CMT Lesson 4

AGGREGATES 

Aggregates 

  • coarse particulate rock-like material 

  • the disintegration of the hard stratum of the rock.

  • derived from igneous, sedimentary, and metamorphic rocks or manufactured from blast

furnace slag, etc.

  • inert materials which are mixed with binding material such as cement or lime for manufacturing of mortar or concrete

  • form the body of the concrete, reducing

the shrinkage.

  • 60% to 75% of the total mass of concrete

  • materials used as filler with blinding material in the production of concrete and mortar

  • Occupy 70-80% of the volume


Derived from:

  • Igneous

  • Sedimentary 

  • Metamorphic


2 types:

  • Coarse aggregate - bigger ones (grit)

  • Fine aggregate - smaller ones (sand)


3 Group of Aggregates

  1. Primary Aggregates

  • naturally occurring deposits of minerals, such as crushed rocks, sand, and gravel.

  • used for home improvements, landscaping projects, and for creating buildings and structures.


  1. Secondary Aggregates 

  • by-products of processes such as quarrying or mining,

  • produced through industrial processes.


  1. Recycled Aggregates

  • Neither primary nor secondary aggregates, have been used before, 

  • used for a purpose but have been collected through deconstruction and have been crushed and processed.




Classification (Based on Density)

1. Normal - Density Aggregates

  • natural materials used for making concrete. 

  • gravels, igneous rocks like basalt & granite.

  • Stronger sedimentary rocks such as limestone and sandstone.

  • have relative densities within a limited range of approximately 2.55–2.75.

  • produced concretes normally ranges 2250–2450 kg/m3, depending on mix proportions.

2. Lightweight Aggregates 

  • The most important aspect is porosity. 

  • reduced relative density is obtained from air voids within the aggregate particles.

  • Eg, (dolomite, pumice perlite)

3. Heavyweight Aggregates

  • can be used where concrete of high density is required, such as in radiation shielding.

  • may be made with high-density ores such as barytes and haematite, or manufactured, such as steel shot.


Classification of  Aggregate (Geological Origin)

  • Natural Aggregates - obtained by crushing from quarries of igneous, sedimentary or metamorphic rocks

  • Artificial Aggregates - broken bricks, furnace slag, and synthetic aggregates


Classification (Based on Size)

Coarse Aggregate

  • Coarse aggregates measure above the 4.75mm limit.

  • It shall easily fit into the forms and in between reinforcing bars

  • Dolomite aggregates, crushed gravel or stone, and the natural disintegration of rock are the major sources of coarse aggregate.

All-in Aggregate 

  • Naturally available aggregates of different fractions of fine and coarse sizes


Graded Aggregate

  • Aggregate most of which passes through a particular size of sieve


Fine Aggregate

  • The size of fine aggregates is defined as 4.75mm or smaller.

  •  River sand or machine sand, crushed stone sand, and crushed gravel sand are the major sources of fine aggregate.


Classification (Based on Shape)


Rounded Aggregate

  • from river or sea shore 

  • unsuitable for high strength concrete & pavements because of poor interlocking behavior and weak bond strength

  • Rock, stone, sand & gravel. 


Irregular Aggregates

  • develop good bond and suitable for making ordinary concrete

  • Small stones & gravel.

  • Offer reduced workability to rounded aggregates.

  • result in 35- 37% of voids 

  • bond strength is slightly higher than rounded aggregates but not as required for high strength concrete


Angular Aggregates

  • has sharp, angular & rough particles, provide very good bond and suitable for high strength concrete and pavements.

  • comes in the form of crushed rock and stones

  • give 10-20% more compressive strength 

  • useful in high strength concrete manufacturing


Flaky Aggregate

  • aggregate thickness is small when compared with width and length

  • least dimension is less than the 60% of its mean dimension


Elongated 

  • longer than they are thick or wide.

  • adds more surface area to a mix.


Flaky and Elongated 

  • A mix of the previous two– and the least efficient form of aggregate with regards to workability.


CHARACTERISTICS OF AGGREGATES


Strength

  • Should be at least equal to that of the concrete


Stiffness

  • modulus of elasticity of concrete is approximately equal to the weighted average of the moduli of the cement paste ad the aggregate

Bond Strength

State of internal stress even if no external forces are present

Shape and Texture

  • Influences the properties of fresh concrete more than when it has hardened

Specific Gravity 

  • Greatly influence the strength and absorption of the concrete

Bulk Density

  • Depend upon their packing, the particles shape and size.

Void

  • If it is more in the concrete, the strength will  be low

Porosity

  • Pores - entrapped bubbles in the rocks during their formation lead to a minute holes or cavities

  • Generally less than 20%

Moisture content 

  • Expressed as a percentage of the weight of the saturated surface dry aggregate 

Bulking

  • Increase in the volume of given mass of fine aggregate caused by the presence of water

Fineness Modulus

  • It is the numerical index of fineness, giving some ideas about the mean size of the particles in the aggregates


Soundness 

  • Id defined as the ability of aggregate to resist changes in volume as a result of changes in physical conditions

Thermal Properties of Aggregate

  • Specific heat

  • Thermal conductivity

  • Coefficient of expansion


TEST CARRIED OUT ON AGGREGATES:


SPECIFIC GRAVITY & WATER ABSORPTION

Specific Gravity

  • quantifies the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water 

  • bulk density or specific weight (unit weight) of an aggregate is defined as the weight of the aggregate particles that would fill a unit volume.

  • The typical bulk density used in making normal concrete ranges from 1200 to 1750 kg/m^3

  • void contents range between 30% to 45% for coarse aggregate and 40% to 50% for fine aggregate. Void content increases with

angularity decreases with well-graded aggregate.

  • indicator of the strength and quality 

  • how robust and resilient the material 

  • aggregates with lower specific gravity values = weaker.


Bulk Density of Aggregates Depends Upon 3 Factors

  1. Degree of compaction

  2. Grading of aggregates

  3. Shape of aggregate particles


Water Absorption

  • gives an idea on the internal structure of aggregate 

  • aggregates having more absorption are more porous in nature & considered unsuitable

  • The moisture content of the aggregates determines whether the aggregate will effectively add water to the concrete mix or whether the aggregates will take water away from the mix.


3 Quantities Used in Water Absorption: 

  1. Absorption Capacity (AC)

  • maximum amount of water the aggregate will absorb. The range for most normal-weight aggregates is 1 - 2%.


  1. Effective Absorption (EA)

  • amount of water required to bring an aggregate from the air dry state to the saturated-surface-dry state.


  1. Surface Moisture (SM) 

  • amount of water in excess of SSD.

  • represents water in excess of the SSD state,


Bulk Density 

  • UNIT WEIGHT of an aggregate is the mass or weight of the aggregate that is required to fill a container of a specified unit volume.

  • Mass of the aggregate over the volume of aggregate particles with voids. 


Relative Density 

  • relative density or SPECIFIC GRAVITY of an aggregate is the ratio of its mass to the mass of an equal volume of water.

  • Most aggregates have a relative density between 2.4-2.9 with a corresponding particle (mass) density of 2400-2900 kg/m3 (150-181 lb/ft3).

  • Mass of the aggregate over the mass of equal volume of water. 


Apparatus Required

  • Wire Mesh Bucket

  • Setup of Water Absorption Test


ABRASION


Hardness Property 

  • determined by conducting abrasion test


Los Angeles Abrasion Test

  • to find the percentage wear due to relative rubbing action between the aggregate and steel balls used engineering play a

  • pivotal role in assessing the durability and wear resistance of materials used in construction, as abrasive charge


Apparatus for Los Angeles Abrasion Test

  • Los Angeles abrasion machine.

  • Abrasive charge balls: These are made of cast iron or steel, with a diameter of approximately 48mm, and each ball weighs between 390 to 445 grams; six to twelve balls are necessary.

  • Sieves: Including IS Sieves with the following mesh sizes - 1.70mm, 2.36mm, 4.75mm, 6.3mm, 10mm, 12.5mm, 20mm, 25mm, 40mm, 50mm, 63mm, and 80mm.

  • A balance with a capacity of 5 kg or 10 kg.

  • Oven for drying.

  • Miscellaneous elements such as a tray.


Apparatus Required

  • Balance

  • Sieves

  • Los Angeles Testing

  • Abrasive Charges


DENSITY AND UNIFORMITY


Density 

  • ratio of the mass of aggregate to the mass of an equal absolute volume of water (the measure of the compactness)

  • inversely proportional to void content

  • directly proportional to strength


Uniformity

  • the consistency in size, shape, and gradation of aggregate particles


Fineness Modulus (FM)

  • Duff Abrams material researcher, defined fineness modulus in 1918 as a way of characterizing concrete aggregates, simplifying the gradation curve, and estimating the correct proportions to use in mix designs.

  • used in determining the degree of uniformity of the aggregate gradation.

  • The higher the FM is, the coarser the aggregate is.

  • defined as the sum of the cumulative percentages retained on specified sieves divided by 100.

  • describes the average size of particles in a sample of aggregate.

  • Fineness modulus is not a precise value, as it provides a way to estimate the amount of aggregate needed to proportion mix designs.


Apparatus Required

  • Wire Mesh Bucket

  • Setup of Water Absorption Test

  • Pycnometer