These broad gradation bands are for use all across the country. 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.īroad coarse-aggregate gradation ranges are listed in Table 2 of ASTM C 33. 4 sieve divided by 100.įine aggregate should fall within gradation limits provided in ASTM C 33, Section 6. Table 1 - Fineness modulus (FM) is calculated by totaling the percent Percentage of individual fraction retained by mass Excessively coarse material will produce harsh mixes that are more difficult to place, consolidate, and finish. Excessively fine materials will have a higher water demand and typically result in a sticky mix. The FM should not change more than 0.2 otherwise, mix adjustments may be necessary. The FM for fine aggregate should fall within the range of 2.3 to 3.1.
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 fineness modulus (FM) is an index of the fineness of an aggregate. The dividing line between fine and coarse aggregate is the 3/8-inch sieve. The proportions between coarse and fine aggregates will change based on the unique characteristics of each aggregate, the placement method, and the finish desired. To avoid post-placement problems in concrete that are difficult to rectify, verify that these limits are not exceeded at the time of material submittals.Īn optimized gradation based on aggregate availability and project requirements will result in an economical concrete with good workability and finishability. Some projects might require even more-stringent limits. The allowable percentage of deleterious substances for both fine and coarse aggregate are listed in ASTM C 33 Tables 1 and 3, respectively. (Photo courtesy of PCA)Īs a naturally occurring material, aggregate will sometimes include weathered or unstable particles in the delivered product. When located near a saturated surface, they can absorb water and upon freezing, expand and spall the concrete immediately above it, along with a portion of the aggregate. Some chert particles are more absorptive than higher-quality aggregates. (Photo courtesy of the Portland Cement Association)Ĭross section of a core sliced longitudinally exhibits the mechanism of a popout. Though not a structural concern, popouts are a nagging surface defect that can be a source of aesthetic complaints, and if excessive, may compromise cover quality in reinforced slabs. Surface of concrete flatwork where a popout occurred over a chert particle. Here's an overview of the most important factors to consider when selecting and proportioning concrete aggregate. A lower water content will reduce the potential for shrinkage and for cracking associated with restrained volume change. Less cement (within reasonable limits for durability) will mean less water if the water-cement (w/c) ratio is kept constant. Using larger coarse aggregate typically lowers the cost of a concrete mix by reducing cement requirements, the most costly ingredient. You can often save money by selecting the maximum allowable aggregate size. Changes in gradation, maximum size, unit weight, and moisture content can all alter the character and performance of your concrete mix.Įconomy is another reason for thoughtful aggregate selection. But a closer look reveals the major role and influence aggregate plays in the properties of both fresh and hardened concrete. And for very rounded tops, the volume may increase by 30% to 40%.Aggregates are generally thought of as inert filler within a concrete mix. If the pile has a moderately rounded peak, the volume may be increased by 15% to 20%. For slightly rounded tops, the volume may be increased by a factor of 2% to 5%. When the peak of the pile of gravel or sand is rounded, there is extra volume near the top.
The volume of a pyramid with a rectangular base is LWH/3, where L is the length, W is the width, and H is the height. An alternate version of the formula is πD²H/12, where D is the diameter. The formula for the volume of a cone is πR²H/3, where R is the radius of the circular base and H is the height of the cone. You can then adjust this estimate according to how rounded the top of the cone or pyramid is. For conical and pyramid-shaped piles of gravel, you can use the cone and pyramid volume formulas to compute a rough estimate. The volume of a pile of sand or gravel depends on the dimensions and shape of the pile.