Concrete dams can experience cracking due to the shrinkage of the concrete caused by temperature fluctuations. These cracks can occur internally within the dam or externally on its surface, with surface cracks being more concerning than interior ones.
To mitigate cracking in concrete dams, various methods have been developed. These methods aim to control the formation and spread of cracks, particularly surface cracks, which pose a higher risk.
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
Mass concrete structures, such as dams, can experience thermal cracking due to slow dissipation of heat generated during the hydration process. To avoid this issue, low heat cement can be used instead of ordinary Portland cement. The heat generation during hydration is mainly attributed to C3S and C3A present in OPC. Low heat cement has a higher amount of C2S and lower amounts of C3S and C3A, resulting in slower heat production and hardening rates. This controlled temperature inside the concrete mass can prevent thermal cracking.
2. By Pre-cooling of Concrete
To prevent cracking in concrete, one effective measure is to pre-cool the ingredients used in its preparation. This involves cooling the fine and coarse aggregates, which can be achieved by blowing air through them or washing them with chilled water. Additionally, the use of cool water in the making of concrete can also help to balance the heat generated during the hydration process and prevent thermal cracking. By taking these steps to pre-cool the ingredients and manage the temperature of the concrete during hydration, it is possible to enhance the durability and overall quality of the finished product.
3. By Post-cooling of Concrete
The process of post-cooling concrete involves passing cold water through pipes that are embedded in the concrete. These pipes are 250 meters long and have a thin external diameter of 25 mm. They are placed in the concrete after each lift is poured and are connected together by expansion coupling. The pipes are spaced horizontally at intervals ranging from 0.5 m to 2.0 m.
After the completion of concreting for one block of a dam, cold water is immediately passed through these pipes at a velocity of 0.6 m/s. This water is continuously circulated through the pipes 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 it.
4. By Reducing % of Cement
The amount of heat generated during hydration increases with a higher cement content. However, the rate at which heat is dissipated in the interior of a dam is much slower than on the exterior. To address this issue, it is recommended to reduce the amount of cement used in the construction of the dam’s interior. Specifically, a 20% reduction in cement content is preferred for the interior portion of the dam, if X amount of cement is used for the exterior. This approach can help maintain a more even distribution of heat throughout the dam during the hydration process.
5. By Providing Contraction Joints
Concrete dams are constructed with contraction joints to prevent the formation of cracks due to the shrinkage of concrete caused by temperature variations. The joints are classified as either longitudinal or transverse depending on their orientation. Longitudinal joints are provided parallel to the axis of the dam, while transverse joints are provided normal to the axis of the dam.
Transverse joints are continuous and are spaced at a distance of 15 m 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. The spacing of longitudinal joints is also 15 m.
The purpose of these joints is to divide the dam into several blocks, as shown in Figure 5. This division helps to reduce the likelihood of cracking by allowing the concrete to shrink in a controlled manner. By providing contraction joints at specific intervals, the concrete can shrink without creating large, uncontrolled cracks that can affect the structural integrity of the dam.
6. Time Interval between Concrete lifts
When constructing mass concrete structures such as dams, it is necessary to pour the concrete in layers, or lifts. However, it is crucial to ensure that there is an appropriate amount of time between each lift to prevent cracking caused by shrinkage. Experts recommend a time period of 3 to 4 days between successive lifts to ensure the integrity and stability of the structure. By allowing enough time for each layer to properly cure and settle, the risk of cracking and other structural damage can be significantly reduced. Therefore, careful planning and adherence to recommended time intervals is essential when constructing large-scale concrete structures.
7. By Limiting the Height of Lift
To prevent cracking in mass concrete structures, it is essential to limit the height of a concrete lift to a maximum of 1.5 meters. This precaution is crucial in order to maintain the structural integrity and durability of the concrete. Exceeding this height threshold can result in increased risk of cracking, which can compromise the strength and stability of the overall structure. Therefore, it is imperative to strictly adhere to this guideline during the construction process to ensure the longevity and performance of mass concrete structures.
Fig 6: Concrete Pouring