AGGREGATES

INTRODUCTION OF AGGREGATES Aggregates are the materials basically used as filler with binding material in the production of mortar and concrete. They are derived from igneous, sedimentary and metamorphic rocks. They occupy 70-80 per cent of the volume effect economy They should be clean, hard, strong, durable and graded in size to achieve utmost economy.

certain types of aggregates exhibit chemical bond at the interface of aggregates and cement paste. coarse aggregate (grit) and the smaller ones fine aggregate (sand).

CLASSIFICATION OF AGGREGATES

ON THE BASIS OF GEOLOGICAL ORIGIN Natural aggregates :- These are obtained by crushing from quarries of igneous, sedimentary or metamorphic rocks. They require sieving and washing before they can be used in concrete. Artificial aggregate:- Broken bricks, blast furnace slag and synthetic aggregates are artificial aggregates. Broken bricks known as brick bats are suitable for mass concreting, for example, in foundation bases. They are not used for reinforced concrete works.

On the basis of size Coarse aggregate:-Aggregate retained on 4.75 mm sieve are identified as coarse. They are obtained by natural disintegration or by artificial crushing of rocks. The maximum size of aggregate can be 80 mm. For economy the maximum size should be as large as possible but not more than one- fourth of the minimum thickness of the member.

All-in-aggregate :- Naturally available aggregates of different fractions of fine and coarse sizes are known as all-in-aggregate. The deficiency of any particular fraction can be corrected for use in the mix but they are not recommended for quality concrete.

Graded aggregate :- Aggregate most of which passes through a particular size of sieve are known as graded aggregate. For example, a graded aggregate of nominal size 20 mm means an aggregate most of which passes IS sieve 20 mm.

Fine Aggregate :- Aggregate passing through 4 Fine Aggregate :- Aggregate passing through 4.75 mm sieve are defined as fine. They may be natural sand—deposited by rivers, crushed stone sand—obtained by crushing stones and crushed gravel sand. The smallest size of fine aggregate (sand) is 0.06 mm. Depending upon the particle size, fine aggregates are described as fine, medium and coarse sands.

On the basis of shape Rounded aggregates:- These are generally obtained from river or sea shore and produce minimum voids (about 32 per cent) in the concrete. They have minimum ratio of surface area to the volume. Poor interlocking bond makes it unsuitable for high strength concrete and pavements.

Irregular aggregates:- They have voids about 36 per cent and require more cement paste as compared to rounded aggregate. Because of irregularity in shape they develop good bond and are suitable for making ordinary concrete.

Angular aggregates:- They have sharp, angular and rough particles having maximum voids (about 40 per cent). Angular aggregate provide very good bond than the earlier two, are most suitable for high strength concrete and pavements. Thus requirement of cement paste is relatively more.

Flaky aggregates:- These are sometimes wrongly called as elongated aggregate. The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times the mean dimension. For example, the mean sieve size for an aggregate piece passing through 50 mm and retained on 40 mm sieve is (50 + 40)/2 = 45.0 mm. If the least lateral dimension is less than 0.6 × 45 = 27.0 mm, the aggregate is classified as flaky.

Elongated aggregate are those aggregate whose length is 1 Elongated aggregate are those aggregate whose length is 1.8 times its mean dimension. They adversely affect durability and are restricted to maximum of 15 per cent.

Based on unit weight Aggregates are classified as normal-weight, heavy-weight and light-weight aggregate depending on weight and specific gravity

Characteristics of aggregates The properties to be considered while selecting aggregate for concrete are:-

strength The strength should be at least equal to that of the concrete. Rocks commonly used as aggregates have a compressive strength much higher than the usual range of concrete strength. The test conducted for strength evaluation are crushing test (most reliable) , impact-test and ten per cent fines test. The toughness of aggregate is measured by impact test. Hardness of aggregate is tested by abrasion test.

Stress strain curve for aggregates

stiffness The modulus of elasticity of concrete is approximately equal to the weighted average of the moduli of the cement paste and the aggregate, as such the modulus of the coarse aggregate has an important influence on the stiffness of concrete. modulus of elasticity, is a number that measures an object or substance's resistance to being deformed elastically (i.e., non- permanently) when a force is applied to it.

Bond strength Due to difference between the coefficients of thermal expansion of paste and aggregate and to the shrinkage of cement paste during hardening, concrete is in a state of internal stress even if no external forces are present. It is reported that the stresses are likely to be greatest at the paste-aggregate interfaces where minute cracks exist, even in concrete that has never been loaded. There is no standard test for bond but it is known that the rougher the surface texture of the particles, the better the bond.

Shape and texture Rounded aggregate are highly workable but yield low strength concrete. Same is the case with irregular shaped aggregate. Flaky aggregate require more cement paste, produce maximum voids and are not desirable. Angular shape is the best.

specific gravity The specific gravity of most of the natural aggregates lies between 2.6-2.7. The specific gravity and porosity of aggregates greatly influence the strength and absorption of concrete. Specific gravity of aggregates generally is indicative of its quality. A low specific gravity may indicate high porosity and therefore poor durability and low strength.

bulk density The bulk density of aggregate depends upon their packing, the particles shape and size, the grading and the moisture content. For coarse aggregate a higher bulk density is an indication of fewer voids to be filled by sand and cement.

voids The void ratio is calculated as If the voids in the concrete are more the strength will be low.

porosity porous aggregate absorb more moisture, resulting in loss of workability of concrete at a much faster rate.

moisture content A high moisture content increases the effective water/cement ratio to an appreciable extent and may render the concrete weak.

bulking The increase in the volume of a given mass of fine aggregate caused by the presence of water is known as bulking. The extent of bulking depends upon the percentage of moisture present in the sand and its fineness.

Effect of moisture content on bulking of sand

fineness modulus It is a numerical index of fineness, giving some idea about the mean size of the particles in the aggregates. The fineness modulus (F.M.) varies between 2.0 and 3.5 for fine aggregate, between 5.5 and 8.0 for coarse aggregate, and from 3.5 to 6.5 for all-in aggregate.

Aggregate, whose F.M. is required, is placed on a standard set of sieves (80, 63, 40, 20, 12.5, 10, 4.75, 2.36, 1.18 mm and 600, 300, 150 micro-m) and the set vibrated. The material retained on each sieve after sieving represent the fraction of aggregate coarser than the sieve in question but finer than the sieve above. The sum of the cumulative percentages retained on the sieves divided by 100 gives the F.M. A fineness modulus of 3.0 can be interpreted to mean that the third sieve i.e., 600 micro-m is the average size.

- Higher F.M. aggregate result in harsh concrete mixes and lower F.M. result in uneconomical concrete mixes.

Testing of aggregates 1 Particle Size Distribution Test: Sample of fine aggregate, coarse aggregate or all-in-aggregate, as required to be tested, are taken in sufficient quantities. The minimum weight of sample for the test should be as follows.

The air dried sample is placed on a set of specific sieves with largest size on the top. The set of sieve is shaked for 2 minutes. Arrangement of sieve for different types of aggregate is as follows.

A cruve is plotted with sieve sizes on abscissa on a graph (Fig. 6 A cruve is plotted with sieve sizes on abscissa on a graph (Fig. 6.3) and percentage of aggregate passing as ordinate. From this graph relative amount of various sizes of aggregate can be readily compared.

Flakiness Index and Elongation Index Test (IS: 2386 (Part I)) Because of large number of flaky particles in the coarse aggregate more voids are formed in the concrete consequently more mortar is required to fill the voids, resulting in uneconomy. Also, durability of concrete will be affected. For flakiness index (F.I) and elongation index (E.I) sufficient quantity of aggregate is taken so as to provide at least 200 pieces of any fraction to be tested. The sample is sieved through I.S. sieves as given in Table

For determining F. L the aggregate retained on sieves are separated For determining F.L the aggregate retained on sieves are separated. Now, each aggregate piece is passed through the corresponding slot in the thickness gauge. Flakiness index is given by ,

Bulking Test for Fine Aggregate (IS: 2386 (Part III)) Fine aggregate (sand) has a tendency to increase in volume (bulk) depending upon the moisture present in it. In making concrete mix, if the batching in done by volume, the actual quantity of sand in each batch will be less than the recommended volume and actual quantity of sand in each batch will be less than the recommended volume and consequently the mix will be rich in cement, i.e. the proportions of cement and sand will be different to be provided. This will result in uneconomy and may affect adversely the concrete also. Therefore, the amount of sand will have to be increased by percentage bulking (B).

In the laboratory test sufficient quantity of oven dried sand is filled in graduated cylinder up to a certain fixed mark. The sand is emptied in a container and the weigh of dry sand is determined 1% water is added in the sand mixed thoroughly and filled in the graduated cylinder. The volume of sand will be found to increase. The process is repeated by increasing the percentage of water in steps of 1% each time till a decrease in the volume of sand is observed. Still this addition of water is continued till the volume of sand comes back to the original volume (fully saturated).

A graph is plotted between moisture content and percentage increase in volume. The percentage bulking is given by,

Effect of moisture content on bulking of sand

Crushing Value Test (IS: 2386 (Part IV)) The material for the test should consist of aggregate passing 12.5 mm sieve and retained on 10 mm sieve. The aggregate is tested in a surface-dry condition. The weight of material comprising the test sample is determined (weight A). The cylinder of the test apparatus is positioned on the base-plate and the test sample is added in thirds, each being subjected to 25 strokes from the tamping rod. The surface of the aggregate is carefully levelled and the plunger is inserted so that it rests horizontally on this surface.

The apparatus, with the test sample and plunger in position is then placed between the platens of the testing machine and loaded at an uniform rate as possible, so that the total load is reached in 10 minutes. The total load should be 400 kN. The load is released and the whole of the material is removed from the cylinder and sieved on a 2.36 mm sieve for the standard test. The fraction passing the sieve is weighed. The ratio of the weight of fines formed to the total sample weight in each test is expressed as a percentage, recorded to the first decimal place:

Determination of Bulk Density and Voids (IS: 2386 (Part III)) The test is carried out on dry material when determining the voids, but when bulking tests are required material with a given percentage of moisture may be used. The measure is filled with thoroughly mixed aggregate to about one-third and tamped with 25 strokes of the rounded end of the tamping rod. A further similar quantity of aggregate is added with a further tamping of 25 times and the surplus aggregate is struck off, using the tamping rod as a straight edge. The net weight of the aggregate in the measure is determined and the bulk density is calculated.

The measure is then filled to overflowing by means of a shovel or scoop, the aggregate being discharged from a height not exceeding 50 mm above the top of the measure. The surface of the aggregate is then levelled with a straight edge. The net weight of the aggregate in the measure is determined and the bulk density is calculated. The percentage of voids are calculated as follows:

Deleterious Materials and Organic Impurities Test (IS: 2386 Part II)) Determination of clay lumps: The sample of aggregate is dried to constant weight at a temperature not exceeding 110°C. The sample is spread in a thin layer on the bottom of container and examined for clay lumps. The clay lumps, if any, are broken and removed by the use of sieves as given in Table .

2.Determination of clay, fine silt and fine dust Sedimentation method is used to determine clay, fine silt and dust, including particles up to 20 micron. For testing fine aggregate for clay, fine silt and dust weight approximately 300g of the air dried sample, passing the 4.75 mm IS sieve, and place it in the screw-topped glass jar, together with 300 ml of diluted sodium oxalate solution. The jar is then rotated about its long axis (with this axis horizontal) at a speed of 80 rpm for 15 minutes. The suspension is then poured into 1000 ml measuring cylinder and the residue washed by gentle swirling and decantation of successive 150 ml portions of sodium oxalate solution, the washings being added to the cylinder until the volume is made up to 1000 ml.

For coarse aggregate the sample is placed in a suitable container, covered with a measured volume of sodium oxalate solution (0.8 g/l), agitated vigorously and the liquid suspension transferred to the 1000 ml measuring cylinder. The process is repeated until all clayey material is transferred to the cylinder. The volume is made upto 1000 ml with sodium oxalate solution.