Aggregates

The material combined with cement and water to make concrete is called aggregate. Aggregate makes up 60 to 80 percent of concrete volume. It increases the strength of concrete, reduces the shrinking tendencies of the cement, and is used as economical filler.

Types

Aggregates are divided into fine, usually consisting of sand, and coarse categories. For most building concrete, coarse aggregate consists of gravel or crushed stone up to 1 1/2 inches in size. In massive structures such as dams, the coarse aggregate may include natural stones or rocks ranging up to 6 inches or more in size.

Purpose of Aggregates

The large, solid coarse aggregate particles form the basic structural members of the concrete. The voids between the larger coarse aggregate particles are filled by smaller particles. The voids between the smaller particles are filled by still smaller particles. Finally, the voids between the smallest coarse aggregate particles are filled by the largest fine aggregate particles. In turn, the voids between the largest fine aggregate particles are filled by smaller fine aggregate particles, the voids between the smaller fine aggregate particles by still smaller particles, and so on. Finally, the voids between the finest grains are filled with cement. You can see from this that the better the aggregate is graded; that is, the better the distribution of particle sizes, the more solidly all voids will be filled, and the denser and stronger the concrete will be.

The cement and water form a paste that binds the aggregate particles solidly together when it hardens. In a well graded, well designed, and well mixed batch, each aggregate particle is thoroughly coated with the cement-water paste. Each particle is solidly bound to adjacent particles when the cement-water paste hardens.

Aggregate Sieves – The size of an aggregate sieve is designated by the number of meshes to the linear inch in that sieve. The higher the number, the finer the sieve. Any material retained on the No. 4 sieve can be considered either coarse or fine. Aggregates larger than No. 4 are all coarse; those smaller are all fines. No. 4 aggregates are the dividing point. The finest coarse aggregate sieve is the same No. 4 used as the coarsest fine aggregate sieve. With this exception, a coarse aggregate sieve is designated by the size of one of its openings. The sieves commonly used are 1 1/2 inches, 3/4 inch, 1/2 inch, 3/8 inch, and No. 4. Any material that passes through the No. 200 sieve is too fine to be used in making concrete.

Particle Distribution – Experience and experiments show that for ordinary building concrete, certain particle distributions consistently seem to produce the best results. For fine aggregate, the recommended distribution of particle sizes from No. 4 to No. 100 is shown in Table 1.

Determine the distribution of particle sizes in aggregate by extracting a representative sample of the material, screening the sample through a series of sieves ranging in size from coarse to fine, and determining the percentage of the sample retained on each sieve. This procedure is called making a sieve analysis. For example, suppose the total sample weighs 1 pound. Place this on the No. 4 sieve, and shake the sieve until nothing more goes through. If what is left on the sieve weighs 0.05 pound, then 5 percent of the total sample is retained on the No. 4 sieve. Place what passes through on the No. 8 sieve and shake it. Suppose you find that what stays on this sieve weighs 0.1 pound. Since 0.1 pound is 10 percent of 1 pound, 10 percent of the total sample was retained on the No. 8 sieve. The cumulative retained weight is 0.15 pound. By dividing 0.15 by 1.0 pound, you will find that the total retained weight is 15 percent.

The size of coarse aggregate is usually specified as a range between a minimum and a maximum size; for example, 2 inches to No. 4, 1 inch to No. 4, 2 inches to 1 inch, and so on. The recommended particle size distributions vary with maximum and minimum nominal size limits, as shown in Table 2.

A blank space in Table 2 indicates a sieve that is not required in the analysis. For example, for the 2 inch to No. 4 nominal size, there are no values listed under the 4 inch, the 3 1/2 inch, the 3 inch, and the 2 1/2 inch sieves. Since 100 percent of this material should pass through a 2 1/2 inch sieve, there is no need to use a sieve coarser than that size. For the same size designation, 2 inch size aggregate, there are no values listed under the 1 1/2 inch, the 3/4 inch, and the 3/8 inch sieves. Experience has shown that it is not necessary to use these sieves in making this particular analysis.

Quality Standards

Since 66 to 78 percent of the volume of the finished concrete consists of aggregate, the aggregate must meet certain minimum quality standards. It should consist of clean, hard, strong, durable particles free of chemicals that might interfere with hydration. The aggregate should also be free of any superfine material, which might prevent a bond between the aggregate and the cement-water paste. The undesirable substances most frequently found in aggregate are dirt, silt, clay, coal, mica, salts, and organic matter.

Most of these can be removed by washing. Aggregate can be field tested for an excess of silt, clay, and the like using the following procedure:

1. Fill a quart jar with the aggregate to a depth of 2 inches.
2. Add water until the jar is about three-fourths full.
3. Shake the jar for 1 minute, then allow it to stand for 1 hour.
4. If, at the end of 1 hour, more than 1/8 inch of sediment has settled on top of the aggregate, shown in Figure 3, the material should be washed.

Figure 4 shows an easily constructed rig for washing a small amount of aggregate.

Weak, friable (easily pulverized), or laminated (layered) aggregate particles are undesirable. Especially avoid shale, stones laminated with shale, and most varieties of chert (impure flint-like rock). For most ordinary concrete work, visual inspection is enough to reveal any weaknesses in the coarse aggregate. For work in which aggregate strength and durability are of vital importance, such as paving concrete, aggregate must be laboratory tested.

Figure 5 shows a popout, which occurs when internal pressure causes a small part of the concrete surface to break away. This usually leaves a cone-shaped hole, ranging in size from 1/4 inch or less to 2 inches or more. (Photo courtesy of http://www.all-things-concrete.com.)

Handling and Storage

A mass of aggregate containing particles of different sizes has a natural tendency toward segregation. Segregation refers to particles of the same size tending to gather together when the material is loaded, transported, or otherwise disturbed. Always handle and store aggregate by a method that minimizes segregation.

Do not build up stockpiles in cone shapes, formed by dropping successive loads at the same spot. This procedure causes segregation. Build a pile up in layers of uniform thickness, each made by dumping successive loads alongside each other.

If aggregate is dropped from a clamshell, bucket, or conveyor, some of the fine material may be blown aside, causing segregation of fines on the side of the pile away from the wind. Discharge all conveyors, clamshells, and buckets in contact with the pile.

When charging (filling) a bin, drop the material from a point directly over the outlet. Material chuted in at an angle or material discharged against the side of a bin will segregate. Since a long drop will cause both segregation and the breakage of aggregate particles, minimize the length of a drop into a bin by keeping the bin as full as possible at all times. The bottom of a storage bin should always slope at least 50˚ toward the central outlet. If the slope is less than 50˚, segregation will occur as material is discharged out of the bin.