Sub-concrete technology

TOPIC- aggregate

SEMESTER- 6^TH

by

Er. Sasmita gochhayat

Hod,civil engineering

AY:2021-2022

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Concrete

• Concrete is basically a mixture of two components
• Paste (Portland cement, water, and air)
• Aggregate (sand, gravel, crushed stone)
• Aggregates make up 60-75% of total volume of concrete.

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What is an

AGGREGATE?

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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.

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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.

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Classification of Aggregates

• Aggregates are broadly classified as:
• Fine aggregates
• Coarse aggregates

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Fine Aggregates

• Particles of fine aggregates pass through 4.75mm(No.4) sieve .Most commonly used fine aggregates are sand, crushed stone, ash or cinder and surki.

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Coarse Aggregates

• Coarse aggregates are retained on 4.75mm(#4) sieve.
• Aggregates the size of whose particle is bigger than 4.75 mm but smaller than 7.5 mm are known as coarse aggregates.

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Uses of Aggregates

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

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Ultra-lightweight Aggregates

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Vermiculite

Perlite

Diatomite

Pumice

Scoria

Lightweight Aggregates

Expanded clay (left)

Expanded shale

(right)

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Crushed Brick

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Normal weight Aggregates

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River gravel

Crushed Concrete

Crushed Limestone

Heavyweight Aggregates

Magnetite (left)

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Magnetite-sand

(right)

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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

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Ranges of particle sizes found in aggregates for use in concrete

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Cracking of concrete from alkali silica reactivity

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Tests on Aggregates

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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

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Fine Aggregates grading limits

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Fineness Modulus�

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• 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.

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• 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.

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• Maximum size of aggregate: the smallest sieve that all of a particular aggregate must pass through.

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• Nominal maximum size of an aggregate: the smallest sieve size through which the major portion of the aggregate must pass (90%-100%).

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• Example: Aggregate size number 7 has a maximum size of 19 mm, and a nominal maximum size of 12.5 mm.

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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.

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• Aggregate tensile strengths range between 2 to 15 MPa, and compressive strengths range between 65 to 270 MPa.

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Particle Shape and Surface Texture

• The shape and surface texture affect the properties of fresh concrete more than the properties of hardened concrete.

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• 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.

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Bulk Density

• 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.

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• 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.

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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).

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• 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).

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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

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Moisture conditions of aggregate

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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.

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• Concrete containing calcareous coarse aggregates performs better under fire exposure than siliceous aggregate (granite or quartz).

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Thank

You..

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