Aggregate is a collective term for the mineral materials such as sand, gravel and crushed stone that are used with a binding medium (such as water, bitumen, Portland cement, lime, etc.) to form compound materials (such as asphalt concrete and Portland cement concrete). Aggregates can also be defined as granular materials of mineral composition such as sand, gravel, shells, slags or crushed stones used with a cementing medium to form concrete or alone for base courses and railway ballast according to ASTM. Aggregate is also used for base and sub base courses for both flexible and rigid pavements. The properties that make rocks suitable as an aggregate are dependent on the availability, workability, suitability as aggregates, affinity for binding materials (cements, bitumen or asphalts), binding properties and the ability to resist alternating wet and dry conditions of its environment (Kutu et.al., 2014; Rangwala et.al., 2008).
2.1 TYPES OF AGGREGATES
Aggregates can either be natural or manufactured. Natural aggregates are generally extracted from larger rock formations through an open excavation (quarry). Extracted rock is typically reduced to usable sizes by mechanical crushing. These are aggregates that are obtained from the natural deposits of sand and gravel or from the quarries by cutting the rocks. Among the mentioned types, the natural sand and the gravel are the cheapest. These are reduced to present size by means of natural agents; water, snow, wind…etc. The sand that are river deposits are most commonly used type which gain good quality. Nowadays the natural aggregates are found scarce which increase the demand of artificial aggregates. The artificial aggregates are the man-made aggregates. These are either by-products of any other product manufacture. Or it can be obtained by recycling a material that is found to be harmful to the environment in its free state.
The most commonly used artificial aggregate are clean broken brick and the sir cooled blast furnace slag. The broken brick of good quality are good for mass concrete but not for reinforced concrete. These must be free from lime and sulphate plaster. These are not suitable for water proof concrete. Other by-products are fly ash, silica fume. etc.
2.2 CLASSIFICATION OF AGGREGATES
Aggregates can be classified according to production methods, petrological characteristics, particle size and unit weight.
2.2.1 Classification according to production methods
Classification of aggregates according to production methods are further grouped into four. Natural aggregates which are taken from native deposits without any change in their natural states during production except for crushing, grading or washing. Examples are sand, gravel, crushed stone, lime rock. By-Product aggregates which comprise blast-furnace slags and cinders, fly ash, etc. Cinders are residue of coal or wood after burning. Processed aggregates which are heat treated, expanded materials with lightweight characteristics. Examples are Perlite, burnt clays, shales, processed fly ash. Colored Aggregates of which examples are glass, ceramics, manufactured marble for decorative and architectural purposes.
2.2.2 Classification according to petrological characteristics
According to petrological characteristics, aggregates can be classified according to igneous rocks which form as a result of solidification of molten lava forms igneous rocks. Examples include quartz, granite, basalt, obsidian, pumice, and tuff. Sedimentary rocks obtained by the deposition of weathered and transported pre-existing rocks. Examples are sandstone, limestone, and shale. Metamorphic rocks formed at a depth under high heat and pressure by the alterations of either igneous rocks or sedimentary rocks. Examples are marble, slate, and schist.
2.2.3 Classification according to particle size
Under the classification according to particle size, aggregates are either fine or course. Fine aggregate (sand) includes the particles that all passes through 4.75 mm sieve and retain on 0.075 mm sieve. Coarse Aggregate (gravel) includes the particles that retain on 4.75 mm sieve. (4%20Aggregates.pdf).
2.2.4 Classification according to unit weight
According to their unit weight classification, aggregates are classified as light weight aggregates (which are aggregates that is natural or synthetic which weighs less than 1100 kg/m3. The lightweight is due to the cellular or high internal porous microstructure, which gives this type of aggregate a low bulk specific gravity. The most important aspect of lightweight aggregate is the porosity. They have high absorption values and examples are slag, slate and other light stones.) And heavy weight aggregates (which are aggregates that are natural or synthetic which typically weigh more than 2,080 kg/m3 and can range up to 4,485 kg/m3. Heavy weight aggregate is most commonly used for radiation shielding, counterweights and other applications where a high mass-to-volume ratio is desired.) Examples are hemotite, barite magnetite, steel and iron punchings. (Classification%20of%20aggregates.htm).
2.3 PROPERTIES OF ROAD AGGREGATES
The properties of aggregates are the geologic features, the physical properties and the mechanical properties of aggregates. Studies by Torppa Akseli and Seppo Leinonen (2015) shows that the mechanical properties of aggregates clearly correlate with their geological characteristics. Recent studies shows that the mineral composition and grain-size distribution are generally key parameters in accessing the mechanical properties of aggregates but the roles of other textural features like mineral grain-shape, geometry of grain boundaries, and orientation of grains also has a role to play.
2.3.1 Geologic features of road aggregates
The physical and chemical properties of aggregates results from the geologic origin and mineralogy of the potential source and its subsequent weathering or alteration. Many of the properties of aggregates relate to their grain size, mineralogy, texture, pore space and weathering products. The best road stone is a fresh fine-to medium-grained igneous rock, with intergrowth of the minerals producing strong bonding and without rock glass. Most sedimentary rocks are easily crushed to be used as road stone but hard grit stones can also be used for this purpose. Many crystalline metamorphic rocks can also be used although apart from hornfels and schistose grit, they are too variable to make good road stone aggregate.
Studies by Vinod.B.R shows some physical and mechanical properties of aggregates. Main desirable properties of aggregate are;
1. Strength: The aggregates to be used in road construction should be sufficiently strong to withstand the stresses due to traffic wheel loads. The aggregates which are to be used in top layer of the pavements, particularly in the wearing course have to be capable of withstanding high stresses in addition to wear and tear; hence they should possess sufficient strength and resistance to crushing. Igneous rocks are massive rocks and usually of high strength. Their mineral constituents are interlocking resulting in slight differences in the mechanical properties of the rock. They are usually associated with very few problems in construction works (Brekke and Terry, 1972 and references therein)
Sedimentary rocks show very great variation in terms of behaviour and strength. They are composed mostly of soft minerals and have a weaker assemblage compared to igneous rocks. The minerals present in sedimentary rocks are cemented together and possess inter-granular matrix materials. Sedimentary rocks generally contain bedding and lamination or other structures related to sedimentation. Some of these rocks are not stable for a long time and are sometimes very susceptible to swelling and slaking. (Brekke and Terry, 1972 and references therein).
Metamorphic rocks are structurally diverse, with different composition and properties. Metamorphism produce hard minerals and high intact rock strength. The orientation of platy or sheet minerals usually attributed shearing movement result in considerable differences in physical and mechanical properties of rocks (Brekke and Terry, 1972 and references therein)
According to (Brekke and Terry, 1972, Olsen, 1964 and references therein) some minerals like Chlorite, Mica, Amphiboles and Pyroxene usually has an impact on the mechanical and physical properties of rocks. Parallel and continuous alignment of these minerals mostly found in sedimentary and regional metamorphic rocks result in a plane of weakness along layers of these flaky minerals. This zone of weakness is mostly significant where Mica and Chlorite occur in continuous layers and their effect on the strength of the rock is strong. Similarly, some sheet like minerals like Serpentine, Talc, and Graphite also reduces the strength of rocks and is attributed to easy sliding of these minerals along the cleavage surfaces.
Quartz is of great importance in the construction industry because of its hardness. It has a high grade in the Mohs hardness scale and also the last mineral to crystallize from melt and it’s the most stable rock-forming mineral on Earth which is more resistant to weathering.
Clay minerals are the most reactive silicates. They affect the engineering behaviour of soil and rock both as materials of construction and as foundation materials. The most significant problems associated with clay minerals are the change of moisture content of swelling minerals of Smectite (Montmorillonite) group. These minerals in addition to their expansive nature also have low shear strength. (Olivier, 1976; Cole and Beresford (1980) and references therein). Aggregate crushing value is used to determine the strength of aggregates. The aggregate crushing value is the relative measure of the resistance to crushing of an aggregate under a gradually applied compressive load. Aggregates for construction purposes should be very strong enough to resist crushing under a load. The aggregate crushing value is determined by the ratio of the weight of fines passing through a specified BS sieves to the total weight of the sample expressed as a percentage.
Aggregate crushing value=100*W2W1Where W1 = the total weight of dry samples
W2 = the weight of the portion of crushed material passing through 2.36mm BS sieve.
2) Hardness: The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic. They should be hard enough to resist the wear due to abrasive action of traffic. Abrasive action may be increased due to the presence of abrasive material like sand between the tyres of moving vehicles and the aggregates exposed at the top surface. This action may be severe in the case of vehicles with steel tyres. Heavy wheel loads can also cause deformations on some types of pavement resulting in relative movement of aggregates and rubbing of aggregates with each other within the pavement layer. The mutual rubbing of stones is called attrition, which also may cause a little wear in the aggregates; however attrition will be negligible or absent in most of the pavement layers. (Vinod.B.R).
The abrasion resistance of aggregates is generally tested using the Los Angeles (LA) testing machine. (Yilmaz Ozcelik, 2011 and references therein).
Abrasion resistance, displayed by Nordic abrasion value in recent research by Yilmaz Ozcelik (2011) is more dependent on the mineral compositions, while fragmentation resistance, displayed by Los Angeles value, is more sensitive to the textural variations regardless of the lithology of the aggregates.
Igneous type rock samples showed more resistance to abrasion than sedimentary and metamorphic type rock samples. More abrasion-resistant rocks are likely to have high unit volume weight, uniaxial compressive strength, tensile strength, shore hardness, point load strength and low porosity. Dependence of abrasion characteristics on each rock property investigated by regression analysis showed that high significant correlations exist between LAA and SH, UWV, UCS, PL and TS for sedimentary rocks, SH, UCS, PL and TS for metamorphic rocks and SH, UWV, AP, UCS, PL and TS for igneous rock. LAA rate can be easily predicted by using the empirical equations;
Los Angelos value= MO-mMO*100Where MOthe initial mass of the sample and m is the sum of mass; 1.6mm. (Yilmaz Ozcelik, 2011).
3) Toughness: Aggregates in the pavements are also subjected to impact due to moving wheel loads. Severe impact like hammering is quite common when heavily loaded steel tyred vehicles move on water bound macadam roads where stones protrude out especially after the monsoons. Jumping of the steel tyred wheels from one stone to another at different levels causes’ severe impact on the stones. The magnitude of impact would increase with the roughness of the road surface, the speed of the vehicle and other vehicular characteristics. The resistance to impact or toughness is hence another desirable property of aggregates. This characteristic is measured by impact value test. The aggregate impact value is a measure of resistance to sudden impact or shock, which may differ from its resistance to gradually applied compressive load. The aggregate impact value is determined using the aggregate impact test apparatus (BS 812-112:1990).
4) Durability: The stone used in the pavement construction should be durable and should resist disintegration due to the action of weather. The property of the stones to withstand the adverse action of weather may be called soundness. The aggregates are subjected to the physical and chemical action of rain and ground water, the impurities there-in and that of atmosphere. Hence it is desirable that the road stones used in the construction should be sound enough to withstand the weathering action. Soundness/Durability/ Accelerated weathering test is used to determine the durability or resistance to weathering.
5) Shape of aggregates: The size of the aggregates is first qualified by the size of square sieve opening through which an aggregate may pass, and not by shape. Aggregates which happen to fall in a particular size range may have rounded, cubical, angular flaky or elongated shape of particles. It is evident that the flaky and elongated particles will have less strength and durability when compared with cubical, angular or rounded particles of the same stone. Hence too flaky and too much elongated aggregates should be avoided as far as possible. The voids present in a compacted mix of coarse aggregates depend on the shape factors. Highly angular, flaky and elongated aggregates have more voids in comparison with rounded aggregates. Based on the shape of the aggregate particle, stones may be classified as rounded, angular, flaky and elongated. Flaky aggregates have lesser thickness when compared to the length and width. Elongated aggregates have one of the dimensions or the length higher than the width and thickness. The shape factors of aggregates depend on the source, properties of the rock and the type and condition of the crushing. The shape of aggregates is generally described in terms of its shape factors such as flakiness index, elongation index and angularity number. Several researchers have indicated that in pavement construction flaky and elongated aggregates are to be avoided, particularly in surface course. If flaky and elongated aggregates are present in appreciable proportions, the strength of the pavement layer would be adversely affected due to possibility of breaking down during compaction and under loads. (Vinod.B.R,). Flakiness index test is used to determine the particle shape of the aggregate and each particle shape being preferred under specific conditions. Flaky aggregate shape is considered unfavorable due to low strength, which lowers the strength of asphalts. (Prowell et al., 2005). A flaky particle is the one whose least dimension (thickness) is than 0.6 times the mean size. Flakiness index is the percentage by weight of flaky particles in a sample. Flakiness index is calculated using the equation below:
Flakiness index=Passing sample particle countRetained sample+passing sample*100%Elongation index is used to determine the elongation of aggregates. It is the percentage by weight of elongated particles in a sample. According to BS 812: Part 105.1, the presence of elongated particles in excess of 10-15% of the weight of coarse aggregate is considered undesirable. Elongated and flaky particles have a large surface area relative to its small volume. The Elongated index is calculated by expressing the weight of Elongated particles as percentage of total weight of the sample as shown in the equation below;
Elongation Index=Total weight of material retained on various length gaugesTotal weight of the sample taken*100%
6) Adhesion with Bitumen: The aggregates used in bituminous pavements should have less affinity with water when compared with bituminous material; otherwise the bituminous coating on the aggregates will be stripped off in presence of water. This is done with the Bitumen adhesion/Stripping Test to determine the Adhesion of bitumen. (Vinod.B.R).
7) Water Absorption: Strong aggregate will have a very low absorption value that is below 1.0 percent. The amount of water an aggregate can absorb tends to be an excellent indicator as to the strength or weakness of the aggregate. Therefore, the aggregate moisture content will affect the water content and the water content affects aggregate proportioning because it contribute to aggregate weight. (Ndukauba Egesi et al., 2012). Water absorption Test is used to measure the porosity of aggregates. Absorption is the increase in the mass of aggregates due to water being absorbed into the pores of aggregates, but it excludes water adhering to the outside surface of the particles, expressed as a percentages of the dry mass. (Senior, 1991). The water absorption of an aggregate is usually determined in conjunction with the specific gravity determination. The water absorption of an aggregate is determined by measuring the increase in weight of an over dried sample when immersed in water for 24 hours. (B.S.812:1967)
The water absorption is given by
Water Absorption = 100 (ww – wo) /woWhere Ww = weight of oven dried sample in air
Wo = weight of saturated sample in air
For use in surfacing airfield pavement, an aggregate should have a water absorption of less than 120. (TAN, 1983).
8) Specific gravity: Specific gravity of an aggregate is an important factor in the design of some engineering structures. The specific gravity is a useful index of rock quality and can indicate the degree of weathering or chemical alteration of the rock.
Specific gravity = Wo/ (Ww – Ws)
Where Wo = weight of oven dried sample in air.
Ww = weight of saturated sample in air
Ws = weight of saturated sample in water
Specific gravity is the ratio of the mass of an aggregate to the mass of an equal volume of water. There are several varieties of specific gravity that are used for relative to aggregates. Bulk specific gravity test are used for computations when aggregates are dry. Bulk specific gravity (Saturated surface dry or SSD) are used when aggregates are wet. Whereas apparent specific gravity is used to determine the solid material making up the constituent particles and ignores the pore spaces within the particles that is accessible to water. Specific gravity are expressed either as bulk specific gravity, saturated surface dry or apparent specific gravity and are calculated using the equation below In the equation below, A is the oven dry mass, B-SSD mass and C is the weight in water (AASHTO, 2005).
Bulk specific gravity=A(B-C) ————————————–Bulk specific gravity
Bulk specific gravity. SSD=BB-C———————————Bulk specific gravity, SSD
Apparent specific gravity=AA-C———————————Apparent Specific gravity
Tests on Road Aggregates and Properties Evaluated
SL No. Type of test Required property
1 Aggregate impact test Toughness or resistance to impact
2 Los Angeles Abrasion Test Hardness or resistance to abrasion
3 Aggregate Crushing Test Strength or resistance to crushing
4 Soundness/Durability/Accelerated weathering test Durability or resistance to weathering
5 Shape test: Flakiness Index, Elongation Index and Angularity Number Assessment of suitable shape or shape factors of coarse aggregates
6 Specific gravity Test
To measure the quality or strength of material.
7 Water absorption Test To measure the porosity
8 Bitumen adhesion/Stripping Test Adhesion of bitumen
9 Polished stone value test or accelerated polishing test. Resistance to getting smooth or polished
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