Chapter 2 Aggregate Civil Engineering Material

Chapter 2: Aggregate 1st Ed, Civil Engineering Materials

Chapter 2AGGREGATE Classification of Aggregates

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

In Civil Engineering, the term of aggregate can be described as crushed stone, gravel, sand, slag and recycled concrete, which is composed of individual particles. Aggregates are also used as base material under foundations, a component of composite materials such as concrete and asphalt 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 or building), or as a low-cost extender that binds with more expensive cement or asphalt to form concrete.

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

Prepared by: Ahmad Fahmy Kamarudin, January 2010

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

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

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


Figure 2.2: Rock quarried

2.1 Classification of Aggregates Aggregate 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

Others

i. Produce dense and crack free concrete

ii. Not suitably graded and difficult to have adequate workability without segregation


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Aggregate

High-Density Aggregate Normal Aggregate

Light Weight Aggregate

Natural Aggregate Natural Artificial

Magnetite Heamatite Limonite Barites


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

Dry loose bulk density < 1200 kg/m3

Water absorption High

Type 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 premixed prior 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 the material divided by the volume they occupy. The volume includes the space between 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 the density of a given substance, to the density of water. Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less 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


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Shale Clay Pumice

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

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

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

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

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

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

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


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 disintegration of rock

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

c) Partially Crushed Gravel or StoneA product of blending of uncrushed and crushed gravel or blending 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 SandFine aggregate resulting from natural disintegration

b) Crushed Stone Sand or Crushing Gravel SandFine aggregate produced by crushing hard stone or natural gravel respectivelyMS 29: 1995; The coarseness or fineness is indicated by the 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 graded Used in unimportant work


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Tab

le 2.1

: Grad

ing Lim

it for

Co

arse A

ggre

gates

(De

rived

from B

S 8

82)


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 the application of aggregate such as concrete, foundation and etc. in terms of random arrangement of particles contribute to spreading of concentrated loading effectively. However, the aggregate should be


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

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


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

Figure 2.3: Force compressed between aggregate

The advantage of angular particles and rough aggregates can create better interlocking system and tendency to resist forces from developed friction compare to rounded particles with smooth surface contributes to less frictions resistance and easy to slide.

High compressive strength of aggregate is useful to enhance the capability in resisting compressive force especially for composite materials such as concrete, asphalt concrete and etc. In normal practice, the weight of aggregate is stronger than the composite materials.

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 metamorphic rock in selection of aggregate types.

Table 2.4: Rock Classifications

Igneous Rock Sedimentary Rock Metamorphic Rock

Definition

Rocks formed by solidification of cooled magma by crystallizing into a mosaic of materials

Rocks formed from sediments of the earth’s land area

Rocks are created by changes induced at high temperature and/or high pressure


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P kN


Igneous Rock Sedimentary Rock Metamorphic Rock

Environment Underground: and as lava flows

Deposition basin: mainly sea

Mostly deep inside mountains chains

Rock strength

Uniform high strength Variable low Variable high

Major types with

compressive strength

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 the damaging effect of load or applied pressure. This hardness aggregate is depending on the type of parent rock.

The hardness of aggregate can be tested by using abrasion test as described 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 large drum with standard sized of steel balls. About 500 revolutions of drum rotation are carried out, and the aggregates will pass through the sieve.


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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 or internal damaging attack such as weathering effect (also known as soundness)

The soundness test is described in BS 812: Part 121: 1989 or ASTM C88. As described in ASTM C 88, the soundness of aggregate is tested by simulating the weathering effect by soaking the different sized fractions of oven-dry sample, in sodium sulfate or magnesium sulfate solution for 16 hours to create freezing effect. The sample is subjected to five cycles of soaking and drying procedure. Tested samples were then washed and weighted to determine loss percentage of entire samples. The results will be compared with allowable limits to determine whether the aggregate is acceptable.

2.3.4 ToughnessToughness is defined as the resistance if aggregate to failure by impact. The toughness of aggregates can be determined by implementing Aggregate Impact Test according to MS 30: Part 10: 1995. The aggregate impact value shall not exceed 45% by weight for aggregate 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 its total 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 less depending on types of aggregate.


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Most of granite and limestone have very low porosity whereas a large majority of sandstone rocks have high porosity as high as 13% and 30%.

Table 2.5: Rocks and Porosity (%)

Type of Rock Porosity (%)

GraniteShaleClay

Sandstone (fractured)Sand

GravelLimestone (cavernous)

Chalk

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501530255

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Porosity of natural aggregate can be determined by using following formula:

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 absorption when it is dry. The amount of water absorption is depending on the size and volume of aggregate.

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

Porosity of concrete is contributed by the porosity of aggregate since aggregate comprises 75% of the volume of concrete. When concrete exposed to cold temperature and moisture, resistance to freeze-thaw is important to ensure long service life. Hence, further investigation must be carried out if the selection of porous aggregate as part of composite 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 the aggregate particles. The amount of void normally expressed as void content 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 50 percent depending on size, shape and texture. Typically, fine aggregate indicates 35 to 40% of void content while coarse aggregate is about 30 to 50% (depending on size).

The amount of absorption is important to be evaluated for appropriate amount of fluid to be mixed into composite materials. Highly absorptive aggregates require greater amount of fluid and making less economical.

The definition of absorption capacity or water absorption or absorbed moisture can be defined as the moisture content in the saturated surface dry condition. Further explanation of voids and moisture absorption 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 the aggregate in an oven to a constant mass.

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

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

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

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

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

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

The water added to the concrete mix must be adjusted to take account on water absorption of aggregates when making concrete, to obtain constant and required workability and strength of concrete. The determination of MC of an aggregate is necessary to determine the net water cement ratio for a batch of concrete. High moisture content will increase the effective water-cement ratio to appreciable extent and make the concrete weak unless a suitable allowance is made. BS 812: Part 109: 1990 and MS 50 described method of determination of moisture content and absorption of aggregate. They are:

a. Displacement methodIt gives the moisture content as a percentage by mass of saturated 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 of aggregate in their saturated and surface dried condition and any change in moisture content must be reflected in the adjustment to the weight of aggregate and the mix.

2.4 Grading of AggregatesSieve analysis test is used in grading of aggregate. Sieve analysis consists of determining the proportionate amounts of particles retained or passing through each of a set of sieves arranged in decreasing 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 curve showing cumulative percentages of the material passing the sieves. The grading curve indicates whether the grading of a given sample conforms to that specified, or is too coarse or too fine or too deficient in particular size. The reading of the grading curve will indicates the followings:

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, the aggregate is finer and more water will be required, thus increasing the quantity of cement also for a constant water cement ratio. Therefore, this is uneconomical.

c. If the actual grading is steeper than specified, it indicates an excess 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 workability and stability of concrete mix. Besides it is also important factor in concrete 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 aggregate are needed to produce good quality of concrete. The grading of fine aggregate has a much greater effect on workability of concrete than does the grading of the coarse aggregate.

Too fine an aggregate requires too large water cement ratio for adequate workability. Meanwhile, larger size of aggregate will reduce the 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


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

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


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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


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

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

Crushing rock produces angular particles with sharp corners. The corners of aggregates break down due to weathering effect and creating sub-angular particles. When the aggregate being transported in water, the corners become completely rounded.

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

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

Rough texture generally improves the bonding, inter-particle friction but 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 and having rounded edges. E.g. Other gravel land or dug flint


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AngularProcessing well defined edges formed at theIntersection or roughly planes faces. E.g. Crushed focks 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 the length is considerably larger than the other two dimensions.

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

2.5.2 Surface Texture of AggregateSurface texture is a measure of the smoothness or roughness of the aggregate. The strength of the bond between aggregate and cement paste depends upon the surface texture. The bond is the development of mechanical anchorage and depends upon the surface roughness and surface porosity of the aggregate.

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

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


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Tutorial 2

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

a. Shrinkageb. Modulus of elasticityc. Chemical reactivity


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Chapter 2 Aggregate Civil Engineering Material