TESTING OF AGGREGATES

1.            Specific Gravity and Water Absorption of Fine Aggregate

Object

Determination of specific gravity and water absorption of Fine aggregate.

Theory and Scope

The specific gravity of an aggregate is defined as the ratio of the mass of a given volume of sample to the mass of an equal volume of water at the same temperature.

The specific gravity of fine aggregate is generally required for calculations in connection with concrete mix design, for determination of moisture content and for the calculations of volume yield of concrete. The specific gravity also gives information on the quality and properties of aggregate. Departure of specific gravity from its standard value indicates change in shape and grading.

Absorption

It influences the behavior of aggregate in concrete in several important aspects. A highly absorptive aggregate, if used in dry condition, will reduce effective water cement ratio to an appreciable extent and may even make the concrete unworkable unless a suitable allowance is made. Hence determination of absorption of aggregate is necessary to determine net water cement ratio.

Apparatus

Pycnometer bottle or flask, weigh balance, conical mould, metal tray and drying oven to operate between 100- 1100C

Procedure

1.            Calibrate the flask by weighing it empty and fill with water at room temperature. Roll and agitate the flask gently in an inclined position, to eliminate air.

2.            Take a sample of fine aggregate and soak it in water and keep it for 24 +/- 0.5 hours. The temperature should be 27 +/- 50C.

3.            Take out and spread the sample (approximately 1.5 kg) on a clean flat surface exposed to gently moving current of warm air until the material just reaches free running condition ( flowing freely).

4.            Place the sand loosely in conical mould and tamp it on surface 25 times. Lift the mould vertically. If the sand retains its shape, it means free surface moisture is present. Continue the drying with constant stirring until the cone of sand slumps on the removal of the mould. This indicates that sand has reached a surface dry condition.

5.            Immediately weigh 500 gm of saturated surface dry sand in the flask.

6.            Fill the flask with water to the top of the cone. Roll the flask in an inclined position to eliminate all air bubbles and replace with water by means of fountain pen filler.

7.            Wipe the flask dry and weigh it accurately.

8.            Calculate the specific gravity.

Absorption Test

1.            Weigh the remaining 1000 gm of saturated surface dry sand in the tray of known weight.

2.            Dry the sample in an oven at 100-1100C for 24 hours

3.            Weigh the dry sand with tray.

4.            Calculate the absorption capacity as the percentage of oven dry Mass.

Bulk specific gravity  =     W2/ (W2-(W3-W1)

 Percentage absorption =               (W4-W5)*100/W5

Observations and Calculations

Precautions

1.            The entire sample should be frequently stirred to secure uniform drying.

2.            The air trapped in the aggregate should be brought to surface by rolling the flask in inclined position.

3.            All weighing should be accurate to the nearest gm.

4.            Sand should not be allowed to stick to the sides of the jar or flask.

5.            The results of different repetitions should not differ more than 0.02 for specific gravity and 0.005 percent for absorption.

References:

1.            IS 2386 part III – 1963   Methods of Test for Aggregates for Concrete

 2.           Specific gravity and Absorption of Coarse Aggregate

Object

Determination of specific gravity and absorption of coarse aggregate

Scope

For design of concrete mix, information should be available about the specific gravity of the aggregates. Specific gravity of an aggregate gives valuable information on its quality and properties. If the specific gravity is above or below that normally assigned to a particular type of aggregate, it may indicate that shape and grading of aggregate has altered.

Apparatus

Weigh balance, Wire basket 200 mm in diameter and 200 mm height of 4.75mm IS sieve net, water tub for immersing the wire basket in water, suitable arrangement for suspending the wire basket from centre of scale pan of balance and absorbent cloth for surface drying of the sample.

Procedure

1.            Take about 5 kg of aggregate by method of quartering; rejecting all material passing a 10 mm IS sieve.

2.            Wash thoroughly to remove the dust etc. from the surface of particles. Dry to constant mass at a temperature of 105 +/- 50C.

3.            Immerse the sample in water at 22 to 320C for a period of 24 hours.

4.            Remove the aggregate from water and roll the same in a large piece of an absorbent cloth until all visible films of water are removed, although the surface of particles will still appear to be damp.

5.            Now, weigh 3 kg of this sample in the saturated surface dry condition and note down the mass as W1 gm.

6.            Place the weighed aggregate immediately in the wire basket and dip it in water. Weight this basket with aggregate, while keeping it in water, with the help of the balance. Note its mass as W3 gm.

7.            Dry the sample to the constant weight at the temperature of 100 to 1100C for 24 hours.

8.            Cool to room temperature and weigh.

9.            Calculate the specific gravity and absorption of the aggregate.

10.         Repeat the procedure for fresh aggregate.

Calculations and Observations

The specific gravity of coarse aggregate is defined as the ratio between the mass of equal volume of coarse aggregate and water at the same constant temperature.

Bulk specific gravity = mass of sample in air          

Loss in mass of sample in water = W1 / W1-(W3-W2)

Where, W1 = mass of SSD sample in air.

W2 = mass of basket in water.

W3 = mass of basket + sample in water. W4 = mass of oven dry sample in air.

Percentage absorption = (W1-W4)*100/W4

 Tabulate the observations as follow:

Precautions

1.            The mass of sample should be accurate at all stages and should be determined to the nearest 0.5 gm.

2.            The sample should be free from foreign matters.

3.            The large fragments should be wiped individually.

4.            Avoid evaporation during surface drying operation.

5.            The absorbent cloth should be 1000mm x 1000 mm in size. It must be of such a type that it can absorb quite large quantity water.

References:

1.            IS 2386 part III – 1963   Methods of Test for Aggregates for Concrete

 3.           Unit Mass of Concrete Aggregates

Object

Determination of unit mass (bulk density) of concrete aggregates

Scope

The bulk density of an aggregate can be used for judging the quality by comparison with normal density for that type of aggregate. The bulk density determines the type of concrete for which it may be used. It is also required for converting proportions by mass into the proportions by volume and is used in calculating the percentage of voids in the aggregate.

Apparatus

Weighing balance, cylindrical container (3, 15 or 30 liter capacity) and a piece of glass plate to be used for calibrating the container

Procedure

1.            Determine the volume of container to be used by accurately filling it with water at 16.70C and weighing the filled container. The mass of water in kg will give the volume of container in liters.

2.            Take the sample by quartering.

3.            Fill the container with aggregate to overflowing by means of a shovel, the aggregate being discharged from a height not exceeding 50mm above the top of container.

4.            Level off the surface of the aggregate with a straight edge.

5.            Determine the net mass of aggregate in the container.

6.            Compute the unit mass of aggregate by dividing the net mass of aggregate in container by volume of container.

Observations and Calculations

References:

1.            IS 2386 part III – 1963   Methods of Test for Aggregates for Concrete

 4.           Moisture Content of concrete aggregates

Object

Determination of moisture content (or surface moisture) in concrete aggregates by drying method.

Scope

The determination of moisture content of an aggregate is necessary in order to determine net water cement ratio for a batch of concrete. A high moisture content will increase effective water cement ratio to an appreciable extent and may even make the concrete weak unless a suitable allowance is made.

Apparatus

Weigh balance, metal tray (frying pan) and a source of heat.

Procedure

1.            Weigh approximately 1000gm of aggregate from the material to be tested by method of quartering in a metal tray.

2.            Heat the aggregate in tray for about 20 minutes.

3.            Weigh the tray with dry aggregate.

4.            Take the aggregate out and clean the tray thoroughly and weigh it.

5.            Express the loss in mass as a percentage of the dried sample to give the moisture content.

Observations and calculations

Precautions

1.            For accurate results, the aggregate should be dried and weighed until there is no further loss in weight.

2.            The aggregate should be turned over at intervals during the drying period to prevent over heating of the parts of sample.

3.            Heat should not be applied fiercely to avoid chemical change.

5.            Fineness Modulus and Grain size Distribution

Object

To determine fineness modulus and grain size distribution of given Coarse and Fine aggregates.

Theory

Fine aggregate is the sand used in mortars. Coarse aggregate that is the broken stone or gravel, and the mixed aggregate which is the combination of coarse and fine aggregates are used in concrete. The coarse aggregate, unless mixed with fine aggregate, does not produce good quality concrete for construction works. The size of the fine aggregate is limited to maximum of 4.75 mm gauge beyond which it is known as coarse aggregate.

Fineness modulus is only a numerical index of fineness giving some idea of the mean size of particles in the entire body of aggregates. Determination of fineness modulus may be considered as a method of standardization of the grading of the aggregates. It is obtained by sieving a known mass of given aggregate on a set of standard sieves and by adding the cumulative percentages of mass of material retained on all the sieves and dividing the total percentage by 100.

The object of finding the fineness modulus is to grade the given aggregate for obtaining a most economical and workable mix with minimum quantity of cement. Certain limits of fineness modulus for fine, coarse and mixed or all-in-aggregates are given in below table. The sample under test should satisfy these results so that the aggregate may give good workability under economical conditions.

Apparatus

Indian Standard test sieves : Fine wire cloth Nos. 4.75 mm, 2.36 mm, 1.18 mm, 0.6 mm, 0.3 mm, 0.15mm and square hole perforated plates 25mm, 20 mm, 16 mm, 12.5 mm, 10 mm and 4.75 mm, weighing balance, sieve shaker, trays, rice plates, drying oven (to operate between 100 to 1100C).

Procedure

Coarse Aggregate

1.            Take 2 kg of coarse aggregate of nominal size 20 mm from a sample of 10kg by quartering.

2.            Carry out sieving by hand. Shake each sieve in order: 25mm, 20mm, 16mm, 12.5mm, 10mm, 4.75mm over a clean dry tray for a period of 2 minutes. The shaking is done with a varied motion; backwards and forwards, left to right, circular clockwise and anticlockwise and with frequent jarring, so that the material is kept moving over the sieve surface in frequently changing directions.

3.            Find the mass of aggregate retained on each sieve taken in order.

Fine Aggregate

1.            Take 1 kg of sand from a laboratory sample of 10 kg by quartering and break clay lumps, if any in a clean dry rice plate.

2.            Arrange the sieves in order of IS sieve nos. 4.75mm, 2.36mm, 1.18mm, 0.6mm, 0.3mm, 0.15mm keeping sieve nos. 4.75mm at the top and 0.15mm at the bottom. Fix them in the sieve shaking machine with the pan at the bottom and cover at the top.

3.            Keep the sand in the top sieve; carry out the sieving in the set of sieves as arranged before for not less than 5 minutes.

4.            Find mass retained on each sieve.

Fineness modulus is an empirical factor which is obtained by dividing the sum of the cumulative percentages of aggregate retained on each Indian Standard sieve taken in order by 100.

Fineness modulus of coarse aggregate = ∑C/100 =

B.            Fine Aggregate

Mass of fine aggregate, W =         Grams.

Fineness modulus of Fine aggregate = ∑F / 100 =

Precautions

1.            Sieves should be cleaned before use.

2.            Stiff worn out brushes should not be used.

3.            The sieving must be done carefully to prevent the spilling of the aggregates.

4.            Do not apply pressure to force the particles through the mesh.

References: 

1. IS 383: 1970 Specifications for coarse and fine aggregates from natural source for concrete.

6.            Silt Content

Object

To determine the silt content in sand by volumetric method field test

Apparatus

Measuring jar of 500ml

Procedure

1.            Prepare 1% solution by mixing 10gms of common salt in 1litre of water.

2.            Put this solution in measuring jar up to the mark of 100 ml.

3.            Add Sand as received in the jar till its level reaches 200 ml.

4.            Add Solution again so that the mixture reaches the level of 300 ml.

5.            Measuring jar top is now covered with the palm of hand and shake vigorously by turning upside down.

6.            Then allow it to stand undisturbed for an interval of time.

7.            Take the reading of sand in a jar after settling (h1).

8.            Take the reading of total sample in a jar after settling (h2).

9.            Then the reading of silt (h) = (h2-h1)

Calculations

Silt content (%) =              (h / h2) x100

7. Bulking of Fine Aggregate

Object

Determination of necessary adjustment for the bulking of fine aggregate by field method

Theory and Scope

In concrete mix design, the quantity of fine aggregate used in each batch should be related to the known volume of cement. The difficulty with measurement of fine aggregate by volume is the tendency of sand to vary in bulk according to moisture content. The extent of this variation is given by this Test.

If sand is measured by volume and no allowance is made for bulking, the mix will be richer than that specified because for given mass, moist sand occupies a considerably larger volume than the same mass of dry sand, as the particles are less closely packed when the sand is moist. If as is usual, the sand is measured by loose volume, it is necessary in such a case to increase the measured volume of the sand, in order that the amount of sand put into concrete may be the amount intended for the nominal mix used (based on the dry sand). It will be necessary to increase the volume of sand by the percentage bulking. The correction to be made is only a rough method at the best, but a correction of the right order can easily be determined and should be applied in order to keep the concrete uniform.

This experiment is intended to cover the field method of determining the necessary adjustment for bulking of fine aggregate.

Apparatus

Weighing balance, cylindrical container, graduated cylinder, metal tray, steel rule and oven

Procedure

1.            Fill the container to about two-third full with given sand loosely.

2.            Level off the top of sand and measure the height by pushing a steel rule vertically down through the sand at the middle to the bottom, let it be h mm.

3.            Take the sand out into a clean metal tray without any loss.

4.            Fill the container with water to half full.

5.            Pour the sand back into the container and stir it with a steel rod 6 mm in diameter so that volume may reduce to a minimum.

6.            Smooth and level the top surface of the inundated sand and measure its depth at the middle with the steel rule. Let it is h’ mm.

7.            Calculate percentage of bulking of sand due to moisture, using the following formula.

Percentage Bulking = ((h-h’)*100)/h Comments and Discussions

It is seen that bulking increases with increasing water content up to a certain point where it is maximum and then it begins to decrease until when the sand is inundated with bulking being practically nil. With ordinary sands the bulking usually varies between 15 and 30 percent. If, therefore, in volume batching no allowance is made for bulking, the mix will be richer than specified.

For example, when the sand has bulked by say 15 percent the mix 1:2:4 by volume batching will correspond to 1:1.74:4 and for 30 percent bulking the ratio 1:2:4 will correspond to 1:1.54:4.

An increase in bulking from 15 to 30 percent will result into an increase in concrete strength by as much as 13 percent. If no allowance is made for bulking, concrete strength may vary by as much as 25 percent.

In absence of such a test it is common to assume that the normal dampness present in sand results in 25 percent bulking.

References:

1. IS 2386 part III – 1963              Methods of Test for Aggregates for Concrete

8. Flakiness and Elongation Indices of Coarse Aggregate

Object

To determine the Flakiness and Elongation indices of Coarse Aggregates

Theory

An aggregate having least dimension less than 3/5th of its mean dimension is termed as flaky. Where the mean dimension is the average of the sieve sizes through which the particles pass and the sieve size on which these are retained. On the other hand the particles having the largest dimension (length) greater than 9/5 times the mean size are termed elongated.

The presence of excess of flaky and elongated particles in concrete aggregate decreases the workability appreciably for a given water cement ratio, thus requiring larger amounts of sand, cement and water. The flaky and elongated particles tend to orient in one plane and cause laminations which adversely affect the durability of the concrete. The percentage of flaky and elongated particles should be limited to 15 to 20.

Apparatus

Weighing balance, a set of sieves ranging from 25mm IS sieve to 10mm IS sieves, thickness gauge and length gauge.

 Procedure

a.            To determine the flakiness index of coarse aggregate

1.            Take a sufficient quantity W1 of coarse aggregate by quartering so as to provide at least 200 pieces of any fraction.

2.            Carry out sieving by hand. Shake each sieve in order: 25mm, 20mm, 16mm, 12.5mm, 10mm and 6.3mm, over a clean dry tray for a period not less than 2 minutes. The shaking is done with a varied motion: backward and forward, left to right, circular, clockwise and anticlockwise and with frequent jarring, so that the material is kept moving over the sieve surface in frequently changing directions.

3.            Pass the separated aggregate fractions as retained on the sieves in step 2 through the corresponding slots in the thickness gauge as shown. E.g. the material passing through 25mm sieve and retained on 20mm sieve is passed through 0.5(25+20) x 3/5 = 13.5 mm slot. Determine the mass of aggregate passing through each of the slots.

4.            Find the total mass W2 of the materials passing through the slots of the thickness gauge.

5.            Calculate the flakiness index as defines below:

The flakiness index is an empirical factor expressing the total material passing through the slots of the thickness gauge as the percentage of the mass of sample taken for testing.

b.            To determine the elongation index of coarse aggregate

1.            Take a sufficient quantity W3 of coarse aggregate by quartering so as to provide at least 200 pieces of any fraction.

2.            Carry out sieving by hand. Shake each sieve in order: 25mm, 20mm, 16mm, 12.5mm, 10mm and 6.3mm as explained in the part A so that the material is kept moving over the sieve surface in frequently changing directions.

3.            Pass the separated aggregate fractions as retained on the sieves in step 2 through the corresponding length gauge size as shown. E.g. the material passing through 25mm sieve and retained on 20mm sieve is passed through (9/5) x (25+20)/2 = 40.5 mm slot. A particle of length which cannot pass through the corresponding gauge size is taken as retained by the length gauge. Determine the mass of aggregate retained on each of the length gauge sizes.

4.            Find the total mass W4 of the material retained on the length gauges.

5.            Determine the elongation index as percentage material retained by the length gauges of the total material taken for testing.

                                            ∑W=W2

Flakiness index of coarse aggregate = W2/W1 X 100 =                     percent.

A. Elongation index of aggregate

                   Mass of aggregate, W3 =                           grams

                                                                                                                   ∑W=W4 

Elongation index = W4/W3 X 100 =           percent.

Precautions

1.            The representative sample should be taken by quartering. For quartering, the sample is thoroughly mixed and spread out evenly on the clean surface; it is then cut into four equal parts by a trowel. Two opposite quarters are taken and mixed to make the sample. If any further quantity reduction is required, the process may be repeated.

2.            The particles of length which cannot pass through the length gauge size are taken to be retained by the length gauge. They should not be forced to pass through an opening.

References:

1. IS 2386 part III – 1963              Methods of Test for Aggregates for Concrete