Aggregates for Concrete and Asphaltic Mixes

As aggregates are mainly used as fillers and are relatively inexpensive, it is important that they occupy as much volume as possible to minimize the volume of the paste. In the case of concrete mixes, an appropriate size distribution must be achieved in order for the volume of paste to be minimized. A uniform distribution (particles of similar sizes) will require more paste to fill the voids than a properly graded (particles of many sizes) aggregate. A properly graded aggregate contains particles of all sizes such that very little space needs to be filled by the paste. Additionally, the size distribution of particles will have a major influence on the properties of the fresh concrete, including its flowability, or the ability to be easily placed in forms, and finishability, or the ability to obtain a flat surface with good wearability characteristics.

Through many years of field experience and laboratory testing, gradation curves have been developed as recommended ranges for the grading of both coarse and fine aggregates. In these curves, the horizontal axis refers to the particle size, with fine aggregates or sands being on the left and coarse aggregates (or rocks) being on the right. The vertical axis represents the cumulative percent of particles smaller than the given size. For practical reasons, the optimum distributions are specified as ranges. For example, very fine sand must have at most 85% of its particles with a no.16 size (1.118 mm) or below, while very coarse sand must have at most 55% of its particles below this size. Practical aggregate mixes thus will have about 55% to 85% of its particles passing the 1.18 mm sieve.

For both types of aggregates, these ranges are defined by running sieve tests with sieves at specified standard sizes and in descending order of sieve openings in order to determine the amounts of aggregates of a given size and their cumulative distribution. The smallest sieve through which the entire amount of aggregate passes is called the maximum size of the aggregate, while the sieve through which 95% of the aggregate passes gives the nominal maximum size of the aggregate. An important relationship that is hidden within the gradation curves is that the total surface area of the aggregates will need to be coated with water during mixing to obtain proper workability. If there are too many fine particles, the surface area will be high and a lot of water will be used up coating the particles, resulting in a stiffer concrete mix that is harder to place.

For fine aggregates, a fineness modulus (FM) is often computed. The fineness modulus is defined as the summation of the percentages retained by weight from the No. 4 to the No. 100 sieves, divided by 100. Typical values for the fineness modulus range from about 2.3 to 3.1, with the former consisting more of fine particles and the latter of coarser particles. The FM can vary greatly with the application. For example, the FM may be as low as 1.8 for use in masonry mortars, which contain no coarse aggregate and which require greater finishability.

Concrete mix designs are very susceptible to water content, and since coarse and fine aggregates are typically stored in the open and exposed to wind and rain, it is necessary to account for even trace amounts of water present in the aggregate. Four environmental conditions are usually recognized. The oven dry condition, as the name implies, occurs after the aggregate has been placed in an oven for a sufficiently long time and high temperature, such that all water has evaporated. The air-dry condition arises when some, but not all of the inner pores are filled. The saturated surface dry (SSD) condition arises when all the internal pores are saturated, but the surface is dry. The SSD condition is the one used as the reference for mix design and is achieved by immersing the aggregates in water until all internal pores are saturated and then drying the surface of all particles. This can be done with a little effort for the coarse aggregate but is very difficult to do for fine aggregates as it is impossible to get the surface of all of the sand particles dry without drawing out the water from the internal pores. Alternately, the SSD for fine aggregate can be measured using the slump test, as described in the protocol section. For this, a conical mold is filled with sand or aggregate, and then packed. The mold is flipped over and removed. If it slumps slightly, it is in SSD condition. If the mold holds its shape, the aggregate is in damp or wet condition. The damp or wet condition occurs after the aggregate has been immersed in water for long enough that all internal pores are saturated, and the surface is wet. In practice, the aggregates will either be in a wet (too much water) or air dry (too little water) with respect to the design SSD condition. Thus, prior to mixing, the amount of water needs to be adjusted.

Although the range of moisture contents from oven dry to wet is small (mostly in the range of 4% to 6%), the amount of aggregates in a typical concrete mix are much larger than those of the water, often in the range of 25 to 1. Thus, even a small difference in the percent water content of the aggregates can have an enormous effect on the total water that needs to be added to maintain a certain water-to-cement ratio, the main variable used to control strength and durability of concrete mixes. The absorption capacity of an aggregate is defined as:

(Eq. 1)

The moisture content of a sample of weight W is defined as:

(Eq. 2)

The bulk specific gravity is defined as the ratio of the mass of a unit volume of aggregate, including the water in voids, to the mass of an equal volume of gas-free distilled water at the stated temperature. This is in contrast to the apparent specific gravity, which has a similar definition but does not include the volume of water in the voids. The bulk specific gravity is an important aggregate characteristic because mixes are often specified by either volume or weight of the constituents, and therefore it is critical to be able to go from one set of measures to the other. Values of specific gravity are referenced as being in either the oven-dry or the saturated surface dry condition. In the former case, the bulk specific gravity is the oven-dry mass divided by the mass of a volume of water equal to the SSD aggregate volume. In the latter case, the SSD bulk specific gravity is the saturated surface-dry mass divided by the mass of a volume of water equal to the SSD aggregate volume. Most aggregates have a bulk specific gravity SSD between 2.3 and 3.0.

Other key characteristics that affect the choice of aggregate sources are chemical inertness and resistance to wear. Chemical inertness is desirable to avoid problems such as sulfate attack and alkali-silicate reactions, which have resulted in substantial losses in the past, as they are problems that surface many years after the concrete was cast. Resistance to wear refers to the ability of the aggregate particles to resist the deterioration from pedestrian and vehicle traffic without undue wear or rutting. Tests for these characteristics are beyond the scope of this laboratory and will not be discussed.

Table 1: Fine Aggregate Moisture Test Data

From the above data (Table 1), the specific gravity values and absorption are calculated as follows (Table 2):
Apparent Specific Gravity (dry) = A / (B+A-C)
Bulk Specific Gravity (dry) = A / (B+D-C)
Bulk Specific Gravity (SSD) = D / (B+D-C)
Absorption = ((D-A) / A) x 100%

Table 2: Summary of Moisture Test Results

Table 3 illustrates the calculation of the fineness modulus. An interpretation of the fineness modulus might be that it represents the (weighted) average sieve of the group upon which the material is retained, No. 100 being the first, No. 50 the second, etc. Thus, for sand with a FM of 3.00, sieve No. 30 (the third sieve) would be the average sieve size upon which the aggregate is retained. In our case, a fineness modulus of 2.92 indicates that there are many fine particles in our aggregate sample, as a high fineness modulus indicates that many particles were trapped in the smaller sieves.

Table 3: Sample Calculation in Determining Fineness Modulus

Fineness Modulus of Sand = Cumulative % retained/100
= (12.2+28.5+40.7+54.9+73.2+93.5)/100 = 3.02
* #200 sieve should not be included in computing the FM.

Three important characteristics of aggregates used in concrete mixes were examined in this laboratory exercise. The first is the moisture content and absorption capacity. These quantities are needed to properly determine the amount of water to be added to a concrete mix. The second characteristic is the specific gravity. This value is needed because it is sometimes necessary to go from volumes to weights and vice versa in batching concrete mixes. The third characteristic is the size distribution or gradation. A suitable gradation of an aggregate in a Portland cement concrete mixture is desirable in order to secure workability of the concrete mix and economy in the use of cement. For asphalt concrete, suitable gradation will not only affect the workability of the mixture and economy in the use of asphalt, but also will significantly affect the strength and other integral properties.

In the design of concrete and asphaltic mixes, it is always desirable to maximize the use of fine and coarse aggregates, as they are the least expensive component of these mixes. Concrete mixes are used in many construction projects, ranging from building bridges to power plants and industrial facilities. Appropriate use of gradation, moisture content, and the fineness modulus will result in durable and efficient infrastructure projects.