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An optimized gradation based on aggregate availability and project requirements will result in an economical concrete with good workability and finishability. The proportions between coarse and fine aggregates will change based on the unique characteristics of each aggregate, the placement method, and the finish desired.

The dividing line between fine and coarse aggregate is the 3/8-inch sieve. The fineness modulus (FM) is an index of the fineness of an aggregate. The FM is calculated by adding the cumulative percentages by mass retained on each of a specified series of sieves and dividing the sum by 100 (see Table 1). The FM for fine aggregate should fall within the range of 2.3 to 3.1. The FM should not change more than 0.2; otherwise, mix adjustments may be necessary. Excessively fine materials will have a higher water demand and typically result in a sticky mix. Excessively coarse material will produce harsh mixes that are more difficult to place, consolidate, and finish.

Sieve size Percentage of individual fraction retained by mass Percentage passing by mass Cumulative percentage retained, by mass
9.5 µm (3/8 in.) 0 100 0
4.75 µm (No. 4) 2 98 0
2.36 µm (No. 8) 13 85 15
1.18 µm (No. 16) 20 65 35
600 µm (No. 30) 20 45 55
300 µm (No. 50) 24 21 79
150 µm (No. 100) 18 3 97
Pan 3 0 -
Total 100 283
Fineness modulus = 283 ÷ 100 = 2.83

Fine aggregate should fall within gradation limits provided in ASTM C 33, Section 6. If there is a deficiency in a locally available fine aggregate, concrete may benefit from the addition of air entrainment, additional cement, or a supplemental cementitious material (SCM) to address these shortcomings.

Broad coarse-aggregate gradation ranges are listed in Table 2 of ASTM C 33. These broad gradation bands are for use all across the country. Once a gradation is selected for a project, maintaining the gradation within tight ranges will achieve greater batch-to-batch consistency. This is typically accomplished by properly stockpiling aggregates and reworking stockpiles to counter excessive segregation (Photo 4).

The maximum size of coarse aggregate that can be used in a mix depends on the size, shape, and reinforcing of a member, based on these "not-to-exceed" guidelines (see Photos 5 and 6):

  • 3/4 of the clear spacing between rebar or between rebar and forms (cover distance)

  • 1/3 the depth of the slab

  • 1/5 the narrowest dimension of a member

Often aggregates are analyzed using combined grading of fine and coarse materials proportioned as anticipated in the proposed concrete mix (Photo 7). This provides an estimate of how the blend will perform in concrete. Every region has its own deficiencies in aggregates, but once a combined aggregate gradation is plotted (percentage retained vs. sieve size), these deficiencies can more easily be identified and remedied. Alternative aggregate sources or additional aggregate blending can then be considered to approach the elusive "ideal" gradation that provides the best workability, pumpability, reduced shrinkage, and economy (Figure 1).

Note that eliminating the sand from a mix transforms conventional concrete to a "no-fines" mix, also known as "pervious" concrete" (see Pervious Concrete Pavements). Pervious concrete qualifies for LEED points, a green building rating system developed by the U.S. Green Building Council (USGBC), because it permits runoff to percolate directly into the subgrade, recharging the ground water table.

Portland Cement Association

Photo 4 - To address excessive segregation of particle sizes, batching is usually preceded by reworking piles around the circumference to reblend sizes, rather than working straight into the pile. (Photo courtesy of PCA)

Portland Cement Association

Photo 5 - Recommended maximum aggregate size to pass through the clear distance between reinforcing steel and cover dimension. (Photo courtesy of PCA)

Portland Cement Association

Photo 6 - Maximum aggregate size based on placement method and slab thickness. (Photo courtesy of PCA)

Portland Cement Association

Photo 7 - Size distribution from fine to coarse aggregate plays a significant role in concrete workability and performance. (Photo courtesy of PCA)

Portland Cement Association

Figure 1 - Optimum combined aggregate grading for conventional concrete. (Courtesy of PCA)

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