Materials Used for RCC Mixtures
Due to moisture sensitivity and other factors, choosing the appropriate aggregate gradation is a major part of successful RCC construction. Therefore, it is important to characterize the sources of aggregate planned for a project with respect to its gradation. Some have advocated the use of an optimized mixture design by holding the combined gradation curve to the 0.45 power curve and thus avoiding gaps in the gradation curve. However, the number of aggregates to be included depends on the method of mixing, as discussed further below.
Therefore, two items are important for aggregates used in RCC construction: gradation and mixture stability during paving under moisture content fluctuations. Most gradations for RCC mixtures are optimized for density by making the combined gradation as well distributed as possible. The mixture stability under moisture content fluctuations relates to the sharpness of the moisture-density curve and the rate at which the curve’s slope changes from positive to negative, as well as aggregate shape. Mixtures optimized for density typically follow the 0.45 power gradation, as shown in figure 1. These mixtures have well distributed aggregate sizes throughout, which helps facilitate placement and densification.
Furthermore, the greater the density, the lower the percentage of voids in the aggregate mixture. Less cementitious material is needed to fill voids, aiding mobility of the mixture and potentially reducing its cost. However, there are other factors that affect the percent voids, such as the maximum aggregate size, compactive effort, and angularity of the aggregate particles. It should also be pointed out that the percent voids is determined as a function of the dry-rodded unit weight (ASTM C29) and the specific gravity (ASTM C33) of the base material. These test procedures are common and can be carried out by most testing laboratories.
With respect to mixture stability and moisture sensitivity, two density moisture curves are compared in figure 2. The upper curve has a steeper slope and a narrower base than the lower curve, indicating greater susceptibility to moisture changes. Most experts agree that a broader range in moisture with little effect on density (as depicted in the lower curve) is desirable for a concrete mixture for RCC construction.
