function and importance of aggregate in concrete

17
FUNCTIONS AND IMPORTANCE OF AGGREGATE IN CONCRETE APPLICATIONS Vinod Kumar Singh Co-founder, www.Happho.com www.happho.com is an Online Marketplace for Construction Material & One Stop Solution for Bungalow Construction & Interiors

Upload: happho-care

Post on 13-Apr-2017

705 views

Category:

Engineering


4 download

TRANSCRIPT

Page 1: Function and Importance of Aggregate in Concrete

FUNCTIONS AND IMPORTANCE OF AGGREGATE IN CONCRETE

APPLICATIONS

Vinod Kumar SinghCo-founder, www.Happho.com

www.happho.com is an Online Marketplace for Construction Material & One Stop Solution for Bungalow Construction & Interiors

Page 2: Function and Importance of Aggregate in Concrete

CONCRETEBasic Definition:Concrete is a composite material that essentially consists of a binding medium

embedded with Fine-Aggregate (typically sand) and Coarse Aggregate (typically gravel) with or without chemical and mineral admixture and filler.

• Constituents:Mixture of aggregate and paste

• Cement paste - 30 to 40%– Portland cement + Mineral Admixture 7% to 15% by Vol.– Water + Chemical Admixture 14% to 21% by Vol.

• Aggregates - 60% to 70%– coarse aggregates– Fine aggregates

Page 3: Function and Importance of Aggregate in Concrete

Typical Concrete Proportion by Volume:

CONCRETE

Page 4: Function and Importance of Aggregate in Concrete

AGGREGATE• Aggregate is relatively inexpensive and does not enter into complex chemical

reactions with water; it has been customary, therefore, to treat it as an inert filler in concrete.

• However, due to increasing awareness of the role played by aggregates in determining many important properties of concrete, the traditional view of the aggregate as an inert filler is being seriously questioned.

• It is true that aggregate strength is usually not a factor in normal concrete strength because, with the exception of lightweight aggregates, the aggregate particle is several times stronger than the matrix and the interfacial transition zone in concrete. In other words, with most natural aggregates the strength of the aggregate is hardly utilized because the failure is determined by the other two phases.

• There are, however, aggregate characteristics other than strength, such as the size, shape, surface texture, grading (particle size distribution), and mineralogy which are known to affect concrete strength in varying degrees.

Page 5: Function and Importance of Aggregate in Concrete

BASIC FUNCTIONS OF AGGREGATE • Cheap fillers

• Hard material

• Provide for volume stability

• Reduce volume changes

• Provide abrasion resistance

It is inappropriate to treat the aggregate with any less respect than cement.

Page 6: Function and Importance of Aggregate in Concrete

NOMENCLATURE & CLASSIFICATIONAggregates are generally classified according to particle size, bulk density, or source sieve).

• Coarse aggregate is used to describe particles larger than 4.75 mm, and the term fine aggregate is used for particles smaller than 4.75mm; typically, fine aggregates contain particles in the size range 75 µm to 4.75 mm, and coarse aggregates from 4.75 to about 50 mm, except for mass concrete which may contain particles up to 150 mm.

• Most natural mineral aggregates, such as sand and gravel, have a bulk density of 1520 to 1680 kg/m3and produce normal-weight concrete with approximately 2400 kg/m3 unit weight.

For special needs, aggregates with lighter or heavier density can be used to make correspondingly lightweight and heavyweight concretes. Generally, the aggregates with bulk densities less than 1120kg/m3 are called lightweight and those weighing more than 2080 kg/m3 are called heavyweight

For the most part, concrete aggregates are comprised of sand, gravel, and crushed rock derived from natural sources and, therefore, are referred to as natural mineral aggregates ( are further classified as granite, limestone, basalt etc. depending upon their parent rock source)

On the other hand, thermally processed materials such as expanded clay and shale, which are used for making lightweight concrete, are called synthetic aggregates. Aggregates made from industrial by products, for instance, blast-furnace slag and fly ash, also belong to this category.

Page 7: Function and Importance of Aggregate in Concrete

AGGREGATE CHARACTERISTICS AND THEIR SIGNIFICANCE

Generally, aggregate properties affect not only the concrete mixture proportions but also the behavior of fresh and hardened concrete. Due to considerable overlap between the two, it is more appropriate to divide the study of aggregate properties into three categories that are based on microstructural and processing factors.

•Characteristics dependent on porosity: density, moisture absorption, strength, hardness, elastic modulus, and soundness

• Characteristics dependent on prior exposure and processing factors: particle size, shape, and texture

• Characteristics dependent on chemical and mineralogical composition: strength, hardness, elastic modulus, and deleterious substances present

•A knowledge of certain aggregate characteristics (i.e., density, grading, and moisture state) is required for proportioning concrete mixtures. Porosity or density, grading, shape, and surface texture determine the properties of plastic concrete mixtures.

•The mineralogical composition of aggregate affects its crushing strength, hardness, elastic modulus, and soundness which, in turn, influence various properties of hardened concrete containing the aggregate.

Page 8: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIES• Absorption capacity is defined as the total amount of moisture required to

bring an aggregate from the oven-dry to the SSD condition.

• SSD condition :When all the permeable pores are full and there is no water film on the surface, the aggregate is said to be in the saturated-surface dry condition (SSD).

• Surface Moisture: The amount of water in excess of the water required for the SSD condition is referred to as the surface moisture.

• Specific Gravity :defined as the density of the material including the internal pores.

• Bulk Density :defined as the mass of the aggregate fragments that would fill a unit volume.

Page 9: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIESSoundness:An aggregate is considered unsound when the volume changes in aggregate

induced by weather (e.g., alternate cycles of wetting and drying, or freezing and thawing) ,result in the deterioration of concrete.

IS limit:• Fine Aggregate = 10% (weight loss of five cycles with Na2SO4)• Fine Aggregate = 15% (weight loss of five cycles with MgSO4)• Coarse Aggregate = 12% (weight loss of five cycles with Na2SO4)• Coarse Aggregate = 18% (weight loss of five cycles with MgSO4)

Shape:• Flakiness Index :Thickness being 0.6 times their mean dimension, contributes

more surface area for a unit volume occupied.

• Elongation Index :Greatest dimension being 1.8 times their mean dimension, contributes more surface area for a unit volume occupied.

Page 10: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIESShape:• Flakiness Index :Thickness being 0.6 times their mean dimension, contributes

more surface area for a unit volume occupied.

Flakiness Index Apparatus Flaky Aggregate

Page 11: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIESShape:• Elongation Index :Greatest dimension being 1.8 times their mean dimension,

contributes more surface area for a unit volume occupied.

Elongation Index Apparatus Elongated Aggregate

Page 12: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIESIdeal Aggregate:

Neither Elongated Nor Flaky

Page 13: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIES• Mechanical Properties: Crushing strength, impact value abrasion resistance, and elastic modulus of aggregate

are interrelated properties, that are greatly influenced by porosity. Aggregates from natural sources that are commonly used for making normal-weight concrete, are generally dense and strong; therefore they are seldom a limiting factor to strength and elastic properties of concrete.

Indian Standard (IS) limit:• Crushing and Impact Value :

Wearing surface = 30% & Non-wearing surface = 45%.• Abrasion Resistance :

Wearing surface = 30% & Non-wearing surface = 50%.

• Fineness Modulus: Empirical factor called the fineness modulus is often used as an index of the fineness of aggregate.

The fineness modulus is computed from screen analysis data by adding the cumulative percentages of aggregate retained on each of a specified series of sieves, and dividing the sum by 100. The sieves used for determining the fineness modulus are: No. 100 (150 µm), No. 50 (300 µm), No. 30 (600 µm), No. 16 (1.18 mm), No. 8 (2.36 mm), No. 4 (4.75 mm), 10 mm,20mm,40mm etc.

• Slit Content :Material finer than 75-µm (No. 200) sieve are generally called slit. They affect the workability as water demand increases, strength is also influenced along with bonding. IS limit is 3% by weight.

Page 14: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIESSize and Grading :• Grading is the distribution of particles of a granular material among various size ranges, usually

expressed in terms of cumulative percentage larger or smaller than each of a series of sizes of sieve openings, or the percentage between certain range of sieve openings.

• Size: The maximum size of aggregate is conventionally designated by the sieve size on which 15 percent or more particles are retained. In general, the larger the maximum aggregate size, the smaller will be the surface area per unit volume which has to be covered by the cement paste of a given water-cement ratio.

Since the price of cement may be 10 to 15 times as much as the price of aggregate, any action that saves cement without reducing the strength and workability of concrete can result insignificant economic benefit

• There are several reasons for specifying grading limits and maximum aggregate size, the most important being their influence on workability and cost.

For example, very coarse sands produce harsh and unworkable concrete mixtures, and very fine sands increase the water requirement (therefore, the cement requirement for a given water-cement ratio) and are uneconomical.

• Aggregates that do not have a large deficiency or excess of any particular size produce the most workable and economical concrete mixtures.

Page 15: Function and Importance of Aggregate in Concrete

AGGREGATE PROPERTIES

IS Sieve Size (mm)

WeightRetained (gms)

Cum.WeightRetained

(gms)

%Retaine

d

%Passin

g

10 0 0 0 1004.75 120 120 4 962.36 450 570 19 811.18 390 960 32 68

0.600 870 1830 61 390.300 750 2580 86 140.150 360 2940 98 2Pan 60 3000 - -

Fineness Modulus = Col.04/100 = 300/100 = 3As per our experience Fine Aggregate with F.M of 2.7 to 3.0 are best suited concrete application

Fineness Modulus (F.M) solved example :

Page 16: Function and Importance of Aggregate in Concrete

CONCLUDING REMARKS• Aggregate primarily acts as a inert filler, but has secondary influences on various concrete

properties.

• Awareness about the role played by aggregate in concrete can be instrumental in exploiting the use of the same in achieving concrete properties as per intended requirements, which would be of high performance and economical.

• It is inappropriate to treat the aggregate with any less respect than cement.

Page 17: Function and Importance of Aggregate in Concrete

Buy Good Quality Non-Elongated, No Flaky Aggregate anywhere in India at www.happho.com for your Construction Needs

THANK YOU