49

Properties and

Testing of AggregatesUNIT 3 PROPERTIES AND TESTING OF AGGREGATES

Structure 3.1 Introduction Objectives

3.2 Properties of Aggregates 3.2.1 Classification of Aggregates 3.2.2 Density and Specific Gravity 3.2.3 Water Absorption and Surface Moisture 3.2.4 Bulking of Sand 3.2.5 Soundness of Aggregate

3.3 Flakiness and Elongation Index 3.3.1 Flakiness Index 3.3.2 Elongation Index

3.4 Sieve Analysis, Fineness Modulus and Grading 3.4.1 Sieve Analysis 3.4.2 Fineness Modulus 3.4.3 Grading

3.5 Mechanical Properties of Aggregates 3.5.1 Aggregate Crushing Value (ACV) 3.5.2 Aggregate Ten Percent Fine Value (TPFV) 3.5.3 Aggregate Abrasion Value (AAV) 3.5.4 Aggregate Impact Value (AIV)

3.6 Effect of Aggregate Properties on Strength of Concrete 3.6.1 Maximum Size of Aggregate 3.6.2 Shape/Texture of Aggregate 3.6.3 Soundness of Aggregate

3.7 Summary 3.8 Answers to SAQs

3.1 INTRODUCTION

Aggregates are very important ingredients of concrete due to the following reasons :

(a) They provide body and strength to the concrete (role of aggregates is similar to that of bones in human body).

(b) They occupy about 75% volume of concrete and being comparatively cheaper as compared to the other ingredients, they govern economy of concrete.

(c) Not only the strength of concrete but also other properties are governed by aggregates such as durability, workability, shrinkage, volume, stability, etc.

Earlier it was believed that aggregates are inert material but later on it was established that some of the aggregates (depending upon their chemical composition) are chemically active. Therefore, it is necessary to study each and

50

Concrete Technology

every property of aggregates namely, size, shape, grading, surface texture, specific gravity, density, impact and crushing strength, abrasion value, soundness and chemical composition.

Aggregates, used in concrete, can be natural or artificial type and are broadly classified as coarse aggregates (size > 4.75 mm) and fine aggregates (size < 4.75 mm). The coarse aggregates, locally known as ‘gitti’, are generally available in 80, 40, 20, 10 and 4.75 mm sizes. The fine aggregates (i.e. sand) popularly known as ‘ret’ or ‘bajri’ can be conveniently grouped into coarse, medium and fine sand.

In this unit, various important properties of aggregates namely, water absorption, density and specific gravity, soundness, crushing strength, abrasion resistance, impact strength, grading, etc. and the various tests to determine the same have been described in brief. Further, the effect of these properties of aggregates on the strength of concrete has also been discussed.

Objectives After studying this unit, you should be able to

• explain the importance of various properties of aggregates in manufacturing the concrete.

• get acquainted with some of the related technical terms in defining these properties of aggregates like, bulking of sand, saturated surface dry (SSD) condition, flakiness and elongation index, fineness modulus, alkali-aggregate reaction, etc., and

• determine these properties in laboratories or at site.

[Note : However, for actually carrying out these tests, you are advised to go through the test procedures and other related details/recommendations by referring to the relevant IS Codes/Lab Manuals.]

3.2 PROPERTIES OF AGGREGATES

Aggregates available for manufacturing of concrete can be broadly put under two categories, i.e. Natural Aggregates and Artificial Aggregates.

Natural aggregates are those which are available naturally (e.g. sand in the river bed or sea shore, etc.) or are manufactured from the naturally available rocks/stone by manipulating their sizes, shape and surface texture only and other properties remain unchanged.

Artificial aggregates are those, which are manufactured artificially (e.g. burnt clay, colored ceramic aggregates) or are obtained as byproduct/waste of industries (e.g. blast furnace slag).

3.2.1 Classification of Aggregates Based on the source, size, shape and weight the aggregates are further classified as given in the subsequent paragraphs.

Source of Aggregate

Based on the source of material of natural aggregates they are classified as :

51

Properties and

Testing of Aggregates(a) Igneous rocks, (b) Sedimentary rocks, and (c) Metamorphic rocks.

The chemical and other properties of such aggregates are mainly governed by the properties of their parent rocks. Comments

Generally, people prefer the locally available aggregates for normal concrete works because transportation cost of the aggregate matters a lot. Igneous rocks are widely available on the face of the earth and are hard, tough and dense. They could be acidic or basic in nature depending upon the percentage of silica content. Highly laminated sedimentary rocks may lead to flaky or elongated aggregates.

Size of Aggregate Based on the size, the aggregates can be classified as coarse aggregates and fine aggregates (i.e. sand). The aggregates of size > 4.75 mm are called as coarse aggregates and are available in 80, 40, 20, 10, 4.75 mm sizes. The fine aggregates are those which are having particles of sizes < 4.75 mm. Depending upon the fineness modulus (discussed further in this unit) the fine aggregates are categorized as coarse, medium and fine sand. Comments

Using large size aggregates in concrete will result into reduction of cement and water requirement and will also reduce the degree of shrinkage of concrete. Smaller size of coarse aggregates is preferred for the concrete used for thin members, heavily reinforced member and for high strength concrete. The concrete with large size of coarse aggregates is preferred for mass concrete.

Shape of Aggregate Based on the shape, the aggregates are classified as Rounded, Angular, Flaky and Elongated aggregates. The rounded aggregates are without edges or corners and are smooth as compared to angular aggregates, which can be identified by their well-defined edges and corners. The surface area of rounded aggregates is comparatively smaller. Flaky aggregate is that whose least dimension (i.e. thickness) is reasonably small relative to its other two dimensions (i.e. width and/or length), whereas elongated aggregate is that whose greatest dimension (i.e. length) is reasonably large relative to its mean dimension. Flaky and elongated aggregates are explained in detail later. Comments

Rounded aggregates provide good workability, as they require less amount of cement for the same w/c ratio but they lead to weak bond with the cement paste as compared to angular aggregates. Angular aggregates give better durability, better bond and higher strength but to achieve desired workability these aggregates require more cement paste. The presence or use of flaky and elongated aggregates should be discouraged as they adversely effect the workability, cement requirement, strength and durability, which finally lead to poor quality concrete.

Unit Weight of Aggregate

52

On the basis of unit weight (i.e. density) the aggregates are classified as normal-weight, lightweight and heavyweight aggregates. The criteria for classifying them into these categories is shown in Table below.

Concrete Technology

Type of Aggregate Unit Weight Example

Normal-weight 1520-1680 kg/m3 Most of the natural aggregates

Light-weight < 1120 kg/m3 Synthetic aggregates

Heavy-weight > 2080 kg/m3 Aggregates obtained from barium-minerals, iron and titanium ores, etc.

[Note : Unit weight of concrete is 2400-2500 kg/m3.]

Comments Special care and strict quality control is exercised for concreting with light and heavy weight aggregates because use of such aggregates may adversely effect workability and have tendency of segregation. Concrete with light-weight aggregates is used for reducing the self weight of the structure particularly in the case of high rise buildings. Light-weight aggregates being porous and weak, are more suitable for nonstructural elements. Concrete with heavy-weight aggregates is used for nuclear radiation shield or high strength concrete.

3.2.2 Density and Specific Gravity Bulk Density

The density of aggregate-material and the density of aggregate-sample are different in the sense that latter is defined as the weight of the sample of aggregates that would fill a unit volume in a standard manner. The sample of aggregates always has the volume occupied by voids and the solid aggregate. That is why the density of aggregate sample is designated as bulk density of aggregate. It depends on how densely the aggregates are packed which further depend on the shape, size, grading and moisture content of the aggregates. So, higher bulk density indicates lesser voids.

Apparent Specific Gravity

The specific gravity of an aggregate in its absolute sense is the ratio of the weight of the solid in a given volume of sample to the weight of an equal void-free volume of water at the same temperature. Due to the presence of internal voids in the aggregates specific gravity of aggregate is more conveniently designated as apparent specific gravity. It is defined as ratio of the weight of oven dried aggregate to the weight of water occupying the volume of aggregates (including the volume of internal voids). It is determined in the laboratory by the following formula :

Apparent Specific Gravity =−c

c b

where, c = Weight of aggregates dried in an oven at 100º-110ºC for 24 hours, and

b = Weight of saturated surface dry aggregates in water.

Saturated Surface Dry (SSD) Condition

53

Properties and

Testing of AggregatesSaturated surface dry condition means the aggregate is saturated and there is no water film (moisture) on the surface of aggregate. The specific gravity based on the SSD condition is most frequently used in concrete technology. Because the water present in the internal pores of the aggregate does not contribute in the hydration process so it can be considered as part of the aggregate.

Comments

The bulk density is used to convert the weight proportion into the volume proportion or vice-versa and to judge the quality and grading of aggregates by comparing it with the normal density of that type of aggregates. It is also used for determining the void ratio.

Bulk densityVoid RatioApparent specific gravity

=

The range of average specific gravity for many commonly available rocks is 2.6 to 2.7. Any variation in the specific gravity indicates the variation in shape/grading of aggregate. It is useful in determining the percentage voids of aggregates. It also helps in converting the weight of aggregates into solid volume and also used for determining the compacting factor in workability test.

3.2.3 Water Absorption and Surface Moisture Water Absorption

All aggregates have permeable pores of varying sizes over a wide range, which causes porosity. The porosity varies from 0 to 20 percent for commonly available rocks. The water absorption of aggregate is the percentage of water absorbed by an aggregate after immersing it into the water in standard manner for 24 hours and then bringing it to saturated surface dry (SSD) condition. The average range of water absorption for commonly available aggregates is 0.5-1.0 percent.

Free Moisture Content or Surface Moisture

Free moisture content is the quantity of water, present in an aggregate sample, in excess of the saturated surface dry (SSD) condition. This is also termed as free moisture, which is readily available for hydration process. Necessary correction in measuring the quantity of water must be done during batching so as to maintain the desired water cement ratio.

Comments

It is necessary to know the free moisture content of aggregate at site. In the calculation of mix design the weights of aggregates are based on the SSD condition, but in practice this SSD condition is very rare. In fact, the actual moisture content of aggregates is mostly either more than or less than that required for SSD condition. So, it is necessary to make the necessary correction in the weight of aggregates and also in the quantity of water, so that the desired proportion of all the ingredients and required water cement ratio could be maintained as per the mix-design. Any variation in the quantity of water will lead to weak and poor quality concrete.

54

3.2.4 Bulking of Sand Concrete Technology

The moisture content of fine aggregates is also equally important. The fine aggregates in the damp condition may be subjected to bulking phenomenon. Increase in volume of a given weight of sand (fine aggregate) caused by the presence of moisture (within certain limit) is termed as bulking. The presence of moisture forms a film of water around the sand particles due to surface tension, which pushes the sand particles apart and volume of sand increases. The volume of sand goes on increasing with the increase in moisture content but only up to a certain limit, i.e. 5-6 percent. Beyond this, the surface tension reduces to the extent that the water film is destroyed. The sand particles loose the repulsive action and volume starts reducing.

The bulking phenomenon of fine aggregates is explained in Figure 3.1. The figure clearly indicates that the finer sand bulks considerably more as compared to coarse and medium sand. The bulking phenomenon is almost absent in the case of coarse aggregates as they are larger in size and heavy in weight.

Figure 3.1 : Bulking Phenomenon

Comments

The range of bulking for the commonly available sand is 15-30 percent. The bulking plays an important role in volume batching of fine aggregates for manufacturing of concrete (see Unit 5). If the necessary correction to compensate the bulking is not made, such concrete mix will be under-sanded, because the measured volume of bulked sand is greater than the actual volume of the sand.

3.2.5 Soundness of Aggregate Soundness of aggregate is the resistance of aggregate to disintegration and to change in volume with the change in the environmental/physical condition such as variation in temperature, freezing and thawing, alternate wetting and drying etc. Soundness of aggregate also indicates the durability of aggregate. Aggregate, which undergoes greater volume change under the above conditions, is called unsound aggregate. Aggregates with large amount of water reactive material (e.g. lime) and porous and weak aggregates undergo large volume change. Alkali-aggregate reaction is very important in this regard.

Alkali-aggregate Reaction

Earlier the aggregates were considered as inert material but later on it was found that these are not fully inert material. Some of the aggregates may contain some amount of reactive silica, which reacts with alkalies present in

Fine Sand

Coarse Sand

Medium Sand

15 18 21 0 3 6 9 12

10

20

30

Moisture Content (Percent of Weight)

Bul

king

(Per

cent

of D

ry V

olum

e)

55

Properties and

Testing of Aggregatesthe cement. It produces the alkali silicate gel, which is of unlimited swelling type. Due to the swelling property of such gel, cracks are developed in concrete hence durability of concrete reduces.

It is interesting to note that alkali-aggregate reaction can take place only if the reactive silica of particular fineness and that too in particular proportion is present. The factors responsible for alkali-aggregate reaction are reactivity of aggregate, high alkali content of cement (i.e. > 0.6%, expressed as soda equivalent), availability of excess moisture and the optimum temperature conditions (i.e.10-38oC). So, it is quite possible to reduce the probability of such reaction by changing the proportion or fineness of reactive silica or both and by controlling the availability of excess moisture or temperature. Mortar bar expansion test [IS 2386 (part vii): 1963] is popularly used to determine the potential reactivity of the aggregate.

Comments

Use of unsound aggregates in concrete will degrade the quality of concrete. The change in volume of concrete may appear in the form of scaling, surface cracks and/or disintegration of concrete over a considerable depth, which not only hamper the appearance but also may cause structural damages. Therefore, it is always preferred to avoid the use of unsound aggregates.

SAQ 1

(a) Give the advantage of using large size aggregate in concrete.

(b) Compare the merits and demerits of using rounded and angular aggregate.

(c) How will you classify the aggregates on the basis of their

(i) Shape, and

(ii) Unit Weight.

(d) Why is the density of aggregates denoted by Bulk Density?

(e) What do you mean by SSD condition of aggregate?

SAQ 2

(a) Why is it necessary to know the moisture content of aggregates at site?

(b) With the help of neat curve discuss the Bulking of sand and its importance.

(c) Give the factors responsible for Alkali-Aggregate Reaction.

(d) Why is the Alkali-Aggregate Reaction harmful to concrete?

56

Concrete Technology

3.3 FLAKINESS AND ELONGATION INDEX

3.3.1 Flakiness Index Aggregate is termed as flaky when its least dimension (i.e. thickness) is reasonably small relative to its other two dimensions (i.e. width and/or length). The amount of flakiness is denoted by flakiness index.

Flakiness index is given as the percentage by weight of particles in the sample of aggregate whose least dimension is less than 3/5 of the mean dimension (i.e. mean sieve size fraction to which the particle belongs). For example, if we have aggregate sample of 20 mm mean size then any particle of this sample of thickness less than 12 mm (i.e. (3/5) × 20) will be designated as flaky aggregate.

It is measured with the help of standard metal thickness gauge as shown in Figure 3.2 and as described in IS : 2386 (Part I) - 1963. The thickness gauge has apertures of different sizes. A sample of aggregate is taken and sieved through a set of sieves ranging from 63 mm to 6.3 mm and particles of each fraction are collected separately. The aggregate sample should be taken in sufficient quantity so that each fraction sample should have at least 200 particles. Now, particles of each fraction are allowed to pass through the corresponding aperture individually. The flakiness index is calculated as below:

Weight of particles through thickness gaugeFlakiness Index 100Total weight of thesample

= ×passing

These Sizes Marked on Gauge

Figure 3.2 : Thickness Gauge (For Determining Flakiness Index)

3.3.1 Elongation Index Aggregate is termed as elongated when its greatest dimension (i.e. length) is reasonably large relative to its mean dimension. The amount of elongation is denoted by elongation index.

Elongation index is given as the percentage by weight of particles in the sample of aggregate whose greatest dimension (i.e. length) is greater than 9/5 times of their mean dimension (i.e. mean sieve size fraction to which the particle belongs).

1.6 mm Thickness Sheet Rolled over 8 mm Bar

All Dimensions in mm

63 to

50

33.9

50 to

40

27.0

40 to

25

19.5

25 to

20

13.5

20 to

15

15 to

12.

5

10.8 8.55

12.5

to 1

0

6.75

16.95

25 4.83

70

57

Properties and

Testing of AggregatesFor example, if we have aggregate sample of 20 mm mean size then any particle of this sample having its length greater than 36 mm [i.e. (9/5) × 20] will be designated as elongated aggregate. The elongation index is applicable to particles of size 6.3 mm or greater.

It is measured with the help of standard metal length gauge as shown in Figure 3.3 and as described in IS 2386(Part I) : 1963. The length gauge has apertures of different sizes. A sample of aggregate is taken and sieved through a set of sieves ranging from 63 mm to 6.3 mm and particles of each fraction are collected separately. The aggregate sample should be taken in sufficient quantity so that each fraction sample should have at least 200 particles. Now, particles of each fraction are allowed to pass through the corresponding aperture individually.

The elongation index is calculated as below :

Weight of particles on the length gaugeElongation Index 100Total weight of thesample

= ×retained

10 6.3

12.5 10

15 12.5

20 14

25 20

40 45

50 40 Retained on Sieve

Passing of Sieve

4.7 20.2 25.6 32.4 40.5 50.5 81.0

45 25

6

6φ

All Dimensions in mm

Figure 3.3 : Length Gauge

(For Determining Elongation Index)

Comments

The presence of elongated, blade-shaped or flaky particles, both in coarse and fine aggregates, should be avoided. It could be restricted to 15-20 % by weight of total aggregate for normal concrete. The presence of such type of particles adversely affects the strength of concrete as they act as a weak-link within the gel structure of concrete. The surface area to volume ratio for such type of particles is high, which adversely affect the workability of concrete mix.

SAQ 3

(a) Differentiate between Flaky and Elongated Aggregates.

(b) Define Flakiness Index of aggregate.

(c) Define Elongation Index of aggregate.

(d) Why is the presence of flaky or elongated aggregates harmful for concrete?

(e) Describe briefly the procedure for determining flakiness and elongation index.

58

Concrete Technology 3.4 SIEVE ANALYSIS, FINENESS MODULUS AND

GRADING

3.4.1 Sieve Analysis Sieve analysis is a procedure for finding the proportion of different standard size fraction of aggregates present in a sample. For this, the aggregate sample is sieved through a set of standard sieves taken in order. The sieves are arranged in descending order from top to bottom with sieve of maximum size at top. As per IS 2386 (Part-I) : 1963 the sieve sets are taken in the following order : 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 μm, 300 μm, and 150 μm. (You may note that size of any sieve is about half of the previous sieve.)

For performing the sieve analysis, appropriate weight of aggregate sample is taken as per the IS 2386 (Part I): 1963 and are given in Table 3.1. The weight of sample to be taken depends upon the maximum size of aggregate present substantially in the sample.

Table 3.1: Minimum Weight of Aggregate Sample to be Taken for Sieve Analysis

Sl. No. Maximum Size of Aggregate Present in Substantial

Proportion in mm

Minimum Weight of Sample to be Taken for

Sieving in kg 1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

63 50

40 or 31.5 25

20 or 16 12.5 10 6.3

4.75 2.36

50 35 15 5 2 1

0.5 0.2 0.2 0.1

Table 3.2 gives a sample calculation of sieve analysis to determine various related parameters. After sieving the aggregate sample (1000 gm) through the set of sieves in standard manner, the material retained on each sieve is measured by weight and tabulated in Column 2 of the table. The weight retained on each sieve represents the size-range of particles. For example, weight retained on 2.36 mm sieve (i.e. 120 gm) indicates that this particular fraction of aggregate is having the particles of size > 2.36 mm and < 4.75 mm.

In Column 3 of the table, the percentage weight retained on each sieve is calculated as below. It indicates the percentage of that particular fraction present in the aggregate sample. For a particular sieve size, say Sx,

Weight retained on thesieve

% weight retained on the ieve 100Total weight of theaggregatesample

xx

Ss S = ×

For example, percentage weight retained on 2.36 mm sieve is calculated as below

120 100 12%1000

= × = .

59

Properties and

Testing of AggregatesTable 3.2 : Typical Example of the Sieve Analysis

IS Sieve Size

(1)

Weight Retained

(gms) (2)

% Weight Retained

(3)

Cumulative % Retained

(4)

Cumulative % Passing

(5)

10.00 mm 0 0 0 100 4.75 mm 030 03 03 97 2.36 mm 120 12 15 85 1.18 mm 150 15 30 70 600 μm 200 20 50 50 300 μm 320 32 82 18 150 μm 150 15 97 03

Pan 30 ∑ (4) = 277

In Column 4 of the table, the cumulative percentage retained on each sieve is calculated, which, for sieve Sx, will be equal to the cumulative sum of the percentage weight retained on each sieve (given in Column 3) above and including that on the sieve Sx. The cumulative percentage retained on the sieve Sx indicates that this much percentage of particles in that sample of aggregate is coarser than sieve size Sx. For example, cumulative percentage retained on 2.36 mm sieve will be 3 + 12 = 15 %. It indicates that 15 % of particles in the given sample of aggregate are coarser than 2.36 mm.

In Column 5 of the table, the cumulative percentage passing on each sieve is obtained (100 − cumulative percentage retained.) The cumulative percentage passing through the sieve Sx indicates that this much percentage of particles in that sample of aggregate is finer than sieve size Sx. For example, cumulative percentage passing from 2.36 mm sieve will be 100 – 15 = 85%. It indicates that 85% of particles in the given sample of aggregate are finer than 2.36 mm.

Comments

The observations of the sieve analysis, put in tabular form, are used for drawing the grading curve, defining the grading of aggregate and for determining the fineness modulus as explained in subsequent paragraphs. Also, by comparing the results of sieve analysis of fine aggregates with the standard table given in IS 383: 1970 (reproduced in Table 3.3) the zones of sand (I, II, III or IV) can be identified. The Zone I indicates coarse sand and Zone IV represents fine sand. This information is useful in performing the concrete mix design. It is recommended that use of the fine aggregates, conforming to Zone IV, should be avoided in reinforced concrete unless the suitability of using such aggregates is ascertained through tests.

Table 3.3 : Grading Limits for Fine Aggregates Percentage Passing for IS-Sieve

Designation Grading Zone I

Grading Zone II

Grading Zone III

Grading Zone IV

10 mm 4.75 mm 2.36 mm 1.18 mm

600 micron 300 micron 150 micron

100 90-100 60-95 30-70 15-34 5-20 0-10

100 90-100 75-100 55-90 35-59 8-30 0-10

100 90-100 85-100 75-100 60-79 12-40 0-10

100 95-100 95-100 90-100 80-100 15-50 0-15

60

3.4.2 Fineness Modulus Concrete Technology

The fineness modulus is a numerical index, which gives general idea about the coarseness (mean size) of aggregates but does not provide any information regarding particle size distribution in the sample of aggregate. It is determined by the following formula :

(Cumulative percentage retained on each sieve)

Fineness Modulus100

= ∑

In the above example (Table 3.2), the fineness modulus = (277)/100 = 2.77. It can be viewed as weighted average size of a sieve on which particles are retained. The sieve being counted from the finest sieve as one. So, fineness modulus of 2.77 indicates that average size of particles is between 300-600 μm sieve and more precisely, close to 600 μm.

The value of fineness modulus is higher for coarser aggregates and lower for finer aggregates. For the aggregates commonly used, the range of fineness modulus is as below :

Fine Aggregate 2.0 to 3.5

Coarse Aggregate 5.5 to 8.0

All-in-aggregate 3.5 to 6.5

Further, the range of fineness modulus for various categories of sand is as below:

Fine Sand 2.0 to 2.6

Medium Sand 2.6 to 2.9

Coarse Sand 2.9 to 3.5

Generally, the sand with fineness modulus ranging between 2.2 to 3.2 is considered suitable for making concrete.

Comments

The fineness modulus is useful in grading the given aggregate for designing the mix economically for the given strength and workability with the minimum amount of cement paste. Aggregates with higher fineness modulus produce harsh concrete and with the lower fineness modulus will give uneconomical mix. The fineness modulus is also used to provide a rough check on the batch-to-batch variation in the grading of aggregates obtained from the same source.

3.4.3 Grading The particle size distribution in a given sample of aggregate is termed as grading of the aggregate. The grading of aggregates can be conveniently defined with the help of grading curves. Grading Curve is a curve showing variation of cumulative percentage of passing (on Y-axis) over the different size of sieves (on X-axis in log scale).

The grading curve for the data, obtained in Table 3.2, is shown in Figure 3.4. The steep slope of the grading curve, which is from A to B (i.e. between 300-600 μm size), indicates the greater proportion of these size particles in the sample. The uniform slope of the grading curve, which in this case is from B to D (i.e. between 600 μm-2.36 mm), indicates the continuous grading, i.e. particles of all sizes (between 600 μm-2.36 mm size) are present in the sample. Flat slope of the

61

Properties and

Testing of Aggregatesgrading curve, which is from D to E (i.e. between 2.36-4.75 mm), indicates lesser/negligible contribution of the particles of that size. Horizontal portion of grading curve, which in this case is between E to F (i.e. between 4.75-10.00 mm), indicates that the particles of this size (i.e. 4.75-10 mm) are almost absent in the sample.

grading curve, which is from D to E (i.e. between 2.36-4.75 mm), indicates lesser/negligible contribution of the particles of that size. Horizontal portion of grading curve, which in this case is between E to F (i.e. between 4.75-10.00 mm), indicates that the particles of this size (i.e. 4.75-10 mm) are almost absent in the sample.

150 μm

300 μm

600 μm

1.18 mm

2.36 mm

4.75 mm

10 mm

0

20

40

60

80

100

Sieve Size

Perc

enta

ge P

assi

ng

A

B

C D

E

F

Figure 3.4 : Grading Curve for Table 3.2 Figure 3.4 : Grading Curve for Table 3.2

150 μm

300 μm

600 μm

1.18 mm

2.36 mm

4.75 mm

10 mm

0

20

40

60

80

100

Sieve Size

Perc

enta

ge P

assi

ng

A B

C D

E

F

Figure 3.5 : Gap Graded Aggregate Figure 3.5 : Gap Graded Aggregate

The horizontal portion of the grading curve represents Gap-Grading. For example, the curve in Figure 3.5 represents the gap-graded aggregate in which the particles of size 600 μm-2.36 mm are almost absent.

The horizontal portion of the grading curve represents Gap-Grading. For example, the curve in Figure 3.5 represents the gap-graded aggregate in which the particles of size 600 μm-2.36 mm are almost absent.

Comments Comments

A single numerical value or a mathematical formula cannot specify the good grading of aggregate. In fact, one should go for actual trial and error procedure in order to obtain reliable/satisfactory grading. The good grading of aggregate basically implies that the aggregate sample is having required proportion of all standard fractions of the aggregate so that sample contains minimum voids and provides least surface area. This will require minimum cement paste to fill these voids, i.e. requirement of cement and water will be reduced and will lead to greater strength, better economy, less shrinkage and more durability of concrete. Also, proper care should be taken in selecting the grading so as to avoid segregation. Generally, the workability of concrete is governed more by grading of fine aggregate rather than the coarse aggregate.

A single numerical value or a mathematical formula cannot specify the good grading of aggregate. In fact, one should go for actual trial and error procedure in order to obtain reliable/satisfactory grading. The good grading of aggregate basically implies that the aggregate sample is having required proportion of all standard fractions of the aggregate so that sample contains minimum voids and provides least surface area. This will require minimum cement paste to fill these voids, i.e. requirement of cement and water will be reduced and will lead to greater strength, better economy, less shrinkage and more durability of concrete. Also, proper care should be taken in selecting the grading so as to avoid segregation. Generally, the workability of concrete is governed more by grading of fine aggregate rather than the coarse aggregate.

S AQ 4 AQ 4

62

(a) Discuss the importance of performing sieve analysis. Concrete Technology

(b) How will you define the good grading and gap-grading of aggregate?

(c) Define and discuss the importance of grading curve.

(d) Discuss the restriction on use of the fine aggregates conforming to Zone IV in concrete.

3.5 MECHANICAL PROPERTIES OF AGGREGATES

Apart from the physical properties of aggregates, mechanical properties of aggregates are equally important. They include crushing strength, toughness, hardness, etc. These properties of aggregates are determined by performing, various tests for aggregate crushing value, ten percent fine value, aggregate abrasion value and aggregate impact value. Out of these, tests for aggregate abrasion value and aggregate impact value (which represents the hardness and toughness of aggregate) are particularly more important for the aggregates used for concrete in wearing surface. A brief description of these aggregate properties and the related tests has been discussed in the subsequent paragraphs.

3.5.1 Aggregate Crushing Value (ACV) The resistance of aggregate to crushing under gradually applied compressive load is relatively measured in terms of aggregate crushing value. This test is more popular for representing the strength of aggregate. To determine the aggregate crushing value, a sample of aggregate passing through 12.5 mm and retained on 10 mm is taken. This sample of aggregate in saturated surface dry condition is filled in the cylindrical container in three layers by tamping each layer 25 times with tamping rod in standard manner and the weight of sample ‘A’ is taken. This aggregate sample (weighing ‘A’) is placed in the test apparatus and is uniformly loaded through a plunger up to a total load of 40 tons in 10 minutes. After removing the load the aggregate is taken out and sieved through 2.36 mm sieve. The weight ‘B’ of the fraction passing through 2.36 mm is taken.

Aggregate crushing value (ACV) is determined in percentage by :

ACV 100BA

= ×

where, A = Weight of prepared aggregate sample, and

B = Weight of aggregate passing through 2.36 mm after applying the load.

As per IS 383: 1970, the ACV should not be more than 45% for the concrete used for other than wearing surface, whereas ACV should be limited to 30% for the concrete used for wearing surface such as roads, runways and air field pavements.

3.5.2 Aggregate ‘Ten Percent Fine Value’ (TPFV) The other way of representing the strength of aggregate is the ‘ten percent fine value’ (TPFV), which is basically the load required for 10% fines. As compared

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Properties and

Testing of Aggregatesto aggregate crushing value test this test is more reliable for representing the strength of aggregate. To determine the ‘ten percent fine value’ the aggregate sample is prepared in the same manner as in the case of determining the aggregate crushing value. The prepared aggregate sample (weighing ‘A’) is placed in the test apparatus and the load is applied uniformly through a plunger at uniform rate so as to cause total penetrations in 10 minutes as below :

• 15.00 mm for rounded or partially rounded aggregates.

• 20.00 mm for normal crushed aggregates.

• 24.00 mm for honeycombed aggregates (e.g. expanded shale and slag).

After achieving the desired penetration the load is removed and the aggregate is taken out and sieved through 2.36 mm sieve. The weight ‘B’ of the fraction passing through 2.36 mm is taken. The load applied should be such that the percentage of fines [i.e. (B/A) × 100] be in the range of 7.5-12.5% otherwise the test is repeated with another load. Two such tests are conducted and the load required for ten percent fines [TPFV] is obtained by :

14TPFV4X

Y×

=+

where, X = Load in tons causing 7.5-12.5 percent fines, and

Y = Mean percentage fines from two tests at X tons load.

3.5.3 Aggregate Abrasion Value (AAV) The resistance of aggregate to wear is known as hardness of aggregate. This property is particularly more important for the concrete used for wearing surfaces such as, road, runway, etc. It is measured in terms of aggregate abrasion value and determined by Deval Abrasion Test Machine or Los Angeles Abrasion Test Machine. The latter is more commonly used as it gives better representation of hardness. The test method combines the attrition and abrasion effects both. The test sample of clean aggregate is taken in standard manner as specified in IS 2386 (Part IV) : 1963 and dried in oven at 105-110oC. It is placed in Los Angeles Abrasion Test Machine along with the specified abrasive charge. The abrasive charge is in the form of cast iron spheres or steel spheres. Each sphere is of approximately 48 mm in diameter and of 390-445 gm in weight. The aggregates together with the abrasive charge are rotated in the machine @ 20-33 revolutions per minute for 500-1000 revolutions (depending upon the grading of the aggregates). After completing the specified revolutions the aggregate is taken out and sieved through 1.7 mm sieve. The weight of the aggregate, coarser than 1.7 mm, is taken after washing and drying in standard manner, which is known as final weight.

Original weight Final weightAAV 100Original weight

−= ×

The limiting value of abrasion-value for concrete aggregate is 16%.

3.5.4 Aggregate Impact Value (AIV) Resistance of aggregate to sudden shock or impact is known as toughness of aggregate. Aggregate impact value (AIV) is a relative measure of toughness of aggregate. To determine the aggregate crushing value, a sample of aggregate

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passing through 12.5 mm and retained on 10 mm is taken. This sample of aggregate is dried for 4 hrs. in oven at 100-110oC. After getting the dried sample cooled, it is filled in the cylindrical container in three layers by tamping each layer 25 times with tamping rod in standard manner and the weight of sample ‘A’ is taken. This aggregate sample (weighing ‘A’) is placed in the impact load machine in which the impact load is applied through 15 blows of standard hammers falling from a height of 380 mm. The aggregate is then taken out and sieved through 2.36 mm sieve. The weight ‘B’ of the fraction passing through 2.36 mm is taken.

Concrete Technology

Aggregate impact value is determined in percentage by :

A

SAQ 5

SAQ 6

IV = 100BA×

where, A = Weight of prepared aggregate sample, and

B = Weight of aggregate passing through 2.36 mm after applying the load.

The AIV should not be more than 45% for the concrete used for other than wearing surface, whereas AIV should be limited to 30% for the concrete used for wearing surface such as roads, runways and air field pavements.

(a) Give the limiting values of aggregate impact value for concrete used

for wearing surface and for concrete used for other than wearing surface.

(b) Define the hardness and toughness of aggregates.

(a) For aggregate abrasion value test give the amount and duration of load to be applied through plunger.

(b) Describe in brief the procedure for determining the following :

(i) Aggregate Impact Value

(ii) Aggregate Crushing Value

(iii) Aggregate Abrasion Value

(iv) Aggregate Ten Percent Fine Value

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Properties and

Testing of Aggregates3.6 EFFECT OF AGGREGATE PROPERTIES ON STRENGTH OF CONCRETE

Effect of aggregate properties on strength of concrete is not much straightforward as compared to their effect on workability. The effect of aggregate properties on workability is being discussed in Unit 4. The effects of aggregate properties on performance of concrete, in general, have been included in the comment part after explaining each property of the aggregate. However, in this unit the effect of aggregate properties on the strength of concrete as such has been given, particularly the effect of shape/texture, size and soundness of aggregate.

3.6.1 Maximum Size of Aggregate The effect of size of aggregate is not straightforward but it depends upon the grade of concrete (i.e. whether the concrete is high strength or low strength). The size of aggregate does not affect much the strength of low strength concrete but for high strength concrete the strength increases with decrease in size of aggregate. This is due to the fact that in high strength concrete the failure is more likely to take place due to the loss of bond between the aggregate and the cement paste whereas in the low strength concrete the failure of cement paste itself take place. The bond strength between the aggregate and cement paste depends upon the surface area provided by the aggregates. The surface area provided by the aggregates will be large for smaller size aggregates as compared to the large size aggregates.

3.6.2 Shape/Texture of Aggregate Angular aggregate and aggregate with rough surface provides better bond due to more surface area hence gives better strength to concrete. Flaky and elongated particles being weak in strength and also providing the weaker bond, their presence always lead to poor strength of concrete.

3.6.3 Soundness of Aggregate Weak and porous aggregate containing any undesirable extraneous matter makes the aggregate unsound which in general reduces the strength of concrete particularly, under the moderate or severe exposure conditions like variation in temperature, freezing and thawing condition, alternate wetting and drying condition, etc.

SAQ 7

(a) Discuss the effect of following on the strength of concrete

(i) Size of Aggregate

(ii) Shape/Texture of Aggregate

(b) What do you understand by soundness of aggregate? Discuss its effect on the strength of concrete.

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Concrete Technology 3.7 SUMMARY

In this unit, the importance of aggregate for concrete has been discussed in detail. A brief classification of aggregates on the basis of their source, shape, size and unit weight has been explained. The important parameters associated with the aggregates such as bulk density, apparent specific gravity, water absorption, etc. have been discussed and the normal range of these parameters has been given.

The importance of the various properties of aggregate (i.e. physical as well as mechanical properties) on the performance and quality of concrete has been separately discussed in details under the comments. Further, a brief description of various test procedures to determine these properties of aggregates has been provided. The importance of sieve analysis and that of the associated terms such as fineness modulus, grading curve, sand zones, etc. have been explained. Finally, the effect of various properties of aggregate on the strength of concrete has been discussed.

3.8 ANSWERS TO SAQs

Refer the relevant preceding text in the unit or other useful books on the topic listed in the section ‘Further Reading’ given at the end of the booklet to get the answers of SAQs.