This document describes the effects of aggregate gradation on concrete performance.
How to Maintain Concrete Economy and Integrity With Aggregates
A concrete mix basically consists of sand and rock of various sizes glued together with a mixture of cement and water. Concrete aggregates displace a significantly large portion of the volume of a cubic yard of concrete. For example, in a typical 600-pound cement mix with entrained air, the fine and coarse aggregate will make up approximately 67 percent of the total volume. With leaner mixes, the aggregate takes up even more volume. The gradation and overall fineness of the aggregate used will affect the physical properties and the cost of the mix.
The gradation of the aggregate, or distribution of particle sizes, should meet the specifications outlined in ASTM C33, "Standard Specification for Concrete Aggregates." ASTM C33 suggests that the gradation curve be smooth, with neither a deficiency nor excess of material of any one size. A smooth gradation curve decreases the voids between the aggregate particles in a homogeneous concrete mix, and, because the voids must be filled with a mixture of cement and water, it therefore decreases the amount of cement required. (The paste portion of the concrete can also be increased by adding water, but this increases the water-cement ratio and decreases strength.) Cement is usually the most expensive component in concrete, so minimizing the amount needed makes the mix more economical.
In order to see the effects of gradation on concrete cost, pack a beaker with 1-inch-diameter rock and measure the amount of water needed to fill up the beaker. This water represents the volume of the voids in the beaker (see the following illustration). Then, take the same beaker, packing it with a homogeneous mixture of 1-inch-diameter and ⅜-inch-diameter rock and again measure the quantity of water needed to fill the beaker.
Less water will be needed to fill the beaker with the two sizes of stone. Every time another size is added to the mix, the void content in the beaker will decrease. This is what happens in a cubic yard of concrete, as well, except that the voids are filled with cement and water.
Another factor that can affect the economy of a concrete is the fineness of the aggregate. Because all surfaces of all particles in the mix must be covered with cement paste, the more surface area present in the concrete, the more cement and water needed to do the job.
This can be illustrated by imagining a 1-inch-by-1-inch-by-1-inch cube of stone, with a total surface area of 6 square inches. If this cube is then sliced in half, the weight of the stone remains the same, but the surface area is increased to 8 square inches, and more cement paste is needed to cover the surfaces and glue them together. Imagine what would happen if the cube of stone were sliced 1,000 times!
Other Considerations for Fine Aggregate
ASTM C33 suggests that the fineness modulus be kept between 2.3 and 3.1. Very fine sand will increase the water demand of the mix, while very coarse sand could compromise its workability.
ASTM C33 requires that the sand be less than 45 percent retained on any one sieve. Too much material on one sieve means gap-grading, which will increase the water demand of the mix.
The amount of material passing the #50 and #100 sieves will affect workability, slab surface texture, and bleeding. Increased bleeding will occur as the portion passing the #50 sieve increases. The flatwork finishability of a mix also increases as the portion passing the #50 sieve increases.
ASTM C33 limits the amount of material passing the #200 sieve to 3 percent for natural sand that contains clay. Clay is a very fine particle that greatly increases the water demand of a mix, reduces strength significantly, and promotes bleeding.
Other Considerations for Coarse Aggregate
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