RCC has the same basic ingredient as conventional concrete: cement, water, and aggregates. The basic difference is that RCC is a much drier mix with practically zero slump. It is drier, and looks and feels like damp gravel. It does not require any forms, dowels, reinforcing steel & finishing. Also, the method of compaction is different with the RCC being compacted by vibratory or pneumatic-tired rollers.

3. How is the mix for RCC different from the mix of Conventional Concrete?

The objective of mix design is to produce an RCC mixture that has sufficient paste volume to coat the aggregates in the mix and to fill in the voids between them. Any of the basic RCC proportioning methods like those based on concrete consistency testing, the solid suspension model, the optimal paste volume method, and soil compaction testing may be used for mix design.

Roller-Compacted Concrete (RCC) uses aggregate sizes often found in conventional concrete.  However, the blending of aggregates will be different than that done in case of conventional concrete. Crushed aggregates are preferable in RCC mixes due to the sharp interlocking edges of the particles, which help to reduce segregation, provide higher strengths, and better aggregate interlock at joints and cracks. Gap-graded mixes that are dominated by two or three aggregate sizes are not desirable for RCC. The content of fine particles required is typically higher than that of conventional concrete. Washed aggregates are not required for this type of concrete since a small quantity of non-plastic fines present (2% to 8% material passing a No. 200 sieve) can enhance its properties. This produces a mix that is stable during rolling. Various Standards /institutes like ACI recommend particular gradings for different roller compacted structures like pavements. Generally use of dense, well-graded blends with a nominal maximum size aggregate (NMSA) not exceeding ¾-inch (19 mm) is recommended in order to help minimize segregation and produce a smooth finished surface.

The moisture content in the mix should be such that the mix is dry enough to support the weight of a vibratory roller yet wet enough to ensure an even distribution of the cement paste.

Properties of RCC Mix can be summarized as below:

Compared with conventional concrete, RCC pavement mixes have:

• A lower water content
• A lower paste content
• No air-entrainment, although some admixtures may be used to increase workability and control set time.
• More finer aggregates
• Smaller maximum size coarse aggregate.

4. Does the RCC mix require curing?

Curing is very important for RCC also. Since there is no  has no bleed water in RCC (due to much lesser water content) , the main concern is to prevent its drying to avoid cracking resulting from drying shrinkage &  to ensure adequate strength by allowing continuing hydration. Curing also helps in avoiding dusting of surface.  Normally surface of RCC is kept moist for 7 days, or until a curing compound is applied. Due to more open texture surface of RCC, the curing compound application rates are 1.5 to 2 times the application rates used for conventional concrete. Due to large surface area of structures like RCC pavement curing techniques such as plastic sheeting and wet burlap may not be used due to higher cost involved.

5. Basic Construction Sequence for Roller Compacted Concrete

• Produced in a pug mill or central batch plant
• Transported by dump trucks
• Placed with an asphalt paver : The thickness of layers generally varies from 8” to 10”
• May be compacted by vibratory or pneumatic-tired rollers

• (Source: www.watanabesato.co.jp)

The table below (source PCA) illustrates the features & benefits of RCC:

photograph above(Source: www.watanabesato.co.jp) is a sample cut from RCC pavement. It is seen that this sample is tightly compacted from top to bottom

·        Aesthetics - RCC does not have the same appearance as other types of concrete. It is not as pretty and smooth as regular concrete.

·        Rougher Surface Texture - The mix design and construction methods that make roller compacted concrete so fast, easy, cheap, and durable also create a surface texture that gives it a characteristic coarse finish.

·        Limited to low-speed traffic - Due to the nature of its surface, RCC is not appropriate for all types of traffic. Vehicles traveling at high speeds would experience a bumpy ride. That makes it better for applications where strength and durability are needed instead of speed.

         (Source: Mississippi Concrete Industries Association)

8. Typical application areas

The use of RCC for pavements at industrial facilities such as port and intermodal container terminals is particularly appropriate because of the ability to construct low-cost concrete pavements over large areas.

• bulk material storage
• general cargo storage
• container terminals
• road / rail transfer facilities
• Ro-Ro terminals
• truck parks
• tank roads and parking
• sewage sludge stacking
• composting slabs
• pre-casting yards