This article discusses the formation of cracks in concrete dams, which mainly occurs due to the shrinkage of concrete caused by temperature changes. Cracks can develop internally within the body of the dam or externally on its surface. However, surface cracks are considered more hazardous than interior cracks.
The article also delves into the various methods used to control cracking in concrete dams. These methods are designed to prevent the formation and spread of cracks in the dam structure, thus ensuring its long-term stability and safety.
Methods to Control Cracking in Concrete Dams
Following are the methods to control or minimize the development of cracks in concrete dams.
- By Using Low heat Cement
- By Pre-cooling of Concrete
- By Post-cooling of Concrete
- By Reducing % of Cement
- By Providing Contraction Joints
- Time Interval between Concrete lifts
- By Limiting the Height of Lift
1. By Using Low heat Cement
Massive concrete structures such as dams are vulnerable to thermal cracking due to slow dissipation of the heat generated during the hydration process. The rise in temperature inside the concrete mass is a result of the slow dissipation of heat, and it can lead to the formation of cracks. To prevent thermal cracking, low heat cement is recommended instead of ordinary Portland cement.
Ordinary Portland cement contains C3S and C3A, which are the primary culprits for heat generation during the hydration process. On the other hand, low heat cement contains a higher amount of C2S and a lower amount of C3S and C3A. This difference in composition results in a slower rate of heat production and hardening of the concrete. As a consequence, the temperature inside the concrete mass is effectively controlled, thereby preventing the occurrence of thermal cracks.
2. By Pre-cooling of Concrete
Preventing cracking in concrete is a crucial step in ensuring its longevity and durability. One effective method to prevent cracking is by pre-cooling the concrete ingredients, such as the fine and coarse aggregates. This can be achieved by blowing air through the aggregates or washing them with chilled water. By doing so, the temperature of the aggregates is lowered, which reduces the heat generated during the hydration process.
Another way to pre-cool concrete is by using cool water during the mixing process. This approach helps balance the heat generated during the hydration process, preventing thermal cracking. When the concrete is mixed with cool water, it reduces the temperature of the concrete mix, and this temperature reduction minimizes the risk of cracking.
Pre-cooling concrete ingredients is a simple yet effective technique that can significantly reduce the likelihood of cracking in concrete. By lowering the temperature of the aggregates and the concrete mix, the heat generated during hydration is balanced, which helps prevent thermal cracking. It is crucial to take these preventive measures during the construction process to ensure the longevity and durability of the concrete structure.
3. By Post-cooling of Concrete
Post-cooling of concrete is a process that involves passing cold water through pipes that are embedded in the concrete. This method is employed after each lift of concrete is poured and is aimed at reducing the temperature of the concrete. The pipes used in post-cooling are 250 meters long and have a thin design, with an external diameter of 25 mm. The pipes are spaced between 0.5 meters to 2.0 meters apart horizontally and are connected using expansion coupling.
Once the pouring of concrete for one block of a dam is complete, cold water is immediately passed through the pipes at a velocity of 0.6 m/s. This process is continued until the temperature of the concrete mass falls to the local temperature. To monitor the temperature of the concrete mass, resistance thermometers are installed within the concrete. These thermometers provide accurate temperature readings, which are used to determine when the cooling process can be stopped. Overall, post-cooling is an important process in ensuring the durability and stability of concrete structures.
4. By Reducing % of Cement
The hydration process of cement generates heat, and the amount of heat generated increases with higher cement content. However, the rate of heat dissipation is slower in the interior portion of a dam as compared to the exterior. Therefore, it is advisable to use a lower amount of cement for the construction of the dam’s interior.
In general, a 20% reduction in the amount of cement used for the interior portion of the dam is recommended. For instance, if X amount of cement is used for the exterior part of the dam, then the preferred amount for the interior part would be 0.8X. This reduction in cement content will help control the heat generated during the hydration process, ensuring that the dam remains structurally sound and stable over time.
5. By Providing Contraction Joints
Concrete dams are susceptible to cracks caused by the shrinkage of concrete due to temperature variations. To prevent this from happening, contraction joints are added to the structure. These joints are categorized as either longitudinal or transverse depending on their orientation with respect to the axis of the dam.
Transverse joints are positioned perpendicular to the axis of the dam and are continuous. They are spaced at a maximum of 15 meters or the height of the dam, whichever is less. On the other hand, longitudinal joints are non-continuous and are placed between the transverse joints. Their spacing is also 15 meters. Both types of joints serve to divide the dam into a series of blocks, as illustrated in figure 5.
6. Time Interval between Concrete lifts
Mass concrete structures such as dams require a specific pouring process to ensure their structural integrity. This process involves pouring the concrete in lifts, or layers, rather than all at once. It is important to leave a sufficient amount of time between each lift to prevent cracking due to shrinkage.
To achieve this, a recommended time period of 3 to 4 days is advised between successive lifts. This interval allows the concrete to settle and solidify properly before the next layer is added. If the concrete is poured too quickly without adequate time for curing, it can result in cracks forming in the structure, which can compromise its strength and stability.
Therefore, careful consideration must be given to the timing of each lift during the construction of a mass concrete structure. Proper attention to this process can ensure the longevity and safety of the final structure, making it capable of withstanding the test of time and natural forces.
7. By Limiting the Height of Lift
Concrete structures can be prone to cracking due to a variety of factors, such as changes in temperature or excessive weight load. In the case of mass concrete structures, the height of concrete lifts is an important consideration in preventing cracking. It is recommended that the height of a concrete lift should not exceed 1.5 meters. This height limitation is put in place to reduce the risk of cracking and ensure the structural integrity of the concrete. By adhering to this guideline, builders and engineers can ensure that the concrete structure remains stable and safe for use over the long term.
