Aggregates for Road Construction

Introduction

Natural rocks constitute the primary source of aggregates used in highway construction.  These rocks exist in the form of rock outcrops at or near the earth’s surface or gravel deposits usually along streams channels.

In most cases, crushed rock aggregates from stone quarries may constitute the sole structural layer materials for high-performance roads.

Other types of aggregates (considered artificial) that may find limited use in pavement construction are slug –a by-product from blast furnace and production of steel, and light-weight aggregates produced by heating clay to a very high temperature.

The importance of aggregates in highway construction lies in the fact that the materials constitute about 80% of the weight of paving mixtures.

For engineering applications, however, the most important consideration of rock materials, whether naturally-occurring or artificial, is how well they serve in the various applications such as in the construction of sub-base, bases, surfacing, and other civil engineering works. 

Classification of rocks as source of aggregates

The nature and properties of rocks are determined by the predominant constituent materials and the manner in which the materials are arranged  relative to each other.

The compositional and textural characteristics of any rock are in turn dependent on the manner of formation of rocks. Thus one way of classifying rocks is to use a criterion based on geologic origin or mode of formation.

In engineering application, however, a distinction between rock aggregates from different geologic origins is not as important as the quality of the aggregates relative to the intended use.

In this sense, even if aggregates are of different geologic origins, but they exhibit the same or similar engineering characteristics, they are put in the same category or class for engineering use purposes.

This gives rise to engineering group classification of rocks in which a group is assigned the name of the most-commonly known rock within it.

Geologic classification

Rocks may be classified by geologic origin into three main groups as igneous, metamorphic, and sedimentary.

  1. Igneous rocks

These are formed from the cooling of hot magma that flowed from depth to or towards the earth’s surface. Those that cooled on the earth surface (extrusive rocks) did so rapidly and tend to have fined-grained crystals as against those that cooled slowly beneath the earth’s surface (intrusive rocks) which tend to have large surface crystal  grains.

Well-known igneous rocks include basalt, granite, porphyry, and gabbro.  Depending on the silica content, igneous rocks may be described as acidic, intermediate, basic, and ultra-basic.  

Silica content (%)Nature of rock
>66Acidic
55-66Intermediate
45-55Basic
<45Ultra-basic

Acidic aggregates are generally considered to be water-loving and do not develop a satisfactory film of asphalt  around them in the presence of water when used in bituminous paving mixture formulations.

Such aggregates are likely to cause stripping failures to occur when used in bituminous pavement constructions. A well-known example of acidic rock is quartzite.

For successful bituminous construction,  rocks of intermediate acidity may be used but it is preferable to use basic rocks. 

2. Metamorphic rocks

These were formed when pre-existing rocks of any type came under extremely intense heat and/or pressure over geologic times such that the rocks have become altered in appearance, texture, and mineral composition from those of the original parental materials through crystallization. 

Generally, metamorphic rocks formed under intense heat such as quartzite often show an interlocking array of mineral constituents that renders them more useful as aggregates.

On the other hand, those rocks formed under intense pressure tend to develop parallel planes or foliations which are zones of weakness along which the rocks are easily split.

Such rocks, typified by slate, gneiss, and schist, break into flaky aggregates when quarried and are generally not suitable as road stones.

3. Sedimentary rocks

These are rocks formed by the deposition and compaction of fragments of pre-existing rocks or from the deposition of inorganic remains of marine animals or from water-soluble materials precipitated under conditions of over-saturation.

Sedimentary rocks are characterized by layered structure developed as a direct result of the mode formation. These rocks may be grouped according to the predominant mineral present as calcerous (chalks, limestone, etc),  siliceous (sandstone, chert, etc.), or argillaceous (shale, etc.).

The grains of sedimentary rocks may be bonded together by a cementing material which usually is naturally-deposited mineral matter such as silica, calcite, haematite, etc.

Engineering group classification

This type of classification is based on the qualities of the rock aggregates as either road building or constructional materials irrespective of geologic origin.

Engineering group classification is very important in serving as a source of aggregate for civil engineering construction and in giving useful indications about certain index properties useful to quarrying and construction companies.

Some of the major rock classes recognized in this kind of classification are summarized in the Table below.

Engineering group  Rock types
Basalt groupThe predominant rocks are basic and intermediate rocks of medium to fine-grained size and may include some metamorphic equivalents such as hornblende-schist. Common rocks within the group are basalt, dolerite, basic porphyrite , quartz dolerite, and andesite
Granite groupCommon rocks in this group gneiss, granite, granodiorite, garnulite, pegmaitite, quartz diorite, syenite. The rocks are generally coarse-grained, light in colour and have specific gravities below 2.8. They are used extensively as road aggregates.
Limestone groupRocks include limestone, dolomite and marble. They are light in colour and are used extensively in road works.
Gabbro groupCommon rocks in the group are basic diorite, basic gneiss, gabbro, and hornblende-rich rocks. The rocks have dark colour and high specific gravity values (2.8-3.3). They are used extensively for road works.
Porphry groupRocks in this group are acidic and intermediate igneous rocks very close to the granite group. They include porphyry, felsite, micro-granite and rhyolite. They are usually fine-grained and used extensively for road works
Quartzite groupRocks in this group are siliceous sedimentary and metamorphic rocks composed almost entirely of quartz
HornfelsContact altered rocks of all kinds except marble and quartzite. Rocks in the group are hard and dense. They are medium to dark in colour and tend to be medium-grained. They provide good road aggregates.
Schist groupThis group is mostly composed of laminated rocks such as schist, phyllite and slate. Not considered suitable as a source of road stone because of their instability and tendencies to break in planar sheets

Rock aggregate quality indices

The different civil engineering applications to which crushed rocks and naturally-occurring aggregates are put require certain index quality properties to be met in order to ensure the soundness and longevity of those constructions of which they form a part.

In effect, quality indices provide a useful indication as to whether a given rock aggregate can satisfactorily perform in a specific engineering application or not.

The following are the more generally used indices in assessing the quality of aggregates.

  1. Cleanliness

The presence of foreign or deleterious materials in aggregates makes them unsuitable for use in highway construction, especially in the formulation of bituminous paving mixtures.  Deleterious materials may be in the form of clay lumps, vegetation, soft particles, etc.

The standard method of determining the presence of deleterious materials is the Sand Equivalent test carried out on the portion of the aggregates passing the 4.75 mm (No. 4) sieve.

2. Particle shape and texture

Shape is an important index of aggregate quality since dimensional characteristics whether of crushed rock or transported rock materials affect the overall performance of the materials in engineering applications, especially in road construction. 

Aggregate shape affects the workability of concrete mixtures, be it Portland cement concrete or asphaltic concrete. The descriptive terms for particle shape and associated characteristics are listed in the table below.

Particle ShapeCharacteristics
RoundedFully water-worn or completely shaped by attrition
IrregularNaturally irregular or partially shaped by attrition and having rounded edges
AngularMaterials posses well-defined edges formed at the intersection of roughly planar edges
FlakyMaterials of which the thickness is small relative to the other dimensions
ElongatedMaterials usually angular in which the lengths are larger than the other two dimensions
Flaky and
Elongated
Materials have the characteristics of both flaky  and elongated shapes

In compacted materials, angular-shaped particles provide greater interlock and exhibit higher internal friction and hence greater mechanical stability than do rounded particles.

Rounded aggregates such as beach gravels and sand on the other hand tend to have better workability (a property of no strength advantage).

In asphalt works, for example, smooth-textured aggregates may be easier to coat with asphalt but would not exhibit strong mechanical bonds.

Flaky and elongated aggregates break when subjected to impact load such as compaction. Such aggregate particles also impede compaction and may prevent the development of adequate strength in asphaltic concrete.

An evaluation of the shape characteristics of aggregates is given by the

  • Flakiness Index
  • Elongation Index.

The Flakiness Index measures the percentage by mass of aggregate particles whose least dimension is less than 0.6 times their mean dimension.

The Elongation Index measures the percentage by mass of the aggregate particles whose largest dimension is greater than 1.8 times their mean dimension.

The mean dimension is the average of two adjacent sieve sizes between which the aggregate particles being measured are retained.

The surface texture of aggregates is an expression of the nature of the inter-growth of the minerals forming the rock including the size of the crystals of the minerals and the probable bond strength resulting thereof.

The surface texture of a crushed rock may be described as follows (see Table below).

Descriptive term Characteristics
Glassy Conchoidal fracture
SmoothWater-worn or smooth
GranularFracture showing more or less uniform rounded grains
Rough Rough fracture of fine-or medium-grained rock containing no easily visible crystalline constituents
Crystalline Containing  easily visible crystalline constituents
HoneycombedWith visible pores and cavities

These categories are based on the impression gained by a simple examination of hand specimens and are therefore not a precise petrographical index because of the subjective nature of the description.

However. Aggregates for road construction should be rough to provide inter-particle friction and a good bond with bituminous binders

3. Toughness

Aggregates used for road construction may come under compressive load or may experience polishing by the traction and shearing action of traffic. The toughness of an aggregate measures the resistance of the material to quality–degrading forces.

Several parameters are available for measuring the toughness of an aggregate. These include the aggregate

  • Aggregate Abrasion Value,
  • Polished Stone Value
  • Aggregate Impact Value,
  • Aggregate Crushing Value
  • 10% Fines Value
  • Unconfined compressive strength

 The standard test methods for these are detailed in BS 812: 3.

The resistance of aggregates to surface wear is assessed by the Aggregate Abrasion Value and the Polished Stone Value. These parameters are extremely valuable in evaluating the suitability of aggregates that will form part of the wearing course of a highway pavement where degradation by attrition is highest.

A very popular test is the Los Angeles Abrasion test which is most often used to obtain an indication of both the toughness and abrasion resistance of an aggregate.

Typical test values range from 10%  for extremely hard igneous rocks to about 60% for soft limestone and sandstones.

The Aggregate Impact Value gives a relative measure of the resistance of the aggregates to sudden impact whereas the Aggregate Crushing Value measures the resistance to a compressive load. 

Materials with crushing values greater than 25-30% are not considered suitable for use in pavement construction.

Aggregate quality may also be assessed by its Unconfined Compressive Strength (qu) which is carried out on the solid rock material.

For good quality road stones, qu values should be greater than 100MN/m2. The 10% Fines Value gives a measure of the resistance of the aggregate to crushing and it is the compressive load required to cause 10% fines (material passing 2.36mm BS sieve) to be formed during the relevant test.

A high value is indicative of a material with high resistance and hence high quality.  The minimum value acceptable of road aggregates is about 8kN.

4. Porosity and water absorption

Aggregate porosity is closely related to water absorption characteristics and affects the performance of a mix.

Water absorption characteristics give an indication of the probable water absorption potential of aggregates and may give an invaluable clue to the performance of marginal aggregates used under saturated conditions especially if such aggregates have a tendency to lose stability after taking in moisture.

In addition, this quality index provides useful information about bitumen absorption properties of aggregates.

Even tough porosity may improve the bond between asphalt and aggregates, it may cause inadequate drying of aggregates which can lead to stripping problems in asphaltic pavements. 

Porous aggregates affect the economics of asphaltic mixtures as they tend to require additional asphalt binder to satisfy absorption of the aggregates even though such additional asphalt will neither be available to contribute to asphalt film formation around the aggregate nor bond strength development in the asphaltic paving mixture.

5. Wettability and affinity for asphalt

This property affects the stripping of asphalt from aggregates in the presence of water.

Depending on their affinity for water, aggregates may be described as hydrophobic (water hating) or hydrophilic (water-loving).

Aggregates that are hydrophobic have a good attraction for asphalt. Such aggregates have +ve surface charges and are referred to as basic.

On the other hand, hydrophilic aggregates have –ve surface charges and good attraction for water.

Such aggregates will reject asphalt for water in the case of the two being present and competing for attention by the aggregate. Aggregates in this category are siliceous and are acidic in nature. A typical example is quartzite.

A test for asphalt affinity (or lack of it) of an aggregate is the stripping test in which the un-compacted bituminous mixture is soaked in water and the coated particles evaluated visually.

Immersion-compression test may also be used to evaluate the potential of the aggregate to cause stripping problems. In the test, the strength of a compacted paving mixture after soaking in water is compared to the strength of an identical un-soaked sample.

The reduction in strength is an indication of the quality of the aggregate in terms of its resistance to stripping in the presence of water.

Samples composed of hydrophilic aggregates will exhibit a higher reduction in strength in the immersion-compression test than those composed of hydrophobic aggregates.

Aggregate Specific Gravity

The presence of pores in aggregates leads to three different kinds of specific gravity values that may be used to characterize the performance of aggregates in road building applications, especially in hot-mix asphalt concrete design.

These specific gravity values are apparent specific gravity (Gsa), bulk specific gravity (Gsb) and effective specific gravity (Gse).

The weight-volume relationships defining these specific gravity values are shown in Fig. 6.1 which is used to denote the equivalent volume of a typical aggregate.

Equivalent volume of an aggregate
Equivalent volume of an aggregate

By definition,

Gsa = Ws/Vsγw

Gsb = Ws/(Vs+Vpp)γw

Gse = Ws/(Vs+Vpp-Vap)γw

where

Vs=volume of solids

Vpp=volume of water permeable pores

Vap=volume of water permeable pores absorbing asphalt

Vpp-Vap=volume of water permeable pores not absorbing asphalt

Ws=weight of aggregate

When an aggregate blend is made up of different size fractions or aggregate types as is usually the case, the various specific gravity values for the combined aggregates may be evaluated using the following expression;

G = 100/(P1/G1+P2/G2+…Pn/Gn)

where

 G=Specific gravity of the blend

G1, G2, ….Gn=specific gravity values of component or fraction 1, 2, …n respectively

P1, P2, ….Pn=% by weight of each component or fraction in the total blend.

Note that the equation is general and may be used to evaluate all the three types of specific gravity values defined above.

Aggregate gradation

Aggregate gradation is the distribution of particle sizes as a percent of the total weight and can be presented  graphically on a gradation curve in which the ordinate (vertical axis) is the total percent passing a given size on an arithmetic scale while the abscissa (x-axis) represents the particle size plotted to a log scale.

The gradation of a material is important as it affects the strength that can be mobilized when the material is compacted.

Descriptive terms for aggregate gradation include dense-graded or well-graded, open-graded, or gap-graded and uniformly graded (see image below).

Types of aggregate gredation

Gradation of aggregates for base course construction

Base course materials for high performance roads may be any of the following;

  • Mechanically-stable natural gravel
  • Cement or lime-stabilised soil
  • Crushed rock
  • Bitumen stabilized sand or gravel

Natural gravel  bases

Natural gravel found suitable for road bases include lateritic gravels and quartzite gravels, river gravels, decomposed rocks, etc. These materials must be well-graded and must contain sufficient fines to provide a high density on compaction. The fines should preferably be non-plastic or meet the following range  of Atterberg Limits:

LL=25%

PI<12% (in dry areas) or <6% (in wet areas)

Shrinkage Limit <4%

Suitable materials have a minimum 4-day soaked CBR of 80%. The image below provides a guide to the required gradation of natural gravels intended for use in base course construction.

Typical gradation limits for natural in base course construction

Crushed rock aggregate base

Crushed rock is employed for the construction of water-bound macadam, dry-bound macadam, and all-in-aggregate base courses.

  1. Water-bound macadam bases
  2. Dry-bound macadam bases
  3. All-in-crushed rock bases

Water-bound macadam base

For bases that are water-bound macadam, the aggregates used must be sound, single-sized material of 37.5-50mm (1 ½ -2in) nominal size, laid and compacted to a thickness not exceeding twice the nominal aggregate size.

Well-graded, non-plastic fines less than 6.3 mm ( ¼  in) maximum size are rolled and watered into the layer of single-sized aggregates.

Dry-bound macadam base

Aggregates for dry-bound macadam bases must have the same characteristics and be compacted to a thickness as required of wet-bound macadam bases.

The fines used in filling the interstices of the layer of single-sized aggregates material must be a dry well-graded crushed rock material having sizes ranging from 5mm ( 3/16 in) down to dust.

The fines are vibrated into the interstices of the layer of single-sized aggregates by means of vibratory rollers and plate compactors.

All-in crushed rock bases

High quality crushed rock bases may be constructed using all-in crushed rock aggregates, Such materials must conform to the grading limits given in the Table below and must be compacted in layers each not exceeding 150mm (6 in) thick.

Sieve size Percentage passing
50mm (2 in)100
37.5mm (1 ½ in)95-100
20mm ( 3/4 in)60-80
10mm  (3/8 in)40-60
5mm (3/16)25-40
2.36 (No. 7)15-30
600 μm (No. 25)8-22
75 μm (No. 200)5-12
Grading Limits for all-in crushed-rock aggregate bases

*Restrict fines content to lower range for high plastic fines and vice-versa

Because of the tendency for all-in aggregates to segregate whilst being transported and spread, they are usually kept wet during the transportation and spreading processes.

Design of aggregate gradation

Unlike natural gravel deposits and sands which will possess a particle size distribution unique to that particular deposit, crushed rock aggregates for use in engineering projects can be produced to any desired grading from the quarry.

In practice,  however, to improve handling characteristics and avoid segregation of coarse and fine materials in stockpiles, quarried or processed aggregates are separated into sizes before stockpiling.

For engineering applications, therefore, the need to blend aggregates of different sizes in order to meet the desired specification requirements may arise. In addition, blending of different aggregates may be necessary if it results in cost savings to the project.

The basic formula expressing the combination of aggregates A, B and C, etc., in a blending technique is as follows;

p= aA+bB+cC+…                            

where,

p=% of blend passing a particular sieve size

A, B, C,….= the respective % of component materials A, B, C, etc. passing the particular sieve size

a, b,c,..=the decimal proportions of aggregates A, B, C, etc. respectively in the blend (a+b+c+..=1.0)

Design by trial-and–error

The most common methods of aggregate gradation design are by trial-and-error. Usually, such methods are aided by experience, plots of individual gradation curves and specifications limits.

With the procedure, a trial blend is selected and calculations for p (the % passing each sieve) is made using (p= aA+bB+cC+….)

The grading calculated from the trial is compared with the specification requirements and where necessary, adjustments are made for the next trial.

The procedure is repeated until a satisfactory blend is obtained.

It is necessary as part of the blending technique to first plot the gradation of the aggregates to be blended and the specification limits  on  a gradation chart before actual blending is attempted.

The plots make it easier to

  • decide whether a blend can be formed using the available aggregates to meet the required specifications
  • identify the critical sieve sizes and
  • select appropriate trial proportions

An important fact to note from such curves is that the gradation for all possible combinations of any two materials always lies between the gradation curves for the individual materials and that if the curves for the two materials cross at a point the curves for all possible combinations of the two materials will also cross that point.

Mathematical method

An equation of the form of (p= aA+bB+cC+…) is obtained for each sieve size using either the % passing or retained on a given sieve size.

A valid equation can also be written in terms of  the proportions of the individual aggregates forming the blend as

a+b+c+…=1                                     

As many equations as represent the number of unknowns  (a, b, c..) may be written and the resulting set of equations solved simultaneously.

Where the specification for a particular sieve size is a range, the recommended p value to use in Eq. 6.5 in formulating the equation corresponding to that sieve size must be the mid-range value.

Of course, any value within the range may also be used. If in the solution of the simultaneous equations one of the unknowns turns out to be negative as can happen and which obviously has no practical significance, the indication is simply that the target specification values (the p values in Eq. 6.5) used in developing the equations are simply unattainable in whatever proportions the materials are combined.

The values of p should in that case be revised but still kept within the specifications limits in the next trial.

Even when a reasonable solution (no negative values) appears to have been found, the satisfactory one will be the solution for which a check of the resulting gradation with the specification limits meets the requirements for all sieve sizes.

Graphical method

A more efficient method for establishing the required proportions of an aggregate blend especially where more than two aggregates are involved is by a graphical method.

Where more than two aggregates are to be blended, it is prudent to begin blending the fine and intermediate materials first.

The graphical method of blend design is set out step-wisely as follows and best understood by referring to the image below:

Step 1

Plot the % passing values for aggregate A on the right-hand vertical scale representing (100% of aggregate A on lower horizontal scale). Do the same for the values for aggregate B on the left-hand vertical scale (100% of aggregate B on the upper horizontal scale; refer to the image below).

Step 2

Connect points corresponding to the same sieve size on the two vertical scales with a straight line and label the line with the sieve size for identification purposes. Note that any vertical line intersecting each sieve size line will define the blend gradation on the vertical scale and the blend proportions on the bottom and top horizontal scales.

Step 3

For a given sieve size line, mark on the line the points where the line crosses the upper and lower specifications limits measured on the vertical scale. The portion of the straight line between the two marks represents the proportions of  the aggregates A and B measured on the horizontal scale that will meet the specification limits for that particular size.

Step 4

Draw two vertical lines one for the upper and the other for the lower spec. limits such that the limits for all sizes in the specs, will not be violated. The region on the horizontal scale   bounded by the two vertical lines represents all proportions of possible satisfactory blends.

In practice, the mid-point of the horizontal region is selected for the desired blend. The point of intersection of the vertical through this point with the various size lines will define the gradation of the blend on the vertical scale.

Step 5

If there are three materials to be blended, the resulting blend after Steps 1-4  is treated as if it were a single material and then blended with the third material by repeating the process from the beginning (Steps 1-4).

If at the end of Step 5 the proportion of  the various components in the three-component  blend is x% of (A+B) and (100-x)% of C and the A+B blend is itself composed of y% of A and (100-y)% of B, then the three-component blend (let us call it material D)  is made up in composition as follows:

Equation

where x, y are values in %.

The table below sets out the proportion of the individual materials in the blend.

Component MaterialProportion in blend (%)
Axy/100
Bx(100-y)/100
C(100-x)

Thus,  the size fractions of the final blend will consist of the above proportions of the corresponding size fractions of the individual components.

For a multi-component blend, Steps 1-4 are applied repeatedly each time taking the resulting blend as a single material before combining with the next material.

At each blending stage, it is important to note the proportions in which the components forming the blend exist in order that at the end of the process, the exact proportion of the individual materials composing the final blend may be evaluated. 

% of Aggregate in Blend
Graphical method for blending two aggregates

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