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The size of concrete pours for dam construction are large. To reduce the risk of thermal cracking the heat generated during the setting of the concrete should be controlled. Relatively low quality concrete, with a low cement content, may be used for the bulk of the structure. The maximum aggregate size will be larger than that used for general concrete construction, say 150 mm. Higher quality concrete will be required for dam faces that are exposed to water or the atmosphere.
Heights of lifts are generally limited to about 1.5 m. Deeper lifts are possible if measures are taken to cool the concrete. This may be by reducing the temperature of the fresh concrete by cooling the aggregates or using flaked ice as part of the mix water. Alternatively, concrete can be cooled after placing by means of tubes cast into the concrete through which cooling water is pumped. This cooling is started immediately after the concrete is placed and is continued until a stable temperature condition is reached.
An alternative approach for gravity dams is the use roller compacted concrete, where concrete is placed in continuous strips along the full length of the dam. Compaction is achieved by rolling so the workability may be much lower than for usual concrete. A range of spreading and compacting plant is available, much of which is generally used for earth-moving.
The use of readily available plant and speed of construction are the main attractions of the process. While roller compacted concrete may be used for the bulk of the dam, the upstream and downstream faces require a more durable concrete. This may be in the form of precast concrete units or slip-formed in situ.
Acknowledgement: The Concrete Society