The stationary tilt drum mixer is fast, consistent, and suitable for most RCC projects. Photos: Portland Cement Association
The total amount of cementitious material consisting of cement and pozzolan is typically between 400 and 600 lbs./ cu. yd. in a wide range of proportions of these materials.
Admixtures: Chemical admixtures in RCC mixes should conform to ASTM C 494 and be approved by the project engineer before use. Adding water-reducing or set-retarding (hydration-stabilizing) admixtures may be useful when there is a long haul time from the production point to the project location. Water reducers can distribute the cement paste uniformly throughout the mix and improve workability during paving. Polycarboxylate superplasticizers have been used in dry batch plant production to improve workability and reduce mixing times, resulting in significantly increased production rates.
Water. Even though considerably reduced for RCC, the water should meet the same quality requirements as for conventional concrete mixes. Typically between 150 and 200 lbs./cu. yd. of water is used. Water to total cementitious ratios—W/(C + P)—generally fall between 0.30 and 0.45. Ratios in this range have the greatest positive influence on the final strength, with 28-day unconfined compressive strengths typically exceeding 6000 psi.Mix Design
Besides material selection, the correct proportioning of raw materials is critical to quality RCC mixes. They are designed using a systematic procedure based on the aggregates, water, and cementitious materials and confirmed through a select number of trial tests.
Several methods exist for proportioning RCC mixes for pavements: 1.) concrete consistency testing, 2.) the solid suspension model, 3.) the optimal paste volume method, and 4.) soil compaction testing. Whichever method is used, the goal is to produce a mixture that meets the specific project requirements.
Concrete consistency tests normally require specific mixture parameters such as the amounts of aggregate, water, or cementitious materials—and then adjust one of them to meet a required level of consistency, workability, or strength. This method can optimize each ingredient in the mixture in order to obtain the desired properties (to read about various mixeing production methods, click here).
Recently, a more theoretical and fundamental approach to RCC mix design proportioning has been introduced—the solid suspension model. This method determines the proportions of the dry solid ingredients (cement, fly ash, silica fume, sand, and coarse aggregate) that optimize the dry packing density of a given mixture. It is then easy to calculate the amount of water necessary to fill entirely the void spaces between the dry ingredients. The main advantage is that this model can quickly recalculate the optimum proportions without having to prepare a large number of laboratory trial batches.
Originally developed to prepare RCC mixes for dams and other large structures, the optimal paste volume method has been used lately to proportion non-air-entrained RCC pavement mixes as well. Because workability is vital, this method stresses a design that will meet specified workability requirements. It assumes that an optimal mix should have just enough paste to fill completely any remaining voids after the aggregates have reached maximum density under compaction.
Based on soil and soil-cement technology, the soil compaction testing method establishes a relationship between the density and moisture content of a mix. A moisture-density test (ASTM D1557) determines the optimum moisture content and maximum density of mixtures. Once determined, the lab prepares strength test specimens by compacting at the optimum moisture content for each particular cementitious material content. From these tests, a plot of strength versus cementitious material content will indicate the minimum cementitious materials content that will meet the design requirements.
The optimum moisture content will facilitate compaction and provide the best opportunity to achieve maximum compaction and density.
— Greg Halsted, PE, is the soil cement/ RCC pavements engineer for the Portland Cement Association based in Skokie, Ill. For more information on RCC, visit www.cement.org/pavements.