern for concrete structures, especially in high-early-strength concrete, as it can lead to early-age cracking and affect the structural integrity of the concrete. Understanding the causes and effects of autogenous shrinkage is important for ensuring the durability and safety of concrete structures.
4. Plastic Settlement
Plastic settlement is a frequent cause of early age cracking in concrete. This phenomenon occurs when solid particles within a concrete mixture settle under the influence of gravity and bleeding water, resulting in upward movement. When the concrete is constrained locally from settling, the downward movement of solid particles generates stress. Early age cracking arises when this stress surpasses the tensile strength of freshly poured concrete, with cracks emerging at the point of restraint in the concrete.
A typical scenario for early-age cracking due to plastic settlement involves a sudden shift in the concrete depth. This is because settlement is more pronounced in the deeper section, causing cracks to develop around the location of change in the formwork section. Such cracks may compromise the integrity of the concrete and could pose safety hazards. It is, therefore, important to minimize the likelihood of plastic settlement and its associated consequences during the construction process.
5. Drying Shrinkage
Drying shrinkage is a common cause of early-age cracking in concrete structures. This occurs when the shrinkage is constrained internally, externally, or both. Non-uniform distribution of drying shrinkage through the thickness of a concrete member results in differential shrinkage, which causes axial movement and warping.
Axial movement and warping, in turn, generate axial and bending stresses when the concrete is restrained, leading to early-age cracking. Additionally, drying shrinkage creates residual stress in small elements, further contributing to the risk of early-age cracking.
Therefore, it is important to carefully consider drying shrinkage during the design and construction of concrete structures to prevent early-age cracking and ensure the longevity of the structure.
6. External Loading
External loading can cause additional stress on concrete, which may result in the initiation of early age cracks if the concrete’s tensile strength is exceeded. Concrete’s tensile strength is significantly lower during its early age, making it vulnerable to cracking if subjected to excessive loads. It is crucial not to ignore the effects of external loading after concrete placement due to this vulnerability.
Vibration, traffic, and wind are examples of external loading that can create extra stress on concrete. Such loading can exceed the tensile strength of concrete, leading to the formation of early age cracks. The low tensile strength of concrete during its early age increases its susceptibility to such damage. Therefore, it is essential to take measures to prevent the effects of external loading on concrete after its placement to avoid any unwanted consequences.
7. Concrete Creep
Cracking in concrete can occur due to a phenomenon known as concrete creep. This happens when the application of internal or external stresses causes time-dependent movement in the concrete. The aggregate present in the concrete plays a role in restraining these movements, but in some cases, the concrete may still experience cracking.
The movement that occurs in concrete due to creep can take place early on in the concrete’s life cycle, and this can lead to early-age cracking. This can be a problem, as it can impact the integrity of the concrete and reduce its lifespan. To prevent early-age cracking, it is important to understand the causes of concrete creep and take steps to address them. By doing so, it may be possible to improve the durability of concrete and reduce the risk of cracking.
FAQs
What is early-age cracking in concrete structures?
Concrete is a widely used building material due to its durability and strength. However, during the first week after pouring, early-age cracking may occur, which can also take more than seven days to become visible in reinforced concrete slabs. These types of cracks that appear within the first 60 days after placing the concrete are considered early-age cracks.
Early-age cracking can pose a significant problem for construction projects, as it can compromise the structural integrity of the concrete. This type of cracking is typically caused by a combination of factors, including the drying shrinkage of the concrete, the temperature differential between the surface and the core of the concrete, and the tensile stresses induced by the load of the concrete itself.
To minimize the risk of early-age cracking, several measures can be taken. For example, concrete mixtures with lower water-cement ratios are less prone to cracking. Additionally, controlling the concrete temperature and humidity levels can help prevent cracking. Other strategies, such as using crack control joints and avoiding overloading the concrete, can also be effective in reducing the likelihood of early-age cracking. By taking these measures, construction professionals can ensure that their concrete structures are strong and durable.
What are the causes of early-age cracking in concrete members?
In the realm of concrete construction, there are various factors that can impact the performance and durability of a structure. One crucial aspect is monitoring the internal concrete temperature, as it can affect the curing process and overall strength of the concrete. Another consideration is the temperature gradient, which refers to the difference in temperature between the core and surface of the concrete. This gradient can cause stress and cracking in the concrete, so it’s important to monitor and manage.
Autogenous shrinkage is another factor that can affect concrete. This type of shrinkage occurs due to the self-desiccation of the cement paste and can lead to cracking and reduced durability. Plastic settlement, on the other hand, occurs during the setting process and can cause surface defects such as voids and cracks.
As the concrete dries, it undergoes drying shrinkage, which is the contraction of the concrete due to the loss of moisture. This shrinkage can also lead to cracking and reduced durability. External loading is another factor that can affect the performance of concrete structures, as it can cause stress and deformation in the material over time.
Finally, concrete creep is the gradual deformation of the concrete under sustained load over time. This phenomenon can lead to long-term structural deformation and cracking if not properly managed. Overall, understanding and monitoring these various factors is crucial for ensuring the long-term durability and performance of concrete structures.
How does high heat of hydration cause early-age cracking in concrete?
During cement hydration, heat is generated which is known as the heat of hydration. This heat can be responsible for the development of cracks at an early age. When the internal temperature of the cement reaches approximately 50°C, it can cause the instability and dissolution of ettringite. After the temperature of the cement decreases, the ettringite undergoes expansion, which can result in the development of both internal and external cracks.
What are the common external and internal restraints in concrete members?
Concrete structures may experience restraints due to various factors, including changes in section depth. For instance, T-beams or coffer slabs may encounter restraints at the junction between the web and flange. Additionally, top reinforcements in beams or slabs, as well as stirrups in columns, can cause restraints in concrete structures.
Another factor that can contribute to restraints is the use of formwork ties, which hold the formwork in place while the concrete sets. The ties create a bond between the form and the concrete, which can cause restraint. Similarly, a bridge of coarse aggregate between form and narrow sections can lead to restraints in concrete structures.
It’s important to identify and address these restraints, as they can cause cracking and other damage to the concrete structure. By understanding the various factors that can contribute to restraints, engineers can design structures that minimize these effects and ensure the longevity and durability of the concrete.
How does temperature gradient lead to early-age cracking?
When concrete is subjected to high temperatures that cannot be dissipated, it creates a temperature gradient. This gradient results in a significant difference in temperature between the internal and external surfaces of the concrete structural elements, leading to thermal stresses. These thermal stresses can cause cracks in the concrete if the member is restrained. This is especially true during the early age of the concrete when the thermal stress is likely to surpass the material’s tensile strength. Therefore, it is crucial to monitor temperature changes in concrete and take appropriate measures to prevent the development of cracks.