Chapter 2 Aggregate

Chapter 2: Aggregate 1st Ed, Civil Engineering Materials

Prepared by: Ahmad Fahmy Kamarudin, January 2010

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Chapter 2AGGREGATE

2.1 Classification of Aggregates2.2 Type of Aggregates2.3 Physical Properties2.4 Grading of Aggregates

In Civil Engineering, the term of aggregate can be described as crushedstone, gravel, sand, slag and recycled concrete, which is composed ofindividual particles. Aggregates are also used as base material underfoundations, a component of composite materials such as concrete andasphalt concrete, which is normally used in building and road constructions

Figure 2.1: Application of aggregate in civil engineering practice

Aggregates are used as a stable foundation or road/rail base with predictable,uniform properties (e.g. to help prevent differential settling under the road orbuilding), or as a low-cost extender that binds with more expensive cementor asphalt to form concrete.

Aggregate is needed for any kind of constructions. Normally, natural sourcesfor aggregates include gravel pits, river run deposits and rock quarries.Gravel deposits are crushed to obtain the needed size distribution, shapeand texture.

http://en.wikipedia.org/wiki/Composite_material

http://en.wikipedia.org/wiki/Concrete

http://en.wikipedia.org/wiki/Asphalt_concrete

http://en.wikipedia.org/wiki/Image:Asphalt_base.jpg

http://en.wikipedia.org/wiki/Image:08_tory_railtrack_ubt.jpeg



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Figure 2.2: Rock quarried

2.1 Classification of AggregatesAggregate can be classified according to their unit weight.

2.2 Type of Aggregates

2.2.1 High-Density Aggregate (H-DA)Specific Gravity 2.8 to 2.9

Unit Weight 2800 to 2900 kg/m3

Type of H-DA Magnetite, heamatite, limonite and baritesCompressive strength (in

concrete)20 to 21 N/mm2

Othersi. Produce dense and crack free concreteii. Not suitably graded and difficult to have

adequate workability without segregation

Aggregate

High-DensityAggregate

Normal Aggregate Light WeightAggregate

Natural Aggregate Natural Artificial

Magnetite Heamatite Limonite Barites

http://en.wikipedia.org/wiki/Image:HematiteUSGOV.jpg

http://en.wikipedia.org/wiki/Image:Magnetite_Russia.jpg

http://en.wikipedia.org/wiki/Image:LimoniteUSGOV.jpg

http://en.wikipedia.org/wiki/Image:Barite09.jpg



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2.2.2 Light Weight Aggregate (LWA)Particle density < 2000 kg/m3

Dry loose bulk density < 1200 kg/m3

Water absorption HighType of LWA Pumice, expanded shale, expanded clay

Workability of concretei. Quick stiff.ii. Aggregate require wetting before mixing in

the mixer

Concrete mixing operationWater and aggregates are usually premixedprior to addition of cement

Others (concrete using LWA)

i. coarse surface textureii. lower tensile strengthiii. lower Modulus of Elasticityiv. Higher creep and shrinkage

Bulk DensityBulk density is a property of particulate materials. It is the mass of particles of thematerial divided by the volume they occupy. The volume includes the spacebetween particles as well as the space inside the pores of individual particles

Specific GravitySpecific gravity (SG) is a special case of relative density defined as the ratio of thedensity of a given substance, to the density of water. Substances with a specificgravity greater than 1 are heavier than water, and those with a specific gravity ofless than 1 are lighter than water.

2.2.2 Normal Aggregate (NA)Specific gravity 2.5 to 3.0

Bulk density 1450 to 1750 kg/m3

MS 29: 1995

Classify according to size:i. Coarse aggregateii. Fine aggregate (sand)iii. All-in aggregateGrading limit in percentages by weight for coarse aggregate

Shale Clay Pumice

http://en.wikipedia.org/wiki/Image:ShaleUSGOV.jpg

http://en.wikipedia.org/wiki/Image:Clay-ss-2005.jpg

http://en.wikipedia.org/wiki/Image:Teidepumice.jpg

http://en.wikipedia.org/wiki/Particulate

http://en.wikipedia.org/wiki/Mass

http://en.wikipedia.org/wiki/Volume

http://en.wikipedia.org/wiki/Porosity

http://en.wikipedia.org/wiki/Relative_density

http://en.wikipedia.org/wiki/Density

http://en.wikipedia.org/wiki/Water_(molecule)



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BS 812: Part 103:1985

Determination of particle size distribution

i. Coarse Aggregate

Retain on 5 mm (3/16 inch) BS 410 test sievea) Uncrushed Gravel or Uncrushed Stone

Coarse aggregate resulting from natural disintegrationof rock

b) Crushed Stone or Crushed GravelCoarse aggregate produced by crushing hard stoneand gravel respectively

c) Partially Crushed Gravel or StoneA product of blending of uncrushed and crushed gravel orblending stone

ii. Fine Aggregate

Pass through 5 mm (3/16 inch) BS 410 test sieveSand - Lower size limit of about 0.07mmSilt - size limit between 0.06 to 0.002mmClay - smaller particles

a) Natural Sand Fine aggregate resulting from natural disintegrationb) Crushed Stone Sand or Crushing Gravel Sand

Fine aggregate produced by crushing hard stone or naturalgravel respectivelyMS 29: 1995; The coarseness or fineness is indicated bythe zone in which the grading fallsHigher zone number indicates a finer material

iii. All-In AggregateCompose of a mixture of coarse and fine aggregate.Not gradedUsed in unimportant work

Table 2.1: Grading Lim

it forC

oarse Aggregates(D

erived from B

S 882)



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2.3 Physical Properties

2.3.1 StrengthAggregate cannot transmit tensile force from one particle to another,but very well in resisting compressive forces. In real practice theapplication of aggregate such as concrete, foundation and etc. interms of random arrangement of particles contribute to spreading ofconcentrated loading effectively. However, the aggregate should be

Table 2.3: Typical Grading Curves for A Zone 2 Fine Aggregate and AGraded 20 mm Coarse Aggregate

Table 2.2 Grading Limits for Fine Aggregate (Derived from BS 882)



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compacted for significant contact between particles in distributing ofloading and reducing settlement.

Figure 2.3: Force compressed between aggregate

The advantage of angular particles and rough aggregates can createbetter interlocking system and tendency to resist forces fromdeveloped friction compare to rounded particles with smooth surfacecontributes to less frictions resistance and easy to slide.

High compressive strength of aggregate is useful to enhance thecapability in resisting compressive force especially for compositematerials such as concrete, asphalt concrete and etc. In normalpractice, the weight of aggregate is stronger than the compositematerials.

Example:Concrete Strength Aggregate Strength

20 N/mm2 to 50 N/mm2 70 N/mm2 to 350 N/mm2

Igneous rocks are much stronger than sedimentary or metamorphicrock in selection of aggregate types.

Table 2.4: Rock ClassificationsIgneous Rock Sedimentary Rock Metamorphic Rock

DefinitionRocks formed bysolidification of cooledmagma by crystallizinginto a mosaic ofmaterials

Rocks formed fromsediments of theearth’s land area

Rocks are created bychanges induced athigh temperatureand/or high pressure

P kN

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Igneous Rock Sedimentary Rock Metamorphic Rock

Environment Underground: and aslava flows

Deposition basin:mainly sea

Mostly deep insidemountains chains

Rockstrength Uniform high strength Variable low Variable high

Major typeswith

compressivestrength

Granite (90 MPa),basalt (160Mpa)

Sandstone (40Mpa),limestone, clay Schist, slate

The strength of aggregate is measured by on following tests:a. Aggregate crushing value (most popular)b. Aggregate impact testc. Ten percent fines value

2.3.2 HardnessHardness is defined as the ability of aggregates to resist thedamaging effect of load or applied pressure. This hardness aggregateis depending on the type of parent rock.

The hardness of aggregate can be tested by using abrasion test asdescribed in BS 812: Part 113: 1990 or ASTM C 131: C535.

Figure 2.5: Los Angles abrasion machine

This test is conducted by placing the blended aggregates in a largedrum with standard sized of steel balls. About 500 revolutions of drumrotation are carried out, and the aggregates will pass through thesieve.

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Satisfactory aggregate < 30% value of abrasion(use for wearing surface)

< 50% value of abrasion(use for non wearing surface)

2.3.3 DurabilityDurability is defined as the ability of aggregate to withstand external orinternal damaging attack such as weathering effect (also known assoundness)

The soundness test is described in BS 812: Part 121: 1989 or ASTMC88. As described in ASTM C 88, the soundness of aggregate istested by simulating the weathering effect by soaking the differentsized fractions of oven-dry sample, in sodium sulfate or magnesiumsulfate solution for 16 hours to create freezing effect. The sample issubjected to five cycles of soaking and drying procedure. Testedsamples were then washed and weighted to determine losspercentage of entire samples. The results will be compared withallowable limits to determine whether the aggregate is acceptable.

2.3.4 ToughnessToughness is defined as the resistance if aggregate to failure byimpact. The toughness of aggregates can be determined byimplementing Aggregate Impact Test according to MS 30: Part 10:1995. The aggregate impact value shall not exceed 45% by weight foraggregate used in concrete and 30% for wearing surface.

2.3.5 PorosityPorosity is defined as the ratio of the volume of pores in particle to itstotal volume (solid volume Plus the volume of pores)

Porosity = Volume of poresTotal volume of particles

All aggregates are porous; some are more porous and some are lessdepending on types of aggregate.



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Most of granite and limestone have very low porosity whereas a largemajority of sandstone rocks have high porosity as high as 13% and30%.

Table 2.5: Rocks and Porosity (%)Type of Rock Porosity (%)

GraniteShaleClay

Sandstone (fractured)Sand

GravelLimestone (cavernous)

Chalk

13

501530255

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Porosity of natural aggregate can be determined by using followingformula:

Porosity = 100WGs .( W + 100 )percent

where:W : water absorption in percentGs : specific gravity on saturated surface-dry basis

A porous aggregate may influence the capability of water absorptionwhen it is dry. The amount of water absorption is depending on thesize and volume of aggregate.

Besides, it is also less resistance to cycles of freezing and thawingwhich can cause cracking or fail due to internal expansion, if theaggregate are not strong enough to withstand the stresses.

Porosity of concrete is contributed by the porosity of aggregate sinceaggregate comprises 75% of the volume of concrete. When concreteexposed to cold temperature and moisture, resistance to freeze-thawis important to ensure long service life. Hence, further investigationmust be carried out if the selection of porous aggregate as part ofcomposite materials ingredient has been made.



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2.3.6 AbsorptionAggregate can capture fluid (water, moisture, asphalt binder and etc)in surface voids. Voids represent the amount of air space between theaggregate particles. The amount of void normally expressed as voidcontent and can be determined by using equation below:

Void content = SG x W – B x 100 SG x W

where:SG : specific gravityW : density of waterB : bulk density

Normally the void content in normal aggregate varies from 30 to 50percent depending on size, shape and texture. Typically, fineaggregate indicates 35 to 40% of void content while coarse aggregateis about 30 to 50% (depending on size).

The amount of absorption is important to be evaluated for appropriateamount of fluid to be mixed into composite materials. Highlyabsorptive aggregates require greater amount of fluid and making lesseconomical.

The definition of absorption capacity or water absorption or absorbedmoisture can be defined as the moisture content in the saturatedsurface dry condition. Further explanation of voids and moistureabsorption of aggregate is illustrated by using following figure.

Figure 2.6: Voids and moisture absorption of aggregates



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a) Bone dry :The aggregate contains no moisture; this requires drying theaggregate in an oven to a constant mass.

b) Air dry :The aggregate may have some moisture but the saturationstate is not quantified.

c) SSD :The aggregate’s voids are filled with moisture but the mainsurface area of the aggregate particles is dry.

d) Moist :The aggregate have moisture content in excess of the SSDcondition

e) Free moisture :The difference between the actual moisture content of theaggregate and the moisture content in the SSD condition.

Determination of moisture content (MC) can be calculated by usingfollowing equation:

MC = Weight of moisture x 100% Oven-dry weight

The water added to the concrete mix must be adjusted to takeaccount on water absorption of aggregates when making concrete, toobtain constant and required workability and strength of concrete. Thedetermination of MC of an aggregate is necessary to determine thenet water cement ratio for a batch of concrete. High moisture contentwill increase the effective water-cement ratio to appreciable extentand make the concrete weak unless a suitable allowance is made. BS812: Part 109: 1990 and MS 50 described method of determination ofmoisture content and absorption of aggregate. They are:

a. Displacement methodIt gives the moisture content as a percentage by mass ofsaturated surface dry sample



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b. Drying method – i. Oven drying methodii. Modified drying method

where, total moisture content due to free plus absorbed water.

Concrete mix proportion are normally based on the weight ofaggregate in their saturated and surface dried condition and anychange in moisture content must be reflected in the adjustment to theweight of aggregate and the mix.

2.4 Grading of AggregatesSieve analysis test is used in grading of aggregate. Sieve analysisconsists of determining the proportionate amounts of particlesretained or passing through each of a set of sieves arranged indecreasing sizes. It is expressed in terms of percentages.

Figure 2.7: Table-top sieve’s Figure 2.8: Aggregate is Placed in Shaker and sieves Sieves before Sieving

The grading curve can be drawn from this analysis and the curveshowing cumulative percentages of the material passing the sieves.The grading curve indicates whether the grading of a given sampleconforms to that specified, or is too coarse or too fine or too deficientin particular size. The reading of the grading curve will indicates thefollowings:

a. If the actual grading curve is lower than specified grading curve,the aggregate is coarser and segregation of mix might take place.



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b. If the actual grading curve lies well above the specified curve, theaggregate is finer and more water will be required, thusincreasing the quantity of cement also for a constant watercement ratio. Therefore, this is uneconomical.

c. If the actual grading is steeper than specified, it indicates anexcess of middle-size particles and leads to harsh mix.

d. If the actual grading curve is flatter than specified grading curve,the aggregate will be deficient in middle size particles.

The grading of aggregates has considerable effect on the workabilityand stability of concrete mix. Besides it is also important factor inconcrete mix design.

Uniform size of particle will contain more voids after compaction,whereas various particle sizes will give a mass containing lesser voids.

Proper grading of aggregate comprises of coarse and fine aggregateare needed to produce good quality of concrete. The grading of fineaggregate has a much greater effect on workability of concrete thandoes the grading of the coarse aggregate.

Too fine an aggregate requires too large water cement ratio foradequate workability. Meanwhile, larger size of aggregate will reducethe cement requirement for a particular water-cement ratio.

Example: Sieve Analysis of Coarse Aggregate( According to ASTM Standard )

Sieve analysis for 3/4-in stone

Sieve Mass Retained (lb) % Retained % Passing

1 in 0 0.0 100.03/4 in 719.8 6.0 94.01/2 in 2999.2 24.9 69.13/8 in 4318.8 35.8 33.3No. 4 3110.1 25.8 7.5No. 8 608.8 5.0 2.5No. 16 165.4 1.4 1.1

Pan 138.8 1.1 0Total 12060.9 100

% Retained in 3/4 in sieve: (719.8 x 100)/12060.9 = 6.0%



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Example 2: Sieve Analysis of Fine Aggregate( According to ASTM Standard )

Sieve # % Passing

1 in 100

3/4 in 94

1/2 in 69

3/8 in 33

No. 1 7.5

No. 8 2.5

No. 16 1



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2.5 Particle Shape and Surface Texture of AggregateAggregate has three dimensional of masses namely shape, size andsurface texture.

Shape and surface texture are considered as external characteristic.The shape and surface texture of fine aggregate govern its voidcontent and thus affect the water requirement of mix significantly.

Crushing rock produces angular particles with sharp corners. Thecorners of aggregates break down due to weathering effect andcreating sub-angular particles. When the aggregate being transportedin water, the corners become completely rounded.

Aggregate particles which have sharp edges or rough surface such ascrushed stone used more water than smooth and rounded particles toproduce concrete of same workability. About 5 – 10% of water contentcan be reduced by using rounded aggregate. However, the angularaggregates will be more difficult for them to slide across each other.

Besides, the interlocking between aggregates particle, and strongermortar bond, for crushed aggregate is higher than smooth or roundedaggregate in concrete with same water cement ratio. This increase instrength may be up to 38% for concrete having-cement ratio below 0.4.

Rough texture generally improves the bonding, inter-particle frictionbut more difficult to compact into a dense configuration.

2.5.1 Particle Shape of Aggregate.The particle shapes of aggregate are round, irregular, angular, flaky,elongated and rough.

RoundedFull water-worn or completely shaped by attritionor abrasion. E.g. river or sea shore gravel

IrregularNaturally irregular or partly shaped by attrition andhaving rounded edges. E.g. Other gravel land or dugflint

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AngularProcessing well defined edges formed at theIntersection or roughly planes faces. E.g. Crushedfocks of all types

FlakyA material of which the thickness is small relativeto other two dimensions. E.g. Laminated rock

ElongatedThe aggregate is usually angular, is shape, and thelength is considerably larger than the other twodimensions.

Flaky and ElongatedMaterial having the length which is considerablylarger than the width, and the width is considerablylarger than the thickness

2.5.2 Surface Texture of AggregateSurface texture is a measure of the smoothness or roughness of theaggregate. The strength of the bond between aggregate and cementpaste depends upon the surface texture. The bond is the developmentof mechanical anchorage and depends upon the surface roughnessand surface porosity of the aggregate.

An aggregate with rough and porous texture may increase theaggregate-cement bond up to 1.75 times, in which may increase thecompressive and flexural strength of concrete up to 20%.

The surface pores help in the development of good bond on accountof suction of paste into these pores. Aggregate with polished surfacedo not produce such strong concrete compared to those with roughsurface, The more angular the aggregate, the more surface area it willproduce, thus, result in greater bonding.

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Tutorial 2

Q1: In selecting an aggregate for a particular application, the mostimportant physical properties as follows are needed to be considered.You are required to explain each of them.

a. Shrinkageb. Modulus of elasticityc. Chemical reactivity

Chapter 2 Aggregate

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