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The stationary tilt drum mixer is fast, consistent, and suitable for most RCC projects. Photos: Portland Cement Association

Roller-compacted concrete (RCC) is just what it sounds like. This stiff, zero-slump concrete mixture can be placed with asphalt-type paving equipment and then compacted with rollers.

Even though RCC has the same basic ingredients as conventional concrete—cement, fine and coarse aggregates, and water—it is drier, and looks and feels like damp gravel. A low water-cement ratio and dense-graded aggregates give RCC high strength, making it ideal for paving inter-modal facilities, equipment yards, and trucking terminals. It has also been used for parking lots, city streets, and intersections.

This concrete costs less because it is easy and fast to produce and place. Resulting pavements do not require joints, dowels, reinforcing steel, formwork, or finishing, and are virtually maintenance-free.

Properties Of An RCC Mix

Special attention to mixing and batching procedures must be considered with concrete that's to be compacted by rollers. 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.

The concrete must be dry enough to support the weight of a large compaction roller, yet wet enough for an even distribution of the paste throughout the mix.

Materials selection is important. Knowlege of the ingredients, the construction requirements, and project specifications helps ensure that the mix will meet design and performance objectives.

Aggregates: RCC uses a well-graded aggregate similar to that used in conventional concrete. However, 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.

Coarse aggregates include crushed or uncrushed gravel, crushed stone, or even crushed recycled concrete. Fine aggregates are usually natural sand, manufactured sand, or a combination of the two. Crushed aggregates typically work better in these mixes due to the sharp interlocking edges of the particles, which help to reduce segregation, result in higher strengths, and provide better aggregate interlock at joints. Because up to 90% in volume of an RCC mix can include both coarse and fine aggregates, its durability should have standard physical property testing as outlined in ASTM C 33.

The American Concrete Institute and the Portland Cement Association recommend using dense, well-graded blends with a nominal maximum size aggregate not to exceed 1 inch to help minimize segregation and produce a smooth finished surface. Gap-graded mixes dominated by two or three aggregate sizes are not desirable for this type of concrete. Also, the recommended gradation calls for 2% to 8% of fine particles—typically higher than acceptable for conventional concrete. This eliminates the need for washed aggregates in many cases and produces a mix that is stable during rolling.

Cementitious materials: The materials used in RCC mixes include Type I, Type IP, or Type II portland cement or blended hydraulic cement; Class F or Class C fly ash; silica fume; and ground granulated blast furnace slag. The type and amount of cementitious materials should be based on availability as well as the required design strength and durability of the finished RCC.