In general, relationships between mineral and physical properties are quite complex, making it difficult to accurately predict how a particular aggregate source will behave based on mineral properties alone.

2.4  Chemical Properties

While relatively unimportant for loose aggregate, aggregate chemical properties are important in a pavement material.  In HMA, aggregate surface chemistry can determine how well an asphalt cement binder will adhere to an aggregate surface.  Poor adherence, commonly referred to as stripping, can cause premature structural failure.  In PCC, aggregates containing reactive forms of silica can react expansively with the alkalis contained in the cement paste.  This expansion can cause cracking, surface popouts and spalling.  Note that some aggregate chemical properties can change over time, especially after the aggregate is crushed.  A newly crushed aggregate may display a different affinity for water than the same aggregate that has been crushed and left in a stockpile for a year.

2.4.1  Stripping (HMA)

Although the displacement of asphalt on the aggregate particle surface by water (stripping) is a complex phenomena and is not yet fully understood, mineralogy and chemical composition of the aggregate have been established as important contributing factors (Roberts et al., 1996).  In general, some aggregates have an affinity for water over asphalt (hydrophilic).  These aggregates tend to be acidic and suffer from stripping after exposure to water.  On the other hand, some aggregates have an affinity for asphalt over water (hydrophobic).  These aggregates tend to be basic and do not suffer from stripping problems.  Additionally, an aggregate’s surface charge when in contact with water will affect its adhesion to asphalt cement and its susceptibility to moisture damage.  In sum, aggregate surface chemistry seems to be an important factor in stripping.  However, specific cause-effect relationships are still being established.

2.4.2  Alkali-Aggregate Reaction (PCC)

	Figure 3.2: Map/Pattern Cracking Resulting from an Alkali-Aggregate Reaction

Alkali-aggregate reaction is the expansive reaction that takes place in PCC between alkali (contained in the cement paste) and elements within an aggregate.  The most common is an alkali-silica reaction.  This reaction, which occurs to some extent in most PCC, can result in map or pattern cracking (see Figure 3.2), surface popouts and spalling if it is severe enough.  The mechanism for this alkali-silica reaction proposed by Diamond is as follows (Mindess and Young, 1981):

1. Initial alkaline depolymerization and dissolution of reactive silica.  Cement (a high-alkali substance) can increase the solubility of non-crystalline silica and the rate at which it dissolves.  Additionally, the cement will raise the pH of the surrounding medium which will affect the crystalline silica.
2. Formation of a hydrous alkali silicate gel.  The initial dissolution of reactive silica then opens up the aggregate pore structure and allows more silica to dissolve into solution.  The end result is alkali-silica gel that is formed in place.  This gel formation is not expansive itself but it does destroy the integrity of the aggregate particle. 
3. Attraction of water by the gel.  The gel attracts considerable amounts of water and expands.  If the expansion is great enough, the resulting stress will crack the now-weakened aggregate and surrounding cement paste.
4. Formation of a gel colloid.  After the gel ingests enough water, the water takes over and the substance becomes an alkali-silica gel disbursed in a water fluid.  This fluid then escapes to surrounding cracks and voids and may partake in secondary reactions.

This reaction can be controlled by:

• Avoiding susceptible aggregates.  Local experience may show that certain types of rock contain reactive silica.  Typically rock types that may be susceptible are: siliceous limestone, chert, shale, volcanic glass, synthetic glass, sandstone, opaline rocks and quartzite.  River rock is also typically susceptible. 
• Pozzolanic admixture.  By reacting with the calcium hydroxide in the cement paste, a pozzolan can lower the pH of the pore solution.  Additionally, the silica contained in a pozzolan may react with the alkali in the cement.  This reaction is not harmful because it essentially skips the expansive water attraction step.
• Low-alkali cement.  Less alkali available for reaction will limit gel formation.
• Low water-cement ratio.  The lower the water-cement ratio, the less permeable the concrete.  Low permeability will help limit the supply of water to the alkali-silica gel.

In sum, alkali-silica reactions are expansive in nature and occur in most PCC.  If the reaction is severe enough it can fracture aggregates and surrounding paste resulting in cracking, popouts and spalling.  There are several ways of avoiding this reaction, the simplest of which is just avoiding susceptible aggregate.

2.5  Physical Properties

Aggregate physical properties are the most readily apparent aggregate properties and they also have the most direct effect on how an aggregate performs as either a pavement material constituent or by itself as a base or subbase material.  Commonly measured physical aggregate properties are (Roberts et al., 1996):

These are not the only physical properties of aggregates but rather the most commonly measured.  Tests used to quantify these properties are largely empirical.  The physical properties of an aggregate can change over time.  For instance, a newly crushed aggregate may contain more dust and thus be less receptive to binding with an asphalt binder than one that has been crushed and stored in a stockpile for a year.

2.5.1 Gradation and Size

The particle size distribution, or gradation, of an aggregate is one of the most influential aggregate characteristics in determining how it will perform as a pavement material.  In HMA, gradation helps determine almost every important property including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and resistance to moisture damage (Roberts et al., 1996). In PCC, gradation helps determine durability, porosity, workability, cement and water requirements, strength, and shrinkage.  Because of this, gradation is a primary concern in HMA and PCC mix design and thus most agencies specify allowable aggregate gradations for both.

2.5.1.1 Maximum Aggregate Size

Maximum aggregate size can affect HMA, PCC and base/subbase courses in several ways.  In HMA, instability may result from excessively small maximum sizes; and poor workability and/or segregation may result from excessively large maximum sizes (Roberts et al., 1996).  In PCC, large maximum sizes may not fit between reinforcing bar openings, but they will generally increase PCC strength because the water-cement ratio can be lowered.  ASTM C 125 defines the maximum aggregate size in one of two ways:

Thus, it is important to specify whether "maximum size" or "nominal maximum size" is being referenced. 

2.5.1.2 Gradation Test

The gradation of a particular aggregate is most often determined by a sieve analysis (see Figure 3.3).  In a sieve analysis, a sample of dry aggregate of known weight is separated through a series of sieves with progressively smaller openings. Once separated, the weight of particles retained on each sieve is measured and compared to the total sample weight.  Particle size distribution is then expressed as a percent retained by weight on each sieve size.  Results are usually expressed in tabular or graphical format.  PCC gradation graphs are traditionally semi-logarithmic, while HMA graphs often employ the standard 0.45 power gradation graph.

Figure 3.4 shows typical gradation graphs.  Note that sieve sizes are presented from smallest to largest, left to right.  The number and size of the sieves used in a sieve analysis depend upon specification requirements.

Figure 3.4: Example Sieve Analysis Plot on a 0.45 Power Graph

For PCC, aggregate is typically classified as either "coarse" or "fine".  Coarse aggregate is generally the fraction retained on the 4.75 mm (No. 4) sieve while fine aggregate is the fraction passing the 4.75 mm (No. 4) sieve.

Standard Sieve Analysis test methods are:

• AASHTO T 27 and ASTM C 136: Sieve Analysis of Fine and Coarse Aggregates

• AASHTO T 11 and ASTM C 117: Materials Finer Than 75-mm (No. 200) Sieve in Mineral Aggregate by Washing

• AASHTO T 30: Mechanical Analysis of Extracted Aggregate (this is used for aggregate extracted from bituminous mixtures)

2.5.1.3  Desired Gradation

Gradation has a profound effect on material performance.  But what is the best gradation?  This is a complicated question, the answer to which will vary depending upon the material (HMA or PCC), its desired characteristics, loading, environmental, material, structural and mix property inputs.  Therefore, gradation requirements for specific HMA and PCC mixes are discussed in their respective pavement type sections.  This section presents some basic guidelines applicable to common dense-graded mixes.

It might be reasonable to believe that the best gradation is one that produces the maximum density. This would involve a particle arrangement where smaller particles are packed between the larger particles, which reduces the void space between particles.  This creates more particle-to-particle contact, which in HMA would increase stability and reduce water infiltration.  In PCC, this reduced void space reduces the amount of cement paste required.  However, some minimum amount of void space is necessary to:

• Provide adequate volume for the binder (asphalt binder or portland cement) to occupy.
• Promote rapid drainage and resistance to frost action for base and subbase courses.

Therefore, although it may not be the "best" aggregate gradation, a maximum density gradation does provide a common reference.  A widely used equation to describe a maximum density gradation was developed by Fuller and Thompson in 1907. Their basic equation is:

The 0.45 Power Maximum Density Curve
In the early 1960s, the FHWA introduced the standard gradation graph used in the HMA industry today.  This graph uses n = 0.45 and is convenient for determining the maximum density line and adjusting gradation (Roberts et al., 1996).  This graph is slightly different than other gradation graphs because it uses the sieve size raised to the n^th power (usually 0.45) as the x-axis units.  Thus, n = 0.45 appears as a straight diagonal line (see Figure 3.5).  The maximum density line appears as a straight line from zero to the maximum aggregate size for the mixture being considered (the exact location of this line is somewhat debatable, but the locations shown in Figure 3.4 are generally accepted).

Figure 3.5: Maximum Density Curves for 0.45 Power Gradation Graph
(each curve is for a different maximum aggregate size)

To illustrate how the maximum density curves in Figure 3.5 are determined, Table 2.2 shows the associated calculations for a maximum aggregate size of 19.0 mm.

Table 2.2: Calculations for a 0.45 Power Gradation Curve Using 19.0-mm (0.75-inch) Maximum Aggregate Size

Particle Size (mm)	% Passing
19.0
12.5
9.5
2.00
0.300
0.075

Several common terms are used to classify gradation.  These are not precise technical terms but rather terms that refer to gradations that share common characteristics (refer to Figure 3.6):

Figure 3.6: FHWA Gradation Graph Showing Representative Gradations
 

Permeability
Figure 3.7 shows some typical aggregate gradations and their associated permeabilities.  This shows that even a small amount of particles passing the 0.075-mm (#200) sieve results in very low permeability.  Therefore, for base and subbase aggregates where permeability is important for drainage and frost resistance, many agencies will specify a maximum percent-by-weight passing for this sieve.

Table 3.3 and Figure 3.8 show some typical specification bands for aggregate courses taken from the FHWA 1996 Standard Specifications (FHWA, 1996). 

2.5.1.4  Fineness Modulus

For aggregates used in PCC, another common gradation description for fine aggregate is the fineness modulus.  It is described in ASTM C 125 and is a single number used to describe a gradation curve.  It is defined as: 

The larger the fineness modulus, the more coarse the aggregate.  A typical fineness modulus for fine aggregate used in PCC is between 2.70 and 3.00.

2.5.2 Toughness and Abrasion Resistance

Aggregates undergo substantial wear and tear throughout their life.  In general, they should be hard and tough enough to resist crushing, degradation and disintegration from any associated activities including manufacturing, stockpiling, production, placing, compaction (in the case of HMA) and consolidation (in the case of PCC) (Roberts et al., 1996).  Furthermore, they must be able to adequately transmit loads from the pavement surface to the underlying layers (and eventually the subgrade).  Aggregates not adequately resistant to abrasion and polishing will cause premature structural failure and/or a loss of skid resistance.

2.5.2.1 Los Angeles Abrasion Test

A common test used to characterize toughness and abrasion resistance is the Los Angeles (L.A.) abrasion test.  For the L.A. abrasion test, the portion of an aggregate sample retained on the 1.70 mm (No. 12) sieve is placed in a large rotating drum that contains a shelf plate attached to the outer wall (the Los Angeles machine – see Figure 3.9).  A specified number of steel spheres are then placed in the machine and the drum is rotated for 500 revolutions at a speed of 30 - 33 revolutions per minute (RPM).  The material is then extracted and separated into material passing the 1.70 mm (No. 12) sieve and material retained on the 1.70 mm (No. 12) sieve.  The retained material (larger particles) is then weighed and compared to the original sample weight.  The difference in weight is reported as a percent of the original weight and called the "percent loss".

Figure 3.9: Los Angeles Abrasion Machine

Table 3.4 shows some typical test values from the L.A. abrasion test.  Unfortunately, the test does not seem to correspond well with field measurements (especially with slags, cinders and other lightweight aggregates).  Some aggregates with high L.A. abrasion loss, such as soft limestone, provide excellent performance.  However, no matter the performance characteristics, aggregate with high L.A. abrasion loss values will tend to create dust during production and handling, which may produce environmental and mixture control problems.

Table 3.4: Typical L.A. Abrasion Loss Values
(from Roberts et al., 1996; NHI, 2000)

Standard L.A. abrasion test methods are:

2.5.3 Durability and Soundness

Aggregates must be resistant to breakdown and disintegration from weathering (wetting/drying and freezing/thawing) or they may break apart and cause premature pavement distress.  Durability and soundness are terms typically given to an aggregate’s weathering resistance characteristic.  Aggregates used in HMA are dried in the production process and therefore should contain almost no water.  Thus, for aggregate used in HMA, freezing/thawing should not be a significant problem.  This is not true for aggregate used in PCC or as base and/or subbase courses.  These aggregates typically contain some water (on the order of 0.1% to 3% usually) and are not dried prior to use.

2.5.3.1 Soundness Tests

The most common soundness test involves repeatedly submerging an aggregate sample in a saturated solution of sodium or magnesium sulfate.  This process causes salt crystals to form in the aggregate pores, which simulate ice crystal formation (see Figure 3.10 and 3.11). The basic procedure is as follows (from Roberts et al., 1996):

The maximum loss values typically range from 10 – 20 percent for every five cycles.

Other soundness tests use relatively the same procedure but substitute actual freezing and thawing in place of the salt crystallization of the procedure described previously.  Cracks in PCC resulting from poor aggregate freeze-thaw resistance are often called durability cracks or "D cracks".

Standard soundness tests are:

2.5.4 Particle Shape and Surface Texture

Particle shape and surface texture are important for proper compaction, deformation resistance, HMA workability and PCC workability.  However, the ideal shape for HMA and PCC is different because aggregates serve different purposes in each material.  In HMA, since aggregates are relied upon to provide stiffness and strength by interlocking with one another, cubic angular-shaped particles with a rough surface texture are best.  However, in PCC, where aggregates are used as an inexpensive high-strength material to occupy volume, workability is the major issue regarding particle shape.  Therefore, in PCC rounded particles are better.  Relevant particle shape/texture characteristics are:

2.5.4.1 Tests for Particle Shape and Surface Texture

There are several common tests used to identify and quantify aggregate particle shape and surface texture.  Among the most popular are:

• Particle index
• Percent fractured face (or coarse aggregate angularity)
• Fine aggregate angularity

Other tests, using automated machines equipped with video cameras and lasers are under development.

Particle Index
The particle index test provides a combined shape-texture characterization.  This test requires that an aggregate sample be divided up into specific size fraction.  Each size fraction is placed into a container in three layers.  This is done twice; the first time, each layer is compacted with 10 blows of a tamping rod, and the second time, each layer is compacted with 50 blows of a tamping rod.  The particle index is computed from the following equation:

The overall sample particle index is computed as a weighted average of the individual size fraction particles indexes based on the size fraction weights.  Aggregates composed of rounded, smooth particles may have a low particle index of around 6 or 7, while aggregates composed of angular, rough particles may have a high particle index of between 15 and 20 or more.

The standard particle index test is:

Percent Fractured Face (or Coarse Aggregate Angularity)
For coarse aggregate, a sample retained on the 4.75 mm (No. 4) sieve is collected and the number of particles with fractured faces is compared to the number of particles without fractured faces.  A fractured face is defined as an "angular, rough, or broken surface of an aggregate particle created by crushing, by other artificial means, or by nature" (ASTM, 2000).  In order for a face to be considered fractured it must constitute at least 25 percent of the maximum cross-sectional area of the rock particle.

The standard percent fractured face test is:

	Figure 3.12: Fine Aggregate Angularity Test

Fine Aggregate Angularity
Superpave uses a test to determine the uncompacted void content of fine aggregate, which gives some indication of fine aggregate particle shape and surface texture. The test involves filling a 100 mL cylinder with fine aggregate (see Figure 3.12), defined as that aggregate passing the 2.36 mm (No. 8) sieve, by pouring it from a funnel at a fixed height.  After filling, the amount of aggregate in the cylinder is measured and a void content is calculated.  The assumption is that this void content is related to the aggregate angularity and surface texture (e.g., more smooth rounded particles will result in a lower void content).  The key disadvantage to this test is that inclusion of flat and elongated particles, which are known to cause mix problems, will cause the fine aggregate angularity test results to appear more favorable.  Finally, surface texture may have a larger effect on mix performance than fine aggregate angularity values.

The standard fine aggregate angularity test is:

Flat or Elongated Particles
Flat and elongated particles can cause HMA problems because they tend to reorient and break under compaction.  Therefore, they are typically restricted to some maximum percentage.  An elongated particle is most often defined as one that exceeds a 5:1 length-to-width ratio.  Testing is done on a representative sample using a caliper device and a two-step process.  First, the longest dimension is measured on one end of the caliper (see Figure 3.13).  Then, based on the position of the pivot point (numbered holes shown in Figure 3.12), the other end of the caliper (see Figure 3.14) is automatically sized to the predetermined length-to-width ratio (in Figures 3.13 and 3.14 it is set at 2:1).  If the aggregate is able to pass between the bar and caliper it fails the test.

The standard flat or elongated particle test is:

Figure 3.13: Testing Caliper Measuring the Elongated Dimension	Figure 3.14: Testing Caliper Measuring the Flat Dimension

2.5.5 Specific Gravity

Aggregate specific gravity is useful in making weight-volume conversions and in calculating the void content in compacted HMA (Roberts et al., 1996).  AASHTO M 132 and ASTM E 12 define specific gravity as:

"…the ratio of the mass of a unit volume of a material at a stated temperature to the mass of the same volume of gas-free distilled water at a stated temperature."

The commonly used "stated temperature" is 23° C (73.4° F).  Given the structure of a typical aggregate particle, there are several different kinds of specific gravity.   This section will first describe the structure of a typical aggregate particle and then discuss each type of specific gravity and its use.

2.5.5.1 Aggregate Particle Structure

A typical aggregate particle consists of some amount of solid material along with a certain amount of air voids.  These air voids within the aggregate particle (see Figure 3.15) can become filled with water, binder or both (see Figure 3.16).  It takes a finite amount of time for water/binder to penetrate these pores, so specific gravity test procedures generally contain a 15 to 19-hour (for AASHTO procedures) or a 24-hour (for ASTM procedures) soak period for the purpose of allowing penetration into these pores.

Depending upon how aggregate voids are dealt with, calculated aggregate specific gravities can vary.  If they are excluded entirely, then the specific gravity is that of the solid portion of the aggregate only, while if they are included entirely then the specific gravity essentially becomes a weighted average of the specific gravity of the solid aggregate and whatever is in its voids.

2.5.5.2 Aggregate Specific Gravities

Generally, there are three different aggregate specific gravities used in association with pavements: bulk, apparent and effective. 

2.5.6 Cleanliness and Deleterious Materials

Aggregates must be relatively clean when used in HMA or PCC.  Vegetation, soft particles, clay lumps, excess dust and vegetable matter are not desirable because they generally affect performance by quickly degrading, which causes a loss of structural support and/or prevents binder-aggregate bonding.

2.5.6.1 Tests for Deleterious Materials – Sand Equivalent

The sand equivalent test is a rapid field test to show the relative proportions of fine dust or claylike materials in aggregate (or soils).  A sample of aggregate passing the 4.75-mm (No. 4) sieve and a small amount of flocculating solution are poured into a graduated cylinder and are agitated to loosen the claylike coatings from the sand particles.  The sample is then irrigated with additional flocculation solution forcing the claylike material into suspension above the sand.  After a prescribed sedimentation period, the height of flocculated clay and height of sand are determined. The sand equivalent is determined from the below equation:

Cleaner aggregates will have higher sand equivalent values.  Agencies often specify a minimum sand equivalent around 25 to 35 (Roberts et al., 1996).

Standard sand equivalent tests are:

2.5.6.2 Tests for Deleterious Materials – Clay Lumps and Friable Particles

To test for clay lumps or friable particles, a sample is first washed and dried to remove material passing the 0.075-mm (No. 200) sieve.  The remaining sample is separated into different sizes and each size is weighed and soaked in water for 24 hours.  Particles that can be broken down into fines with fingers are classified as clay lumps or friable material.  The amount of this material is calculated by percentage of total sample weight.  Specifications usually limit clay and friable particles to a maximum of one percent.

Standard sand equivalent tests are:

2.5.7  Moisture Content

Since aggregates are porous (to some extent) they can absorb moisture.  Generally this is not a concern for HMA because the aggregate is dried before HMA production.  However, this is a concern for PCC because aggregate is generally not dried and therefore the aggregate moisture content will affect the water content (and thus the water-cement ratio also) of the produced PCC and the water content also affects aggregate proportioning (because it contributes to aggregate weight).  In general, there are four aggregate moisture conditions (see Figure 3.17):

1. Oven-dry (OD).  All moisture is removed by heating the aggregate in an oven at 105°C (221°F) to constant weight (this usually constitutes heating it overnight).  All pores connected to the surface are empty and the aggregate is fully absorbent.
2. Airdry (AD).  All moisture is removed from the surface, but pores connected to the surface are partially filled with water.  The aggregate is somewhat absorbent.
3. Saturated surface dry (SSD).  All pores connected to the surface are filled with water, but the surface is dry.  The aggregate is neither absorbent nor does it contribute water to the concrete mixture.
4. Wet.  All pores connected to the surface are filled with water and there is excess moisture on the surface.  The aggregate contributes water to the concrete mixture.

Note that pores not connected to the surface are not considered.

Figure 3.17: Aggregate Moisture States
(these moisture states only consider the aggregate pores that are connected to the surface)

These conditions are used to calculate various aggregate properties.  The moisture content of an aggregate is expressed as:

If the moisture content is positive, the aggregate has surface moisture and will contribute water to the PCC, while if the moisture content is negative the aggregate is air dry to some degree and will absorb moisture from the PCC.

Typical moisture tests are:

• ASTM C 70: Surface Moisture in Fine Aggregate
• AASHTO T 85 and ASTM C 127: Specific Gravity and Absorption of Coarse Aggregate
• AASHTO T 84 and ASTM C 128: Specific Gravity and Absorption of Fine Aggregate
• AASHTO T 255: Total Evaporable Moisture Content of Aggregate by Drying
• ASTM C 566:  Total Moisture Content of Aggregate by Drying

2.6 Aggregate as a Base Material

Aggregate is often used by itself as an unbound base or subbase course.  When used as such, aggregate is typically characterized by the preceding physical properties as well as overall layer stiffness.  Layer stiffness is characterized by the same tests used to characterize subgrade stiffness.

2.7  Summary