Roller Compacted Concrete – Definition
The development of Roller Compacted Concrete (RCC) was a major change in the construction practices of dams and other structures. Roller Compacted Concrete is defined as the concrete which is compacted by roller compaction technique. The concrete mixture used for construction should keep in an unhardened state to support the roller compaction. Therefore roller compacted concrete differs from the conventional concrete mixture in terms of consistency required, i.e., the concrete should be dry enough to prevent sinking of the roller equipment but it should be wet enough to permit the required distribution of binder mortar used.
The first successful application of Roller Compacted Concrete (RCC) technology was demonstrated in 1974 in the repair of collapsed tunnel of Tarbela dam. Upon construction it proved that, these type of concrete also can provide required strength and durability. It is also used in the construction of world’s largest coffer dam, Three-Gorges dam, China.
Materials and Mix proportion
Cement: Cement with lower heat generation is used. There is no special type of cement is preferred for RCC.
Admixture: Large amounts of mineral admixtures are used extensively in RCC mixtures to reduce temperature rise of concrete and improves durability. Air entraining and water reducing admixtures are used in RCC commonly. Also set-retarding admixtures can extend the time up to which the concrete should remain unhardened.
Aggregates: Aggregates of size less than 38 mm (1.5 in) is used commonly. The size of aggregate has significant influence on the degree compaction of small layers but not in thicker layers. If the aggregates of size less than 75µm are used, it produces a more cohesive mixture of concrete. If the stress level in the structure is low, cohesive concrete can be used in the interior of structure.
Roller Compacted Concrete Properties
Strength: They have the compressive strength and tensile strength results almost same as the normal concrete, ranging from 7 to 13 percent. Like the normal concrete, the ratio depends on aggregate type, cement content, age etc.
Elastic Modulus and Poisson’s ratio: RCC mixture has low elastic modulus, because the heat of hydration generated is proportional to elastic modulus. RCC mixture has low heat of hydration. Poisson’s ratio ranges from 0.15 to 0.20
Creep: creep of RCC depends on type of aggregate, water-cement ratio, age of loading and the duration of loading and the duration of loading. RCC with lower compressive strength and lower elastic modulus gives high creep. Lean concrete with large amount of fine aggregate also shows high creep.
Thermal properties: The rise of temperature in RCC is similar to the normal concrete mixture and mainly depends on the amount and type of cement used. All other characters such as the specific heat, conductivity and coefficient of thermal expansion are a function of amount of aggregate used in the mixture.
- Costs: It can reduce the cost up to 25-50% compared to conventional concrete mixture.
- Rapid construction: Compared to the regular mass concrete dams, RCC structures can be finished in 1 to 2 years earlier than the prescribed project duration.
- Spillways: Normally in embankment dams spillways are constructed in an abutment, but in this technology the spillway can be constructed in the main structure of dam.
- Due to the use of leaner concrete mix, cement consumption will be less.
- Due to the use of layer placement method, costs of formwork is lower.
- Due to low temperature rise, pipe cooling is not required.
- Costs of transportation, placement and compaction of concrete are lower.
Roller Compacted Concrete Applications
- Willow Creek dam: It is the world’s first all RCC structure constructed in 1982 after 1 year of duration of project. The aggregates used for this dam construction contained approximately 25% silty, sandy gravel overburden. The dam is constructed as one monolithic mass with no vertical joints. The temperature rise was only 11˚Ϲ in 4 weeks.
- Tamagawa dam: it was built in Japan in Tama river for flood control, water supply and power generation. The use of RCC was selected for construction because it can save almost one year of construction time because the site environment was so harsh to proceed the construction work due to approaching winter. The dam is 100m high and with a river bed width of 200 m and concrete volume of 1.14 million m3.