In concrete dams, cracks are formed mainly due to shrinkage of concrete due to temperature variations. These cracks may develop internally in the body of the dam or externally on the surface of the dam. Surface cracks are more dangerous than interior cracks. Different methods to control cracking in concrete dams are explained in this article.
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
In mass concrete structures like dams, the heat evolved during the hydration process is dissipated very slowly. This will cause an increase in temperature inside the concrete mass and thermal cracks will occur. To prevent this, Low heat cement should be used in place of ordinary Portland cement.
It is known that C3S and C3A in OPC are responsible for heat generation during hydration. Low heat cement consists high amount of C2S and less amount of C3S and C3A. As a result, the rate of heat production, as well as the rate of hardening is slow in this case. Hence, the temperature inside the concrete mass is controlled and cracking can be prevented.
2. By Pre-cooling of Concrete
Cracking can be prevented by pre-cooling of concrete ingredients such as fine aggregate, coarse aggregate. This is done by blowing air through the aggregates or by washing them with chilled water. Concrete can also be pre-cooled by using cool water for its making. This will balance the heat generated during the hydration process and prevents thermal cracking.
3. By Post-cooling of Concrete
Post-cooling of concrete is done by passing cold water through pipes embedded in concrete. For post-cooling, 250 m long and thin pipes of 25 mm external diameter are placed in concrete after each lift is poured. All pipes are connected together by expansion coupling. These pipes are spaced at 0.5 m to 2.0 m horizontally.
After 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 and is continued until the mass concrete temperature falls to the local temperature. To check the temperature, resistance thermometers are installed concrete mass.
4. By Reducing % of Cement
Higher the cement content higher will be the heat generation during hydration. The rate of heat dissociation is very slow in the interior portion of the dam compared to the exterior part. Hence, reduce the cement content used for the construction of the interior portion of the dam. In general, if X amount of cement is used for the exterior portion of the dam, then 20% reduction which is 0.8X amount is preferred for the interior portion of the dam.
5. By Providing Contraction Joints
Contraction joints are provided in concrete dams to prevent the cracks formed due to shrinkage of concrete due to temperature variations. If contraction joints are provided parallel to the axis of the dam, then they are called longitudinal joints and if they provided normal to the axis of the dam, called transverse joints.
Transverse joints are continuous and are spaced at a distance of 15 m or height of dam whichever is less. Longitudinal joints are non-continuous and are provided between the transverse joints. The spacing of longitudinal joints is also 15 m. Both these joints divide the dam into a number of blocks as shown in below figure 5.
6. Time Interval between Concrete lifts
In mass concrete structures like dams, the concrete is poured lifts wise and there should be a good time interval between successive lifts to prevent cracking due to shrinkage. A time period of 3 to 4 days is recommended between successive lifts.
7. By Limiting the Height of Lift
The height of a concrete lift should not be greater than 1.5 m to prevent cracking in mass concrete structures.