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Roller-Compacted Concrete (RCC) Mixture Proportioning

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Factors considered in RCC mixture proportioning

Proper mixture proportioning is crucial in creating a high-performing RCC mixture for an RCC Pavement. There are a number of factors that should be considered when designing an RCC mixture and they include constructability, mechanical strength, economics, durability and performance. There are two general approaches to mixture proportioning. The first is the soil compaction approach, which determines the proportions of cement and aggregate based on the optimum moisture content and the maximum dry density. The second method is the consistency or workability approach, which is a w/cm approach, with the consistency kept constant and the mixture determined by absolute volume. These two approaches are the basis for the four main mixture proportioning methods.

The four main proportioning methods are the soil compaction test, concrete consistency test, solid suspension model, and the optimal paste volume method. The soil compaction method is the most common in the U.S. for RCC pavements. The methods all strive to ensure that the mixture has sufficient paste volume to coat the aggregates and fill the voids. Another goal is to produce the required fresh properties such that the RCC can be placed in addition to the proper hardened properties such that the pavement can be durable enough to endure the traffic loading and environmental conditions that it is subjected to.

Materials Selection

The materials selection of an RCC mixture is very important to the mixture design. The RCC should be designed with well-graded aggregates to optimize paste content and minimize void space, while also reducing segregation and producing a dense, smooth, tight surface. The water content should be such that the mixture is dry enough to support the weight of the rollers yet wet enough to ensure an even distribution of cement paste. The cementitious materials should meet strength and durability requirements. Chemical admixtures can also be utilized but will perform differently than in conventional paving concrete.

Mixture Proportioning Methods

There are four main mixture proportioning methods. The most common one for RCC pavements is the soil compaction method. The other methods, including the concrete consistency test, solid suspension model, and the optimal paste volume method, are more commonly used for hydraulic structures such as dams and spillways but can also be used for pavements.

Soil Compaction Method

The soil compaction method is a six step mixture design method for roller-compacted concrete pavements. This method is the most commonly used method for RCC pavements. The density and moisture content relationship is manipulated to produce the maximum density. This is done by compacting samples over a range of moisture contents. The soil compaction method determines the mixture proportions required to meet strength requirements but does not directly consider such issues as durability, constructibility, etc.

Well-Graded Aggregates

The first step in the soil compaction method is to optimize the aggregates from the perspectives of gradation, segregation resistance, and compactibility. A maximum-density grading should be targeted for the combined aggregate gradation. One method used to achieve a maximum density (for any aggregate size) is a 0.45-Power curve [1]. Most construction specifications will specify upper and lower limits for the combined aggregate gradation; generally the 0.45-Power curve falls in the middle of these two limits.

Selection of Cementitious Content

The second step in the soil compaction method is to determine a cementitious content. A good starting point for RCC used as a surface course is 11-13 percent by weight of dry materials (i.e. cementitious materials and dry aggregate) [2]. The main factors controlling cementitious content will be project strength requirements and/or minimum cementitious material requirements.

Develop Moisture-Density Plots

Example Moisture-Density Plot Showing MDD and OMC

The third step in the soil compaction method is the development of moisture density plots for each cementitious materials content chosen. Moisture-density plots are constructed by performing modified Proctor compaction tests (ASTM D1557) at various moisture contents; typically moisture content is varied in the range of 5-8%. Moisture content is defined as the weight of water divided by the weight of dry materials (i.e. cementitious materials and dry aggregate). The dry density is then plotted against moisture content. The maximum dry density (MDD) is then used to proportion the cementitious materials (if selected as a percentage) and aggregate. The optimum moisture content (OMC) is the moisture content that corresponds to the MDD and is used to determine the total moisture content of the RCC mixture. A vibrating table can also be used to determine moisture-density plots of RCC according to ASTM C1170. The figure at the right shows an example moisture-density plot with its corresponding MDD and OMC.

Cast Samples to Measure Strength

The fourth step in the soil compaction method is to cast specimens for strength testing to determine compliance with project specifications. Typically, compressive strength is the main parameter of concern. RCC specimens are cast using a vibrating hammer according to ASTM C1435, or using a vibrating table according to ASTM C1176. The use of the vibrating table for producing RCC specimens is less common.

Test Specimens and Select Required Cementitious Content

Relationship between Compressive Strength and Cementitious Content

The fifth step in the soil compaction method is to test the RCC specimens produced in the previous step. If only one cementitious content was chosen, the results of the strength testing should be compared against required strength value(s) from the project specifications to determine compliance. If multiple cementitious contents were chosen, strength should be plotted against cementitious content to determine the cement content that meets the required strength(s); this procedure is shown in the figure at the right.

Calculate Mixture Proportions

The sixth and final step in the soil compaction method is to determine the mixture proportions. If only one cementitious content was chosen, and strength testing met specification requirements, the mixture proportions used to cast the specimens for strength testing are also the final mixture proportions. If multiple cementitious contents were chosen, this step is required. It is suggested that if the required cementitious content is significantly different from the cementitious contents used during testing that another moisture-density plot be determined at the required cementitious content. The MDD and OMC resulting from this new moisture-density plot will then be used to determine the final mixture proportions.

Consistency and Workability Proportioning Methods

Concrete Consistency Method

This method involves fixing two mixture parameters while varying one other mixture parameter. For example, amount of aggregate and cementitious content might be the two variables that remain constant while the water content is changed. For each combination of parameters, the consistency is measured using the Vebe consistometer (ASTM C1170). This process is iterated until the required level of consistency is met. Strength properties should also be checked for compliance. This method has largely been used for the design of hydraulic structures.

Solid Suspension Model

This method is a more theoretical and fundamental approach to RCC mixture design. By knowing properties such as gradation, specific gravity, and packing density of the various constituents (i.e. aggregates and cementitious materials) the dry packing density of a given RCC mixture can be calculated. This method can be used to optimize the dry packing density of an RCC mixture rather quickly since it only requires the previously mentioned properties of each constituent and does not require each trial mixture to be tested in the laboratory. After choosing a combination of constituents that produces the maximum dry packing density (or a desired dry packing density), the amount of water to entirely fill the void spaces between the dry ingredients (i.e. aggregate and cementitious materials) is then calculated. Strength properties should then be checked for compliance. While this seems to be a promising method for proportioning RCC mixes, it has seen little use in industry.

Optimal Paste Volume Method

This method is similar to the solid suspension model in that its objective is to fill the voids in the aggregate structure with sufficient paste such that, after compaction, there are no voids in the mixture. This method therefore minimizes the paste content required to fill the voids present in a given aggregate structure. Also similar to the solid suspension model, this method has seen little use in industry.

Related Pages

References

  1. National Stone, Sand & Gravel Association. 1991. Aggregate Handbook. Arlington, VA: National Stone, Sand & Gravel Association.
  2. National Concrete Pavement Technology Center. 2010. Guide for Roller-Compacted Concrete Pavements. Ames, IA: National Concrete Pavement Technology Center.