AGGREGATES

Role of Aggregates in Concrete: Concrete is basically a mixture of two components Paste (Portland cement, water, and air) Aggregate (sand, gravel, crushed stone) The aggregate occupies 70-75% of the volume of concrete, so its quality is of great importance.

Aggregates may affect the following properties of concrete: Strength Durability Structural Performance Economy

Aggregates have 3 main functions in concrete: To provide a mass of particles which are suitable to resist the action of applied loads & show better durability then cement paste alone. To provide a relatively cheap filler for the cementing material. To reduce volume changes resulting from setting & hardening process & from moisture changes during drying.

The properties of concrete are affected by the properties of aggregate: The mineral character of aggregate affects the strength, durability, elasticity of concrete. The surface characteristics of aggregate affects the workability of fresh mass & the bond between the aggregate & cement paste in hardened concrete. If it is rough, workability decreases & bond increases. The grading of aggregate affects the workability, density & economy.

Aggregates: Aggregates are inert materials mixed with a binding material like cement or lime in the preparation of mortar or concrete. Granular material of mineral composition such as sand, gravel, shale, slag or crushed stone.

Uses of Aggregates: Filler material Dimensional Stability shrinkage, thermal changes Strength and Stiffness Economy To make the concrete denser

Classification of Aggregates: Natural Aggregates All natural aggregates particles originally formed a part of a larger parent mass. many properties of the aggregates depend entirely on the properties of the parent rock. E.g. chemical and mineral composition, petrological character, specific gravity etc. some properties are possessed by the aggregates but absent in the parent rock: particle shape and size, surface texture, and absorption.

Artificial Aggregates They are obtained either as a by-product or by a special manufacturing process such as heating. (blast furnace slag, expanded perlite).

Classification of Aggregates According to Petrological Characteristics: Igneous rocks: are formed by solidification of molten lava. (granite) Sedimentary rocks: are obtained by deposition of weathered & transported pre-existing rocks or solutions. (limestone) Metamorphic rocks: are formed under high heat & pressure alteration of either igneous & sedimentary rocks (marble).

Classification of Aggregates According to size: Fine aggregates Coarse aggregates

Fine Aggregates (d ≤ 5 mm) Particles of fine aggregates pass through 4.75mm(No.4) sieve .Most commonly used fine aggregates are sand, crushed stone, ash or cinder and surkhi.

Coarse Aggregates (d > 5 mm) Coarse aggregates are retained on 4.75mm sieve. Aggregates the size of whose particle is bigger than 4.75 mm but smaller than 37.5 mm are known as coarse aggregates. It specially includes gravel and crushed stones.

Classification of Aggregates According to Unit Weight: Heavy weight agg.: Hematite, Magnetite Specific Gravity, Gs > 2.8 Normal weight agg.:Gravel, sand, crushed stone 2.8 < Gs < 2.4 Light weight agg.:Expanded perlite, burned clay Gs < 2.4

Examples of Aggregates Used Weight Examples of Aggregates Used Uses for the Concrete ultra-lightweight vermiculite, ceramic, diatomite, pumice, scoria, perlite, can be sawed or nailed, also used for its insulating properties (250 to 1450 kg/m3). lightweight expanded clay, shale or slate, crushed brick used primarily for making lightweight concrete for structures, also used for its insulating properties (1350 to 1850 kg/m3). normal weight crushed limestone, sand, river gravel, crushed recycled concrete used for normal concrete projects heavyweight barlite, magnetite , steel or iron shot; steel or iron pellets used for making high density concrete for shielding against nuclear radiation

Ultra-lightweight Aggregates Vermiculite Perlite Pumice Scoria Diatomite 16

Lightweight Aggregates Expanded clay (left) Expanded shale (right) Crushed Brick

Normal weight Aggregates River gravel Crushed Limestone Crushed Concrete

Heavyweight Aggregates Magnetite (left) Magnetite-sand (right)

Constituents in naturally occurring Aggregates: Naturally occurring concrete aggregates are a mixture of rocks and minerals Minerals Silica (ex. Quartz) Silicates (ex. Clay) Carbonate (ex. Calcite, dolomite) Igneous rocks Granite Basalt Sedimentary rocks Sandstone Limestone Shale Metamorphic rocks Marble slate

Ranges of particle sizes found in aggregates for use in concrete

PARTICLE SHAPE & SURFACE TEXTURE: In addition to petrological character, the external characteristics, i.e. The shape & surface texture of aggregates are of importance. Particle Shape Rounded: Completely water worn & fully shaped by attrition. (River Gravel) Irregular: Partly shaped by attrition so it contains some rounded edges. (Land Gravel)

Angular: Has sharp corners, show little evidence of wear Angular: Has sharp corners, show little evidence of wear. (Crushed Stone) Flaky: Thickness is relatively small with respect to two other dimensions. (Laminated Rocks) Elongated: Have lengths considerably larger than two other dimensions.

FLAT ELONGATED ANGULAR ROUND

Rounded aggregates are suitable to use in concrete because flaky & elongated particles reduce workability, increase water demand & reduce strength. In the case of angular particles, the bond between agg. Particles is higher due to interlocking but due to higher surface area, angular particles increase water demand & therefore reduce workability. As a result, for the same cement content & same workability rounded agg. Give higher strength.

Surface Texture This affects the bond to the cement paste & also influences the water demand of the mix. 1. Smooth: Bond b/w cement paste & agg is weak. 2. Rough: Bond b/w cement paste & agg. is strong. Surface texture is not a very important property from compressive strength point of view but agg. Having rough surface texture perform better under flexural & tensile stresses.

SMOOTH ROUGH

Grading of Aggregates: Grading is the particle-size distribution of an aggregate as determined by a sieve analysis using wire mesh sieves with square openings. According to ASTM Fine aggregate - 7 standard sieves with openings from 150 μm to 9.5 mm Coarse aggregate - 13 sieves with openings from 1.18 mm to 100 mm

IS Sieves 125 mm 100 mm 90 mm 75 mm Sieves for 63 mm Coarse 50 mm Aggregates 37.5 mm 25 mm 12.5 mm 9.5 mm 4.75 mm 2.38 mm 1.19 mm 0.595 mm 0.297 mm 0.149 mm IS Sieves 125 mm 90 mm Sieves for Fine 63 mm Aggregates 31.5 mm 16 mm 8 mm 4 mm 2 mm 1 mm 0.5 mm 0.25 mm

Dry agg. is sieved to prevent lumps. The material is sieved through a series of sieves that are placed one above the other in order of size with the largest sieve at the top. Dry agg. is sieved to prevent lumps. ***** Agg. #4 #8 #16 #30 #50 #100 Pan Sieve shaker Lateral & Vertical motion

The particle size distribution in an aggregate sample is known as “gradation”. Strength development of concrete depends on degree of compaction & workability together with many other factors. So, a satisfactory concrete should be compacted to max density with a reasonable work. On the other hand, in good concrete all aggregate particles must be covered by cement paste. The grading of aggregate must be so that the workability, density & volume stability of concrete may not be adversely affected by it.

A reasonable combination of fine & coarse aggregate must be used A reasonable combination of fine & coarse aggregate must be used. This can be expressed by maximum density or minimum voids concept.

Determination of the Grading of Aggregate: There are two different methods for determining the agg. grading: Fineness Modulus (FM) Granulometry The grading of the particles in an agg. sample is performed by “sieve analysis”. The sieve analysis is conducted by the use of “standard test sieves”. Test sieves have square openings & their designation correspond to the sizes of those openings.

Fineness Modulus (FM): FM is a single figure which is the sum of cumulative % retained on a series of sieves having a clear opening half that of the preceeding one. Usually determined for fine agg. FM = Σ (% cumulative retained on each sieve) 100

Physical Properties of Aggregates 1) Specific gravity: Specific gravity is the ratio of the weight of a unit volume of material to the Weight of the same volume of water at 20º to 25ºC. G = (Wt/V) / (Wtw/V) Where, G – Specific Gravity Wt – Weight of Material V – Volume Wtw – Weight of water

Specıfıc Gravıty of Agg. Sp.Gr.= Weight of Agg. (WA) Weight of an equal volume of water (VA*ρw) Sp.Gr. is used in certain computations for concrete mix design or control work, such as, absolute volume of aggregate in concrete. It is not a measure of the quality of aggregate.

2) Bulk Density: The bulk density or unit weight of an aggregate gives valuable information regarding the shape and grading of the aggregate. It is measured by filling a container of known volume in a standard manner and weighing it. It shows how densely the aggregate is packed when filled in a standard manner. It is depend on size and shape of aggregates. Higher the bulk density lower is the void content to be filled by sand and cement.

It is calculated by, Percentage voids = [(Gs - γ) / Gs] x 100 Where, Gs – Specific gravity of aggregate γ – Bulk density in kg/lit Reduction of Voids

3) Absorption & Moisture Condition of Aggregates: Some of the aggregates are porous and absorptive & it will affect water – cement ratio and hence workability of concrete & durability of concrete. The water absorption of aggregate is determined by measuring the increase in weight of an oven dry sample when immersed in water for 24 hrs. The ratio of the increase in wt to the wt of the dry sample expressed as % is known as absorption of agg. Agg. absorbs water in concrete & thus affects the workability & final volume of concrete.

Diagrammatic representation of Moisture in Aggregate

Porosity / Absorption of Aggregates: Porosity or permeability of aggregates and its absorption may affect the following factors: The bond between aggregate and cement paste Resistance to freezing & thawing of concrete Chemical stability Resistance to abrasion Specific gravity Yield of concrete for a given weight of agg.

4) Bulking of Aggregates: The free moisture content in fine agg. Results in bulking of volume. Free moisture forms a film around each particle. This film of moisture exerts which is known as surface tension which keeps every particles away from each other. Hence no point of contact is possible between the particles & this causes the bulking of the volume. The bulking increases with increase in moisture content upto a certain limit & beyond that the further increase in moisture content results in decrease in volume.

& at a moisture content representing saturation point, the fine agg & at a moisture content representing saturation point, the fine agg. Shows no bulking. The following graph shows fine sand bulks more and coarse sand bulks less. Percentage of bulking = [(h1 – h2) / h2] x 100 Bulking may affect the yield of concrete for a given cement content.

Surface Moisture on fine aggregate

Measurement of Moisture Content of Aggregates: Determination of this is of vital importance in the control of the quality of concrete with respect to workability and strength. The water content can be expressed in terms of the wt of the agg. when absolutely dry, surface dry or when wet. Water content means the free water that held on the surface of the agg. It can be measured by- Drying Method, Displacement Method, Automatic Measurement, & by Electrical method.

Harmful material in aggregates:

Cracking of concrete from alkali silica reactivity

Tests on Aggregates

Grading Proper selection of various sizes will be very effective in reducing the total volume of voids between aggregates. The cement paste requirement is related the void content of the combined aggregates. Production of satisfactory; economical concrete requires aggregates of low void content, but not the lowest.

Grading Grading is the distribution of particles among various sizes. Grading is usually expressed in terms of cumulative percentage passing each sieve. Different standards and specifications specify grading limits for both fine and coarse aggregates. There are several reasons for specifying grading limits, they affect: Cement and water requirement Workability Economy Shrinkage and durability of concrete

Fine Aggregates grading limits

Fineness Modulus (ASTM C 125) The FM is an index of the fineness of the aggregate. The higher the FM, the coarser the aggregate. FM of fine aggregate is useful in estimating proportions of fine and coarse aggregate in concrete mixtures. The fineness modulus (FM) for both fine and coarse aggregates is obtained by adding the cumulative percentages by mass retained on each of a specified series of sieves and dividing the sum by 100.

Maximum size of aggregate: the smallest sieve that all of a particular aggregate must pass through. Nominal maximum size of an aggregate: the smallest sieve size through which the major portion of the aggregate must pass (90%-100%). Example: Aggregate size number 7 has a maximum size of 19 mm, and a nominal maximum size of 12.5 mm.

Strength Generally, strength of aggregate does not influence the strength of conventional concrete as much as the strength of the paste and the paste-aggregate strength. However, aggregate strength becomes important in high strength concrete. Aggregate tensile strengths range between 2 to 15 MPa, and compressive strengths range between 65 to 270 MPa.

Particle Shape and Surface Texture The shape and surface texture affect the properties of fresh concrete more than the properties of hardened concrete. Rough-texture, and angular particles require more water to produce workable concrete than do smooth, rounded and compact particles. For both crushed or noncrushed aggregate, proper gradation gives the same strength for the same cement factor.

Bulk Density (ASTM C 29) Defined as the weight of the aggregate particles that would fill a unit volume. The term bulk is used since the volume is occupied by both the aggregates and voids. The typical bulk density used in making normal concrete ranges from 1200 to 1750 kg/m3. The void contents range between 30% to 45% for coarse aggregate and 40% to 50% for fine aggregate. Void content increases with angularity and decreases with well graded aggregate.

Relative Density (Specific Gravity) The relative density of an aggregate (ASTM C 127 and C 128) is defined as the ratio of its mass to the mass of an equal volume of water. It is used in certain computations for mixture proportioning and control. Most natural aggregates have relative densities between 2.4 and 2.9 (2400 and 2900 kg/ m3). The density of aggregate used in mixture proportioning computations (not including the voids between particles) is determined by multiplying the relative density of the aggregate times the density of water (1000 kg/m3).

Absorption and Surface Moisture The absorption and surface moisture of aggregates should be determined using ASTM C 70, C 127, C 128, and C 566 so that the total water content of the concrete can be controlled and the batch weights determined. The moisture conditions of aggregates are: Oven dry Air dry Saturated surface dry (SSD) Damp or wet

Fire Resistance and Thermal Properties The fire resistance and thermal properties of concrete depend on the mineral constituents of the aggregates. Lightweight aggregates are more fire resistance than normal weight aggregates due to their insulation properties. Concrete containing calcareous coarse aggregates performs better under fire exposure than siliceous aggregate (granite or quartz).