TESTING OF AGGREGATES

NAME K.I.U.NANAYAKKARA

INDEX NO 090344M

MODULE CE 1132 : BUILDING CONSTRUCTION AND

MATERIALS

DATE OF EXPERIMENT 30/06/2010

DATE OF SUBMISSION 28/07/2010

INTRODUCTION

It is necessary to check suitability of aggregates from a new aggregate quarry for making structural concrete. In addition, it is also necessary to test the aggregates to obtain relevant properties for concrete mix design.

Aggregates used in concrete are divided into two categories as coarse and fine aggregates. Crush rock and river sand are the most common coarse and fine aggregates, respectively, used in Sri

Lanka.

Since approximately three quarters of the volume of concrete is occupied by aggregate, its quality has a considerable influence on the durability and structural performance of concrete. Some of the important properties of aggregates which affect the properties of both fresh and

hardened concrete are,

a) Particle size distribution (Grading).

b) Shape and surface texture of particles.

c) Physical properties (e.g. specific gravity, water absorption, porosity, etc.)

d) Mechanical Properties (e.g. strength, toughness, hardness, etc.)

e) Chemical and thermal properties.

Various tests are carried out on natural aggregates to check conformity with BS 882 (1983). The methods of testing aggregates are described in BS 812. [Although we use these standards in

aggregate testing they have been replaced since and the current relevant British standards are BS EN 12620: 2002, BS EN 1097:2009]

Sampling

Only grading characteristics and important physical properties of aggregates are determined in this experiment. Following tests are carried out in this experiment

1. Sieve analysis on representative samples of fine and coarse aggregates.

2. Determination of relative density and water absorption of coarse aggregates.

3. Determination of dry loose bulk density of fine and coarse aggregates.

As it is not practical to test the whole bulk of aggregates it is very important to take a

representative sample on which the tests could be carried out. BS 812 has laid down some guidelines on this representative sample.

Since we are performing tests on a sample of an aggregate and we are interested in the bulk of the aggregate we must ensure that the sample is typical of the average properties of the aggregates.

The main sample for testing must be made of a minimum number of 10 portions drawn from different parts of the whole and the weight of the sample must not be less than that given in Table (1).

Type of material Normal size[ ] Minimum mass of main

sample

Aggregates

50 kg

25 kg

13 kg

Table 1: Minimum mass of main sample (BS 812)

The minimum mass of test sample for sieve analysis is given in Table (2).

Nominal size (mm) 63 50 40 28 20 14 10 6 5 3

Minimum mass of sample (kg)

50 35 15 05 02 01 .5 .2 .2 .2 .1

Table 2: Minimum mass of test sample.

The main sample may be too large and so the sample must be reduced before testing. There are two methods of doing this.

1. Quartering.

The main sample is thoroughly mixed and the material heaped into a cone and then turned over to form a new cone .In the case of fine aggregate, the sample is dampened

This procedure is carried out three times. ; the material always being deposited at the apex of the cone so that the fall of particles is evenly distributed round the circumference. The final cone is flattened, divided into quarters and a pair of diagonally opposite quarters is discarded, the remainder form the sample for testing and if still too

large, the sample can be reduced by further quartering.

2. Riffling (Using a sample divider)

A riffler is a box with a number of parallel vertical divisions, alternate ones discharging

to the left and to the right. The sample is discharged into the riffler over its full width and the halves are collected into two boxes at the bottom of the chutes on either side. One

half is discharge and riffling of the other half is repeated until the sample is reduced to the desired size.

In this experiment the coarse aggregate to be tested was of 20 mm nominal size. Thus the minimum mass of the main sample of coarse aggregate was 25 kg and the test sample was 2 kg. As

sand was the fine aggregate, minimum mass of the main and test samples were 13 kg and 200 g

respectively. Riffling was used to reduce the main sample.

Figure 1: Riffler.

TEST 1: Sieve Analysis

APPARATUS

1. Sieve set

28mm, 20mm, 14mm, 10mm, 5mm, 2.36mm, 1.18mm, 0.6mm, 0.3mm, 0.15mm,

0.075mm.

2. Scale.

THEORY AND PROCEDURE

Sieve analysis is a simple operation of dividing a sample of aggregates into fractions, each consisting of particles between specific limits, this being the openings of standard test sieves.

The test sieves used for concrete aggregate have square openings. Sieve sizes are by the nominal aperture size in mm or microns. All sieves are rested one above the other in order of size, with the largest sieve at the top, and the material retained on each sieve after shaking represents the fraction of aggregate coarser than the sieve in question and finer than the sieve above.

The sample was discharged onto the top most sieve and was covered with a lid. Then the sieve set is shaken well in all directions. Then the particles left on each sieve was collected and

weighed separately.

RESULTS

Coarse aggregates:

Sieve Size (mm)

Mass retained (kg)

Percentage retained (%)

Cumulative

percentage

passing (%)

Cumulative

percentage

retained (%)

28 0 0 100 0

20 0.5158 16.5 83.5 16.5

14 1.5565 49.8 33.7 66.3

10 0.831 26.6 7.1 92.9

5 0.202 6.5 0.6 99.4

0.0177 0.6 0 100

The sieve analysis results for coarse aggregate is within the range specified, for single sized aggregates of size 20 mm, under BS 882:1983, with a slight deviation being present; the

percentage passing the 20 mm sieve being 83.5 % instead of specified 85%.

So it can be concluded that the aggregate sample tested, and therefore the bulk from which the

sample was extracted, is a single sized aggregate of nominal size 20 mm.

Fine aggregates:

Sieve Size (mm)

Mass retained (g) Percentage

retained

Cumulative percentage

passing

Cumulative percentage

retained

2.36 22.4 10.4 89.6 10.4

1.18 55.7 26 63.6 36.4

0.6 52.3 24.4 39.2 60.8

0.3 54.1 25.2 14 86

0.15 26.1 12.2 1.8 98.2

0.075 3.9 1.8 0 100

The sieve analysis results for fine aggregate is within the range specified, for fine aggregates, under BS 882:1983. It also falls within limits, and more closely resembles the grading pattern,

laid down for medium fine aggregates, under additional limits in the same standard.

Thus the bulk of fine aggregates can be recommended for use in concrete, provided it exhibits good quality under other mechanical tests.

TEST 2: Determination of Relative Density and Water Absorption.

APPARATUS

1. Vessel [ a cylindrical container]

2. A piece of glass

3. Scale

4. Oven

THEORY

Aggregates normally contain pores (voids), and therefore the term relative density or specific

gravity must be carefully defined. Although some of the pores are impermeable, water can penetrate into the aggregates through permeable pores which are open onto the surface of the particles. When all the permeable pores are filled with water, the aggregate is said to be saturated and surface dry. If aggregates in this condition are allowed to dry in air, part of the water will evaporate resulting air dry aggregates. Prolonged drying in an oven will remove the

moisture completely and the aggregate is said to be oven dry (bone-dry)

Bone dry or

Oven dry

Air dry

Saturated and

surface dry

(SSD)

Moist

Total Water

Content

Absorbed moisture

(Absorption)

Free Moisture

(Moisture Content)

Figure 2: Water content of aggregates.

Relative density in different conditions and water absorption of aggregates are defined in the following manner.

Where,

A= Mass of saturated and surface dry sample in air

B = Mass of vessel containing sample filled with water

C = Mass of vessel filled with water only

D = Mass of oven dried sample in air

PROCEDURE

1. The vessel was filled with water and the mass was measured.

2. Aggregate sample was put into the vessel and the vessel was topped up with water. The combined mass was measured.

3. The vessel was covered with the piece of glass and kept for 24 hours.

4. Aggregates were taken out and the surface water was removed and the mass was recorded. The weighed sample was in saturated and surface dry condition.

5. The aggregates were then dried in an oven at 1050C for 24 hours.

6. The mass of the oven dried sample was measured.

CALCULATIONS

A= 549.5 g

B = 2924.6 g

C = 2574.7 g

D = 546.9 g

TEST 3: Determination of Bulk Density.

APPARATUS

1. Container [Bucket]

2. Steel rod.

3. Weighing machine [Scale]

THEORY

Bulk density of aggregate can be defined as the weight of aggregates that would fill a container of unit volume. Bulk density depends on how densely the aggregate is packed and therefore two degrees of compaction; loose and compacted are used

BS 812 specifies the degree of compaction. In determining loose bulk density, the aggregate should be gently placed in the container to an overflowing level, and then cut level with a straight edge. In order to find the compacted bulk density, the container is filled in three stages, each third of the volume and being tamped 25 times (according to BS 812:1960) by a round

nosed rod. BS 812 also specifies the container and the rod to be used.

Voids ratio, volume of voids as a factor of the bulk volume, of aggregates in saturated and

surface dry condition is defined as below.

PROCEDURE

1. The container is filled with coarse aggregate to an overflowing level and then cut level with a straight edge.

2. The mass of the aggregate is measured.

3. The container was filled again by the method specified by BS 812 for bulk density under compaction.

4. The mass of the aggregate was measured.

5. The container was fully filled with water and the mass of the collected water was

measured.

6. Weight of the empty container was measure.

Steps one to four was repeated for both coarse and fine aggregates

CALCULATIONS

Coarse Aggregates

Fine Aggregates

Discussion

1. The importance of aggregate grading in concrete nix design.

All the particles in an aggregate are not in the same size. They do differ in size and shape. In aggregate grading we check what proportions of what sized particles are available in the bulk.

There is no such perfect grading. Aggregates with different grading can produce satisfactory concrete. But the standards provide a guideline to check the suitability of aggregates to be used in concrete.

It is very important that particles of different sizes are available in the concrete mix. Conformity with standards will ensure that there are more larger particles and fewer smaller particles, but

not being deficient of small particles. This ensures that the aggregates will pack more densely. In concrete, more compaction means higher strengths.

Also a graded aggregate is more workable, due to the lubricating effect of smaller particles. This also means that it is easier to achieve a fully compacted concrete.

Also a graded aggregate has more larger particles meaning a lesser surface area. Lower surface area means less binding material; i.e. cement. Thus using a graded aggregate will be an

economical decision too.

Having a low cement paste will mean that the concrete will have a low shrinkage during its life. Also the dense packing and low cement content will ensure a lower permeability. These two

factors will mean that the concrete made from graded concrete will be durable.

2. The use of graded, single sized and gap graded aggregates in concrete.

There are several types of gradings observed; graded, single sized and gap graded.

In general usage, single sized coarse aggregates, appropriately mixed with fine aggregate, are used. It is both easier and economical to do such a mix design. Hear the effective aggregate

mixture is a graded one.

Single sized aggregates (coarse aggregates) will be used in making porous concrete. Here, there

won’t be any addition of fine aggregates.

There can be certain economies achieved by using gap graded aggregates. For a fixed water cement ratio a gap graded coarse aggregate will require a lesser amount of fines than a graded

coarse aggregate, to achieve a specific workability. So appropriately mixing a gap graded coarse aggregate with fines will achieve a specific workability, without losing strength, for a lower cost,

given that fine aggregates are cheaper.

3. The practical importance of moisture content of aggregates.

Moisture is contained in the aggregate as both absorbed moisture and free moisture. So, effectively, aggregates can act as either a water absorbent, while in air dry or bone dry condition, or water provider, in moist condition, to the concrete mix. The strength of the

concrete is largely determined by its water cement ratio. This means that it is very important to have a good understanding about the moisture content of aggregates in achieving the desired strength of concrete.

While in air dry or bone dry condition, the aggregate particles will absorb water from the mix, reducing the water available for cement water reaction. If the used aggregate particles are in moist condition, the additions of aggregates mean addition of an extra amount of water. Thus the water cement ratio will increase and will produce a lower strength concrete. Thus aggregate

particles in saturated surface dry condition will be the best fitted condition for the aggregates to

be in when added to a concrete mix.

But in most of the cases aggregates will be in moist conditions. Then the water content of the mix has to be reduced by the amount of moist introduced by the aggregates. Also the aggregate

content has to be increased since a percentage, of what we think is aggregates, is really water.

Moisture content, as free water, in sand give rise to a phenomenon called bulking. When the aggregates are in moist condition a thin film of water will be formed around the particle,

pushing away the adjoining particles. When sand is considered the volume of the water layers is considerable, thus affecting the volume batching of sand in concrete. Bulking in coarse

aggregates is very small and thus is being neglected.

4. The importance of bulk density and voids ratio and how these properties are

influenced by the grading of aggregates.

The bulk density is an important factor when considering the preparation of concrete. Concrete is usually batched by volume. It will be a mistake to convert the content by weight to content by volume using the density of aggregate particles. This is because the aggregates will not be

packing perfectly. Thus the particle density and bulk density will be different. Since the volume of aggregates is measured using gauge boxes, at site, the conversion should be using bulk

density. Since the bulk density will differ with different compactions and the compaction achieved in the lab and in the site can be different, minor differences in the mix can be found.

Bulk density is very important in mix designs for lightweight and heavy concretes. This is because of the particular emphasis on weight in these concrete and the fact the larger proportion of the volume of concrete will be taken by aggregates.

Voids ratio gives the proportion of volume of voids to the volume of bulk. High voids ratio will mean a larger amount of cement paste (and fine aggregates) to be used. A higher amount of

cement paste will mean the concrete is both expensive and can give rise to shrinkage cracks.

Bulk density of aggregates is affected by several factors, one of that is the grading of aggregate. An aggregate which has a continuous grading will have a large amount of large particles and small amounts of smaller particles. This will mean that there will be particles to fill up the voids between larger particles. Having particles of different sizes will mean that this pattern of filling

will go on till the void spaces are a minimum. i.e. it will have a smaller voids ratio. Think of a single sized aggregate sample similar in weight to a continuously graded aggregate sample. It is obvious that the graded aggregate will take up a smaller volume since there are lesser voids. This effectively means that a concrete made of graded concrete is denser.

5. The important mechanical properties which can be used to assess the quality of

aggregates and methods of testing.

There are several mechanical properties off aggregate, which may or may not be directly related

to strength of concrete but, can be used to assess the quality of the aggregates. They are surface texture, strength, toughness and hardness.

Surface texture of aggregates can be classified as glossy, smooth, granular, rough, crystalline or porous. A rougher surface texture means that cementing material can hold on to the surface of aggregate, creating a stronger bond. Having a stronger bond will obviously going to affect the strength of the concrete. There are no specified tests on surface texture since visual estimates

provide quite reliable results, as we will be mainly considering the coarse aggregates.

It is obvious that a concrete cannot be stronger than the aggregates, which make up the

concrete. Crushing strength of aggregates can be used to assess the strength of aggregates. A good average value for the crushing strength will be about 200-80 N mm-2. Determination of

strength is carried out by performing a compression test on a prepared sample of aggregates as prescribed in the standards.

Toughness is the resistance of an aggregate to failure by impact. Aggregate crushing value is used to compare aggregates on this regard. Impact test is also used as a measure of toughness. Results of both tests are related, with the only difference of the two tests being the relative ease of the impact test to be carried out on site, with slight modifications.

In the aggregate crushing value test the oven dried sample, prepared according to the

specifications, will be placed in cylindrical mould. Then it will be crushed according to the

specified method. The initial sample will be a sample which will totally pass through a specific

sieve, say A, and totally retain in a specific sieve, say B. After crushing, a percentage of particles will pass the sieve B. This percentage is the aggregate crushing value.

Hardness is the resistance to wear. Hardness of aggregates can be measured using abrasion test and attrition test.

Abrasion value is determined by subjecting an aggregate specimen to wear by quartz sand. The loss of weight of the specimen will determine the abrasion value. Similarly, in the attrition test

an aggregate sample is subjected to wear in an iron cylinder. The attrition value is the proportion of broken materials. Attrition value is determined under two conditions; wet and dry. In contrast with abrasion value a lower attrition value indicates a better quality.

References

1. Properties of concrete by A.M.Neville.

2. BS 812: Part 2 :1995