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WPC #27380 07/09

AMRC 2011 MODULE 8

Properties of Aggregates

CONTENTS Overview ......................................................................................... 8-1 Objectives ........................................................................................ 8-1 Procedures ....................................................................................... 8-2 8.1 Definition .............................................................................. 8-3 8.2 Aggregates for Construction Purposes.................................. 8-5 8.3 Size and Grading ................................................................... 8-7 8.4 Shape ................................................................................... 8-13 8.5 Texture ................................................................................ 8-15 8.6 Deleterious Substances ....................................................... 8-17 8.7 Aggregate Strength Properties ............................................ 8-19

8.7.1 Toughness .............................................................. 8-19 8.7.2 Soundness .............................................................. 8-19 8.7.3 Crushing Strength .................................................. 8-20

8.8 Relative Density and Absorption ........................................ 8-21 8.9 Affinity for Asphalt ............................................................ 8-25 8.10 Summary ............................................................................. 8-27 8.11 Self-Test .............................................................................. 8-29

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Module 8

Properties of Aggregates

Overview When we talk about construction materials, we refer to products such

as concrete, asphalt, base materials for roads and, in some cases, soils. The fundamental component of these major products is aggregates. Since aggregates are found randomly in deposits around the province, and the material within each deposit varies according to its size, density, shape and quality, it is important to be aware of the origin of aggregates. This was discussed in a previous module. In this module, we will look at the definition of aggregates and how they are used as a fundamental component for a variety of construction materials, such as Portland or asphaltic concretes and base gravels. As aggregates are the largest component of these composite materials, comprising 75–95%, it is important to assess their properties as they directly relate to the satisfactory performance of these products. This module will identify and discuss these properties. It is important to note that since aggregates are derived from natural sources, and these sources vary a great deal from one area to another, the aggregate properties also vary. Evaluation of all properties must be undertaken and the relative performance expectation determined. Most aggregate sources will require some processing in order to satisfy project requirements. For this reason, standards or codes were developed to aid in identifying an acceptable range of each property.

Objectives Upon successful completion of this module, you will be able to: define construction aggregates describe and explain the following properties of aggregates;

shape, size, texture, density, absorption, strength, toughness and soundness

demonstrate the properties of aggregates as they pertain to

construction materials relate the properties of aggregates to the origin of aggregates identify the proper tests required to determine a specific

property of an aggregate.

MODULE 8 Properties of Aggregates

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Procedures Study the module materials and make notes as required. Perform the self-test on these principles and review the course

materials in such a manner as to be able to successfully complete similar questions upon examination.

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SECTION 8.1

Definition

The dictionary definition of aggregate says “formed by a collection of particles into a mass; an assemblance of different material substance separated by mechanical means or a collection into one sum, mass or body.” Therefore, aggregates for the use in construction are the combining of various sizes of parent material into a heterogenous (mixed) material. For our purposes, let it suffice that aggregates can be defined as a combination of pieces of rock material ranging in size from 600 mm down to dust.


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SECTION 8.2

Aggregates for Construction Purposes

Aggregates for construction purposes will generally fall within the range of 75 mm (3") to 75 µm (0.005"), commonly referred to as sands and gravels. Larger material is used for riprap embankment stabilization and smaller material, silts and clays, are not covered in this course. The following table provides a range of sizes for various aggregate components. boulders cobbles gravel sand silt clay

300 mm (1’) to 75 mm (3") to 5 mm (¼") to 75 µm (0.005") to 0.005 mm (0.0002") to < 0.005 mm (0.0002")

1 metre (3’) 300 mm (1’) 75 mm (3") 5 mm (¼") 75 µm (0.005")

Most commercial aggregate sources will contain quantities of all the various components to some degree. An ideal source will contain a predominance of sands and gravels, with a trace of the other components.


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SECTION 8.3

Size and Grading

Usually the first property which is determined in an aggregate is the size of particles, and the amount of each size within the aggregate source. This is sometimes referred to as the grain size distribution of the aggregate. A sieve test is used to measure quantity and size of particles within an aggregate sample. A size range from 300 mm to 75 µm (0.075 mm) will satisfy most needs. This range includes the gravel and sand sizes. A graph of the grain size distribution (see Figure 8.1) presents a picture of the relationship of one particle size with another. The graph is determined by a sieve analysis test. The relationship indicates the aggregate’s performance with respect to a specific application. For example, if the aggregate was to be used as a drain rock, it would be desirable to have all the particles of similar size, probably around 20 mm. This would leave large gaps or voids between the particles to allow the water to drain through easily. On the other hand, a road base material, which is required to compact, will have an evenly distributed graph reducing the voids between the particle size (see Figure 8.2). As you can see, the requirement of the particle distribution is dependent on the intended use of the material.


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Figure 8.1 Grain Size Distribution Showing Relationship of Particle Sizes


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Figure 8.2

Grain Size Distribution of Road Base and Drain Rock Materials


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Figure 8.3

Gap-Graded Materials


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Grading of aggregates can be classified into three categories; well-graded, uniform and gap-graded. Well-graded aggregate is a sample which has all the sizes within a specified range in equal amounts. A uniform (sometimes referred to as open-graded) aggregate describes a sample where the bulk of the particles are the same size. Gap-graded refers to an aggregate sample where one or two particle sizes are absent (see Figure 8.3). To summarize, the grain size distribution of an aggregate sample refers to the physical dimensions of the individual particles and the quantity of each size within an aggregate supply.


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SECTION 8.4

Shape

Closely related to the gradation of the aggregate is the shape of the individual particles. The shape of the particles can influence the behaviour of the aggregate when used in the manufacturing of other products such as Portland or asphaltic concretes. It controls the material’s ability to interlock, therefore affecting strength or maybe its rate of deterioration during handling cycles. Let’s look at the particle shapes of aggregates. There are basically four types: rounded sub-rounded sub-angular angular. Aggregates which have been formed by water are usually rounded or sub-rounded (see Figure 8.4). The bouncing along the water course tends to tumble the particles, smoothing the surfaces and grinding down the edges, giving an appearance similar to a selection of children’s marbles. On the other hand, material which has been transported by glacier or gravity tends to be sub-angular to angular in shape. In addition, processed aggregates are generally more angular because of the crushing or blasting process. An aggregate particle having a rounded shape has less surface area than an aggregate with an irregular shape. This feature is important in selecting aggregate for specific applications. Rounded particles could improve economy and workability in Portland cement concretes and angular particles could provide interlocking stability in road base materials or more contact area in asphaltic cement concretes. Sub-round to angular particles can create problems if their width, length or thickness is out of proportion. Ideal aggregates should be either spherical or cubical in shape. However, through natural or process procedures, particles can become flat or elongated in shape. These characteristics are detrimental for construction. Plate or sliver-like particles tend to break during handling. They could reduce the strength of products made from this type of aggregate.


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Rounded Sub-rounded Sub-angular Angular

Figure 8.4

Aggregate Shape To recap, aggregate particles come in various degrees of shape ranging from rounded to angular. The shape can contribute to or detract from the intended use of the material, and it is therefore important to assess the particle shape of the aggregate.

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SECTION 8.5

Texture

Aggregate texture refers to the surface condition of the particles. The surface texture is the result of the geological composition of the particle and the aggregate forming process. Aggregates formed from metamorphic rocks will probably have a smoother texture than those formed from sedimentary rocks. Aggregate particles with a crushed surface have a tendency to be rougher than rounded surfaces formed by water. The texture of the surface can influence the bonding ability of the aggregate with asphalt or Portland cement paste. The texture may affect the friction between particles; the rougher the texture, the greater the friction between the particles. Texture sometimes is an indicator of the porosity of the aggregate (ability to absorb liquids). Coarsely textured rocks are generally more porous, which may affect the binding ability of the aggregate. So in general, texture is dependent on the origin and composition of the rock and the method by which the particle was formed. It influences the interlocking and adhesion of individual particles into the matrix of the desired material.


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SECTION 8.6

Deleterious Substances

Following along our list of visual physical properties of aggregate, we will conclude with deleterious material or substances. Deleterious substances are materials (organic or inorganic) which could be injurious or detrimental to the end use of the aggregate. The deleterious substance could be: 1. wood, roots, sticks, etc. 2. coal or lignite 3. clay lumps 4. controlled quantities of material finer than 80 µm 5. in some cases, chert, shale, siltstones, sandstone, argillaceous

limestones 6. friable particles 7. reactive aggregates.

The amount of deleterious material permissible in an aggregate supply depends on the use of the aggregate. The reason for removing the deleterious material from the aggregate supply is quite obvious. Substances such as clay lumps will break down in water, thus altering the material’s original gradation. Friable particles (mineral particles which tend to oxidize upon exposure to the air and will crumble between one’s index finger and thumb) will break down during the aggregate’s use, resulting in a loss of strength. In some aggregate applications, such as Portland cement concrete, more than 5% material finer than 80 µm can cause serious bonding problems. The fine material will coat the aggregate particles preventing a proper bond between the cement past and the aggregate. Soft particles such as coal lumps are lighter than the natural aggregates and therefore do not have the strength to withstand the working stress of the aggregate. When the aggregates are subjected to abrasion, such as in gravel, asphalt or Portland cement roads, the soft material disintegrates and could cause road failure. Aggregate composed of particles of cherts, some shale, siltstones, sandstones and argillaceous limestones will either expand or disintegrate when used with Portland cements to make concrete. Expanding cherts cause the aggregates to literally pop out of the concrete, resulting in unsightly appearance and jeopardizing the integrity of the concrete structure.


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Organic impurities are sometimes found in both coarse and fine aggregates. In coarse aggregate, organic material is usually found in the form of roots, twigs and branches of trees. In fine aggregate, the non-mineral organic material is found in the form of tannic acid. The large pieces of wood will rot and weaken the aggregate products. Organics in fine aggregate will hinder the hardening process of Portland cement concretes. Reactive aggregates are those which contain minerals which react with alkalies in Portland cements causing excessive expansion of mortar or concrete. The expansion takes place over a number of years resulting in the disintegration of the concrete member. The reaction is either alkali-silica or alkali-carbonate, depending on the aggregate. The reaction is most susceptible when concrete is exposed to a humid atmosphere. Aggregates currently used in coastal British Columbia are not as susceptible to alkali reaction as other parts of Canada, which is fortunate, considering the west coast climate. As you can see, many substances must be considered when checking an aggregate for deleterious material. Aggregates must be virtually free of clay lumps, light substances such as coal; must be low in material finer than 80 µm; must not contain cherts, shales and siltstones, friable particles or alkali reactive material or organic impurities. Aggregates free of deleterious materials will be well-performing construction material.

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SECTION 8.7

Aggregate Strength Properties

The following three properties, toughness, soundness and crushing strength, refer to the strength characteristics of aggregates.

8.7.1 Toughness Toughness (or resistance to abrasion) is an aggregate’s ability to resist breakdown during handling. This property is important for aggregates which will be mixed in concrete mixers or asphalt pugmills, compacted with vibratory compactors or base gravels which are placed and finished by heavy construction equipment. All these processes can break the particles, thus changing the desired gradation. Determining toughness of an aggregate source is usually assessed by subjecting the aggregate to a series of impact loads and measuring the broken fragments left over.

8.7.2 Soundness The next property relating to strength is the soundness of the individual particles. This means the resistance to disintegration under weathering, including alternating heating and cooling, wetting and drying, and freezing and thawing. Expansion and contraction stresses caused by temperature changes impose a stress on aggregate which may eventually cause aggregate size breakdown. Chemical changes which slowly disintegrate some types of aggregate are brought about by atmospheric moisture which contains dissolved gases. Drying and rewetting renews the chemical attack which is generally the dissolving of a constituent within the aggregate. The most destructive effect of weathering is caused by freezing and thawing. Pores in the particles become filled with water, which freezes. The ice expands within the pores, exerting pressure which will attempt to break the particle. Tests can be performed on aggregates to assimilate these weathering conditions and measure the soundness qualities of the aggregate. You can appreciate the importance of this property since most aggregate applications are exposed to the natural environmental elements.


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8.7.3 Crushing Strength The final strength characteristic of aggregate is its crushing strength.

The crushing strength refers to the loads the aggregate can support before failure. Aggregates used only as construction materials such as road base, or gravel surface, have no cementing substance to bond the individual particles together. Therefore, the size, grading, shape and texture are the adhesive properties holding the material together. The load transmitted to the material in the form of traffic must be supported by the compressive strength properties of the aggregate particles. The loads are transmitted through the gravel layer by point contact. Point contact refers to the contact points of each particle with its neighbouring particle. As the load is transferred deeper into the gravel layer, the more aggregate particles supporting the load (pyramid like, see Figure 8.5). If the aggregate has poor crushing strength, the point contact will fail, causing a domino failure effect throughout the layer. Of course, there is a limit to how much load any material can accept. Two phases occur when a strong aggregate is under large loads. Phase one is the reduction in the air voids between the particles, in other words, more compaction. This is accomplished by movement of the particles to better align themselves with their neighbours. When further compaction is no longer attainable, phase two commences. The area under the load will start to settle. This is called horizontal displacement. If horizontal displacement does not occur, then the aggregate itself must have failed by crushing. A strong aggregate is a material which can transfer the load by point contact without crushing.

Figure 8.5

Aggregate Particles Supporting a Load To summarize, the three properties which refer to an aggregate’s strength are its toughness, soundness and crushing strength. Toughness is the ability to resist breakdown during handling; soundness, the ability of an aggregate to resist breakdown through environmental weathering; and crushing strength, the ability to resist failure under loading conditions.

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SECTION 8.8

Relative Density and Absorption

We now move into the area of aggregate properties relating to the mass and volume of material, sometimes referred to as mass-volume relationship. Relative density or specific gravity of an aggregate refers to its relationship with water. Water has a relative density of 1.0. All aggregates, except pumice and some artificial aggregates, are heavier than water. In order to design concrete and asphalt mixes, and determine quantities of road base material, it is necessary to determine the mass ratio of aggregate with respect to water. In addition, it is necessary to determine the amount of water which can be absorbed into the aggregate particles. Therefore, the relative density and absorption of an aggregate are important. An aggregate’s relative density in most cases relates to its strength. The higher the relative density, generally the stronger the material. Relative densities of aggregates range from 2.5 to 3.0. For special purposes such as dense concretes, special aggregates with relative densities of 4 or more may be used. This is rare, however. Aggregates from the Campbell River area of BC have relative densities of 2.85 or 2.85 times greater than water. Aggregates from Mackenzie (Williston Lake area, BC) have relative densities of 2.55 or 2.55 times greater than water. Tests on aggregates from these two areas show that the Campbell River aggregates are considerably stronger. Absorption is the ability of the aggregate to take in moisture to its external or internal pores. It is usually expressed as a percentage thus:

Volume of Water Required to Fill Pores% absorption = 100

Volume of Aggregate

A general rule of thumb is that the higher the relative density, the lower the percent absorption. This is somewhat obvious because as the particles in the material matrix become more congested, the space available in the form of pores is reduced.


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In BC, natural aggregate relative densities range from 2.5 to 3.0 with an average of about 2.72. Absorptions range from 0.3% to 2.0% with an average of about 1.0% by mass of material.

Student Note: Relative density is the international metric term used. Specific gravity is the imperial term. They mean the same.

Determining the relative density of a material is a simple test involving the comparison of various aggregate conditions to water and this will be discussed in a future module. However, there are a few concepts of relative density which should be discussed now. First there are three relative density values for any given aggregate; saturated surface dry, bulk relative and apparent relative. This is the result of aggregates having a porous structure and a textured irregular surface. The first term to discuss is the condition SSD. SSD is the acronym for saturated surface dry. A saturated surface dry aggregate is one whose internal pores are filled with water, not air, and whose surface is dry. If you were to soak some coarse aggregate in water overnight, remove and dry it with a paper towel until it is no longer shiny (i.e., it has a dull, damp look), then the material is considered SSD. The relative density determined when the aggregate is in this condition is called the relative density SSD (or bulk SSD). This value is the most commonly used value. However, there are two other values quoted; the bulk and the apparent relative density. The bulk relative density is measured when the aggregate surface irregularities, as well as the pores, are filled with water. The apparent relative density is measured when the aggregate is completely dry with no water in the pores (see Figure 8.6). To recap, relative density and absorption are two properties which establish a ratio relationship between the material and water. Using this relationship, it is possible to design concrete and asphalt mixes, and to convert volume of material to its mass and vice versa.


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Apparent Dry piece of aggregate with pores filled with air

Bulk SSD Piece of aggregate with internal voids (pores) and some external crack filled with water

Bulk Piece of aggregate with internal voids (pores) and all irregularities in aggregate filled with water

Note: The percent water required to fill all internal voids and

external voids to bring the aggregate to SSD condition is the absorbed water.

Figure 8.6

Measuring Relative Density


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SECTION 8.9

Affinity for Asphalt

Affinity for asphalt is an unusual property of aggregates and is very critical in asphaltic concrete manufacturing. Some aggregates possess properties which make them attract or repel water. Water loving aggregates are called hydrophilic and water hating aggregates are called hydrophobic. Hydrophobic aggregates such as limestones, dolomite and trap rock, because they repel water, are considered highly resistant to asphalt stripping are said to have an affinity for asphalt. Stripping is the separation of the asphalt film from the aggregate through the action of water. Hydrophilic aggregates, on the other hand, are highly susceptible to stripping. Siliceous aggregates such as quartzite and some granites fall into this category. It is not totally understood why some aggregates behave in this manner. Aggregate sources, when used for asphaltic concrete, are tested when hydrophilic properties are suspected.


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SECTION 8.10

Summary

In this module, we have discussed seven properties characteristic to construction aggregates. All these properties can be tested and determined for their suitability for specific applications. Of course, these properties vary from source to source, and careful consideration must be given to them when designing aggregate properties. Economics plays an important part in weighing the properties against other design criteria. To review the seven properties, they were: 1. size and grading 2. shape 3. texture 4. deleterious substances 5. aggregate strength (toughness, soundness, crushing strength) 6. relative density and absorption 7. affinity to asphalt. The reference section at the end of the course manual includes several additional references on aggregate properties. The self-test at the end of this module will aid you in reviewing your comprehension of the presented material.


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SECTION 8.11

Self-Test

1. From Figure 8.7 on the following page, identify the type of

material for each of the curves (1–3) (i.e., gap-graded, uniform, well-graded).

2. On Graph 3, what size of material makes up the majority of the

sample?

3. Would you consider Graph 2 to be free draining (allowing water to flow through quickly)?

4. What are the advantages of crushed surfaces for aggregates?

5. Is it advantageous to have all particles within the aggregate source rounded to sub-rounded? Why?

6. What is the most efficient shape of an aggregate particle? 7. Why are cubic-shaped particles preferred over elongated or flat

particles?

8. Is there a relationship between the surface texture and the absorption of an aggregate? Explain.

9. Does crushing an aggregate change its texture?

10. List three processes which affect the texture of aggregate.

11. Why are clay lumps undesirable in processed aggregates? 12. How does fine dust affect concrete aggregate? 13. List two reasons for not allowing organic impurities in

aggregates?


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14. What is the difference between the toughness and soundness of an aggregate?

15. List three conditions which attack the soundness of an aggregate.

16. Where is an aggregate likely to fail under load?

17. The density of an aggregate is relative to?

18. Aggregates which have a dry surface but with their voids filled with water are considered to be?

19. If a truck hauled in 27 tonnes of aggregate and the relative

density was 2.7, how many cubic metres would it contain?


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Figure 8.7 Questions 1, 2 and 3


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Properties of Aggregates - BCIT Commons · 8.7 Aggregate Strength Properties ... relate the...

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