Concrete Mixtures

To develop a mixture with the greatest density and strength at the lowest cement content, the designer has some choice in optimizing a mixture for a given project. The following components typically are considered:

• Nominal maximum aggregate size
• Water content
• Sand content
• Cement and fly ash content based on strength and density (minimum of 450 lb/cy)
• Aggregate gradation, blending, stockpiling, and potential for segregation
• Moisture–density relationship
• Aggregate size
• Aggregate top size
• Minus 200 aggregate content (ranges from 0 to 8 percent)
• Use of admixtures
• Consistency of the concrete mixture; it must be stiff enough to sustain vibratory rolling(see figure 3)
• Water content to achieve the needed consistency for mixing and placement

The mixture design procedures involve the following steps:

1. Select well-graded aggregates.
2. Select the cement content (as a percentage of total weight).
3. Develop moisture–density relationships; determine the maximum dry density and the optimum moisture content.
4. Cast strength specimens at maximum density and optimum moisture content.

Figure 4 shows typical mixture proportions, where a common amount of cement is 450 lb/cy. The amounts of sand and coarse aggregate are the opposite of what is used in conventional concrete, with the sand being 50 to 55 percent of the aggregate weight and the coarse material being 45 to 50 percent. The range of water to cement ratio in RCC mixtures can be as low as 0.36 and as high as 0.45, but it is effectively a result of the optimum moisture content derived from the moisture density curve (rather than taken simply as an input that conventionally has dictated the water content in a mixture design). Enough water is used to facilitate mixing and compaction while maintaining stability behind the paver and under the rollers.