Aggregate Packing Density is obviously one of the most important parameters in Shotcrete/Concrete mix design. It represents how densely the aggregate particles are packed & is an indicator of the amount of voids between the aggregate particles. 

The main parameters affecting the aggregate packing density are the shape and combined particle size distribution of the aggregates used in the mix. Once the aggregate supply has been selected, the only parameter that the mix designer can manipulate is the fine/coarse aggregates ratio.

In concrete technology, the term “particle” is used for fine aggregate, the coarse aggregate(s), and the fibers (if any) and the term “paste” is used for Water, cement, other cementitious materials, and air. It is the paste phase that lubricates the particles. 

To make concrete or shotcrete, not only must all the space (voids) between the particles be filled with cement paste, but some extra paste must also be added to provide the desired level of workability. Normally, there is an optimum fine/coarse aggregate ratio (and aggregate gradation) that minimizes the amount of voids in the aggregate skeleton, thus minimizing the amount of paste required to fill the voids. Extra paste is required, over and beyond that required to fill the voids, and its presence affects the workability or, in this case, the pumpability of the wet-mix shotcrete (Chapdelaine and Beaupré 2000). Normally provision of extra paste to the extent of 6% is considered a good practice although it depends on hose dia & hose layout also.

The void content of the aggregate phase can be easily determined using the methods/procedures described in basic soil mechanics manuals. The curves below have been built by testing different combinations of fine aggregate, coarse aggregates, and fibers (if any).

Packing curves for mixtures with different fiber contents

The Figure above shows the void contents for various proportions of coarse aggregates (as a fraction of the total combined aggregate mass) for three mixtures containing 0, 1, and 2% (by volume) of synthetic macrofibers. Each mixture has a minimum void content that indicates optimum particle packing. The optimum ratio depends upon the shape and gradation of both the fine and coarse aggregates and the fiber type and content.

When fibers are present, the aggregate packing is modified, which leads to a different optimum coarse/total aggregate ratio and void content. These curves were obtained with standard concrete sand and a 10 mm (0.394 in.) maximum size crushed granite coarse aggregate. In this figure, the mixture without fibers having a coarse/total aggregates content of 47% has a void content of 29%. In practice, this mixture should contain 29% of cement paste plus approximately 6% of extra paste* in order to have good pumpability (Chapdelaine and Beaupré 2002). For the same ratio, the paste content should be increased when fibers are present; 1% of fibers not only moves the optimum toward a mixture richer in sand, but increases the void content to 35% (35% + 6% = 41% paste requirement for good pumping). This is only an example; one should remember that these numbers are affected not only by the aggregates used, but also by fiber geometry and content. However, it illustrates the importance of this concept.

Having a minimum paste content to fill the voids between aggregates and fiber particles is an essential factor with respect to concrete pumpability. As implied in Figure above, there are two ways of achieving this: either increase the paste content (increase cement, water, or air content) and/or reduce the aggregate void content by optimizing the fine/coarse aggregate ratio. The two options have the same effect: increasing the workability (and pumpability) of the concrete mixture.

However, there are limits related to cost, mechanical, and durability considerations as to how much the paste content can be increased. Because of the beneficial effects on material costs, shrinkage, durability, and mechanical properties, minimization of voids in the aggregate skeleton and hence the paste required in the mixture should always be implemented first.

Therein lies the significance of Coarse Aggregate to Fine aggregate ratio.

Reference: “Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and Placement” by Marc Jolin and Denis Beaupré